OCR GCSE Combined Science
Topic questions The rate and extent of chemical change The effect of changing conditions on equilibrium
7 questions · 45 marks · about 55 minutes
14* Hydrogen is an important chemical used in industry.
Hydrogen is produced when methane and steam are left to reach a dynamic equilibrium.
The forward reaction is endothermic.
Describe and explain how changing one reaction condition can produce as much hydrogen as possible.
State and use Le Chatelier’s principle.
Methanol, CH3OH, can be made by reacting carbon monoxide, CO, with hydrogen, H2.
The forward reaction is exothermic.
Which change in temperature and pressure will move the position of equilibrium to the right ?
| Change in temperature | Change in pressure | |
| A | decreases | decreases |
| B | decreases | increases |
| C | increases | decreases |
| D | increases | increases |
When a solution of yellow Fe3+(aq) ions is mixed with a solution of colourless SCN–(aq) ions they 16 react to form a solution of red FeSCN2+(aq) ions.
This is an example of a dynamic equilibrium.
Complete the equation for this reaction.
Plot the results from the table on the graph. Two points have already been plotted for
you.
Draw a line of best fit.
What is the time it takes for the reaction to reach equilibrium?
Give a reason for your answer.
Time to reach equilibrium =........ seconds
A student increases the concentration of SCN – (aq) ions in the mixture. (c)
The mixture turns darker red.
Use Le Chatelier’s principle to explain the change in colour.
The student thinks that the forward reaction shown in (a) for the equilibrium is exothermic
Describe what the student should do to the equilibrium mixture to show that the forward reaction is exothermic.
Predict the student’s observations.
Ethanol, C2H5OH, can be made by reacting ethene, C2H4, with steam, H2O.
The forward reaction is exothermic.
State Le Chatelier’s principle.
As the temperature of the reaction is increased does the amount of ethanol decrease, increase or stay the same?
Explain your answer to (b)(i).
As the pressure of the reaction is increased does the amount of ethanol decrease, increase or stay the same?
Explain your answer to (c)(i).
When a reversible reaction is left in a closed system, an equilibrium is reached.
Look at the equation.
It shows the equilibrium between NO2(g) and N2O4(g).
Fig. 17.1 shows how the reaction rate of the forward and backward reactions change as the equilibrium is reached.
Describe how Fig. 17.1 shows that the equilibrium has been reached.
Fig. 17.2 shows how the concentrations of NO2(g) and N2O4(g) change as the equilibrium is reached.
Describe how Fig. 17.2 shows that the equilibrium has been reached.
Each of the equations, A–E, shows a reversible reaction.
∆H shows the energy change of the forward reaction.
• If ∆ H is negative the forward reaction is exothermic.
• If ∆ H is positive the forward reaction is endothermic.
Write the letter, A – E, of one equation where more product is formed when the temperature is decreased.
Write the letter, A – E, of one equation where the amount of product is unchanged when the pressure is increased.
Write the letter, A – E, of one equation where more product is formed when either the temperature is increased or when the pressure is decreased.
16* Nitrogen and hydrogen react to form ammonia in a dynamic equilibrium.
The table shows the amount of ammonia formed at different temperatures and pressures.
| Pressure (MPa) | Amount of ammonia formed at different temperatures and pressure (%) | ||||
| 100 °C | 200 °C | 300 °C | 400 °C | 500 °C | |
| 2.5 | 92 | 64 | 27 | 9 | 3 |
| 5.0 | 95 | 74 | 40 | 15 | 6 |
| 10.0 | 97 | 82 | 53 | 25 | 11 |
| 20.0 | 98 | 89 | 67 | 39 | 18 |
| 40.0 | 99 | 95 | 80 | 55 | 32 |
Describe and explain how the percentage of ammonia formed changes with temperature and pressure.
Use the information provided in the table and your knowledge of reactions in dynamic equilibrium.
Look at the equation for the equilibrium reaction between sulfur dioxide, SO2, oxygen and sulfur trioxide, SO3, in a closed system.
Predict the effect of adding more oxygen to the equilibrium mixture.
Explain your answer.
Predict the effect of increasing the total pressure in the equilibrium mixture.
Explain your answer.
Predict the effect of increasing the temperature of the equilibrium mixture.
Explain your answer.
Sulfur dioxide for this reaction is made by burning sulfur.
Calculate the mass of sulfur needed to make 48 tonnes of sulfur dioxide.
Give your answer to 2 significant figures.
The relative atomic mass, Ar, of S is 32.1 and of O is 16.0.
Answer =........ tonnes
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