WAEC SSCE - Chemistry - 1996

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Alkaline pyrogallol solution is commonly used to remove oxygen from a gas mixture. This is because pyrogallol can easily react with oxygen in the air to form a brown precipitate. Therefore, the component of air that can be removed by alkaline pyrogallol solution is oxygen.

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The incorrect statement about transition metals is "low melting points". Transition metals are characterized by a high melting and boiling point due to their strong metallic bonding, which involves the delocalization of electrons over a large number of atoms. Other properties of transition metals include variable oxidation states, tendency to form complex ions, formation of coloured ions, and ability to act as catalysts.

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The statement ''Equal volumes of all gases at the same temperature and pressure contain the same number of molecules'' is an expression of Avogadro's Law. This law is named after the Italian scientist Amedeo Avogadro, who discovered that the volume of a gas is directly proportional to the number of molecules or atoms present, provided that the temperature and pressure remain constant. This law can be used to determine the molar volume of gases and is important in many fields of science, such as chemistry and physics. In simple terms, this law states that two gases at the same temperature and pressure will have the same number of molecules in a given volume, regardless of their chemical identity or atomic/molecular weight.

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Sucrose is a disaccharide. A disaccharide is a type of carbohydrate made up of two monosaccharides linked together by a glycosidic bond. Sucrose, also known as table sugar, is composed of glucose and fructose joined together by a glycosidic bond. The other options listed are not disaccharides; fructose is a monosaccharide, starch, cellulose, and glycogen are polysaccharides.

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An arrangement of two different metals in aqueous solutions of their salts to produce an electronic current is known as an electrochemical cell. This is because the cell involves a chemical reaction that converts chemical energy into electrical energy. One of the metals acts as a negative electrode and the other as a positive electrode, and the movement of electrons between them through the salt bridge or porous membrane generates an electric current. This process is used in batteries and other electronic devices to produce electrical energy.

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The statement that is not correct about electrolysis is "Reduction occurs at the anode". During electrolysis, reduction occurs at the cathode and oxidation occurs at the anode. At the anode, positively charged ions (cations) lose electrons to become neutral atoms or molecules. At the cathode, negatively charged ions (anions) gain electrons to become neutral atoms or molecules. Anions migrate to the anode, while positive ions migrate to the cathode. The concentration of the electrolyte affects the rate of the reaction and the discharge of ions. Electrolytes conduct electric current because they contain ions that are free to move and carry charge.

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Nitrogen is prepared on a large scale by the fractional distillation of liquefied air. Air is composed of about 78% nitrogen and 21% oxygen, along with other gases. To obtain nitrogen, the air is liquefied by compressing and cooling it to a very low temperature. This liquefied air is then separated into its components by fractional distillation, a process in which the different gases are separated based on their boiling points. Since nitrogen has a lower boiling point than oxygen, it boils off first and can be collected. This process is used to obtain large amounts of nitrogen for industrial and commercial purposes.

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Palm wine is a fermented beverage that is produced from the sap of certain palm trees. The fermentation process is mainly carried out by yeast, which converts glucose to ethanol. When palm wine is exposed to air, the ethanol undergoes oxidation, which results in the formation of ethanoic acid. This is what causes the sour taste of the palm wine. Therefore, the correct answer is "ethanol to ethanoic acid."

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In the given chemical equation, X represents the Enthalpy of Formation. Enthalpy of formation (∆H_f) is defined as the amount of heat energy released or absorbed when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. In the given equation, the reactants are hydrogen gas and oxygen gas in their standard states, and the product is liquid water. The value of ∆H represents the amount of heat energy that is either released or absorbed when one mole of water is formed from its constituent elements. If the value of ∆H is negative, it means that the reaction is exothermic, i.e., heat energy is released during the reaction, while a positive value of ∆H represents an endothermic reaction, i.e., heat energy is absorbed during the reaction.

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A homologous series is a series of organic compounds having the same functional group and similar chemical properties in which the successive members differ by a CH₂ unit. So, the general formula of the homologous series would be C_nH_2n+2. If the third member of a homologous series is C₃H₈, it means that the general formula of the series is C_nH_2n+2, where n = 3. C₃H₈ belongs to the alkane series, so the fifth member of the series would also belong to the alkane series and have the general formula C_nH_2n+2, where n = 5. Substituting n = 5 in the general formula, we get C₅H₂₍₅₎₊₂ = C₅H₁₂. Therefore, the fifth member of the homologous series would be C₅H₁₂.

