Chemical Kinetics

Lesson

Part 1 — how fast reactions go, how we measure it, and how the speed depends on concentration.

In this lesson you will

Express the average and instantaneous rate of a reaction and relate the rates of change of different species
Identify the factors that affect the rate of a reaction
Write a rate law, find the order of a reaction from data and state the units of the rate constant
Distinguish order from molecularity and explain the rate-determining step

Table of three reactions with very different speeds: silver chloride precipitation almost instant, rusting over months, weathering over thousands of years
Reaction	Speed
Precipitation of silver chloride	Almost instant
Rusting of an iron nail	Days to months
Weathering of rocks	Thousands of years

Concept

What is the rate of a reaction?

The rate of a reaction 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:

r_\text{av} = -\dfrac{\Delta[\ce{R}]}{\Delta t} = +\dfrac{\Delta[\ce{P}]}{\Delta t}

Table explaining the minus sign for reactants, the units of rate and why rate changes with time
Point	Why
Minus sign for reactants	[R] decreases, so Δ[R] is negative; rate must be positive
Units	$\text{mol L}^{-1}\ \text{s}^{-1}$ (or per minute); for gases $\text{atm s}^{-1}$
Rate changes with time	As reactants are used up, the reaction slows

Diagram

Average and instantaneous rate

Tangent at t = 10 s
Concentration of R

Illustrative data. The rate is minus the slope of the tangent.

The average rate is the slope of the chord between two times. The instantaneous rate at a moment is the slope of the tangent to the concentration–time curve at that moment: $r = -\dfrac{d[\ce{R}]}{dt}$ .

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