Topic 7 + 17: Equilibrium

equilibrium properties

• dynamic (in constant motion)

• reversible (as long as system closed)

• can be approached from either direction (forward or reverse)

• macroscopic properties (ex: colour, density, pH) are constant, even though on a microscopic scale there is continual interconversion of reactants and products

graph of concentration/time

concentrations of reactants and products becomes constant

graph rate/time

rate of forward reaction equals rate of reverse reaction

physical equilibrium examples

• vapor pressure

  • rate of vaporization = rate of condensation

• saturated solution

  • rate of dissociation = rate of crystallization

law of chemical equilibrium

at a given temperature the ratio of the concentration of products (raised to the power of their molar coefficient) to the concentration of reactants (raised to the power of their molar coefficient) is constant

Kc >> 1

products favoured

Kc << 1

reactants favoured

reaction quotient Q

• if a system has not reached equilibrium, the ratio of concentration of products to reactants is not considered Kc

• Q: helps determine the progress of the reaction as it moves towards equilibrium

Q > Kc

concentration of products greater than at equilibrium and reverse reaction favoured until equilibrium reached

Q < Kc

concentration of reactants greater than at equilibrium and forward reaction favoured until equilibrium reached

Q = Kc

system at equilibrium and forward and reverse reactions occur at equal rates

NH3 optimum temperature, pressure and enthalpy

• 450 °C

• 200 atm

• -92 kJmol-1

NH3 catalyst

• Fe (s)

• Fe2O3 (s)

SO3 optimum temperature, pressure and enthalpy

• 450 °C

• 1 atm

• • 197 kJmol-1

SO3 catalyst

V2O5 (s)

chatelier’s principle

if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance