Equilibrium law questions

Assignment: Questions on Topic 17.1: Equilibrium law

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Phosphorus(V) chloride decomposes when heated according to the equation:

PCl5(g) ⇌ PCl3(g) + Cl2(g)

Assume that the total volume is fixed at V dm3 and that an initial amount of phosphorus(V) chloride of a mol gives x mol of chlorine in the equilibrium mixture. The equilibrium constant is represented by Kc.

i.  Show that the equilibrium expression for this reaction is:
    Kc = x2 / (a – x)V

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ii.  Explain how this equation can be used to deduce the effect of increasing the pressure on the equilibrium mixture at the same temperature.

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Steam reacts with methane in the gaseous state to form carbon dioxide and hydrogen.

i.  State the equation for this reaction.

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ii.  54.06 g of steam and 16.04 g of methane are placed in a 1 dm3 container. When equilibrium has been reached 4.04 g of hydrogen are present in the mixture. Determine the value of Kc for this reaction.

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iii. The reaction is endothermic. State the effect on (I) the amount of hydrogen in the equilibrium mixture and (II) the value of the equilibrium constant if the reaction is carried out at a higher temperature to the one above.

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Hydrogen and iodine react reversibly in the gaseous state to form hydrogen iodide. A particular equilibrium mixture was found to contain 0.45 mol of iodine, 0.55 mol of hydrogen and 2.4 mol of hydrogen iodide. Calculate the value of the equilibrium constant at this temperature.

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When 1.0 mol of ethanol reacts with 0.5 mol of ethanoic acid at 100 oC the mixture contains 0.42 mol of ethyl ethanoate once equilibrium has been reached. Calculate the value of Kc at 100 oC for this esterification reaction.

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The standard enthalpy of formation of ethyl ethanoate at 298 K is – 480 kJ mol−1 and the standard entropy of ethyl ethanoate at 298 K is + 259 J mol−1 K−1.

i.  Use these values and information from Section 12 of the IB data booklet to determine a value for Kc for the esterification reaction between ethanoic acid and ethanol at 298 K.

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ii.  Compare your answer to the literature value of 4 at 298 K and suggest one reason which may contribute to the fact that there is such a considerable difference.

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