Chemistry 9701/22 February/March 2021: question paper and mark scheme

Question

The rate of chemical reactions is affected by changes in temperature and pressure. (a) (i) Draw a curve on the axes to show the Boltzmann distribution of energy of particles in a sample of gaseous krypton atoms at a given temperature. Label the curve T1 and label the axes. [2] (ii) On the diagram in (a)(i), draw a second curve to show the distribution of energies of the krypton atoms at a higher temperature. Label the second curve T2.[1] (b) The Boltzmann distribution assumes that the particles behave as an ideal gas. (i) State two assumptions of the kinetic theory as applied to an ideal gas. 1 . …… . …… 2 . …… . …… [2] (ii) 2.00 g of krypton gas, Kr(g), is placed in a sealed 5.00 dm3 container at 120 °C. Calculate the pressure, in Pa, of Kr(g) in the container. Assume Kr(g) behaves as an ideal gas. Show your working. pressure = …… Pa [3] (iii) State and explain the conditions at which krypton behaves most like an ideal gas. . …… . …… . …… . …… [2] (c) Krypton reacts with fluorine in the presence of ultraviolet light to make krypton difluoride, KrF2(g). Kr(g) + F2(g) → KrF2(g) activation energy for the reaction, Ea = +385 kJ mol–1 enthalpy change of formation of KrF2, ∆Hf = +60.2 kJ mol–1 (i) Use this information to complete the reaction profile diagram for the formation of KrF2. Label Ea and ∆Hf on the diagram. Assume the reaction proceeds in one step. energy reactants / kJ mol–1 progress of reaction [2] (ii) Explain, in terms of activation energy, Ea, and the collision of particles, how an increase in temperature affects the rate of a chemical reaction. . …… . …… . …… . …… [2] [Total: 14]

Question

Chlorine, Cl 2, is a reactive yellow-green gas. It is a strong oxidising agent. (a) State how Cl 2 is used in water purification. . …… . …… [1] (b) Chlorine has the highest first ionisation energy of the Period 3 elements Na to Cl. (i) Construct an equation for the first ionisation energy of chlorine. Include state symbols. . …… [1] (ii) Explain the general increase in the first ionisation energies of the Period 3 elements. . …… . …… . …… . …… [2] (c) The halide ions, X– (where X = Cl, Br, I), show clear trends in their physical and chemical properties. (i) State and explain the relative thermal stabilities of the hydrogen halides, HX. . …… . …… . …… [2] The halide ions react easily with concentrated H2SO4. The main sulfur-containing product of each reaction is shown in the table. halide ion Cl – Br – I– main sulfur-containing product of HSO4– SO2 H 2S reaction with concentrated H2SO4 oxidation number of sulfur (ii) Complete the table to show the oxidation number of sulfur in each of the sulfur-containing products.[1] (iii) Explain why different sulfur-containing products are produced when each of these halide ions reacts with concentrated H2SO4. . …… . …… [1] (d) Cl 2 reacts with aqueous sodium hydroxide in a disproportionation reaction. (i) State what is meant by disproportionation. . …… . …… [1] (ii) Write an equation for the reaction of Cl 2 with cold aqueous sodium hydroxide. . …… [1] (e) Aluminium reacts with chlorine to form aluminium chloride. Aluminium chloride can exist as the gaseous molecule Al 2Cl 6(g). This molecule contains coordinate bonds. (i) Draw a diagram that clearly shows all the types of bond present in Al 2Cl 6(g). [2] (ii) Describe what you would see when solid aluminium chloride reacts with water. Name the type of reaction that occurs. . …… . …… . …… [2] (f) 0.020 mol of element Z reacts with excess Cl 2 to form 0.020 mol of a liquid chloride. The liquid chloride has formula ZCl n, where n is an integer. ZCl n reacts vigorously with water at room temperature to give an acidic solution and a white solid. When excess AgNO3(aq) is added to the solution, 11.54 g of AgCl (s) forms. (i) Suggest the type of bonding and structure shown by ZCl n. . …… [1] (ii) Calculate the value of n in ZCl n. n = …… [2] (g) Dichloromethane, CH2Cl 2, is widely used as an organic solvent. CH2Cl 2 can be prepared by reacting CH3Cl and Cl 2 at room temperature. The reaction proceeds via several steps, as shown. initiation Cl 2 2Cl • propagation 1 Cl • + CH3Cl HCl + •CH2Cl propagation 2 Cl 2 + •CH2Cl products final step Cl • + •CH2Cl CH2Cl 2 (i) Give the name of the mechanism of this reaction. . …… [1] (ii) State the essential condition required for the initiation step to take place. . …… [1] (iii) Give the electronic configuration of Cl •. 1s2 . …… [1] (iv) Identify the products of the step labelled propagation 2. . …… [1] (v) Name the type of reaction shown in the final step. . …… [1] (vi) Suggest the identity of another organic molecule that is a product of the reaction of CH3Cl and Cl 2 under the same conditions. . …… [1] [Total: 23]

