An original Thinka practice paper modelled on the structure and difficulty of the Jun 2023 (V1) Cambridge IGCSE Chemistry (0620) paper. Not affiliated with or reproduced from Cambridge.
部分 Theory (Extended)
Answer all questions. Show your working where appropriate. Use a calculator if needed. Refer to the Periodic Table provided.
12 题目 · 81 分
题目 1 · short_answer
1 分
The symbols of the elements in Period 3 of the Periodic Table are shown.
Na, Mg, Al, Si, P, S, Cl, Ar
Answer the following question using only these symbols. Each symbol may be used once, more than once or not at all.
Give the symbol of the element that exists as a giant covalent macromolecular structure.
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解题
Silicon (Si) is a Group IV metalloid that forms a giant covalent structure similar to diamond, where each silicon atom is tetrahedrally bonded to four other silicon atoms.
评分标准
Si
题目 2 · short_answer
1 分
The symbols of the elements in Period 3 of the Periodic Table are shown.
Na, Mg, Al, Si, P, S, Cl, Ar
Answer the following question using only these symbols. Each symbol may be used once, more than once or not at all.
Give the symbol of the element that forms an amphoteric oxide.
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解题
Aluminium (Al) forms aluminium oxide (\text{Al}_2\text{O}_3), which is amphoteric as it reacts with both acids and bases.
评分标准
Al
题目 3 · short_answer
1 分
The symbols of the elements in Period 3 of the Periodic Table are shown.
Na, Mg, Al, Si, P, S, Cl, Ar
Answer the following question using only these symbols. Each symbol may be used once, more than once or not at all.
Give the symbol of the element that has atoms with exactly five electrons in their outer shell.
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解题
Phosphorus (P) is in Group V (Group 15) of the Periodic Table, so it has 5 electrons in its outer shell.
评分标准
P
题目 4 · short_answer
1 分
The symbols of the elements in Period 3 of the Periodic Table are shown.
Na, Mg, Al, Si, P, S, Cl, Ar
Answer the following question using only these symbols. Each symbol may be used once, more than once or not at all.
Give the symbol of the element that is a highly reactive metal stored under oil to prevent reaction with air and water.
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解题
Sodium (Na) is an alkali metal (Group I) that reacts vigorously with water and oxygen in the air, so it must be stored under oil.
评分标准
Na
题目 5 · short_answer
1 分
The symbols of the elements in Period 3 of the Periodic Table are shown.
Na, Mg, Al, Si, P, S, Cl, Ar
Answer the following question using only these symbols. Each symbol may be used once, more than once or not at all.
Give the symbol of the element that exists as a diatomic gas at room temperature and pressure.
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解题
Chlorine (Cl) is a halogen (Group VII) that exists as a diatomic gas (\text{Cl}_2) at room temperature and pressure.
评分标准
Cl
题目 6 · short_answer
1 分
The symbols of the elements in Period 3 of the Periodic Table are shown.
Na, Mg, Al, Si, P, S, Cl, Ar
Answer the following question using only these symbols. Each symbol may be used once, more than once or not at all.
Give the symbol of the element that is a monatomic gas with a stable outer shell of eight electrons.
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解题
Argon (Ar) is a noble gas (Group VIII/0) with a full valence shell (8 outer-shell electrons), meaning it is stable and exists as monatomic atoms.
评分标准
Ar
题目 7 · subjective
12 分
Aqueous copper(II) chloride is electrolysed using inert platinum electrodes.
