Chemical kinetics is a branch of chemistry that deals with the rate of chemical reactions, the factors affecting these rates, and the mechanisms by which reactions proceed. Unlike thermodynamics, which tells us whether a reaction is feasible or not, chemical kinetics helps us understand how fast a reaction occurs and the pathway it follows.
This topic is extremely important in Class 12 Chemistry because it provides insights into reaction mechanisms, industrial applications, and the factors controlling reaction rates.
Chemical Kinetics Class 12 Notes| Rate Law, Order, Arrhenius Equation, MCQs
Importance of Chemical Kinetics
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Helps in understanding reaction mechanisms in detail.
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Useful in controlling reaction rates in industrial processes.
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Helps in drug formulation and enzyme kinetics in biology and medicine.
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Important for safety in chemical industries by controlling explosive reactions.
Basic Terms in Chemical Kinetics
1. Rate of Reaction
The rate of reaction is the change in the concentration of reactants or products per unit time.
Mathematically,
Rate=−d[Reactant]dt=d[Product]dtRate = -\frac{d[Reactant]}{dt} = \frac{d[Product]}{dt}
The negative sign for reactants indicates that their concentration decreases with time.
2. Average Rate of Reaction
It is the rate measured over a longer time interval.
Average Rate=Δ[Concentration]ΔtAverage \, Rate = \frac{\Delta [Concentration]}{\Delta t}
3. Instantaneous Rate of Reaction
The rate of reaction at a particular moment of time is called the instantaneous rate. It is obtained by taking the slope of the concentration-time curve at that instant.
4. Factors Affecting Rate of Reaction
The rate of a reaction depends on:
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Concentration of reactants
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Temperature
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Pressure (for gases)
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Catalyst
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Surface area (in heterogeneous reactions)
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Nature of reactants
Rate Law and Rate Constant
Rate Law Expression
For a reaction:
aA+bB→ProductsaA + bB → Products
The rate law is given as:
Rate=k[A]x[B]yRate = k [A]^x [B]^y
Here,
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kk = Rate constant
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x,yx, y = Order of reaction with respect to A and B
Characteristics of Rate Constant
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Independent of concentration.
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Depends on temperature and catalyst.
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Different for different reactions.
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Its units depend on the order of reaction.
Order of Reaction
Definition
The sum of the powers of concentration terms in the rate law is called the order of reaction.
For example, in
Rate=k[A]1[B]2Rate = k [A]^1 [B]^2
Order = 1 + 2 = 3
Types of Order
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Zero Order Reaction – Rate is independent of reactant concentration.
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First Order Reaction – Rate depends on the first power of concentration.
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Second Order Reaction – Rate depends on the square of concentration.
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Fractional Order – Order may be in fractions (e.g., 1/2).
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Mixed Order – Different orders in different stages.
Molecularity of a Reaction
Definition
The number of reactant species (atoms, ions, molecules) colliding in a single step to form products is called molecularity.
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Unimolecular reaction → One reactant species.
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Bimolecular reaction → Two reactant species.
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Termolecular reaction → Three reactant species.
Difference Between Order and Molecularity
| Property | Order of Reaction | Molecularity |
|---|---|---|
| Definition | Sum of powers in rate law | Number of colliding species |
| Experiment | Determined experimentally | Theoretical concept |
| Value | Can be zero, fractional | Always a whole number |
| Stage dependency | Depends on slow step | Entire reaction |
Integrated Rate Equations
1. Zero Order Reaction
Rate = k
[A]=[A]0−kt[A] = [A]_0 – kt
Half-life: t1/2=[A]02kt_{1/2} = \frac{[A]_0}{2k}
2. First Order Reaction
Rate = k[A]
k=2.303tlog[A]0[A]k = \frac{2.303}{t} \log \frac{[A]_0}{[A]}
Half-life: t1/2=0.693kt_{1/2} = \frac{0.693}{k}
3. Second Order Reaction
Rate = k[A]^2
k=1t(1[A]−1[A]0)k = \frac{1}{t} \left( \frac{1}{[A]} – \frac{1}{[A]_0} \right)
Half-life depends on initial concentration.
