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Rate of Reaction

Rate of reaction is the change in concentration of a reactant or product per unit time. For a reactant it is written with a negative sign because its concentration decreases, while for a product it is written with a positive sign because its concentration increases.

In one line: Reactants reduce, products produce; coefficients correct the rate.

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Understand it

Reaction rate measures how quickly a chemical reaction proceeds. Average rate is calculated over a time interval, while instantaneous rate is the rate at a particular moment, usually obtained from the slope of a concentration-time curve. For a reaction aA + bB → cC + dD, the rate must be divided by stoichiometric coefficients so that the same reaction rate is obtained from any species.

Example

For N2 + 3H2 → 2NH3, rate = -d[N2]/dt = -(1/3)d[H2]/dt = (1/2)d[NH3]/dt. If ammonia concentration increases by 0.020 mol L−1 in 10 s, the rate of reaction based on NH3 is (1/2)(0.020/10) = 0.0010 mol L−1 s−1.

Rule to remember

Average rate for reactant = -Delta[R]/Delta t; average rate for product = +Delta[P]/Delta t.

For aA + bB → cC + dD, rate = -(1/a)d[A]/dt = -(1/b)d[B]/dt = (1/c)d[C]/dt = (1/d)d[D]/dt.

Usual unit: mol L−1 s−1.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

For A → 2B, [B] rises from 0.10 mol L−1 to 0.22 mol L−1 in 30 s.

  1. 1Rate of formation of B = 0.12/30 = 0.004 mol L−1 s−1.
  2. 2Reaction rate = (1/2)(0.004) = 0.002 mol L−1 s−1.

Method you can reuse

  1. 1First balance the reaction.
  2. 2Next decide whether the species is a reactant or product.
  3. 3Use a negative sign for disappearance and a positive sign for formation.
  4. 4Finally divide by the stoichiometric coefficient and include mol L−1 s−1 unless another time unit is given.

Visual tip

A concentration-time graph is useful. Reactant concentration curves slope downward and product concentration curves slope upward. Instantaneous rate is read from the tangent slope at the required time.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

In 2A → B, [A] decreases by 0.04 mol L−1 in 20 s. What is the reaction rate?

Think first, then show answer
Rate = -(1/2)Delta[A]/Delta t = (1/2)(0.04/20) = 0.001 mol L−1 s−1.

Common wrong answer

Writing 0.004 mol L−1 s−1 as the reaction rate for A → 2B when it is only the rate of formation of B.

Don't mix this up

Students often forget to divide by stoichiometric coefficients, especially for species such as H2 or NH3 in balanced equations.

Exam tip

Always start by writing the balanced equation, then attach the correct sign and coefficient factor before substituting values.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions ask students to calculate average rate from concentration data, compare rates of disappearance and formation, or identify the correct expression using a balanced chemical equation.
  1. 1

    Enough for 1 mark

    Rate of reaction is the change in concentration of a reactant or product per unit time. For a reactant it is written with a negative sign because its concentration decreases, while for a product it is written with a positive sign because its concentration increases.

  2. 2

    Add for 2 marks

    Example: For N2 + 3H2 → 2NH3, rate = -d[N2]/dt = -(1/3)d[H2]/dt = (1/2)d[NH3]/dt. If ammonia concentration increases by 0.020 mol L−1 in 10 s, the rate of reaction based on NH3 is (1/2)(0.020/10) = 0.0010 mol L−1 s−1.

  3. 3

    Add for 3 marks

    Reaction rate measures how quickly a chemical reaction proceeds.

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