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When an atom gains an electron, it becomes negatively charged because it now has more electrons than protons. This negatively charged atom is called an anion. The extra electron is held by the nucleus through the electromagnetic force and affects the chemical properties of the atom. The anion has a greater affinity for positively charged ions or cations and can undergo chemical reactions, forming compounds with other atoms or ions. Therefore, the correct option is "negatively charged."

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The statement that is not correct about a catalyst is: "can start a chemical reaction which will normally not take place." A catalyst cannot start a chemical reaction that will normally not take place because a catalyst only speeds up the rate of an existing reaction. It works by lowering the activation energy required for a reaction to occur, thus making it easier for the reaction to take place. Once the reaction is completed, the catalyst remains unchanged and can be used again. Therefore, options a, b, d, and e are all correct statements about a catalyst, while the statement in option c is not correct.

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The alkali metals are a group of elements in the periodic table that include lithium, sodium, potassium, rubidium, cesium, and francium. They exhibit similar chemical properties because they all have one valence electron in their outermost shell, which they readily give up to form positive ions. This makes them highly reactive and they are easily oxidized in air or water. The similarity in their electronic configuration results in similar chemical behavior, such as the ability to form ionic compounds with non-metals and the tendency to react vigorously with water to produce hydrogen gas and an alkali metal hydroxide.

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The existence of two or more forms of the same element in the same physical state is known as allotropy. Allotropy is the property of some elements to exist in two or more different forms, called allotropes. These allotropes have different physical and chemical properties due to the different arrangements of atoms in their crystal lattice structures. For example, carbon has three common allotropes: diamond, graphite, and fullerene. Each of these allotropes has different physical and chemical properties despite being made up of the same element.

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The rate of production of hydrogen gas from the reaction between zinc granules and hydrochloric acid can be increased by using zinc powder instead of zinc granules. The rate of reaction between zinc and hydrochloric acid is determined by the surface area of the zinc that is exposed to the acid. Zinc powder has a higher surface area than zinc granules, which increases the rate of reaction between the zinc and the acid. Cooling the reaction mixture would slow down the reaction rate because the reaction is exothermic and releasing energy as heat, and lowering the temperature would reduce the kinetic energy of the reacting particles. Using a zinc rod instead of zinc granules would not increase the surface area, but would decrease it, slowing down the reaction rate. Carrying out the reaction at a higher pressure or in a closed vessel would not increase the surface area of the zinc and would not affect the rate of reaction.

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Ammonia gas is suitable for performing the 'Fountain' experiment because it is very soluble in water. In the experiment, ammonia gas is passed through water, where it dissolves to form an alkaline solution. The gas bubbles produced rise to the top of the solution, and as more gas bubbles are produced, the solution becomes saturated with the gas. Eventually, the pressure of the gas forces the solution to rise up and out of the container, creating a fountain-like effect. Ammonia gas is preferred for this experiment because of its high solubility in water, which allows for a sufficient amount of gas to dissolve in the solution and produce the necessary pressure for the fountain effect.

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The equation that represents a neutralization reaction is: H₂SO₄ + KOH → KHSO₄ + H₂ O. In this reaction, an acid (H₂SO₄) reacts with a base (KOH) to form a salt (KHSO₄) and water (H₂O). Neutralization reactions involve the combination of an acid and a base to form a salt and water, with the acid and base neutralizing each other's acidic and basic properties. The other options either do not involve an acid and a base or do not form a salt and water, and thus do not represent neutralization reactions.

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When acidified KMnO₄ solution is decolorised by ethene, it means that ethene is oxidised to a colorless compound and KMnO₄ is reduced. Therefore, ethene is acting as a reducing agent. So, the correct option is (c) a reducing agent.

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The fact that a sugar was produced by boiling Compound X with concentrated hydrochloric acid suggests that Compound X was likely a carbohydrate, specifically a type of polysaccharide such as cellulose. When polysaccharides undergo hydrolysis (a reaction with water), they are broken down into their constituent monosaccharides (simple sugars) by the addition of water molecules. Therefore, the production of a sugar as a product of the reaction between Compound X and hydrochloric acid suggests that the reaction involved hydrolysis of the polysaccharide. The other compounds listed (rubber, polyethylene, nylon, and ethanol) are not polysaccharides and would not be expected to produce a sugar upon hydrolysis.

