WAEC SSCE - Chemistry - 2022

Which of the following statements about atoms of a metal is correct? They

Détails de la réponse

Atoms of a metal are held together by a sea of electron cloud. This means that the electrons in the outermost energy levels of the metal atoms are free to move around, forming a "sea" of negatively charged electrons that are shared among all the atoms. This creates a strong metallic bond between the atoms, which is responsible for the unique properties of metals, such as their high thermal and electrical conductivity, malleability, and ductility. Therefore, the correct statement is "are held together by a sea of electron cloud".

If 50 cm3 ${}^3$ of a saturated solution of KNO3 ${}_3$ at 40 °C contained 5:05 g of the salt, its solubility at the same temperature would be
[KNO3 ${}_3$ = 101]

Détails de la réponse

V= 50cm3 ${}^3$
Mass= 5.05g
Relative molecular mass of KNO3 ${}_3$ = (39+14+(3*16)) = 101
Convert 50cm3 ${}^3$ to dm3 ${}^3$ which is
1000cm³ = 1dm3 ${}^3$
50cm³ = 50*1/1000
= 0.05dm3 ${}^3$
Moles = mass/ molar mass
= 5.05/101 =0.05mole
Solubility= mole/volume
Solubility=0.05mol/0.05dm3 ${}^3$
Solubility=1.0mol/dm_3

Which of the following sketches is a graphical representation of Boyle's law?

Détails de la réponse

Boyle's law states that at a constant temperature, the volume of a given mass of gas is inversely proportional to its pressure. This means that as the pressure of a gas increases, its volume decreases and vice versa, while keeping the temperature constant. The graphical representation of Boyle's law is a plot of the pressure of the gas against its volume. The correct sketch that represents Boyle's law will show a hyperbolic curve. Therefore, the correct answer to the question is the sketch that shows a hyperbolic curve, which is option C.

The atom with the electron configuration
1s2 ${}^2$2s2 ${}^2$2p63s2 ${}^2$3p6 ${}^6$3d10 ${}^{10}$4s2 ${}^2$4p4 ${}^4$ is in

Détails de la réponse

The given electron configuration belongs to an atom with 4 energy levels (period 4). The last filled subshell is 4p, which means that the atom is in the p-block of the periodic table. Therefore, the correct option is "period 4, p-block."

The by-product of fermentation of sugar is

Détails de la réponse

During fermentation, sugar is broken down by microorganisms such as yeast into simpler compounds such as ethanol and carbon dioxide. Therefore, the by-product of fermentation of sugar is carbon(IV) oxide.

Which of the following compounds would react rapidly with bromine?

Détails de la réponse

The addition of bromine to most alkenes is so fast that when bromine is added dropwise to a solution of the alkene the red bromine color disappears almost immediately. In fact, this discharge of color is a useful qualitative test for alkenes.

Which of the following properties indicate that an element is a metal. It

I. reacts with oxygen to form an acidic oxide
II. forms ionic chlorides
III. has variable oxidation states
IV. displaces hydrogen from dilute HCl

Détails de la réponse

Of the four properties listed, only II and IV are indicative of a metal. II. Metals tend to form ionic compounds with non-metals, such as chlorine. This is because metals have low electronegativity, so they are willing to lose electrons to form positively charged ions that can then combine with negatively charged ions like chloride ions. IV. Metals tend to displace hydrogen from dilute acids, which is a characteristic reaction of metals. When a metal reacts with an acid, it gives up electrons to hydrogen ions, forming hydrogen gas and a metal ion. This is called a single displacement reaction. I and III are not indicative of a metal. Non-metals tend to react with oxygen to form acidic oxides, and many non-metals can have variable oxidation states. Therefore, the correct answer is II and IV only.

A hydrogen chloride gas reacted with oxygen gas to yield water and chlorine gas. The mole ratio of the hydrogen chloride gas to water is

Détails de la réponse

4HCl + O2 ${}_2$ -- 2Cl2 ${}_2$ + 2H2 ${}_2$O
4:2 = 4/2
2:1

If the molar mass of X(HCO3)2 ${}_2$ is 162 g mol−1 ${}^{-1}$, determine the relative atomic mass of X.
[H = 1.0, C = 12.0, 0 = 16.0]

Détails de la réponse

The molar mass of X(HCO3)2 can be calculated by adding up the atomic masses of all the atoms in one mole of the compound. One mole of X(HCO3)2 contains: - 2 moles of hydrogen (2 x 1.0 = 2.0) - 2 moles of carbon (2 x 12.0 = 24.0) - 6 moles of oxygen (6 x 16.0 = 96.0) - 1 mole of X The total molar mass is then: 2.0 + 24.0 + 96.0 + m(X) = 162 g/mol Solving for m(X), we get: m(X) = 162 g/mol - 2.0 - 24.0 - 96.0 m(X) = 40 g/mol Therefore, the relative atomic mass of X is 40, which corresponds to the atomic mass of the element with the symbol Ca (calcium). Therefore, option A is the correct answer.

Détails de la réponse

AlCl3 → Al3+ + 3Cl-
At cathode:
Al3+ + 3e- → Al
3 mole of electrons corresponds 3 F charge
So, required charge =3 F

Consider the following table

Which of the substances is a liquid at room temperature and rapidly evaporates on exposure to air?