Question

Compounds P, Q and R have all been found in the atmosphere of one of Saturn’s moons. P Q R H H N C C C C N H C C C N C C H C N (a) The equation for the complete combustion of P, C4N2(l), is shown. C4N2(l) + 4O2(g) → 4CO2(g) + N2(g) ∆H = –2036 kJ mol–1 (i) The enthalpy change of formation, ∆Hf, of CO2(g) is –384 kJ mol–1. Calculate the enthalpy change of formation, ∆Hf, of P, in kJ mol–1. ∆Hf of P = …… kJ mol–1 [2] (ii) One of the products of the complete combustion of P is nitrogen gas, N2(g). Explain the lack of reactivity of nitrogen. . …… [1] (b) Q forms when HCN reacts with ethyne, H C C H. (i) Ethyne, HCN and Q are all weak Brønsted–Lowry acids. Explain what is meant by the term weak Brønsted–Lowry acid. . …… . …… . …… [2] (ii) Ethyne, HCN and Q all contain triple bonds between two atoms. A triple bond consists of one sigma (σ) and two pi (π) bonds. Draw a labelled diagram to show the formation of one pi (π) bond. [2] (c) P and Q can be detected in the atmosphere by infrared spectroscopy. Identify two absorptions, and the bonds that correspond to these absorptions, that will appear in the infrared spectra of both P and Q. 1 . …… . …… 2 . …… . …… [2] (d) The flow chart shows some reactions of R. R H H H H dilute H2SO4(aq) heat under reflux C C C C S reaction 1 reaction 2 H C H C N N reaction 3 H2(g) T CH3CH2CH2NH2 (i) Name the type of reaction shown in reaction 1. . …… [1] (ii) Draw the structure of S, the organic product of reaction 2. [1] (iii) Name T. . …… [1] (iv) T can also be formed by the reaction of CH3CH2CH2Br with ammonia. State the necessary conditions of this reaction. . …… [1] [Total: 13]

Question

Hydroxyethanal, HOCH2CHO, has been observed in dust clouds near the centre of our galaxy. hydroxyethanal H O HO C C H H (a) Predict the bond angles labelled x and y in the diagram of hydroxyethanal. H O x HO C C y H H x = ……° y = ……° [2] (b) Hydroxyethanal reacts separately with 2,4-dinitrophenylhydrazine (2,4-DNPH) and with Tollens’ reagent. State what you would observe in each reaction. reaction with 2,4-DNPH …… reaction with Tollens’ reagent …… [2] (c) Hydroxyethanal is converted to ethanedioic acid, (CO2H)2, when it reacts with excess acidified dichromate(VI) ions, Cr2O72–. (i) State the role of acidified Cr2O72– in this reaction. . …… [1] (ii) State and explain any other necessary conditions for this reaction to be successful. . …… . …… . …… [2] (d) Hydroxyethanal can be reduced to ethane-1,2-diol, (CH2OH)2, as shown. hydroxyethanal ethane-1,2-diol H O H H [H] HO C C HO C C OH H H H H (i) Write an equation for the reduction of hydroxyethanal to (CH2OH)2. Use [H] to represent an atom of hydrogen from the reducing agent. . …… [1] (ii) Identify a reagent for this reduction reaction. . …… [1] (iii) (CH2OH)2 also forms when an alkene A reacts with cold, dilute, acidified manganate(VII) ions. Name A. . …… [1] [Total: 10]