Reaction Kinetics

Understandings:

• Species react as a result of collisions of sufficient energy.

• The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

Rate of Reaction

As you can see from the above graph - either of the definitions can apply to rate and either would give you the same number. This is because the Change in Concentration (this is the difference between the starting concentration and the concentration at a fixed time after the reaction starts) of either the reactants or products should be relatively equal.

The graph also shows that as a reaction proceeds and the concentration of reactants decreases this would cause a change in rate as rate decreases over time as the concentration of reactants decreases

Collision Theory

In the above animation you can see how particles of a gas move at a constant temperature. Some of these particles are colliding in the animation to make something new - some of these particles are colliding and not reacting.

A collision which produces a new substance is called a successful collision

A collision which does not produce a new substance is an unsuccessful collision - an assumption of collision theory is that most particle collisions are unsuccessful.

As Rate of Reaction measures the change of reactants into products then rate of reaction measures the frequency of successful collisions. Any factor that can increase these collisions would therefore speed up rate and any factor that can decrease these collisions would slow the rate down.

A rate is a measure of how some property varies with time. Speed is a familiar rate that expresses the distance traveled by an object in a given amount of time. Wage is a rate that represents the amount of money earned by a person working for a given amount of time. Likewise, the rate of a chemical reaction is a measure of how much reactant is consumed, or how much product is produced, by the reaction in a given amount of time.

The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time dependence of some property that can be related to reactant or product amounts. Rates of reactions that consume or produce gaseous substances, for example, are conveniently determined by measuring changes in volume or pressure. For reactions involving one or more colored substances, rates may be monitored via measurements of light absorption. For reactions involving aqueous electrolytes, rates may be measured via changes in a solution’s conductivity.

For reactants and products in solution, their relative amounts (concentrations) are conveniently used for purposes of expressing reaction rates. If we measure the concentration of hydrogen peroxide, H₂O₂, in an aqueous solution, we find that it changes slowly over time as the H₂O₂ decomposes, according to the equation:

The rate at which the hydrogen peroxide decomposes can be expressed in terms of the rate of change of its concentration, as shown here:

This mathematical representation of the change in species concentration over time is the rate expression for the reaction. The brackets indicate molar concentrations, and the symbol delta (Δ) indicates “change in.” Thus,

[

H

2

O

2

]

t

1

[H2O2]t1

represents the molar concentration of hydrogen peroxide at some time t₁; likewise,

[

H

2

O

2

]

t

2

[H2O2]t2

represents the molar concentration of hydrogen peroxide at a later time t₂; and Δ[H₂O₂] represents the change in molar concentration of hydrogen peroxide during the time interval Δt(that is, t₂ − t₁). Since the reactant concentration decreases as the reaction proceeds, Δ[H₂O₂] is a negative quantity; we place a negative sign in front of the expression because reaction rates are, by convention, positive quantities. Figure 1 provides an example of data collected during the decomposition of H₂O₂.