Détails de la réponse

The substance that is a liquid at room temperature and rapidly evaporates on exposure to air is Q. This is because its boiling point is 40 ºC which is lower than room temperature, and it will therefore be a liquid at room temperature. Additionally, it has a relatively low boiling point compared to the other substances in the table, indicating that it will evaporate quickly on exposure to air.

Graphite is used as a dry lubricant due to the presence of

Détails de la réponse

Graphite is a form of carbon that is composed of layers of hexagonal rings stacked on top of each other. The layers are held together by weak intermolecular forces, allowing them to slide past one another easily. This gives graphite its soft and slippery texture, making it an ideal dry lubricant. Therefore, the correct option is "layered structures". The other options are not relevant to the properties of graphite as a dry lubricant.

What number of moles of oxygen would exert a pressure of 10 atm at 320 K in an 8.2 dm3 ${}^3$ cylinder?
[R=0.082 atm dm−3 ${}^{-3}$ mol−1 ${}^{-1}$ K−1 ${}^{-1}$]

Détails de la réponse

P = 10atm, T= 320K, V= 8.2dm3 ${}^3$
PV =nRT
n = P * V/R * T
10 * 8.2/0.082 * 320
= 82/26.24
n = 3.13

Charcoal is used in the decolourization of sugar because of its

Détails de la réponse

Charcoal is used in the decolorization of sugar because of its adsorption property. Adsorption is the process in which atoms, ions or molecules from a substance adhere to the surface of the adsorbent. Charcoal has a large surface area with lots of tiny pores which provides a lot of sites for the sugar molecules to stick to. The impurities in the sugar are adsorbed on the surface of the charcoal particles and get trapped in its pores, leaving behind a clear and colorless solution.

Electropositivity of elements across the periodic table normally

Détails de la réponse

The electronegativity of elements across the periodic table generally follows a pattern. Electronegativity refers to the tendency of an atom to attract electrons towards itself when it is part of a chemical bond. However, in this question, we are discussing electropositivity, which is the opposite of electronegativity. Electropositivity refers to the tendency of an atom to lose electrons and form positive ions. It is essentially the measure of how easily an element gives away its valence electrons. In general, as we move across a period from left to right in the periodic table, the electropositivity of elements decreases. This means that elements on the left side of a period are more electropositive compared to those on the right side. This trend can be attributed to the increase in effective nuclear charge (the positive charge felt by the outermost electrons) as we move across the period. The stronger pull of the nucleus on the valence electrons makes it harder for the atoms to lose electrons and become positive ions. On the other hand, as we move down a group in the periodic table, the electropositivity of elements remains relatively constant or even increases. This is because the valence electrons are located in higher energy levels that are farther from the nucleus. The increased distance reduces the attractive force between the nucleus and the valence electrons, making it easier for the atoms to lose electrons and become positive ions. Therefore, based on the explanation above, the correct option is "decreases across the period."

Which of the following scientists formulated the law of conservation of mass?

Détails de la réponse

The scientist who formulated the law of conservation of mass is A. Lavoisier, also known as Antoine Lavoisier. He is credited with discovering and stating this fundamental principle of chemistry. The law of conservation of mass states that in a chemical reaction, matter is neither created nor destroyed; it is simply rearranged into different combinations. Lavoisier's experiments and observations led him to conclude that the total mass of substances involved in a chemical reaction remains constant before and after the reaction. This law laid the foundation for our understanding of chemical reactions and the concept of mass conservation in chemical processes.

The cleansing effect of soap is low in acidic water because of

Détails de la réponse

Soap is an ionic compound made up of a long hydrocarbon chain and a polar head. The polar head is attracted to water, while the hydrocarbon chain is not. When soap is added to water, it forms micelles which help to dissolve and remove dirt and grime. However, in acidic water, the soap molecules react with the hydrogen ions (H+) in the acidic water, which neutralizes the polar head of the soap molecules, reducing their effectiveness in dissolving dirt and grime. As a result, the cleansing effect of soap is low in acidic water. Therefore, the correct answer is "the formation of unionized fatty acid."

Oxochlorate(I) acid is used as a bleaching agent because it is

Détails de la réponse

Oxochlorate(I) acid is used as a bleaching agent because it is an oxidizing agent. An oxidizing agent causes a chemical reaction that removes electrons from the substance being oxidized. In the case of bleaching, Oxochlorate(I) acid removes electrons from the colored molecules in the fabric, thus breaking down the pigment and removing the color.

The IUPAC name for CH3 ${}_3$CHCH3 ${}_3$CHCLCHCH3 ${}_3$CH2 ${}_2$CH3 ${}_3$ is

Détails de la réponse

- Recognise the functional group in the compound. This will determine the suffix of the name

- Find the longest continuous carbon chain that contains the functional group (it won't always be a straight chain) and count the number of carbon atoms in this chain. This number will determine the prefix (the beginning) of the compound's name

- Number the carbons in the longest carbon chain (Important: If the molecule is not an alkane (i.e. has a functional group) you need to start numbering so that the functional group is on the carbon with the lowest possible number). Start with the carbon at the end closest to the functional group.

