AQA GCSE Combined Science: Trilogy · 5.6

The rate and extent of chemical change

Revision questions with answers for 5.6 The rate and extent of chemical change in AQA GCSE Combined Science: Trilogy (spec 8464). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below.

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Question 1 · Spec 5.6.1.1
What does reaction rate describe?
Answer:
How quickly reactants are used up or products are formed.
Question 2 · Spec 5.6.1.1
State two forms of the mean-rate equation.
Answer:
Mean rate = quantity of reactant used / time, or quantity of product formed / time.
Question 3 · Spec 5.6.1.1
A reaction loses 2.4 g in 60 s. Find its mean mass-loss rate.
Answer:
Rate = 2.4 / 60 = 0.040 g/s.
Question 4 · Spec 5.6.1.1
A reaction produces 48 cm³ gas in 30 s. Find the mean rate.
Answer:
Rate = 48 / 30 = 1.6 cm³/s.
Question 5 · Spec 5.6.1.1
When can mass loss be used to measure rate?
Answer:
When a gas product escapes from the reaction vessel, decreasing its measured mass.
Question 6 · Spec 5.6.1.1
How can a gas syringe measure reaction rate?
Answer:
Record collected gas volume at fixed times, then calculate the volume produced per unit time.
Question 7 · Spec 5.6.1.1
Why should gas collection begin as soon as the reactants mix?
Answer:
Otherwise early gas escapes or goes unrecorded, underestimating the initial rate and total collected volume.
Question 8 · Spec 5.6.1.1
What does a steeper product-quantity graph mean?
Answer:
A faster reaction rate, because more product forms per unit time.
Question 9 · Spec 5.6.1.1
What does a plateau on a gas-volume graph usually mean?
Answer:
No more gas is forming because the reaction has stopped, commonly because a reactant has been used up.
Question 10 · Spec 5.6.1.1
How does a reactant-quantity graph differ from a product-quantity graph?
Answer:
Reactant quantity decreases, whereas product quantity increases; rate is represented by the magnitude of the gradient.
Question 11 · Spec 5.6.1.1 · Higher only
A tangent rises 12 cm³ over 8 s. Calculate the instantaneous rate.
Answer:
Gradient = 12 / 8 = 1.5 cm³/s at the point where the tangent touches the curve.
Question 12 · Spec 5.6.1.2 · Required practical
Outline a concentration-rate investigation using gas production.
Answer:
React equal amounts of magnesium with equal volumes of different acid concentrations; record gas volume against time while controlling temperature.
Question 13 · Spec 5.6.1.2 · Required practical
How can a disappearing cross measure the effect of concentration?
Answer:
Time how long a reacting solution takes to become cloudy enough to hide a cross, using the same endpoint each time.
Question 14 · Spec 5.6.1.2 · Required practical
Why can 1/time compare rates in a disappearing-cross experiment?
Answer:
The same visible endpoint represents approximately the same extent of reaction, so a shorter time indicates a faster rate.
Question 15 · Spec 5.6.1.2 · Required practical
What should be controlled when changing concentration in a turbidity investigation?
Answer:
Total solution volume and depth, temperature, other reactant amount, viewing method and endpoint judgement.
Question 16 · Spec 5.6.1.2 · Required practical
Why is a disappearing-cross endpoint less objective than an electronic light reading?
Answer:
Different observers may decide the cross disappears at different cloudiness levels.
Question 17 · Spec 5.6.1.2 · Required practical
What precaution matters when a thiosulfate-acid reaction produces sulfur dioxide?
Answer:
Use small quantities in a well-ventilated area and avoid inhaling the gas; use eye protection.
Question 18 · Spec 5.6.1.2 · Required practical
Why repeat each concentration and calculate a mean?
Answer:
Repeats reveal variation and reduce the influence of random error on the estimated rate.
Question 19 · Spec 5.6.1.2
Name four factors that can increase reaction rate.
Answer:
Higher temperature, higher reactant concentration or gas pressure, greater solid surface area and a suitable catalyst.
Question 20 · Spec 5.6.1.3
Why does greater solution concentration increase reaction rate?
Answer:
More reacting particles occupy each unit volume, causing more frequent collisions.
Question 21 · Spec 5.6.1.3
Why does increasing gas pressure usually increase reaction rate?
Answer:
Gas particles are closer together, so collision frequency increases.
Question 22 · Spec 5.6.1.3
Why does powder often react faster than a large solid lump?
Answer:
The same mass has a larger exposed surface area, providing more sites for collisions.
Question 23 · Spec 5.6.1.3
Why does increasing temperature increase rate?
Answer:
Particles move faster and collide more often; a greater proportion of collisions have energy above the activation energy.
Question 24 · Spec 5.6.1.3
What makes a collision successful?
Answer:
Reacting particles collide with sufficient energy to overcome the activation-energy barrier.
Question 25 · Spec 5.6.1.4
How does a catalyst increase reaction rate?
