NEET Chemical Kinetics Complete Notes and Important Questions by PW

Chemical Kinetics focuses on the rate of chemical reactions, the factors affecting reaction rates and the mathematical relationships used to study reaction progress. Understanding rate laws, reaction order, rate constants, half-life and the Arrhenius equation can help you solve conceptual and numerical questions from this chapter.

PW offers Chemical Kinetics notes and important questions for NEET covering important concepts, useful tricks and previous year questions (PYQs). Use the session alongside your notes to revise key formulas, understand numerical-solving approaches and practise applying Chemical Kinetics concepts for NEET 2027.

Rate Of Reaction And Stoichiometric Relationships

During a reaction, reactant concentration decreases, while product concentration increases. Therefore, rate can be expressed as:

• Rate of disappearance of a reactant: -d[R]/dt

• Rate of appearance of a product: +d[P]/dt

The negative sign is used for a reactant because its concentration falls with time. The positive sign is used for a product because its concentration rises. The rate of reaction is always a positive quantity.

For the reaction:

aA + bB → cC + dD

the overall rate is:

Rate = -(1/a)d[A]/dt = -(1/b)d[B]/dt = +(1/c)d[C]/dt = +(1/d)d[D]/dt

The individual rate of a substance is not divided by its coefficient. However, the overall rate of reaction depends on the stoichiometric coefficient.

(*Memory Tip: Reactant disappears, so use a negative sign. Product appears, so use a positive sign. Overall rate requires division by the stoichiometric coefficient, but individual rate does not.)*

Units And Forms Of Reaction Rate

When concentration is used, the unit of rate is mol L⁻¹ time⁻¹, such as mol L⁻¹ s⁻¹.

Rate may also be expressed using:

• Concentration

• Number of moles

• Pressure for gaseous reactions

It should not be expressed directly in terms of mass.

If concentration is replaced by number of moles at constant volume, divide by both the stoichiometric coefficient and volume. For gases, the ideal gas equation gives:

P = (n/V)RT

Therefore:

Concentration = P/RT

This permits conversion between concentration-based and pressure-based rate expressions.

Ammonia Formation Example

For:

N₂ + 3H₂ → 2NH₃

the overall rate is:

Rate = -d[N₂]/dt = -(1/3)d[H₂]/dt = +(1/2)d[NH₃]/dt

Thus:

Rate Of NH₃ Formation = (2/3) × Rate Of H₂ Disappearance

If ammonia appears at 2.5 × 10⁻⁴ mol L⁻¹ s⁻¹, hydrogen disappears at:

(3/2) × 2.5 × 10⁻⁴ = 3.75 × 10⁻⁴ mol L⁻¹ s⁻¹

Rate Law And Order Of Reaction

The rate law gives the relation between reaction rate and reactant concentration:

Rate = k[A]^m[B]^n

Here, k is the rate constant. The powers m and n are the orders with respect to A and B. The overall order is:

Overall Order = m + n

Order is the power of a concentration or pressure term in the rate law. It may be positive, zero, negative, or fractional.

For example, if:

Rate = k[A]^3

doubling A makes the rate eight times greater. If:

Rate = k[A]^(1/2)[B]^2

then:

• Order with respect to A = 1/2

• Order with respect to B = 2

• Overall order = 5/2

If A is increased fourfold, its contribution to the rate is 4^(1/2) = 2. If B is doubled, its contribution is 2² = 4.

Experimental Determination Of Order

Order is an experimental quantity and cannot always be obtained from the balanced equation. To find the order with respect to one reactant, keep the concentration of the other reactants constant.

If doubling A makes the rate four times greater:

2^m = 4, so m = 2

If doubling B causes no rate change:

2^n = 1, so n = 0

The rate law then becomes:

Rate = k[A]²

(*Memory Tip: To determine order experimentally, keep one reactant constant and vary the other. Compare the rate-change factor with the concentration-change factor.)*

Order From The Unit Of Rate Constant

For an overall order n:

Unit of k = mol^(1-n) L^(n-1) time⁻¹

Common units include:

The order can be determined from a theoretical rate law, experimental data, or the unit of k.

Elementary And Complex Reactions

In an elementary reaction, the rate law can be written directly from the stoichiometric coefficients. For:

2A + B → Products

Rate = k[A]²[B]

The order and molecularity are both 3 in this suitable elementary-reaction case.

A complex reaction occurs through several elementary steps. The slowest step is the rate-determining step. The initial rate expression is written from this slow step, but intermediates must be removed from the final rate law.

Therefore, complex reactions can have fractional orders even when the overall equation contains whole-number coefficients.

Molecularity cannot be zero, negative, or fractional.

Integrated Rate Laws

Zero-Order Reaction

For a zero-order reaction:

Rate = k

The integrated equation is:

[R]t = [R]0 - kt

The graph of [R] versus time is a straight line with slope -k. The rate remains constant, and complete consumption can occur in finite time.

Half-life:

t₁/₂ = [R]0/(2k)

Thus, zero-order half-life depends directly on initial concentration.

