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Rate of Reaction

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Rate of Reaction or Reaction Rate in chemistry is defined as the speed or the rate at which a chemical reaction takes place. The rate of a Chemical Reaction is directly proportional to the increase in the concentration of a product per unit of time and to the decrease in the concentration of a reactant per unit of time. This can vary drastically. Chemical reactions proceed at extensively different speeds depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. 

In this article, you will understand the meaning of rate of a chemical reaction, rate of reaction order, the unit of the rate of reaction, and formulas.

What is the Rate of Reaction

Rate of Reaction is the speed at which a chemical reaction occurs. A low-rate reaction means that the rearrangement of molecules by breaking old bonds and creating new bonds is slow. Some reactions can take hundreds of years to occur, whereas others can occur in less than a second. Consider how long it takes plants and ancient fish to become fossils if you want to conceive of an extremely slow reaction (carbonization). The Speed of Reaction is also affected by the type of molecules combined. The reaction will be slower if an essential element or compound is present in low concentrations.

Rate of Reaction Meaning

The Rate of Reaction is defined as the change in the concentration of any one of the reactants or products per unit of time.

Rate of Reaction

Rate of Reaction Formula

The Rate of Reaction is proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit time. And can be defined as,

  • Rate of Reaction = Decrease in the concentration of a reactant R / Time Interval
  • Rate of Reaction = Increase in the concentration of a product P / Time interval

If we consider, [R1] and [P1] are the molar concentrations of the reactant and the product respectively at any time t1 and [R2] and [P2] are the concentrations of reactant and product at time t2, then changes in concentrations of the reactant and product will be d [R] = R2 – R1 and d [P] = P2 – P1 and time interval is dt = t2 – t and the rate of reaction in terms of reactant or product is given by

  • Rate of reaction = -(R2 – R2)/ (t2 – t1) = + (P1 – P2)/(t2 – t1)
  • Rate of reaction = – Δ[R]/Δt =+ Δ[P]/Δt.

Significance of Negative and Positive Signs

The sign in the rate of reaction tells about the increase and decrease in the concentration. 

  • Negative sign indicates that the concentration of the reactant is decreasing.
  • Positive sign indicates that the concentration of the product is increasing.

Expressing the Rate of Reaction in terms of different Reactants and Products 

 Let us consider a reaction ⇒ a A + b B ⇢  x X + y Y

Then the Rate of Reaction is given by

Rate = – 1/a d[A]/dt = – 1/b d[B]/dt = + 1/x d[X]/dt = + 1/y d[Y]/dt

where

  • d[A], d[B] represent small decrease in the concentrations of A and B respectively
  • d[X] and d[Y] represent small increase in the concentrations of X and Y respectively in the small interval of time dt

Average Rate of Reaction

From beginning to end the rate of reaction does not remain the same, it can vary from time to time. Therefore, the rate of reaction is defined as the ‘Average Rate of Reaction’.

Mathematically, the Average rate of reaction is given by, 

rav = -Δ[R]/Δt = +Δ[P]/Δt

Instantaneous Rate of Reaction

The Rate of Reaction at any instant of time is the rate of change of concentration of any one of the reactants or products at that particular instant of time. From the Average Rate of Reaction, we can understand it as when the change in the time interval is very less i.e. Δt→0 then the Rate of Reaction is termed as Instantaneous Rate of Reaction. Let us suppose that the small change in concentration is given dx in the small interval of time dt. Then the rate of reaction at that instant is given by dx/dt. It is given by the tangent to the curve of Concentration of Reactant/Product vs Time.

Mathematically, the Instantaneous Rate of Reaction is given by,

rinst = dx/dt

rinst  = -Δ[R]/Δt as Δt ⇢ 0 = -d[R]/dt = -slope

rinst  = +Δ[P]/Δt as Δt ⇢ 0 = +d[P]/dt = +slope     

Rate of Reaction Graph

Unit of Rate of Reaction

The Rate of Reaction, in general, can be measured as a change in concentration of reactant or product where concentration is denoted in moles/liter and time in seconds or minutes. 

So the Unit of Rate of Reaction = mol L-1 s-1 or mol L-1 min-1.

Factors Affecting Rate of Reaction

There are various factors that affect the reaction rate which are listed below:

  • Reactant Concentration
  • Order of Reaction
  • Nature of Reactant
  • Pressure
  • Temperature
  • Solvent
  • Electromagnetic Radiation
  • Presence of Light
  • Presence of Catalyst
  • Surface Area
  • Activation Energy

Reactants Concentration 

From the Collision Theory, we know that the more the reactant more will be the collision and hence more will be the reaction, hence Rate of Reaction is directly proportional to the concentration of Reactants. Also as the concentrations of the reactants decrease, the rate of reaction decreases. Hence, the Rate of Reaction for a given reaction in terms of concentration of Reactant can be given as

aA + bB ⇢ cC + dD

Rate ∝ [A]x [B]y

Rate = k[A]x [B]y

where 

  • k is the Rate Constant 
  • x and y may or may not be equal to the coefficient of the reactant.

