Kinetics and Equilibrium

Exam Study Notes

Kinetics is the measuring of reaction rates.

Reaction rate is how fast a reaction occurs.

A common measure of rate is how the concentration of a reaction participant changes over time.

Concentration [ ] can be expressed as mol.L-1 OR

mol/L OR M OR mol

L

Rate = Δ[ ]

Δt

Rate expression for a reaction

K is the rate constant

Rate Law: aA = bB cC + dD

rate = k [A]x [B]y

Determine the order of the reactants and the overall order of the reaction

Collision Theory – sufficient energy and proper orientation

Rate determining steps- the reaction can only occur as fast as the slowest step

Reaction Intermediates – cancel out throughout the reaction mechanism

For example, nitrogen monoxide reacts with oxygen according to the equation

2 NO(g) + O2 → 2 NO2

This reaction does not occur in a single step, however, but rather through these two steps:

Step 1:

2 NO → N2O2

Step 2:

N2O2 + O2 → 2 NO2

 

 

Notice that if you add these two reactions together, you end up with the overall reaction:

Step 1: 2 NO → N2O2

Step 2: N2O2 + O2 → 2 NO2

Overall: 2 NO(g) + O2 → 2 NO2

 

The series of steps a reaction undergoes is called the reaction mechanism.

In our example dinitrogen dioxide (N2O2) cancels out and does not appear in our overall equation. Substances such as this are called reaction intermediates and are typically short-lived.

Activation Energy is the minimum amount of energy required for a successful collision. Ea or also called threshold energy.

ΔH = products – reactants kJ

Factors Influencing Reaction Rates:

1. Temperature

2. Concentration

3. Pressure

4. Reactivity of the Reactants (determined by bond strength) Ions in Solution (redox) fast, covalent bonding slow. Aq, liquids, gases faster than solids.

5. Catalyst

Equilibrium Reactions can be reversible under the right

conditions.

Shown by a double arrow

Equilibrium is when the rate of the forward reaction equals the rate of the reverse reaction. This does NOT mean the amounts and it must be carried out in a closed system.

Equilibrium Constant Eq – there is a mathematical relationship between the concentration of the reactants and the concentration of the products once equilibrium has been reached.

Keq = [C]c x [D]d products

[A]a x [B]b reactants

a,b,c,d are the molar coefficients from the balanced equation

H2 (g) + I2 (g) 2 HI(g)

the equilibrium constant expression will be:

Keq = [HI]2

[H2] × [I2 ]

At equilibrium: [H2] = 0.022 M

[I2] = 0.022 M

[HI] = 0.156 M

We substitute these values into our equilibrium expression and solve for Keq: 

Keq = [HI]2 = (0.156)2 = 50.3 

[H2] × [I2 ] (0.022)((0.022)

 

The value of Keq, which has no units, is a constant for any particular reaction, and its value does not change unless the temperature of the system is changed. It does not depend on the initial concentrations used to reach the point of equilibrium.

Equilibrium Concentrations Using Ice Tables

Le Chatelier’s Principle:

If a system is subjected to an external stress, the equilibrium will shift to minimize the stress.

1. Concentration: add more reactant ----shift to product

Or more product – shift to reactant

Or remove a substance

2. Changes in Volume and Pressure:

Only affects the reaction if gases are present.

Increase the pressure ----shift to side with fewest moles

Decrease the pressure – shift to most moles

If both sides have the same number, no change

3. Changes in Temperature:

An increase in temperature will favor the side the reaction direction that absorbs heat (endothermic direction).

Keq will change more products – increase

fewer products - decrease