Sem1 Unit6 Chemical Equilibrium

SK017 Unit 6

Past Year Examination Questions

Unit 6: Chemical Equilibrium Jun 99 1.

The decomposition of ammonium hydrogen sulphide: NH4HS(s) NH3(g) + H2S(g) is an endothermic process. An amount of 5.2589 g of NH4HS sample was placed in a 3-L container at 25°C. After attaining equilibrium, the pressure in the container was 0.659 atm. i. Calculate the value of Kp. <0.109>

ii.

What per cent of NH4HS solid remained in the container after the decomposition? <61%>

2.

Sulphonyl chloride, SO2Cl2 is a very reactive gas. It decomposes as follows when heated: SO2Cl2(g) SO2(g) + Cl2(g) A sample of 3.50 g SO2Cl2 is added to a 1.0-L container and the temperature is elevated to 375 K. If the decomposition has not yet occurred, what is the initial pressure in this container? As the system attains equilibrium at 375 K, the total pressure in the container is 1.43 atm. Calculate the partial pressures of SO2, Cl2 and SO2Cl2. <0.798> <0.632, 0.166>

Jan 00 3.

The reaction between NO and O2 is exothermic: State the effects on equilibrium if i. O2 is added ii. NO2 is taken out iii. the temperature is increased iv. the total pressure is decreased

2NO(g) + O2(g)

2NO2(g)

4.

Nitrosyl bromide NOBr decomposes when it is heated: NOBr(g) NO(g) + 12 Br2(g) An amount of 1.79 g of NOBr was placed in a closed, 1.0-L container and heated to 100°C. Calculate the Kp if the pressure at equilibrium is 0.657 atm. <0.704>

Jul 00 5.

For a reaction at equilibrium: aA(g) + bB(g) i. Write the expressions for Kc and Kp. ii. Derive the expression of Kc in terms of Kp.

dD(g) + eE(g)

6.

At a certain temperature, the equilibrium constant Kc for the following reaction is 7.5: 2NO2(g) N2O4(g) If 2.0 mol of NO2 is placed in a 2.0-L container and let the reaction to occurs; calculate the concentrations of NO2 and N2O4 at equilibrium. <0.228, 0.386> If the size of the container is increased, predict the changes in concentrations of the above species.

7.

State the Le Chatelier Principle. An amount of 0.50 mol of NO2 gas was added to a 5.0-L container and was heated to 100°C. The NO2 gas decomposed to NO gas and O2 gas. 2NO2(g) 2NO(g) + O2(g) At equilibrium, 0.20 mol of NO2 gas was leftover in the container. i) Calculate the number of moles of NO and O2 at equilibrium <0.30; 0.15> ii) Calculate the equilibrium constants Kc and Kp at 100°C. <0.0675; 2.07>

iii)

What will happen to the equilibrium if the volume of the container is increased? 30

SK017 Unit 6

Past Year Examination Questions

Aug 01 8.

The value of Kc for the reaction N2O4(g) 2NO2(g) is 4.6x10−3. If the concentrations of N2O4 and NO2 are 0.5 M and 0.15 M respectively, determine the direction of reaction in order to reach equilibrium? <0.045>

9.

Determine the per cent of dissociation at equilibrium for the reaction: PCl5(g) PCl3(g) + Cl2(g) when 5.00x10−2 mol of PCl5 was heated at temperature 150°C in a 0.50-L container. At the same temperature, the value of Kc for the above reaction is 5.55x10−4 mol L−1. <7.2>

Aug 02 10.

At 613 K, the equilibrium constant Kc for the reaction: Fe2O3(s) + 3H2(g) 2Fe(s) + 3H2O(g) is 0.064. If 1.0 mol of Fe2O3 reacted with 0.1 mol of H2 gas in a 2.0-L container until the system reached equilibrium, determine the concentrations of H2O and H2 at equilibrium. <0.014; 0.036>

Sept 03 11.

The value of Kc at 45 °C for the reaction P(g) + Q(g) R(g) + S(g) is 0.84. A mixture of these gases which is not in equilibrium has the following concentrations of the reactants and products: [P] = 0.04 M; [Q] = 0.50 M; [R] = 0.40 M and [S] = 0.04 M. In which direction must the reaction proceed to reach equilibrium?

12.

A system was charged with NOCl gas until its concentration reached 0.400 mol L −1. The temperature of the system was then increased to 245°C and it was allowed to reach equilibrium according to equation 2NOCl(g) Cl2 (g) + 2NO(g) At equilibrium, the concentration of Cl2 was 0.0225 mol L−1. Calculate the value of Kc at this temperature. <3.62x10−4 M>

Oct 04 13.

Based on the following reaction, 3H2(g) + N2(g) 2NH3(g) ∆H = −92 kJ Predict and explain the direction of the equilibrium position when i. the volume of the vessel is increased. ii. A mixture of nitrogen and helium gas is added to the system.

14.

Nitrosyl bromide, NOBr dissociates according to the chemical equation: 2NOBr(g) 2NO(g) + Br2(g) A 1.86-mol sample of NOBr is placed in a 5.0 litre evacuated flask at 25°C. Analysis of the mixture at equilibrium shows that 0.164 mol of NO is present. Calculate the equilibrium constant, Kc for this reaction.

31

SK017 Unit 6

Past Year Examination Questions

Oct 05 15.

(a) A 2.50-L flask contains 0.525 mol of CO2, 1.250 mol of CF4 and 0.750 mol of COF2 at 1000°C. Explain in which direction will a net reaction occur to reach the equilibrium shown below. CO2(g) + CF4(g) 2COF2(g) Kc = 0.50 at 1000°C <0.857> (b) Sodium hydrogen carbonate decomposes at 100°C according to the equation below: 2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g) If the value of Kp is 0.231, calculate the total gas pressure (atm) at equilibrium. <0.96>

Oct 06 16.

Nitrogen (IV) dioxide dimerised as follows, 2NO2(g) N2O4(g) ∆H = -58.84 kJ Sketch a graph showing how the concentrations of the reactant and product of the above reaction vary during the course of the reaction. At 100oC , the value of Kc is 5.0. Calculate Kp for the above reaction. Explain how the product could be increased. If initially 1 mole of NO2 is filled into a 1 L vessel, determine the concentration of N2O4 at equilibrium.

Oct 07 17.

(a) Two moles of hydrogen bromide, HBr, is placed in a 4 L container. At high temperature, it decomposes according to the following equation: 2HBr(g) H2(g) + Br2(g) At equilibrium, the concentration of bromine is 0.1 M. Calculate the equilibrium constant, Kc and the percentage of dissociation of HBr. (b) Carbon monoxide reacts with oxygen to produce carbon dioxide as follows:

∆H = - 514 kJ 2CO(g) + O2(g) 2CO2(g) , Explain how the amount of CO2 could be increased. Oct 08 18.

Sulphuryl chloride, SO2Cl2, gas dissociates reversibly into sulphur dioxide gas, SO2, and chlorine gas, Cl2. The reaction is endothermic. Suggest all the possible ways to increase the chlorine production. An amount of 0.20 mol of SO2Cl2 was placed into a 2-L vessel and allowed to reach equilibrium. Calculate the equilibrium constant for the reaction if the final concentration of Cl2 is 0.08 M. The system in equilibrium is disturbed by adding 0.14 mol of SO2Cl2 at the same temperature. Calculate the concentrations of sulphuryl chloride and chlorine at the new equilibrium. 32