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| Rate Law Expression |
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| Experimentally, it is found that the rate of a reaction is proportional to the concentrations of the reactants raised to some power, provided that other factors, which also affect the rate of the reaction are kept constant. |
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| For e.g., for the hypothetical reaction, |
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| the rate of reaction might depend on the concentrations of A and B in the following manners. |
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| the expressions, k [A][B] and k' [A]2 [B] are the rate law expressions for the rate of the reaction and k is the rate coefficient or specific rate constant of the reaction. This constant is independent of concentration but is usually dependent on temperature. |
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| There can be other rate law expression for the same hypothetical reaction given above such as |
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| rate = k'' [A] [B]o |
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| rate = k''' [A]1/2 [B]1 |
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| Theoretically, there are many possibilities to rate law expression of a reaction. However, only one expression will fit the experimentally determined rate data. |
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| In general, for any hypothetical reaction, |
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| the rate law expression can be written as |
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| rate (r) = k [A]m [B]n …..(8) |
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| It is an equation that relates the true rate of reaction to the concentrations of the reactants raised to some power. |
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| Rate law expression serves three purposes. |
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| (i) If the composition of the reaction mixture and the rate constant are known, then the rate of the reaction can be predicted. |
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| (ii) It is useful in postulating the mechanism of a reaction. |
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| (iii) It is useful in classifying the reactions into various orders |
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| Experimental determination of rate law |
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| This involves the determination of the values of the exponents 'm' and 'n' as given in the expression (equation 8). The units of the rate constant k depend on the order of the reaction. |
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