Factors Affecting Rates

Section 1 of 7The four factors

Rate of reaction

Concentration or pressure

• Concentration or pressure Raising the concentration of a solution, or the pressure of a gas, increases the rate.

Temperature

• Temperature Raising the temperature of the reacting mixture increases the rate.

Surface area

• Surface area Increasing the surface area of a solid reactant, for example by using powder instead of lumps, increases the rate.

Catalyst

• Catalyst A catalyst speeds up a reaction without being used up.

Section 2 of 7Reading a rate graph

1

Plot the amount of product (or the mass lost) on the y-axis against time on the x-axis.

2

A faster reaction gives a steeper line at the start, because more product forms each second.

3

A faster reaction reaches a flat line sooner, because the reactants are used up earlier.

4

If the quantities of reactants are the same, the final amount of product is the same for every line.

5

Compare gradients at the same time on each curve to rank the rates.

Section 3 of 7Comparing the graphs

Change made	Gradient at the start	When the line levels off	Total product
Higher concentration or pressure	Steeper	Sooner	Same
Higher temperature	Steeper	Sooner	Same
Larger surface area (powder)	Steeper	Sooner	Same
Lower concentration, temperature or surface area	Shallower	Later	Same

Section 4 of 7Surface area and particle size

Smaller pieces

• Smaller pieces Surface area increases as particle size decreases, so smaller pieces react faster.
• Cutting a solid into smaller pieces exposes more of its particles to the other reactant.

Worked example

• Worked example A cube of side 2 cm has 6 faces of 4 cm², so a surface area of 24 cm².
• Cut into eight cubes of side 1 cm, the total surface area is 8 × 6 × 1 = 48 cm², twice as much.

Section 5 of 7Answering questions well

Say per unit volume

• Say per unit volume For concentration or pressure, write that there are more particles per unit volume, usually per cm³, not just more particles.

Apply in reverse

• Apply in reverse Questions may involve decreasing a factor, so a lower temperature, concentration or surface area gives a slower reaction.

Control variables

• Control variables When testing one factor, keep the others the same so the comparison is fair.

Section 6 of 7Key terms

Term	Meaning
Rate of reaction	How quickly reactants are used up or products are formed.
Catalyst	A substance that increases the rate of a reaction and is not used up.
Surface area	The total area of the outside of a solid that is exposed to other reactants.
Gradient	The steepness of a line on a graph; a steep line means a fast reaction.

Section 7 of 7Exam technique

1

Saying a faster reaction makes more product - the final amount is unchanged if the quantities of reactants are the same.

2

Writing 'more particles' instead of 'more particles per unit volume' for concentration.

3

Believing a catalyst is used up - it is unchanged at the end.

4

Reading the rate from the height of the line - the rate is the steepness of the line.