BEEZY STUDENTS

GCSE Chemistry Revision

Learn it. Recall it. Revise it.

GCSE Chemistry revision

Collision theory and activation energy

Rate of reaction

AQA 4.6.1.3 Foundation & Higher
Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

MOST 🎯🎯

SOME 🎯🎯🎯

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

Introduction

  • Reaction rates can vary enormously, from very slow processes like the rusting of iron (taking years) to near-instantaneous explosions like fireworks.
  • A typical example of a moderate reaction rate is magnesium reacting with acid, which produces a steady stream of hydrogen gas bubbles.

How to Measure the Rate of Reaction

  • The rate of reaction can be measured by tracking either how quickly reactants are used up or how quickly products are formed.
  • The two equations for rate of reaction are: Rate = quantity of reactants used / time taken or Rate = quantity of products formed / time taken Quantities are measured in grams (g) or cubic centimetres (cm³), and time must always be converted into seconds (s) unless another unit is specified.

Worked Examples of Rate Calculations

  • If 180 cm³ of hydrogen gas is produced in 2 minutes, the rate of reaction is 180 / 120 = 1.5 cm³/s If 3 g of magnesium is fully used up in 4 minutes, the rate of reaction is 3 / 240 = 0.0125 g/s If 0.6 moles of magnesium is used in 2 minutes and the answer is required in moles per minute, the rate is 0.6 / 2 = 0.3 mol/min

Average vs. Instantaneous Rate

  • The rates calculated using the equations above represent the mean (average) rate of reaction over the entire duration of the reaction.
  • In reality, the rate is fastest at the start of a reaction when there are the most reactants present, and slows down as reactants are used up.

Graphs: Mass of Reactant Remaining

  • When plotting mass of reactant remaining (y-axis) against time (x-axis), the graph starts at the initial mass and decreases over time.
  • The graph is steepest at the beginning (fastest rate) and gradually levels off as the reactant is fully consumed.
  • The graph becomes completely flat (a plateau) when all the reactant has been used up and the reaction has stopped.

Graphs: Volume of Product Produced

  • When plotting volume of product (e.g. hydrogen gas) on the y-axis against time on the x-axis, the graph starts at zero as no product has yet been formed.
  • The graph rises steeply at first, reflecting the fast initial rate of reaction, then becomes less steep as the reaction slows.
  • The graph eventually plateaus when one of the reactants is completely used up and no more product can be formed.

Units for Rate of Reaction

  • The units for rate of reaction depend on how the quantity is measured: common units include g/s, cm³/s, mol/s, or mol/min.
  • Always check which units are required in the question, and ensure time is expressed in the correct unit (seconds or minutes) to match.
  • The method for calculating the rate remains the same regardless of the units — simply divide the quantity by the time taken.