Rate of reaction

Often, when chemicals are mixed together they react too slowly for the synthesis to be economic, or too quickly for the reaction to be safe. The rate of synthesis reactions needs to be controlled so that the product is produced as quickly (therefore cheaply) and safely as possible.

Following the rate of a reaction

In the exam you will be expected to give examples of reactions with different rates and to explain how the rate of a reaction could be measured.

Different reactions can happen at different rates. Reactions that happen slowly have a low rate of reaction and reactions that happen quickly have a high rate of reaction. For example, the chemical weatheringweathered: Weathering is the gradual breaking down of rocks by rainwater and changes in temperature. of rocks is a very slow reaction: it has a low rate of reaction. Explosions are very fast reactions: they have a high rate of reaction.

Reactants and products

The rate of a reaction can be measured by either:

  1. Measuring the rate that a reactant [reactant: Substances present at the start of a chemical reaction ] is used up, or
  2. Measuring the rate that a productproduct: A product is a substance formed in a chemical reaction. is formed.

The formation of product or consumption of reactant could be measured by observing a change of colour or the formation of a precipitate. Sometimes it is easier to measure the change in the amount of a reactant that has been used up, and sometimes it is easier to measure the change in the amount of a product that has been produced.

Things to measure

The measurement itself depends upon the nature of the reactant or product:

  • The mass of a substance (solid, liquid or gas) is measured with a balance.
  • The volume of a gas is usually measured with a gas syringe, or sometimes with an upside down measuring cylinder or burette.

It is usual to record the mass or total volume at regular intervals and to plot a graph. The readings go on the vertical axis and the time goes on the horizontal axis.

Factors affecting rate of reaction

In the exam you will need to know the factors that affect the rate of reactions, and how to plot or interpret graphs from rate experiments.

The rate of a reaction increases if:

  • the temperature is increased
  • the concentration of a dissolved reactant is increased
  • solid reactants are broken into smaller pieces
  • a catalyst is used
Graph showing rates of reaction under changing conditions. At a lower temperature, lower concentration, or with larger pieces, the rate of reaction is slower than at higher temperatures, higher concentrations, or with smaller pieces

Rate of reaction and changing conditions

The above graph summarises the differences in the rate of reaction at different temperatures, concentrations and size of solid pieces. The steeper the line, the greater the rate of reaction. Reactions are usually fastest at the beginning, when the concentration of reactants is greatest. When the line becomes horizontal, the reaction has stopped.

A catalyst is a chemical that speeds up the rate of a reaction without being used up in the chemical changes that take place. The addition of a catalyst often allows a reaction to take place quickly at a lower temperature, saving money on fuel.

Read on if you are taking the Higher paper.

Collision theory - Higher

In a chemical reaction, the reactant particles can only react with each other when they bump into one another. According to collision theory when molecules collide bonds between their atoms can break, and then new bonds can form to make new molecules.

The molecules in gases and liquids are moving constantly, and millions of collisions take place every second. But only a small number of these collisions lead to the formation of product. For a collision to be 'successful', the particles involved must possess enough energy, called the activation energy, to break some of the existing bonds.

Any change that increases the number of collisions per second, or increases the energy of the particles that are colliding, will increase the rate of reaction.

If the temperature is increased, there will be more energy in the collisions. More collisions will have the activation energy, resulting in an increase in the rate of reaction.

If the concentration of one or more of the reactants is increased, there will be more collisions per second, resulting in an increase in the rate of reaction.

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