Factors affecting reaction rate

Rate and collisions

The rate of reaction tells you how fast reactants are used up or products are formed. A fast reaction finishes in a short time. A slow reaction takes a long time.

For two particles to react they must collide. Not every collision works. The particles must hit each other with enough energy to break bonds. The minimum energy needed is called the activation energy.

So there are two ideas to remember:

• more collisions per second means a faster reaction (higher frequency of collisions);

• collisions with more energy are more likely to be successful.

Every factor on this page changes one or both of these.

We can measure the rate in two common ways:

The first is used when you time how long something takes to finish (for example, a mark disappearing). The second is used when you collect a gas. A shorter time means a bigger rate.

Concentration and surface area

Concentration

Concentration is how much solute is dissolved in a given volume, for example in $\text{mol/dm}^3$. In a more concentrated acid there are more acid particles in the same space. Particles are closer together, so they collide more often. More collisions per second means a faster reaction.

Look at the results of a magnesium and acid experiment:

Doubling the concentration halves the time, so it doubles the rate. This pattern (the rate is proportional to the concentration) is true for many simple reactions, but not all.

Surface area

Only particles at the surface of a solid can be hit by the particles in the liquid or gas. If you break a solid into smaller pieces, you expose more surface. More of the solid is in contact with the other reactant, so there are more collisions per second.

A marble chip (calcium carbonate) reacts slowly with acid, but the same mass of marble powder reacts much faster. The total mass is the same, but the powder has a far larger surface area.

Temperature

Think of food in a fridge and food left on the counter. Cold slows down the chemical reactions that make food go off.

When you heat a mixture, the particles gain kinetic energy and move faster. This has two effects:

1. They collide more often.

2. A greater fraction of collisions has energy equal to or above the activation energy, so more collisions are successful.

The second effect is the bigger one. That is why a small rise in temperature can make a big difference.

A common investigation uses sodium thiosulfate and acid. A cloudy yellow precipitate of sulfur forms, and you time how long it takes for a cross under the flask to disappear.

Each rise of 10 °C halves the time. As a rough rule, many reactions double in rate for every 10 °C rise.

Catalysts

A catalyst is a substance that speeds up a chemical reaction but is not used up in it. It is chemically the same at the end as at the start.

A catalyst provides an alternative pathway with a lower activation energy. More collisions now have enough energy to react, so the rate goes up. The temperature does not change.

Examples:

• manganese(IV) oxide, $\text{MnO}_2$, speeds up the breakdown of hydrogen peroxide;

• the catalytic converter in a car uses metals such as platinum to turn harmful exhaust gases into less harmful ones;

• enzymes are biological catalysts, such as the amylase in your saliva.

Hydrogen peroxide decomposes slowly:

With $\text{MnO}_2$ it fizzes quickly. The $\text{MnO}_2$ can be filtered off, dried and weighed. Its mass is unchanged, which shows it was not used up. The oxygen relights a glowing splint.

Following a reaction over time

When a reaction produces a gas, you can collect it in a gas syringe and record the volume at regular times. Plotting volume against time gives a curve.

The curve is steepest at the start and then flattens:

• Steep at the start: reactants are at their highest concentration, so collisions are most frequent and the rate is greatest.

• Getting less steep: reactants are being used up, so collisions become less frequent and the rate falls.

• Flat: one reactant has been completely used up. The reaction has stopped, and no more product forms.

The average rate between two times is the change in volume divided by the change in time.

When a metal reacts with an acid, the products are a salt and hydrogen gas:

For example, zinc + sulfuric acid → zinc sulfate + hydrogen, or $\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2$. The equation is balanced because each side has 1 Zn, 2 H, 1 S and 4 O. Test for hydrogen: a lighted splint makes a squeaky pop.

A final volume that is the same in two experiments means the same amount of limiting reactant was used (the limiting reactant is the one that runs out first, so it decides how much product can form). Faster conditions (higher temperature, more concentrated acid, powder) change how quickly you get there, not how much gas you finally get. A smaller amount of the limiting reactant, for example a more dilute acid that runs out, gives a smaller final volume.

Summary