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5.2Equilibrium and Reaction Rates

Reaction rates — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 3 1 quiz

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Lesson summary

Reaction rates EST Chemistry notes: Explain rate using collision theory.A rise of about 10 °C roughly doubles many reaction rates.Orders cannot be read from the balanced equation (except elementary steps).Overall order = sum of exponents.Enzymes are biological catalysts.A catalys…

  • Doubling [A] doubles the rate; doubling [B] quadruples it. Write the rate law
  • Reaction rates EST Chemistry: what to do next
  • Activation energy (Ea)
  • Activated complex
  • Reaction order
  • Rate-determining step

Part 1 of 8Key ideas

1
A rise of about 10 °C roughly doubles many reaction rates.
2
Orders cannot be read from the balanced equation (except elementary steps).
3
Overall order = sum of exponents.
4
Enzymes are biological catalysts.
5
A catalyst lowers Ea for both forward and reverse reactions and does not change ΔH.

Part 2 of 8Learning objectives

Learning objectives
Explain rate using collision theory.
Describe factors that affect rate.
Determine rate laws from data.

Part 3 of 8Essential knowledge

1
Energy diagrams show Ea as the height from reactants to peak.
2
Homogeneous catalysts share the reactants' phase; heterogeneous ones do not.
3
Instantaneous rate is the slope of the tangent on a concentration–time graph.
4
Intermediates are made in one step and used in another.
5
Half-life of a first-order reaction is constant.

Part 4 of 8Process & sequence

1
Collect initial-rate data.
2
Compare experiments where only one concentration changes.
3
Find the order from the rate ratio.
4
Write the rate law.
5
Substitute data to find k.

Collision theorywrite these steps in this order; each step is a marking point.

1
Particles collide
2
Energy ≥ Ea?
3
Correct orientation?
4
Activated complex
5
Products form

Part 5 of 8Worked examples

  1. Doubling [A] doubles the rate; doubling [B] quadruples it. Write the rate law.

Part 6 of 8Summary

Summary
Effective collisions need energy and orientation.
Catalyst lowers Ea.
Orders come from experiment.

Part 7 of 8Keep going

Reaction rates EST Chemistry: what to do next

These Reaction rates EST Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Reaction rates EST Chemistry understanding with the practice questions below, then reread any Reaction rates EST Chemistry point you were unsure about before moving on.

1
Practice: Reaction rates EST Chemistry practice questions
2
Previous lesson: 5.1 Chemical equilibrium
3
Next lesson: 5.3 Ionic equilibria
4
Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks

Reaction rates EST Chemistry revision notes

Part 8 of 8Compare: Compare

Factors affecting rate
FactorEffectReason
Higher concentrationFasterMore frequent collisions
Higher temperatureFasterMore particles exceed Ea; more collisions
Greater surface areaFasterMore exposed particles
CatalystFasterLower Ea
Nature of reactantsVariesBond strengths and ionic vs covalent
Reaction orders
OrderEffect of doubling [A]Unit of k
ZeroNo changeM/s
FirstRate doubless−1
SecondRate quadruplesM−1s−1

Key terms

Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Reaction rate · Collision theory · Activation energy (Ea) · Activated complex · Catalyst · Rate law · Reaction order · Rate-determining step
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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