OCR GCSE Combined Science
Topic questions The rate and extent of chemical change Dynamic equilibrium
8 questions · 30 marks · about 35 minutes
The equation shows a reaction that has reached dynamic equilibrium.
Which statement explains why this reaction is a dynamic equilibrium?
A N2 and H2 are gases.
B The rates of the forward and reverse reactions are equal.
C The reaction has stopped.
D The reaction produces a 100% yield of NH3.
The equation shows a reaction that has reached dynamic equilibrium.
Which statement explains why this reaction is a dynamic equilibrium?
A N2 and H2 are gases.
B The rates of the forward and reverse reactions are equal.
C The reaction has stopped.
D The reaction produces a 100% yield of NH3.
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.
Sulfur dioxide, SO2, reacts with oxygen, O2, to make sulfur trioxide, SO3, to form a dynamic equilibrium.
Which statement describes the reaction at equilibrium?
A Rate of forward reaction < rate of backward reaction
B Rate of forward reaction > rate of backward reaction
C Rate of forward reaction = rate of backward reaction
D Rate of forward reaction ~ rate of backward reaction
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.
The reaction between sulfur dioxide and oxygen is reversible.
In a closed system the reaction between sulfur dioxide and oxygen reaches a dynamic equilibrium.
Use a symbol from the list to complete the sentence.
At equilibrium, the rate of the forward reaction is …….... the rate of the reverse reaction.
What is meant by the term reversible reaction ?
Name one source of sulfur dioxide in the atmosphere.
Describe two problems caused by the release of sulfur dioxide into the atmosphere.
The table shows some relative atomic masses.
| Element | Relative atomic mass |
| Sulfur | 32.1 |
| Oxygen | 16.0 |
Calculate the percentage, by mass, of oxygen in SO2.
Give your answer to 1 decimal place.
The amount of sulfur dioxide released in the UK is decreasing.
This graph shows how it has decreased since 1970.
How much did the sulfur dioxide decrease between 1975 and 2015?
Sulfur dioxide decrease =........ kilotonnes
Look at the equation for the reaction between sulfur dioxide and oxygen to make sulfur trioxide.
The reaction forms a dynamic equilibrium.
Which of the following describes dynamic equilibrium?
A All the reactants and products are gases.
B The rate of the backward reaction is greater than the rate of the forward reaction.
C The rate of the forward and backward reactions are equal.
D The rate of the forward reaction is greater than the rate of the backward reaction.
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