(a) State the observations at each electrode during the electrolysis. - At the anode (positive electrode): [1] - At the cathode (negative electrode): [1]
(b) Write ionic half-equations, including state symbols, for the reactions occurring at: - the anode: [2] - the cathode: [2]
(c) State and explain how the colour of the electrolyte changes during the course of the electrolysis. [2]
(d) During the electrolysis, a student obtained \( 0.16\text{ g} \) of copper at the cathode. (i) Calculate the number of moles of copper produced. \( [A_r(\text{Cu}) = 64] \) [1] (ii) Deduce the number of moles of chlorine molecules, \( \text{Cl}_2 \), produced at the anode. [1] (iii) Calculate the volume of chlorine gas produced at room temperature and pressure (r.t.p.) in \( \text{cm}^3 \). [The volume of one mole of any gas is \( 24\text{ dm}^3 \) at r.t.p.] [2]
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解题
(a) - Anode: Bubbles of a pale green/yellow-green gas (chlorine) are observed. - Cathode: A pink/brown solid (copper metal) is deposited on the electrode.
(c) The blue colour of the electrolyte fades and eventually becomes colourless. This is because the blue copper(II) ions, \( \text{Cu}^{2+} \), are discharged and removed from the solution to form copper metal at the cathode.
(d) (i) \( \text{Moles of Cu} = \frac{0.16\text{ g}}{64\text{ g/mol}} = 0.0025\text{ mol} \) (ii) According to the stoichiometry of the electron transfer: \( 1\text{ mol of Cu} \) requires \( 2\text{ mol of e}^- \), and \( 1\text{ mol of Cl}_2 \) produces \( 2\text{ mol of e}^- \). Therefore, the mole ratio of \( \text{Cu} : \text{Cl}_2 \) is \( 1 : 1 \). \( \text{Moles of Cl}_2 = 0.0025\text{ mol} \). (iii) \( \text{Volume of Cl}_2 = 0.0025\text{ mol} \times 24\text{ dm}^3\text{/mol} = 0.06\text{ dm}^3 \) In \( \text{cm}^3 \): \( 0.06 \times 1000 = 60\text{ cm}^3 \).
评分标准
(a) Anode: bubbles / effervescence / fizzing of green-yellow gas [1] Cathode: pink / brown / reddish-brown solid [1] (Reject: 'copper' on its own as it is an inference, must be an observation).
(a) Describe a suitable experimental setup that could be used to measure the rate of this reaction by monitoring the loss in mass. State the purpose of the cotton wool plug used in the flask. [3]
(b) The reaction is repeated at two different temperatures, \( 20^\circ\text{C} \) and \( 40^\circ\text{C} \), with all other variables kept constant. (i) On a single set of axes, sketch curves to show how the mass of the flask and its contents changes with time for both temperatures. Label the curves clearly with the temperatures. [3] (ii) Explain, in terms of collision theory, why the rate of reaction is higher at \( 40^\circ\text{C} \) than at \( 20^\circ\text{C} \). [4]
(c) The experiment at \( 20^\circ\text{C} \) is repeated using the same volume of hydrochloric acid but with double the concentration. The marble chips remain in excess. Describe and explain how this change affects: - the initial rate of the reaction [1] - the final total mass of carbon dioxide gas produced. [2]
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解题
(a) Place the conical flask containing dilute hydrochloric acid on a digital mass balance. Add the marble chips, insert a cotton wool plug in the neck of the flask, and start a timer. Record the mass at regular intervals. The cotton wool plug allows carbon dioxide gas to escape while preventing any acid spray/droplets from being lost, which would lead to an inaccurate mass loss reading.
(b) (i) The sketch should show: - Y-axis labelled 'mass of flask and contents' (or 'mass') and X-axis labelled 'time'. - Both curves starting at the same initial mass and falling to the same final horizontal asymptote (since marble chips are in excess, the limiting reactant, acid, determines the total mass loss). - The curve for \( 40^\circ\text{C} \) is steeper initially and levels off sooner than the curve for \( 20^\circ\text{C} \).
(ii) At \( 40^\circ\text{C} \), the particles have more kinetic energy and move faster. Consequently, the collision frequency of reactant particles increases. More importantly, a much larger proportion of the colliding particles possess energy greater than or equal to the activation energy (\( E_a \)). This leads to a higher frequency of successful/effective collisions.
