Chemistry essential Notes, Schemes and Mind Maps of Chemistry

Download Chemistry essential Notes and more Schemes and Mind Maps Chemistry in PDF only on Docsity!

WCHS Chemistry

Homework Booklet

3.1.6 Equilibria and Kc

Name: …………………………………………………………

Class: …………………………………………………………

Due: …………………………………………………………

Mark: …...…... /50 Grade: …...…...

Things to work on:

Q1. Colourless solutions of X (aq) and Y (aq) react to form an orange solution of Z (aq) according to the following equation. X (aq) + 2 Y (aq) Z (aq) Δ H = −20 kJ mol− A student added a solution containing 0.50 mol of X (aq) to a solution containing 0.50 mol of Y (aq) and shook the mixture. After 30 seconds, there was no further change in colour. The amount of Z (aq) at equilibrium was 0.20 mol. (a) Deduce the amounts of X (aq) and Y (aq) at equilibrium. Amount of X (aq) = ________ mol Amount of Y (aq) = ________ mol (2) (b) On the grid below, draw a graph to show how the amount of Z (aq) changed from the time of initial mixing until 60 seconds had elapsed. (3)

Q2.

(a) A mixture of 1.50 mol of hydrogen and 1.20 mol of gaseous iodine was sealed in a container of volume V dm^3. The mixture was left to reach equilibrium as shown by the following equation. H 2 (g) + l 2 (g) 2Hl(g) At a given temperature, the equilibrium mixture contained 2.06 mol of hydrogen iodide. (i) Calculate the amounts, in moles, of hydrogen and of iodine in the equilibrium mixture. Moles of hydrogen ______________________________________________ Moles of iodine _________________________________________________ (2) (ii) Write an expression for the equilibrium constant ( K c) for this equilibrium.

(1) (iii) K c for this equilibrium has no units. State why the units cancel in the expression for K c

(1) (iv) A different mixture of hydrogen, iodine and hydrogen iodide was left to reach equilibrium at the same temperature in a container of the same volume. This second equilibrium mixture contained 0.38 mol of hydrogen, 0.19 mol of iodine and 1.94 mol of hydrogen iodide. Calculate a value for K c for this equilibrium at this temperature.

(Extra space) ___________________________________________________

(2)

(b) This question concerns changes made to the four equilibria shown in parts (b)(i) to (b)(iv). In each case, use the information in the table to help you choose from the letters A to E the best description of what happens as a result of the change described. Write your answer in the box. Each letter may be used once, more than once or not at all. Position of equilibrium (^) Value of equilibrium constant, K c A remains the same same B moves to the right same C moves to the left same D moves to the right different E moves to the left different (i) Change: increase the temperature of the equilibrium mixture at constant pressure. H 2 (g) + I 2 (g) 2Hl(g)∆ H ϴ^ = +52 kJ mol– (1) (ii) Change: increase the total pressure of the equilibrium mixture at constant temperature. 3H 2 (g) + N 2 (g) 2NH 3 (g)∆ H ϴ^ = -92 kJ mol– (1) (iii) Change: add a catalyst to the equilibrium mixture at constant temperature. CO(g) + H 2 O(g) CO 2 (g) + H 2 (g)∆ H ϴ^ = -41 kJ mol– (1) (iv) Change: add chlorine to the equilibrium mixture at constant temperature. PCl 5 (g) PCl 3 (g) + Cl 2 (g)∆ H ϴ^ = +93 kJ mol–

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________

(b) The second stage is carried out in a separate reactor. Carbon monoxide is converted into carbon dioxide and more hydrogen is formed. The equation for this reaction is CO(g) + H 2 O(g) CO 2 (g) + H 2 (g) Δ H = −41 kJ mol− Use Le Chatelier’s principle to state the effect, if any, of a decrease in the total pressure on the yield of hydrogen in this second stage. Explain your answer. Effect ______________________________________________________________ Explanation _________________________________________________________

(2) (Total 9 marks) Q4. This question is about the gaseous equilibrium between compounds E , F and G as shown in the equation. E (g) + 2 F (g) 2 G (g) ΔH = –50 kJ mol– (a) A 2.0 mol sample of E was heated in a sealed container with a 1.0 mol sample of F. Equilibrium was established at a given temperature and the equilibrium mixture formed contained 0.80 mol of G. Calculate the amount, in moles, of E and of F in this equilibrium mixture. Moles of E __________________________________________________________ Moles of F __________________________________________________________ (2) (b) Write an expression for the equilibrium constant Kc for this equilibrium. State the units of Kc Expression _________________________________________________________

___________________________________________________________________

Units ______________________________________________________________

(2) (c) A different mixture of E and F reached equilibrium at temperature T 1 in a container of volume 1.50 dm^3. This equilibrium mixture contained 2.50 mol of E , 1.20 mol of F and 0.85 mol of G. Calculate a value of Kc for the equilibrium at temperature T 1

(2) (d) The mixture in part (c) was allowed to reach equilibrium at temperature T 1 in a different container of volume 3.00 dm^3. State whether the amount of G in the equilibrium mixture will increase, decrease or stay the same. Explain your answer. Effect on the amount of G ______________________________________________ Explanation _________________________________________________________

(3) (e) The mixture in part (c) was allowed to reach equilibrium at temperature T 2 in the original container of volume 1.50 dm^3. The value of K c for the equilibrium was found to have increased. State and explain which of T 1 or T 2 is the higher temperature. Higher temperature ___________________________________________________

A and B react together in this reversible reaction. A + 3B ⇌ C + 2D A mixture of 10 mol of A and 10 mol of B were left to reach equilibrium. The equilibrium mixture contained 4 mol of B. What is the total amount, in moles, of substances in the equilibrium mixture? A 14 B 16 C 18 D 20 (Total 1 mark) Q7. Hydrogen is produced by the reaction of methane with steam. The reaction mixture reaches a state of dynamic equilibrium. CH 4 (g) + H 2 O(g) ⇌ CO(g) + 3H 2 (g) ∆ H = +206 kJ mol− Which of the following shows how the equilibrium yield of hydrogen and the value of the equilibrium constant are affected by the changes shown? Change Effect on equilibrium yield of H 2 (g) Effect on value of K c A Increase pressure decrease decrease B Add a catalyst increase no effect C Increase temperature increase increase D Remove CO(g) as formed increase increase (Total 1 mark) Q8. A pale brown mixture of NO 2 and N 2 O 4 is allowed to reach equilibrium in a sealed gas syringe according to the following equation. 2NO 2 (g) ⇌ N 2 O 4 (g) When the plunger is pushed further into the syringe the pressure increases and the mixture becomes paler in colour. When the syringe is placed in a hot oven the mixture becomes darker in colour. Which of the following statements is correct?

A NO 2 is brown and the forward reaction is exothermic. B NO 2 is brown and the forward reaction is endothermic. C NO 2 is colourless and the forward reaction is exothermic. D NO 2 is colourless and the forward reaction is endothermic. (Total 1 mark)