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Order of reaction from initial rates: Class 12 practice

For rate = k[A]^m[B]^n, compare two experiments where only one concentration changes: (rate₂/rate₁) = (conc₂/conc₁)^order. Read the method, follow the three worked examples, then try the six questions.

What you should know first

  • Exponents
  • Logarithms

The idea behind order of reaction from initial rates

For rate = k[A]^m[B]^n, compare two experiments where only one concentration changes: (rate₂/rate₁) = (conc₂/conc₁)^order. Overall order = m + n.

Step-by-step method

  1. Find two runs where only [A] changes.
  2. Solve (rate ratio) = (conc ratio)^m.
  3. Repeat for B; then k = rate ÷ ([A]^m[B]^n).

Worked example 1 (Basic)

Runs: ([A] = 0.1, [B] = 0.3, rate = 8.100e-4), ([A] = 0.2, [B] = 0.3, rate = 3.240e-3), ([A] = 0.1, [B] = 0.6, rate = 3.240e-3). Find the order with respect to A.

  1. Rate ratio for A ×2: 4 → m = 2
  2. Rate ratio for B ×2: 4 → n = 2
  3. Answer: 2

Worked example 2 (Standard)

Runs: ([A] = 0.1, [B] = 0.1, rate = 1.000e-2), ([A] = 0.2, [B] = 0.1, rate = 1.000e-2), ([A] = 0.1, [B] = 0.2, rate = 2.000e-2). Find the order with respect to A and B.

  1. Rate ratio for A ×2: 1 → m = 0
  2. Rate ratio for B ×2: 2 → n = 1
  3. Answer: m = 0, n = 1, overall 1

Worked example 3 (Challenge)

Runs: ([A] = 0.2, [B] = 0.3, rate = 1.080e-3), ([A] = 0.4, [B] = 0.3, rate = 4.320e-3), ([A] = 0.2, [B] = 0.6, rate = 4.320e-3). Find the order with respect to A and B and the rate constant.

  1. Rate ratio for A ×2: 4 → m = 2
  2. Rate ratio for B ×2: 4 → n = 2
  3. k = 0.3
  4. Answer: m = 2, n = 2, k = 0.3

Common mistakes and misconceptions

  • Comparing runs where both change
  • Adding orders before finding each

Practice questions

  1. (Basic) Runs: ([A] = 0.1, [B] = 0.3, rate = 8.100e-3), ([A] = 0.2, [B] = 0.3, rate = 1.620e-2), ([A] = 0.1, [B] = 0.6, rate = 3.240e-2). Find the order with respect to A.
  2. (Basic) Runs: ([A] = 0.1, [B] = 0.1, rate = 3.000e-1), ([A] = 0.2, [B] = 0.1, rate = 3.000e-1), ([A] = 0.1, [B] = 0.3, rate = 3.000e-1). Find the order with respect to A.
  3. (Standard) Runs: ([A] = 0.2, [B] = 0.3, rate = 4.800e-3), ([A] = 0.6, [B] = 0.3, rate = 4.320e-2), ([A] = 0.2, [B] = 0.6, rate = 9.600e-3). Find the order with respect to A and B.
  4. (Standard) Runs: ([A] = 0.1, [B] = 0.1, rate = 2.000e-5), ([A] = 0.2, [B] = 0.1, rate = 8.000e-5), ([A] = 0.1, [B] = 0.3, rate = 1.800e-4). Find the order with respect to A and B.
  5. (Challenge) Runs: ([A] = 0.2, [B] = 0.1, rate = 1.000e-1), ([A] = 0.6, [B] = 0.1, rate = 1.000e-1), ([A] = 0.2, [B] = 0.3, rate = 1.000e-1). Find the order with respect to A and B and the rate constant.
  6. (Challenge) Runs: ([A] = 0.1, [B] = 0.2, rate = 2.000e-4), ([A] = 0.3, [B] = 0.2, rate = 1.800e-3), ([A] = 0.1, [B] = 0.4, rate = 8.000e-4). Find the order with respect to A and B and the rate constant.

Answer key with full working

  • Q1: Rate ratio for A ×2: 2 → m = 1 → Rate ratio for B ×2: 4 → n = 2 → Answer: 1
  • Q2: Rate ratio for A ×2: 1 → m = 0 → Rate ratio for B ×3: 1 → n = 0 → Answer: 0
  • Q3: Rate ratio for A ×3: 9 → m = 2 → Rate ratio for B ×2: 2 → n = 1 → Answer: m = 2, n = 1, overall 3
  • Q4: Rate ratio for A ×2: 4 → m = 2 → Rate ratio for B ×3: 9 → n = 2 → Answer: m = 2, n = 2, overall 4
  • Q5: Rate ratio for A ×3: 1 → m = 0 → Rate ratio for B ×3: 1 → n = 0 → k = 0.1 → Answer: m = 0, n = 0, k = 0.1
  • Q6: Rate ratio for A ×3: 9 → m = 2 → Rate ratio for B ×2: 4 → n = 2 → k = 0.5 → Answer: m = 2, n = 2, k = 0.5

Syllabus note

This skill is listed under "Chemical Kinetics" for Class 12 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

Related practice and notes

Frequently asked questions

Which chapter is order of reaction from initial rates in?

"Chemical Kinetics", Class 12 Chemistry (NCERT/CBSE). Other boards may place it differently.

How were the answers checked?

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

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