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A solution of pH 7 is neutral. The pH scale ranges from 0 to 14, with pH 7 being the midpoint of the scale. A pH of 7 indicates a balance of hydrogen ions (H+) and hydroxide ions (OH-) in the solution, with equal concentrations of both ions. In other words, a neutral solution is neither acidic nor basic.

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The most suitable metal for use where lightness and resistance to corrosion are of importance is Aluminium. This is because Aluminium is a lightweight metal that has a low density and is resistant to corrosion. Aluminium forms a protective layer of oxide on its surface that prevents further oxidation, making it highly resistant to corrosion. In addition, Aluminium is a good conductor of heat and electricity and can be easily shaped into different forms, making it useful for a wide range of applications, such as in the aerospace and automotive industries, where weight reduction and corrosion resistance are critical factors. Therefore, the correct answer is Aluminium.

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Soap is made up of sodium or potassium salts of long-chain fatty acids. The cleaning action of soap is due to its ability to dissolve in water and attach to oil and dirt particles, enabling them to be washed away. However, in hard water, which contains dissolved calcium and magnesium ions, soap forms insoluble salts with these ions. These salts, called scum, are insoluble in water and stick to surfaces, reducing the effectiveness of the soap. As a result, the cleansing action of soap in hard water is not satisfactory.

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When elements in group two (alkaline earth metals) react with elements in group seven (halogens), they form ionic compounds with a general formula of MX₂. Therefore, the likely formula of the compound formed between element M in group two and element X in group seven is MX₂.

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Benzene exhibits resonance. Resonance is a phenomenon in which electrons move between different possible locations within a molecule, resulting in the stabilization of the molecule. In benzene, the carbon atoms are arranged in a ring with alternating double bonds and single bonds. The electrons in the double bonds are delocalized and move freely around the ring, resulting in a more stable molecule. This is an example of resonance. The other molecules listed do not have a similar arrangement and therefore do not exhibit resonance.

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The balanced equation for the reaction between CaCO₃ and 2HCI is: CaCO₃ + 2HCI → CaCI₂ + H₂O + CO₂ The molar mass of CaCO₃ is 100 g/mol, which means that 100 g of CaCO₃ will produce 1 mole of CO₂. Therefore, 2.5 g of CaCO₃ will produce: 1 mole CO₂ = 22.4 dm³ at s.t.p. x moles CO₂ = (2.5 g / 100 g/mol) = 0.025 moles x = 0.56 dm³ (rounded to two decimal places) Therefore, the volume of CO₂ produced at s.t.p is 0.56 dm³. Hence, the answer is (d) 0.56 dm³.

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During water treatment for town supply, water is passed through layers of sand beds in order to filter the water. The sand beds act as a physical barrier that separates large particles, sediments, and debris from the water. As the water flows through the sand bed, the sand traps the suspended particles, and clean water passes through. This process is called filtration and is an essential step in water treatment, as it removes impurities and improves the quality of the water. The filtered water can then be treated further to remove any remaining impurities or disinfect it to kill any harmful bacteria and viruses.

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The reaction between compound X and concentrated hydrochloric acid involves hydrolysis. Hydrolysis is a type of chemical reaction in which a compound is broken down into two or more simpler compounds through the addition of water. In this case, the concentrated hydrochloric acid acts as a catalyst, promoting the breakdown of compound X into simpler compounds, one of which is a sugar. Therefore, the reaction between compound X and concentrated hydrochloric acid is a hydrolysis reaction.

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The oxidation number of an element is the hypothetical charge it would have if all its bonds were purely ionic. In other words, it is a measure of the degree of oxidation of an atom in a molecule. To determine the oxidation number of nitrogen in each compound, we need to know the electronegativity of the other elements involved in the bond. In NO₂, the electronegativity of oxygen is higher than nitrogen, so nitrogen is assigned an oxidation number of +4. In N₂O, the oxidation number of nitrogen is +1. In NO, nitrogen is assigned an oxidation number of +2. In HNO₂, nitrogen is assigned an oxidation number of +3. In HNO₃, nitrogen is assigned an oxidation number of +5. Therefore, the only compound in which the oxidation number of nitrogen is equal to +3 is HNO₂.

