# Rate of Reaction

**rate of a reaction**is defined as the change of reactant or product concentration in unit time. If we were to define the rate of a reaction in terms of the reactants, we should define the rate as the rate of disappearance of reactants. If we were to define the rate in terms of the products formed, we should define it as the rate of appearance of products.

*a*and

*b*their coefficients respectively. X and Y are the products, and

*x*and

*y*their coefficients respectively.

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# Rate Law

**Rate law**is an expression that we can find experimentally, which relates the concentration of reactants and the rate of a reaction. Let's consider the same hypothetical equation:

**For this reaction, we can write the rate as follows:**

*k*[A]

^{m}[B]

^{n}

*k*is the rate constant,

The rate of the reaction will have the general look as shown. Assume that the rate law of the reaction is:

Rate = *k* [ P ]* ^{m}* [ Q ]

^{n}The plan is to find out the actual values of *m* and *n* from the given data. Let's do it step by step.

Look at the given experimental data. If we compare Experiments 1 and 2, we can see that the concentration of Q is doubled in Experiment 2. But the concentration of P is kept constant. With these changes, we see the quadrupling of the rate. That means the exponent of Q is 2. At this point we can rewrite the rate law as follows:

Rate = *k* [ P ]* ^{m}* [ Q ]

^{2}

Now compare Experiments 2 and 3. Here the concentration of Q is kept constant, but the concentration of P is doubled. The rate is doubling because of this change. So the exponent of P is 1. We can now write the completed rate law of the reaction.

Rate = *k* [ P ] [ Q ]^{2}