Chemistry 9701/22 February/March 2020: question paper and mark scheme

Question

Group 2 metals form alkaline solutions in water. (a) (i) Write the equation for the reaction of calcium oxide with water. . …… [1] (ii) Identify the ion that causes an aqueous solution to be alkaline. . …… [1] (b) The table shows the melting points of some Group 2 metal oxides. compound melting point / °C MgO 2825 CaO 2613 SrO 2531 BaO 1923 Explain the trend in the melting points of the oxides down Group 2. . …… . …… . …… [2] (c) Oxygen reacts readily with some metals, but each Group 2 metal requires strong heating to start the reaction with oxygen. Suggest why strong heating is required to start these reactions. . …… . …… [1] (d) Beryllium oxide reacts with hydrochloric acid to form molecules of BeCl 2. Deduce the bond angle in BeCl 2. …… [1] (e) Unlike the other oxides of Group 2 metals, beryllium oxide is amphoteric. (i) Give the meaning of the term amphoteric. . …… . …… [1] (ii) Beryllium oxide and aluminium oxide have similar chemical properties. The Be(OH)42– anion is a product of the reaction between beryllium oxide and excess concentrated OH–(aq). Construct an equation for this reaction. . …… [1] (f) Magnesium oxide reacts reversibly with chlorine according to the following equation. 2MgO(s) + 2Cl 2(g) 2MgCl 2(s) + O2(g) Under certain conditions, a dynamic equilibrium is established. (i) State two features of a reaction that is in dynamic equilibrium. 1 . …… 2 . …… [2] (ii) The equilibrium constant, Kp, is given by the following expression. pO Kp = 2 2 pCl 2 At 1.00 × 105 Pa and 500 K, 70% of the initial amount of Cl 2(g) has reacted. Calculate Kp and state its units. Kp = …… units = …… [3] (g) Magnesium peroxide, MgO2, is made in the following reaction. MgO(s) + H2O2(l) MgO2(s) + H2O(l) ∆H = –96 kJ mol–1 enthalpy change of compound formation, ∆Hf / kJ mol–1 MgO(s) –602 H2O2(l) –188 H2O(l) –286 (i) The peroxide ion is O22–. Deduce the average oxidation number of oxygen in the peroxide ion. . …… [1] (ii) Define the term enthalpy change of formation. . …… . …… . …… [2] (iii) Use the data given to calculate the enthalpy change of formation of MgO2(s). ∆Hf MgO2(s) = …… kJ mol–1 [2] (iv) Magnesium peroxide decomposes slowly to form magnesium oxide and oxygen. 1 MgO2(s) MgO(s) + 2 O2(g) Use your answer to (g)(iii) and the data in the table to calculate the enthalpy change of this reaction. If you were unable to obtain an answer to (g)(iii), use the value ∆Hf = –550 kJ mol–1. This is not the correct answer. enthalpy change of reaction = …… kJ mol–1 [1] [Total: 19]

Question

The Group 17 elements, chlorine, bromine and iodine, are non-metals that show trends in their physical and chemical properties. (a) Describe the trend in the colour of the Group 17 elements down the group. . …… . …… [1] (b) The Group 17 elements can oxidise many metals to form halides. (i) Describe the relative reactivity of the elements in Group 17 as oxidising agents. . …… . …… [1] (ii) Chlorine reacts with hot tin metal to form tin(IV) chloride, SnCl 4. SnCl 4 is a colourless liquid at room temperature that reacts vigorously with water to form an acidic solution. Suggest the type of structure and bonding shown by SnCl 4. Explain your answer. . …… . …… . …… . …… . …… [2] (c) The Group 17 elements form soluble halides with sodium. (i) Describe what is seen when dilute AgNO3(aq) is added to NaBr(aq) followed by aqueous ammonia. . …… . …… . …… [2] (ii) NaCl reacts with concentrated H2SO4 to form HCl and NaHSO4. Explain the difference between the reactions of concentrated H2SO4 with NaCl and with NaI. Your answer should refer to the role of the sulfuric acid in each reaction. . …… . …… . …… . …… . …… . …… [3] (d) The hydrogen halides are useful reagents in organic and inorganic reactions. (i) Describe and explain the trend in the boiling points of the hydrogen halides, HCl, HBr and HI. . …… . …… . …… . …… [2] (ii) Describe and explain the trend in the thermal stabilities of the hydrogen halides, HCl, HBr and HI. . …… . …… . …… . …… [2] (e) Lucas’s reagent is a mixture of HCl and ZnCl 2. Primary, secondary and tertiary alcohols can be distinguished by their reaction with Lucas’s reagent. Alcohols react with the HCl in Lucas’s reagent to form halogenoalkanes. ZnCl 2 acts as a homogeneous catalyst for these reactions. (i) Explain the meaning of the term homogeneous. . …… . …… [1] (ii) Pentan-3-ol, C2H5CH(OH)C2H5, reacts slowly with HCl to form a secondary halogenoalkane. Complete the equation for this reaction using structural formulae. (iii) The fastest reaction shown by Lucas’s reagent is with a tertiary alcohol. Draw the structure of the tertiary alcohol that is an isomer of pentan-3-ol. [1] (iv) Tertiary alcohols tend to react with Lucas’s reagent using the same mechanism as in their reaction with HCl. Suggest the type of reaction shown by tertiary alcohols with Lucas’s reagent. . …… [1] [Total: 17]

