Lesson

Rates of Reaction

Collision Theory

IB Chemistry Power Points

Topic 06

Kineticswww.pedagogics.ca

Technically, the rate of a chemical reaction is a measure of the change in [concentration] per unit time:

a decrease in [reactant] per unit time

OR

an increase in [product] per unit time

Rate of Reaction – Data analysis exercise

rate = -D[reactant]

Dtrate =

D[product]Dt

Rates of reaction can be determined in different ways. For example:

1. a decrease in mass can be measured if the reaction has a gas product

2.The volume of gas produced can be measured if the reaction has a gas product

3.absorption of light can be measured if there is a color change in the reaction mixture

4.can also measure changes in conductivity or pH

Measuring Rate of Reaction

Graphs of concentration vs time are plotted to illustrate how reactions proceed.

Rate of Reaction

For example: consider the following reaction A B

rate = -D[A]Dt

rate = D[B]Dt

time

Rate of Reaction – Instantaneous vs Average Rates

Rate of Reaction – Instantaneous vs Average Rates

An average rate would be determined by the change in concentration over a longer time interval. This is represented by the slope of the dashed line

The rate of a reaction changed over time!

Kinetic Molecular Theory – a review

Basic KMT- particles are in constant random motion (have

KE)

- absolute temperature is proportional to kinetic energy

Solid Liquid Gas

Enthalpy Review

The activation energy is the energy difference between the reactants and the minimum energy required for the reaction. In an endothermic reaction

Enthalpy ReviewAnd in an exothermic reaction

Label (a), (b) and (c)

Collision Theory

Collision Theory

If a reaction between the particles of two reactants occurs:- particles must collide with correct orientation

(geometry)

- there must be sufficient energy to initiate the reaction E>Ea

NOT all collisions result in reactions!

Collision Theory

According to collision theory, increasing a reaction rate would result by increasing the number of SUCCESSFUL collisions per unit of time.

To increase a reaction rate- increase the frequency of collisions

- increase the energy of collision

Factors affecting the rate of reactionThere are four factors that affect the rate

of a chemical reaction. Each effect can be explained using collision theory. The 4 factors are: Concentration

Surface Area

Temperature

Catalysts

Concentration

The more concentrated the reactants are, the more particles there are per unit volume of the reaction medium (the mixture of the reactants).

More particles means a greater collision frequency (more collisions per unit time) and therefore a greater reaction rate.

When one reactant is in solid pieces, the other reaction can only occur at the surface.

No reaction occurs

If the solid pieces are subdivided, there is more surface area and more collisions per unit time. Therefore the reaction occurs faster.

Surface Area

Temperature

Increasing temperature increases the rate of reaction for two reasons.

1. frequency of collision increases because particles are moving faster.

2. increase in energy which increases the proportion of successful collisions – a larger % of particles have E > Ea

Temperature - Maxwell Boltzmann curves

Recall, a temperature measurement represents the average kinetic energy of a group of molecules.

As temperature increases, the proportion of

molecules with the required velocity

(minimum energy) also increases

Catalysts

Catalysts increase reaction rates by lowering activation energy. When a catalyst is present, the proportion of successful collisions increases because more molecules have sufficient energy to react.

Temperature, Concentration and Surface Area effects on reaction rates.