Rates of reactions

Chemical reactions occur in a variety of situations with a wide range of different reaction types. Some chemical reactions occur extremely quickly, such as the explosion of a firework. Other chemical reactions happen at a more moderate speed, such as the biological reactions that occur during digestion and metabolism. Chemical reactions can also occur over very long time frames, such as when iron rusts.

Understanding the range of factors that influence rates of reactions allows chemists to design chemical processes that occur as quickly as possible, while still occurring efficiently. This is an important part of industrial chemistry.

Use this page to revise the following concepts within rates of reactions:

Measuring rates of reactions

When a chemical reaction occurs, chemists can measure how fast it is occurring by tracking the change of reactants into products. A rate is a measure of how one quantity changes over a period of time. The reaction rate is often measured in M/s or mol/L/s.

Note

Reaction rates are commonly measured as the change in reactant or product concentration over time. M/s is the most common measure of reaction rate, but any measure of the reaction progress can be used. Examples could include a change in mass (when a reactant is converted to a gas which is lost to the system), a change in volume (if gas evolved is measured), or a change in pH (if there is an acidic or basic reactant or product). Any measure that correlates to reactants or products can be used to find a rate of reaction.

A common high school chemistry experiment is shown below, where calcium carbonate (CaCO3) is reacted with hydrochloric acid (HCl) to produce carbon dioxide gas. The gas escapes from the conical flask and therefore the mass decreases as the experiment proceeds.

Diagram of experiment showing a conical flask sitting on a balance containing marble chips in dilute hydrochloric acid. The flask has cotton wool over the neck.

This might generate experimental results as shown below.

A similar graph would be produced if this was measuring the volume of carbon dioxide produced, rather than mass lost. These graphs show the change in the rate of reaction. Initially it is very quick, and then it begins to slow.A graph with time (min) on the x-axis and volume of gas (ml) on the y-axis. It it initially very steep, but as the reaction proceeds, it begins to taper out, eventually reaching a plateau

If we change various parameters in an experiment, such as the concentration of the acids, then the rate of the reaction changes. The higher the concentration of reactants, the higher the rate of reaction. These factors will be explained further below.

Collision theory

Collision theory explains that three conditions determine whether a chemical reaction will be successful:

  1. There needs to be a collision between chemicals.
  2. The collision needs to be in the right orientation for the reaction to be successful.
  3. There must be sufficient energy in the colliding particles for the reaction to proceed (activation energy ).

Collision theory allows us to explain the rate of reaction. Changes in the collisions between chemicals lead to changes in the rate of reaction. More collisions per time gives a faster rate, and vice versa.

Orientation

For a chemical reaction to occur, particles must collide with the correct orientation. This is similar to how a key enters a keyhole which only works if it happens in the correct orientation.

Diagram showing how orientation of chemicals affects the reaction. In the first scenario, chemical A collides with the B element of chemical BC in the correct orientation, forming chemical AB + C. In the second scenario, the chemicals are in the wrong orientation. A collides with the C element of chemical BC. No reaction occurs as the orientation is incorrect

Activation energy

The minimum energy required for a chemical reaction to occur is known as the activation energy. This energy is required to break the bonds in the reactants. At this energy, the reactants enter a new state known as the transition state, an intermediate stage before the products are formed. Substances in a transition state continue to react and then form products and, in doing so, become more stable and lower in energy.

The activation energy determines how easy it is for a given reaction to occur. The lower the activation energy, the more likely the reaction. We can represent this information in the energy profile diagrams illustrated below.

Two energy profile diagrams plotting energy level against reaction progress. The first, labelled "Exothermic Reaction," shows reactants starting at a higher energy level than the products, with a curve rising to a peak and then dropping to the products' lower energy level. The second, labelled "Endothermic Reaction," shows reactants starting at a lower energy level than the products, with a curve rising to a peak and then leveling off at the products' higher energy level. The difference between the reactant energy level and the peak is labelled Ea (activation energy)