Half-Life Period (t₁/₂)
The time required for the concentration of a reactant to become half of its initial value.
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For first-order: Independent of initial concentration.
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For zero/second-order: Depends on initial concentration.
Pseudo First Order Reactions
Reactions which are not truly first-order but behave like first-order reactions due to the presence of one reactant in excess.
Example: Hydrolysis of ester in the presence of water.
Temperature Dependence of Rate Constant
Arrhenius Equation
The relation between rate constant and temperature is given by:
k=Ae−EaRTk = A e^{-\frac{E_a}{RT}}
Where,
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AA = Frequency factor
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EaE_a = Activation energy
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RR = Gas constant
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TT = Temperature
Activation Energy (Ea)
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Minimum energy required for effective collisions.
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Higher EaE_a → Slower reaction.
Collision Theory of Chemical Reactions
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Reactant molecules must collide to form products.
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Collisions must have sufficient energy and proper orientation.
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Not all collisions are effective; only those with energy ≥ EaE_a lead to product formation.
Catalysis and Rate of Reaction
Catalysts alter the rate of reaction by providing an alternative pathway with lower activation energy.
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Positive catalysts → Increase rate.
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Negative catalysts (inhibitors) → Decrease rate.
Enzyme Catalysis
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Biological catalysts called enzymes accelerate reactions in living systems.
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Example: Catalase enzyme decomposing hydrogen peroxide.
Applications of Chemical Kinetics
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Pharmaceutical industry for drug shelf life.
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Chemical industry for process optimization.
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Environmental science for pollutant degradation.
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Food industry for preservation methods.
Graphical Representation in Chemical Kinetics
1. Concentration vs Time Curve
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Zero Order → Straight line.
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First Order → Exponential decay curve.
2. Rate vs Concentration Curve
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Linear for first-order reactions.
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Non-linear for complex orders.
Experimental Methods for Rate Measurement
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Conductometry
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Colorimetry
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Manometry
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Titrimetric methods
Summary of Formulas
| Reaction Order | Integrated Equation | Half-life Expression |
|---|---|---|
| Zero Order | [A] = [A]_0 – kt | t1/2=[A]0/2kt_{1/2} = [A]_0/2k |
| First Order | k=2.303tlog[A]0[A]k = \frac{2.303}{t} \log \frac{[A]_0}{[A]} | t1/2=0.693/kt_{1/2} = 0.693/k |
| Second Order | k=1t(1[A]−1[A]0)k = \frac{1}{t} \left(\frac{1}{[A]} – \frac{1}{[A]_0}\right) | Depends on [A]_0 |
Multiple Choice Questions (MCQs) – Chemical Kinetics
Q1. The rate of reaction depends on:
a) Concentration of reactants
b) Temperature
c) Catalyst
d) All of the above
Answer:
The correct answer is d) All of the above because reaction rate depends on concentration, temperature, and catalyst.
Q2. Rate law of a reaction is determined by:
a) Stoichiometry
b) Experimental studies
c) Molecularity
d) Theoretical assumptions
Answer:
The correct answer is b) Experimental studies because rate law cannot be predicted theoretically.
Q3. The unit of rate constant for a first-order reaction is:
a) L mol⁻¹ s⁻¹
b) mol L⁻¹ s⁻¹
c) s⁻¹
d) mol⁻¹ L s⁻¹
Answer:
The correct answer is c) s⁻¹ because for first-order reactions, rate constant has the dimension of reciprocal time.
Q4. Half-life of a first-order reaction depends on:
a) Initial concentration
b) Temperature
c) Both a and b
d) Only pressure
Answer:
The correct answer is b) Temperature because half-life for first-order reactions is independent of initial concentration but depends on temperature.
Q5. Hydrolysis of an ester in excess water follows:
a) Zero order
b) First order
c) Second order
d) Third order
Answer:
The correct answer is b) First order because it is a pseudo-first-order reaction.
Q6. The energy barrier that must be overcome for a reaction to occur is called:
a) Threshold energy
b) Activation energy
c) Potential energy
d) Kinetic energy
Answer:
The correct answer is b) Activation energy because it is the minimum energy required for effective collisions.
