Unit 7 Flashcards
Formula for Kc
Kc=[C]c[D]d
[A]a[B]b
products are on the top of the expression and the reactants on the bottom
square brackets [ ] are used to show the concentrations (in mol dm-3) of the reactants and products at equilibrium
the powers to which the concentrations are raised are the stoichiometric coefficients in the balanced equation for the reaction.
equilibrium constant, Kc, is a constant for a given temperature, therefore, it is temperature-dependent.
write equilibrium constant expressions for reactions involving homogeneous equilibrium - that is when all reactants and products are in the same physical state.
What do kc result tell you
If Kc »_space; 1, the reaction is said to have gone almost to completion. The conversion of reactants to products is almost complete.
If Kc «_space;1, the forward reaction has hardly proceeded. Only a small proportion of the reactants have been converted to products.
The reaction quotient, Q
a measure of the relative amounts of reactants and products for a reaction that is not yet at equilibrium.
Q=[C]c[D]d
[A]a[B]b
an important point to note is that the reaction quotient Q is calculated with non-equilibrium concentrations of reactants and products. This is different from the equilibrium constant Kc, which is calculated using the concentrations of reactants and products when the reaction is at equilibrium.
Affect of Changes in concentration on equilibrium
important to note that changes in concentration do not affect the value of the equilibrium constant, Kc, which remains unchanged.
Increase concentration of reactant or decrease concentration of product Equilibrium shifts to the right
Increase concentration of product or decrease concentration of reactant
Equilibrium shifts to the left
Affect of changes in pressure
Changing the pressure in a chemical reaction only has an effect on the equilibrium position when the reactants and products are in gaseous states.
increase in pressure, the equilibrium position will shift to the side with the lowest number of gaseous molecules.
if the pressure of an equilibrium mixture of gases is decreased, the equilibrium position will shift to the side with the greatest number of gaseous molecules
Affect of temperature changes in pressure
Increase temperature Equilibrium shifts to favour the endothermic reaction Kc changes
Decrease temperature Equilibrium shifts to favour the exothermic reaction Kc changes
Affect of catalyst changes in pressure
No change
The chemical process that produces ammonia gas involves the following reaction:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔH = –92 kJ mol–1
