WAEC SSCE - Chemistry - 1988

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Benzene is the only compound that is aromatic. Aromaticity refers to a special type of stability in organic molecules that contain a ring of atoms with alternating double bonds. This creates a "delocalized" system of electrons that is spread out over the entire ring, making it particularly stable and resistant to chemical reactions. Benzene has a six-carbon ring with alternating double bonds, making it an aromatic compound. In contrast, cyclobutane, cyclopentane, hexane, and ethene do not have this type of ring and are not considered aromatic. They have different types of bonds and molecular structures that make them less stable and more reactive compared to benzene.

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The incorrect statement is "Chlorine exhibits allotopy." The correct term is "allotropy," which refers to the ability of an element to exist in different physical forms known as allotropes. Carbon and sulfur are examples of elements that exhibit allotropy. Chlorine, on the other hand, does not exhibit allotropy because it exists only as a gas in its standard state, and does not have any other stable physical forms. Nitrogen is a gas and argon is a noble gas, which means that they do not readily react with other elements.

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The naturally occurring iron ores are compounds that contain iron combined with other elements such as oxygen, carbon, and sulfur. Out of the given options, FeCl₂ is not a naturally occurring iron ore because it is a compound of iron and chlorine. Iron chloride is not found naturally in the earth's crust as an ore. Fe₂O₃, Fe₂O₃O₃, FeCO₃, and FeS₂ are all naturally occurring iron ores. Fe₂O₃ is commonly known as hematite, Fe₂O₃O₃ is not a known compound, FeCO₃ is known as siderite, and FeS₂ is known as pyrite or fool's gold.

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Aluminum (Al) is the metal that cannot be extracted from its ore using carbon (II) oxide as the reducing agent. This is because aluminum is more reactive than carbon and thus cannot be reduced by it. To extract aluminum from its ore, electrolysis is used instead, where the aluminum oxide is dissolved in molten cryolite and a direct current is passed through it, causing the aluminum to be deposited at the cathode. On the other hand, metals like copper (Cu), zinc (Zn), lead (Pb), and iron (Fe) can be extracted from their ores using carbon (II) oxide as the reducing agent because they are less reactive than aluminum and can be reduced by carbon.

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The main characteristic feature of transition metals is that they have variable oxidation states. This means that they can lose or gain different numbers of electrons when they form chemical compounds, allowing them to have a wide range of chemical properties and reactivities. Transition metals are located in the middle of the periodic table and have partially filled d orbitals, which give rise to their unique properties. Their variable oxidation states also make them important catalysts in many chemical reactions, as they can easily switch between different oxidation states to facilitate reactions.

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The balanced chemical equation for the reaction between hydrogen and oxygen is: 2H₂ + O₂ → 2H₂O This means that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. Using the ideal gas law, PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature, we can calculate the number of moles of hydrogen and oxygen present in the mixture before the reaction. Assuming that the pressure and temperature are constant, we have: n_H2 = (P * V_H2) / (R * T) = (1 atm * 100 cm³) / (0.0821 L atm mol^-1 K^-1 * 298 K) = 0.00406 mol n_O2 = (P * V_O2) / (R * T) = (1 atm * 200 cm³) / (0.0821 L atm mol^-1 K^-1 * 298 K) = 0.00812 mol Since the stoichiometry of the reaction is 2:1 for hydrogen and oxygen, the limiting reactant is hydrogen. This means that all of the hydrogen will react with half of the oxygen to produce water, leaving the other half of the oxygen unreacted. The number of moles of water produced is given by: n_H2O = n_H2 = 0.00406 mol The number of moles of oxygen consumed is given by: n_O2 = n_H2 / 2 = 0.00203 mol The number of moles of oxygen left unreacted is given by: n_{O2 unreacted} = n_O2 - n_{O2 consumed} = 0.00812 mol - 0.00203 mol = 0.00609 mol Using the ideal gas law again, we can calculate the volume of oxygen left unreacted: V_{O2 unreacted} = (n_{O2 unreacted} * R * T) / P = (0.00609 mol * 0.0821 L atm mol^-1 K^-1 * 298 K) / 1 atm = 0.150 L = 150 cm³ Therefore, the volume of oxygen left unreacted is 150 cm³.

