Zero-order reactions: Class 12 practice
For a zero-order reaction, rate = k (independent of concentration), [A] = [A]₀ − kt, and t½ = [A]₀/(2k). Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Linear equations
The idea behind zero-order reactions
For a zero-order reaction, rate = k (independent of concentration), [A] = [A]₀ − kt, and t½ = [A]₀/(2k). The units of k are mol L⁻¹ s⁻¹.
Step-by-step method
- Use [A] = [A]₀ − kt.
- t½ = [A]₀/2k.
- Time to finish = [A]₀/k.
Worked example 1 (Basic)
A zero-order reaction has [A]₀ = 0.2 M and k = 0.009 M/s. Find [A] after 19 s.
- [A] = 0.2 − 0.009 × 19
- = 0.029 M
- Answer: 0.029 M
Worked example 2 (Standard)
Find the half-life of a zero-order reaction with [A]₀ = 0.8 M and k = 0.009 M/s.
- t½ = [A]₀/2k = 0.8/0.018
- = 44.44 s
- Answer: 44.44 s
Worked example 3 (Challenge)
How long does a zero-order reaction with [A]₀ = 0.5 M and k = 0.002 M/s take to finish?
- t = [A]₀/k
- = 250 s
- Answer: 250 s
Common mistakes and misconceptions
- Using the first-order half-life
- Wrong units of k
Practice questions
- (Basic) A zero-order reaction has [A]₀ = 0.9 M and k = 0.003 M/s. Find [A] after 275 s.
- (Basic) A zero-order reaction has [A]₀ = 0.2 M and k = 0.003 M/s. Find [A] after 50 s.
- (Standard) Find the half-life of a zero-order reaction with [A]₀ = 0.5 M and k = 0.005 M/s.
- (Standard) Find the half-life of a zero-order reaction with [A]₀ = 0.1 M and k = 0.007 M/s.
- (Challenge) How long does a zero-order reaction with [A]₀ = 0.6 M and k = 0.009 M/s take to finish?
- (Challenge) How long does a zero-order reaction with [A]₀ = 0.4 M and k = 0.01 M/s take to finish?
Answer key with full working
- Q1: [A] = 0.9 − 0.003 × 275 → = 0.075 M → Answer: 0.075 M
- Q2: [A] = 0.2 − 0.003 × 50 → = 0.05 M → Answer: 0.05 M
- Q3: t½ = [A]₀/2k = 0.5/0.01 → = 50 s → Answer: 50 s
- Q4: t½ = [A]₀/2k = 0.1/0.014 → = 7.14 s → Answer: 7.14 s
- Q5: t = [A]₀/k → = 66.67 s → Answer: 66.67 s
- Q6: t = [A]₀/k → = 40 s → Answer: 40 s
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 zero-order reactions 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.