Rates of Reaction Notes

Edexcel GCSE Chemistry: Revision notes

Key facts

  • The rate of reaction is how quickly reactants are used up or products form.
  • Reactions happen when particles collide with enough energy (the activation energy).
  • Higher temperature increases both the frequency and energy of collisions; concentration, pressure and surface area increase only the frequency.
  • On a graph, a steeper gradient means a faster rate, and the line flattens when the reaction ends.
  • A catalyst gives a pathway with lower activation energy and is not used up.

Measuring rate

Rate can be measured by the gas produced, the mass lost, or the time taken for a colour change.

Two specified practical methods are:

  • marble chips and hydrochloric acid, measuring the volume of COX2\ce{CO2} with a gas syringe, or the mass lost
  • sodium thiosulfate and hydrochloric acid, timing how long a cross takes to disappear as sulfur makes the mixture cloudy

The rate of reaction is quicker when the time is shorter.

Marble chips and acid

What forms:
Carbon dioxide gas
What you measure:
Volume of gas or mass lost over time

Thiosulfate and acid

What forms:
Sulfur precipitate
What you measure:
Time for a cross to disappear
  1. 1

    Mark a cross

    on paper under the flask

  2. 2

    Mix the reactants

    start the timer

  3. 3

    Watch from above

    the solution turns cloudy as sulfur forms

  4. 4

    Stop the timer

    when the cross disappears

Timing the thiosulfate and acid reaction

The cross disappears after 20 s in one trial and 45 s in another. Which trial was faster?

Collision theory

Particles must collide with at least the activation energy for a reaction to happen.

According to collision theory, reactions happen when reactant particles collide with enough energy. That minimum energy is the activation energy.

The rate rises when collisions become more frequent, or more energetic.

24681012141618200.10.20.30.40.50.60.70.8xyactivation energyparticle energies
Illustrative spread of particle energies. Only the shaded particles have at least the activation energy, so only their collisions react.
  • Reaction happens whenparticles collide

What two conditions must be met for particles to react?

Factors affecting rate

Raising temperature, concentration, pressure or surface area increases the rate, but only temperature also raises collision energy.

Raising temperature makes particles move faster, so they collide more often and with more energy.

Raising concentration, pressure or surface area to volume ratio packs more particles where they can collide, so collisions are more frequent.

246810121416182022240.020.040.060.080.10.120.140.160.180.2xyactivation energylower temperaturehigher temperature
Illustrative: at a higher temperature more particles have at least the activation energy, so more collisions are successful.

Temperature

Rate:
Increases
More frequent collisions:
Yes
More energetic collisions:
Yes

Concentration

Rate:
Increases
More frequent collisions:
Yes
More energetic collisions:
No

Pressure (gases)

Rate:
Increases
More frequent collisions:
Yes
More energetic collisions:
No

Surface area

Rate:
Increases
More frequent collisions:
Yes
More energetic collisions:
No

Which factor increases both the frequency and the energy of collisions?

Reading rate graphs

A steeper gradient means a faster rate, and the line flattens once a reactant is used up.

Plot the mass, volume or concentration against time. The gradient at the start shows the rate: the steeper it is, the faster the reaction.

The graph levels off when the reaction has finished.

123456789101020304050xyChosen rateSlower reaction
Move the slider: a faster reaction rises more steeply but reaches the same final amount.

What does a steeper line on a volume against time graph show?

Catalysts

A catalyst gives the reaction an alternative pathway with a lower activation energy and is not used up.

A catalyst speeds up a reaction without being used up or changing the products. It has the same mass at the end.

It provides an alternative pathway with lower activation energy, so more particles can react and more successful collisions happen each second. Enzymes are biological catalysts, such as those in yeast used to make alcoholic drinks.

1234567891024681012xywithout catalystwith catalyst
Illustrative reaction profiles: the catalyst gives a pathway with a lower activation energy; reactants and products are unchanged.
  • Catalystalternative pathway

How does a catalyst increase the rate of a reaction?

Try an exam question

Use collision theory to explain why increasing the temperature increases the rate of a reaction.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.