Rate of Reaction

Rate of Reaction

the change in concentration in unit time of any product or reactant

Units

mol l -1 s -1

Average Rate

the change in concentration in a certain amount of time

Instantaneous Rate

the rate at a particular point in time during the reaction

Factors Affecting the Rate

1. Natura of Reactants

2. Surface Area / Particle Size

3. Concentration

4. Temperature of the reaction

5. Catalysts

Nature of Reactants

The rate will depend on the bonds involved in the reaction. Some bonds may be easier to break and form then others.

Reactions involving covalent compounds

Tend to be slow. Bonds have to be broken before new bonds can be formed.

Reactions involving ionic compounds

Normally take place in aqueous solution. When dissolved in water, ionic compounds are pulled into their ions. So, when the reactants are mixed, their bonds are already broken and the ions react when they collide.

Particle Size / Surface Area

The more finely divided the solid, the faster the reaction. In finely divided solids the surface area available for reaction is greater.

Concentration of Reactants

An increase in the concentration of reactants usually increases the rate of reaction

Temperature

In most cases, an increase in temperature increase the rate.

(An increase of 10K can cause the reaction to double)

Increasing the temperature of exothermic reactions too much, can lead to explosions.

Catalyst

A substance that alters the rate of a chemical reaction but is chemically unchanged at the end of the reaction

Types of Catalysts

Homogeneous

Heterogeneous

Homogeneous

All reactants and catalysts are in the same physical phase

Heterogeneous

Catalyst is in a different physical phase to the reactants

Autocatalysis

Occurs when a product of a reaction increases the rate of reaction