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Rate of Reaction - Practice Questions & MCQ

Edited By admin | Updated on Sep 25, 2023 25:23 PM | #NEET

Quick Facts

  • Instantaneous Rate of Reaction is considered one the most difficult concept.

  • Factors Affecting Rate of Reaction, Factors Affecting Rate of Reaction(2) is considered one of the most asked concept.

  • 106 Questions around this concept.

Solve by difficulty

Rate determining step of a nulti-step reaction is :.

 For a reversible reaction A \rightleftharpoons B, the equilitrion constant ( k_e ) is given as k_e=(13) /(17 P.) Choose the correct expression for k_q in terms of the rate constants \left(k_1 \& k_{-1}\right) of the forward it reverse reactions.

A reaction with a negative \mathrm{\Delta H} enthalpy charge will have a potential energy profile diagram that:

Which of the following statements is true regarding a catalysed reaction and its potential energy profile diagram?

For an endothermic reaction, the energy difference between the reactants and the product is:

In a potential energy diagram with a single transition state indicates that the reaction:

What does the vertical axis of a potential Energy profile diagram represent?

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For an exothermic reaction, the potential energy of the product is"

The Energy difference between the reactants and products in an exothermic reaction is

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In a potential energy diagram, the distance along the horizontal axis represents :-

Concepts Covered - 5

Rate of Reaction

It is defined as "The rate of change of concentration of a reactant or a product per unit time" that is it is nothing but it is the per unit mole of rate of decomposition of any reactant or formation of product.

\text { Rate of reaction }(\mathrm{r})=\frac{\mathrm{C}_{1}-\mathrm{C}_{2}}{\mathrm{t}_{2}-\mathrm{t}_{1}}

As rate of reaction varies greatly with time, so generally, average reaction rate and instantaneous reaction rates are used. Average rate ways to is the rate at some particular instant of time and is equal to the time rate of change of the active mass of any of the reactants or any of the products.

\begin{array}{l}{\text { For a reaction A} \rightarrow \mathrm{P}} \\\\ {\text { Rate of disappearance of A}=\mathrm{-\frac{\Delta[A]}{\Delta T}}} \\\\ {\text { Rate of appearance of P} =\mathrm{-\frac{\Delta[P]}{\Delta T}}}\end{array}


Average Rate of Reaction

Change in the concentration of reactants or products per unit time is called average reaction velocity. If Δc is the change in the concentration of reactants and product in Δt time, then:

\text { Average velocity }=\mathrm{\pm \frac{\Delta c}{\Delta t}}

\mathrm{Unit \: of\: average \: velocity=\frac{Unit \: of \: concentration}{Unit\: of\:time}=\frac{gram\: mole}{litre\: second}={gram\: mole\: litre}^{-1}\: second^{-1}}

Instantaneous Rate of Reaction

As average reaction rate fails to predict rate at a particular moment of time so we use instantaneous rate which is equal to small change in concentration (dx) during a small interval of time (dt). It is given as dx/dt.

\mathrm{\lim _{\Delta t \rightarrow 0} \frac{\Delta c}{\Delta t}=\frac{d c}{d t}}

  • dx/dt = tanӨ = slope of curve
  • It can be written for any of the reactant or the product in terms of stoichiometric coefficients Vj which is negative for reactants and positive for products as follows:
    \frac{\mathrm{dx}}{\mathrm{dt}}=\frac{1}{\mathrm{V}_{\mathrm{j}}} \frac{\mathrm{d}(\mathrm{J})}{\mathrm{dt}}

    \mathrm{a A+b B \rightarrow c C+d D}

    \text { Rate w.r.t. }[\mathrm{A}]=-\frac{\mathrm{d}[\mathrm{A}]}{\mathrm{dt}} \times \frac{1}{\mathrm{a}}

    \text { Rate w.r.t. }[\mathrm{B}]=-\frac{\mathrm{d}[\mathrm{B}]}{\mathrm{dt}} \times \frac{1}{\mathrm{b}}

