Laju Reaksi (2) | Cara Mudah Menentukan Persamaan Laju Reaksi dan Orde Reaksi | Kimia Kelas 11

Main Ideas

A reaction-rate equation describes how reactant concentrations affect the reaction rate. In general, for reactants A and B:

[ v = k[A]^m[B]^n ]

Here, (v) is the reaction rate, (k) is the rate constant, and (m) and (n) are the reaction orders for A and B. The overall reaction order is (m+n).

Reaction orders are the exponents in the rate equation. They indicate how strongly concentration changes affect the rate. They generally cannot be read directly from the overall balanced equation; they must be determined from experimental data or, in the lesson’s mechanism example, from the slow elementary step.

If a reactant’s concentration is multiplied by a factor (a) and the rate changes by a factor (b), its order (p) satisfies:

[ a^p = b ]

Ways to Determine Reaction Order

From a Reaction Mechanism

• Identify the slow step, which controls the overall reaction rate.
• As presented in the lesson, use the reactant coefficients in that step as their reaction orders.

The example gives a rate law that is first order in H₂ and second order in NO:

[ v = k[H_2][NO]^2 ]

From Experimental Data

To find the order for one reactant:

1. Compare two experiments in which the other reactant concentrations are unchanged.
2. Calculate how many times the target reactant’s concentration changes and how many times the rate changes.
3. Use (a^p=b) to solve for the order (p).

Example: (A + B \rightarrow AB)

To find the order in A, compare trials where B stays at 0.5:

• [A] rises from 1 to 3, a factor of 3.
• The rate rises from 2 to 18, a factor of 9.
• Since (3^m=9), the order in A is (m=2).

To find the order in B, compare trials where A stays constant:

• [B] rises from 0.5 to 2, a factor of 4.
• The rate rises from 2 to 32, a factor of 16.
• Since (4^n=16), the order in B is (n=2).

The resulting rate equation is:

[ v = k[A]^2[B]^2 ]

The overall order is 4.

Worked Example: ICl and H₂

For the reaction (2ICl + H_2 \rightarrow I_2 + 2HCl), the data are used to determine the reaction orders and rate constant.

• Order in ICl: Compare trials 2 and 3, where [H₂] is constant. [ICl] doubles and the rate doubles, so the ICl order is 1.
• Order in H₂: Compare trials 1 and 2. Both [ICl] and [H₂] double, while the rate doubles. Since doubling [ICl] already accounts for the rate doubling, H₂ has order 0.
• Overall order: (1+0=1).
• Rate equation:

[ v = k[ICl] ]

The H₂ term is omitted because ([H_2]^0=1).

Using the third trial:

[ k = \frac{3.0\times10^{-3}}{0.2} = 1.5\times10^{-2} ]

The units depend on the units used for rate and concentration.

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