The rate and extent of chemical change
Exam-style questions with answers for 4.6 The rate and extent of chemical change in AQA GCSE Chemistry (spec 8462). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below.
All questions and answers
Question 1 · Spec 4.6.1.1 · Rate
State the equation for the mean rate of reaction. [1 mark]
Answer:Mean rate = quantity of reactant used or product formed ÷ time taken.Question 2 · Spec 4.6.1.1 · Rate
60 cm³ of gas is produced in 30 s. Calculate the mean rate. [2 marks]
Answer:60 ÷ 30 (1); = 2 cm³/s (1).Question 3 · Spec 4.6.1.1 · Rate
State two ways to measure the rate of a reaction that produces a gas. [2 marks]
Answer:Measure the volume of gas with a gas syringe or upturned measuring cylinder (1); measure the loss in mass on a balance (1).Question 4 · Spec 4.6.1.1 · Rate · Higher only
How can the rate at a particular time be found from a graph? [1 mark]
Answer:Draw a tangent to the curve at that time and calculate its gradient.Question 5 · Spec 4.6.1.2 · Factors
State five factors that affect the rate of reaction. [3 marks]
Answer:Concentration or pressure (1); surface area and temperature (1); catalysts (1).Question 6 · Spec 4.6.1.2 · Required practical 5
Describe how to investigate the effect of concentration on rate using sodium thiosulfate and hydrochloric acid. [3 marks]
Answer:Mix the solutions in a flask on top of a cross drawn on paper (1); time how long until the cross disappears due to the cloudy precipitate (1); repeat with different concentrations of thiosulfate, keeping volume and temperature the same (1).Question 7 · Spec 4.6.1.2 · Required practical 5
Why is timing the disappearing cross subjective? [1 mark]
Answer:Different people judge the moment the cross disappears differently.Question 8 · Spec 4.6.1.3 · Collision theory
State two conditions for a reaction to happen in collision theory. [2 marks]
Answer:Particles must collide (1); with enough energy (at least the activation energy) (1).Question 9 · Spec 4.6.1.3 · Concentration
Explain why increasing concentration increases rate. [2 marks]
Answer:There are more particles in the same volume (1); so collisions are more frequent (1).Question 10 · Spec 4.6.1.3 · Surface area
Explain why powders react faster than lumps. [2 marks]
Answer:Powders have a larger surface area to volume ratio (1); so there are more frequent collisions (1).Question 11 · Spec 4.6.1.3 · Temperature
Explain why increasing temperature increases rate. [2 marks]
Answer:Particles move faster, so collisions are more frequent (1); and more collisions have energy greater than the activation energy (1).Question 12 · Spec 4.6.1.3 · Pressure
Explain why increasing pressure increases the rate of a gas reaction. [1 mark]
Answer:The particles are closer together, so collisions are more frequent.Question 13 · Spec 4.6.1.4 · Catalysts
What is a catalyst? [2 marks]
Answer:A substance that increases the rate of reaction (1); without being used up (1).Question 14 · Spec 4.6.1.4 · Catalysts
How do catalysts work? [1 mark]
Answer:They provide a different reaction pathway with a lower activation energy.Question 15 · Spec 4.6.1.4 · Enzymes
What are enzymes? [1 mark]
Answer:Biological catalysts.Question 16 · Spec 4.6.1.4 · Catalysts
How can you show that a substance is a catalyst? [1 mark]
Answer:Compare the rate with and without it, and show its mass is unchanged at the end.Question 17 · Spec 4.6.2.1 · Reversible reactions
What is a reversible reaction? [1 mark]
Answer:A reaction where the products can react to produce the original reactants.Question 18 · Spec 4.6.2.1 · Reversible reactions
What does the symbol ⇌ mean? [1 mark]
Answer:The reaction is reversible.Question 19 · Spec 4.6.2.1 · Reversible reactions
Describe what happens when ammonium chloride is heated. [2 marks]
Answer:It decomposes into ammonia and hydrogen chloride (1); which recombine to form ammonium chloride when cooled (1).Question 20 · Spec 4.6.2.2 · Energy changes
If the forward reaction is endothermic, what is the reverse reaction? [1 mark]
Answer:Exothermic, transferring the same amount of energy.Question 21 · Spec 4.6.2.2 · Hydrated copper sulfate
Describe what happens when blue hydrated copper sulfate is heated. [2 marks]
Answer:It turns white as water is removed (anhydrous copper sulfate) (1); adding water turns it blue again and releases energy (1).Question 22 · Spec 4.6.2.3 · Equilibrium
When is equilibrium reached? [2 marks]
Answer:In a closed system (1); when the forward and reverse reactions happen at exactly the same rate (1).Question 23 · Spec 4.6.2.3 · Equilibrium
Why is equilibrium described as dynamic? [1 mark]
Answer:Both reactions are still happening, but the amounts of reactants and products do not change.Question 24 · Spec 4.6.2.4 · Le Chatelier · Higher only
State Le Chatelier's principle. [1 mark]
Answer:If a system at equilibrium is subjected to a change in conditions, the system responds to counteract the change.Question 25 · Spec 4.6.2.5 · Concentration · Higher only