Question

Glycerol, CH2(OH)CH(OH)CH2OH, is widely used in the food industry and in pharmaceuticals. (a) A series of reactions starting from glycerol is shown. P Q CH2OH HOOC COOH NaCN and HCN H C OH C O NC C OH reaction 1 reaction 2 CH2OH HOOC COOH (i) Suggest the reagent(s) and conditions for reaction 1. . …… . …… [2] (ii) Name the reaction mechanism for reaction 2. . …… [1] (iii) Give the observation you would make when 2,4-dinitrophenylhydrazine is added to P. . …… [1] (iv) Q does not show optical isomerism. Explain why. . …… . …… . …… . …… [1] (v) When Q is heated with excess aqueous ethanoic acid in the presence of a catalytic amount of sulfuric acid, two reactions take place to form compound R. R COOH HOOC C OCOCH3 COOH Identify the two types of reaction that occur. 1 …… 2 …… [2] (b) Glycerol can be used as a starting material in the manufacture of nitroglycerine, C3H5N3O9. Nitroglycerine decomposes rapidly on heating to form a mixture of gases. 4C3H5N3O9(l) 12CO2(g) + 10H2O(g) + 6N2(g) + O2(g) A sample of nitroglycerine decomposes, releasing 1.06 dm3 of O2(g) at 850 K and 1.00 × 105 Pa. (i) Calculate the mass of nitroglycerine that decomposes. mass of nitroglycerine = …… g [3] (ii) Calculate the total volume of gas released by this decomposition at 850 K and 1.00 × 105 Pa. total volume of gas = …… dm3 [1] (c) Fats are compounds made from glycerol and unsaturated carboxylic acids. 4-pentenoic acid is an example of an unsaturated carboxylic acid. 4-pentenoic acid (CH2)2COOH H H H (i) Give the molecular formula of 4-pentenoic acid. . …… [1] (ii) Draw the repeat unit of the addition polymer that can be formed from 4-pentenoic acid. [1] (iii) Unsaturated acids are often brominated before being added to soft drinks. Complete the mechanism for the addition of Br2 to 4-pentenoic acid. ●● Include the structures of the intermediate and the product of the reaction. ●● Include all charges, partial charges, lone pairs and curly arrows. In the mechanism, R has been used to represent (CH2)2COOH. R H H H Br Br [4] (d) A reaction of another unsaturated carboxylic acid, T, is shown. T U C6H13 C6H13 HOOC HOOC Br Br (i) T is one of a pair of geometrical (cis-trans) isomers. Draw the other geometrical isomer of T and explain why the molecules exhibit this form of isomerism. . …… . …… . …… [3] (ii) Identify the reagent used to convert T to U. . …… [1] (iii) The C–Br bond has an absorption between 500 cm–1 and 600 cm–1 in an infrared spectrum. The infrared spectra for both T and U have absorptions between 2850 cm–1 and 2950 cm–1. These correspond to C–H bonds. Identify: ●● two other absorptions that would be seen in the infrared spectra of both T and U ●● one other absorption that would only be seen in the infrared spectrum of T. For each absorption, give the range of the absorption and the bonds that correspond to these absorptions. absorption 1 present in both spectra …… . …… . …… absorption 2 present in both spectra …… . …… . …… absorption only present in spectrum of T …… . …… . …… [3] [Total: 24]