There is usually more than one way to measure the rate of a reaction. We can study the decomposition of hydrogen iodide, for example, by measuring the rate at which either H₂ or I₂ is formed in the following reaction or the rate at which HI is consumed.

2 HI(g) H₂(g) + I₂(g)

Experimentally we find that the rate at which I₂ is formed is proportional to the square of the HI concentration at any moment in time.

What would happen if we studied the rate at which H₂ is formed? The balanced equation suggests that H₂ and I₂ must be formed at exactly the same rate.

What would happen, however, if we studied the rate at which HI is consumed in this reaction? Because HI is consumed, the change in its concentration must be a negative number. By convention, the rate of a reaction is always reported as a positive number. We therefore have to change the sign before reporting the rate of reaction for a reactant that is consumed in the reaction.

The negative sign does two things. Mathematically, it converts a negative change in the concentration of HI into a positive rate. Physically, it reminds us that the concentration of the reactant decreases with time.

What is the relationship between the rate of reaction obtained by monitoring the formation of H₂ or I₂ and the rate obtained by watching HI disappear? The stoichiometry of the reaction says that two HI molecules are consumed for every molecule of H₂ or I₂ produced. This means that the rate of decomposition of HI is twice as fast as the rate at which H₂ and I₂ are formed. We can translate this relationship into a mathematical equation as follows.

As a result, the rate constant obtained from studying the rate at which H₂ and I₂ are formed in this reaction (k) is not the same as the rate constant obtained by monitoring the rate at which HI is consumed (k')

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