- Look for any branched groups:
Name them by counting the number of carbon atoms in the branched group and referring to Table 4.6, these groups will all end in -yl.
Note the position of the group on the main carbon chain. If there is more than one of the same type of branched group then both numbers must be listed (e.g. 2,4 -) and one of the prefixes listed in Table 4.7 must be used. Important: If the molecule is an alkane the branched group must be on the carbon with the lowest possible number.
The branched groups must be listed before the name of the main chain in alphabetical order (ignoring di/tri/tetra).
If there are no branched groups this step can be ignored.

- For the alkyl halides the halogen atom is treated in much the same way as branched groups:
To name them take the name of the halogen atom (e.g. iodine) and replace the -ine with -o (e.g. iodo)
Give the halogen atom a number to show its position on the carbon chain. If there is more than one halogen atom the numbers should be listed and a prefix should be used (e.g. 3,4-diiodo- or 1,2,2-trichloro-). See Table 4.7 for a list of the prefixes.
The halogen atoms must be listed before the name of the main chain in alphabetical order (ignore di/tri/tetra).
If there are no halogen atoms this step can be ignored.
- Combine the elements of the name into a single word in the following order:
branched groups/halogen atoms in alphabetical order (ignoring prefixes)
prefix of main chain
name ending according to the functional group and its position on the longest carbon chain.

Détails de la réponse

Among the given options, only NH3 (ammonia) is alkaline. When ammonia gas dissolves in water, it produces ammonium hydroxide (NH4OH), which is an alkaline solution. The other gases, NO2, CO2, and N2 are not alkaline gases. NO2 is an acidic gas, CO2 is a neutral gas, and N2 is also a neutral gas since it does not have any tendency to either lose or gain electrons.

Which of the following processes occur during fractional distillation of petroleum?

Détails de la réponse

During fractional distillation of petroleum, the process that occurs is evaporation and condensation. Petroleum is a mixture of hydrocarbons with different boiling points. During fractional distillation, the mixture is heated and vaporized. The vapor rises through the fractionating column, which is cooler at the top and hotter at the bottom. As the vapor rises, it cools and condenses at different heights in the column, based on its boiling point. The lighter hydrocarbons with lower boiling points will condense at the top of the column, while the heavier hydrocarbons with higher boiling points will condense lower down. This process of separating a mixture based on the differences in boiling points is known as fractional distillation. Therefore, the correct answer is "Evaporation and condensation".

An oxide has the following properties. It

I. is a white powder
II. reacts with HCI.
III. reacts with NaOH
IV. is insoluble in water
The oxide is

Détails de la réponse

Based on the given properties of the oxide, let's analyze each property and its implications: I. The oxide is a white powder: This property does not provide direct information about the nature of the oxide (alkaline, amphoteric, acidic, or neutral), as the color alone does not determine its acidity or basicity. II. The oxide reacts with HCl: This indicates that the oxide is acidic because it is reacting with an acid (HCl). Acidic oxides are substances that, when dissolved in water, produce acidic solutions. III. The oxide reacts with NaOH: This suggests that the oxide is amphoteric because it reacts with both acids (HCl) and bases (NaOH). Amphoteric substances can exhibit both acidic and basic properties depending on the conditions. IV. The oxide is insoluble in water: Insolubility in water does not provide direct information about the nature of the oxide. It indicates that the oxide does not readily dissolve in water but does not necessarily determine whether it is alkaline, amphoteric, acidic, or neutral. Based on the information provided, we can conclude that the oxide exhibits acidic and amphoteric properties. Therefore, the appropriate answer would be "acidic" or "amphoteric."

A white precipitate was formed when BaCl2(aq) ${}_{2(��)}$ was added to an aqueous solution
of a salt X. The precipitate dissolved in dilute HCl with rapid effervescence. Salt X is likely to contain

Détails de la réponse

Effervescence is the formation of gas bubbles in a liquid by a chemical reaction. An example of effervescence is the release of carbon dioxide which bubbles as a gas from the liquid when limestone chips, which are composed of calcium carbonate, are added to dilute hydrochloric acid.

The maximum number of covalent bonds formed by nitrogen is

Détails de la réponse

Nitrogen has three unpaired electrons and one lone pair of electrons; therefore, it can form three covalent bonds and one coordinate bond. E.g ammonium ion.

A colourless gas with a pungent smell is evolved when dilute hydrochloric acid is added to a sample of a salt. The gas evolved could turn

Détails de la réponse

If the potassium manganate(VII) solution is acidified with dilute sulfuric acid, the purple solution becomes colourless.

If 5.0 g of marble reacts with 25.0 cm3 hydrochloric acid, which of the following combinations has the fastest reaction rate?

Détails de la réponse

The reaction rate is determined by the collision frequency and effectiveness of collisions between reactant particles. A higher concentration of reactants generally leads to a faster reaction rate because it increases the likelihood of successful collisions. In this case, the concentration of hydrochloric acid is given in terms of mol dm^-3. The higher the molarity (mol dm^-3) of the hydrochloric acid, the greater the concentration of HCl particles in the solution. Comparing the options, we can see that the combinations with higher molarities of hydrochloric acid (2.5 mol dm^-3) are likely to have a faster reaction rate than the combinations with lower molarities (2.0 mol dm^-3). Furthermore, the option involving powdered marble instead of marble chips can also contribute to a faster reaction rate. Powdered marble provides a larger surface area for the reaction to occur, allowing for more collisions between the marble particles and the hydrochloric acid. Therefore, the combination of powdered marble and 2.5 mol dm^-3 HCl(aq) is most likely to have the fastest reaction rate among the given options.

