Topic 7: Equilibrium

7.1 Dynamic equilibrium

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/vaporv3.swf

Chemical systems

H2 (g) + I2 (g) → 2HI (g)

7.2 The position of equilibrium

● An equilibrium expression can be written for ahomogenous reaction, for which all the reactants andproducts are in the same phase.

● Kc is a measure of the amount of products in an equilibrium

compared with the amount of reactants and has a fixed value for a particular reaction.

The equilibrium constant, Kc

eqm=equilibrium concentrations!

Ex.

H2 (g) + I2 (g) → 2HI (g)

Kc >> 1

● High propotion of products compared to reactants● Reaction proceeds virtually to completion

Kc << 1

● Only a small fraction of reactants have been converted into products

Kc ≈ 1

● Almost equal amounts of reactants and products

Le Châtelier's principle

● When a system at equilibrium is subjected to a change, it will respond in such a way as to minimize the effect of the change.

a) Change in concentration

N2( g) + 3H2(g) 2 NH3(g)

● Increasing the concentration of one the reactants:

the rate of the forward reaction increases and more products are formed (→ to the right)

● Removing product:

more product is formed (→ to the right)

● Kc will be unchanged

b) Change in pressure

● Only affects reactions where there is a change in the number of gas molecules

● Increasing the pressure (=decreasing the volume):

shift in equilibrium to the side with a smaller number of molecules

● Kc will be unchanged

The concentrations of the reactants and the products are affected in the same way

Equilibrium shifts to the side that counteracts this, i.e.to the side with lower number of molecules of gas.

c) Change in temperature

● Lowering the temperature = "removal of heat"

● Increasing the temperature = "addition of heat"

● Exothermic: 2 NO2 (g) N2O4 (g) ΔH = -24 kJ/mol

● Increase in temperature → to the left in favor of reactants (Kc will decrease)

● Endothermic: N2O4(g) 2 NO2 (g) ΔH = +58 kJ

● Increase in temperature → to the right in favor of products (Kc will increase)

● Kc is temperature dependend and will change

d) Addition of catalyst

● The catalyst lowers the activation energy of both the forward and the backward reaction and increases their rate by the same amount.

→ No effect on the position of equilibrium or on the value of Kc.

Industrial applications

● As high a yield as possible should be obtained

● High rate

Haber process

3 H2(g) + N2(g) 2 NH3 (g) ΔH = −92 kJ mol−1

● Predict the conditions for the process.

● Removing product

● High pressure (200 atm)

● Moderate temperature (450ºC)

● Catalyst (Fe)

Contact process

● Sulfuric acid is the most industrially produced chemical (150 million tonnes/year produced world-wide)

● Conditions: – High pressure (2 atm)

– Optimum temperature (450ºC)

– Catalyst (vanadium(V)oxide)

Row 1 Row 2 Row 3 Row 40

2

4

6

8

10

12

Column 1

Column 2

Column 3

17.1 Liquid-vapour equilibrium (HL)

● If liquid is placed in an open container, evaporation will occur.

● The vapour particles escape from the surface and the liquid will continue to evaporate so that no equilibrium is reached.

● If liquid is placed in a closed container, as the liquid begins to evaporate, the concentration of vapour increases.

● As the concentration increases, more vapour particles collide with the surface of the liquid and thus the rate of concentration increases.

● When the rate at which molecules leave the surface is equal to the rate at which they return, the liquid and vapour are in equilibrium.

Vapour pressure

● The vapour pressure rises and finally reaches a maximum value.

● This value is constant at a given temperature and is known as the saturated vapour pressure, Pa (mmHg).

Factors that do NOT influence vapour pressure

● The surface area of the liquid, because evaporation and condensation both occur at the surface and are equally effected. (But the equilibrium is reached faster when the surface area is increased)

● Volume changes cause a change in pressure and an adjustment of the equilibrium, but the vapour pressure will be the same once the equilibrium is re-established.

● The molecules at the surface are less strongly bonded and can escape more easily than those in the body of the liquid.

● Evaporation takes place when particles at the liquid surface have sufficient kinetic energy to overcome the forces holding them within the liquid.

● Increasing temperature causes an increase in evaporation, the equilibrium shifts to the right and vapour pressure increases.

Factors affecting vapour pressure:1. Temperature

Change in vapour pressure with temperature for water

● The boiling point is the temperature at which the vapour pressure is equal to the external temperature.

● Evaporation is a surface phenomenon that occurs at a range of temperatures.

● Boiling is a volume phenomenon that occurs at a specific temperature and is dependent on the nature of the substance and the external pressure.

● When the pressure is lower the boiling point decreases, e.g on the summit of Mt Everest where the pressure is low water boils at 69ºC.

● When the pressure is higher the boiling point increases, (in a pressure cooker or autoclave water can reach a temperature of up to 120ºC).

2. Nature of the substance

● When a liquid boils the intermolecular forces of attraction between the particles in the liquid are completely broken.

● The enthalpy of vaporization is defined as the energy required to convert on mole of a substance in its liquid state into one mole of gas (298K).

● The stronger the intermolecular forces, the higher the enthalpy of vaporization and the higher the boiling point.

● Evaporation is an endothermic process, requires energy from the surroundings.

● Condensation is an exothermic process, energy is released.

HOT

COLD

● Substances that have high vapour pressure at a specific temperature (= low boiling point), are said to be volatile.

● Ex.1 Can you identify which factors you think are the cause of the differences in vapour pressures at 298K of the following liquids.

Liquid Saturated vapour pressure ( Pa)

water 3170

ethanol 7850

benzene 12600

mercury 0.226

The role of intermolecular forces

● Ex.2 Put the following compounds in order of increasing boiling point:

a. ethanalb. ethanolc. 2-methylpropane

Phase diagrams● The phase diagram for a substance shows the state of

matter which is stable at any given temperature and pressure.

Phase diagram for carbon dioxide

17.2 Equilibrium calculations

1. Write the balanced equation

2. Under the equation, write in the values of the concentrations (or sometimes moles) of each component by using

- Initial

- Change

(a minus sign for reactants=decrease in concentration)

(a plus sign for products=increase in concentration)

The changes are in the same ratio as the coefficients in the balanced equation, if we know one we can deduce the others

- Equilibrium is the concentration of each component in the equilibrium mixture

3. Write the expression for Kc

Calculating Kc from equilibrium concentrations

Calculating equilibrium concentrations from the Kc