RC3

You have examined a range of chemical reactions and why some reactions occur spontaneously, whilst others do not react. In the final section, we look at the rate of a chemical reaction and the factors which can increase or decrease reaction rate.

Rate of reaction = change in amount of reactant or product / time

Using the equation above, what units would be suitable for measuring the rate of a reaction? (Hegarty, 2018, p. 37)

Collision Theory

To really understand what is happening in Chemical reactions, we need to understand and apply collision theory. For a reaction to occur the reactant particles must collide with each other with sufficient energy to break the bonds within the reactants. Often this collision must occur in a particular way in order to ensure that broken bonds don’t just reform but there is a new substance(s) created, through the formation of new bonds.

The minimum amount of energy required to break the bonds within the reactant substances is known as the energy of activation, or simply Activation Energy.

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• Enthalpy PPT

3.2 explain a change in reaction rate using collision theory

3.3a conduct a practical investigation, using appropriate tools (including digital technologies), to collect data, analyse and report on how the rate of a chemical reaction can be affected by a range of factors, including but not limited to:

• surface area of reactant(s)

Temperature is not the only factor which affects reaction rate. The size of the particles and the surface area of reactants can affect the rate of reaction.

One way to think about the effect of surface area is to change a crystal into a powder. How does a regular object change when it is cut into smaller and smaller pieces?

Think of a cube; 12 x 12 x 12 cm. Calculate its surface area and its volume.

Cut the cube into smaller cubes 6 x 6 x 6 cm. How many of these cubes can you make from the original? What are the new surface area and volumes?

Cut the original cube into 1 x 1 x 1 cm cubes. How many of these cubes can you make from the original? What are the new surface area and volumes?

When we increase the surface area of a reactant, more individual particles, ions, atoms, molecules, etc are exposed to any other reactant. This increases the number of collisions and hence the rate of the reaction.

The reaction can take place more quickly, more energetically, or both.

3.2 explain a change in reaction rate using collision theory

3.3b conduct a practical investigation, using appropriate tools (including digital technologies), to collect data, analyse and report on how the rate of a chemical reaction can be affected by a range of factors, including but not limited to:

• concentration of reactant(s)

Another factor that can increase the chances of collisions between reacting particles is the amount of the substance in the container (concentration). The higher the concentration [ ], the higher the chance of collisions, and therefore of successful collisions. Therefore, the higher the reaction rate.

CHEMICAL KINETICS:

Some reactions are faster than others. The study of reaction rates is called chemical kinetics. Several factors affect the rate of a reaction. The determining action for any chemical reaction is the collision between reactant particles.

All moving particles have kinetic energy. Particles in the gaseous phase move freely and more rapidly than particles in the solid phase. They have greater kinetic energy.

Particles in a container collide with one another. If the collisions have sufficient energy to break the bonds, they will react. The greater the number of collisions between reactant particles and the greater the energy of the particles

The stability of a chemical substance is related to the energy in the substance. This may be in the form of potential energy related to the phase of the substance and its pressure and density, or kinetic energy, the random motion of atoms and molecules. The total of all of these forms of energy is called the heat content or enthalpy of the substance. The chemical bonds in substances store energy. This energy may become available when the substance reacts. Enthalpy is the sum of the kinetic and potential energy of one mole of particles of a specified substance. This branch of chemistry is known as thermochemistry.

Raising the temperature of a substance contributes heat energy to the particles. The particles speed up as a result as the heat energy is converted into kinetic energy.

Arrhenius explained chemical kinetics by proposing that a reaction would occur if the sum of the kinetic energies of reactants is greater than the activation energy. Adding heat increases the kinetic energy of the particles, resulting in a higher proportion of particles having a KE > E_A.