Test: Reaction Order & Elementary Rate Law - Chemistry MCQ

The above single step reaction is an elementary reaction. Molecularity is the number of molecules participating in the reaction.

Elementary reactions are the ones with single reaction step and the reaction rate corresponds to stoichiometric equation.

To determine the relative rate of reaction for the given chemical reaction:

2A + 9B → 7C + 4D

• The rate of a chemical reaction is often expressed as the change in concentration of a reactant or product over time. For a balanced chemical equation, the rate of disappearance of reactants and the rate of appearance of products are related through their stoichiometric coefficients.
• For the reaction given, the stoichiometric coefficients are 2 for A, 9 for B, 7 for C, and 4 for D. These coefficients are used to express the rate of reaction relative to each species involved.
• The relative rate of reaction is determined by setting the rate of change for each reactant and product equal, adjusted by their respective stoichiometric coefficients:
  • The rate of disappearance of A: -rA
  • The rate of disappearance of B: -rB
  • The rate of formation of C: rC
  • The rate of formation of D: rD 
• These rates are related to each other by their stoichiometric coefficients:
  • For A: -rA / 2
  • For B: -rB / 9
  • For C: rC / 7
  • For D: rD / 4  
• Thus, the correct expression for the relative rate of reaction is:
  • -rA / 2 = -rB / 9 = rC / 7 = rD / 4 
• Rearranging this, we express them as equal to a common rate, which is often represented as simply the rate of reaction r.
• Therefore, the correct expression from the options provided is:
  • Option B: –rA = -rB / 9 = rC / 7 = rD / 4

Order is empirical, hence can be a fractional value but molecularity refers to the mechanism and is applicable only for elementary reactions.

The reaction rate constant depends on temperature only.

Order is equal to molecularity when reaction is elementary.

General expression for the rate constant is time^-1 concentration^(1-n).

Order w.r.t A is 0.7 and B is 1.3.Hence, Overall order is 2.

Elementary reaction occurs in single reaction step with no intermediates and molecularity is the number of particles involved in the reaction.