5.6.1.2Factors which affect the rates of chemical reactions
AQA GCSE Combined Science (8464), Higher tier · Chemistry › The rate and extent of chemical change › Rate of reaction
Practise Factors which affect the rates of chemical reactions. 19 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The five factors that change the rate of a reaction: concentration, pressure of gases, surface area, temperature and catalysts. You must recall their effects, compare rate graphs for different conditions, and describe the rates required practical, which investigates how concentration affects the rate.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name the five factors that affect rateConcentration of solutions, pressure of gases, surface area of solids, temperature and the presence of a catalyst.
4
State how each factor changes the rateIncreasing the concentration, pressure, surface area or temperature, or adding a catalyst, increases the rate.
5
Describe the gas-volume method for ratesMeasure the volume of gas given off at regular time intervals with a gas syringe or an upturned measuring cylinder.
5
Describe the disappearing-cross (turbidity) methodTime how long it takes for a cross under the flask to disappear as a cloudy precipitate forms.
6
Sketch rate curves for changed conditionsA faster reaction has a steeper start and levels off sooner, at the same height if the amounts of reactants are the same.
6
Identify and control the variablesIndependent: concentration; dependent: time taken or volume of gas; control: temperature, volumes, and mass and size of solid.
8
Plan a rates investigation from a hypothesisGive a testable hypothesis, a step-by-step method with a range of values and repeats, and say how the results test it.
Notes
The five factors
Increasing the concentration of a reactant in solution increases the rate.
Increasing the pressure of reacting gases increases the rate.
Increasing the surface area of a solid reactant (smaller pieces, or a powder) increases the rate.
Increasing the temperature increases the rate.
Adding a catalyst increases the rate, and the catalyst is not used up.
Changing the temperature or surface area, or adding a catalyst, changes how fast the product forms, not how much forms in the end. The total amount of product only changes if the amount of the reactant that runs out changes.
Comparing rate graphs
A faster reaction gives a steeper curve at the start, and it levels off sooner.
With the same amounts of reactants, both curves level off at the same final volume or mass.
If you halve the amount of the reactant that runs out, the curve levels off at half the height.
Example: the same mass of marble as a powder instead of chips gives a steeper curve that reaches the same final volume of carbon dioxide sooner.
Required practical: effect of concentration
Gas-volume method: react magnesium ribbon with hydrochloric acid of different concentrations, and measure the volume of hydrogen in a gas syringe at regular intervals (e.g. every 10 s).
Turbidity method: mix sodium thiosulfate solution with hydrochloric acid in a flask standing on a paper cross. A pale yellow precipitate of sulfur makes the mixture cloudy. Time how long until you can no longer see the cross.
Make different concentrations by mixing the sodium thiosulfate solution with different volumes of water, keeping the total volume the same.
A shorter time means a faster rate. You can use 1 ÷ time as a measure of the rate.
Control variables: temperature, volumes of solutions, concentration of the other reactant, mass and length of magnesium, and the same cross viewed by the same person.
The thiosulfate reaction gives off sulfur dioxide, so work in a well-ventilated room and wear eye protection.
Smaller pieces or powder = larger surface area = faster reaction
Faster reaction: steeper start, levels off sooner
The final amount of product depends on the amount of the reactant that runs out, not on the rate
Disappearing cross: sodium thiosulfate + hydrochloric acid gives a cloudy precipitate of sulfur
Rate can be measured as 1 ÷ time taken (shorter time = faster)
Gas collection: gas syringe, or upturned measuring cylinder filled with water
How to answer each type of question
Recall: give ways to increase the rate
1 to 2 marks3
Pick changes that apply to the reaction given: surface area only for solids, pressure only for gases, concentration only for solutions.
Use comparatives: 'higher temperature', 'more concentrated acid', 'smaller pieces'.
Example. Calcium carbonate lumps react with dilute hydrochloric acid. Give two changes that would increase the rate of this reaction.
Show the model answer
Any two from: use powdered calcium carbonate or smaller lumps (1); use more concentrated acid (1); increase the temperature (1). Maximum 2 marks.
Describe or sketch the curve for changed conditions
2 marks6
Decide whether the change makes the reaction faster or slower: faster means a steeper start.
Decide whether the final amount changes: only if the amount of the reactant that runs out changes.
Draw the new curve from the origin, levelling off sooner (faster) or later (slower) at the correct height.
Example. A student reacted 0.5 g of large marble chips with excess hydrochloric acid at 20 °C and plotted the volume of carbon dioxide against time. The curve levelled off at 120 cm3. Describe how the curve would be different if the student used 0.5 g of powdered marble instead.
Show the model answer
The curve would be steeper at the start / the initial rate is higher (1). It would level off sooner, but at the same final volume of 120 cm3 (1).
6-mark: describe a method for the rates required practical
4 to 6 marks6
Name the apparatus and exactly what you measure, with what, and how often.
Say what you change (the independent variable) and give a range, e.g. five concentrations.
List the control variables and how you keep them the same.
Say how you will process the results: repeat, find a mean, calculate the rate or 1 ÷ time.
Example. Sodium thiosulfate solution reacts with dilute hydrochloric acid to form a precipitate of sulfur, which makes the mixture cloudy. Describe a method to investigate how the concentration of sodium thiosulfate solution affects the rate of this reaction.
