Science
What is the rate of a reaction, and what makes reactions go faster?
Quick answer
The rate of reaction is how fast reactants are used up or products are made. Higher temperature, higher concentration, a bigger surface area and a catalyst all make reactions faster.
In one sentence
The rate of a reaction is how quickly reactants turn into products: how much product forms (or reactant is used up) every second.
Explained
Some reactions are over in a flash, like a firework. Some take years, like an old car rusting. Most reactions in a school lab sit in between. Chemists measure the speed with the rate of reaction:
mean rate = amount of product formed ÷ time taken, or amount of reactant used ÷ time taken
The "amount" can be a mass in grams (g/s), a volume of gas in cm³ (cm³/s) or an amount in moles (mol/s). If 60 cm³ of gas is made in 30 seconds, the mean rate is 60 ÷ 30 = 2 cm³/s.
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Collision theory explains why rates change. Particles can only react when they collide, and only if they hit with at least a minimum amount of energy, the activation energy. Most collisions are just bumps that do nothing. Anything that makes successful collisions happen more often makes the reaction faster. There are four main factors:
- Temperature. Hotter particles move faster. They collide more often, and, more importantly, a bigger fraction of collisions have enough energy to react. A rough rule for many reactions: a 10 °C rise about doubles the rate.
- Concentration (or pressure for gases). More particles in the same volume means more frequent collisions. Doubling the concentration of a solution often roughly doubles the rate.
- Surface area. Only particles on the surface of a solid can be hit. Cutting a solid into smaller pieces or grinding it to a powder exposes more surface, so there are more collisions each second.
- A catalyst. A catalyst gives the reaction a different route with a lower activation energy, so more collisions succeed. It is not used up. Enzymes are the catalysts in living things.
How do you measure a rate? Pick something that changes and time it: collect gas in a gas syringe or upturned measuring cylinder, watch the mass fall on a balance as gas escapes, or time how long a solution takes to go cloudy (the classic "disappearing cross" with sodium thiosulfate and acid). If you time how long a fixed change takes, the rate is proportional to 1 ÷ time: a shorter time means a faster rate.
Rate graphs. Plot the amount of product against time. The line is steepest at the start, when there are the most reactant particles, then gets less steep and finally goes flat when one reactant runs out. The gradient of the line is the rate (just like a slope on a line graph). A faster reaction gives a steeper curve that flattens sooner; with the same amounts of reactants it still ends at the same final amount.
Examples
| Change | Effect on rate | Why |
|---|---|---|
| Milk kept in the fridge | slower (it lasts longer) | lower temperature, fewer energetic collisions |
| Marble chips → marble powder in acid | faster | bigger surface area |
| 2 mol/dm³ acid instead of 1 mol/dm³ | faster | more acid particles to collide |
| Adding manganese(IV) oxide to hydrogen peroxide | much faster | catalyst |
| Catalytic converter in a car exhaust | faster clean-up of harmful gases | platinum, palladium and rhodium catalysts |
| Chopping potatoes small to cook them | faster | bigger surface area for heat and reactions |
Common mistakes
- Saying a catalyst makes more product. It only makes the same product faster; it does not change the final amount.
- Saying particles "collide harder" so they all react. Only collisions with at least the activation energy react, and temperature increases both how often and how hard they collide.
- Thinking a bigger lump has a bigger surface area. Smaller pieces of the same mass have more surface area in total.
- Forgetting units. A rate needs units, like g/s or cm³/s. "The rate is 2" is not an answer.
Step by step
Step by step
- Choose what to measureGas volume, mass lost or the time for a colour change.
- Keep everything else the sameChange only one factor (the independent variable) for a fair test.
- Take readingsRecord the amount at regular times, for example every 10 seconds.
- Plot a graphAmount on the y-axis, time on the x-axis.
- Work out the rateMean rate = change in amount ÷ change in time. For the rate at one moment, draw a tangent and find its gradient.
Practice questions
Calculate and explain the rate
Try each one, then check your answer.
- 60 cm³ of hydrogen is made in 30 s. Find the mean rate.
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60 ÷ 30 = 2 cm³/s - A flask loses 1.2 g of mass in 40 s as carbon dioxide escapes. Find the mean rate.
