Rates of Chemical Reactions: KS3 Chemistry Guide

The rate of a chemical reaction tells you how quickly reactants become products. Some reactions take milliseconds; others take millions of years. All four factors that control rate share one underlying idea: how often particles collide, and how much energy those collisions carry.

What is the rate of a chemical reaction?

Rate of reaction measures how fast a reaction proceeds — either how quickly reactants are used up, or how quickly products are formed. Reactions happen when particles of reactants collide with sufficient energy to break bonds and form new ones. This minimum energy needed is called the activation energy.

Before exploring the factors, think about the particle model first: in a solid, particles vibrate in fixed positions; in a gas, they move freely at high speed. Which state would you expect to react fastest? Gases and dissolved particles in solution react faster because they can move and collide more readily.

What four factors affect reaction rate?

Factor | How it increases rate | Example
Temperature | Particles move faster → more collisions AND more with enough energy | Heating marble chips in acid
Concentration | More dissolved particles per volume → more collisions | Using stronger hydrochloric acid
Surface area | More exposed particles → more collisions possible | Powdering a solid vs using lumps
Catalyst | Provides an alternative pathway with lower activation energy | Manganese(IV) oxide speeds up decomposition of hydrogen peroxide

How does temperature affect reaction rate?

Raising the temperature does two things simultaneously:

Particles move faster → they collide more frequently.

More particles have energy above the activation energy → a greater fraction of collisions are successful (result in a reaction).

As a rough rule of thumb at KS3 level, increasing the temperature by 10 °C approximately doubles the reaction rate. So a reaction taking 60 seconds at 20 °C might take only 30 seconds at 30 °C, and about 15 seconds at 40 °C.

How does concentration affect reaction rate?

In a more concentrated solution, there are more dissolved particles in the same volume. The particles are packed closer together, so they collide more frequently. More frequent collisions → faster reaction rate.

Worked example: Marble chips (calcium carbonate) react with hydrochloric acid:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

If you use 2 mol/dm³ HCl instead of 1 mol/dm³, the reaction rate roughly doubles because there are twice as many H⁺ ions in the same volume to collide with the marble surface.

How does surface area affect reaction rate?

Surface area matters for solid reactants. Only particles on the surface of a solid can collide with other reactants — particles buried inside cannot. Breaking a solid into smaller pieces exposes more surface particles, increasing the collision frequency.

Example comparison:

Form of marble | Surface area | Reaction rate
One large lump (10 g) | Low | Slow
Small chips (10 g) | Medium | Medium
Powder (10 g) | High | Fast

All three samples have the same mass and total number of particles — only the surface area changes. This is why flour dust can cause explosive reactions in grain silos: the enormous surface area of tiny particles allows extremely rapid combustion.

What is a catalyst?

A catalyst is a substance that speeds up a chemical reaction without being used up. It is not consumed in the reaction and can be recovered at the end. Catalysts work by providing an alternative reaction pathway that has a lower activation energy — so more collisions are successful.

Catalysts are used extensively in industry to make reactions faster and more economical without the energy cost of raising temperature. Biological catalysts are called enzymes — they are proteins that catalyse reactions in living cells.

Example: Manganese(IV) oxide (MnO₂) catalyses the decomposition of hydrogen peroxide:

2H₂O₂ → 2H₂O + O₂

Without MnO₂, this reaction is very slow at room temperature. A spatula of the black powder causes rapid bubbling as oxygen gas is released. The MnO₂ can be filtered off and reused.

How do you measure reaction rate?

Reaction rate can be measured by tracking how fast a product appears or how fast a reactant disappears:

Gas production — collect the gas in a syringe or over water, record volume every 30 seconds. Rate = volume of gas ÷ time.

Mass loss — place the reaction flask on a balance; CO₂ escaping reduces mass. Record mass at intervals.

Colour change — some reactions are monitored visually (e.g., iodine clock reaction) or using a colorimeter.

Precipitation — observe when a cross drawn under a flask disappears (sodium thiosulfate + hydrochloric acid produces a sulfur precipitate that clouds the solution).

Rate = amount of product formed (or reactant used) ÷ time taken. Units are mol/s, g/s, cm³/s, or similar depending on what you are measuring.

Frequently asked questions

What is collision theory in KS3 chemistry?

Collision theory states that a chemical reaction can only occur when reactant particles collide with sufficient energy — equal to or greater than the activation energy — and with the correct orientation. Most collisions do not lead to a reaction because the particles do not have enough energy. Anything that increases the frequency of collisions, or the proportion of collisions above the activation energy, will increase the reaction rate.

Why does a catalyst speed up a reaction without being used up?

A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed route. Because the energy barrier is lower, a greater proportion of collisions are successful. The catalyst is regenerated at the end of each catalytic cycle, so it is not consumed overall — you need only a tiny amount to have a large effect on the rate.

How does surface area affect reaction rate?

Only the particles on the surface of a solid reactant are available to collide with other reactants. Crushing a solid into smaller pieces increases the total surface area, exposing more particles and allowing more collisions per second. The total amount of reactant (the mass) stays the same, so the overall amount of product formed is unchanged — but the reaction finishes more quickly.

What is the difference between rate of reaction and amount of product?

The rate of reaction describes how quickly the reaction proceeds — fast or slow. The total amount of product formed depends only on how much reactant you started with, not on how fast the reaction occurs. Using a catalyst or raising the temperature makes the reaction faster but does not give you more product at the end. Concentration, surface area, temperature, and catalysts all affect rate; only the initial amounts of reactant affect the total yield.

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