An original Thinka practice paper modelled on the structure and difficulty of the Jun 2023 (V3) Cambridge IGCSE Chemistry (0620) paper. Not affiliated with or reproduced from Cambridge.
Paper 4 Theory (Extended)
Answer all questions. Use a black or dark blue pen. You may use a calculator. Show all working.
8 题目 · 91 分
题目 1 · structured
6 分
A student investigates the electrolysis of concentrated aqueous sodium chloride using inert carbon electrodes.
(a) State the name of the gas produced at the anode (positive electrode) and describe its appearance. Gas name: Appearance: [2]
(b) Write the ionic half-equation, including state symbols, for the reaction occurring at the cathode (negative electrode). [2]
(c) Explain why the solution around the cathode becomes alkaline as the electrolysis progresses. [2]
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解题
(a) During the electrolysis of concentrated aqueous sodium chloride, chloride ions (\(\text{Cl}^-\)) are selectively discharged at the anode to produce chlorine gas, which is a pale yellow-green gas.
(b) At the cathode, hydrogen ions (\(\text{H}^+\)) from water are discharged in preference to sodium ions (\(\text{Na}^+\)) because hydrogen is less reactive than sodium. The ionic half-equation is: \(2\text{H}^+(\text{aq}) + 2\text{e}^- \rightarrow \text{H}_2(\text{g})\)
(c) Water molecules dissociate slightly into \(\text{H}^+\) and \(\text{OH}^-\). Since the \(\text{H}^+\) ions are continuously discharged at the cathode to form hydrogen gas, there is an excess of hydroxide (\(\text{OH}^-\)) ions remaining in the solution around the cathode, causing the pH to rise and the solution to become alkaline.
评分标准
(a) Chlorine [1] Yellow-green / pale green (gas) [1] (Reject: green, accept: light green)
(b) Correct species: \(\text{H}^+\) and \(\text{e}^-\) on the left, \(\text{H}_2\) on the right [1] Correct balancing and state symbols: \(2\text{H}^+(\text{aq}) + 2\text{e}^- \rightarrow \text{H}_2(\text{g})\) [1] (Accept: \(2\text{H}_2\text{O}(\text{l}) + 2\text{e}^- \rightarrow \text{H}_2(\text{g}) + 2\text{OH}^-(\text{aq})\) for [2] marks)
(c) Hydrogen ions / \(\text{H}^+\) are discharged / reduced / removed [1] Leaving an excess of hydroxide ions / \(\text{OH}^-\) in the solution [1]
题目 2 · structured
11 分
2 (a) The symbols of the elements in Period 2 of the Periodic Table are shown.
Use the symbols of the elements in Period 2 to answer the questions that follow. Each symbol may be used once, more than once, or not at all.
Give the symbol of the element that: (i) forms a giant covalent structure in the form of graphite. [1] (ii) is a diatomic gas making up approximately 21% of clean, dry air. [1] (iii) contains atoms with a full outer shell of electrons. [1] (iv) reacts violently with water to form an alkaline solution and hydrogen gas. [1] (v) forms an oxide that is a major contributor to acid rain. [1] (vi) has a diatomic molecule containing a single covalent bond. [1]
(b) The relative atomic mass of an element can be calculated from the relative masses of its isotopes and their percentage abundances. (i) Identify the isotope to which all relative masses are compared. [1] (ii) Table 2.1 shows the relative masses and the percentage abundances of the two naturally occurring isotopes in a sample of copper.
Table 2.1 $$\begin{array}{|c|c|} \hline \text{Relative mass of isotope} & \text{Percentage abundance of isotope} \\ \hline 63 & 69 \\ \hline 65 & 31 \\ \hline \end{array}$$
Calculate the relative atomic mass of copper in this sample to one decimal place. [2]
(c) An ion contains 18 electrons, 16 protons and 18 neutrons. (i) State the nucleon number of the ion. [1] (ii) Identify the element that forms this ion. [1]
(c) (i) Nucleon number = Protons + Neutrons = 16 + 18 = 34 (ii) Sulfur / S (atomic number 16)
评分标准
(a) 1 mark for each correct symbol: (i) C (ii) O (iii) Ne (iv) Li (v) N (vi) F
(b) (i) Carbon-12 / \(^{12}\text{C}\) [1] (ii) M1: Show working of abundance multiplication: \((63 \times 69) + (65 \times 31)\) [1] M2: Correct division and rounding to 63.6 [1]
(c) (i) 34 [1] (ii) Sulfur / S [1]
题目 3 · structured
11 分
2 (a) The symbols of the elements in Period 2 of the Periodic Table are shown.
