What is Rate of Reaction? Definition, Properties, Factors & Class 11 Notes

In a chemical process, the speed at which a reactant converts into a product is known as the rate of reaction. The rate of reaction is calculated by dividing the change in concentration of a substance by a unit of time.

Aslo read: NCERT Solutions | Class 11 Chemistry NCERT Solutions

Rate of reaction = Change in concentration / time taken

The rate of reaction is measured in mol  ${\mathrm{L}}^{-1}{\mathrm{s}}^{-1}$ (or atm  ${\mathrm{s}}^{-1}$ for gases, using  $\mathrm{P}=\mathrm{C}\mathrm{R}\mathrm{T}$ ).

Significance of Rate of Reaction

The importance of the rate of reactions is

• If we know the rate of reaction, then it is possible to control the speed of the chemical reaction as per our needs.
• In real life, the best example of the rate of reaction is cooking food. When the heat is high, food starts cooking fast and similarly, it is reverse when the heat is low. We know that in high heat, food can get burned so we can prevent this by controlling the heat.
• In industries, we can reduce the waste by controlling the rate of reaction and increasing the production rate.

The unit of rate of reaction depends on how it is measured. Usually, the rate is defined as the change in concentration of a reactant or product per unit time. Concentration is typically measured in moles per litre (mol/L) and time in seconds (s). The standard unit of rate of reaction is mol·L⁻¹·s⁻¹ (moles per litre per second). For a gaseous reaction unit of rate of reaction is expressed in atm/s.

 

Important Topics:

NCERT Class 12 notes	NCERT Class 12 Chemistry notes
Class 12 Maths NCERT notes	Physics Class 12 NCERT notes

 

The rate of reaction is classified in various ways. Below are the types of rate of reaction.

Average Rate of Reaction:

• The rate is defined as the change in concentration of reactants or products over a time interval : $\text{Average rate}=\frac{\mathrm{\Delta }C}{\mathrm{\Delta }t}=\frac{C_2-C_1}{t_2-t_1}$
• For a reaction $n1A+n2B->m1C+m2D$ , the rate is: $\text{Rate}=-\frac{1}{n_1}\frac{\mathrm{\Delta }\left[A\right]}{\mathrm{\Delta }t}=-\frac{1}{n_2}\frac{\mathrm{\Delta }\left[B\right]}{\mathrm{\Delta }t}=\frac{1}{m_1}\frac{\mathrm{\Delta }\left[C\right]}{\mathrm{\Delta }t}=\frac{1}{m_2}\frac{\mathrm{\Delta }\left[D\right]}{\mathrm{\Delta }t}$
• Average rate of reaction Units: $\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{L}}^{-1}{\mathrm{s}}^{-1}$ or $\mathrm{g}\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{L}}^{-1}{\mathrm{s}}^{-1}$ (Page 1).
  Example: For $R->P$ , if $\left[R\right]$ changes from 0.03 M to 0.02 M in 25 min (Page $3)$ :

Instantaneous Rate of Reaction:

• Rate is defined at a specific moment in time
• Rate changes throughout the reaction
• Calculated as the limit of the average rate over a very short interval  $\text{Instantaneous rate}=\underset{\mathrm{\Delta }t\to 0}{\mathrm{l}\mathrm{i}\mathrm{m}} \frac{\mathrm{\Delta }C}{\mathrm{\Delta }t}=\frac{dC}{dt}$ ):
• For $n1A+n2B->m1C+m2D$ : $\text{Rate}=-\frac{1}{n_1}\frac{d\left[A\right]}{dt}=-\frac{1}{n_2}\frac{d\left[B\right]}{dt}=\frac{1}{m_1}\frac{d\left[C\right]}{dt}=\frac{1}{m_2}\frac{d\left[D\right]}{dt}$

Initial Rate of Reaction:

• Rate of reaction at the initial at the beginning of the reaction, i.e. t=0"
• The initial rate helps in determining the reaction order and rate constant.

For a reaction like $2\mathrm{H}2\mathrm{O}2->2\mathrm{H}2\mathrm{O}+\mathrm{O}2$ , the rate is: $\text{Rate}=-\frac{1}{2}\frac{\mathrm{\Delta }\left[H2O2\right]}{\mathrm{\Delta }t}=\frac{\mathrm{\Delta }\left[O2\right]}{\mathrm{\Delta }t}$

For $\mathrm{N}2\mathrm{O}5->2\mathrm{N}\mathrm{O}2+1/2\mathrm{O}2$ , if the rate of decomposition of N₂O₅ is $1.8\times {10}^{-3}\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{L}}^{-1}{\mathrm{m}\mathrm{i}\mathrm{n}}^{-1}$ , then: $\frac{\mathrm{\Delta }\left[NO2\right]}{\mathrm{\Delta }t}=2\times 1.8\times {10}^{-3}=3.6\times {10}^{-3}\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{L}}^{-1}{\mathrm{m}\mathrm{i}\mathrm{n}}^{-1},\mathrm{}\frac{\mathrm{\Delta }\left[O2\right]}{\mathrm{\Delta }t}=\frac{1}{2}\times 1.8\times {10}^{-3}=9\times {10}^{-4}\mathrm{m}\mathrm{o}\mathrm{l}{\mathrm{L}}^{-1}\mathrm{m}\mathrm{i}\mathrm{n}$

The rate of reaction at a specific moment in time during a chemical reaction is referred to as the instantaneous rate of reaction. It shows how fast a reactant is consumed or a product is formed at a particular point. We can determine the instantaneous rate of reaction by finding the slope of the tangent to the concentration vs time graph at an exact point. It is useful in reactions where the rate changes quickly. Instantaneous rate of reaction is mol·L⁻¹·s⁻¹.

A chemical reaction which is a higher-order reaction, but the concentration of molecules hardly changes because of the large number, so it looks like a first-order reaction. 

In technical terms, x+y is not equal to 1 (one).

$\mathrm{Rate}=k{\left[A\right]}^2{\left[B\right]}^2$

For example: 2+2=4

The factors affecting the rate of reaction are:

• Concentration: In a chemical reaction, when the concentration of reactants is increased, the reaction rate also increases.
• Pressure: For gases, when the pressure is increased, the concentration of gas molecules increases and there is a rise in reaction rate.
• Temperature: When the temperature is increased by every 10°C, the rate of reaction doubles or triples.
• Surface Area: If the surface area of the solid reactant is increased, the rate of reaction also increases.