(c) - The initial rate of reaction increases (doubles) because there are more reactant particles per unit volume, which increases the frequency of collisions between reactant particles. - The final total mass of carbon dioxide produced doubles. This is because doubling the concentration of the acid while keeping the volume constant doubles the number of moles of the limiting reactant (\( \text{HCl} \)), producing twice as many moles of product.
评分标准
(a) Flask placed on a mass balance/balance [1]; cotton wool allows gas/\( \text{CO}_2 \) to escape [1]; cotton wool prevents loss of acid spray/mist [1].
(b) (i) Correct axes (mass on y, time on x) and both curves decreasing to the same final mass [1]; \( 40^\circ\text{C} \) curve is steeper/levels off earlier than \( 20^\circ\text{C} \) [1]; both curves clearly labelled [1].
(ii) Particles gain kinetic energy / move faster [1]; higher frequency of collisions [1]; more particles have energy greater than/equal to the activation energy [1]; higher frequency of successful/fruitful collisions [1].
(c) Initial rate increases / is higher [1]; total mass of carbon dioxide increases / doubles [1]; because there are more moles of acid reacting / double the moles of \( \text{HCl} \) [1].
题目 9 · subjective
12 分
Barium sulfate, \( \text{BaSO}_4 \), is an insoluble salt used in medicine. It can be prepared by a precipitation reaction.
(a) Name two soluble salts that could react together to prepare a pure sample of barium sulfate. [2]
(b) Write a chemical equation for the precipitation reaction using the salts you named in (a). Include state symbols. [3]
(c) Describe the practical steps required to obtain a pure, dry sample of barium sulfate starting from the mixture obtained immediately after the reaction. [4]
(d) Explain why barium sulfate is safe to ingest for medical imaging (as a 'barium meal') even though barium ions, \( \text{Ba}^{2+} \), are highly toxic. [1]
(e) Write the ionic equation, with state symbols, for this precipitation reaction. [2]
(b) Using barium nitrate and sodium sulfate: \( \text{Ba(NO}_3\text{)}_2\text{(aq)} + \text{Na}_2\text{SO}_4\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaNO}_3\text{(aq)} \)
(c) 1. Filter the mixture to separate the insoluble barium sulfate precipitate (residue) from the soluble sodium nitrate solution (filtrate). 2. Wash the residue (barium sulfate) with distilled/deionised water to remove any remaining soluble impurity ions. 3. Dry the pure barium sulfate precipitate in a warm oven (or leave it to dry between sheets of filter paper).
(d) Barium sulfate is highly insoluble in water and stomach acid. Therefore, it does not dissolve or release toxic \( \text{Ba}^{2+} \) ions into the body / bloodstream.
(a) Barium nitrate / barium chloride [1] and sodium sulfate / potassium sulfate / ammonium sulfate / sulfuric acid [1]. (Reject: insoluble sulfates or barium carbonate/oxide/hydroxide if not specified as soluble).
(b) Correct formulae of reactants and products [1]; balanced equation [1]; correct state symbols: (aq) for soluble reactants/products, (s) for \( \text{BaSO}_4 \) [1].
(c) Filter / filtration to obtain residue [1]; wash residue with distilled/deionised water [1]; dry residue [1]; description of drying (e.g., in oven, using filter paper, desiccator) [1].
(d) It is insoluble / does not dissolve in body fluids/blood [1].
(e) \( \text{Ba}^{2+} \) and \( \text{SO}_4^{2-} \) on left and \( \text{BaSO}_4 \) on right [1]; state symbols \( \text{(aq)} + \text{(aq)} \rightarrow \text{(s)} \) [1].
题目 10 · subjective
13 分
Compound Y is a liquid organic compound containing carbon, hydrogen, and oxygen. It has the molecular formula \( \text{C}_4\text{H}_8\text{O}_2 \).
