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Worked Examples · Example 3.4

Q.Identify the reaction order from each of the following rate constants.

(i) k=2.3×10−5 L mol−1s−1k = 2.3\times10^{-5}\ \text{L mol}^{-1}\text{s}^{-1}
(ii) k=3×10−4 s−1k = 3\times10^{-4}\ \text{s}^{-1}
Telangana TsbieTextbookSubjective· 2mImportance★★★★★
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The units of the rate constant kk directly reveal the overall reaction order. For (i), units of L mol−1s−1\text{L mol}^{-1}\text{s}^{-1} indicate second order; for (ii), units of s−1\text{s}^{-1} indicate first order.

The rate constant kk is not just a number — its units carry a fingerprint of the reaction’s overall order. This works because the rate of a reaction always has units of concentration per time (typically mol L−1s−1\text{mol L}^{-1}\text{s}^{-1}). When you write the rate law Rate=k[A]n\text{Rate} = k[\text{A}]^n, the units of kk must adjust so that the right-hand side matches the units of rate.

For a reaction of order nn, the general unit of kk is:

Units of k=(concentration)1−n×time−1\text{Units of } k = (\text{concentration})^{1-n} \times \text{time}^{-1}

In molar terms, concentration is in mol L−1\text{mol L}^{-1}, so:

Units of k=Ln−1mol1−ns−1\text{Units of } k = \text{L}^{n-1} \text{mol}^{1-n} \text{s}^{-1}

Let’s apply this to each case.

  1. Case (i): k=2.3×10−5 L mol−1s−1k = 2.3 \times 10^{-5}\ \text{L mol}^{-1}\text{s}^{-1}

    • The units are L1mol−1s−1\text{L}^{1} \text{mol}^{-1} \text{s}^{-1}.
    • Compare with the general form Ln−1mol1−ns−1\text{L}^{n-1} \text{mol}^{1-n} \text{s}^{-1}.
    • For the exponent of mol\text{mol}: 1−n=−1  ⟹  n=21-n = -1 \implies n = 2.
    • Check the exponent of L\text{L}: n−1=1n-1 = 1, which matches.
    • So the reaction is second order overall.
  2. Case (ii): k=3×10−4 s−1k = 3 \times 10^{-4}\ \text{s}^{-1}

    • The units are simply s−1\text{s}^{-1}, with no concentration term.
    • From the general form, this means (concentration)1−n(\text{concentration})^{1-n} must be dimensionless, so 1−n=0  ⟹  n=11-n = 0 \implies n = 1.
    • The reaction is first order overall. …

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