Use the symbols of the elements in Period 2 to answer the questions that follow. Each symbol may be used once, more than once, or not at all.
Give the symbol of the element that: (i) forms a giant covalent structure in the form of graphite. [1] (ii) is a diatomic gas making up approximately 21% of clean, dry air. [1] (iii) contains atoms with a full outer shell of electrons. [1] (iv) reacts violently with water to form an alkaline solution and hydrogen gas. [1] (v) forms an oxide that is a major contributor to acid rain. [1] (vi) has a diatomic molecule containing a single covalent bond. [1]
(b) The relative atomic mass of an element can be calculated from the relative masses of its isotopes and their percentage abundances. (i) Identify the isotope to which all relative masses are compared. [1] (ii) Table 2.1 shows the relative masses and the percentage abundances of the two naturally occurring isotopes in a sample of copper.
Table 2.1 $$\begin{array}{|c|c|} \hline \text{Relative mass of isotope} & \text{Percentage abundance of isotope} \\ \hline 63 & 69 \\ \hline 65 & 31 \\ \hline \end{array}$$
Calculate the relative atomic mass of copper in this sample to one decimal place. [2]
(c) An ion contains 18 electrons, 16 protons and 18 neutrons. (i) State the nucleon number of the ion. [1] (ii) Identify the element that forms this ion. [1]
(c) (i) Nucleon number = Protons + Neutrons = 16 + 18 = 34 (ii) Sulfur / S (atomic number 16)
评分标准
(a) 1 mark for each correct symbol: (i) C (ii) O (iii) Ne (iv) Li (v) N (vi) F
(b) (i) Carbon-12 / \(^{12}\text{C}\) [1] (ii) M1: Show working of abundance multiplication: \((63 \times 69) + (65 \times 31)\) [1] M2: Correct division and rounding to 63.6 [1]
(c) (i) 34 [1] (ii) Sulfur / S [1]
题目 4 · structured
10 分
During the electrolysis of concentrated aqueous copper(II) chloride, \(\text{CuCl}_2(\text{aq})\), inert platinum electrodes are used.
(a) State the names of the products formed at each electrode: Cathode: [1] Anode: [1]
(b) Write ionic half-equations, including state symbols, for the reactions occurring at: (i) the cathode: [2] (ii) the anode: [2]
(c) Describe a chemical test, and its positive result, to confirm the identity of the gas produced at the anode. Test: [1] Result: [1]
(d) State and explain the color change of the electrolyte as the electrolysis progresses. Color change: [1] Explanation: [1]
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解题
(a) At the cathode, copper(II) ions are discharged to form copper metal because copper is lower in the reactivity series than hydrogen. At the anode, chloride ions are discharged in preference to hydroxide ions because the solution is concentrated, forming chlorine gas.
(b) (i) Cathode reaction: \(\text{Cu}^{2+}(\text{aq}) + 2\text{e}^- \rightarrow \text{Cu}(\text{s})\). Both species and balancing must be correct, along with state symbols. (ii) Anode reaction: \(2\text{Cl}^-(\text{aq}) \rightarrow \text{Cl}_2(\text{g}) + 2\text{e}^-\).
(c) Chlorine gas is acidic and a strong oxidizing agent. Damp blue litmus paper is used; it initially turns red due to the acidic nature of chlorine in water, and then bleaches white.
(d) The blue color of aqueous copper(II) chloride is due to the presence of hydrated \(\text{Cu}^{2+}\) ions. As these ions are discharged at the cathode and converted to copper solid, their concentration decreases, causing the blue color of the solution to fade and eventually become colorless.
评分标准
(a) Cathode: copper [1] Anode: chlorine [1] (b) (i) \(\text{Cu}^{2+}(\text{aq}) + 2\text{e}^- \rightarrow \text{Cu}(\text{s})\) - Correct species and balancing [1] - Correct state symbols [1] (ii) \(2\text{Cl}^-(\text{aq}) \rightarrow \text{Cl}_2(\text{g}) + 2\text{e}^-\) - Correct species and balancing [1] - Correct state symbols [1] (c) Test: damp blue litmus paper [1] (accept damp universal indicator paper; reject dry litmus) Result: bleached / turns white [1] (accept turns red then bleaches) (d) Color change: blue to colorless / fades / becomes paler [1] Explanation: Concentration of \(\text{Cu}^{2+}\) / copper(II) ions decreases / copper(II) ions are discharged [1]
题目 5 · structured
10 分
Copper(II) carbonate undergoes thermal decomposition when heated.
