Electrochemistry and chemical kinetics are two of the most scoring chapters in Class 12 Chemistry. They combine conceptual understanding with numerical problem-solving, and both are heavily tested in CBSE board exams, JEE, and NEET.
What Is Electrochemistry?
Electrochemistry is the study of the relationship between electrical energy and chemical reactions. It covers two main types of cells: galvanic (voltaic) cells that convert chemical energy to electrical energy, and electrolytic cells that use electrical energy to drive non-spontaneous reactions.Galvanic Cell vs Electrolytic Cell
| Feature | Galvanic Cell | Electrolytic Cell |
|---|---|---|
| Energy conversion | Chemical to electrical | Electrical to chemical |
| Reaction type | Spontaneous | Non-spontaneous |
| Cell potential | Positive | Negative (requires external voltage) |
| Anode | Negative terminal (oxidation) | Positive terminal (oxidation) |
| Cathode | Positive terminal (reduction) | Negative terminal (reduction) |
| Example | Daniel cell (Zn-Cu) | Electrolysis of water |
What Is the Nernst Equation?
The Nernst equation relates the cell potential to the standard cell potential and the concentrations of reactants and products:
E(cell) = E°(cell) - (RT / nF) ln Q
At 25°C, this simplifies to: E(cell) = E°(cell) - (0.0592 / n) log Q
where E° is the standard cell potential, n is the number of electrons transferred, and Q is the reaction quotient.
At equilibrium, E(cell) = 0 and Q = K(equilibrium constant), so: E°(cell) = (0.0592 / n) log K.
What Is Chemical Kinetics?
Chemical kinetics is the study of the rates of chemical reactions and the factors that affect them.Rate of Reaction
The rate of reaction is the change in concentration of a reactant or product per unit time.
For a reaction A → B:
- Rate = -d[A]/dt = +d[B]/dt
What Is the Rate Law?
The rate law expresses the rate of reaction as a function of reactant concentrations:
Rate = k[A]^m[B]^n
where k is the rate constant, and m and n are the orders of reaction with respect to A and B. The overall order is m + n. These orders must be determined experimentally — they cannot be predicted from the balanced equation.
Orders of Reaction
| Order | Rate Law | Units of k | Integrated Rate Law | Half-life |
|---|---|---|---|---|
| Zero | Rate = k | mol L⁻¹ s⁻¹ | [A] = [A]₀ - kt | t½ = [A]₀ / 2k |
| First | Rate = k[A] | s⁻¹ | ln[A] = ln[A]₀ - kt | t½ = 0.693 / k |
| Second | Rate = k[A]² | L mol⁻¹ s⁻¹ | 1/[A] = 1/[A]₀ + kt | t½ = 1 / k[A]₀ |
What Is the Arrhenius Equation?
The Arrhenius equation describes how the rate constant k changes with temperature:
k = A × e^(-Ea/RT)
where A is the pre-exponential factor (frequency factor), Ea is the activation energy, R is the gas constant, and T is temperature in Kelvin.
In logarithmic form: ln k = ln A - Ea/RT
A plot of ln k versus 1/T gives a straight line with slope = -Ea/R. This is the most common way to determine activation energy experimentally.
The key takeaway: increasing temperature increases k, which increases the rate of reaction. A rough rule is that a 10°C rise doubles the reaction rate.
How to Solve Electrochemistry and Kinetics Numericals
- For electrochemistry: identify the half-reactions, calculate E°(cell), then use the Nernst equation if concentrations are not standard
- For kinetics: identify the order from experimental data (initial rates method or graphical method), then use the appropriate integrated rate law
- For Arrhenius problems: use the two-temperature form: ln(k2/k1) = (Ea/R)(1/T1 - 1/T2)
Key Takeaways
Frequently Asked Questions
What is the difference between order and molecularity of a reaction?
Order is an experimentally determined value that describes how the rate depends on concentration. It can be zero, fractional, or negative. Molecularity is the number of molecules participating in an elementary step and is always a positive integer (1, 2, or 3). For complex reactions with multiple steps, molecularity applies to each elementary step, while order applies to the overall reaction.
How do you calculate the EMF of a galvanic cell?
E°(cell) = E°(cathode) - E°(anode), using standard reduction potentials from the electrochemical series. The electrode with the higher reduction potential becomes the cathode. If concentrations are not standard (1M, 1 atm), use the Nernst equation.
How do you determine the order of a reaction from experimental data?
Use the initial rates method. Compare two experiments where only one reactant concentration changes. If doubling the concentration doubles the rate, it is first order. If it quadruples the rate, it is second order. If the rate does not change, it is zero order with respect to that reactant.
Why is activation energy important?
Activation energy is the minimum energy that reactant molecules must have to undergo a successful collision and form products. A lower activation energy means more molecules have enough energy to react, resulting in a faster reaction. Catalysts work by providing an alternative pathway with lower activation energy.
Test your Class 12 Chemistry knowledge with our Chemistry Grade 12 quizzes covering electrochemistry, kinetics, and all major topics.