Pearson Edexcel GCSE Combined Science · C7

Rates of reaction and energy changes

Revision questions with answers for C7 Rates of reaction and energy changes in Pearson Edexcel GCSE Combined Science (spec 1SC0). 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 C7.2
State the mean reaction-rate equation.
Answer:
Quantity of reactant used or product formed divided by elapsed time.
Question 2 · Spec C7.2
A reaction produces 45 cm³ gas in 30 s. Find mean rate.
Answer:
45/30 = 1.5 cm³/s.
Question 3 · Spec C7.2
When is measuring mass loss a suitable rate method?
Answer:
When a gas escapes from the reacting mixture, decreasing its measured mass.
Question 4 · Spec C7.1 · Core practical
What gas forms from marble chips and hydrochloric acid?
Answer:
Carbon dioxide, collected to track reaction progress.
Question 5 · Spec C7.1 · Core practical
Outline the marble-chip gas-rate practical.
Answer:
Mix measured acid and marble quantities; record gas volume against time, changing one chosen condition.
Question 6 · Spec C7.1 · Core practical
How can acid concentration's effect be tested fairly?
Answer:
Use different concentrations with the same acid volume, marble mass and size, temperature and measuring method.
Question 7 · Spec C7.1 · Core practical
Why must gas-collection apparatus be airtight?
Answer:
Leaks cause gas volume and calculated rate to be underestimated.
Question 8 · Spec C7.1 · Core practical
What change makes the sodium thiosulfate-acid reaction observable?
Answer:
Sulfur forms, clouding the solution and obscuring a mark beneath the flask.
Question 9 · Spec C7.1 · Core practical
Why can 1/time compare rates in the disappearing-cross method?
Answer:
The same endpoint represents a comparable amount of cloudiness; a shorter time means a faster rate.
Question 10 · Spec C7.1 · Core practical
Why keep solution depth constant in a disappearing-cross test?
Answer:
Different depths can obscure the cross at different amounts of sulfur.
Question 11 · Spec C7.1 · Core practical
What is a limitation of judging when a cross disappears?
Answer:
The endpoint is subjective and may differ between observers.
Question 12 · Spec C7.1 · Core practical
Why use ventilation and small quantities in thiosulfate-acid experiments?
Answer:
The reaction can release sulfur dioxide, which should not be inhaled.
Question 13 · Spec C7.3
What must happen for reacting particles to react?
Answer:
They must collide with enough energy to overcome the activation-energy barrier.
Question 14 · Spec C7.4
How does greater concentration increase rate?
Answer:
More particles per unit volume cause more frequent collisions.
Question 15 · Spec C7.4
How does higher gas pressure increase rate?
Answer:
Particles become closer together, increasing collision frequency.
Question 16 · Spec C7.4
Why does powder react faster than equal-mass lumps?
Answer:
It has a larger surface area:volume ratio, exposing more particles for collisions.
Question 17 · Spec C7.4
Why does higher temperature increase reaction rate?
Answer:
Collisions are more frequent and a greater proportion have sufficient energy to react.
Question 18 · Spec C7.5
What does a steeper product-volume graph indicate?
Answer:
A greater reaction rate.
Question 19 · Spec C7.5
What does a plateau on a gas-volume graph normally indicate?
Answer:
No more gas is forming because the reaction has stopped, commonly when a reactant is used up.
Question 20 · Spec C7.5
A tangent rises 18 cm³ in 6 s. Calculate its rate.
Answer:
Gradient = 18/6 = 3.0 cm³/s at the tangent's contact point.
Question 21 · Spec C7.5
How does a reactant-mass graph differ from a product-volume graph?
Answer:
Reactant mass decreases while product volume increases; rate depends on the magnitude of the slope.
Question 22 · Spec C7.4
Equal marble masses react with excess acid as chips or powder. Compare total gas.
Answer:
Powder reacts faster, but both produce the same final amount if their composition is equivalent.
Question 23 · Spec C7.6
What is a catalyst?
Answer:
A substance that speeds a reaction and is unchanged chemically and in mass at the end.
Question 24 · Spec C7.7
How does a catalyst work?
Answer:
It provides an alternative pathway with lower activation energy.
Question 25 · Spec C7.6
Does a catalyst change the reaction's products?
Answer:
No. It changes the speed rather than the identity of the products.
Question 26 · Spec C7.8
What are enzymes, and name a listed industrial use.
Answer:
Biological catalysts; they are used in producing alcoholic drinks.
Question 27 · Spec C7.8
How does yeast help produce alcoholic drinks?
Answer:
Its enzymes catalyse fermentation of sugars into ethanol and carbon dioxide.
Question 28 · Spec C7.1 · Core practical
Why repeat each rate measurement?
Answer:
To assess variation and calculate a more reliable mean, checking anomalous results.
Question 29 · Spec C7.2
A 2.0 g mass loss occurs in 50 s. Find mean rate.
Answer:
2.0/50 = 0.040 g/s.
Question 30 · Spec C7.5
Why must a graph's axis units be checked before calculating rate?
Answer:
The gradient's unit follows the plotted quantities, such as cm³/s or g/min.
Question 31 · Spec C7.9
Name four changes whose heat effects can be measured in solution.
Answer:
Salt dissolving, neutralisation, displacement and precipitation.
Question 32 · Spec C7.10
What is an exothermic reaction or change?
Answer:
One that transfers heat energy to the surroundings.
Question 33 · Spec C7.11
What is an endothermic reaction or change?
Answer:
One that takes in heat energy from the surroundings.
Question 34 · Spec C7.9
A mixture rises from 20.0 to 27.5 °C. Find the change and classify it.
Answer:
Rise = 7.5 °C; without another heat source, this indicates an exothermic change.
Question 35 · Spec C7.9
A mixture falls from 21.0 to 16.0 °C. What does this indicate?
Answer:
A 5.0 °C fall, consistent with an endothermic change.
Question 36 · Spec C7.9
Why use insulation when measuring a reaction's temperature change?
Answer:
It reduces energy exchange with surroundings, improving the measured change.
Question 37 · Spec C7.9
Why stir and record the maximum or minimum temperature?
Answer:
Stirring gives a more uniform temperature; the extreme reading better captures the reaction before surroundings reduce the change.
Question 38 · Spec C7.12
What are the energy effects of breaking and forming bonds?
Answer:
Breaking bonds is endothermic; forming bonds is exothermic.
Question 39 · Spec C7.13
Why is a reaction exothermic overall?
Answer:
Forming product bonds releases more energy than breaking reactant bonds requires.
Question 40 · Spec C7.13
Why is a reaction endothermic overall?
Answer:
Breaking reactant bonds needs more energy than forming product bonds releases.
Question 41 · Spec C7.14 · Higher only
State the bond-energy calculation for overall energy change.
Answer:
Total energy needed to break bonds minus total energy released forming bonds.
Question 42 · Spec C7.14 · Higher only
Breaking bonds needs 850 kJ/mol and forming bonds releases 1,100 kJ/mol. Find change.
Answer:
850 − 1,100 = -250 kJ/mol; exothermic.
Question 43 · Spec C7.14 · Higher only
Breaking bonds needs 1,250 kJ/mol and forming bonds releases 900 kJ/mol. Find change.
Answer:
1,250 − 900 = +350 kJ/mol; endothermic.
Question 44 · Spec C7.14 · Higher only
H₂ + Cl₂ → 2HCl. Bond energies: 436, 243 and 431 kJ/mol respectively. Find change.
Answer:
Broken = 436 + 243 = 679; formed = 2 × 431 = 862; change = -183 kJ/mol.
Question 45 · Spec C7.15
Define activation energy.
Answer:
The minimum energy colliding particles need for a successful reaction.
Question 46 · Spec C7.16
What are the axes of a reaction profile?
Answer:
Energy vertically and reaction progress horizontally; the horizontal axis is not time.
Question 47 · Spec C7.16
How are products positioned on an exothermic reaction profile?
Answer:
Below the reactant energy level.
Question 48 · Spec C7.16
How are products positioned on an endothermic reaction profile?
Answer:
Above the reactant energy level.
Question 49 · Spec C7.16
Where is activation energy measured on a reaction profile?
Answer:
From the reactant energy level up to the peak.
Question 50 · Spec C7.16
How does a catalyst alter the reaction profile?
Answer:
It lowers the peak without changing reactant or product energy levels.

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