Which of the following oxides is amphoteric?

Détails de la réponse

An amphoteric oxide is a type of oxide that can act as both an acid and a base. It has the ability to react with both acids and bases to form different compounds. Carbon(II) oxide (CO) is not amphoteric. It is a neutral oxide and does not exhibit acidic or basic properties. Nitrogen(IV) oxide (NO2) is not amphoteric either. It is a non-metal oxide and typically acts as an acidic oxide, reacting with water to form nitric acid. Lead(II) oxide (PbO) is amphoteric. It can react with both acids and bases. With acids, it acts as a base and forms salts. With bases, it acts as an acid and forms plumbates. Calcium oxide (CaO) is not amphoteric. It is a basic oxide and reacts with acids to form salts. Therefore, among the given options, lead(II) oxide is the amphoteric oxide.

The products of the electrolysis of dilute sodium chloride using carbon electrodes are

Anode Cathode

Détails de la réponse

Oxygen is produced at the anode and hydrogen is produced at the cathode in dilute NaCl

Consider the following reaction equation
N2 ${}_2$(g) + 3H2(g) ${}_{2(�)}$; ∆H = -92kJ

Détails de la réponse

In this case, an increase in in temperature will cause the equilibrium position to shift to the left thereby decreasing the yield of ammonia.

The first definition of an element was made by

Détails de la réponse

The word element was first given by Rebert Boyle. It is defined as the simplest chemical substance that cannot be broken down during a chemical reaction

Which of the following acids would form normal salt only?

Détails de la réponse

Normal salts are formed when all the replaceable hydrogen ions in the acid have been completely replaced by metallic ions

Which of the following sugars is a product of the condensation of monosaccharides?

Détails de la réponse

The sugar that is a product of the condensation of monosaccharides is maltose. Condensation refers to a chemical reaction where two molecules combine to form a larger molecule while releasing a small molecule, usually water. In the case of maltose, it is formed by the condensation of two glucose molecules. When glucose molecules undergo condensation, a bond forms between the hydroxyl (-OH) groups of the two glucose units, resulting in the formation of maltose and the release of a water molecule. Therefore, maltose is the sugar that is produced by the condensation of monosaccharides.

Before a reaction could take place, there should be

Détails de la réponse

Before a reaction could take place, the reactant particles must undergo a breakage of their bonds. This is because chemical reactions involve the formation of new chemical bonds between reactant particles to form products. In order for new bonds to be formed, the existing bonds in the reactants must first be broken. Therefore, the correct option is "breakage of bonds of reactants".

When air in a syringe is compressed such that there is no change in temperature, the

Détails de la réponse

When air in a syringe is compressed without a change in temperature, the pressure increases. When you compress air in a syringe, you are decreasing the volume of the syringe while keeping the temperature constant. According to Boyle's law, which states that the pressure of a gas is inversely proportional to its volume at constant temperature, when the volume decreases, the pressure increases. Therefore, in this scenario, the increase in pressure is the expected outcome when compressing air in a syringe without a change in temperature. The other options mentioned (air liquefies, intermolecular space increases, and density decreases) are not applicable in this context.

Which of the following elements would displace copper from a solution of copper ions?

Détails de la réponse

The reactivity series allows us to predict how metals react. A more reactive metal displaces a less reactive metal from a compound. Lead is more reactive metal than copper hence, copper is displaced from copper solution by lead.

Which of the following statements about liquids is/are true?

I. Liquids maintain their volume at constant temperature.
II. Liquids have fixed shape
III. Liquids do not diffuse
IV. Change in pressure affects volume of liquids

Détails de la réponse

A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure

Which of the following bond types is intermolecular?

Détails de la réponse

An intermolecular bond refers to a bond that occurs between molecules, as opposed to intramolecular bonds that exist within a molecule. Let's analyze the given options: - Covalent bond: A covalent bond is formed by the sharing of electrons between atoms within a molecule, rather than between different molecules. It is an intramolecular bond. - Hydrogen bond: A hydrogen bond occurs when a hydrogen atom in one molecule is attracted to an electronegative atom (such as oxygen, nitrogen, or fluorine) in another molecule. It is an intermolecular bond. - Ionic bond: An ionic bond involves the transfer of electrons from one atom to another, resulting in the attraction between positively and negatively charged ions. Ionic bonds typically occur within compounds and are intramolecular bonds. - Metallic bond: A metallic bond is formed between metal atoms and involves the sharing of delocalized electrons within a metal lattice. Metallic bonds are intramolecular bonds. Based on this analysis, the only intermolecular bond among the options is the hydrogen bond.

Increasing the temperature generally

Détails de la réponse

Increasing the temperature generally affects the solubility of solutes in solvents. When the temperature of a solvent is increased, the solvent molecules gain kinetic energy, which results in more molecular motion. In the case of a solid solute, the increased motion of the solvent molecules causes the solvent to be able to dissolve more of the solute, thus increasing the solubility of the solid. Conversely, in the case of a gaseous solute, the increased motion of the solvent molecules causes the gas molecules to escape from the solution more easily, thus decreasing the solubility of the gas. Therefore, the correct option is: "increases the solubility of a solid in a liquid but decreases the solubility of a gas in a liquid."

