Coaching Centre
Rates of reaction
How to measure the rate of a reaction and read rate graphs, collision theory and activation energy, how concentration, pressure, temperature, surface area, stirring and catalysts change the rate, and why rates matter for food, industry, dust explosions and bushfires.
A spoonful of flour dropped on a flame just smoulders. Blow the same flour into a cloud above the flame and it bursts into a fireball. Nothing about the chemistry has changed: only the size of the pieces. Dust explosions like this have destroyed flour mills, grain silos and sugar refineries.
Measuring the rate of a reaction
The rate of reaction is how quickly reactants are used up or products are formed:
rate = change in amount of reactant or product ÷ time taken
You can't count particles, so you measure something that changes as the reaction goes:
- Volume of gas collected in a gas syringe or upturned measuring cylinder (mL per second).
- Loss of mass on a balance as gas escapes (g per second).
- Time for a colour change or cloudiness to appear. A shorter time means a faster rate; the rate is roughly 1 ÷ time.
- Change in pH or temperature measured with a data logger.
Rate graphs have a typical shape. The curve is steepest at the start, when there are the most reactant particles. It gets less steep as reactants are used up, and it goes flat when one reactant has run out. The gradient (slope) at any point is the rate at that moment. A steeper curve means a faster reaction. If the amounts of reactants are the same, the final amount of product is the same, however fast it gets there.
Use the rate graph above. Find the average rate of reaction over the first 60 s for the powder and for the chips. Then explain why the powder's initial rate is higher than its average rate.
At 60 s the powder has made about 110 mL of CO₂ and the chips about 60 mL.
Powder: 110 ÷ 60 ≈ 1.8 mL/s. Chips: 60 ÷ 60 = 1.0 mL/s.
The dashed tangent at t = 0 rises about 108 mL over 20 s, so the powder's initial rate is about 108 ÷ 20 ≈ 5.4 mL/s, three times its average over the first minute. The rate is fastest at the start, when there is most marble and acid, and falls as they are used up, so an average over a long time hides how fast the reaction began.
Collision theory
Collision theory explains rates. For particles to react they must:
- collide with each other;
- collide with at least the activation energy, so that bonds can start to break; and
- collide in the right orientation, so the atoms that need to bond actually meet.
Only a tiny fraction of collisions meet all three conditions. In a gas at room temperature, each molecule collides billions of times a second, yet many reactions take minutes. For example, when hydrogen iodide decomposes (2HI → H₂ + I₂), two HI molecules must meet hydrogen end to hydrogen end, hard enough to stretch their bonds. They briefly form an unstable arrangement, sometimes called an activated complex, which then either falls apart back into HI or forms H₂ and I₂.
Anything that increases the number of successful collisions per second increases the rate.
What changes the rate?
| Factor | Effect | Explanation |
|---|---|---|
| Concentration (solutions) or pressure (gases) | Higher means faster | More particles in the same volume, so they collide more often. |
| Temperature | Higher means faster; for many reactions the rate roughly doubles for every 10 °C rise | Particles move faster, so they collide more often, and, more importantly, a much larger fraction of collisions have at least the activation energy. |
| Surface area (solids) | Smaller pieces mean faster | Only particles on the surface can collide. Cutting a solid into smaller pieces exposes more of them. |
| Catalyst | Faster | Provides a different pathway with a lower activation energy, so more collisions are successful. The catalyst is not used up. |
| Stirring | Faster | Brings fresh reactant particles to the surface of a solid and spreads them through a solution. |
| Light (some reactions) | Faster | Light supplies energy to break bonds, as in photosynthesis, the fading of dyes and the reaction of hydrogen with chlorine. |
- Run the reaction at 20 °C with the default number of particles. Watch the counter: what fraction of collisions are successful?
- Reset and run at 50 °C. Compare the percentage of collisions that are successful with your 20 °C run. Did the particles speed up much? Why does the product graph rise so much faster?
- Back at 20 °C, double the number of particles. What happens to the collision rate and to the shape of the product graph?
- Turn on the catalyst. Does it change how often particles collide, or how many collisions succeed?
- Let any run finish. Why does the product graph always level off?
- Run large chips in 1 M acid at 20 °C. Record the initial rate (it should be about 1.2 mL/s) and the volume of gas after 10 minutes.
- Change only the chip size to powder. How many times faster is the initial rate? Does the final volume change? Why not?
- Go back to large chips and change the acid to 2 M. What happens to the initial rate?
- Raise the temperature from 20 °C to 30 °C, then 40 °C. Does the "doubles every 10 °C" rule hold in this model?
- Plan a fair test of the effect of temperature: which variables would you keep the same?
Controlling rates in everyday life
Sometimes we want reactions faster, sometimes slower:
- Food: fridges and freezers slow the reactions that spoil food and the growth of microbes. Cooking speeds up reactions; a pressure cooker works faster because water boils at a higher temperature under pressure.
- Industry: a faster reaction makes more product per hour from the same equipment. Chemical engineers raise temperatures, grind solids, add catalysts and use high pressures, but each costs money for energy and stronger equipment, so they choose the best compromise (4E).
- Safety: fine powders such as flour, sugar, coal dust and sawdust have huge surface areas and can explode if a cloud of them is ignited. Mills and mines control dust, ventilate and remove sparks.
- Bushfires: fine fuels such as dry grass, leaf litter and twigs burn far faster than logs because of their surface area. First Nations fire practitioners have long used frequent, small, "cool" burns to reduce build-up of fine fuels in a patchwork across the landscape. Fire agencies in several states now work with Aboriginal rangers to bring back cultural burning.
Test yourself
According to collision theory, particles react only if they collide
Energy and orientation both matter.
On a graph of gas volume against time, the reaction is fastest
Reactant concentrations are highest at the start, so the gradient is steepest.
Powdered marble reacts faster with acid than marble chips because the powder
More particles are exposed to acid particles.
Increasing the temperature by 10 °C often roughly
A useful rule of thumb for many reactions near room temperature.
Two runs use the same mass of marble with excess acid, one with chips and one with powder. The final volume of CO₂ is
The same amount of marble makes the same amount of gas, just more slowly for chips.
Label the rate graph.
Will each change make the reaction faster or slower?
For a reaction to happen, particles must with enough energy. A higher means particles collide more often. A lowers the activation energy. Cutting a solid into smaller pieces increases its area. The rate at any moment is the of a graph of product against time.
Food scientists found that milk goes sour about twice as fast at 15 °C as at 5 °C. Use collision theory to explain why reactions are faster at higher temperatures, and explain why storing milk in a fridge makes it last longer.
At a higher temperature, particles have more kinetic energy and move faster. They collide more often, but more importantly a much larger proportion of the collisions have at least the activation energy, so many more collisions per second are successful and the reaction rate increases. Milk goes sour because bacteria convert lactose into lactic acid using enzyme-controlled reactions. In a fridge at about 4 °C the particles move more slowly, fewer collisions have enough energy to react, and the bacteria grow and react much more slowly. Lactic acid builds up more slowly, so the milk takes longer to sour and lasts several days longer.