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The given information provides us with the number of atoms present in 32.0g of sulfur, which is 6.02 x 10^23 atoms. Now, we need to find out the number of atoms in 2.70g of aluminum, given that the atomic mass of aluminum is 27. To find the number of atoms in 2.70g of aluminum, we need to first calculate the number of moles of aluminum present in 2.70g. Number of moles = mass/molar mass Molar mass of aluminum = 27g/mol Number of moles of aluminum = 2.70g/27g/mol = 0.1mol Now, we can use Avogadro's number, which is the number of particles (atoms or molecules) in one mole of a substance, to calculate the number of atoms in 0.1mol of aluminum. Avogadro's number = 6.02 x 10^23 particles/mol Number of atoms in 0.1mol of aluminum = 0.1mol x 6.02 x 10^23 particles/mol = 6.02 x 10^22 atoms Therefore, the correct answer is 6.02 x 10^22.

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The statement that is not true about the rusting of iron is that "rusting is a reduction process." Rusting is an oxidation process, which means that it involves the loss of electrons by iron atoms. When iron reacts with oxygen and water, a redox reaction takes place. Iron atoms lose electrons to oxygen atoms to form iron oxide, while water molecules are split into hydrogen ions and hydroxide ions. The hydrogen ions combine with oxygen atoms to form water molecules, while the hydroxide ions combine with iron ions to form hydrated iron (III) oxide, which is the reddish-brown rust that we see on iron surfaces. Therefore, rusting is not a reduction process but an oxidation process that occurs in the presence of oxygen and moisture. The rust formed is mainly hydrated iron (III) oxide, and the process is similar to corrosion rather than burning.

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Iodine crystals change directly into the gaseous state when heated because of a process called sublimation. Sublimation occurs when a solid transitions directly into a gas without first becoming a liquid. Iodine molecules are held together by weak intermolecular forces, which makes it easier for them to break apart and turn into a gas when heated. This process is called sublimation and it explains why iodine crystals can change directly into a gaseous state when heated.

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The given chemical equation represents a reaction between Sodium Hydroxide (NaOH) and Hydrochloric Acid (HCl) in an aqueous solution. During the reaction, the NaOH reacts with HCl to form Sodium Chloride (NaCl) and water (H2O) as products. The reaction can be represented as follows: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(/) This type of reaction is known as a neutralization reaction as it involves the combination of an acid and a base to form a salt and water. Hence, the correct answer is "is a neutralization". This reaction is not reversible, so the option "is reversible" is incorrect. The reaction does not usually require a catalyst, so the option "is usually catalysed" is also incorrect. Additionally, the option "attains equilibrium after a few seconds" is incorrect as the reaction goes to completion, meaning that all of the reactants will be converted to products, and no reactants will be left.

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The correct answer is "efflorescent". Efflorescence is a process where a salt that contains water molecules loses its water content when exposed to air. The loss of water causes the salt to crumble or turn into a powder, which can result in a decrease in its mass. This occurs because the salt is more soluble in water than in air, and the water molecules in the salt are attracted to the water molecules in the air. As a result, the water molecules in the salt are released into the air, leaving behind the dry salt. This process is commonly observed in some household items like baking soda or washing soda, which can form a white powdery substance on their surface when left exposed to air.