(a) State the colour change observed when a sample of pure copper(II) carbonate is heated strongly.
from .............................. to .............................. [2]
(b) (i) Write the chemical equation for the thermal decomposition of copper(II) carbonate. Include state symbols.
(c) In an experiment, a sample of \(6.20\text{ g}\) of pure anhydrous copper(II) carbonate is completely decomposed.
Calculate the volume of carbon dioxide gas, in \(\text{dm}^3\), produced at room temperature and pressure (r.t.p.). The volume of one mole of any gas is \(24\text{ dm}^3\) at r.t.p. Relative formula mass, \(M_{\text{r}}\) of \(\text{CuCO}_3 = 124\).
Use the following steps.
- Calculate the number of moles of \(\text{CuCO}_3\) used.
.............................. mol
- Deduce the number of moles of \(\text{CO}_2\) produced.
.............................. mol
- Calculate the volume of \(\text{CO}_2\) formed at r.t.p.
(ii) Test for carbon dioxide: - test: bubble gas through limewater - positive result: turns cloudy / milky
(c) - Moles of \(\text{CuCO}_3 = \frac{6.20}{124} = 0.05\) mol - Moles of \(\text{CO}_2\) produced = \(0.05\) mol - Volume of \(\text{CO}_2 = 0.05 \times 24 = 1.20\) \(\text{dm}^3\)
(d) - Copper is reduced because its oxidation number decreases from \(+2\) (in \(\text{CuO}\)) to \(0\) (in \(\text{Cu}\)). - Carbon is oxidised because its oxidation number increases from \(+2\) (in \(\text{CO}\)) to \(+4\) (in \(\text{CO}_2\)).
评分标准
(a) - from green [1] - to black [1]
(b) (i) - \(\text{CuCO}_3 \rightarrow \text{CuO} + \text{CO}_2\) [1] (correct formulae of reactant and products) - state symbols: \((\text{s}) \rightarrow (\text{s}) + (\text{g})\) [1] (dependent on correct formulae)
(ii) - test: bubble through limewater AND positive result: turns cloudy / milky [1]
(d) - copper / \(\text{Cu}^{2+}\) / \(\text{CuO}\) is reduced AND oxidation number decreases / changes from \(+2\) to \(0\) [1] - carbon / \(\text{CO}\) is oxidised AND oxidation number increases / changes from \(+2\) to \(+4\) [1]
题目 6 · structured
16 分
A student investigates the electrolysis of aqueous zinc sulfate, \(\text{ZnSO}_4(\text{aq})\), using two different sets of electrodes.
(a) In the first experiment, the student uses inert graphite electrodes.
(i) State the observation at the anode (positive electrode) during this electrolysis. [1]
(ii) Write the ionic half-equation for the reaction occurring at the cathode (negative electrode). [2]
(iii) State and explain how the pH of the electrolyte changes as this electrolysis progresses. [3]
(b) In the second experiment, the student repeats the electrolysis of aqueous zinc sulfate, but uses active zinc electrodes instead of graphite.
(i) Describe what happens to the mass of the anode and the mass of the cathode during this experiment. [2]
(ii) Explain why the concentration of zinc ions, \(\text{Zn}^{2+}\), in the electrolyte remains constant. [2]
(c) The electrolysis apparatus can be modified to electroplate a steel key with zinc.
(i) State what should be used as the anode and what should be used as the cathode in this electroplating process. [2]
(ii) Give two reasons why steel objects, such as keys, are electroplated with zinc. [2]
(d) A simple chemical cell can be made by connecting a zinc electrode and a copper electrode in an electrolyte.
State the direction of electron flow in the external circuit and explain your answer in terms of the reactivity of the metals. [2]
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解题
(a) (i) Bubbles of a colourless gas are observed at the anode (due to the production of oxygen gas).
(ii) Zinc ions gain electrons to form zinc metal: \(\text{Zn}^{2+} + 2\text{e}^- \rightarrow \text{Zn}\).
