Question

From the rate expression for the following reactions, determine their order of reaction and the dimensions of the rate constants.

1. 3NO(g) $\to$ N2O (g) Rate = k[NO]2
2. H2O2 (aq) + 3I−(aq) + 2H+ $\to$ 2H2O(l) + I3- Rate = k[H2O2][I−]
3. CH3CHO(g) $\to$ CH4(g) + CO(g) Rate = k [CH3CHO]$\frac 32$
4. C2H5Cl(g) $\to$ C2H4(g) + HCl(g) Rate = k [C2H5Cl]

Answer 1

(i) Given rate = k [NO]2
Therefore, order of the reaction = 2
Dimension of $K =\frac {Rate }{ [NO]^2}$
$= \frac {mol L^{-1} s^{-1}}{(mol L^{-1})^2}$
$= \frac {mol L^{-1} s^{-1}}{mol^2 L^{-2}}$
$= L \ mol^{-1} s^{-1}$

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(ii) Given rate = k [H2O2] [I−]
Therefore, order of the reaction = 2
Dimension of $K = \frac {Rate}{[H_2O_2][I^-]}$
= $\frac {mol L^{-1} s^{-1}}{(mol L^{-1})(mol L^{-1})}$
= $L\ mol^{-1} s^{-1}$

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(iii) Given rate = $k [CH_3CHO]^{\frac 32}$
Therefore, order of reaction =$\frac 32$
Dimension of $K = \frac {Rate}{ [CH_3CHO]^{\frac 32}}$
= $\frac {mol L^{-1} s^{-1}}{(mol L^{-1})^{\frac 32}}$
= $\frac {mol L^{-1} s^{-1}}{mol^{\frac 32} L^{-\frac 32}}$
= $L^{\frac 12} mol^{-\frac 12} s^{-1}$

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(iv) Given rate = $k [C_2H_5Cl]$
Therefore,order of the reaction = 1
Dimension of $k = \frac {Rate}{ [C_2H_5Cl]}$
= $\frac {mol L^{-1} s^{-1}}{(mol L^{-1})}$
= $s^{-1}$