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(a) Two monatomic gases

Helium and neon (argon is also acceptable). The noble gases exist as single atoms, so they are monatomic.

(b) Gases

• (i) Dense white fumes with ammonia vapour: hydrogen chloride (HCl). It combines with ammonia to form white fumes of ammonium chloride: NH₃ + HCl → NH₄Cl.
• (ii) Yellow precipitate with ammoniacal silver trioxonitrate (V) solution: ethyne (C₂H₂). It reacts to form a precipitate of silver ethynide (silver acetylide).

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(a) Monomer units:

• (i) Polyethene is formed from the monomer ethene (ethylene), \(CH_2=CH_2\).
• (ii) Cellulose is a natural polymer whose monomer unit is glucose (\(\beta\)-glucose), \(C_6H_{12}O_6\).

(b) When steam and air are passed over red-hot coke, the two fuel gases obtained are:

• Water gas (a mixture of carbon(II) oxide and hydrogen, \(CO + H_2\)), formed by the action of steam on red-hot coke: \[C_{(s)} + H_2O_{(g)} \rightarrow CO_{(g)} + H_{2(g)}\]
• Producer gas (a mixture of carbon(II) oxide and nitrogen, \(CO + N_2\)), formed by the action of air on red-hot coke: \[2C_{(s)} + O_{2(g)} \rightarrow 2CO_{(g)}\] the nitrogen of the air passing through unreacted.

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(a) Oxidation number of manganese in the tetraoxomanganate(VII) ion, \(MnO_4^-\).

Let the oxidation number of Mn be \(x\). Each oxygen is \(-2\) and the overall charge on the ion is \(-1\):

\[x + 4(-2) = -1\]

\[x - 8 = -1\]

\[x = +7\]

So manganese has an oxidation number of +7.

(b)(i) Two metals extracted from their ores by electrolysis: sodium and aluminium. (Potassium, calcium and magnesium are also acceptable, as they are all high in the electrochemical series.)

(ii) Difference between a conductor and an electrolyte:

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Reading the diagram. The vertical axis is Energy in \(kJ\,mol^{-1}\), gridded at \(0, 100, 200, 300, 400\). From the plotted levels:

• energy of the reactants (A + B) \(\approx 100\ kJ\,mol^{-1}\);
• energy at the top of the curve (the peak / transition state) \(\approx 330\ kJ\,mol^{-1}\);
• energy of the products (C + D) \(\approx 50\ kJ\,mol^{-1}\).

(a)(i) Enthalpy of the reaction. The enthalpy change is the difference between the energy of the products and that of the reactants:

\[\Delta H = H_{products} - H_{reactants} = 50 - 100 = -50\ kJ\,mol^{-1}\]

The negative sign shows that energy is given out.

(a)(ii) Activation energy of the reaction. The activation energy is measured from the reactant level up to the top of the energy barrier:

\[E_a = E_{peak} - H_{reactants} = 330 - 100 = 230\ kJ\,mol^{-1}\]

(b) Type of reaction. The products lie at a lower energy than the reactants and \(\Delta H\) is negative, so the profile is for an exothermic reaction (energy is released to the surroundings).

(c) Substances providing an alternative pathway. Catalysts. A catalyst provides an alternative reaction pathway of lower activation energy, so the reaction proceeds faster without the catalyst itself being used up.

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(a)(i) Three factors that affect the discharge of ions during electrolysis

• The position of the ion in the electrochemical (discharge) series; the ion lower in the series is discharged in preference.
• The concentration of the ion in the electrolyte; a high concentration can favour the discharge of an ion that would otherwise not be discharged.
• The nature of the electrode used (whether it is inert, e.g. platinum/carbon, or attackable, e.g. copper).

(a)(ii) Purification of copper by electrolysis

• A thick block of impure copper is made the anode and a thin strip of pure copper is made the cathode.
• The electrolyte is acidified copper(II) tetraoxosulphate(VI) solution, \( CuSO_4 \).
• When current flows, the impure anode dissolves: \( Cu \rightarrow Cu^{2+} + 2e^- \).
• The copper ions migrate to the cathode and are deposited there as pure copper: \( Cu^{2+} + 2e^- \rightarrow Cu \).
• Soluble impurities such as iron and zinc remain in solution, while insoluble impurities such as silver and gold fall below the anode as anode mud.

