G12 ASSESSMENT 22 MAY 2020

CHEMICAL EQUILIBRIUM   ASSESSMENT TOTAL 50 MARKS
Please note that any answers which appear to be cut and pasted will be assigned a zero. I am trusting you to be honest. You only have 1 hour to complete this assessment.

SECTION A - Short Questions (18 MARKS)

2

All of the following reactions are at equilibrium. The reaction which does not undergo an equilibrium shift when only the volume is changed is :

4

The following system is at equilibrium until a temperature disturbance is made.
When the temperature is increased describe what will happen to each of these when a new equilibrium has been reached (simply click/drag each item to the appropriate category). Note that negative marking will apply.

∆H value
Kc value
Amount of HF(g)
Amount of OF2(g)

Increases
Decreases
Remains unchanged

2

Write a Kc expression for the following system at equilibrium.

2

Consider the system at equilibrium:

Which of the following conditions will give the greatest yield of NO2? [Select all that apply but negative marking will apply]

1

Which of the following changes will affect the value of Kc?

2

Consider the Haber process:
Equilibrium is reached in a container of volume 2 dm3 at a temperature of 200 K.
At equilibrium, Kc = 75. The concentration of N2(g) is determined to be 0,023 mol.dm-3 and the concentration of H2(g) is 0,078 mol.dm-3 at equilibrium. What is the concentration of NH3(g) at equilibrium?

1

Consider the following equilibrium.
The volume of the system is increased.
Describe the effect on the following items at the new equilibrium, by comparing them to the original equilibrium before the disturbance was applied. Negative marking will apply.

Rate of forward reaction
Rate of reverse reaction
Kc value

Increases
Decreases
Remains the same

2

Consider the following system in equilibrium:
Which disturbance(s) would not favour the formation of orange dichromate ions? Negative marking applies.

2

The following hypothetical reaction X ⇌ Y has a Kc = 3. If the reaction started out with 12 X only, how many X and Y molecules will there be at this equilibrium?

SECTION B - Longer Questions (32 marks)

QUESTION 1

The following concentration-time graph illustrates an equilibrium system that experiences various disturbances.

The graph is drawn to scale, however the x-axis units reflect values of interest only.
Answer the questions that follow.

1

1.1 How many disturbances are represented in total on this graph?

2

1.2 Give one time interval during which the system is in equilibrium.

4

1.3 Using Le Chatelier's Principle, identify and explain the disturbance that was applied at 8 min.

1

1.4 Identify the disturbance at 20 min:

3

1.5 Provide a brief reason for your answer in Q1.4 above.

QUESTION 2

A mixture of 1,2 mol of NO(g) and 1,6 mol of H2(g) are placed in a 5 dm3 container at a certain temperature. The flask is sealed and the system is left to reach equilibrium, according to the folowing balanced chemical equation:
At equilibrium, the concentration of NO(g) is 0,18 mol.dm-3. 

2

2.1 Write a Kc expression for this equilibrium. 2NO(g) + 2H2(g) ⇌ N2(g) + H2O(g)

2

2.2 Calculate the number of moles of NO at equilibrium.
[Info from above: A mixture of 1,2 mol of NO(g) and 1,6 mol of H2(g) are placed in a 5 dm3 container at a certain temperature. At equilibrium, the concentration of NO(g) is 0,18 mol.dm-3]

7

2.3 Calculate the value of Kc for this system. [Info from above: 2NO(g) + 2H2(g) ⇌ N2(g) + H2O(g) A mixture of 1,2 mol of NO(g) and 1,6 mol of H2(g) are placed in a 5 dm3 container at a certain temperature. At equilibrium, the concentration of NO(g) is 0,18 mol.dm-3]

1

2.4 What does the value of Kc indicate about the yield of the products?

4

2.5 When the temperature is decreased, the value of Kc decreases.
Is this reaction exothermic or endothermic? Explain your reasoning.