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The statement that is not true about the rusting of iron is that "rusting is a reduction process." Rusting is actually an oxidation process, which means that it involves the loss of electrons from the iron atoms in the metal. This loss of electrons causes the iron to become positively charged, which allows it to react with oxygen in the air and water molecules to form hydrated iron oxide, which is the main component of rust. Rusting occurs when iron is exposed to both oxygen and moisture. In the presence of oxygen, iron undergoes a chemical reaction called oxidation, which produces iron oxide or rust. This process is accelerated by the presence of water, which provides the necessary electrons for the reaction to occur more quickly. Rust is typically reddish-brown in color and is composed mainly of hydrated iron (III) oxide, which has the chemical formula Fe2O3•xH2O. The "x" in the formula represents the number of water molecules that can bond to each iron atom in the rust. While rusting may look similar to burning, it is actually a different process. Burning involves a rapid oxidation reaction that produces heat and light, whereas rusting is a slower process that does not produce heat or light.

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Equilibrium is attained in a reversible reaction when the rates of the forward and backward reactions are equal. This means that the reaction is still ongoing, but the concentrations of the reactants and products have reached a constant state where there is no net change in the concentrations of the reactants and products over time. Think of it like a tug-of-war between the reactants and products. At first, the reactants are winning, and the reaction proceeds in the forward direction, forming products. But as the products build up, they start to pull back in the opposite direction, and the reaction starts to go in reverse. Eventually, the pull from the reactants and products becomes balanced, and the reaction reaches a point where there is no net movement in either direction. So, when the forward and backward rates are equal, the reaction is said to be at equilibrium, and the concentrations of the reactants and products remain constant over time. Note that this does not mean that all the reactants have been used up or all the products have been formed, as the reaction is ongoing but in a state of balance.

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C_4H10 is the molecular formula for butane, which is an alkane with four carbon atoms. Isomers are molecules with the same molecular formula but different structural arrangements of their atoms. In the case of butane, there are two main types of isomers: constitutional isomers and stereoisomers. Constitutional isomers have the same molecular formula but different connectivity between atoms. For example, butane has two possible constitutional isomers: n-butane, where the four carbon atoms are arranged in a straight chain, and isobutane, where the carbon atoms form a branched chain. Stereoisomers have the same molecular formula and connectivity between atoms, but differ in the spatial arrangement of their atoms. Butane has no stereoisomers because all of its atoms are connected by single bonds, which do not allow for different spatial arrangements. Therefore, the number of isomers that can be obtained from C_4H10 is 2, which correspond to its two constitutional isomers.

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Bubbling of carbon (IV)oxide into calcium hydroxide solution results in the formation of calcium carbonate (CaCO₃). When carbon (IV) oxide gas is bubbled into calcium hydroxide solution, a chemical reaction takes place between them. This reaction is called a precipitation reaction, where a solid is formed from the reaction of two solutions. The chemical equation for the reaction is: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O In this reaction, calcium hydroxide (Ca(OH)₂) reacts with carbon (IV) oxide (CO₂) to form calcium carbonate (CaCO₃) and water (H₂O). Calcium carbonate is a white solid that is commonly found in nature as a mineral in rocks and shells. It is also used in various applications, such as in the production of cement, as a dietary supplement, and in the manufacturing of paper, plastics, and paint.

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The Solvay process is a method used to produce sodium trioxocarbonate (IV), also known as sodium bicarbonate or baking soda. In this process, limestone (calcium carbonate) is heated to produce quicklime (calcium oxide), which is then reacted with brine (a solution of sodium chloride) in a reactor vessel called a "Solvay tower." The reaction produces sodium hydroxide and calcium chloride as byproducts. Therefore, the waste product in the Solvay process is calcium chloride.

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Among the options given, the acid present in proteins is an amino acid. Proteins are made up of long chains of amino acids linked together, forming a polypeptide chain. Each amino acid has an amine group (-NH2) and a carboxyl group (-COOH). When these groups react with each other, they form a peptide bond, creating a polypeptide chain. Therefore, the acid present in proteins is the carboxyl group of the amino acids, which gives proteins their acidic properties. Other acids listed, such as lactic acid, propanoic acid, palmitic acid, and stearic acid, are not typically found in proteins. Lactic acid is a byproduct of anaerobic respiration in some organisms. Propanoic acid is a naturally occurring acid found in some foods. Palmitic and stearic acids are both saturated fatty acids found in animal fats and some plant oils.

