What Affects Reaction Rate?
Some chemical reactions happen in the blink of an eye — like a firework exploding. Others take years — like iron rusting on an old fence. What makes one reaction fast and another slow? In this lesson, you will discover the key factors that control how quickly chemical reactions occur.
Before You Begin
Think about these three everyday situations:
- A sparkler burns brightly for a minute and then goes out.
- An iron nail left outside slowly turns reddish-brown over months.
- Vinegar and baking soda fizz vigorously when mixed.
Write down your answers before reading on:
- What do we mean when we say a reaction is "fast" or "slow"?
- Why do you think some reactions are faster than others?
- Can you think of one way you could make a slow reaction faster?
Know
- The definition of reaction rate as the speed of reactant consumption or product formation
- The basic idea of collision theory: particles must collide with sufficient energy and correct orientation
- The four main factors that affect reaction rate: concentration, surface area, temperature and catalysts
Understand
- Why more frequent collisions lead to a faster reaction
- That not every collision results in a reaction — only effective collisions do
- How each factor changes the number or energy of collisions
Can Do
- Identify which factor is being changed in a given scenario
- Predict whether a reaction will speed up or slow down based on changed conditions
- Apply collision theory to explain simple rate changes
What Is Reaction Rate?
Measuring how fast a reaction happens
Moles Molar Mass
Reaction rate tells us how quickly a chemical reaction is happening. We can measure it by observing how fast reactants disappear or how fast products appear.
For example, when magnesium reacts with hydrochloric acid, hydrogen gas is produced. If 50 mL of gas is produced in 10 seconds, the reaction is faster than if it takes 60 seconds. Similarly, if a solution changes colour, the faster the colour change, the faster the reaction.
Everyday examples of different rates
| Fast reactions | Slow reactions |
|---|---|
| Firework exploding | Iron rusting |
| Baking soda + vinegar | Steel bridge corroding |
| Strike-anywhere match | Milk turning sour |
| Antacid tablet in water | Coal forming from plants |
Collision Theory
Why particles must collide to react
Chemical reactions happen when particles (atoms, ions or molecules) collide with each other. But not every collision causes a reaction. For a reaction to occur, the collision must be effective — meaning the particles must:
- Collide with enough energy — the particles must be moving fast enough to break existing bonds and form new ones.
- Collide with the correct orientation — the particles must hit each other at the right angle so that the right parts of the molecules can interact.
Factors That Affect Reaction Rate
An overview of the four key factors
There are four main factors that chemists use to control how fast a reaction happens. Each factor works by changing the number or energy of effective collisions.
The four factors
- Concentration: More concentrated solutions contain more particles per unit volume. This means more frequent collisions and a faster reaction.
- Surface area: Breaking a solid into smaller pieces increases the total surface area exposed to other reactants. More surface means more particles available to collide.
- Temperature: Higher temperature means particles move faster. Faster particles collide more often and with more energy, leading to more effective collisions.
- Catalysts: A catalyst provides an alternative pathway for the reaction that requires less energy. More collisions now have enough energy to be effective.
Common Misconceptions
"Catalysts are used up in reactions." No — catalysts are not consumed. They help the reaction happen faster but are chemically unchanged at the end. You could recover and reuse a catalyst.
"Temperature is the only factor that affects reaction rate." No — concentration, surface area and catalysts all have significant effects. In fact, changing concentration or surface area is often easier in a school lab than changing temperature.
Controlling Reactions in Gold Mining
Australia is the world's second-largest gold producer. Extracting gold from ore involves chemical reactions that must be carefully controlled. In the cyanide leaching process used at many Australian mines, gold ore is crushed to increase surface area, then mixed with a cyanide solution. The reaction that dissolves gold is slow at room temperature, so mines often heat the solution or add catalysts to speed it up.
Understanding reaction rates is literally worth billions of dollars to the Australian economy. Faster, more efficient reactions mean more gold extracted using less energy and fewer chemicals.
✍ Copy Into Your Books
▾Reaction Rate
- Speed of reactant use or product formation
- Measured by observing changes over time
Collision Theory
- Particles must collide to react
- Effective collision = enough energy + correct orientation
Four Factors
- Concentration: more particles per volume
- Surface area: more exposed particles
- Temperature: faster, more energetic particles
- Catalysts: lower energy needed for reaction
Factor Identifier
Predict the Rate
Test Your Understanding
1. Which statement best describes reaction rate?
2. According to collision theory, what two conditions are needed for an effective collision?
3. Why does increasing the concentration of a reactant speed up a reaction?
4. A student wants to make a reaction happen faster. Which of the following would NOT work?
5. Two identical pieces of chalk are dropped into identical beakers of acid. One piece is whole; the other is crushed to powder. Which statement best explains the difference in reaction rate?
Short Answer Questions
1. Explain what is meant by an "effective collision" between particles. Why is it not enough for particles to simply collide? 4 MARKS
2. A student is investigating how surface area affects the reaction between calcium carbonate and hydrochloric acid. Predict whether large marble chips or powdered marble will react faster. Explain your prediction using collision theory. 4 MARKS
3. Describe how a catalyst increases the rate of a chemical reaction without being used up. Use the term "alternative pathway" in your answer. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now define "reaction rate" in your own words?
- Can you name all four factors that affect reaction rate?
- Can you explain one factor using collision theory?
Answers
▾MCQ 1
C — Reaction rate measures how quickly reactants are consumed or products are formed during a chemical reaction.
MCQ 2
B — For an effective collision, particles must have sufficient energy and collide with the correct orientation.
MCQ 3
D — Higher concentration means more particles per unit volume, which leads to more frequent collisions and therefore a faster reaction.
MCQ 4
A — Diluting the solution would decrease the concentration, which would slow down the reaction rather than speed it up.
MCQ 5
C — The powder has a much greater total surface area than the whole piece, so more particles are exposed to the acid and available for collision.
Short Answer 1
Model answer: An effective collision is a collision between particles that results in a chemical reaction. It is not enough for particles to simply collide because they must also have sufficient energy to break existing bonds and form new ones. Additionally, the particles must collide with the correct orientation so that the right parts of the molecules can interact. Without both sufficient energy and correct orientation, the collision is ineffective and no reaction occurs.
Short Answer 2
Model answer: Powdered marble will react faster than large marble chips. This is because powder has a much greater total surface area than chips of the same mass. According to collision theory, a greater surface area means more particles are exposed and available to collide with the acid molecules. This leads to more frequent effective collisions per unit time, so the reaction rate increases. The large chips have less exposed surface, so fewer collisions occur and the reaction is slower.
Short Answer 3
Model answer: A catalyst increases the rate of a chemical reaction by providing an alternative pathway that requires less energy for the reaction to occur. This means that more of the collisions between particles now have enough energy to be effective, so the reaction happens faster. A catalyst is not used up in the reaction because it does not become part of the products. It can be recovered chemically unchanged at the end and used again.
Boss Battle
Test your knowledge in a rapid-fire quiz battle. Defeat the boss by answering questions correctly!
Mark lesson as complete
Tick when you have finished all activities and checked your answers.