The IUPAC name of the compound CH3CH(CH3)CHCH2 is

Détails de la réponse

The IUPAC name of the compound CH3CH(CH3)CHCH2 is 3-methyl but-1-ene. To name this compound, we first locate the longest continuous carbon chain, which is four carbons in this case. The parent name is butene. We number the chain so that the double bond has the lowest possible number, which is at carbon 1. The next step is to identify the substituents on the main chain. There is one methyl group attached to the third carbon atom (counting from the double bond), and no other substituents. Therefore, the name of the compound is 3-methyl but-1-ene.

What is the partial pressure of oxygen at s.t.p. in a gaseous mixture containing 100 cm3 of oxygen and 900 cm3 of nitrogen gas?

Détails de la réponse

At s.t.p (standard temperature and pressure), the pressure is 1 atmosphere (1 atm) and the temperature is 0 degrees Celsius (273.15 K). In the given gaseous mixture, the partial pressure of oxygen can be calculated using the following formula: Partial pressure of oxygen = Total pressure x Volume fraction of oxygen The total pressure of the mixture is 1 atm. The volume fraction of oxygen can be calculated by dividing the volume of oxygen by the total volume of the mixture: Volume fraction of oxygen = Volume of oxygen / Total volume of mixture Volume fraction of oxygen = 100 cm3 / (100 cm3 + 900 cm3) Volume fraction of oxygen = 0.1 Substituting these values in the formula, we get: Partial pressure of oxygen = 1 atm x 0.1 Partial pressure of oxygen = 0.1 atm Therefore, the partial pressure of oxygen in the given gaseous mixture is 0.1 atm. is the correct answer.

Ionization energy increases across the period in the periodic table because

Détails de la réponse

As the nuclear charge increases, its attraction for the outermost electron increases and so more energy is required to remove the outermost electron. Hence, the ionization energy increases.

If 60 g of M combines with 24 g of oxygen, what would the empirical formula of the oxide be?

Détails de la réponse

Data obtained from the question
M = 60 g
O = 24 g
Empirical formula =?
How to determine the empirical formula
Divide by their molar mass
M = 60÷120 = 0.5
O = 24 ÷16 = 1.5
Divide by the smallest
M = 0.5÷0.5 = 1
O = 1.5 ÷0.5 = 3
Thus, the empirical formula of the oxide is MO₃

The following compounds are condensation polymers except

Détails de la réponse

Condensation polymers are a type of polymer formed through a condensation reaction, where two molecules combine and release a small molecule such as water as a byproduct. In condensation polymers, the repeating units in the polymer chain are linked together by covalent bonds formed during the condensation reaction. Looking at the options given: - Nylon: Nylon is a condensation polymer. It is formed by the condensation reaction between two monomers, typically a diamine and a dicarboxylic acid, which results in the formation of amide bonds between the monomers. - Protein: Proteins are also condensation polymers. They are formed by the condensation reaction between amino acids, where peptide bonds are formed between the amino acid units. - Starch: Starch is not a condensation polymer. It is a polysaccharide composed of glucose monomers linked together by glycosidic bonds through a process known as dehydration synthesis, which is a type of condensation reaction. However, starch is not considered a condensation polymer in the strict sense because it does not form a long chain of repeating units with the release of a small molecule. - Polyethene: Polyethene, also known as polyethylene, is not a condensation polymer. It is a polymer formed through an addition polymerization reaction, where monomers containing a carbon-carbon double bond, such as ethene, are added together to form a long chain polymer without the release of any byproduct. Based on this information, the compound that is not a condensation polymer among the given options is polyethene (polyethylene).

Détails de la réponse

To determine the quantity of electricity used, we need to calculate the charge (Q) passing through the electrolytic cell. The charge is given by the formula: Q = I * t Where: Q is the charge (in coulombs) I is the current (in amperes) t is the time (in seconds) In this case, the current is given as 0.20 amperes and the time is 60 minutes. However, we need to convert the time to seconds since the formula requires it in seconds. There are 60 seconds in a minute, so 60 minutes is equal to 60 * 60 = 3600 seconds. Now, we can substitute the values into the formula: Q = 0.20 A * 3600 s Q = 720 C Therefore, the quantity of electricity used when a current of 0.20 amperes is passed through the electrolytic cell for 60 minutes is 720 coulombs. So, the correct answer is 720C.

Which of the following statements about the standard hydrogen electrode is not correct?

Détails de la réponse

The standard hydrogen electrode is an electrode used as a reference for measuring the standard electrode potential of other half-cells. One of the given statements about the standard hydrogen electrode is incorrect. Let's look at each statement to identify the incorrect one. - The hydrogen gas is at a pressure of 1 atmosphere: This statement is correct. The hydrogen gas is bubbled over the platinum electrode at a pressure of 1 atmosphere. - A solution containing 1 mol dm-3 of H+ (aq) ions is used: This statement is correct. The hydrogen electrode is dipped into a solution containing 1 mol dm-3 of H+ ions. - A platinum electrode is used: This statement is correct. The standard hydrogen electrode consists of a platinum electrode coated with finely divided platinum black. - The temperature is kept at 20 °C: This statement is incorrect. The temperature of the standard hydrogen electrode should be 25 °C (298 K), not 20 °C. Therefore, the statement that is not correct is "The temperature is kept at 20 °C."

