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
BRØNSTED -LOWRY ACID
A substance that can give a hydrogen ion, H +
In other words, a proton donor.
BRØNSTED -LOWRY BASE
A substance that can take a hydrogen ion, H +
In other words, a proton acceptor.
Conjugate acid base pairs
chemical species whose formulas differ by only one H+
Acid ‘dissociation’ is an equilibrium:
H 2 O solvent (almost) constant concentration
® not in equilibrium expression
Ka ONLY for reaction of acid “HA” with
the solvent H 2 O as the base
® the stronger the acid, the bigger the Ka !
Base ‘dissociation’ is an equilibrium:
H 2 O solvent (almost) constant concentration
® not in equilibrium expression
Kb ONLY for reaction of base “B” with
the solvent H 2 O as the acid
® the stronger the base, the bigger the Kb!
Amphiprotic
Water can both donate and accept protons - one water molexule acting as acid can react with another water molecule acting as a base
STRONG ACID :
an acid that almost fully dissociates
in water
- conjugate base MUST BE a very weak base!
Factors that affect strength
Degree of polarity of H-X Bond - depends upon the electronegativity of X…
The more polar the H-A bond, the stronger the acid
Along the period : the key factor is electronegativity
The strength of the H-X bond
- depends on the size of the X atom
The larger the X atom - the longer/weaker the bond - the stronger the acid
Down the group : the key factor is the strength of the H-A bond
Basic
greater than 7
Neutral
equals 7
acidic
less than 7
ph
- logarithim of OH or H3O
Ka and Kb over several orders of magnitude
pKa= -log Ka
pKb= -logKb
Levelling effect
For any strong acid in water, the ‘active proton donor’ in water isn’t HA at all, it’s H3O
- any two strong acids will differ in acid strength but, in water, they will exhibit the same acid strength, the acid strength of H3O
Similarly, for any strong base in water, the ‘active proton acceptor’ in water is hydroxide.
Cations from strong bases
Group 1, Group 2. ions that do not react appreciably with water to produce either H2O or OH ions - because they are very poor acids
Anions from strong monoprotic acids
Cl, Br, I, NO3, ClO4
ions that do not react appreciably with water to produce either H2O or OH ions
because they are very poor bases
