DONATSciences and Maths
Chemistry · Practice book · Senior 3 · Unit 5 of 10

Rate of chemical reactions

Senior 3 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 5 Rate of chemical reactions

Key unit competence: Describe and explain the effect of different conditions on the speed of reactions.

Key ideas

  • The rate of reaction is the change in amount (or concentration) of a reactant or product per unit time. It can be followed by measuring the volume of gas produced, the loss in mass, a colour change or the time for a precipitate to hide a mark.
  • Collision theory: particles must collide with enough energy (at least the activation energy) and the right orientation to react. Anything that increases the number of effective collisions per second increases the rate.
  • Increasing concentration (or pressure for gases) puts more particles in the same volume: more frequent collisions, faster rate.
  • Increasing temperature makes particles move faster: more frequent collisions and a larger fraction with energy ≥ activation energy. Roughly, a 10 °C rise doubles many rates.
  • Increasing surface area (smaller pieces, powders) exposes more particles: more collisions, faster rate. Powders of flour or coal dust can explode.
  • A catalyst speeds up a reaction without being used up by providing a path with a lower activation energy (e.g. MnO2 for H2O2 decomposition, iron in the Haber process, enzymes in the body). Light also speeds up some reactions (photosynthesis, darkening of silver salts).

Common misconceptions

  • "A catalyst is used up in the reaction." No: A catalyst is unchanged in mass and chemical composition at the end.
  • "Higher temperature speeds up a reaction only because collisions are more frequent." No: The main reason is that more particles have energy greater than the activation energy.
  • "Using powder instead of lumps gives MORE product." No: It gives the same amount of product faster, if the amounts of reactants are the same.

Worked examples

Example 1. Use collision theory to explain why magnesium powder reacts faster with dilute HCl than magnesium ribbon of the same mass.

Solution

Particles must collide to react.

Powder has a much larger surface area than ribbon of the same mass.

More magnesium atoms are exposed to acid particles, so there are more collisions per second.

More successful collisions per second means a faster rate.

Example 2. In an experiment, the volume of CO2 was 0, 30, 50, 60, 65, 65 cm3 at 0, 1, 2, 3, 4, 5 minutes. Find the average rate in the first two minutes and explain when the reaction stopped.

Solution

Average rate (0–2 min) = 50 ÷ 2 = 25 cm3 per minute.

The rate decreases with time because reactants are used up.

The volume stays at 65 cm3 from 4 minutes: the reaction had stopped by then.

Practice questions

  1. Define rate of reaction.

    Show answer

    The change in amount or concentration of a reactant or product per unit time.

    Common mistake: Saying 'the amount of product formed'.

  2. List four factors that affect the rate of reaction.

    Show answer

    Concentration (pressure), temperature, surface area, catalyst (light).

    Common mistake: Including 'colour of the container'.

  3. What is a catalyst?

    Show answer

    A substance that speeds up a reaction without being used up.

    Common mistake: Saying it increases the amount of product.

  4. Explain why milk goes sour faster on a hot day.

    Show answer

    Higher temperature increases the rate of reactions caused by bacteria.

    Common mistake: Saying heat adds acid to milk.

  5. Name a method to measure the rate of a reaction that produces a gas.

    Show answer

    Collect the gas in a gas syringe and measure volume at intervals (or measure loss in mass).

    Common mistake: Measuring the colour of the gas.

  6. What is activation energy?

    Show answer

    The minimum energy colliding particles need to react.

    Common mistake: Saying it is the energy released.

  7. Explain why powdered zinc reacts faster with acid than granules.

    Show answer

    Larger surface area gives more collisions per second.

    Common mistake: Saying powder is more concentrated.

  8. Give one example of a catalyst used in industry.

    Show answer

    Iron (Haber process), vanadium(V) oxide (Contact process), nickel (hydrogenation), platinum (Ostwald).

    Common mistake: Saying heat.

  9. Why does the volume of gas stop increasing in a reaction?

    Show answer

    One reactant has been completely used up.

    Common mistake: Saying the catalyst ran out.

  10. 60 cm3 of oxygen forms in 2 minutes. Calculate the average rate in cm3 per second.

    Show answer

    60 ÷ 120 = 0.5 cm3/s.

    Common mistake: Dividing by 2 and calling it per second.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Chewing food well helps digestion because it:a. increases the surface area for enzymes to act
    2. Which reaction is speeded up by light?b. Photosynthesis
    3. Increasing temperature increases the rate because particles:c. Manganese(IV) oxide
    4. Which catalyst is used to decompose hydrogen peroxide?d. move faster and more collisions have enough energy
    Show answer

    1 → a (increases the surface area for enzymes to act) · 2 → b (Photosynthesis) · 3 → d (move faster and more collisions have enough energy) · 4 → c (Manganese(IV) oxide)

    • Smaller pieces react faster.
    • Light provides energy for photosynthesis.
    • More collisions exceed the activation energy.
    • MnO2 speeds up 2H2O2 → 2H2O + O2.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    heat of reaction  ·  activation energy  ·  water boils at a higher temperature  ·  at the start
    1. The minimum energy needed for colliding particles to react is the .
    2. Cooking food in a pressure cooker is faster because .
    3. In the reaction of magnesium with HCl, the rate is fastest .
    Show answer

    1: activation energy · 2: water boils at a higher temperature · 3: at the start

    • Only collisions with at least this energy succeed.
    • Higher temperature increases reaction rates.
    • Concentration of acid is highest at the start.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. A catalyst works by:a. Using marble powder instead of chips
    2. Which method could follow the rate of CaCO3 + 2HCl → CaCl2 + H2O + CO2?b. Measuring the volume of gas produced every 30 seconds
    3. Food is kept in a refrigerator because low temperature:c. providing an alternative path with a lower activation energy
    4. Which change increases the rate of reaction between marble chips and HCl?d. slows down the reactions that spoil food
    Show answer

    1 → c (providing an alternative path with a lower activation energy) · 2 → b (Measuring the volume of gas produced every 30 seconds) · 3 → d (slows down the reactions that spoil food) · 4 → a (Using marble powder instead of chips)

    • More collisions then have enough energy.
    • Gas volume increases as the reaction proceeds.
    • Lower temperature reduces reaction rates.
    • Powder has a larger surface area.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    inhibitors  ·  gas particles are pushed closer together  ·  biological catalysts  ·  the same as at the start
    1. Enzymes are .
    2. The mass of a catalyst at the end of a reaction is .
    3. Increasing the pressure of a gaseous reaction increases the rate because .
    Show answer

    1: biological catalysts · 2: the same as at the start · 3: gas particles are pushed closer together

    • Enzymes speed up reactions in living things.
    • Catalysts are not consumed.
    • Higher pressure means higher concentration.

Take the Unit 5 quiz → · Back to contents