Rates of Reaction Notes

AQA GCSE Chemistry: Revision notes

Key facts

  • Rate = quantity of reactant used (or product formed) ÷ time, in units such as g/s or cm³/s.
  • A steeper gradient on a graph of product against time means a faster rate.
  • Higher concentration, surface area and temperature, or a catalyst, increase the rate.
  • Collision theory: reactions happen when particles collide with enough energy; faster reactions have more frequent or more energetic collisions.
  • Catalysts lower the activation energy and are not used up.

Calculating rate

Rate is the quantity of reactant used up or product formed divided by the time taken.

rate=quantity of reactant used (or product formed)time\text{rate} = \frac{\text{quantity of reactant used (or product formed)}}{\text{time}}

The quantity can be mass (g), gas volume (cm³) or concentration (mol/dm³). The rate of reaction has units such as g/s, cm³/s or mol/(dm³·s).

A faster reaction has a larger rate value.

10203040506070809010010203040xyStart: fastest rateFinished: rate 0Volume of gas
Illustrative values: the gradient of the curve is the rate. It is steepest at the start and zero when the reaction has finished.
  • Ratequantitytime\dfrac{\text{quantity}}{\text{time}}

Worked example

A reaction produces 8 g of product in 40 seconds. Another produces 12 g in 60 seconds. Compare their rates.

A reaction makes 30 cm³ of gas in 20 s. What is the rate?

Measuring rate

Track mass lost, gas volume made, or the time for a precipitate to hide a cross.

Choose the method to suit the reaction. In the cross experiment a precipitate forms in a beaker over a cross on paper. Time how long until the cross can no longer be seen from above.

Measures

Change in mass:
Mass lost as gas
Gas collected:
Volume of gas made
Precipitate (cross):
Time for cross to disappear

Example

Change in mass:
Calcium carbonate with acid
Gas collected:
Metal with dilute acid
Precipitate (cross):
Sodium thiosulfate with acid
  1. 1

    Draw a cross

    on paper beneath a beaker

  2. 2

    Start the timer

    as the reaction begins

  3. 3

    Watch from above

    the precipitate hides the cross

  4. 4

    Record the time

    when the cross cannot be seen

The cross experiment

Which method suits a reaction that forms an insoluble product?

Rate graphs

On a graph of product against time, steepness shows rate: steep at the start, flat when finished.

A steeper gradient means a faster rate. The gradient falls over time as reactants are used up, and the line goes flat when the reaction stops.

Do not confuse a concentration-time graph (amount remaining) with how fast the reaction is going at each moment.

5101520253035402468101214xy(0, 0)product formed
Product against time: steepest at the start, flat at the end

Where is the rate fastest on this type of graph?

Concentration

More particles in the same volume collide more often, so the rate rises.

Collision theory: a reaction needs particles to collide with enough energy. Increasing concentration puts more particles in the same volume, so collisions are more frequent.

The activation energy does not change. A higher starting concentration gives a steeper curve at the start and the reaction finishes sooner.

Low concentration

High concentration

Higher concentration: more reactant particles in the same volume, so more collisions
5101520253035402468101214xyproduct formed
Product formed against time. A larger k means a faster reaction.

Why does a higher concentration increase the rate?

Temperature and surface area

Heating makes collisions more frequent and more energetic; a larger surface area exposes more solid.

Temperature: particles move faster, so collisions are more frequent. More particles also have at least the activation energy, which is the more important effect.

Surface area: breaking a solid into smaller pieces exposes more of it to collide with. Marble powder reacts faster than marble chips.

5101520253035402468101214xyhigher temperaturelower temperature
Two temperatures: hotter is faster, same final amount

Why does powdered marble react faster than marble chips?

Catalysts and tangents (HT)

A catalyst lowers the activation energy; a tangent gives the rate at one point on a curve.

A catalyst speeds up a reaction without being used up by lowering the activation energy. More particles can then react at the same temperature.

(HT) On a curved concentration-time graph, draw a tangent at the point and calculate its gradient: change in concentration ÷ change in time.

  1. 1

    Pick the point

    on the curve

  2. 2

    Draw a tangent

    touches the curve without crossing

  3. 3

    Find the gradient

    (y₂ − y₁) ÷ (t₂ − t₁)

  4. 4

    Interpret

    gradient = rate at that moment

The tangent method

Worked example

At 10 s the concentration is 0.8 mol/dm³. A tangent there passes through 8 s (0.9 mol/dm³) and 12 s (0.7 mol/dm³). Find the rate at 10 s.

What does a steeper tangent on a concentration-time graph show?

Try an exam question

Marble chips react with dilute hydrochloric acid. Explain, using collision theory, why powdered marble reacts faster than the same mass of marble chips.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.