Chemistry Class 12 – Chapter 3: Chemical Kinetics
Introduction
Chemical kinetics studies the speed of reactions, factors affecting them, and reaction mechanisms.
Reactions can be very fast, moderate, or very slow.
Rate of Reaction
Average Rate = ∆[Reactant]/∆t or ∆[Product]/∆t
Instantaneous Rate = d[ ]/dt at a specific time.
Rate is always positive, units: mol L■¹ s■¹.
Factors Affecting Rate
1. Concentration of reactants
2. Nature/physical state
3. Temperature
4. Catalyst
5. Surface area
6. Pressure (gases).
Rate Law, Order & Molecularity
Rate law: Rate = k[A]^m[B]^n.
Order = m+n+… (experimentally found).
Molecularity = number of molecules in an elementary step.
Integrated Rate Equations & Half-Life
Zero order: [A]=[A]■-kt, t½=[A]■/2k
First order: ln[A]=ln[A]■-kt, t½=0.693/k
Second order: different forms. Half-life = time to reduce concentration to half.
Pseudo-First Order
When one reactant is in large excess, reaction behaves like first order.
Temperature Dependence & Arrhenius Equation
Arrhenius: k=Ae^(-Ea/RT).
ln k vs 1/T gives straight line slope= -Ea/R.
Ea= activation energy, A= frequency factor. Rule: 10°C rise doubles rate.
Activation Energy & Activated Complex
Introduction
Chemical kinetics studies the speed of reactions, factors affecting them, and reaction mechanisms.
Reactions can be very fast, moderate, or very slow.
Rate of Reaction
Average Rate = ∆[Reactant]/∆t or ∆[Product]/∆t
Instantaneous Rate = d[ ]/dt at a specific time.
Rate is always positive, units: mol L■¹ s■¹.
Factors Affecting Rate
1. Concentration of reactants
2. Nature/physical state
3. Temperature
4. Catalyst
5. Surface area
6. Pressure (gases).
Rate Law, Order & Molecularity
Rate law: Rate = k[A]^m[B]^n.
Order = m+n+… (experimentally found).
Molecularity = number of molecules in an elementary step.
Integrated Rate Equations & Half-Life
Zero order: [A]=[A]■-kt, t½=[A]■/2k
First order: ln[A]=ln[A]■-kt, t½=0.693/k
Second order: different forms. Half-life = time to reduce concentration to half.
Pseudo-First Order
When one reactant is in large excess, reaction behaves like first order.
Temperature Dependence & Arrhenius Equation
Arrhenius: k=Ae^(-Ea/RT).
ln k vs 1/T gives straight line slope= -Ea/R.
Ea= activation energy, A= frequency factor. Rule: 10°C rise doubles rate.
Activation Energy & Activated Complex
