Skip to main content

MYP IB Demystified

IB Demystified EXAMINERS·MODERATORS·AUTHORS
Book free meeting

This page needs the IB Demystified site-wide snippets. Ask the site administrator to install them.

IB DemystifiedMYP Sciences

Rates of reaction

Same marble, same acid, same amount of gas, yet one reaction is finished in a minute and the other takes three. What makes the difference is how often particles collide, and how hard.

Recommended for MYP 4 · eAssessment priority · About 4 lessons · Criteria A, B, C and D

0306090120150180time / s010203040506070volume of gas / cm³AB
Figure 1. Two experiments with the same mass of marble. A is faster; both make 60 cm³ of gas.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding rates
  5. Collision theory and catalysts
  6. Rates in the real world
  7. Worked examples
  8. In the eAssessment
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • describe ways to measure the rate of a reaction, and calculate mean rates
  • interpret graphs of product formed against time
  • predict the effect of temperature, concentration, surface area and catalysts on rate
  • explain these effects using collision theory and activation energy
  • interpret energy profiles with and without a catalyst
  • design, process and evaluate rate investigations

Before you start

You will use these skills. If any feel shaky, review them first.

  • the particle model and how particles move when heated (see States of matter)
  • word equations, reactants and products
  • gradients of graphs and rates such as speed (see Speed)

Key vocabulary

Rate of reaction
How quickly a reactant is used up or a product is formed, for example in cm³/s or g/s.
Collision theory
Particles react only when they collide with enough energy and in a suitable orientation.
Activation energy
The minimum energy that colliding particles need in order to react.
Catalyst
A substance that speeds up a reaction by providing a pathway with a lower activation energy, and is not used up.
Limiting reactant
The reactant that is used up first, so it decides how much product forms.
Enzyme
A protein that acts as a biological catalyst.

Understanding rates

  1. What is it?

    The rate of a reaction is how much product forms, or reactant is used up, per unit of time. On a graph of product against time, the gradient is the rate: steep at the start, then flatter as reactants run out, and flat when the reaction stops.

    mean rate = amount of product formed ÷ time taken

  2. Why does it happen?

    Particles must collide to react, and only collisions with at least the activation energy succeed. Anything that makes successful collisions more frequent increases the rate: more particles in the same volume (concentration), more exposed particles (surface area), faster and more energetic particles (temperature), or a lower energy barrier (a catalyst).

  3. How do we know?

    We cannot watch particles collide, but collision theory predicts measurable patterns, such as rates roughly doubling for every 10 °C rise in many reactions, and experiments confirm them. We measure rates by collecting gas, recording mass loss, or timing a colour change or cloudiness.

  4. Why does it matter?

    Industry needs reactions fast enough to be profitable but safe to control. Catalytic converters clean car exhausts, fridges slow the reactions that spoil food, and uncontrolled fast reactions cause dust explosions in mills and mines.

  5. What does it connect to?

    Rates link to energy changes in reactions, equilibrium and industrial processes in chemistry; to enzymes and digestion in biology; and to gradients and rates of change in mathematics.

Collision theory and catalysts

energyprogress of reactionreactantsproductsPQR
Figure 2. Energy profile with (dashed) and without (solid) a catalyst.
  • Temperature: particles move faster, so they collide more often, and a much larger proportion of collisions have at least the activation energy. This second effect matters most.
  • Concentration or gas pressure: more particles in the same volume, so more frequent collisions. Particles do not move faster.
  • Surface area: smaller pieces expose more particles to collisions.
  • Catalyst: provides an alternative pathway with a lower activation energy (Q instead of P), so more collisions succeed. It does not change the overall energy change (R) or the amount of product.

Rates in the real world

Chemical manufacturers choose temperatures, pressures and catalysts so that products form quickly enough to be economic. Pharmacists store some medicines in fridges because their active ingredients break down faster when warm. Food scientists use packaging that keeps out oxygen, slowing the reactions that make food go stale.

Worked examples

Example 1: mean rate from a graph

Use Figure 1. Calculate the mean rate for experiment B over the first 30 s.

  1. Read the volume on line B at 30 s: about 35 cm³.
  2. mean rate = volume ÷ time = 35 cm³ ÷ 30 s.
  3. mean rate ≈ 1.2 cm³/s.

Example 2: explaining with collision theory

Explain why powdered marble reacts faster than marble chips of the same mass.

  1. Identify the factor: the powder has a larger surface area.
  2. Link to particles: more marble particles are exposed to the acid.
  3. Link to collisions: collisions happen more frequently, so there are more successful collisions per second and the rate is higher.

In the eAssessment

Rates of reaction suits the on-screen eAssessment well, because it combines calculation, graph reading, practical design and data evaluation around one experiment. Expect:

  • Linked questions on one stimulus: a graph or table followed by several questions that build from reading values to explaining and evaluating.
  • Command terms: calculate (show working, give units), explain (use collision theory: frequency and energy of collisions), evaluate (weigh strengths and weaknesses using the data), design (variables, method, sufficient data and safety).
  • Unfamiliar contexts: the chemistry stays the same, but the reaction, apparatus or application may be new to you.

Common ways to lose marks: saying “particles collide more” without mentioning energy for temperature; saying particles move faster when concentration increases; confusing time with rate; and claiming a catalyst makes more product.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: does chip size affect the rate?

Partially guided investigation · about 60 minutes · pairs

Research question
How does the size of marble chips (small, medium, large; same total mass) affect the rate of reaction with excess hydrochloric acid, measured by mass loss?
Scientific background
The acid can only react with particles on the surface of the marble. The same mass cut into smaller pieces has a larger total surface area.
Hypothesis
Write your own hypothesis, with a justification based on collision theory.
Independent variable
Chip size (surface area), using the same total mass of marble each time.
Dependent variable
Mass lost every 30 s as carbon dioxide escapes.
Control variables
Identify at least three, and explain how and why you will control each one.
Apparatus
Conical flask, cotton-wool plug, electronic balance (0.01 g), measuring cylinder, marble chips in three sizes, 1 mol/dm³ hydrochloric acid, stopwatch.
Method
  1. Measure 50 cm³ of acid into the flask, add a cotton-wool plug and record the mass.
  2. Add the chips, replace the plug quickly and start the stopwatch.
  3. Record the total mass lost every 30 s until it stops changing.
  4. Repeat for each chip size, then repeat the whole set.
  5. Plot mass lost against time for each size and compare the initial gradients.

Safety. Wear eye protection: hydrochloric acid irritates eyes and skin. The cotton-wool plug lets gas out but stops acid spray escaping. Rinse any spills with plenty of water.

Results from one run with small chips are used in the Criterion C questions below. Then evaluate: why does the cotton-wool plug matter, and what could make the first readings inaccurate?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

© IB Demystified. IB Demystified is an independent educational resource and is not affiliated with or endorsed by the International Baccalaureate Organization.