Grade 10 general science – Rates of Reactions Quiz
1. Which change will generally increase the rate of a reaction between solid magnesium and dilute hydrochloric acid in a student laboratory?
Grinding increases the surface area of magnesium exposed to acid, increasing collision frequency between reactant particles and so increasing the rate.
2. According to collision theory, why does increasing the temperature usually make reactions go faster?
Raising temperature increases particle kinetic energy so a larger fraction of collisions have enough energy to overcome the activation energy, increasing reaction rate.
3. Which of the following is an example of a catalyst in a classroom experiment decomposing hydrogen peroxide?
Potassium iodide provides an alternative lower-energy pathway for hydrogen peroxide decomposition and speeds up the reaction without being consumed.
4. How would you most accurately measure the rate of a reaction that produces a gas, in a school practical?
A gas syringe measures the volume of gas produced over time accurately, allowing calculation of rate (volume per unit time) in a practical lab.
5. Which factor does NOT usually affect the rate of a chemical reaction between aqueous solutions?
The vessel's colour does not change molecular collisions or energies; concentration, catalyst presence, and temperature directly affect reaction rates.
6. What is the effect of increasing pressure on the rate of a reaction involving only gaseous reactants?
Increasing pressure decreases gas volume, bringing molecules closer and increasing collision frequency, which usually raises the reaction rate for gases.
7. In the decomposition of calcium carbonate by heat, which change would speed up the reaction?
Powdering increases exposed surface area for heating, allowing more particles to react and increasing the decomposition rate.
8. Which observation indicates a faster chemical reaction during a school experiment between magnesium and hydrochloric acid?
Fast bubble formation shows gas is being produced quickly, a direct sign of a faster reaction between magnesium and acid.
9. What role do enzymes play in biological reactions in the body?
Enzymes lower activation energy for biochemical reactions, increasing rates while remaining unchanged at the end of the reaction.
10. If you double the concentration of a reactant in many simple reactions, what is the usual immediate effect on the rate?
Increasing concentration usually raises collision frequency and therefore rate; for first-order reactions doubling concentration doubles rate; for others the effect varies.
11. Which of these best describes activation energy?
Activation energy is the energy barrier that particles must overcome during collisions to react and form products.
12. Why does stirring a mixture of reactants usually increase the rate of reaction in a solution?
Stirring helps mix reactants, bringing particles into contact more often and increasing collision frequency and reaction rate.
13. In an experiment, the time taken for a blue dye to fade gives a measure of rate. Which instrument would give the most precise measurement of the colour change?
A colorimeter quantifies absorbance or transmittance of light by the solution, giving precise, objective measurement of colour change over time.
14. Which of these will usually decrease the rate of a reaction?
Lowering temperature reduces particle kinetic energy and the number of collisions with sufficient energy, decreasing reaction rate.
15. What is the most likely reason powdered limestone reacts faster with dilute acid than a single large chunk?
Breaking limestone into powder increases surface area in contact with acid, increasing collision opportunities and speeding the reaction.
16. How does a homogeneous catalyst differ from a heterogeneous catalyst in practical terms?
Homogeneous catalysts share the same physical state (e.g., both in solution) as reactants; heterogeneous catalysts are in a different phase (e.g., solid catalyst with gas/liquid reactants).
17. Which experimental observation would suggest that light affects the rate of a reaction?
If exposure to light increases rate, the reaction is photochemical or light-dependent; this observation shows light influences the reaction mechanism.
18. Why might two different metals react at different rates with the same acid?
Different metals have different chemical reactivities and activation energies; more reactive metals react faster with acid under the same conditions.
19. In monitoring the rate of a reaction that forms a precipitate, which simple classroom method can be used?
Observing the time for a cross or mark to be obscured by precipitate gives a simple, commonly used measure of reaction rate in school practicals.
20. Which statement about inhibitors is correct?
Inhibitors reduce reaction rates by blocking active sites or interfering with catalysts, decreasing effective collisions or catalytic activity.
21. If the rate of a reaction is measured as 'change in mass per unit time' during a reaction that releases gas, why does the mass decrease?
When a gas is produced and allowed to escape, the total mass in the vessel decreases; measuring the mass loss over time gives the reaction rate.
22. What effect does increasing the surface area of a solid catalyst typically have on a catalytic reaction?
Greater surface area exposes more active sites for reactants to interact with the catalyst, often increasing the reaction rate for heterogeneous catalysts.
23. Why is it important to control temperature and concentration when comparing rates of reactions in different experiments?
To compare reaction rates fairly, variables that influence rate (like temperature and concentration) must be controlled so differences are due to the factor being tested.
24. Which of the following pairs correctly matches a factor with its primary effect on reaction rate?
A catalyst lowers the activation energy by providing an alternative pathway, increasing the fraction of successful collisions and speeding the reaction.
25. In an experiment, two reactant mixtures are identical except mixture A is stirred while mixture B is left still. After five minutes, mixture A shows more product. The most likely reason is:
Stirring ensures reactant particles are more uniformly distributed and encounter each other more often, increasing the reaction rate.
26. Which practical change would you make to slow down a fast reaction that is difficult to measure accurately in class?
Diluting reactants lowers their concentration, reducing collision frequency and slowing the reaction so measurements can be made more accurately.
27. Why does a reaction with a high activation energy often proceed slowly at room temperature?
High activation energy means only a small fraction of particles at room temperature have enough energy to react, so the reaction is slow.