Q7. Which factor does not affect the rate constant?
a) Concentration
b) Temperature
c) Catalyst
d) Nature of reactants
Answer:
The correct answer is a) Concentration because rate constant depends only on temperature and catalyst, not concentration.
Q8. For a zero-order reaction, the rate of reaction:
a) Increases with time
b) Decreases with time
c) Is independent of concentration
d) Depends on concentration
Answer:
The correct answer is c) Is independent of concentration because zero-order reactions do not depend on reactant concentration.
Q9. Arrhenius equation relates rate constant with:
a) Concentration
b) Temperature
c) Time
d) Volume
Answer:
The correct answer is b) Temperature because it shows how rate constant varies with temperature.
Q10. The slope of a first-order reaction graph between log[A] and time gives:
a) Rate constant
b) Activation energy
c) Order of reaction
d) Half-life
Answer:
The correct answer is a) Rate constant because slope = –k/2.303 in first-order reactions.
Q11. A catalyst increases the rate of reaction by:
a) Increasing activation energy
b) Decreasing activation energy
c) Increasing temperature
d) Decreasing pressure
Answer:
The correct answer is b) Decreasing activation energy because it provides an alternate pathway with lower energy.
Q12. Which of the following is always a whole number?
a) Order of reaction
b) Molecularity
c) Rate constant
d) Half-life
Answer:
The correct answer is b) Molecularity because it refers to the number of molecules involved in a single step.
Q13. A unimolecular reaction involves:
a) One reactant species
b) Two reactant species
c) Three reactant species
d) No reactant species
Answer:
The correct answer is a) One reactant species because only one molecule undergoes change in the rate-determining step.
Q14. The half-life of a zero-order reaction is directly proportional to:
a) Initial concentration
b) Temperature
c) Catalyst concentration
d) Rate constant
Answer:
The correct answer is a) Initial concentration because t1/2=[A]0/2kt_{1/2} = [A]_0 / 2k for zero-order reactions.
Q15. In a first-order reaction, concentration becomes half after every:
a) Equal time intervals
b) Unequal time intervals
c) Constant volume intervals
d) None of these
Answer:
The correct answer is a) Equal time intervals because first-order reactions have constant half-life.
Q16. The rate of reaction for aA+bB→ProductsaA + bB \rightarrow Products is given by:
a) k[A] + [B]
b) k[A][B]
c) k[A]^m [B]^n
d) None of these
Answer:
The correct answer is c) k[A]^m [B]^n because it is the general rate law expression.
Q17. For a reaction with order 2, the unit of rate constant is:
a) L mol⁻¹ s⁻¹
b) mol L⁻¹ s⁻¹
c) s⁻¹
d) mol² L⁻² s⁻¹
Answer:
The correct answer is a) L mol⁻¹ s⁻¹ because unit depends on the overall reaction order.
Q18. The reaction rate constant increases with:
a) Increase in activation energy
b) Decrease in activation energy
c) No change in activation energy
d) Increase in pressure
Answer:
The correct answer is b) Decrease in activation energy because lower activation energy means faster reactions.
Q19. The order of reaction can be:
a) Zero
b) Fractional
c) Negative
d) All of the above
Answer:
The correct answer is d) All of the above because order can have any real value depending on experimental results.
Q20. Enzyme catalysts are:
a) Organic in nature
b) Inorganic in nature
c) Metals only
d) Non-metals only
Answer:
The correct answer is a) Organic in nature because enzymes are proteins that catalyze biochemical reactions.
Short Answer Questions – Chemical Kinetics
Q1. Define chemical kinetics.
Answer:
Chemical kinetics is the branch of chemistry that studies the rate of chemical reactions, factors affecting them, and the mechanism by which reactions occur.
Q2. What is the rate law of a reaction?
Answer:
The rate law is a mathematical equation that expresses the rate of a reaction as a function of the concentration of reactants raised to certain powers determined experimentally.
Q3. What is the difference between order and molecularity?
Answer:
Order is determined experimentally and can be zero, fractional, or whole number, while molecularity is a theoretical concept representing the number of reacting species and is always a whole number.