First-Order Reaction

For a first-order reaction:

Rate = k[R]

The integrated equation is:

ln([R]0/[R]t) = kt

or:

log([R]0/[R]t) = kt/2.303

Half-life:

t₁/₂ = 0.693/k

First-order half-life is independent of initial concentration. The concentration decreases exponentially, and complete conversion requires infinite time.

After n half-lives:

[R]t = [R]0/2ⁿ

Therefore, one-fourth concentration requires two half-lives, one-eighth requires three half-lives, and so on.

(*Memory Tip: In a first-order reaction, every half-life reduces the remaining amount by half. 

Equal fractional changes require equal times, regardless of the starting concentration.)*

For percentage completion, first calculate the percentage remaining:

• 99% completion means 1% remains.

• 99.9% completion means 0.1% remains.

Hence:

t₉₉.₉% = (3/2)t₉₉%

General Nth-Order Reaction

For n not equal to 1:

1/[R]t^(n-1) - 1/[R]0^(n-1) = (n-1)kt

The half-life varies as:

t₁/₂ proportional to 1/[k[R]0^(n-1)]

Arrhenius Equation And Activation Energy

The Arrhenius equation is:

k = A exp(-Ea/RT)

where:

• k = rate constant

• A = pre-exponential factor

• Ea = activation energy

• R = gas constant

• T = absolute temperature

For a plot of ln k versus 1/T:

• Slope = -Ea/R

• Intercept = ln A

For a plot of log k versus 1/T:

• Slope = -Ea/(2.303R)

• Intercept = log A

At infinite temperature, exp(-Ea/RT) approaches 1. Therefore:

k = A

For two temperatures:

ln(k₂/k₁) = (Ea/R)(1/T₁ - 1/T₂)

Increasing temperature increases k. Since first-order half-life equals 0.693/k, its half-life decreases as temperature rises.

Collision Theory And Catalyst

Collision theory states:

Rate is proportional to PZAB exp(-Ea/RT)

Here:

• ZAB = collision frequency

• P = probability or steric factor

• exp(-Ea/RT) = fraction of molecules with energy equal to or greater than activation energy

Only effective collisions produce products.

A catalyst provides an alternative pathway with lower activation energy. It increases reaction rate and helps the system reach equilibrium faster.

A catalyst does not change:

• Enthalpy change, ΔH

• State functions

• Equilibrium constant

(*Memory Tip: A catalyst changes the pathway and activation energy, but not the thermodynamic quantities or equilibrium constant.)*

Chemical Kinetics: Important Questions and Answers

Q1. For the reaction N₂ + 3H₂ → 2NH₃, how is the rate of ammonia formation related to the rate of hydrogen disappearance?

Answer: The overall reaction rate is expressed as:

Rate = −d[N₂]/dt = −(1/3)d[H₂]/dt = +(1/2)d[NH₃]/dt

Therefore, the rate of ammonia formation is two-thirds of the rate of hydrogen disappearance.

If NH₃ forms at 2.5 × 10⁻⁴ mol L⁻¹ s⁻¹, the rate of H₂ disappearance is:

(3/2) × 2.5 × 10⁻⁴ = 3.75 × 10⁻⁴ mol L⁻¹ s⁻¹

Q2. If doubling the concentration of reactant A increases the reaction rate fourfold, what is the order of the reaction with respect to A?

Answer: Using the rate law, Rate = k[A]ᵐ:

2ᵐ = 4

Therefore, m = 2. The reaction is second order with respect to A.

Q3. What is the difference between the order and molecularity of a reaction?

Answer:

• Order: The sum of the powers of reactant concentrations in the experimentally determined rate law. It can be zero, negative, fractional or positive.

• Molecularity: The number of reacting species participating in a single elementary step. It is always a positive whole number.

Q4. What is the half-life of a first-order reaction, and does it depend on the initial concentration?

Answer: The half-life of a first-order reaction is the time required for the reactant concentration to decrease to half its initial value.

The formula is:

t₁/₂ = 0.693/k

Here, k is the rate constant. The half-life is independent of the initial reactant concentration.

Q5. What is the Arrhenius equation, and what does the slope of an Arrhenius plot represent?

Answer: The Arrhenius equation relates the rate constant to temperature and activation energy:

k = A exp(−Eₐ/RT)

For a plot of ln k against 1/T:

• Slope: −Eₐ/R

• Intercept: ln A

For a plot of log k against 1/T:

• Slope: −Eₐ/(2.303R)

• Intercept: log A

Here, Eₐ is the activation energy, R is the gas constant, T is the absolute temperature, and A is the pre-exponential factor.

Revise Chemical Kinetics with PW through a one-shot session covering important concepts, useful tricks, PYQs and problem-solving approaches. Use the video to strengthen your understanding of reaction rates, rate laws and numerical questions as part of your NEET Chemistry preparation.

Chemical Kinetics preparation requires a clear understanding of reaction rates, rate laws, reaction order and the formulas used to solve numerical problems. Revise the concepts regularly, practise calculations and review the questions you find difficult. Use your notes alongside the PW one-shot video to reinforce key concepts, learn useful tricks and improve your approach to NEET Chemistry questions.

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