Topics
Introduction of rate laws
Orders of reactions
Determining rate experimentally
a) Graphical method
b) Method of initial rates
Reaction mechanisms

Exponents and Orders of reactions

The values of exponents (m or n) can only be determined through experimentation and may or may not be the values of the coefficients in the balanced chemical equation. The exponent values are usually 0, 1 or 2 but may be 3 or in some cases even fractions. In our discussion we will look at values of only 0, 1 or 2. The following points can be made about the exponents and orders of the reactions.

• The value of the exponents determines the order of the reaction. If a reaction has a single reactant and the value of the exponent is one then it is said to be first order.
• If the exponent were two then it is said to be second order.
• If more than one reactant is present in a reaction, the sum of the exponents (m + n) is called the overall reaction order. The table below provides some more examples.

Reaction #1

2 N2O5 (g)

4 NO2 (g)

+

5 O2 (g)

Experimentally the rate was found to be first order for N₂O₅ (g). That is the rate varies directly with the [N₂O₅ (g)]. Therefore the following rate law can be written.

Rate = k [N₂O₅ (g)] ¹

Note:

• The value of exponent is not the same as the balance for N₂O₅ in the equation.
• The overall reaction order would be first order.

Reaction #2

NO2 (g)

+

CO (g)

NO (g)

+

CO2 (g)

Experimentally the rate was found to be second order for NO₂ (g) and zero order for CO (g). Therefore the following rate law can be written.

Rate = k [NO₂] ² [CO] ⁰

Note:

• The value of the exponent for [NO₂] and [CO] is not the same as the balanced equations.
• The overall reaction order is second order ( 2 + 0 ).

Reaction #3

2 HI (g)

H2 (g)

+

I2 (g)

Experimentally the rate was found to be second order for HI (g). The following rate law is written.

Rate = k [HI] ²

Note:

• The value of the exponent for HI is the same as the coefficient in the balanced equation.
• The overall reaction order is second order.

Assignment #1 : Writing Rate Laws

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