(a) Compound Y reacts with aqueous sodium carbonate to produce carbon dioxide gas. (i) Identify the functional group present in Compound Y. [1] (ii) Draw the displayed formula of Compound Y. [2] (iii) Write the word equation for the reaction of Compound Y with sodium carbonate. [2]
(b) An isomer of Compound Y, Compound Z, is an ester with a sweet, fruity smell. (i) State the name of the catalyst used to prepare an ester in the laboratory. [1] (ii) Name a combination of an alcohol and a carboxylic acid that can react together to form Compound Z. [2] (iii) Draw the displayed formula of one possible ester isomer with the molecular formula \( \text{C}_4\text{H}_8\text{O}_2 \). Circle the ester linkage. [3] (iv) Give the systematic IUPAC name of the ester you drew in (b)(iii). [2]
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解题
(a) (i) The functional group is the carboxylic acid group / carboxyl group (\( \text{-COOH} \)). (ii) Displayed formula of butanoic acid: ``` H H H O | | | // H - C - C - C - C | | | \ H H H O - H ``` (Alternatively, methylpropanoic acid displayed formula is acceptable). (iii) Word equation: \( \text{butanoic acid} + \text{sodium carbonate} \rightarrow \text{sodium butanoate} + \text{carbon dioxide} + \text{water} \) (Accept methylpropanoic acid / sodium methylpropanoate).
(b) (i) Concentrated sulfuric acid. (ii) Ethanol and ethanoic acid (to make ethyl ethanoate). Other acceptable pairs include: - Methanol and propanoic acid (to make methyl propanoate). - Propan-1-ol (or propan-2-ol) and methanoic acid (to make propyl methanoate / isopropyl methanoate). (iii) Displayed formula of ethyl ethanoate: ``` H O H H | // | | H - C - C - O - C - C - H | | | H H H ``` The ester linkage is the \( \text{-C(=O)-O-} \) group and must be circled. (iv) Ethyl ethanoate (or systematic name corresponding to the drawn structure in b(iii)).
评分标准
(a) (i) Carboxylic acid group / carboxyl group / \( \text{-COOH} \) [1]. (ii) Correct skeleton of 4 carbons [1]; correct functional group with \( \text{C=O} \) and \( \text{O-H} \) shown fully displayed [1]. (iii) Carboxylic acid + sodium carbonate \( \rightarrow \) sodium salt + carbon dioxide + water [2] (1 mark for correct reactants, 1 mark for correct products. Allow error carried forward from the specific acid named).
(b) (i) (Concentrated) sulfuric acid [1] (Reject: dilute sulfuric acid, hydrochloric acid). (ii) Suitable alcohol [1] and carboxylic acid [1] that have a total of 4 carbons (e.g. ethanol and ethanoic acid). (iii) Correct displayed formula of a 4-carbon ester [2] (1 mark for correct ester structure, 1 mark for showing all bonds including O-H if any, though none in esters, check C-H, C-O, C=O); correct circle around the ester linkage (\( \text{-COO-} \)) [1]. (iv) Correct systematic IUPAC name matching structure in (b)(iii) [2] (1 mark for the alkyl part, e.g. 'ethyl', 1 mark for the carboxylate part, e.g. 'ethanoate').
题目 11 · subjective
12 分
Methanol, \( \text{CH}_3\text{OH} \), is manufactured industrially by reacting carbon monoxide with hydrogen in a reversible reaction:
(a) State and explain the effect of increasing the pressure on the position of equilibrium. [3]
(b) The reaction is carried out at an industrial temperature of \( 250^\circ\text{C} \). (i) State and explain the effect of a higher temperature, such as \( 350^\circ\text{C} \), on the equilibrium yield of methanol. [3] (ii) Explain why a temperature lower than \( 250^\circ\text{C} \) is not used, even though it would favour the forward reaction. [2]
(c) State the effect of adding a copper-based catalyst on: (i) the rate of the forward reaction [1] (ii) the position of equilibrium. [1]
(d) Describe how the energy levels of reactants and products compare in this reaction, and state whether the reaction is exothermic or endothermic. [2]
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解题
(a) Increasing the pressure shifts the position of equilibrium to the right (towards the products side). This is because the forward reaction results in a decrease in the number of moles of gas (3 moles of gaseous reactants on the left react to form 1 mole of gaseous product on the right). Shifting to the right opposes the increase in pressure.
