PHYSICAL CHEMISTRY Flashcards
when is a chemical reaction in equilibrium
A chemical reaction is in equilibrium when the composition of the reactants and products remains constant indefinitely.
equilibrium constant (K) characterises the equilibrium composition of the reaction mixture
what is the equilibrium constant K
definition
The equilibrium constant ( K ) characterises the equilibrium composition of the reaction mixture.
For the general reaction aA + bB <=> cC + dD the equilibrium expression is:
K = [C]^c[D]^d/[A]^a[B]^b
[A], [B], [C], [D] are the equilibrium concentrations of A, B, C, D and a, b, c d
are the stoichiometric coefficients in the balanced reaction equation.
The value of equilibrium constants can be calculated
Equilibrium constants have no units.
what does the value of K indicate
The value of an equilibrium constant indicates the position of equilibrium
A very high value of K (well above 1) indicated equilibrium far to the RIGHT (forwards reaction favoured) - reaction effectively complete
A very low value of K (well below 1) indicated equilibrium far to the LEFT (backwards reaction favoured) - effectively no reaction
K = 1 – significant quantities of reactant and product at equilibrium
(if K > 10^10 then reaction goes to completion
if K < 10^-10 then reaction will not occur)
The equilibrium constant gives no indication of the rate at which equilibrium is achieved. It indicates only the ratios of products to reactants once this state is reached.
effect of pure liquids and solids in equilibrium expression
The concentrations of pure solids and pure liquids at equilibrium are taken as constant and given a value of 1 in the equilibrium expression.
concentrations of pure liquids and pure solids constant at a given temp
their concentrations do not affect the equilibrium constant
their concentrations are so comparatively large that they do not vary to a measurable extent.
so dont include species with ‘s’ or ‘l’ states in K expression
what does numerical value of K depend on (and what is it independent of)
The numerical value of the equilibrium constant depends on the reaction temperature and is independent of concentration and/or pressure.
when conc. or pressure is altered, the position of equilibrium changes until the ratio of reactants:products returns to its og value (so numerical value of K reestablished)
explain effect of concentration on equilibrium constant
increasing concentration doesnt alter K
EX:
K = [A][B]/[C]
adding A-
equilibrium position favours creation of C, so uses more B
increase in [A] compensated by [B] decrease and [C] increase, so K stays the same
explain effect of pressure on equilibrium constant
change in pressure doesnt alter eq. constant
effectively changes concentration
equilibrium will shift to compensate for change in pressure so K stays the same.
Changes in pressure only affect reactions involving gases.
If the pressure is increased then the position of equilibrium will shift to the side with the fewer number of gaseous moles.
This then causes a new equilibrium to be established, but with the same value of K.
factors affecting the POSITION of equilibrium
higher knowledge
concentration of a reactant or product
pressure when there are different numbers of moles of gases on either side of the balanced equation
temperature
factors that do not affect the POSITION of equilibrium
higher knowledge
use of a catalyst (only affects how quickly equilibrium is reached, rate of reaction.)
effect of temperature on K
explain it
The numerical value of the equilibrium constant depends on the reaction temperature
temperature change is an energy change
numbers of particles not changing by external addition but by energy change
ratio of particles is changing
effect on K of increasing temp on endothermic reactions
For endothermic reactions, a rise in temperature causes an increase in K and the yield of the product is increased
effect on K of increasing temp on exothermic reactions
For exothermic reactions, a rise in temperature causes a decrease in K and the yield of the product is decreased.
effect of catalysts on value of K
The presence of a catalyst does not affect the value of the equilibrium constant.
(only affects rate at which equilibrium established)
ionisation of water equilibrium
In water and aqueous solutions there is an equilibrium between the water molecules and hydronium (hydrogen) and hydroxide ions.
This ionisation of water can be represented by:
H2O(l) + H2O(l) <=> H30+(aq) + OH-(aq)
H3O+ = hydronium, H3O+ is H+
amphoteric meaning and example
AMPHOTERIC= can react as an acid and a base.
Water is amphoteric
