VIII. Chemical Kinetics
Key focus of this chapter: reaction rate
This chapter focuses on reaction rate and gives concise summaries of the important things about rate law, catalysis, and reaction mechanisms in more detail.
A. Reaction rate
: the change in reactant or product concentration per unit time, commonly expressed in M/s.
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- For many reactions, the rate decreases as reactants are consumed; this is not universal (a zero-order rate is constant while the rate law applies).
- Reactant concentrations decrease, so a minus sign makes disappearance rates positive. Product concentrations increase, giving positive formation rates.
- For aA + bB → cC + dD, the normalized reaction rate is −(1/a)Δ[A]/Δt = −(1/b)Δ[B]/Δt = (1/c)Δ[C]/Δt = (1/d)Δ[D]/Δt. Stoichiometric coefficients relate species rates; they do not by themselves determine how fast a reaction proceeds.
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- Qs/ Write the reaction rate for the reaction.
CS2(g) + 3O2(g) → CO2(g) + 2SO2(g)
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B. Factors that affect reaction rate
Concentration | • Increasing the concentration of the reactant increases the colliding particles, so it increases the reaction rate. |
Pressure | • Increasing the pressure of the reactant (gas) increases the colliding particles by decreasing a volume, so it increases the reaction rate. |
Surface area | • Increasing the surface area of the reactant increases the colliding particles by increasing contacting area, so it increases the reaction rate. |
Catalysis | • Using catalysis increases the reaction rate by decreasing activation energy. |
Temperature | • Increasing the temperature increases the number and the intensity of collisions of particles in the reactant, so it increases the reaction rate. (In gas, the reaction rate increases 2 times as a temperature increases 10 oC.) |
Types of reactant | • The reaction rate of ionic bonded molecules is faster than the reaction rate of covalent bonded molecules, which involve more bond rearrangement. |
Fig 1. Factors that affect the reaction rate
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C. Rate law
: an experimentally determined relationship between reaction rate and reactant concentrations raised to reaction orders, for example Rate = k[A]m[B]n. Reaction orders generally cannot be inferred from the coefficients of an overall reaction.
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1. Rate constant k
: the proportionality constant for a specified reaction and rate law at a given temperature. Its value depends on temperature and the reaction pathway, including catalysis.
** For a fixed temperature and pathway, k does not change with reactant concentration.
- Unit of rate constant
Overall reaction order | Rate raw | Rate constant, k | k unit |
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Qs/ From the experimental data for the reaction, calculate the reaction order (m, n) of CH4 and O2, the overall reaction order, and the rate constant k.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) Rate = k[CH4]m[O2]n
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1 2 3 | 0.01 0.03 0.02 0.03 0.02 0.09 | 0.3 1.2 3.6 |
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2. Half life, t1/2
: the time required for the reactant concentration to decrease to one-half of its initial value. Successive half-lives are constant for first-order kinetics.
- Ex/ For a first-order reaction, the half-life is 2 minutes and the initial reactant concentration is 36 M.
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Qs/ A compound undergoes first-order decay: 64 g decreases to 16 g in 24 hours. Calculate the half-life.
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3. Simple types of reaction order
Reaction orders | Rate law | Equation (concentration vs. time) | Graph | Half-life, t1/2 |
Zeroth order | 
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First order | 
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Second order | 
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D. Catalysis
: acceleration of a reaction by a catalyst, which is regenerated and is not consumed overall.
- A catalyst provides an alternative reaction pathway with a lower activation barrier.
- At a fixed temperature, a catalyst increases the rates of both forward and reverse reactions. It does not change ΔG, ΔH, ΔS, the equilibrium constant, or the equilibrium composition.
- A transition state is a local energy maximum along a reaction pathway; a multistep mechanism may have more than one transition state.
Fig. 2 Exothermic reaction with catalysis
Fig. 3 Factors affecting reaction rate. Reactant concentration affects the rate through the rate law; it does not itself create catalysis. Temperature affects k. A catalyst changes the pathway and activation barrier, without changing reaction thermodynamics or equilibrium at a fixed temperature.
E. Reaction mechanisms
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