Factors that Affect the Rate of Reaction

Section 1 of 7Measuring the rate

What rate means

• What rate means The rate of reaction tells you how quickly reactants are used up or products are formed.
• A fast reaction finishes in seconds; a slow one such as rusting can take years.

Mean rate equation

• Mean rate equation Mean rate = quantity of reactant used ÷ time taken, or quantity of product formed ÷ time taken.
• Quantity may be a mass in g, a volume in cm³ or an amount in mol, so units might be g/s, cm³/s or mol/s.

Reading a rate graph

• Reading a rate graph On a graph of product against time, the steepest part of the line is the fastest rate.
• The line becomes flat when one reactant has run out and the reaction has stopped.
• To find the rate at a given moment, draw a tangent to the curve and calculate its gradient (change in y ÷ change in x) (HT only).

Section 2 of 7Ways of following a reaction

Method	What is measured	Suitable when
Mass loss on a balance	Decrease in mass as a gas escapes	A gas of fairly high density is released
Gas syringe or upturned measuring cylinder	Volume of gas collected over time	Any gas is produced
Timing a cross disappearing	Time for a precipitate to cloud the solution	A solid forms and turns the liquid opaque

Section 3 of 7The four main factors

Rate of reaction

Concentration (solutions)

• Concentration (solutions) A higher concentration means more particles in each cm³, so collisions occur more often and the rate increases.

Pressure (gases)

• Pressure (gases) Raising the pressure squeezes gas particles closer together, so collisions are more frequent and the rate increases.

Surface area of solids

• Surface area of solids Breaking a solid into smaller pieces exposes more particles to the other reactant, so more collisions happen each second.

Temperature

• Temperature Particles move faster, so they collide more often and, more importantly, a bigger fraction collide with enough energy to react.

Section 4 of 7Surface area to volume ratio

1Cut the solid up

• Cut the solid up Smaller pieces or a powder replace one large lump.

2Greater surface area for the same mass

• Greater surface area for the same mass More of the solid's particles are on the outside where they can be hit.

3More frequent collisions

• More frequent collisions The other reactant meets exposed particles more often.

4Faster reaction

• Faster reaction The rate increases, and a powder can even react explosively, as with flour dust.

Section 5 of 7Catalysts and the other factors together

Catalysts

• Catalysts A catalyst speeds up a reaction without being used up, by providing an alternative pathway with lower activation energy.

Explaining answers

• Explaining answers Always link the factor to collisions: more frequent collisions, or more collisions with sufficient energy.
• Do not say that the particles 'collide more' without saying 'per second'.

Section 6 of 7Key terms

Term	Meaning
Rate of reaction	How fast reactants are used up or products are made.
Concentration	The amount of solute dissolved in a given volume of solution.
Surface area to volume ratio	The total outside area of a solid compared with its volume; it rises as the pieces get smaller.
Collision frequency	The number of collisions between reacting particles each second.
Catalyst	A substance that increases the rate of a reaction but is not used up.
Tangent	A straight line touching a curve at one point, used to find the rate at that instant.

Section 7 of 7Exam technique

1

Saying that a higher temperature gives particles more energy 'so they react' without mentioning collisions that are more frequent and more energetic.

2

Thinking a larger lump has a larger surface area to volume ratio; breaking it up increases the ratio.

3

Stating that a catalyst is a reactant or is used up, when it is unchanged at the end.

4

Writing 'the reaction stops' for a flat line without adding that a reactant has been used up completely.