Answer:
It provides an alternative reaction pathway with lower activation energy.
Question 26 · Spec 5.6.1.4
Is a catalyst used up in the overall reaction?
Answer:
No. It participates in the process but is regenerated and is not consumed overall.
Question 27 · Spec 5.6.1.4
What are biological catalysts called?
Answer:
Enzymes; they catalyse reactions in living organisms.
Question 28 · Spec 5.6.1.4
How does a catalyst change a reaction profile?
Answer:
It lowers the activation-energy peak while keeping reactant and product energy levels unchanged.
Question 29 · Spec 5.6.1.4
Does adding a catalyst necessarily increase the final product amount?
Answer:
No. It increases how quickly the reaction proceeds; it does not supply extra reactant.
Question 30 · Spec 5.6.1.2
Equal masses of marble powder and chips react with excess acid. Compare gas graphs.
Answer:
Powder gives a steeper initial curve and finishes sooner; both give the same final gas volume if the marble amounts are equivalent.
Question 31 · Spec 5.6.2.1
What is a reversible reaction?
Answer:
A reaction in which products can react to form the original reactants.
Question 32 · Spec 5.6.2.1
What does the symbol ⇌ mean?
Answer:
The reaction can proceed in both forward and reverse directions.
Question 33 · Spec 5.6.2.1
What happens when blue hydrated copper sulfate is heated, then water is added after cooling?
Answer:
Heating removes water, leaving white anhydrous copper sulfate. Adding water restores the blue hydrated form.
Question 34 · Spec 5.6.2.2
If a forward reaction is exothermic, what is the reverse reaction?
Answer:
Endothermic, with the same magnitude of energy change in the opposite direction.
Question 35 · Spec 5.6.2.3
What is dynamic equilibrium?
Answer:
A state in a closed system where forward and reverse reactions continue at equal rates, with constant amounts of reactants and products.
Question 36 · Spec 5.6.2.3
Why must a system be closed to establish this equilibrium?
Answer:
Reactants and products must remain in the system rather than escaping.
Question 37 · Spec 5.6.2.3
At equilibrium, must reactant and product amounts be equal?
Answer:
No. Their amounts are constant, but not necessarily equal; the forward and reverse rates are equal.
Question 38 · Spec 5.6.2.3
Why is calling equilibrium a stopped reaction incorrect?
Answer:
Both reactions continue, but their equal rates mean there is no overall change in composition.
Question 39 · Spec 5.6.2.4 · Higher only
State Le Chatelier's principle.
Answer:
When conditions change at equilibrium, the system shifts in the direction that counteracts the change.
Question 40 · Spec 5.6.2.5 · Higher only
What happens if reactant concentration is increased at equilibrium?
Answer:
The system shifts to use more of that reactant, forming more products until a new equilibrium is reached.
Question 41 · Spec 5.6.2.5 · Higher only
What happens if a product is removed at equilibrium?
Answer:
The forward reaction is favoured, replacing some of the removed product.
Question 42 · Spec 5.6.2.5 · Higher only
For A ⇌ B, predict the effect of adding B.
Answer:
The equilibrium shifts towards A, consuming some of the added B.
Question 43 · Spec 5.6.2.6 · Higher only
How does increasing temperature affect an exothermic forward reaction's equilibrium?
Answer:
It favours the endothermic reverse reaction, reducing the equilibrium proportion of products.
Question 44 · Spec 5.6.2.6 · Higher only
How does increasing temperature affect an endothermic forward reaction's equilibrium?
Answer:
It favours the forward reaction, increasing the equilibrium proportion of products.
Question 45 · Spec 5.6.2.6 · Higher only
How does cooling affect the equilibrium of an exothermic forward reaction?
Answer:
It favours the exothermic direction, increasing the equilibrium proportion of products.
Question 46 · Spec 5.6.2.7 · Higher only
What is the pressure rule for a gaseous equilibrium?
Answer:
Higher pressure favours the side with fewer gas molecules; lower pressure favours the side with more gas molecules.
Question 47 · Spec 5.6.2.7 · Higher only
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), predict the effect of higher pressure.
Answer:
It favours ammonia: the product side has two gas molecules compared with four on the reactant side.
Question 48 · Spec 5.6.2.7 · Higher only
For H₂(g) + I₂(g) ⇌ 2HI(g), predict the effect of higher pressure.
Answer:
There is no equilibrium shift because both sides have equal numbers of gas molecules.
Question 49 · Spec 5.6.2.4 · Higher only
What does a catalyst do to an equilibrium mixture?
Answer:
It speeds up both directions, reaching equilibrium sooner without changing the equilibrium composition.
Question 50 · Spec 5.6.2.6 · Higher only
Why can higher temperature improve rate but reduce equilibrium product proportion?
Answer:
A hotter reaction is faster, but an exothermic forward reaction's equilibrium shifts towards reactants.

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