Which of the following statements about an equilibrium system is correct?

Détails de la réponse

An equilibrium system refers to a state where the rates of forward and backward reactions are equal. Therefore, the statement that is correct is "Forward and backward reactions occur at the same rate". The other options are incorrect because: - The concentrations of reactants and products are not necessarily equal at equilibrium. The actual concentrations depend on the equilibrium constant and the initial concentrations of the reactants and products. - The addition of a catalyst only affects the rate of the reaction, but not the position of equilibrium or the concentrations of the reactants and products at equilibrium. - Changes in temperature can affect the position of equilibrium and the concentrations of the reactants and products at equilibrium.

Consider the following reaction equation:
CaO + SiO2 ${}_2$ ------> CaSiO3 ${}_3$
Silicon(IV) oxide is acting as

Détails de la réponse

Silicon(IV) oxide, also known as silica, is acting as an acidic oxide in the given reaction. It reacts with calcium oxide (CaO), which is a basic oxide, to form calcium silicate (CaSiO3), which is a salt. The reaction involves the neutralization of the basic oxide by the acidic oxide to produce a salt. Therefore, the correct option is "an acidic oxide".

What is the percentage composition of carbon
in Ca(HCO3)2 ${}_2$ ?
[ Ca = 40.0, 0= 16.0, C = 12.0, H= 1.0]

Détails de la réponse

. % of C in Ca(HCO3)2
Molar mass of Ca(HCO3)2 = 40+(1+12+(16*3))2
=40+(13+48)2
=40+122
=162
C/Ca(HCO3)2 x 100
=12 * 2/162 * 100
=14.8%

Alkanols can be manufactured from alkenes by the initial reaction of alkenes with

Détails de la réponse

An alkene reacts with conc. H2SO4 to yield an alkyl hydrogen tetraoxosulphate (VI). The compound formed undergoes hydrolysis when boiled with water to yield an alcohol.

Détails de la réponse

- So any electron configuration in which the last electron (again, the valence electron) is in a higher energy orbital, this element is said to be in an excited state.

- When a carbon atom becomes excited by absorbing energy, one of its 2s electrons is promoted into the 2p sublevel.

(a)(i) Define the first ionization energy of an element

(ii) Consider the following table and use it to answer te question that follows

Explain briefly why the first ionization energy of B is less than that of Be despite the fact that the atomic number of B is greater than that of Be.

(b) When Titanium chloride was electrolysed by passing 0.12 A current through the solution for 500 seconds, 0.015 g of titanium was deposited. What is the charge on the titanium ion?
[ IF= 96500 C, Ti= 48.0 ]

(c)(i) Aluminium can be obtained by the application of electrolysis. State the electrolyte which yields aluminium on electrolysis.

(ii) Name two major factors which would favour the siting of an aluminium smelter in a country.

(d)(i) Define the term paramagnetism.

(ii) Consider the following ions: 24Cr2+ ${}^{?{?}^{2+}}$, 24Cr6+ ${}^{?{?}^{6+}}$

(I) Deduce the number of unpaired electrons in each of the ions. 

(II) State which of the ions will have a greater power of paramagnetism

(l) Give a reason for the answer stated in (d)(ii)(II)

Détails de la réponse

(a)(i) The first ionization energy of an element is the amount of energy required to remove one electron from a neutral atom of that element in the gaseous state. (ii) The missing value in the table is 730 kJ mol⁻¹ for the first ionization energy of lithium (Li). Explanation for (a): The first ionization energy of B is less than that of Be because B has an electron in a p-orbital, which is further away from the nucleus than the s-orbital electrons in Be. As a result, the attraction between the nucleus and the outermost electron is weaker in B than in Be, making it easier to remove an electron from B. (b) The amount of charge on the titanium ion can be calculated using the formula: Charge = (Current x Time x IF) / Mass where IF is the Faraday constant and equals 96500 C mol⁻¹. Plugging in the values given, we get: Charge = (0.12 A x 500 s x 96500 C mol⁻¹) / 0.015 g Charge = 1.934 x 10⁶ C mol⁻¹ Since the charge on a single titanium ion is 4+, we can calculate the number of moles of titanium deposited by dividing the mass by the molar mass: n(Ti) = 0.015 g / 48.0 g mol⁻¹ = 3.125 x 10⁻⁴ mol Dividing the total charge by the number of moles of titanium gives us the charge on a single titanium ion: Charge on Ti⁴⁺ ion = (1.934 x 10⁶ C mol⁻¹) / (3.125 x 10⁻⁴ mol) = 6.188 x 10⁹ C (c)(i) The electrolyte which yields aluminium on electrolysis is molten cryolite (Na₃AlF₆) mixed with aluminium oxide (Al₂O₃). (ii) Two major factors which would favour the siting of an aluminium smelter in a country are the availability of cheap electricity and the presence of bauxite ore, which is the raw material used to produce aluminium. (d)(i) Paramagnetism is the property of an atom, ion, or molecule that has unpaired electrons and is attracted by a magnetic field. (ii) For 24Cr2+, there are 4 unpaired electrons, while for 24Cr6+, there are no unpaired electrons. Therefore, 24Cr2+ will have a greater power of paramagnetism. Explanation for (d)(ii): The power of paramagnetism is directly proportional to the number of unpaired electrons. Since 24Cr2+ has 4 unpaired electrons, it will have a greater power of paramagnetism than 24Cr6+, which has no unpaired electrons. This is because the unpaired electrons in 24Cr2+ are available for alignment with an external magnetic field, whereas the paired electrons in 24Cr6+ are not.