Show the model answer
Marked in levels: a clear, logical and complete method reaches the top level. Creditworthy points: Use a measuring cylinder to put 10 cm3 of sodium thiosulfate solution into a conical flask (1). Stand the flask on a piece of paper with a black cross drawn on it (1). Add 10 cm3 of hydrochloric acid, swirl, and start a stopwatch at the same time (1). Look down through the flask and stop the stopwatch when the cross can no longer be seen (1). Repeat with at least four other concentrations, made by diluting the sodium thiosulfate solution with water to the same total volume, keeping the volume and concentration of acid and the temperature the same (1). Repeat each concentration, calculate a mean time and use 1 ÷ time as the rate (1).
Suggest an improvement to the method
2 marks6
Find the weakness: often a judgement by eye, gas escaping, or a variable not controlled.
Suggest a specific fix and say why it helps.
Example. Two students did the disappearing-cross experiment with the same solutions but recorded different times. Suggest why, and suggest how the method could be improved.
Show the model answer
Different people judge the moment the cross disappears differently / the end point is a matter of judgement (1). Use a light sensor and data logger to measure when the mixture reaches the same cloudiness, or have the same person judge every run using the same cross (1).
Race picture: the faster reaction reaches the top of the graph first, but if the same amount of reactant runs out it stops at the same finish line.
Disappearing cross: long time = slow reaction; short time = fast reaction.
Break it up to speed it up: grinding a solid into a powder increases its surface area.
Where marks are lost
Saying a catalyst or a higher temperature makes more product. It makes the same amount of product, faster.
Saying bigger pieces have a bigger surface area. For the same mass, smaller pieces have the larger total surface area.
Drawing the curve for a faster reaction levelling off higher than the original.
Writing 'more acid' when you mean 'more concentrated acid'.
In the cross method, forgetting that a longer time means a slower rate.
Exam technique
When comparing conditions, use comparatives: 'the higher the temperature, the faster the rate', not just 'temperature affects the rate'.
In method questions, name the apparatus (measuring cylinder, gas syringe, stopwatch), give volumes, and say what is kept the same.
6-mark method questions are marked by levels: write the steps in a logical order that someone else could follow.
If asked for a hypothesis, make it testable, e.g. 'The higher the concentration of acid, the faster the rate of reaction.'
Required practical:Rates of reaction (method, variables and exam tips)
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Two students investigated how the concentration of sodium thiosulfate solution affects the time for a black cross to disappear, viewed through the solution from above.
Student A repeated the experiment three times with 40 g/dm3 solution and got times of 21 s, 22 s and 21 s.
Student B, using a different batch of the same concentration of sodium thiosulfate solution and a different stopwatch, got a time of 23 s.
What is meant by a result being reproducible?
A different person, or different equipment, gets very similar results.
Complete the word equation for the reaction. calcium carbonate + hydrochloric acid → calcium chloride + water + ______________
carbon dioxide
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Marble chips react with dilute hydrochloric acid.
(a) Which two changes would increase the rate of the reaction? Tick (✓) two boxes.[2]
Use larger marble chips
Use powdered marble
Use more dilute acid
Increase the temperature
Use a bigger flask
(b) Give one other factor that affects the rate of a chemical reaction.[1]
(c) A student says: 'Using powdered marble instead of the same mass of marble chips will produce more carbon dioxide.' Is the student correct? Explain your answer.[2]
Show the answer and mark scheme
(a)Answer: Use powdered marble; Increase the temperature
(b)Answer: Concentration of a reactant in solution (or a catalyst, or pressure of gases).
concentration (of a reactant in solution) / adding a catalyst / pressure (of reacting gases)
(c)Answer: No: the same masses of marble and acid react, so the same volume of gas forms; the powder only makes the reaction faster.
no
the same mass of marble reacts (with the same acid) so the same volume of gas is produced; the powder only increases the rate
Question 2Medium4 marks
Two students investigated how the concentration of sodium thiosulfate solution affects the time for a black cross to disappear, viewed through the solution from above.
Student A repeated the experiment three times with 40 g/dm3 solution and got times of 21 s, 22 s and 21 s.
Student B, using a different batch of the same concentration of sodium thiosulfate solution and a different stopwatch, got a time of 23 s.
(a) What is meant by a result being repeatable?[1]
(b) What is meant by a result being reproducible?[1]
(c) Use the data to explain why student A's results are repeatable and the investigation is reproducible.[2]
Show the answer and mark scheme
(a)Answer: The same experimenter repeats the method with the same equipment and gets very similar results.
the same experimenter repeats the investigation using the same method and equipment and gets very similar results
(b)Answer: A different person, or different equipment, gets very similar results.
a different person, or different equipment, obtains very similar results
(c)Answer: Student A's three repeats (21, 22, 21 s) are all close together, showing repeatability; student B's result (23 s), with different equipment, is close to student A's results, showing reproducibility.
student A's repeated results are all close together (within 1 s of each other), showing repeatability
student B's result (23 s), obtained with different equipment, is close to student A's results, showing reproducibility
Question 3Hard6 marks
A factory manager wants to increase the rate of a reaction between two gases in an industrial reactor. The concentrations of the two gases cannot be changed.
Explain, using collision theory, how the manager could increase the rate of reaction. Suggest two different changes, and explain how each one increases the rate.[6]
Show the answer and mark scheme
Answer: See indicative content.
increase the temperature: particles move faster and collide more frequently
at a higher temperature, more particles have energy greater than or equal to the activation energy, so a greater proportion of collisions are successful
add a catalyst: this provides a different reaction pathway with a lower activation energy
so a greater proportion of collisions have enough energy to be successful, without needing to raise the temperature; the catalyst is not used up
if a solid catalyst is already used, increase its surface area (e.g. use pellets or a fine mesh rather than a lump), so more reacting particles collide with its surface per second
each change increases the frequency of successful collisions, increasing the rate
Marked with levels of response: the full level descriptors are in the app.