Rates of reaction - (CCEA)

• The rate of reaction can be determined by measuring the rate of gas production, mass loss or precipitate formation in a chemical reaction.

• A rate of reaction is determined by how often successful collisions of particles take place, which are collisions that possess the activation energy.

• The rate of reaction is affected by changing concentration, temperature, surface area or by using a catalyst.

The experiment you carry out to measure the rate of reaction depends on the nature of the product being measured.

In reactions that produce a gas, the volume of a gas can be measured with a gas syringe (or an upside down measuring cylinder).

The following method is carried out:

• The reactants are mixed together in a conical flask and a gas syringe is connected to the flask.

• At the same time, a stop-clock is started.

• The scale on the gas syringe is used to record the volume of gas produced at regular time intervals.

• A graph of ‘volume of gas produced/ cm³**' against 'time / s' is plotted.

Examples

Examples of reactions that would be suitable for this method are the reactions of magnesium, zinc or calcium carbonate with hydrochloric acid where hydrogen or carbon dioxide gas is produced.

Mg + 2HCl → MgCl₂ + H₂

Zn + 2HCl → ZnCl₂ + H₂

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

In reactions that produce a gas, an alternative method of measuring the rate of reaction is to measure the mass of the reacting mixture during the reaction.

As gas escapes from the reacting mixture the mass of mixture will decrease over time.

The following method is carried out:

• The reactants are mixed together in a conical flask and the flask is placed on a balance.

• The initial mass of the reacting mixture is measured, and at the same time a stop-clock is started.

• The mass of the reacting mixture is measured at regular time intervals.

• A graph of ‘mass lost / g’ against ‘time / s’ is plotted.

The cotton wool prevents any loss of mass caused by liquid spray.

Examples

Examples of reactions that would be suitable for this method are the reactions of magnesium, zinc or calcium carbonate with hydrochloric acid where hydrogen or carbon dioxide gas is produced.

Mg + 2HCl → MgCl₂ + H₂

Zn + 2HCl → ZnCl₂ + H₂

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

In a typical rates experiment, the mass or volume of product is measured at regular time intervals.

The results are usually recorded in a table.

For example:

These results show that the rate decreased during the reaction as the volume of gas produced each minute decreased as the time increased.

The reaction had finished by four minutes as no more gas was produced after that point.

Graphs

The rate of reaction can be analysed by plotting a graph of ‘volume of gas’ or ‘mass lost’ against time. The graph below shows an example for two reactions.

For both reactions, the graph lines show that:

• The reactions become slower as time goes on (the gradients of the lines decrease).
• The reactions have stopped at the points that the lines become horizontal.

Compared to the slow reaction, the graph line for the faster reaction:

• has a steeper gradient at the start
• becomes horizontal sooner (gas stops being produced sooner so the reaction finishes sooner) meaning that the rate of reaction is greater.

Collision theory

For a chemical reaction to occur, the reactant particles must collide with each other.

However, a collision with too little energy will not produce a reaction:

The colliding particles must have enough energy for the collision to be successful in producing a reaction.

The minimum amount of energy for a collision to be successful is called the activation energy.

The rate of a reaction depends on the rate of successful collisions between reactant particles.

The more successful collisions there are, the faster the rate of reaction.

The rate of a chemical reaction can be changed by altering the temperature. If the temperature is increased:

• The reactant particles have more energy.

• They move more quickly and collide more often.

• There are more successful collisions per unit time; and

• the rate of reaction increases.

The rate of a chemical reaction can be changed by altering the concentration of a reactant in solution. If the concentration is increased:

• There are more reactant particles.

• They collide more often.

• There are more successful collision per unit time; and

• the rate of reaction increases.

The rate of a chemical reaction can be increased by using smaller solid particles which increases the surface area of a solid reactant.

This is done by cutting the substance into small pieces, or by grinding it into a powder. If the surface area of a reactant is increased:

• More particles are exposed to the other reactant.

• They collide more often.

• There are more successful collisions per unit time; and

• the rate of reaction increases.

This shows that the rate of reaction is greater when the surface area is increased.

Key fact

‘Per unit time’ is used in place of ‘per second’ or ‘per minute’ when it does not matter which unit of time you are referring to.

The rate of a reaction can be increased by adding a suitable catalyst.

A catalyst is a substance which increases the rate of a chemical reaction but it is not used up (remains chemically unchanged at the end).

• A catalyst speeds up a reaction by providing an alternative reaction pathway of lower activation energy.

• There are more successful collisions per unit time; and

• the rate of reaction increases.

This alternative reaction pathway can be shown on an energy level diagram.

Hydrogen peroxide decomposes to produce water and oxygen gas.

2H₂O₂ → 2H₂O + O₂

This reaction is normally very slow, but the rate can be increased by adding a manganese(IV) oxide catalyst.

Manganese(IV) oxide is a black solid and can be removed from the reaction mixture at the end by filtration.

The rate of this reaction can be investigated by measuring the volume of oxygen gas produced (using a gas syringe) or by measuring the mass lost from the reaction mixture.

Please use the link below to access the article on: Prescribed Practical C6 - Investigate how changing a variable changes the rate of reaction

(https://truemac.vn/bitesize/articles/zxdctcw)