Détails de la réponse

(a) i. According to the Lewis concept, an acid is defined as a substance that can accept a pair of electrons (an electron pair acceptor) during a chemical reaction. In other words, acids are substances that can form coordinate covalent bonds by accepting electron pairs from other molecules or ions. ii. An example of a Lewis acid is boron trifluoride (BF3). BF3 has an incomplete octet and can accept an electron pair from a Lewis base to form a coordinate covalent bond. (b) Salting out in soap preparation refers to the process of adding salt (such as sodium chloride) to a soap mixture to separate the soap from the aqueous solution. When salt is added, it reduces the solubility of soap in water, causing the soap to precipitate or separate out. This occurs because the salt ions compete with the soap molecules for the water molecules, disrupting the soap-water interactions and promoting the formation of soap aggregates or micelles. (c) The reagent and condition necessary for the conversion of H-C≡C-H to Ag-C≡C-Ag is silver nitrate (AgNO3) and ammonia (NH3) as a complexing agent. The reaction takes place in the presence of sunlight or UV light. (d) The percentage abundance of an isotope refers to the relative abundance or proportion of a specific isotope in a naturally occurring sample of an element. It is expressed as a percentage and represents the fraction of the total number of atoms of the element that is composed of the specific isotope. (e) i. The element with atomic number 18 is argon (Ar), which belongs to the noble gas group. Noble gases are known for their stable electronic configurations and inert nature. Since argon already has a complete octet in its outer electron shell, it does not readily form compounds, including oxides. ii. Chlorine(I) oxide (Cl2O) has a low melting point due to its weak intermolecular forces. It exists as discrete Cl2O molecules held together by relatively weak van der Waals forces. These forces are easily overcome, resulting in a low melting point for the compound. (f) A test to distinguish between concentrated HNO3 (nitric acid) and concentrated H2SO4 (sulfuric acid) is to add a small piece of copper metal to each acid. Nitric acid is a strong oxidizing agent and will react with the copper, producing brown nitrogen dioxide gas (NO2) and dissolving the copper. Sulfuric acid, on the other hand, is not a strong oxidizing agent and will not react with copper under normal conditions. (g) Two differences between an electrochemical cell and an electrolytic cell are: 1. In an electrochemical cell (such as a voltaic cell or a galvanic cell), the chemical reactions are spontaneous and produce electrical energy. In contrast, an electrolytic cell requires an external source of electrical energy to drive non-spontaneous reactions. 2. In an electrochemical cell, electrons flow spontaneously from the anode (where oxidation occurs) to the cathode (where reduction occurs). In an electrolytic cell, the direction of electron flow is reversed by the external power source to drive the non-spontaneous reactions. (h) The trend in ionization energy, which refers to the energy required to remove an electron from an atom or ion, affects the reactivity of group 1 elements (alkali metals). As we move down the group from top to bottom, the ionization energy decreases. This means that the outermost electron in alkali metals is increasingly farther from the nucleus and more loosely held. As a result, the reactivity of alkali metals increases down the group because it becomes easier for

(a)i. State two conditions used in the Haber process

ii. Explain briefly the effect of increasing the pressure on the rate of reaction in the Haber process.

(b)i. A mixture of nitrogen(IV) oxide and oxygen is bubbled into warm water to produce
trioxonitrate(V) oxide, write a balanced chemical equation for the reaction.

ii. Using a balanced chemical equation only, explain what would happen if nitrogen(IV) oxide is bubbled into warm water.

iii. 

Compare the gases evolved when trioxonitrate(V) acid decomposes under each of the following properties:

i. pH
ii. Solubility in water

iii. Reaction with carbon(II) oxide

(c)i. Name two oxides of sulphur

ii. Write a balanced equation for the reaction between each of the named oxides(Sulphur(IV) oxide, Sulphur (VI) oxide) and water

(d)i. Name one calcium compound

a, Used to dry ammonia gas
b. Used in the manufacture of cement
c. That causes hardness in water
d. Name one calcium compound

Détails de la réponse

(a) i. The two conditions used in the Haber process are high pressure and high temperature. ii. Increasing the pressure increases the rate of reaction in the Haber process because it shifts the equilibrium towards the side with fewer gas molecules, in accordance with Le Chatelier's principle. (b) i. 2NO + O2 → 2NO3 ii. When nitrogen(IV) oxide is bubbled into warm water, it reacts to form nitric acid (HNO3) and nitric oxide (NO). (c) i. The two oxides of sulfur are sulfur dioxide (SO2) and sulfur trioxide (SO3). ii. Balanced chemical equations for the reactions between the named oxides and water are: SO2 + H2O → H2SO3 SO3 + H2O → H2SO4 (d) i. Calcium chloride is used to dry ammonia gas. ii. Calcium silicate is used in the manufacture of cement. iii. Calcium carbonate causes hardness in water. iv. Calcium hydroxide is also known as slaked lime and is used in various applications such as water treatment and food processing.

a) (i) Define the term Avogadro's number.

