Rate of Reaction and Rate Law

topicmedium119 MCQ

Average and instantaneous rates, order and molecularity. (Chemistry › Chemical Kinetics, NEET UG syllabus.)

What is Rate of Reaction and Rate Law?

The change in concentration of a reactant or product per unit time.

Key formula / rule: Average Rate of Reaction

Define and differentiate between average and instantaneous rates of reaction.
Express the rate of reaction in terms of changes in concentration of reactants and products.
Define rate law and rate constant.
Determine the units of the rate constant for different orders of reaction.

Confusing order of reaction with molecularity.

Term: Rate of Reaction
Meaning: The change in concentration of a reactant or product per unit time.

Term: Instantaneous Rate
Meaning: The rate of reaction at a particular instant in time, determined by the slope of the tangent to the concentration-time curve.

Term: Rate Law
Meaning: An experimentally determined expression that relates the rate of a reaction to the concentrations of reactants raised to some powers.

Term: Rate Constant (k)
Meaning: The proportionality constant in the rate law, specific for a given reaction at a particular temperature.

Term: Order of Reaction
Meaning: The sum of the powers to which the concentration terms are raised in the experimentally determined rate law. It can be zero, fractional, or negative.

Term: Molecularity
Meaning: The number of reacting species (atoms, ions, or molecules) that collide simultaneously in an elementary reaction step. It is a theoretical concept and is always a positive integer (1, 2, or 3).

Term: Complex Reaction
Meaning: A reaction that occurs in more than one step, involving a sequence of elementary reactions.

Define and differentiate between average and instantaneous rates of reaction.
Express the rate of reaction in terms of changes in concentration of reactants and products.
Define rate law and rate constant.
Determine the units of the rate constant for different orders of reaction.
Define order of reaction and molecularity.
Distinguish between order of reaction and molecularity.
Identify factors affecting the rate of reaction (concentration, temperature, catalyst, surface area).

Name: Average Rate of Reaction
Note: Negative sign for reactants indicates decrease in concentration, positive for products indicates increase.
Expression: For aA + bB → cC + dD: Rate_avg = -(1/a)Δ[A]/Δt = -(1/b)Δ[B]/Δt = (1/c)Δ[C]/Δt = (1/d)Δ[D]/Δt

Name: Instantaneous Rate of Reaction
Note: Represents the rate at a specific moment in time, obtained from the slope of the concentration-time graph.
Expression: For aA + bB → cC + dD: Rate_inst = -(1/a)d[A]/dt = -(1/b)d[B]/dt = (1/c)d[C]/dt = (1/d)d[D]/dt

Name: General Rate Law
Note: k is the rate constant, x and y are the orders with respect to A and B, determined experimentally.
Expression: Rate = k[A]^x[B]^y

Name: Units of Rate Constant (k)
Note: Where 'n' is the overall order of the reaction. For 0-order: mol L⁻¹ s⁻¹; for 1st-order: s⁻¹; for 2nd-order: L mol⁻¹ s⁻¹.
Expression: (mol L⁻¹)^(1-n) s⁻¹

Confusing order of reaction with molecularity.
Assuming stoichiometric coefficients are always the orders of reaction.
Incorrectly calculating units of the rate constant 'k' for different orders.
Not understanding that the rate law is an experimental observation, not derived from stoichiometry.
Ignoring the negative sign for reactant concentration change and positive for product concentration change when defining rate.

For the reaction 2A + B → 3C, if the rate of disappearance of A is 0.04 mol L⁻¹ s⁻¹, what is the rate of formation of C?…
easy
For the reaction R → P, the concentration of a reactant R changes from 0.1 M to 0.05 M in 20 minutes. What is the average rate of the reaction in mol L⁻¹ s⁻¹?…
medium
For the reaction 2NO(g) + 2H₂(g) → N₂(g) + 2H₂O(g), the following data were collected at 1280 °C: | Experiment | [NO] (M) | [H₂] (M) | Initial Rate (M s⁻¹) | | :--------- | :------- | :------- | :------------------- | | …
hard