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Short Notes: Chemical Kinetics
Rate of Reaction
- Average Rate: Δ[Concentration] / Δt
- Instantaneous Rate: d[Concentration] / dt
- For aA + bB → cC + dD
- Rate = -(1/a)(d[A]/dt) = -(1/b)(d[B]/dt) = (1/c)(d[C]/dt) = (1/d)(d[D]/dt)
Factors Affecting Rate
- Concentration: Higher concentration → Higher rate
- Temperature: Higher temperature → Higher rate
- Catalyst: Increases rate by lowering activation energy
- Surface area: Larger surface area → Higher rate
- Nature of reactants: Ionic reactions faster than molecular
Rate Law and Order
- Rate Law: Rate = k[A]m[B]n
- k = rate constant
- Order of reaction = m + n (determined experimentally)
- Molecularity: Number of molecules participating in elementary step (1, 2, or 3)
Zero Order Reaction
| Property | Formula/Value |
|---|---|
| Rate Law | Rate = k |
| Integrated Rate | [A] = [A]0 - kt |
| Half-life | t1/2 = [A]0 / 2k |
| Units of k | mol L-1 s-1 |
| Graph | [A] vs t is straight line (slope = -k) |
First Order Reaction
| Property | Formula/Value |
|---|---|
| Rate Law | Rate = k[A] |
| Integrated Rate | ln[A] = ln[A]0 - kt or k = (2.303/t) log([A]0/[A]) |
| Half-life | t1/2 = 0.693/k (independent of [A]0) |
| Units of k | s-1 or time-1 |
| Graph | ln[A] vs t is straight line (slope = -k) |
| Examples | Radioactive decay, decomposition of N2O5 |
Pseudo First Order Reaction
- Second order reaction behaving as first order
- Example: Hydrolysis of ester
- CH3COOC2H5 + H2O → CH3COOH + C2H5OH
- Rate = k[Ester][H2O] ≈ k'[Ester] (when H2O in excess)
Temperature Dependence - Arrhenius Equation
- k = Ae-Ea/RT
- A = Arrhenius factor (frequency factor)
- Ea = Activation energy
- ln k = ln A - Ea/RT
- log k = log A - Ea/(2.303RT)
- For two temperatures: log(k2/k1) = (Ea/2.303R) × [(T2-T1)/(T1T2)]
Collision Theory
- Reactant molecules must collide to react
- Collisions must have:
- Sufficient energy (≥ Ea)
- Proper orientation
- Threshold energy: Minimum energy required for reaction
- Activation energy: Ea = Threshold energy - Average kinetic energy
Catalyst
- Substance that alters rate without being consumed
- Positive catalyst: Increases rate (lowers Ea)
- Negative catalyst: Decreases rate (increases Ea)
- Homogeneous: Same phase as reactants
- Heterogeneous: Different phase from reactants
- Does NOT change equilibrium constant
NEET Focus:
- Integrated rate equations for zero and first order
- Half-life formulas and calculations
- Arrhenius equation problems
- Units of rate constant for different orders
- Pseudo first order reactions
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FAQs on Short Notes: Chemical Kinetics
| 1. What is chemical kinetics? |  |
Ans. Chemical kinetics is the branch of physical chemistry that studies the rates of chemical reactions and the factors affecting these rates. It involves understanding how different conditions, such as temperature, concentration, and catalysts, influence the speed at which reactions occur.
| 2. What factors affect the rate of a chemical reaction? | |
Ans. The rate of a chemical reaction can be affected by several factors, including temperature, concentration of reactants, surface area of solid reactants, and the presence of catalysts. Increasing the temperature generally increases the reaction rate, while higher concentrations of reactants lead to more frequent collisions, thus speeding up the reaction.
| 3. What is the role of catalysts in chemical reactions? | |
Ans. Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by providing an alternative pathway for the reaction with a lower activation energy, which allows more reactant molecules to participate in the reaction, thus speeding it up.
| 4. How is the rate constant related to the order of a reaction? | |
Ans. The rate constant (k) is a proportionality factor in the rate law of a chemical reaction. It varies with temperature and is specific to each reaction. The order of a reaction, determined by the sum of the powers of the concentration terms in the rate law, indicates how the rate depends on the concentration of reactants. For example, in a first-order reaction, the rate is directly proportional to the concentration of one reactant.
| 5. What is the difference between zero-order and first-order reactions? | |
Ans. In zero-order reactions, the rate of reaction is constant and independent of the concentration of reactants; it remains the same regardless of how much reactant is present. In first-order reactions, the rate depends linearly on the concentration of one reactant; as the concentration increases, the rate of reaction also increases proportionally.
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