Equilibreium

dynamic equilibrium

when forward and reverse reactions occur at equal rates and concentrations remain constant in a closed system

closed system

a system where no substances can enter or leave allowing equilibrium to be established

Le Chatelier's principle

if a system at equilibrium is disturbed it shifts to oppose the change

effect of increasing concentration

equilibrium shifts to the side that reduces the added substance

effect of decreasing concentration

equilibrium shifts to the side that replaces the removed substance

effect of pressure on equilibrium

only affects equilibria involving gases

increase in pressure

equilibrium shifts to the side with fewer moles of gas

decrease in pressure

equilibrium shifts to the side with more moles of gas

effect of temperature on equilibrium

equilibrium shifts in the endothermic direction when temperature increases and in the exothermic direction when temperature decreases

exothermic reaction

heat is released and can be treated as a product

endothermic reaction

heat is absorbed and can be treated as a reactant

catalyst effect on equilibrium

does not change equilibrium position but increases rate of both forward and reverse reactions equally

equilibrium constant Kc

ratio of equilibrium concentrations of products to reactants each raised to the power of their stoichiometric coefficients

general expression for Kc

Kc equals concentration of products divided by concentration of reactants each raised to stoichiometric powers

large Kc value

equilibrium lies to the right and products are favoured

small Kc value

equilibrium lies to the left and reactants are favoured

units of Kc

depend on the stoichiometric powers in the expression

homogeneous equilibrium

all reactants and products are in the same physical state

heterogeneous equilibrium

reactants and products are in different physical states

omitting substances in Kc

pure solids and pure liquids are not included in equilibrium expressions

reaction quotient Q

calculated like Kc using current concentrations to predict direction of change

Q less than Kc

reaction proceeds in the forward direction to reach equilibrium

Q greater than Kc

reaction proceeds in the reverse direction to reach equilibrium

Q equals Kc

system is at equilibrium

effect of inert gas at constant volume

no change to equilibrium position

effect of inert gas at constant pressure

volume increases so equilibrium shifts towards side with more moles of gas

effect of changing volume

decreasing volume increases pressure shifting equilibrium towards fewer moles of gas

effect of dilution

equilibrium shifts towards the side with more dissolved particles

stoichiometric coefficients in Kc

determine the powers of each concentration term

reversible reaction

a reaction that can proceed in both forward and reverse directions

equilibrium yield

the amount of product present at equilibrium

dynamic nature of equilibrium

reactions continue at particle level even though concentrations remain constant

equilibrium constant Kp

uses partial pressures of gases instead of concentrations

relationship between Kp and Kc

Kp equals Kc multiplied by RT raised to the power of change in moles of gas

temperature dependence of Kc

the value of Kc changes only with temperature

rate at equilibrium

forward and reverse reaction rates are equal but not zero

graph of equilibrium

concentrations become constant when equilibrium is reached

industrial equilibrium conditions

chosen to balance rate of reaction and yield economically

Haber process equation

N2 + 3H2 ⇌ 2NH3

Haber process conditions

approximately 450 degrees Celsius 200 atmospheres iron catalyst

Contact process equation

2SO2 + O2 ⇌ 2SO3

Contact process conditions

approximately 450 degrees Celsius 1 to 2 atmospheres vanadium V oxide catalyst

effect of temperature on yield

higher temperature favours endothermic direction lower temperature favours exothermic direction