5

2.6 Complete the following rate graph, showing the effect of a decrease in pressure in this system.
Remember to include a suitable key or line labels.
PLEASE USE THE LINE FEATURE NOT PEN FEATURE WHEN DRAWING STRAIGHT LINES (you will have to use the orange colour to continue the red line as it is the closest match)

END OF ASSESSMENT - WELL DONE!    [50 MARKS]

CHEMICAL EQUILIBRIUM ASSESSMENT TOTAL 50 MARKS

SECTION A - Short Questions (18 MARKS)

All of the following reactions are at equilibrium. The reaction which does not undergo an equilibrium shift when only the volume is changed is :

The following system is at equilibrium until a temperature disturbance is made.When the temperature is increased describe what will happen to each of these when a new equilibrium has been reached (simply click/drag each item to the appropriate category). Note that negative marking will apply.

Write a Kc expression for the following system at equilibrium.

Consider the system at equilibrium:Which of the following conditions will give the greatest yield of NO2? [Select all that apply but negative marking will apply]

Which of the following changes will affect the value of Kc?

Consider the following equilibrium.The volume of the system is increased. Describe the effect on the following items at the new equilibrium, by comparing them to the original equilibrium before the disturbance was applied. Negative marking will apply.

Consider the following system in equilibrium:Which disturbance(s) would not favour the formation of orange dichromate ions? Negative marking applies.

The following hypothetical reaction X ⇌ Y has a Kc = 3. If the reaction started out with 12 X only, how many X and Y molecules will there be at this equilibrium?

SECTION B - Longer Questions (32 marks)

QUESTION 1

1.1 How many disturbances are represented in total on this graph?

1.2 Give one time interval during which the system is in equilibrium.

1.3 Using Le Chatelier's Principle, identify and explain the disturbance that was applied at 8 min.

1.4 Identify the disturbance at 20 min:

1.5 Provide a brief reason for your answer in Q1.4 above.

QUESTION 2

2.1 Write a Kc expression for this equilibrium. 2NO(g) + 2H2(g) ⇌ N2(g) + H2O(g)

2.2 Calculate the number of moles of NO at equilibrium. [Info from above: A mixture of 1,2 mol of NO(g) and 1,6 mol of H2(g) are placed in a 5 dm3 container at a certain temperature. At equilibrium, the concentration of NO(g) is 0,18 mol.dm-3]

2.3 Calculate the value of Kc for this system. [Info from above: 2NO(g) + 2H2(g) ⇌ N2(g) + H2O(g) A mixture of 1,2 mol of NO(g) and 1,6 mol of H2(g) are placed in a 5 dm3 container at a certain temperature. At equilibrium, the concentration of NO(g) is 0,18 mol.dm-3]

2.4 What does the value of Kc indicate about the yield of the products?

2.5 When the temperature is decreased, the value of Kc decreases. Is this reaction exothermic or endothermic? Explain your reasoning.

The following system is at equilibrium until a temperature disturbance is made.
When the temperature is increased describe what will happen to each of these when a new equilibrium has been reached (simply click/drag each item to the appropriate category). Note that negative marking will apply.

Consider the system at equilibrium:

Which of the following conditions will give the greatest yield of NO2? [Select all that apply but negative marking will apply]

Consider the following equilibrium.
The volume of the system is increased.
Describe the effect on the following items at the new equilibrium, by comparing them to the original equilibrium before the disturbance was applied. Negative marking will apply.

Consider the following system in equilibrium:
Which disturbance(s) would not favour the formation of orange dichromate ions? Negative marking applies.

2.2 Calculate the number of moles of NO at equilibrium.
[Info from above: A mixture of 1,2 mol of NO(g) and 1,6 mol of H2(g) are placed in a 5 dm3 container at a certain temperature. At equilibrium, the concentration of NO(g) is 0,18 mol.dm-3]

2.5 When the temperature is decreased, the value of Kc decreases.
Is this reaction exothermic or endothermic? Explain your reasoning.