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Edexcel Jun 2014 (R) Paper 4 Q11
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![SECTION C Answer ALL the questions. Write your answers in the spaces provided. 11 The decomposition of hydrogen iodide to form iodine and hydrogen is an equilibrium reaction. 2HI(g) (cid:85) I2(g) + H2(g) The equilibrium was investigated by taking sealed tubes containing the same mass of hydrogen iodide and heating them at 700 K for some time. At this temperature, the equilibrium takes about two days to be established. The tubes were rapidly cooled to room temperature, which maintained the equilibrium concentrations, because at this temperature the reaction is extremely slow. Each tube was opened under an aqueous solution of potassium iodide, which dissolved the hydrogen iodide and the iodine. The amount of iodine was found by titration and the composition of the equilibrium mixture calculated. From the number of moles of each substance at equilibrium, and the volume of the tubes, the equilibrium concentrations were calculated. (a) (i) How would the appearance of the contents of a tube change as it was cooled to room temperature? (2)(ii) How could you show that equilibrium had been established? (2)20 *P42988A02024*<br />
(b) The equilibrium concentrations of one experiment are shown in the table below. [HI] /mol dm3 0.00353 [H2] /mol dm3 0.00048 [I2] /mol dm3 0.00048 *(i) The volume of the tube in this experiment was 30 cm3. Calculate the initial mass of hydrogen iodide. Show your working. (ii) Write an expression for the equilibrium constant, Kc, at 700 K. (iii) Calculate the value for this equilibrium constant. (5) (1) (1) *P42988A02124* 21 Turn over<br />
(iv) Does this equilibrium constant have units? Explain your answer. (1)(c) The equation for the reaction at 700 K can also be written HI(g) (cid:85) H2(g) + I2(g) (i) Write the equilibrium constant, Kc, for this reaction. (ii) Using the same equilibrium concentrations as below, calculate the equilibrium constant, Kc. Deduce the relationship between this value and the value calculated in part (b)(iii). [HI] /mol dm3 0.00353 [H2] /mol dm3 0.00048 [I2] /mol dm3 0.00048 (1) (2) 22 *P42988A02224*<br />
(d) Consider the following equilibrium reaction. 2HI(g) (cid:85) I2(s) + H2(g) For this reaction Kp P H 2 2 P HI Use the expression for Kp to explain the effect of an increase in total pressure on the position of the equilibrium. (3)(Total for Question 11 = 18 marks) TOTAL FOR SECTION C = 18 MARKS TOTAL FOR PAPER = 90 MARKS *P42988A02324* 23<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJune_2014_R_QP_-_Unit_4_Edexcel_Chemistry_A-level_11-0.jpg)
![Section C Question Number 11(a)(i) Purple gas/ gas turns colourless (1) Correct Answer to (silver/shiny) grey/black solid (1) Just gas to solid OR solid forming (1)max Reject Mark Purple liquid/solid 2 Reject Mark 6CH0401R 1406 Correct Answer First mark Heat for different lengths of time OR After more time/specified time eg 2 days . OR Use a colorimeter OR Set up reverse reaction (1) Second mark Measure the concentration of a reactant or product of two tubes, which should be the same OR Colour does not change /is same (1) Question Number 11 (a)(ii)<br />
Question Number *11 (b)(i) Question Number 11 (b)(ii) Reject Mark Correct Answer Equilibrium moles HI 30 x 0.00353 = 0.0001059 (1) 1000 H2 and I2 30 x 0.00048 = 0.0000144 (1) 1000 Initial amount of HI = 0.0001059 + 2 x 0.0000144 = 0.0001347 (mol) ALLOW TE from wrong moles of either or both entity (1) Mass of 1 mol of HI = 127.9 (1) (1) Mass of HI = 0.0001347 x 127.9 = 0.0172 g Correct answer with or without working (5) All marks stand alone Last two marks are available for any amount in moles x 127.9correctly calculated Correct Answer Kc = [H2][I2] [HI]2 Ignore state symbols unless (aq) or (s) Ignore eq or eqm Reject Mark p H2 etc (Kp) 6CH0401R 1406<br />
Correct Answer Kc = 0.00048 x 0.00048 0.003532 = 0.018489 = 0.0185 Allow all SF except 1 Correct Answer Question Number 11 (b)(iii) Question Number 11 (b)(iv) Reject The units cancel OR There are the same numbers of moles of reactants and products Question Number 11 (c)(i) Kc = [H2][I2] Correct Answer [HI] Ignore state symbols unless (aq) or (s) Ignore eq or eqm Reject p H2 etc (Kp) but not if already penalised Reject Mark Mark Mark 6CH0401R 1406<br />
Question Number 11 (c)(ii) Correct Answer= [0.00048][0.00048] Kc [0.00353] = 0.136 Allow all SF except 1 (1) Which is the square root of the previous value OR Kc = (Kc OR 0.1362 = 0.0185 (1) )2 Reject Mark 6CH0401R 1406<br />
(1) becomes less Second mark (when pressure is increased) the quotient/ratio pH2: (pHI)2 than Kp OR Ratio decreases OR Ratio proportional to 1/P (P is total pressure change) ALLOW Kp proportional to 1/P (1) Third mark To restore the value of the quotient/ratio to Kp ALLOW To restore Kp And EITHER pH2 increases / pHI decreases (1) OR Equilibrium shifts to the right (1) Question Number 11 (d) Correct Answer Reject Frist mark Kp remains unchanged/constant Kp decreases for this mark only Mark Total 18 marks Section C = 18 marks 6CH0401R 1406<br />
TOTAL FOR PAPER = 90 MARKS Pearson Education Limited. Registered company number 872828 with its registered office at Edinburgh Gate, Harlow, Essex CM20 2JE 6CH0401R 1406<br />](https://learnah.org/wp-content/uploads/2018/08/June_2014_R_MS_-_Unit_4_Edexcel_Chemistry_A-level_11-0.jpg)
