Equilibria
At dynamic equilibrium the concentrations of reactants and products remains constant and the
rate of the forwards and backwards reaction is equal. The equilibrium concentrations remain
unchanged as long as the conditions are left undisturbed. This can only happen in a closed
system.
The equilibrium constant (K) is used to quantify the relative concentrations of products and
reactants at equilibrium. The reaction quotient (Q) measures the amounts of products and
reactants present during a reaction in a particular point in time. It figures out the direction in
which a reaction is to proceed and can be compared to K to determine the direction in which a
reaction is going. K describes a reaction at equilibrium while Q does not.
If Q > K, then the reaction favours the reactants as the concentration of the reactants is larger
than would be at equilibrium.
If Q < K, then the reaction favours the products as the concentration of products is larger
than would be at equilibrium.
If Q = K, the reaction is already at equilibrium and there is no tendency for more
products/reactants to form.
Le Chatelier’s Principle states that if a system at equilibrium undergoes a change the system will
shift to counteract that change and restore equilibrium.
If the concentration of the reactant is increased the system will attempt to counteract the
change by favouring the forwards reaction.
If the temperature is increased, the system will favour the exothermic reaction which gives
out heat energy.
If the temperature is decreased, the system will favour the endothermic reaction which takes
in heat energy.
If the pressure is increased, the system will favour the side of the reaction with less moles to
decrease the pressure.
Haemoglobin in Equilibrium: Haemoglobin is made up of 4 sub-units which are joined through
intermolecular forces. Each subunit has a heme group which contains an iron molecule which
can bond to one O2. The heme group is based on a planar porphyrin system with 4 N-containing
Equilibria 1
, units. The high conjugation means that the system has strong electronic absorption. The Fe inside
forms a chelate system where each N has a lone pair donating to the Fe, so the Fe is strongly
bound. A histidine residue binds to the Fe distorting it out of the plane. The binding of 02 brings
Fe back into the plane facilitating co-operative binding as the shape distortion reveals the other 3
Fe binding sites. At pH 7 the histidine residue is protonated and this form leads to the formation
of salt bridges which distort the haem iron allowing it to release O2. CO2 binds to the amino
group of certain amino acid residues at the subunit to produce -NHCOO- which releases O2.
Dissociation constant (Kd) gives a measure of the strength of binding. A large Kd means less
binding.
For reactions at equilibrium there is not energetic driver to change the relative proportion of
reactants and products so Gibbs Free Energy is 0.
For forwards reactions, the reaction quotient Q is less than K and the system is trying to
reach Q = K. The Gibbs Free Energy is negative.
For reverse reactions, the reaction quotient Q is greater than K and system is trying to reach
Q = K. The Gibbs Free Energy is positive.
Equilibria 2
At dynamic equilibrium the concentrations of reactants and products remains constant and the
rate of the forwards and backwards reaction is equal. The equilibrium concentrations remain
unchanged as long as the conditions are left undisturbed. This can only happen in a closed
system.
The equilibrium constant (K) is used to quantify the relative concentrations of products and
reactants at equilibrium. The reaction quotient (Q) measures the amounts of products and
reactants present during a reaction in a particular point in time. It figures out the direction in
which a reaction is to proceed and can be compared to K to determine the direction in which a
reaction is going. K describes a reaction at equilibrium while Q does not.
If Q > K, then the reaction favours the reactants as the concentration of the reactants is larger
than would be at equilibrium.
If Q < K, then the reaction favours the products as the concentration of products is larger
than would be at equilibrium.
If Q = K, the reaction is already at equilibrium and there is no tendency for more
products/reactants to form.
Le Chatelier’s Principle states that if a system at equilibrium undergoes a change the system will
shift to counteract that change and restore equilibrium.
If the concentration of the reactant is increased the system will attempt to counteract the
change by favouring the forwards reaction.
If the temperature is increased, the system will favour the exothermic reaction which gives
out heat energy.
If the temperature is decreased, the system will favour the endothermic reaction which takes
in heat energy.
If the pressure is increased, the system will favour the side of the reaction with less moles to
decrease the pressure.
Haemoglobin in Equilibrium: Haemoglobin is made up of 4 sub-units which are joined through
intermolecular forces. Each subunit has a heme group which contains an iron molecule which
can bond to one O2. The heme group is based on a planar porphyrin system with 4 N-containing
Equilibria 1
, units. The high conjugation means that the system has strong electronic absorption. The Fe inside
forms a chelate system where each N has a lone pair donating to the Fe, so the Fe is strongly
bound. A histidine residue binds to the Fe distorting it out of the plane. The binding of 02 brings
Fe back into the plane facilitating co-operative binding as the shape distortion reveals the other 3
Fe binding sites. At pH 7 the histidine residue is protonated and this form leads to the formation
of salt bridges which distort the haem iron allowing it to release O2. CO2 binds to the amino
group of certain amino acid residues at the subunit to produce -NHCOO- which releases O2.
Dissociation constant (Kd) gives a measure of the strength of binding. A large Kd means less
binding.
For reactions at equilibrium there is not energetic driver to change the relative proportion of
reactants and products so Gibbs Free Energy is 0.
For forwards reactions, the reaction quotient Q is less than K and the system is trying to
reach Q = K. The Gibbs Free Energy is negative.
For reverse reactions, the reaction quotient Q is greater than K and system is trying to reach
Q = K. The Gibbs Free Energy is positive.
Equilibria 2