(iii) The pH decreases (becomes more acidic). Hydroxide ions from water are discharged at the anode, leaving an excess of hydrogen ions, \(\text{H}^+\), in the solution, which increases the acidity.
(b) (i) The mass of the anode decreases as zinc atoms dissolve to form ions, while the mass of the cathode increases as zinc ions are deposited as zinc metal.
(ii) The rate at which zinc atoms oxidize to form zinc ions at the anode is equal to the rate at which zinc ions reduce to form zinc metal at the cathode.
(c) (i) Anode: pure zinc; Cathode: steel key.
(ii) To prevent rusting/corrosion of the steel and to improve the appearance of the key.
(d) Electrons flow from the zinc electrode to the copper electrode through the external wire. This is because zinc is more reactive than copper and loses electrons more readily.
评分标准
(a) (i) Bubbles / effervescence / fizzing [1] (Reject: oxygen gas is produced without describing the physical observation)
(ii) \(\text{Zn}^{2+} + 2\text{e}^- \rightarrow \text{Zn}\) - Correct species on left and right [1] - Correctly balanced equation with electrons [1]
(iii) - pH decreases / becomes more acidic [1] - Hydrogen ions (\(\text{H}^+\)) remain in solution / concentration of \(\text{H}^+\) increases [1] - Hydroxide ions (\(\text{OH}^-\)) are discharged at the anode / water is oxidized forming oxygen and acid [1]
(b) (i) - Anode mass decreases [1] - Cathode mass increases [1]
(ii) Rate of loss of zinc at anode is equal to rate of gain of zinc at cathode / zinc ions enter the solution at the same rate they are discharged [2] (or 1 mark for mentioning zinc dissolves and is deposited, 2nd mark for linking equal rates/amounts)
(ii) Any two from: - To prevent rusting / corrosion [1] - To improve appearance / make it shiny [1]
(d) - Direction: from zinc to copper [1] - Explanation: zinc is more reactive than copper / zinc loses electrons more easily [1]
题目 7 · structured
15 分
This question is about hydrocarbons and polymers.
(a) Hydrocarbons are organic compounds containing carbon and hydrogen only. (i) Ethane reacts with chlorine in a substitution reaction. State the essential condition for this reaction. [1] (ii) Draw the displayed formula of the organic product formed when one molecule of ethane reacts with one molecule of chlorine. [1]
(b) Pent-2-ene is an unsaturated hydrocarbon. (i) Describe a chemical test to show that pent-2-ene is unsaturated. test: .................... result: .................... [2] (ii) Pent-2-ene can be polymerised to form an addition polymer. Draw the structure of one repeat unit of this polymer. [2]
(c) A synthetic polyamide, Polymer X, has the repeat unit shown: \(-\text{NH}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{NH}-\text{CO}-\text{CH}_2-\text{CH}_2-\text{CO}-\) (i) Draw the structures of the two monomers used to make Polymer X. Show all the atoms and all the bonds in the functional groups. [2] (ii) State the type of polymerisation that occurs when Polymer X is formed. [1] (iii) Name the small molecule released during this polymerisation. [1] (iv) Polyamides can be used to make synthetic fibres. State one common use of polyamides such as nylon. [1]
(d) Plastics made from non-biodegradable polymers cause pollution. (i) State the meaning of the term *biodegradable*. [1] (ii) Polylactic acid (PLA) is a biodegradable polyester made from the monomer lactic acid, \(\text{CH}_3-\text{CH}(\text{OH})-\text{COOH}\). Lactic acid contains both an alcohol functional group and a carboxylic acid functional group. Draw the structure of the repeat unit of polylactic acid (PLA). Show all atoms and all bonds in the ester linkage. [3]
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解题
(a)(i) Ultraviolet light / UV light / sunlight
(a)(ii) Displayed formula of chloroethane showing all atoms and all bonds: ``` H H | | H--C - C--Cl | | H H ```
(b)(i) Test: Add aqueous bromine / bromine water Result: Turns from orange-brown to colourless / decolourises
(b)(ii) Repeat unit of poly(pent-2-ene): ``` H H | | --[ C - C ]-- | | CH3 CH2CH3 ```
(c)(iv) Ropes / fishing nets / clothing / sleeping bags / carpets (accept any valid use of nylon)
(d)(i) Capable of being broken down/decayed by microbes / bacteria / fungi / natural decomposers.
