Effect of solution concentration on reaction rate

A more concentrated reactant solution contains more reactant particles in the same volume. When these particles are more crowded, they collide with particles of the other reactant more frequently. This generally increases the number of successful collisions each second, so the reaction rate increases.

Diluting a reactant solution reduces the number of its particles in a given volume. Collisions between reactants then occur less frequently, so the reaction generally becomes slower.

This comparison assumes that other conditions remain the same. The effect comes from changing the number of reactant particles in a given volume, not from changing the nature of the particles themselves.

Effect of gas pressure on reaction rate

When gaseous reactants are compressed into a smaller volume, their pressure increases. The particles are closer together, so reactant particles collide more often. More collisions each second give them more opportunities to react, so the reaction rate generally increases.

Reducing the pressure by allowing the gas to occupy a larger volume has the opposite effect. The particles spread out, collide less often, and the reaction generally slows. This comparison assumes that other conditions, such as temperature, remain the same.

Effect of solid surface area on reaction rate

When a solid reacts with particles of another reactant, those particles can collide with the solid only at its exposed surface. Increasing the exposed surface area makes more of the solid available for collisions at the same time. This generally increases the number of successful collisions per second, so the reaction is faster.

For example, breaking a solid into smaller pieces exposes surfaces that were previously inside it. The same mass of solid therefore has a greater exposed surface area than it did as one large piece and will generally react faster under the same conditions.

Reducing the exposed surface area has the opposite effect: less of the solid is available for collisions at once, so the reaction generally becomes slower if other conditions remain the same.

Effect of temperature on reaction rate

Increasing the temperature generally makes a reaction faster. Reactant particles gain kinetic energy and move faster, so they collide more frequently.

Not every collision produces a reaction. The particles must collide with enough energy for the reaction to occur. At a higher temperature, a greater proportion of collisions have enough energy to be successful. The combination of more frequent collisions and a greater proportion of successful collisions increases the reaction rate.

Lowering the temperature generally slows the reaction. Particles move more slowly, collide less frequently and a smaller proportion of collisions have enough energy to cause a reaction. Successful collisions therefore occur less frequently.

Catalysts and reaction rate

A catalyst is a substance that increases the rate of a reaction without being used up. It is unchanged at the end of the reaction and can be used again.

Under otherwise comparable conditions, a reaction with a catalyst is faster than the same reaction without it. If the catalyst is removed, its rate-enhancing effect is removed: the reaction is no longer sped up by that catalyst.

Enzymes are catalysts that increase the rates of biological reactions. Like other catalysts, they are not used up in the reactions they speed up.

Investigating reaction rate experimentally

As a reaction proceeds, reactants are used up and products form. To investigate its rate, measure a quantity that changes as the reaction proceeds and record the time of each measurement. The rate describes how much that quantity changes per unit time.

Start timing when the reaction begins, then take measurements at successive times. The change between any two readings and the time elapsed between them give the average rate over that interval.

Measuring mass during a reaction

The mass of a reactant decreases as it is used up, while the mass of a product increases as it forms. Measuring one of these changes at known times provides data for a rate calculation.

If a reaction produces a gas that escapes, another method is to place the reaction vessel on a balance and record its mass at successive times. The measured mass falls as gas leaves the vessel. The mass lost during an interval, divided by the length of that interval, gives the average rate of mass loss.

Measuring gas produced

When a reaction forms a gas, the gas can be collected in a gas syringe. Record its volume at successive times. The increase in gas volume during an interval, divided by the length of that interval, gives the average rate of gas production.

For example, if the gas volume rises from $$24\ \text{cm}^3$$ to $$42\ \text{cm}^3$$ over $$30\ \text{s}$$, the volume increase is $$18\ \text{cm}^3$$. The average rate over that interval is $$0.6\ \text{cm}^3/\text{s}$$.

Repeating the calculation for different intervals shows whether the measured rate changes as the reaction proceeds.

Interpreting reaction rate data and graphs

Reaction rate data show how a measured amount changes with time. For example, the amount of a reactant may decrease, while the amount of a product may increase. A faster reaction produces a larger change in the measured amount in the same time.

Calculating rate over an interval

The average rate over a time interval is the size of the change in the measured amount divided by the time taken:

For example, if a measured volume decreases from $$50\ \text{cm}^3$$ to $$30\ \text{cm}^3$$ in $$10\ \text{s}$$, the size of the change is $$20\ \text{cm}^3$$. The average rate over that interval is:

The rate is given as a positive size even though the measured volume decreases. Its units depend on the amount measured and the time unit used.

Reading rate from a graph

On a graph of measured amount against time, the slope shows how quickly the amount changes. A steeper slope means a faster rate, whether the line rises or falls. An upward slope shows an increasing amount; a downward slope shows a decreasing amount.

If a curve becomes less steep, the amount changes more slowly, so the rate is slowing. A level section has zero slope: the measured amount is no longer changing.