Chemical Kinetics — Part 2: Integrated rate laws, temperature and collisions: visual lesson, CBSE Class 12 Chemistry

Lesson

Rates over time and temperature

Part 2 — how concentration changes with time, why heat speeds reactions up, and what happens when molecules collide.

In this lesson you will

Use the integrated rate equations for zero- and first-order reactions and their half-lives
Recognise pseudo-first-order reactions
Explain the effect of temperature and catalysts using the Arrhenius equation and energy profiles, and calculate activation energy
Explain reaction rates using collision theory, including energy and orientation

Concept

Zero-order reactions

Rate $= k[\ce{R}]^0 = k$ : the concentration falls at a constant rate. Integrating gives a straight line. Examples: decomposition of ammonia on a hot platinum surface at high pressure; some enzyme reactions when the enzyme is saturated.

[\ce{R}] = [\ce{R}]_0 - kt, \qquad k = \dfrac{[\ce{R}]_0 - [\ce{R}]}{t}

Table of the features of a zero-order reaction: straight line of concentration against time, slope minus k, units and the surface-saturation reason
Feature	Zero order
Plot that is a straight line	[R] against t
Slope	$-k$
Unit of k	$\text{mol L}^{-1}\ \text{s}^{-1}$
Why zero order?	The surface (or enzyme) is fully covered, so extra reactant cannot react faster

Diagram

Zero order on a graph

Zero order: straight line, slope −k

k = 0.02 mol L⁻¹ min⁻¹ in this example.

The plot of $[\ce{R}]$ against $t$ is a straight line with intercept $[\ce{R}]_0$ and slope $-k$ . The reaction stops abruptly when the reactant runs out.

Cookies