Chemical Kinetics (Rates of Reactions)

The rate of a reaction

Is the change in concentration of a reactant or a product per unit time

The average rate of reaction

Is calculated by dividing total volume by total time. Average rate = 200cm3/210s = 0.95cm3/s

The instantaneous rate of a chemical reaction

Is the rate of that reaction at one moment in time. Draw a tangent to find it.

5 factors that can change the rate of a chemical reaction

1. Nature of Reactants

2. Particle Size

3. Concentration

4. Temperature

5. Presence of a Catalyst

Ionic substances

Fast reactions as lower activation energy is need as only the attractive force between ions needs to be broken

Covalent substances

Slow reactions as higher activation energy is need as bonds need to be broken in order to react

Particle size

Small particle size = greater surface area. Greater surface area = more collisions and more effective collisions. This results in a higher rate of reaction

Concentration

More particles increase the number of effective collisions, therefore, the rate increases. As a reaction proceeds the concentration of the reactants will decrease and so the rate of the reaction will decrease with time.

Temperature

Increase in the number of collisions due to the increased energy of the particles. Increased energy of the collisions means that more collisions reach activation energy

A catalyst

Is a substance that alters the rate of a chemical reaction whilst remaining chemically unchanged.

Heterogenous Catalysis

Where the reactants and the catalyst are in different phases

Homogenous Catalysis

Where the reactants and the catlayst are in the same phase

Autocatalysis

Where the catalyst is one of the products produced by the reaction itself.

Intermediate Compound Formation Theory

One of the reactants forms an intermediate compound with the catalyst. The new compound reacts with the other reactant at a greater rate. As the product is formed the intermediate compound is broken down. The catalyst is returned chemically unchanged. Eg potassium sodium tartarate + Co + H2O2

Surface Adsorption Catalysis

The reactants, A and B, build up on the surface of the solid catalyst. (Adsorption) This increases their local concentration. Higher concentrations of reactants give rise to faster reaction rates. Product, C leaves the surface of catalyst (Desorption). Eg Pt solid catalyst used to add H2 to substances

Collision Theory

When a chemical reaction occurs as a result of a collision such a collision is called an effective collision.

Activation Energy

The minimum amount of energy required to produce an effective collision is called the activation energy of the reaction.

Endothermic reaction

Heat taken into the reaction from the environment. Products have more energy than reactants.

Exothermic reaction

Heat given out into the environment. Products have less energy than reactants.