Rate of a reaction

Rate of a reaction is defined as the change in concentration of the reactants or products per unit time. In general it is the speed at which reactants are converted into products.

Table of contents

• Rate of a reaction
     - Representation of rate of a reaction in a chemical reaction 
     - Factors affecting rate of reaction
• Recommended video
• Example
• Test your knowledge on rate of reaction-Quiz
• Frequently Asked Questions-FAQs
• Related topic link

Representation of rate of a reaction in a chemical reaction

Consider a reaction, R P, 

The rate of reaction for the above chemical reaction can be expressed in two ways:

(i) The rate of decrease in concentration of reactant 

(ii) The rate of increase in concentration of product 

• Rate of disappearance of R = Decrease in concentration of RTime taken
• Rate of appearance of P = Increase in concentration of PTime taken

Note: Rate of reaction and rate of disappearance or appearance of R and P respectively are two different terms. 

Rate of reaction (in terms of reactant, R) = -1Stoichiometric coefficient Change in concentration of RTime taken

Rate of reaction (in terms of product, P) = 1Stoichiometric coefficient Change in concentration of PTime taken

Note: The negative sign is used to indicate the decreasing concentration of the reactant.

For a chemical reaction, aA(g) + bB(g) cC(g) + dD(g)

Rate of a reaction can be expressed in the below mentioned way:

Rate of reaction = -1ad[A]dt = -1bd[B]dt = 1cd[C]dt = 1dd[D]dt

Where,

Rate of disappearance of A = -d[A]dt or -dPAdt

Rate of disappearance of B = -d[B]dt or-dPBdt

Rate of appearance of C = d[C]dt or-dPCdt

Rate of appearance of D = d[D]dt or-dPDdt

Unit of rate of reaction is mol L-1 s-1 and for reaction in gaseous phase is atm s-1.

Factors affecting rate of reaction

1. Concentration (or pressure) of the reactants: According to the law of mass action, the rate of reaction is directly proportional to the concentration or pressure (in case of gaseous reaction) of reactants, each raised to the power of their stoichiometric coefficient in the balanced chemical equation.

2. Temperature: It has been experimentally observed that the rate of reaction increases with increase in temperature.

3. Presence of the catalyst: There are two types of catalyst, depending on which rate of reaction gets affected. Positive catalyst increases the rate of the reaction and on the other hand, the presence of negative catalyst decreases the rate of the reaction.

4. Nature of the reactants: In the reactants, if the number of bonds or strength of bonds, to be broken, are greater, the rate of reaction will be slower.

5. Surface area of the reactants: As the surface area of the reactant increases, the rate of reaction increases, i.e., in order to enhance the rate of reaction, reactants must be taken in powdered form.

Recommended videos

https://www.youtube.com/watch?v=MgHX-ZOuzlk
 

Example 1:

For a reaction, 12 A 2B, the relation between rate of disappearance of ‘A’ and rate of appearance of ‘B’ can be expressed by:

(A) -d[A]dt = 14d[B]dt

(B) -d[A]dt = d[B]dt

(C) -d[A]dt = 4d[B]dt

(D) -d[A]dt = 12d[B]dt

Answer: (A)

Solution: 

Rate of reaction = -112d[A]dt = 12d[B]dt

⇒ Rate of reaction = -2d[A]dt = 12d[B]dt…………(i)

Rate of disappearance of ‘A’ = -d[A]dt

Rate of appearance of ‘B’ = d[B]dt

Dividing equation (i) by 2

-d[A]dt = 14d[B]dt

Hence, option (A) is the correct answer.

Example 2: 

 For the reaction, 2A + 3B+ 32C 3P, Which statement is correct?

(A) d[A]dt = 32d[B]dt = 34d[C]dt

(B) d[A]dt = d[B]dt = d[C]dt

(C) d[A]dt = 23d[B]dt = 43d[C]dt

(D) d[A]dt = 23d[B]dt = 34d[C]dt

Answer: (C)

Solution: Rate of a reaction =-12d[A]dt = -13d[B]dt = -132d[C]dt

Multiplying above equation by -2.

d[A]dt = 23d[B]dt = 43d[C]dt

∴ Option (C) is the correct answer. 

Test your Knowledge on Rate of a Reaction-Quiz

1. What do we study in chemical kinetics?

(A) Feasibility of a reaction

(B) Extent of a reaction

(C) Speed of the reaction

(D) Both (B) and (C)

Answer: (C)

2. According to the law of mass action, the rate of reaction depends upon:

(A) Pressure of the reactants

(B) Concentration of the reactants

(C) Concentration of the products

(D) Both (A) and (B)

Answer: (D)

3. Unit of rate is:

(A) mol L-1s-1

(B) mol L-1s

(C) M s-1

(D) Both (A) and (C)

Answer: (D)

Frequently Asked Questions-FAQs

1. What does the negative sign indicate in the rate of reaction?

Answer:

Consider a reaction, R P 

The rate of reaction in terms of reactant = -d[R]dt

The negative sign is used to indicate the decreasing concentration of the reactant.

2. What is the unit of rate of reaction in the gaseous phase?

Answer: 

Consider a reaction, A(g) B(g)

Rate of a reaction = Change in pressureTime taken = atms

The unit of rate of a reaction in the gaseous phase is atm s-1.

3. How do we measure the speed of a reaction in the laboratory?

Answer: The speed of the reaction can be measured by calculating the rate of a reaction.

In the laboratory, it can be measured by noting the concentration of the reaction medium at different intervals of time. Later rate of reaction can be calculated using the formula:

Rate = Total change in concentrationTotal time taken= Cfinal - Cinitial tfinal - tinitial = ct

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