Rate of Reaction and Rate Laws — Part 1: Rate, factors and rate laws: visual lesson, ISC Class 12 Chemistry

Lesson

Rate of reaction

Part 1 — how fast reactions go and how the rate depends on concentration.

In this lesson you will

Express average and instantaneous rates and relate the rates of change of different species
Explain slow and fast reactions in terms of bonds, and the effect of each factor on rate
State the law of mass action
Deduce order and rate constant from initial-rate data and derive the units of k

Table of reactions with very different speeds
Reaction	Typical speed
AgNO₃ + NaCl → AgCl precipitate	Instantaneous
Fading of a dye in sunlight	Hours to days
Rusting of iron	Weeks to months
Formation of coal	Millions of years

Concept

Slow and fast reactions

Ionic reactions in solution are very fast: ions are already free, and no bonds need to be broken, so they react on collision. Molecular (covalent) reactions are slower: covalent bonds must be broken and new ones formed, which needs energy and suitable orientation.

Table of fast, moderate and slow reactions explained by bonds
Type	Why	Example
Very fast	Oppositely charged ions combine; no bond breaking	Ag⁺ + Cl⁻ → AgCl; H⁺ + OH⁻ → H₂O
Moderate	Few bonds broken; measurable rate	Decomposition of H₂O₂; hydrolysis of an ester
Very slow	Strong bonds, high energy barrier	Rusting; H₂ + O₂ at room temperature

Concept

Rate of reaction

The rate is the change in concentration of a reactant or product per unit time. For $\mathrm{R} \rightarrow \mathrm{P}$ , the average rate over an interval is $-\Delta[\ce{R}]/\Delta t = \Delta[\ce{P}]/\Delta t$ ; the minus sign makes the rate positive because $[\ce{R}]$ falls. Units: $\text{mol L}^{-1}\ \text{s}^{-1}$ ; for gases, atm s⁻¹.

r_\text{av} = -\frac{\Delta[\ce{R}]}{\Delta t} = \frac{\Delta[\ce{P}]}{\Delta t}, \qquad r_\text{inst} = -\frac{d[\ce{R}]}{dt}

Table of key points about the rate of reaction
Point	Explanation
Minus sign for reactants	Δ[R] is negative; rate must be positive
Rate changes with time	Reactants are used up, so most reactions slow down
Units	mol L⁻¹ s⁻¹ (or min⁻¹)

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