Already know le Chatelier’s principle…
Concentration: If concentration increase, position of equilibrium shifts in direction that reduces
concentration right/ left, so towards product/ reactants.
Pressure: If pressure increased, position of equilibrium shifts towards side with fewer gaseous
molecules right/ left, to reduce pressure, so towards product/ reactants.
Rate of the forward and reverse reactions both increase if concentration/ pressure increases or
catalyst added. Rate increases if temperature increases.
At equilibrium rate of forward and reverse reactions are the same.
Smaller container/ decrease volume = pressure increases.
Equilibrium K (KP or KC): Greater than 1 is towards products.
– Equilibrium counteracts changes in conc and pressure by changing position of equilibrium
and keeps K constant.
– But K does change if temperature changes. K = Kc or Kp.
Forward Exothermic Reactions: Increasing temp, makes equilibrium shift left towards
endothermic direction. Product decreases and reactants increase.
Make sure to name products and reactants as well.
Forward Reaction Endothermic:
– If temperature increased, system no longer in equilibrium. The ratio now less than Kp or Kc.
– Equilibrium position shifts right in the endothermic direction, towards products to reduce
temperature.
– Named Products increases and reactants decrease. Top increases more than the bottom in
Kc expression.
– So Kc/ Kp equilibrium constant increases in value.
Concentration and Pressure in Terms of Kc: Not AS.
– Kc does not change with pressure/ concentration. -> Write in all Kc questions.
– Increased pressure/ conc, increases named reactant. Denominator (bottom) of Kc/ Kp
expression increases more than top.
– System no longer in equilibrium.
– So equilibrium shifts right so numerator (top) increases and bottom decreases to restore
equilibrium Kc. Named product increases/ reactant decreases.
Pressure of container doubled. Explain in terms of Kc.
– Kc does not change with pressure.
– [NO2]
2
increases more than [N2O4] OR concentration term on bottom of Kc increases more
that concentration term on top of Kc.
– Concentration of N2O4 increases AND concentration of NO2 decreases to restore equilibrium
Kc.
Catalysts: Catalyst affects rate of reaction. Does not affect equilibrium constant or position of
equilibrium. Catalysts speed up both forward and reverse reactions in equilibrium equally.
Equilibrium is reached quicker with catalysts.
Yield/ position of equilibrium is difficult to determine when two things changes as do not know
the relative effect of two opposing factors- pressure/ conc/ temp changes.
When both reactant and product changed in pressure/ conc. If powers of top and bottom are
the same, Kc does not change, no equilibrium shifts and equilibrium conc stays the same.