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The option that is commonly used as an anaesthetic is CHCI₃, which is also known as chloroform. Chloroform is a colorless liquid with a sweet smell, and it was widely used as an anaesthetic in the past. It works by depressing the central nervous system and inducing a state of unconsciousness. However, chloroform has been largely replaced by other anaesthetics due to its potential to cause liver damage and other health problems. Other options, such as C₂H₅OH or ethanol, may be used for sedation or pain relief but are not typically used as general anaesthetics.

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Radioactive isotopes are used in various fields, including medicine, research, and industry. They have several unique properties that make them useful for various applications. However, one of the uses of radioactive isotopes is not related to their properties, and that is determining equilibrium positions. Radioactive isotopes can be used for sterilizing medical equipment, tracing reaction paths, dating elements, and radiography. For example, they can be used to kill bacteria, viruses, and other microorganisms that can cause infections in medical equipment. In radiography, they can be used to produce images of internal body parts, which can help diagnose medical conditions. In research, they can be used to trace the path of chemical reactions or determine the age of geological samples. However, determining equilibrium positions is not a common use of radioactive isotopes. Equilibrium positions are typically determined using chemical reactions and thermodynamic principles. Radioactive isotopes can be used to study the kinetics of reactions, but they are not usually used to determine equilibrium positions.

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Water gas is a mixture of carbon monoxide (CO) and hydrogen gas (H₂). It is produced by reacting steam with hot coal or coke in a process known as gasification. The resulting gas mixture is composed of about 50% CO and 50% H₂. Water gas has been used historically as a fuel for heating and lighting, and it is still used today in some industrial processes.

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The compound that is not decomposed by heat is sodium carbonate (Na2CO3). When some compounds are heated, they break down into simpler substances. This process is called thermal decomposition. The products of thermal decomposition may include gases, liquids, or solids. Calcium carbonate (CaCO3), copper carbonate (CuCO3), zinc carbonate (ZnCO3), and magnesium carbonate (MgCO3) all decompose when heated to form simpler compounds. For example, when calcium carbonate is heated, it breaks down into calcium oxide (CaO) and carbon dioxide (CO2). However, sodium carbonate (Na2CO3) does not decompose when heated. It is a stable compound that can withstand high temperatures without breaking down. Therefore, it is used in many industrial processes that involve high temperatures, such as glass-making and soap-making. In summary, of the options given, sodium carbonate is the compound that is not decomposed by heat.

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(a)

(i) Heavy chemical: Tetraoxosulphate(VI) acid, H₂SO₄. It is produced in large quantities and is essential in many branches of industry.

(ii) Fine chemical: Additives for fuels. Fine chemicals are manufactured for specific industrial or domestic purposes; they are usually of high purity and expensive.

(b) Compounds having the molecular formula C₄H₉Cl

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(a) Ionic equation for the reaction between zinc powder and silver trioxonitrate (V) solution

Zinc displaces silver from the solution because zinc is higher than silver in the electrochemical series:

Zn_(s) + 2Ag⁺_(aq) → Zn²⁺_(aq) + 2Ag_(s)

(b) Oxidation and reduction

• (i) Oxidized: zinc (Zn). Its oxidation number rises from 0 to +2; it loses electrons.
• (ii) Reduced: the silver ion (Ag⁺). Its oxidation number falls from +1 to 0; it gains electrons.

(c) Two applications of oxidation numbers

• They are used in writing and balancing redox equations.
• They are used in the systematic naming (IUPAC nomenclature) of compounds, for example iron (II) and iron (III).
• They are used to identify the species oxidized or reduced and the oxidizing and reducing agents in a reaction.

Any two are acceptable.

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(a) Properties of transition metals

• (i) High melting points: transition metals have strong metallic bonds because both the outer s electrons and some inner d electrons take part in bonding, giving a large number of delocalised electrons and a strong lattice that needs much energy to break.
• (ii) Variable oxidation states: the energies of the 3d and 4s electrons are close, so different numbers of these electrons can be lost during bonding, giving several stable oxidation states.
• (iii) Paramagnetism: they have unpaired electrons in their d orbitals, and these unpaired electrons cause the atoms to be attracted into a magnetic field (paramagnetic).

