VJC Prelim P3 Questions

2  VJC 2017 9729/03/PRELIM/17 [Turn over Section A Answer all the questions in this section. 1 (a) Ethanoic acid and its salt, sodium ethanoate, is often used as an acid buffer to help extend the shelf-life of food products such as meat, fish and dairy. A buffer solution of pH 5.5 was prepared by mixing 0.200 mol dm−3 of ethanoic acid and 0.200 mol dm−3 aqueous sodium ethanoate. (Ka of ethanoic acid = 1.74 x 10−5 mol dm−3) (i) What do you understand by the term buffer solution? [1] (ii) Calculate the ratio of acid][ethanoic ][ethanoate required to prepare the above buffer solution. Hence deduce whether the above buffer is more effective in buffering against added acids or bases. Write an equation to illustrate your answer. [3] (iii) Calculate the volume of ethanoic acid and that of aqueous sodium ethanoate required to prepare 60 cm3 of the above buffer. [1] (iv) The boiling points of the components in the buffer are as shown. Ethanoic acid 118 oC Sodium ethanoate 881 oC With reference to the structure and bonding present in the compounds, account for the difference in the boiling points. [3] (b) The limestone that collects in kettles in hard water areas is mainly calcium carbonate. It can be removed fairly harmlessly by using a warm solution of vinegar, which contains ethanoic acid. The limestone dissolves with fizzing and a solution of calcium ethanoate remains. (i) Write a balanced equation for the reaction between ethanoic acid and calcium carbonate. [1] When the solution in (b) is evaporated and the resulting solid calcium ethanoate is heated strongly in a test-tube, an organic compound Q is formed, which condenses to a colourless liquid. The residue in the tube contains calcium carbonate. When 0.10 g of compound Q was injected into a gas syringe at a temperature of 383 K and a pressure of 101 kPa, 55 cm3 of vapour were produced. (ii) Calculate the relative molecular mass of Q. [2]

3  VJC 2017 9729/03/PRELIM/17 [Turn over (iii) Compound Q is neutral and water -soluble. Q does not react with sodium metal nor with Fehling’s solution but it does react with alkaline aqueous iodine. Suggest a structural formula for Q. Justify your answer by reference to these properties of Q. [4] (iv) Construct a balanced equation for the formation of Q by the action of heat on calcium ethanoate. [1] (v) The residue calcium carbonate can also undergo thermal decomposition when heated very strongly. Copper(II) carbonate behaves in a similar manner when heated. State which carbonate, calcium or copper( II) will have a higher decomposition temperature. Explain your answer, with the help of relevant data from the Data Booklet. [3] [Total: 19]

4  VJC 2017 9729/03/PRELIM/17 [Turn over 2 (a) In a closed reaction vessel maintained at a high temperature, 1 mol of Al2Cl6 dimers dissociate into AlCl3 according to the following equation. Al2Cl6(g) ⇌ 2AlCl3(g) The average Mr of the equilibrium gas mixture is found to be 178.0. (i) Given that the average Mr of the equilibrium gas mixture is the sum of the mole fractions of each gas, multiplied by the Mr of that gas , calculate the degree of dissociation of Al2Cl6, . [3] (ii) Write down the expression of Kp for the above equilibrium system. Hence, show that Kp = KcRT. [2] (iii) An ex periment on the dissociation of A l2Cl6 was conducted in a clos ed container of a fixed volume and the partial pressures of all the components were plotted as shown in the figure below. Copy and complete the diagram to show how the partial pressure of each gas changes when the volume of the container is halved at t1 until t2. [2] (b) Strontium chloride, SrCl2 is a typical salt which forms neutral aqueous solutions. It emits a bright red colour in a flame and this allows it to be used as a source of redness in fireworks. Use the data below to answer the questions that follow. Standard Gibbs free energy change of solution of SrCl2(s) –41.6 kJ mol–1 Standard enthalpy change of formation of SrCl2(s) –828.9 kJ mol–1 Standard enthalpy change of formation of Sr2+(aq) –545.8 kJ mol–1 Standard enthalpy change of formation of Cl–(aq) –167.5 kJ mol–1 Standard enthalpy change of hydration of Sr2+(g) –1446 kJ mol–1 Standard enthalpy change of hydration of Cl–(g) –378 kJ mol–1 (i) Define the term standard enthalpy change of formation of strontium chloride. [1] (ii) Calculate ∆Hosol and ∆Sosol of strontium chloride. [4] (iii) Write an equation to show the relationship between the lattice energy of strontium chloride and ∆Hosol of strontium chloride. Hence, calculate the lattice energy of strontium chloride. [2] Partial pressure / atm t1 t2 Time / s P P AlCl3 Al2Cl6

5  VJC 2017 9729/03/PRELIM/17 [Turn over (iv) Predict with reasoning, how Gibbs free energy change of formation of strontium chloride, ∆Gf will change with increasing temperature. [2] (c) Using the data given below, draw a labelled energy cycle to calculate the enthalpy change of combustion of solid carbon. 2SrO(s)  2Sr(s) + O2(g) H = +1184 kJ mol–1 SrO(s) + CO2(g)  SrCO3(s) H = –234 kJ mol–1 2SrCO3(s)  2Sr(s) + 2C(s) + 3O2(g) H = +2440 kJ mol–1 [4] [Total: 20]

6  VJC 2017 9729/03/PRELIM/17 [Turn over 3 (a) Consider the following synthesis pathways starting from nitrobenzene: (i) Ethanoyl chloride (CH3COCl) is used as a reactant for step 1a and step 1b but under different conditions. State the reaction mechanism that occurred for each step 1a and 1b and hence explain the difference in the conditions required. [3] (ii) Identify the reagents and conditions required for steps 2 and 3 and hence draw the structures of W, X, Y and Z. [Note: Steps 2 and 3 in both pathways each refers to the sa me set of re