What happens to an equilibrium if the concentration of a reactant is increased? [1 mark]
Answer:More products are formed until equilibrium is reached again (it shifts to the right).Question 26 · Spec 4.6.2.5 · Concentration · Higher only
What happens if a product is removed from an equilibrium mixture? [1 mark]
Answer:More reactants react to make more product (the position shifts to the right).Question 27 · Spec 4.6.2.6 · Temperature · Higher only
What happens to the yield of an exothermic forward reaction if temperature is increased? [2 marks]
Answer:The equilibrium shifts in the endothermic (reverse) direction (1); so the yield of products decreases (1).Question 28 · Spec 4.6.2.6 · Temperature · Higher only
What happens to an equilibrium if temperature is decreased? [1 mark]
Answer:The position shifts in the exothermic direction.Question 29 · Spec 4.6.2.7 · Pressure · Higher only
N₂ + 3H₂ ⇌ 2NH₃. What happens to the yield of ammonia if pressure increases? [2 marks]
Answer:The equilibrium shifts to the side with fewer gas molecules (1); the right side has 2 molecules compared with 4, so the yield of ammonia increases (1).Question 30 · Spec 4.6.2.7 · Pressure · Higher only
Why does changing pressure have no effect on H₂ + I₂ ⇌ 2HI? [1 mark]
Answer:There are the same number of gas molecules on each side.Question 31 · Spec 4.6.1.1 · Rate graphs
What does a steeper line on a graph of volume of gas against time show? [1 mark]
Answer:A faster rate of reaction.Question 32 · Spec 4.6.1.1 · Rate graphs
Why does the graph level off at the end? [1 mark]
Answer:One of the reactants has been used up, so the reaction has stopped.Question 33 · Spec 4.6.1.2 · Required practical 5
Name the control variables when investigating concentration and rate. [2 marks]
Answer:Any two from: temperature (1); volume of each solution (1); same person judging the end point (1); same cross and flask (1).Question 34 · Spec 4.6.1.3 · Temperature
Why does a 10 °C rise in temperature often double the rate? [1 mark]
Answer:Many more particles have energy above the activation energy, so many more collisions are successful.Question 35 · Spec 4.6.1.4 · Catalysts
Why are catalysts used in industry? [2 marks]
Answer:They speed up reactions, so products are made faster (1); and lower temperatures can be used, saving energy and money (1).Question 36 · Spec 4.6.1.2 · Required practical 5
Describe how to measure the rate of reaction of marble chips with acid by measuring gas volume. [2 marks]
Answer:Collect the carbon dioxide in a gas syringe (1); record the volume every 10 seconds and plot a graph (1).Question 37 · Spec 4.6.2.3 · Equilibrium
Why can equilibrium only be reached in a closed system? [1 mark]
Answer:Otherwise products or reactants could escape, so the reverse reaction could not keep up.Question 38 · Spec 4.6.2.6 · Temperature · Higher only
If the forward reaction is endothermic, how can the yield be increased? [1 mark]
Answer:Increase the temperature.Question 39 · Spec 4.6.2.4 · Catalysts · Higher only
Does a catalyst change the position of equilibrium? [1 mark]
Answer:No; it speeds up the forward and reverse reactions equally, so equilibrium is reached faster.Question 40 · Spec 4.6.1.1 · Rate
Calculate the mean rate if 0.6 g of magnesium is used up in 120 s. [2 marks]
Answer:0.6 ÷ 120 (1); = 0.005 g/s (1).Question 41 · Spec 4.6.1.3 · Collision theory
What is a successful collision? [1 mark]
Answer:A collision with at least the activation energy, which leads to a reaction.Question 42 · Spec 4.6.1.2 · Surface area
Why do farmers worry about grain dust explosions? [1 mark]
Answer:The fine dust has a huge surface area, so it can react extremely quickly with oxygen.Question 43 · Spec 4.6.1.4 · Catalysts
Why don't catalysts appear in the chemical equation? [1 mark]
Answer:They are not used up in the reaction.Question 44 · Spec 4.6.2.1 · Reversible reactions
What determines the direction of a reversible reaction? [1 mark]
Answer:The conditions, such as temperature and pressure.Question 45 · Spec 4.6.1.1 · Rate
What are the units of rate if gas volume is measured in cm³ and time in seconds? [1 mark]
Answer:cm³/s.Question 46 · Spec 4.6.2.5 · Concentration · Higher only
What happens to an equilibrium if the concentration of a product is increased? [1 mark]
Answer:More reactants form until equilibrium is restored (it shifts to the left).Question 47 · Spec 4.6.1.3 · Collision theory
Why does decreasing temperature slow a reaction? [1 mark]
Answer:Particles move more slowly, so collisions are less frequent and fewer have enough energy.Question 48 · Spec 4.6.2.3 · Equilibrium
What happens to the amounts of reactants and products at equilibrium? [1 mark]
Answer:They stay constant (but not necessarily equal).Question 49 · Spec 4.6.2.7 · Pressure · Higher only
What happens to an equilibrium if pressure is decreased? [1 mark]
Answer:It shifts to the side with more gas molecules.Question 50 · Spec 4.6.1.1 · Rate
What is the rate of reaction? [1 mark]
Answer:How quickly the reactants are used up or the products are formed.