(b) Bauxite and aluminium oxide

• (i) Impurities in bauxite: iron (III) oxide (Fe₂O₃) and silicon (IV) oxide (SiO₂).
• (ii) Removal of impurities: the powdered bauxite is treated with hot concentrated sodium hydroxide solution. The amphoteric aluminium oxide dissolves to form sodium aluminate, while the insoluble impurities (iron (III) oxide) are filtered off. Pure aluminium oxide is then precipitated and heated.
• (iii) Amphoteric nature of aluminium oxide: aluminium oxide is amphoteric because it reacts with both acids and bases to form a salt and water. With acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. With base: Al₂O₃ + 2NaOH + 3H₂O → 2NaAl(OH)₄.

(c) Electrolysis of copper (II) tetraoxosulphate (VI) using copper electrodes

(i) At the cathode, copper ions are discharged and copper is deposited: Cu²⁺_(aq) + 2e^- → Cu_(s). At the anode, the copper electrode itself dissolves (goes into solution) rather than oxygen being evolved: Cu_(s) → Cu²⁺_(aq) + 2e^-. Copper thus dissolves from the anode and is deposited on the cathode. This is the principle used in the electrolytic purification of copper.

(ii) No, the blue colour does not change. For every Cu²⁺ ion removed at the cathode, one Cu²⁺ ion enters the solution from the anode. The concentration of copper (II) ions therefore remains constant, so the blue colour of the solution is unchanged.

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(a) What is a carbohydrate?

A carbohydrate is an organic compound containing carbon, hydrogen and oxygen, in which the hydrogen and oxygen are usually present in the ratio 2 : 1 (the same ratio as in water). Carbohydrates have the general formula C_x(H₂O)_y and are polyhydroxy aldehydes or ketones (or compounds that yield them on hydrolysis).

(b) Two types of carbohydrates with one example of each

Any two types with their correct examples are acceptable.

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(a) Functions of the substances in the Solvay Process

• (i) Limestone: it is heated (decomposed) to supply carbon (IV) oxide for the process and to produce calcium oxide (quicklime), which is later used to recover ammonia.
  CaCO_3(s) → CaO_(s) + CO_2(g)
• (ii) Ammonia: it makes the brine alkaline (forms ammoniacal brine) so that it can absorb carbon (IV) oxide, and it is recovered and recycled at the end of the process.
  NH_3(g) + H₂O_(l) + CO_2(g) → NH₄HCO_3(aq)
• (iii) Brine: it supplies the sodium ions (as concentrated sodium chloride solution) needed to form sodium hydrogen trioxocarbonate (IV).
  NH₄HCO_3(aq) + NaCl_(aq) → NaHCO_3(s) + NH₄Cl_(aq)

(b) Why the reaction is a neutralization

Aqueous sodium trioxocarbonate (IV) is a base (it is the salt of a strong base and a weak acid and reacts as a base towards acids), while dilute hydrochloric acid is an acid. Their reaction produces a salt and water only, which is the definition of neutralization:

Na₂CO_3(aq) + 2HCl_(aq) → 2NaCl_(aq) + H₂O_(l) + CO_2(g)

The essential ionic change is H⁺ + OH^- (from the carbonate acting as a base) forming water; hence it is a neutralization.