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1.2 ÷ 40 = 0.03 g/s - 45 cm³ of gas is collected in 1.5 minutes. Find the rate in cm³/s.
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1.5 min = 90 s, so 45 ÷ 90 = 0.5 cm³/s - The gas volume is 20 cm³ at 10 s and 44 cm³ at 30 s. Find the mean rate between those times.
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(44 − 20) ÷ (30 − 10) = 24 ÷ 20 = 1.2 cm³/s - The cross disappears after 50 s. Find 1 ÷ time.
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1 ÷ 50 = 0.02 s⁻¹ - At a higher temperature the cross disappears after 25 s. How has the rate changed?
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The time halved, so the rate doubled (1 ÷ 25 = 0.04 s⁻¹). - The concentration of an acid falls from 0.50 mol/dm³ to 0.20 mol/dm³ in 60 s. Find the mean rate.
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(0.50 − 0.20) ÷ 60 = 0.30 ÷ 60 = 0.005 mol/dm³ per second - 0.024 mol of CO₂ is made in 120 s. Find the mean rate in mol/s.
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0.024 ÷ 120 = 0.0002 mol/s (2 × 10⁻⁴ mol/s) - A 1 cm cube is cut into 1 mm cubes. How does the total surface area change?
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Before: 6 × 1 = 6 cm². After: 1000 cubes × 6 mm² = 6000 mm² = 60 cm². It is 10 times bigger. - 0.5 g of catalyst is added. What mass of catalyst is left at the end?
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A catalyst is not used up: 0.5 g. - Why does the rate graph go flat at the end?
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One reactant has been used up, so no more product forms. - A reaction takes 80 s at 20 °C. If the rate doubles for every 10 °C rise, how long would it take at 40 °C?
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20 °C hotter means 2 × 2 = 4 times faster: 80 ÷ 4 = 20 s
Word problems
Rates in real life
Try each one, then check your answer.
- Why do we keep food in a fridge?
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The low temperature makes the reactions that spoil food (many caused by microbes and their enzymes) slower, so food lasts longer. - Coal dust in a mine can explode, but a lump of coal burns slowly. Why?
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Dust has a huge surface area, so it can react with oxygen extremely fast. - A tablet takes 40 s to dissolve and fizz when whole, but 15 s when crushed. Which has the higher rate, and why?
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The crushed tablet: its bigger surface area gives more collisions per second. - A factory adds iron to make ammonia faster. Will it need to buy more iron every day for that reason?
Show answer
No. Iron is a catalyst, so it is not used up (though catalysts can slowly get dirty and need cleaning or replacing). - Doubling the concentration of acid halves the time for a reaction from 60 s to 30 s. What happened to the rate?
Show answer
The rate doubled: twice as many acid particles means about twice as many collisions per second.
Questions people ask
What is the formula for rate of reaction?
Mean rate = amount of reactant used ÷ time, or amount of product formed ÷ time. The units depend on what you measure: g/s, cm³/s or mol/s.
How do you find the rate of reaction from a graph?
For the mean rate, divide the change in the y-value by the change in time between two points. For the rate at one moment, draw a tangent to the curve at that time and find its gradient (rise ÷ run).
What are the 4 factors that affect the rate of reaction?
Temperature, concentration (or pressure for gases), surface area and catalysts. Light also speeds up some reactions, such as photosynthesis and the reactions that fade photos.
How do you measure the rate of reaction?
Measure something that changes over time: the volume of gas made (gas syringe), the mass lost (balance) or the time for a solution to change colour or go cloudy.
How does concentration affect the rate of reaction?
A higher concentration puts more reacting particles in the same volume, so they collide more often and the reaction is faster. It does not give the particles more energy.
What is the unit of rate of reaction?
Common units are g/s, cm³/s and mol/s. For solutions, chemists use mol/dm³ per second (mol dm⁻³ s⁻¹), which is the same as mol/L per second.
Is this the same as human reaction time?
No. In sport, "reaction speed" is how fast a person responds to something they see or hear. In chemistry, the rate of reaction is how fast chemicals change into new substances.
Quick quiz
Rates of reaction
Check what you know with 4 quick questions.
Question 1 of 4
Why does powdered marble react faster with acid than a lump of marble of the same mass?
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