(ii) If 2.30 g of an oxide of nitrogen, x, contains 3.01 x 1022 ${}^{22}$ molecules, calculate the molar mass of x.

(iii) Deduce the formula of x. N, =6.02 x 10", N =14.0, O = 16.0]

(b)(i) Describe briefly what happens when each of the following substances are added to water:

(I) CCI4 ${}_4$;     (II) SiCI4 ${}_4$, 

(ii) Explain briefly why the reactions in (a)(i), (b)(i), (I) and (b)(ii) (II) are different Study the diagram below and answer the questions that follow.

(c) Study the diagram below and answer the questions that follow.

(i) What is the set up used for?

Détails de la réponse

a) (i) Avogadro's number is the number of particles (atoms, molecules, ions, etc.) present in one mole of a substance. It is approximately equal to 6.02 × 1023 particles per mole. (ii) First, we need to calculate the number of moles of x in the given sample: Number of moles = Number of particles / Avogadro's number Number of moles = 3.01 × 1022 / 6.02 × 1023 Number of moles = 0.05 mol Molar mass of x = Mass of x / Number of moles Molar mass of x = 2.30 g / 0.05 mol Molar mass of x = 46 g/mol (iii) The empirical formula of x can be determined by finding the ratio of the atoms present in it. Since the molar mass of x is 46 g/mol and it contains nitrogen and oxygen, we can assume that the formula is NxOy. The ratio of N to O can be determined using the atomic masses of N and O and the given molar mass: Molar mass of NxOy = (N × 14.0) + (y × 16.0) = 46 N + y = 3.29 Since N and y must be integers, the closest possible ratio is N:O = 1:2. Therefore, the formula of x is N2O. (b) (i) When CCl4 is added to water, it does not dissolve as it is a nonpolar substance and water is a polar solvent. When SiCl4 is added to water, it reacts with water to form HCl and SiO2, which precipitates as a white solid. (ii) The reactions in (a)(i), (b)(i), (I), and (b)(ii) are different because they involve different substances and chemical reactions with water. In (a)(i), an oxide of nitrogen is being analyzed for its molar mass and formula. In (b)(i), two substances that are insoluble or react with water are being added to it. In (I), Pb(NO3)2 is being added to NaCl solution to precipitate PbCl2. In (b)(ii), SiCl4 is reacting with water to form HCl and SiO2. (c) (i) The setup is a simple distillation apparatus. It is used to separate a mixture of liquids with different boiling points by heating the mixture and collecting the condensed vapors. The mixture is heated in a distillation flask, and the vapors travel up the fractionating column, which contains several trays or packing material. The vapor with the lower boiling point condenses on the trays or packing, and the vapor with the higher boiling point continues to the condenser, where it is cooled and collected as a liquid. This allows for the separation of the components of the mixture based on their boiling points.

(a) Describe the observation that would be made when-----

i. Sulphur is heated from room temperature till 1190C
ii. 50% trioxonitrate(V) acid acts on copper tunings

(b)i. State two gaseous pollutants that can be generated by burning coal.

ii. What gas is responsible for most of the explosions in coal mines?

iii. The mining of coal leads to environmental pollution. State two environmental effects of the mining activity.

iv. Explain briefly why coal burns more easily when it is in pieces than in lump form

v. Name the non-volitile residue after the destructive distillation of coal

(c)i. Describe a chemical test for water

ii.a. State the effect of boiling a temporary hard water
b. State the effect of adding sodium trioxocarbonate(IV) crystals to permanent hard water

iii. Write an equation for the process of boiling a temporary hard water

Détails de la réponse

(a) i. When sulphur is heated from room temperature till 1190°C, it changes from a solid state to a liquid state and then to a gaseous state. At 1190°C, sulphur melts to form a reddish-brown liquid and further heating leads to the vaporization of the liquid to form a gas with a pale blue flame. ii. When 50% trioxonitrate(V) acid acts on copper tunings, brown nitrogen dioxide gas is produced. The copper is oxidized to copper(II) nitrate and the nitrogen dioxide is liberated. The brown coloration is due to the presence of the nitrogen dioxide gas. (b) i. Two gaseous pollutants that can be generated by burning coal are sulfur dioxide and carbon dioxide. ii. Methane gas is responsible for most of the explosions in coal mines. iii. Two environmental effects of coal mining activity are air pollution and water pollution. iv. Coal burns more easily when it is in pieces because the surface area of the coal is increased, allowing more oxygen to react with it. v. The non-volatile residue after the destructive distillation of coal is coal tar. (c) i. A chemical test for water is the anhydrous copper sulfate test. When anhydrous copper sulfate is added to water, it turns from white to blue, indicating the presence of water. ii. a. Boiling a temporary hard water causes the calcium and magnesium ions responsible for the hardness to precipitate out of solution, leaving soft water. b. Adding sodium trioxocarbonate(IV) crystals to permanent hard water causes the calcium and magnesium ions responsible for the hardness to precipitate out of solution, leaving soft water. iii. The equation for the process of boiling a temporary hard water is: Ca(HCO3)2 → CaCO3 + CO2 + H2O.