| Four options are provided as possible answers to the following questions. Each question has only ONE correct answer. Choose the answer and make a cross (X) in the block (A – D) next to the question number (4.1 – 4.5) on the attached ANSWER SHEET. |
1.1 Crystals of potassium dichromate, K2CrO4, are dissolved in 10 cm3 of water. The solution is yellow in colour. With the careful addition of nitric acid the solution turns to orange. The following equilibrium reaction occurs:
| 2CrO42– (aq) | + | 2H+ (aq) | Cr2O72– (aq) | + | H2O (l) | DH > 0 | ||
| yellow | orange |
Which set of factors below would favour the formation of orange dichromate ions?
A Adding a few drops of NaOH solution
B Stirring the mixture
C Decreasing the temperature
D Increasing the temperature
Question 3
The reaction of iron with steam is represented by the equation:
| 3 Fe (s) | + | 4 H2O (g) | Fe3O4 (s) | + | 4 H2 (g) |
If exactly 5 moles of iron and 10 moles of steam are heated in a closed 1 dm3 container, the reaction reaches equilibrium after 4 minutes. After 8 minutes the temperature is decreased.
The graph shows the change in the reaction rate over an interval of 12 minutes.
3.1 Write down the balanced equation for the reaction represented by the broken line. (2)
3.2 Is the reaction represented by the continuous line exothermic or endothermic? (2)
3.3 Give a reason for your answer in 3.2 by referring to the graph. (2)
3.4 What is represented by the horizontal section of the graph? (2)
3.5 How would the addition of a suitable catalyst affect the graph after the twelfth minute? (2)
3.6 If the reaction mixture contains exactly 1,5 moles of Fe3O4 (s) at equilibrium after the increase
in temperature, calculate the value of he equilibrium constant (KC) at this temperature. (7)
3.7 What effect did the decrease in temperature at 8 minutes have on:
(a) the value of KC? (2)
(b) the equilibrium concentration of H2O (g)? (2)
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QUESTION 4
The following reaction:
| C2H4 (g) | + | H2 (g) | C2H6 (g) | DH < 0 |
is in equilibrium in a closed system at a temperature T.
4.1 During which industrial process is this reaction significant? (1)
4.2 What does it mean when we say that a system is “in equilibrium”? (2)
4.3 State Le Chatelier’s principle in words. (3)
4.4 Would increasing or decreasing the temperature T result in higher yield of C2H6 (g)? Give a reason for your answer. (3)
4.5 0,6 moles of C2H4 (g) and 0,5 moles of H2 (g) react in a 500 cm3 closed container. When equilibrium is established at temperature T, it is found that 0,4 moles of C2H6 (g) are present in the container. Determine the equilibrium constant at temperature T. (6)
4.6 Pressure in the container is increased by adding an inert gas at constant volume and temperature. What would be the effect on …
(a) the amount of ethane at equilibrium? (2)
(b) the magnitude of the equilibrium constant? (2)
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Physical sciences break 1.0 