Rates of Chemical Reactions

Consider this reaction:

N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ G298K = -16.8 kJ

What does this equation tell us?

The reaction is spontaneous at 298K It is exothermic entropy is decreasing enthalpy is decreasing the names and states of reactants and

products are given for every 3 mol of H2(g) and 1 mol of

N2(g) consumed 2 mol of NH3(g) is produced

92 kJ of energy is produced

N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ G298K = -16.8 kJ

N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ G298K = -16.8 kJ

What does this equation NOT tell us?

How much of each substance we started with

how much of each substance is consumed How fast the reaction is Whether or not the reaction proceeds

directly as shown in the equation or if intermediate steps occur (reaction mechanism)

Consider this reaction:

4 HBr(g) + O2(g) --> 2 H2O + 2 Br2(g)

This reaction proceeds through these steps

HBr + O2 ---> HOOBr (slow)

HOOBr + HBr ---> 2 HOBr (fast)

2 HOBr + 2 HBr --> 2 H2O + 2 Br2(g) (fast)

In this case the equation simply tells us the reactants and products. It doesn’t tell us the series of steps by which the reaction goes.

When we refer to reaction rates we are referring to how fast a reaction goes. Rates can be expressed both qualitatively and quantitatively. In a qualitative way one would describe a reaction as fast or slow.H2(g) + O2(g) ----> H2O (g) fastFe + 1/2O2 + H2O ---> Fe(OH)2 slowQuantitatively rates are expressed by observing the rate at which a reactant disappears or a product appears.

It may be expressed as the change in mol/unit time orthe change in concentration/unit time

Mg(s) + 2 HCl(aq) ---> MgCl2(aq) + H2(g)

Rate =# mol of Mg consumed

Time required to disappear

=

2.4 g/24 g/mol

45 s

=2.2 x 10-3 mol/s

See Saunders - Chapter 15-Lesson 2 For Measuring Rates then conduct an exercise using burettes

Chemical reactions only occur when reacting particles collide with sufficient energy, and at a favourable geometry.

H2

Cl2

H2

Cl2

H2 Cl2

Fast enough, but the wrong geometry

Fast enough, but the wrong geometry

Fast enough, but the wrong geometry

Fast enough, but the wrong geometry

Fast enough, but the wrong geometry

Fast enough, but the wrong geometry

Fast enough, but the wrong geometry

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, too slow

Right Geometry, sufficient speedActivation energy reached

Right Geometry, sufficient speedActivation energy reached

Right Geometry, sufficient speedActivation energy reached

Right Geometry, sufficient speedActivation energy reached

Right Geometry, sufficient speedActivation energy reached

Right Geometry, sufficient speedActivation energy reached

Potential Energy

Reaction Coordinate

1 - The reacting molecules are sufficiently far apart so they have no influence on one another

2 - As the reacting molecules approach their electrons start to repel so the H-H and Cl-Cl bonds stretch, Ek decreases and Ep increases.

3 - At the same time new bonds are starting to form between H and Cl as H nuclei attract Cl electrons and Cl nuclei attract H electrons

4 - If the molecules have sufficient energy to react a short lived activated complex is formed. At this point no bond breaking or bond making is occuring.

5 - If the reaction proceeds the bonds between H-Cl continue to shrink until they reach a stable state

Potential Energy (kJ)

Reaction Coordinate

1.

2.

3.2.0

4.0

7.6

H = 2.0 kJ - 4.0 kJ = - 2.0 kJ

Eaf= 7.6 kJ - 4.0 kJ = 3.6 kJ

Ear= 7.6 kJ - 2.0 kJ = 5.6 kJ

Eaf = energy of activation, forward

Construct a potential energy vs. reaction coordinate curve for an endothermic reaction

Potential Energy (kJ)

Reaction Coordinate

1.

2.

3.

1.0

3.0

5.6

H = 3.0 kJ - 1.0 kJ = 2.0 kJ

Eaf= 5.6 kJ - 1.0 kJ = 4.6 kJ Ear= 5.6 kJ - 3.0 kJ = 2.6 kJ

Eaf = energy of activation, forward