Equilibrium

Define reversible reaction

chemical reaction that can take place in both directions. (In closed systems, both forward and reverse reaction take place simultaneously)

Characteristics of reversible reactions

1. Only occur in closed system where there is no gain or loss of chemicals to or from the surroundings

2. At equilibrium, rates of forward and reverse reactions are equal

3. Equilibrium can be achieved from either direction

Define dynamic equilibrium

Dynamic equilibrium is achieved when rate of forward and backward reactions are equal and concentration of products and reactants are constant while the mixture is still reacting.

Features of equilibrium state

1. Equilibrium is dynamic and forward and backward reactions occur continuously at the same rate

2. Equilibrium achieved in a close system without any exchange of matter

3. Concentrations of reactant and product remain constant

4. No change in macroscopic properties such as color or density

5. Can be reached from either direction

Types of equilibrium

Chemical
Physical (e.g. melting ice)

What are irreversible reactions

• High Ea for backward reaction

• In which only forward reaction takes place

What is a reversible reaction

• Smaller difference in their Ea for forward and backward reactions

• Both forward and backward reactions can take place

What is homogenous chemical equilibria

System where reactants and products are in same physical state

What is heterogenous chemical equilibria

System where one of the reactants and products are in different physical state

How to get equilibrium constant K

Concentration of products / concentration of reactants

For a system where a pure liquid/solid is in equilibrium with a gas, is the concentration of solid/liquid included in the equilibrium constant

• Position of equilibrium independent of solid or liquid present as long as some is present and temperature is constant

• Concentration of solids and liquids do not appear in K as these are similar to the density, which is mostly constant at any temperature

Does change in cocnentration of reactants or products affect K

No

Does change in pressure of system affect K

No

Does catalyst affect K

No

Does temperature affect K

Yes

What does the values of K mean

K << 1 - reaction hardly proceeded
K < 1 - equilibrium lies towards reactants
K = 1 - appreciable amount of both reactants and products at equilibrium
K > 1 - equilibrium lies towards products
K >> 1 - reaction goes almost to completion

What is Le Chatelier’s Principle

if a change is made to the conditions of a system at dynamic equilibrium, then the position of equilibrium will shift to counteract the change and establish a new equilibrium.

Change in concentration on equilibrium

• Concentration of reactant A increased

  • Forward reaction works to remove excess A and position of equilibrium shifts from left to right

  • Reactants react to produce more product

  • More products at new equillibrium, less reactants than original equilibrium

  • K remains unchanged

• Concentration of reactant A decreased

  • Reverse reaction works to replace reactant, position of equilibrium shifts from right to left

  • Products react to produce more reactants

  • More reactants, less product at new equilibrium

  • K remains unchanged

Change in pressure on equilibrium, where reactants have more moles of gas than products

• Increasing pressure

  • Position of equilibrium moves in the direction that results in a decrease in total number of moles of gaseous particles

  • Position of equilibrium shifts right, reactants react to form more products

  • More products but less reactants at new equilibrium

  • K remains unchanged

• Decreased pressure

  • Position of equilibrium moves in direction that results in an increase in total number of moles of gas particles

  • Position of equilibrium shifts left and more products react to form reactants

  • More reactants and less products at new equilibrium

  • K remains unchanged

How to change pressure of a closed system

Add inert gas at constant volume to equilibrium mixture

Effect of temperature on dynamic equilibrium

• Increase in reaction temperature

  • Heat energy added to equilibrium mixture

  • POE shifts to favour endothermic reaction to remove heat energy

  • K changes

• Decrease in reaction temperature

  • Heat energy is removed from equilibrium mixture

  • POE shifts to favour exothermic reaction to produce some heat energy

  • K changes

Why does K change when temperature changes

K changes as ratio of products to reactants change as rate of forward and backward reaction change, hence K is only temperature dependent as they change at different extents for same increase in temperature

Effect of catalyst on equilibrium

Both forward and backward reactions speed up equally

Conditions of haber process and why
N2 (g) + 3H2 (g) = 2 NH3 (g) ∆H = -92.4 kJ mol-1

Higher pressure gives higher yield of NH3 but requires higher cost for expensive chamber to withstand the high pressure. Thus, moderate pressure of 200atm used to keep cost and maintenance of equipment low

Low reaction temperature gives higher yield of NH3, but low reaction temperature slows down rate of reaction. Thus, moderate temperature of 500degC is used to speed up the reaction. Although the yield of ammonia at equilibrium is reduced, the process is faster

Use of catalyst increases rate of reaction and allows equilibrium to be reached faster. Does not affect yield of NH3. Catalyst of finely divided iron is used.

Reaction quotient and equilibrium constant

K > Q = forward reaction proceeds

K < Q = backward reaction proceeds

K = Q = system in equilibrium

What is reaction quotient

Expression with non equilbrium concentrations

What is ∆G = ∆G^θ + RT lnQ

delta Gθ = free energy change under standard conditions

delta G = 0 at equilibrium