Q4. Define activation energy.
Answer:
Activation energy is the minimum amount of energy required for reactant molecules to undergo effective collisions and form products.
Q5. Write the half-life expression for a first-order reaction.
Answer:
For a first-order reaction, half-life is given by t1/2=0.693kt_{1/2} = \frac{0.693}{k} and is independent of the initial concentration of the reactant.
Q6. What are pseudo-first-order reactions?
Answer:
Reactions that are not truly first-order but appear to follow first-order kinetics because one reactant is present in large excess are called pseudo-first-order reactions.
Q7. Name two factors affecting the rate of reaction.
Answer:
Two factors affecting reaction rate are temperature and the presence of a catalyst.
Q8. Write the integrated rate equation for a zero-order reaction.
Answer:
For a zero-order reaction, the integrated rate equation is [A]=[A]0−kt[A] = [A]_0 – kt.
Q9. What is meant by the rate constant?
Answer:
The rate constant (k) is a proportionality constant in the rate law that relates the rate of reaction to the concentrations of reactants.
Q10. What is the Arrhenius equation?
Answer:
The Arrhenius equation, k=Ae−EaRTk = A e^{-\frac{E_a}{RT}}, shows how the rate constant varies with temperature and activation energy.
Long Answer Questions – Chemical Kinetics
Q1. Explain the difference between average rate and instantaneous rate of a reaction.
Answer:
The average rate of reaction is measured over a finite time interval and gives the change in concentration per unit time between two points. Instantaneous rate, on the other hand, is the rate at a specific moment in time and is determined by finding the slope of the concentration vs. time curve at that point.
Q2. Derive the integrated rate equation for a first-order reaction.
Answer:
For a first-order reaction, Rate=−d[A]dt=k[A]Rate = -\frac{d[A]}{dt} = k[A]. Separating variables and integrating, we get
∫d[A][A]=−k∫dt\int \frac{d[A]}{[A]} = -k \int dt
which simplifies to ln[A]=−kt+ln[A]0\ln[A] = -kt + \ln[A]_0. Taking log base 10,
log[A]=log[A]0−k2.303t\log[A] = \log[A]_0 – \frac{k}{2.303}t
This is the integrated rate equation for first-order reactions.
Q3. Explain collision theory of chemical reactions.
Answer:
Collision theory states that reactant molecules must collide with sufficient energy and proper orientation to form products. Not all collisions are effective; only those with energy greater than or equal to the activation energy lead to product formation. The frequency and effectiveness of collisions determine the reaction rate.
Q4. Describe the temperature dependence of the reaction rate constant using the Arrhenius equation.
Answer:
The Arrhenius equation, k=Ae−EaRTk = A e^{-\frac{E_a}{RT}}, relates the rate constant (k) to temperature (T). As temperature increases, the exponential factor e−EaRTe^{-\frac{E_a}{RT}} increases, leading to a larger rate constant and thus a faster reaction. A plot of lnk\ln k vs 1/T1/T gives a straight line with slope −Ea/R-E_a/R, from which activation energy can be calculated.
Q5. Discuss the significance of half-life period in chemical kinetics.
Answer:
The half-life period (t₁/₂) is the time required for the concentration of a reactant to become half of its initial value. For first-order reactions, half-life is independent of initial concentration, making it useful in studying radioactive decay and drug metabolism, where concentration changes over time.
Q6. How do catalysts affect the rate of chemical reactions?
Answer:
Catalysts increase the rate of reaction by providing an alternative reaction pathway with lower activation energy. They remain chemically unchanged after the reaction and do not affect the equilibrium position but only the speed of attaining equilibrium.
Q7. Derive the half-life expression for a zero-order reaction.
Answer:
For a zero-order reaction, [A]=[A]0−kt[A] = [A]_0 – kt. At half-life, [A]=[A]0/2[A] = [A]_0/2. Substituting this into the equation, we get:
[A]0/2=[A]0−kt1/2[A]_0/2 = [A]_0 – k t_{1/2}
Class 12 Surface Chemistry Notes | Complete Guide with Questions