The above representation of the Rate of Reaction in terms of concentration of Reactant is called Rate Law.

Order of Reaction

In the above reaction, the sum of x and y is called as Order of Reaction. Say for example if (x + y = 0) then the Order of Reaction is Zero and the Rate of Reaction is independent of the concentration of the reactants.

Nature of Reactant

The Rate of Reaction is dependent on the Nature of the Reactant in the manner that the reaction happens fastest if the Reactants are in Gaseous Phase, slower in Liquid Phase, and slowest in the Solid phase.

Pressure

The effect of Pressure is applicable to the reactants in the gaseous phase. The effect is pressure is the same as the effect of concentration in a reaction. High Pressure of gas means higher concentration and hence the rate of reaction increases.

Temperature

The rate of reaction increases with an increase in the temperature. In many cases, the Rate Constant of reaction becomes nearly double for a 10° rise in temperature. However, an accurate explanation of the dependence on temperature was given by Arrhenius. He gave the below expression for the dependence of the rate constant on temperature

k = A -Ea/RT

where,

  • k is Rate Constant
  • A is the Pre-Exponential Factor or the Frequency Factor
  • Ea is Activation Energy
  • R is Gas Constant 
  • T is Temperature

Solvent

Solvent provides the medium for the solute to dissolve. A higher concentration of solute in the solvent will increase the rate of reaction. 

Electromagnetic Radiation

Electromagnetic Radiation provides external energy which increases the rate of reaction.

Presence of light

Some reactions do not take place in the dark but can proceed in the presence of light. For Example: H2 + Cl2 ⇢ 2 HCl. This reaction is known as a “photochemical reaction.”

Presence of Catalyst

The main purpose of the catalyst is to increase the rate of reaction without itself involving in the reactions. So the catalyst increases the reaction rate. A catalyst reduces the activation energy barrier and hence provides an alternate path for the reaction to happen.

Surface area

Greater surface area means more collision to take place and hence, greater is the rate of reaction.

Activation Energy

Activation Energy refers to the minimum amount of energy possessed by the reactant to proceed with the reaction. When the molecules of reactant collide they form an intermediate, to form this intermediate minimum amount of energy is required. This minimum energy is called Activation Energy and the intermediate so formed is called Activated Complex. The Rate of Reaction depends on the Activation Energy in the manner that if the Activation Energy is high then the Rate of Reaction will be low and vice versa. Hence, Activation Energy and Rate of Reaction are inversely related to each other.

Learn more about, Factors Affecting Rate of a Chemical Reaction

Difference between Rate of Reaction and Rate Constant

The difference between the Rate of Reaction and Rate Constant is tabulated below:

Rate of Reaction

Rate Constant

It is the change in the concentration of reactants or the change in the concentration of products per unit of time.

The rate constant is the proportionality constant related to the rate of a particular reaction. 

It depends on the molar concentration of Reactants and Products.

It doesn’t depend on the molar concentration of Reactants and Products.

It indirectly depends on the temperature.

 This directly depends on the temperature 

It is time-dependent.

It is time-independent. 

Related Resources,

FAQs on Rate of Reaction

1. What is Rate of Reaction Meaning?

The rate of reaction is defined as the change in the concentration of any one of the reactants or products per unit of time.

2. What is Rate Law?

Rate law refers to the representation of the Rate of Reaction in terms of the concentration of Reactants where the Rate of Reaction is directly proportional to the concentration of the Reactant species each raised to some power which may or may not be equal to the coefficient of the reactants.

3. What is the Unit of Rate of Reaction?

The Unit of Rate of Reaction is molL-1s-1

4. What is the Difference between Average and Instantaneous Rate of Reaction?

Instantaneous Rate of Reaction: The rate of reaction at any instant of time is the rate of change of concentration of any one of the reactants or products at that particular instant of time.

Average Rate of Reaction: It is the change is concentration of reactant or product over a time period.

5. How is Instantaneous Rate of Reaction calculated?

The Instantaneous Rate of Reaction can be calculated by taking tangent to the curve at that particular instant of time.

6. What is Order of Reaction?

In Rate law, the sum of powers of the concentration of the reactants is called as Order of Reaction. It basically tells the Rate of Reactants is dependent on concentration of how many reactants. For Example, in Zero Order Reaction rate of reaction is independent of the concentration of reactant, in case of First Order reaction, rate is determined by concentration of one of the reactant.

7. How is Rate of Reaction dependent upon Temperature?

The Rate of Reaction get doubles if temperature is increased by 10°, however accurate dependence is given by Arrhenius Equation given by k = A -Ea/RT

8. How is the Rate of Reaction dependent upon Activation Energy?

Rate of Reaction and Activation Energy are inversely related to each other. Higher Activation Energy will lead to a lower Rate of Reaction and Vice Versa.

9. What is the Rate of Reaction Formula?

  • Rate of Reaction = Decrease in the concentration of a reactant R / Time Interval
  • Rate of Reaction = Increase in the concentration of a product P / Time interval


Last Updated : 13 Sep, 2023
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