(c) Mass of Na₂CO₃ from 16.8 g of NaHCO₃

Decomposition equation:

2NaHCO_3(s) → Na₂CO_3(s) + H₂O_(l) + CO_2(g)

Molar mass of NaHCO₃ = 23 + 1 + 12 + 48 = 84 g mol^-1

Moles of NaHCO₃ = 16.8 ÷ 84 = 0.2 mol

From the equation, 2 mol NaHCO₃ give 1 mol Na₂CO₃, so:

Moles of Na₂CO₃ = 0.2 ÷ 2 = 0.1 mol

Molar mass of Na₂CO₃ = 46 + 12 + 48 = 106 g mol^-1

Mass of Na₂CO₃ = 0.1 × 106 = 10.6 g

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(a) The element X, represented as ⁴⁰₂₀X (atomic number 20, mass number 40)

• (i) Electrons and neutrons: number of electrons = atomic number = 20. Number of neutrons = mass number - atomic number = 40 - 20 = 20.
• (ii) Electronic configuration: 2, 8, 8, 2 (that is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²). X is calcium.

(b) Reaction with chlorine (atomic number 17)

• (i) The bond formed is an electrovalent (ionic) bond.
• (ii) X has 2 electrons in its outermost shell and each chlorine atom needs 1 electron to complete its octet. The X atom loses its 2 outer electrons to become X²⁺, and two chlorine atoms each gain one electron to become Cl^-. The oppositely charged ions are held together by strong electrostatic attraction.
• (iii) Formula of the compound: XCl₂ (calcium chloride, CaCl₂). Three properties: it is a crystalline solid with a high melting point; it is soluble in water; and it conducts electricity when molten or in aqueous solution.

(c) Possible relative molar masses of a chlorine molecule

Chlorine exists as Cl₂. Using isotopes of mass 35 and 37 the possible molecular masses are:

• 35 + 35 = 70
• 35 + 37 = 72
• 37 + 37 = 74

(d) Mass of one atom of carbon

1 mole (6.02 × 10²³ atoms) weighs 12.0 g, so:

Mass of one atom = 12.0 ÷ (6.02 × 10²³) = 1.99 × 10^-23 g

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Gay-Lussac's law of combining volumes

The law states that when gases react, they do so in volumes which bear a simple whole number ratio to one another and to the volumes of the gaseous products, provided the temperature and pressure remain constant.

Illustration with a chemical reaction

Consider the formation of ammonia:

N_2(g) + 3H_2(g) → 2NH_3(g)

Here 1 volume of nitrogen combines with 3 volumes of hydrogen to give 2 volumes of ammonia. The ratio 1 : 3 : 2 is a simple whole number ratio, which illustrates the law.

(The reaction H_2(g) + Cl_2(g) → 2HCl_(g), with ratio 1 : 1 : 2, is also acceptable.)

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(a) Two allotropes of carbon

• Diamond
• Graphite

(b) Products of the destructive distillation of coal and one use of each

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Distinction between cracking and reforming

Importance in the petroleum industry

• Cracking converts the less useful heavy fractions (such as heavy naphtha and diesel) into more valuable products, especially petrol, and provides alkenes (ethene, propene) as raw materials for the petrochemical and plastics industries.
• Reforming converts straight-chain hydrocarbons into branched-chain and aromatic hydrocarbons, thereby raising the octane number of petrol and improving its anti-knock quality, and supplies aromatics for the chemical industry.

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(a) Order of increasing energy

The energy carried by the three radiations increases in the order:

alpha particles < beta particles < gamma rays

Alpha particles are the least energetic (heavy, slow moving), beta particles are of intermediate energy, while gamma rays are the most energetic (high frequency electromagnetic radiation).

(b) One difference between nuclear fission and nuclear fusion

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(a) The collision theory and the rate of reaction

The collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another. However, not every collision leads to a reaction. Only collisions in which the particles possess energy equal to or greater than the activation energy, and in which the particles are correctly orientated, are effective and lead to product formation. The rate of a reaction therefore depends on the frequency of effective collisions: the greater the number of effective collisions per unit time, the faster the reaction.

(b) Effect of the following on rate of reaction

• (i) Surface area: increasing the surface area of a solid reactant (for example by powdering it) exposes more particles to collision. This increases the frequency of effective collisions and so increases the rate of reaction.
• (ii) Catalyst: a catalyst provides an alternative reaction pathway of lower activation energy. More colliding particles then possess the required energy, so the number of effective collisions and hence the rate increases.