Unit 4 Flashcards
Concept of Equilibrium
The state reached when the concentrations of reactants
and products remain constant over time.
Chemical reactions can readily go both ways = ‘reversible’
- Some reactions called ‘irreversible’ or ‘proceed to completion’
à actually, still dynamic equilibrium but reverse reaction
very too slow & only trace amounts of reactants remain
Reaction rates are balanced at equilibrium
At equilibrium, the forward and the reverse reactions occur at the same rate.
Reactions are still occurring, but there is no NET reaction.
Chemical equilibria are dynamic and reversible
HOMOGENEOUS EQUILIBRIA + HETEROGENEOUS EQUILIBRIA
Reactants that are pure SOLIDS and LIQUIDS are excluded from equilibrium expressions
because they’re in their standard state a = 1: their ‘concentration’ doesn’t change
THE REACTION QUOTIENT (QC )
is defined in the same way as the equilibrium constant,
Kc , except that the concentrations in Qc can have any values (not necessarily
equilibrium values).
If Qc = Kc
no net reaction occurs (already at equilibrium!)
If Qc < Kc
net reaction goes from left to right
If Qc > Kc
net reaction goes from right to left
Extent of the Reaction
Kc > 10^3 , reaction proceeds nearly to completion, products favoured.
Kc < 10^-3 , reaction proceeds hardly at all, reactants favoured.
10^-3 <Kc < 10^3 , appreciable concentrations of both reactants and products
THE REACTION QUOTIENT (QC )
is defined in the same way as the equilibrium constant,
Kc , except that the concentrations in Qc can have any values (not necessarily
equilibrium values).
If Qc = Kc
no net reaction occurs (already at equilibrium!)
If Qc < Kc
net reaction goes from left to right
If Qc > Kc
net reaction goes from right to left
SOLUBILITY
dissolved ions in equilibrium with
solid (some of which must be present)
At equilibrium, concentration of ions is constant = ‘saturated solution’
A saturated solution of calcium fluoride in
contact with solid CaF 2 contains constant
concentrations of Ca2+ (aq) and F ‒ (aq) because
at equilibrium the ions crystallize at the same
rate as the solid dissolves.
SOLUBILITY PRODUCT, KSP
We can write equilibrium constant (‘ion-product’) expression. a measure of how much of the ionic
compound has dissolved at equilibrium
LE CHÂTELIER ’ S PRINCIPLE :
If a stress is applied to a reaction mixture at equilibrium,
a net reaction occurs in the direction that relieves the stress.
The ‘stresses’ that can be applied
The concentration of reactants or products can be changed.
The pressure (and/or volume) can be changed.
The temperature can be changed.
Châtelier’s Concentration
the concentration stress of an added reactant or product is relieved by net
reaction in the direction that consumes the added substance.
the concentration stress of a removed reactant or product is relieved by net
reaction in the direction that replenishes the removed substance.
Le Chanteliers Principle - temperature
Temperature can alter the equilibrium concentrations, but for a different reason:
it changes the value of Kc !
Disruption in temperature - the equilibrium constant for an exothermic reaction (negative ΔH°)
decreases as the temperature increases.
* the equilibrium constant for an endothermic reaction (positive ΔH°)
increases as the temperature increases.
Chantelier - catalyst
The relative amounts of reactants and products at equilibrium (i.e. the value of
equilibrium constant K) depends on the relative energies (stabilities) of those
reactants and products.
Catalyst doesn’t affect the
position of equilibrium,
i.e. the value of KC
so we reach the same
equilibrium mixture of
reactants and products,
only faster
- Catalyst changes the energy of the pathway from reactants to products
but doesn’t change the energies of the reactants and products themselves
Citric and Absorbic Acid
Citric acid and ascorbic acid (vitamin C) give citrus
fruits their characteristic sour taste
Sodium bicarbonate
Sodium bicarbonate (baking soda) has a
characteristic bitter taste
Quinine
Quinine is responsible for the
bitter taste of tonic water
