Calculating Rates of Reactions Notes

Oxford AQA IGCSE Chemistry: Revision notes

Key facts

  • The rate of reaction is how quickly a reactant is used up or a product is formed.
  • Mean rate = amount of reactant used (or product formed) ÷ time.
  • Units depend on what is measured: g/s, cm³/s or mol/dm³/s.
  • A steeper line on a graph means a faster reaction.
  • A flat line means the reaction has finished.

What rate means

The rate of a reaction measures how quickly reactants are used up or products are formed.

A fast reaction has a high rate of reaction. A slow reaction has a low rate.

Fast reaction

Rate:
High rate
Reactants:
Reactant used up quickly
On a graph:
Steep graph line

Slow reaction

Rate:
Low rate
Reactants:
Reactant used up slowly
On a graph:
Shallow graph line

A reaction has a high rate. Is it fast or slow?

Calculating mean rate

Divide the amount of reactant used, or product formed, by the time taken.

The amount can be a mass in grams, a gas volume in cm³ or a concentration in mol/dm³, whichever is easiest to measure. Time is usually in seconds.

The answer is the mean rate over that period.

  • Mean rateamount of product formedtime\dfrac{\text{amount of product formed}}{\text{time}}
  • Oramount of reactant usedtime\dfrac{\text{amount of reactant used}}{\text{time}}

Worked example

48 cm³ of hydrogen gas is collected in 40 seconds. Calculate the mean rate of reaction.

0.60 g of product forms in 30 seconds. What is the mean rate?

Reading rate graphs

The steeper the line, the faster the reaction; a flat line means it has finished.

Plot the amount of product formed (or reactant used) on the y-axis against time on the x-axis.

  • The line is steepest at the start, when reactants are at their highest.
  • It becomes less steep as reactants are used up.
  • It goes horizontal when the reaction has finished.

The slope is the gradient; a tangent drawn at one point gives the rate at that instant.

2040608010012014010203040xyReaction finishedVolume of gas
Gas collected against time (illustrative): steep at the start, flat at the end

What does a horizontal line on a product-against-time graph mean?

Comparing two reactions

The line with the steeper start belongs to the faster reaction.

Compare the gradient at the start of each curve. The reaction that reaches its plateau sooner finished sooner.

If both used the same amount of reactant, they end at the same total amount of product.

2040608010012014010203040xySame final volumeFaster reactionSlower reaction
Two reactions with the same amount of reactant (illustrative)

Two graphs end at the same final volume, but graph A is steeper at the start. Which reaction has the faster initial rate?

Try an exam question

A student collects 60 cm³ of gas in 25 seconds. Calculate the mean rate of reaction and state how the graph of volume against time would show that the reaction has finished.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.