(b)(i) Completed table

The quantity of electricity is \( Q = I \times t \), with \( I = 0.20\ A \) in each case.

Sample calculation: \( Q = 0.20 \times 900 = 180\ C \), and so on for the other rows.

(b)(ii) Graph of mass of M deposited against quantity of electricity passed

The points lie on a straight line passing through the origin, showing that the mass deposited is directly proportional to the quantity of electricity passed.

(b)(iii) Mass of M deposited by the same current for 20 minutes

Time \( t = 20 \times 60 = 1200\ s \), so the quantity of electricity is \[ Q = I \times t = 0.20 \times 1200 = 240\ C. \]Reading up from \( Q = 240\ C \) on the horizontal axis to the line of best fit, and across to the vertical axis, the mass deposited is 0.08 g. (Check: the gradient is \( \dfrac{0.24}{720} = 3.33\times10^{-4}\ \text{g C}^{-1} \), so \( 3.33\times10^{-4} \times 240 = 0.08\ g \).)

(b)(iv) Law verified

Because the graph is a straight line through the origin, the mass deposited is proportional to the quantity of electricity passed. The experiment therefore verifies Faraday's first law of electrolysis.

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(a) Three postulates of the kinetic theory

• All matter is made up of tiny particles (atoms or molecules) which are in constant, continuous random motion.
• The particles possess kinetic energy which increases as the temperature rises (and decreases as it falls).
• There are spaces (intermolecular spaces) between the particles, and forces of attraction exist between them; these spaces are largest in gases and smallest in solids.

(Also acceptable: collisions between the particles are perfectly elastic, so no kinetic energy is lost.)

(b) Processes involved

• (i) Spread of the smell of a perfume across a room: diffusion.
• (ii) Decrease in the volume of a liquid after exposure in an open vessel: evaporation.

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(a) The elements \(_9\text{F}\), \(_{17}\text{Cl}\), \(_{35}\text{Br}\) and \(_{53}\text{I}\) all belong to Group VII of the Periodic Table.

(i) The first member is fluorine (atomic number 9). Its electronic structure is:

\[ \text{F}: 1s^2\,2s^2\,2p^5 \quad \text{(i.e. } 2,7). \]

(ii) The family name of the elements is the halogens.

(iii) Fluorine (F) has the strongest oxidizing ability. Oxidizing power decreases down the group as atomic size increases and the ease of gaining an electron falls; fluorine, being the smallest, attracts an incoming electron most strongly.

(b)(i) The chloride that exists as discrete molecules is BCl₄ (a covalent chloride, existing as separate molecules with low melting and boiling points).

(b)(ii) The bonding that holds atoms of A and chlorine together in ACl is ionic (electrovalent) bonding, formed by the transfer of an electron from the metal A to chlorine.

(b)(iii) The chloride that is a good conductor of electricity (when molten or in aqueous solution) is ACl, because it is ionic and contains free-moving ions.

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(a)(i) Copper(II) oxide, \(CuO\), is a basic oxide (a metallic oxide that reacts with acids to give salt and water only).

(ii) Copper(II) oxide increases in mass on exposure to the atmosphere because, being a basic oxide, it slowly absorbs moisture (water vapour) and carbon(IV) oxide from the air, forming a basic copper carbonate/hydroxide. The added water and carbon(IV) oxide account for the gain in mass.

(iii) Action of dilute hydrochloric acid on copper(II) oxide:

\[CuO_{(s)} + 2HCl_{(aq)} \rightarrow CuCl_{2(aq)} + H_2O_{(l)}\]

(b) Two methods of removing total hardness (both temporary and permanent) from water:

• Addition of washing soda (sodium trioxocarbonate(IV), \(Na_2CO_3\)), which precipitates the calcium and magnesium ions as insoluble carbonates.
• Ion-exchange method (passing the water through a permutit/zeolite or ion-exchange resin), which replaces the \(Ca^{2+}\) and \(Mg^{2+}\) ions with sodium ions.

(Distillation of the water is also acceptable, as it leaves the dissolved salts behind.)

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(a)(i) One major use of alkenes: they are used in the manufacture of plastics/polymers (for example ethene is polymerized to polyethene). Ethene is also used to ripen fruits and to make ethanol.

