Equilibrium

Explain what it means for a system to reach equilibrium: 

• System must be closed (no exchange of matter with surroundings so equilibrium is only between products and reactants) 

• Equilibrium is dynamic (reaction has not stopped. Both forward + backward reactions occur at same rate) 

• Concentration of reacts and products = constant in equilibrium (produced/destroyed at same rate

• No change in macroscopic properties (observable properties like colour/density) 

• Equilibrium can be reached in either direction (same equilibrium mixture in same conditions no matter if products/reactants started)

• Does NOT need to be an equal amount of reactant and products

Distinguish between physical and chemical equilibrium: 

Physical equilibrium: Change of state (evaporation). Process of dynamic state where process are continously occuring with no overall change

Chemical equilibrium: Changes in chemical composition. Also known as dynamic equilibrium because both forward and backward reactions are still occuring and the rate of each reaction is equal. 

Define reversible reaction: 

• Concentration of reactants and products are constant (not equal) 

• Rate of forward reaction = reverse reaction 

• At equilibrium: No change in colour/pH 

• Equilibrium can be reached from either direction 

Define the equilibrium constant (K_c)

Ratio between concentration of reactant/product at equilibrium in a chemical reaction. It indicates the extent to which a reaction proceeds to completion at a given temperature. 

Dependent on temperature because it is a constant for a reaction at specific temperature. Does not change in different pressure and catalyst

Lower case letters = coefficients of balanced chemical equation

• Products^coefficient/ Reactants^coefficient 

• Terms of products and reactants are multiplied together 

Explain the information given by the magnitude of K_c: 

• Different range of K values = different extents of reactions. 

• K >> 1 = reaction goes almost to completion (high conversion of reaction → products) 

• K<<1= reaction does not proceed 

• K =1, neither products/reactants favoured

• Equilibrium constant for reaction = reciprocal of equilibrium constant of reverse reaction

Define the Châtelier Principle:

• System at equilibrium when subjected to change will respond in a way to minimise the effect of change. 

• The system will respond in the opposite way of whatever happens when the system is at equilibrium. 

• Changes can be concentration, pressure, or temperature. 

• Equilibrium position can shift to right (products) or left (reactants) 

Explain how to use Châtelier Principle to predict the changes in concentration/pressure/temperature

• Concentration: If reactants concentration increase = equilibrium shifts to right (product) and vice versa. K_c does not change 

• Pressure: 

  • Happens if diff number of gaseous molecules in products

  • Position of equilibrium shifts to side with fewer number of gaseous molecules = decrease in pressure and vice versa. K_c does not change 

  • Position of equilibrium ≠ change if same number of gaseous molecules on both sides of equation.  K_c does not change 

• Temperature

  • Forward is exo = reverse is endo and vice versa

  • If forward reaction is exothermic: 

    • Temperature increased = equilibrium shifts to left (reactants) because it absorbs extra heat = endothermic (reverse reaction) 

    • Temperature decreased  = equilibrium shifts to right (products) because it releases extra heat = exothermic (forward reaction) 

  • If forward reaction is endothermic (vice versa of exothermic)