(d)(ii) Repeat unit of PLA: ``` H O | || --[ O - C - C ]-- | || CH3 O ``` *(or alternative orientation showing the single-bonded oxygen linked to the carbon with a methyl group and then to the carbonyl carbon)*
评分标准
(a)(i) UV light / ultraviolet / sunlight [1] (reject: light/heat alone) (a)(ii) Correct displayed formula showing all C-H, C-C and C-Cl bonds [1]
(b)(ii) - Single carbon-to-carbon bond with continuation bonds at each end [1] - One carbon of the chain has -H and -CH3, the other carbon has -H and -CH2CH3 [1]
(c)(i) - Diamine structure: \(\text{H}_2\text{N}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{NH}_2\) showing all bonds in the amine groups [1] - Dicarboxylic acid structure: \(\text{HO}-\text{CO}-\text{CH}_2-\text{CH}_2-\text{CO}-\text{OH}\) showing all bonds in the acid groups [1]
(c)(ii) Condensation [1] (c)(iii) Water / \(\text{H}_2\text{O}\) [1] (c)(iv) Any correct use of nylon, e.g. ropes / clothing / nets / tents [1]
(d)(i) Broken down by microorganisms / microbes / bacteria / fungi [1] (d)(ii) - Correct ester linkage structure shown as \(-\text{O}-\text{CO}-\) [1] - Continuation bonds shown at both ends (one on O, one on carbonyl C) [1] - Rest of repeat unit correct with the \(-\text{CH}(\text{CH}_3)-\) group [1]
题目 8 · structured
12 分
This question is about organic chemistry and polymerisation.
(a) Butane reacts with bromine in a substitution reaction. (i) State the essential condition required for this reaction. [1] (ii) Two structural isomers of monobromobutane, \(\text{C}_4\text{H}_9\text{Br}\), can be formed. Give the structural formula of each isomer. [2]
(b) Alcohols form a homologous series. (i) State the general formula for the homologous series of alcohols. [1] (ii) Draw the displayed formula of propan-1-ol. [2]
(c) Condensation polymerisation can form polyesters. Part of the structure of a polyester is shown.
(i) Draw the structures of the two monomers used to make this polyester. [2] (ii) Name this type of condensation polymer. [1] (iii) Name the two homologous series to which the monomers in (i) belong. [2] (iv) Name the small molecule released during this condensation polymerisation. [1]
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解题
(a)(i) Ultraviolet (UV) light or sunlight is needed to provide the activation energy to split the bromine molecules into radicals. (ii) The bromine atom can attach to the first carbon or the second carbon of the butane chain: - Isomer 1: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Br}\) (1-bromobutane) - Isomer 2: \(\text{CH}_3\text{CH}_2\text{CH(Br)CH}_3\) (2-bromobutane)
(b)(i) The general formula for the alcohol homologous series is \(\text{C}_n\text{H}_{2n+1}\text{OH}\) (or \(\text{C}_n\text{H}_{2n+2}\text{O}\)). (ii) Propan-1-ol has three carbon atoms in a chain, with the \(-\text{OH}\) group on the terminal carbon. All covalent bonds, including the \(\text{O}-\text{H}\) bond, must be explicitly drawn: ``` H H H | | | H-C - C - C - O - H | | | H H H ```
(c)(i) The repeat unit has an ester linkage. Breaking the ester linkage yields: - Monomer 1 (diol): \(\text{HO}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{OH}\) (propane-1,3-diol) - Monomer 2 (dicarboxylic acid): \(\text{HOOC}-\text{CH}_2-\text{CH}_2-\text{COOH}\) (butanedioic acid) (ii) Polyester (iii) Diols (or alcohols) and dicarboxylic acids (or carboxylic acids) (iv) Water, \(\text{H}_2\text{O}\)
(b)(ii) - M1: Correct \(\text{C}-\text{C}-\text{C}\) backbone with terminal oxygen and all \(\text{C}-\text{H}\) and \(\text{C}-\text{C}\) single bonds shown [1] - M2: Fully correct displayed formula showing the \(\text{O}-\text{H}\) single bond explicitly [1]
(c)(i) - Monomer 1: \(\text{HO}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{OH}\) [1] - Monomer 2: \(\text{HOOC}-\text{CH}_2-\text{CH}_2-\text{COOH}\) [1] (Accept structural or displayed formulae; hydroxyl groups must be clearly shown at both ends)