(b) (i) A higher temperature decreases the equilibrium yield of methanol. This is because the forward reaction is exothermic (releases heat, \( \Delta H \) is negative). According to Le Chatelier's principle, increasing the temperature favours the endothermic reverse reaction to absorb the added thermal energy. (ii) A lower temperature is not used because it would dramatically decrease the rate of the reaction. At very low temperatures, the reaction would take too long to reach equilibrium, making the process industrially unprofitable.
(c) (i) The catalyst increases the rate of the forward reaction (and the reverse reaction equally). (ii) The catalyst has no effect on the position of equilibrium.
(d) The reactants have a higher energy level than the products. The reaction is exothermic.
评分标准
(a) Equilibrium shifts to the right [1]; because there are fewer moles of gas on the right / 3 moles on left and 1 mole on right [1]; this decreases the pressure [1].
(b) (i) Yield of methanol decreases [1]; because the forward reaction is exothermic [1]; system shifts to the left / favours endothermic reaction to absorb heat [1]. (ii) Rate of reaction would be too slow [1]; fewer successful collisions per unit time / less kinetic energy of particles [1].
(c) (i) Increases / speeds up [1]. (ii) No effect / unchanged [1].
(d) Reactants have higher energy than products [1]; reaction is exothermic [1].
题目 12 · subjective
13 分
Phosphorus exists in nature as a single stable isotope, \( ^{31}\text{P} \), but synthetic radioactive isotopes such as \( ^{32}\text{P} \) are used in medicine.
(a) Define the term isotopes. [2]
(b) Complete the table below to show the subatomic particles in the phosphorus atom and the phosphide ion: [4]
| Particle | Number of Protons | Number of Neutrons | Number of Electrons | |---|---|---|---| | \( ^{31}\text{P} \) atom | | | | | \( ^{32}\text{P}^{3-} \) ion | | | |
(c) Phosphorus reacts with chlorine to form phosphorus trichloride, \( \text{PCl}_3 \). (i) State the type of chemical bonding in phosphorus trichloride. [1] (ii) Draw a dot-and-cross diagram to show the arrangement of the outer-shell electrons in a molecule of \( \text{PCl}_3 \). [3]
(d) Phosphorus trichloride has a low melting point of \( -93^\circ\text{C} \). Explain, in terms of structure and bonding, why phosphorus trichloride has a low melting point. [3]
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解题
(a) Isotopes are atoms of the same element (same number of protons / atomic number) with different numbers of neutrons (different mass / nucleon numbers).
(c) (i) Covalent bonding. (ii) Phosphorus is in Group V (5 outer electrons) and Chlorine is in Group VII (7 outer electrons). In \( \text{PCl}_3 \): - The central P atom shares one pair of electrons with each of the three Cl atoms (three P-Cl single covalent bonds). - The P atom has 2 non-bonding outer electrons left (one lone pair). - Each Cl atom has 6 non-bonding outer electrons left (three lone pairs).
(d) Phosphorus trichloride has a simple molecular structure. There are weak intermolecular forces (or weak forces of attraction between molecules) which require very little thermal energy to overcome.
评分标准
(a) Atoms of the same element / same number of protons [1]; different numbers of neutrons [1] (Reject: 'elements' instead of 'atoms').
(c) (i) Covalent [1]. (ii) Three shared pairs of electrons between P and each Cl [1]; lone pair (2 electrons) on the P atom [1]; three lone pairs (6 electrons) on each Cl atom [1].
(d) Simple molecular structure / simple covalent structure [1]; weak intermolecular forces / weak forces between molecules [1] (Reject: 'weak covalent bonds'); requires little energy to overcome/break these forces [1].