(ii) The simplest alkane that exhibits structural isomerism is butane, \(C_4H_{10}\) (it exists as n-butane and 2-methylpropane).

(iii) For an alkane \(C_xH_{22}\), the general formula is \(C_nH_{2n+2}\):

\[2n + 2 = 22 \;\Rightarrow\; n = 10\]

So the alkane is \(C_{10}H_{22}\) (decane). Molar mass:

\[= (10 \times 12) + (22 \times 1) = 120 + 22 = 142\,g\,mol^{-1}\]

(iv) Two types of reaction that alkenes and alkynes undergo: addition reactions and polymerization (oxidation is also acceptable).

(b)(i) The conversion of palm oil to margarine is an addition reaction (specifically hydrogenation, the addition of hydrogen across the C=C double bonds).

(ii) The palm oil is first treated with activated charcoal in order to adsorb and remove the coloured impurities, i.e. to bleach/decolourize the oil.

(iii) The catalyst commonly used to convert vegetable oils to margarine is finely divided nickel.

(c)(i) Oils belong to the class of organic compounds called esters (specifically esters of glycerol and long-chain alkanoic (fatty) acids, i.e. lipids).

(ii) Manufacture of soap (saponification): the vegetable oil is boiled with concentrated sodium hydroxide solution; the oil is hydrolysed into soap (sodium salt of the fatty acid) and glycerol. Common salt (sodium chloride) is then added to \"salt out\" the soap, which rises to the surface as a solid, is skimmed off, washed and moulded.

(iii) Calcium ions in domestic water waste soap because they react with the soap (the sodium salt of the fatty acid) to form an insoluble scum (the calcium salt of the fatty acid). This scum forms before the soap can lather, so extra soap must be used before a lather is obtained.

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(a)(i) An effective collision is a collision between reacting particles in which the particles possess energy equal to or greater than the activation energy and collide with the correct orientation, so that a chemical reaction actually takes place.

(ii) In \(\Delta G = \Delta H - T\Delta S\):

• \(\Delta G\) = the change in Gibbs free energy of the system.
• \(\Delta H\) = the change in enthalpy (heat content) of the system.
• \(T\) = the absolute temperature (in kelvin).
• \(\Delta S\) = the change in entropy (degree of disorder) of the system.

(iii) If \(\Delta G = 0\), the deduction is that the system is at equilibrium (the reaction is neither spontaneous in the forward nor in the reverse direction).

(b)(i) Le Chatelier's principle: when a system in dynamic equilibrium is subjected to a change in temperature, pressure or concentration, the equilibrium shifts in the direction that tends to oppose (reduce) the effect of that change.

(ii) For \(2A_{2(g)} + B_{2(g)} \rightleftharpoons 2A_2B_{(g)}\), \(\Delta H = x\,kJ\) (taking the forward reaction to be exothermic), three conditions for maximum yield of \(A_2B\):

• Low temperature (favours the exothermic forward reaction).
• High pressure (3 moles of gas on the left change to 2 moles on the right, so high pressure shifts the equilibrium to the side of fewer gas molecules).
• Continuous removal of \(A_2B\) as it is formed (and/or use of a catalyst to attain equilibrium faster).

(iii) Equilibrium constant expression:

\[K_c = \frac{[A_2B]^2}{[A_2]^2\,[B_2]}\]

(c)(i) A log of wood burns more slowly than the same mass of sawdust because the sawdust is finely divided and therefore has a much larger surface area exposed to air (oxygen); the rate of reaction increases with surface area, so the sawdust burns out faster.

(iii) Two reactions catalysed by light: the photosynthesis of glucose in green plants, and the reaction of hydrogen with chlorine (or the substitution reaction of methane with chlorine).

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(a)(i) Two physical properties of hydrogen sulphide

• It is a colourless gas with an offensive smell (like that of rotten eggs).
• It is denser than air and is slightly soluble in water.

(It is also poisonous; any two physical properties are acceptable.)

(a)(ii) Laboratory equipment for intermittent production of hydrogen sulphide

Kipp's apparatus.

(b) Property illustrated

In the reaction H₂S_(g) + 2NaOH_(aq) → Na₂S_(aq) + 2H₂O_(l), hydrogen sulphide reacts with a base (sodium hydroxide) to form a salt and water. This illustrates the acidic property of hydrogen sulphide (it behaves as an acid).