    \text { Rate w.r.t. }[\mathrm{C}]=-\frac{\mathrm{d}[\mathrm{C}]}{\mathrm{dt}} \times \frac{1}{\mathrm{c}}

    \text { Rate w.r.t. }[\mathrm{D}]=-\frac{\mathrm{d}[\mathrm{D}]}{\mathrm{dt}} \times \frac{1}{\mathrm{d}}
  • For the reactants, negative sign indicates decrease of concentration and for products positive sign indicates increase in concentration.
  • For a reversible reaction at dynamic equilibrium, net reaction rate is always zero as:
    (\mathrm{d} \mathrm{x} / \mathrm{dt})_{\mathrm{f}}=(\mathrm{d} \mathrm{x} / \mathrm{dt})_{\mathrm{B}}
Factors Affecting Rate of Reaction

There are various factors on which the rate of reaction depends:

  • Nature of reactant and product: 
    • For ionic reactants reaction rate is fast as activation energy is zero for them. For example:
      \mathrm{BaCl}_{2}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{BaSO}_{4}+2 \mathrm{HCl}
    • Molecules have slow reaction rate due to need of more activation energy. For example:
      2 \mathrm{CO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{CO}_{2}
  • Physical state of reactants: Rate also changes with physical state.
    Gaseous states > Liquid states > Solid states
  • Pressure: For gaseous reactants rate varies with pressure just like concentration.
    \frac{\mathrm{d} \mathrm{x}}{\mathrm{dt}} \propto \text {Pressure }(\text {as } \mathrm{P} \propto \mathrm{C})
  • Surface Area: Greater the surface area, faster is the rate of reaction due to more number of active sites.
    \text { Rate }(\mathrm{dx} / \mathrm{dt}) \propto \text {Surface area }
Factors Affecting Rate of Reaction(2)
  • Temperature: Rate of reaction increases with the increase of temperature as it increases the number of effective collisions. It is observed that for every 10oC rise in temperature -dx/dt or rates become double to triple as the number of molecules with every greater than activation energy becomes two-three times.
    \mathrm{\text { Temp. Coefficient }(\mu)=\frac{K \text { at } t^{\circ} C+10^{\circ} \mathrm{C}}{K \text { at } t^{\circ} \mathrm{C}}}
    The value of temperature coefficient lies in between 2-3. In case we increase the temperature by more than 10oC the above relation can be given as:
    \frac{\mathrm{K}_{\mathrm{T}_{2}}}{\mathrm{K}_{\mathrm{T}_{1}}}=\mathrm{(\mu)^{\Delta T/10}}
    \text {[Here}\left.\Delta \mathrm{T}=\mathrm{T}_{2}-\mathrm{T}_{1}\right]

    \log _{10} \frac{\mathrm{K}_{\mathrm{T}_{2}}}{\mathrm{K}_{\mathrm{T}_{1}}}=\frac{\Delta \mathrm{T}}{10} \log _{10} \mu

    \frac{\mathrm{K}_{\mathrm{T}_{2}}}{\mathrm{K}_{\mathrm{T}_{1}}}=\text {Antilog}\left[\frac{\Delta \mathrm{T}}{10} \log _{10} \mu\right]
  • Catalyst: It increases the rate of a reaction by decreasing the activation energy by accepting a new alternative smaller path for the reaction. It is reverse in case of negative catalyst to that of positive catalyst. Catalysts are more effective in 'Solid powdered form' due to larger surface area, that is, more active sites.
  • Intensity of light: Rate of photochemical reactions depends upon intensity of light radiations.
    \frac{\mathrm{dx}}{\mathrm{dt}} \propto \text {Intensity of radiation }
  • Concentration of reactants: Rate increases with the increase of concentration as due to more number of reactants there are more collisions.
    \text { Rate of reaction }(\mathrm{dx} / \mathrm{dt}) \propto \text {Concentration}

Study it with Videos

Rate of Reaction
Average Rate of Reaction
Instantaneous Rate of Reaction

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