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

: measuring of changing concentration (M) of reactants or products over time.

  • The reaction rate decreases as time goes on.
  • Minus(-) sign means decomposing of reactants and plus(+) sign means composing of products.
  • The bigger the coefficient, the faster the reaction rate.
  • Qs/ Write the reaction rate for the reaction.

               CS2(g) + 3O2(g)  →  CO2(g) + 2SO2(g)

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

C. Rate law

: an equation for the relationship between the rate constant and the concentration of the reactants increased by powers of reaction orders (m, n).    

1. Rate constant k

: a particular reactant value that is changed by temperature.

** Rate constant is not changed by the concentration of reactants.

  • Unit of rate constant

Overall reaction order

Rate raw

Rate constant, k

k unit

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

Trial

1

2

3

     0.01              0.03

     0.02              0.03

     0.02              0.09

0.3

1.2

3.6

     Sol/

2. Half life, t1/2

: a time expressed for the dropping of one-half of the reactant concentration.

  • Ex/ A half life of a reactant is 2 minutes and its initial concentration is 36 M.  

Qs/ 64g of unknown compound is decayed to 16g during 24hours. Calculate the half-life.

Sol/

3. Simple types of reaction order    

Reaction orders

Rate law

  Equation

(concentration vs. time)

Graph

Half-life, t1/2

Zeroth order

First order

Second order

D. Catalysis

: a substance that speeds up the rate of a reaction without being consumed by the reaction itself.

  • Activation energy (Ea) : minimum energy for chemical reaction.

– the reaction rate decreases as activation energy is increased.

– the reaction rate increasas as activation energy is decreased.

– has no affect on ΔG or ΔH.

  • Transition state: maximum energy at chemical reaction.

Fig. 2 Exothermic reaction with catalysis

Fig. 3 Factors that affect catalysis, k, ΔG, ΔH,ΔS, and reaction rate

Factors that affect catalysis, k, ΔG, ΔH,ΔS, and reaction rate

E. Reaction mechanisms

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