Week 4 – Equilibria, Alcohols and Haloalkanes
Equilibria
Dynamic equilibria
Reversible reactions are shown by using a reversible arrow. It looks like two arrows have been stuck on top of each other and
one side of the arrowhead has been removed.
As reactants get used up the forward reaction slows down, and more product that is formed, the reverse reaction (speeds up)
SO the rules are as reactants decrease, forwards reaction slows down. And as products increase, backwards reaction speeds up.
Dynamic equilibria = when the forwards reaction is going at exactly the same rate as the backwards reaction SO the concentration of the
reactants and the products stay constant.
Le Chatelier’s principle
As you alter the temperature, pressure or concentration the position of the equilibrium shifts… meaning different amounts of
reactants and products at equilibria.
E.g. as equilibria moves to the left you’ll get more reactants, and if it moves to the right you will get more products.
SO, Le Chatelier’s principle tells you how the position of the equilibrium will change if a condition changes.
Concentration Temperature
If you increase concentration of a reactant the If you add heat to a reaction equilibrium shifts to the
equilibrium tries to get rid of the extra so more product endothermic direction to absorb the heat.
is formed. Equilibria shifts to the right.
If you increase the concentration of a product, the
equilibrium will try to get rid of the extra so more If you decrease the temperature equilibrium shifts in
reactants are formed. Equilibria shifts to the left. the exothermic direction to try to replace the heat.
Pressure
Increasing the pressure shifts the equilibrium to the side with fewer gas molecules (to reduce pressure)
Decreasing pressure means equilibria will shift to the side with more gas molecules (to raise the pressure)
Catalysts don’t affect the position of the equilibrium. (Equilibrium reached faster, speeds up forwards and backwards by the same
amount)
Formation of Ethanol
Produced via a reversible exothermic reaction between ethene and steam.
Carried out at 300’C, 60-70atm and a phosphoric acid catalyst.
300’C is a compromise (as if you had a really low temp it would favour the forwards reaction but a really slow rate…)
BUT remember, a pressure that high is super expensive.
Kc – Equilibria constant Kp – Equilibria constant (gases)
To calculate Kc you need to do the products over the reactants Mole fraction number of moles of A / Total number of moles.
and each molecule to the power of their moles ‘a’.
SUM of mole fraction should always be equal to 1.
Kc = [A]a[B]b / [C]c [D]d
Partial pressure = the contribution that the gas makes towards
the total pressure p (A) = mole fraction of A x total pressure P
What does Kc tell us? To work out KP the equation stands the same for the calculation
of Kc.
The larger the value of Kc the further the position of the
equilibrium is to the right SO the greater the
concentration of the products compared to the
reactants. UNITS = kPa(and then you have to do some cancelling)
When Kc = 1, equilibrium is halfway between reactants Other species MUST be ignored… they are irrelevant
and products. (JUST GASES)
UNITS Cancel out what you have and don’t have etc.
Homogenous equilibria = contains equilibria species that all
have the same state or phase.
Heterogeneous equilibria = contains equilibria species that have
different states or phases (ignore solids)
Equilibria
Dynamic equilibria
Reversible reactions are shown by using a reversible arrow. It looks like two arrows have been stuck on top of each other and
one side of the arrowhead has been removed.
As reactants get used up the forward reaction slows down, and more product that is formed, the reverse reaction (speeds up)
SO the rules are as reactants decrease, forwards reaction slows down. And as products increase, backwards reaction speeds up.
Dynamic equilibria = when the forwards reaction is going at exactly the same rate as the backwards reaction SO the concentration of the
reactants and the products stay constant.
Le Chatelier’s principle
As you alter the temperature, pressure or concentration the position of the equilibrium shifts… meaning different amounts of
reactants and products at equilibria.
E.g. as equilibria moves to the left you’ll get more reactants, and if it moves to the right you will get more products.
SO, Le Chatelier’s principle tells you how the position of the equilibrium will change if a condition changes.
Concentration Temperature
If you increase concentration of a reactant the If you add heat to a reaction equilibrium shifts to the
equilibrium tries to get rid of the extra so more product endothermic direction to absorb the heat.
is formed. Equilibria shifts to the right.
If you increase the concentration of a product, the
equilibrium will try to get rid of the extra so more If you decrease the temperature equilibrium shifts in
reactants are formed. Equilibria shifts to the left. the exothermic direction to try to replace the heat.
Pressure
Increasing the pressure shifts the equilibrium to the side with fewer gas molecules (to reduce pressure)
Decreasing pressure means equilibria will shift to the side with more gas molecules (to raise the pressure)
Catalysts don’t affect the position of the equilibrium. (Equilibrium reached faster, speeds up forwards and backwards by the same
amount)
Formation of Ethanol
Produced via a reversible exothermic reaction between ethene and steam.
Carried out at 300’C, 60-70atm and a phosphoric acid catalyst.
300’C is a compromise (as if you had a really low temp it would favour the forwards reaction but a really slow rate…)
BUT remember, a pressure that high is super expensive.
Kc – Equilibria constant Kp – Equilibria constant (gases)
To calculate Kc you need to do the products over the reactants Mole fraction number of moles of A / Total number of moles.
and each molecule to the power of their moles ‘a’.
SUM of mole fraction should always be equal to 1.
Kc = [A]a[B]b / [C]c [D]d
Partial pressure = the contribution that the gas makes towards
the total pressure p (A) = mole fraction of A x total pressure P
What does Kc tell us? To work out KP the equation stands the same for the calculation
of Kc.
The larger the value of Kc the further the position of the
equilibrium is to the right SO the greater the
concentration of the products compared to the
reactants. UNITS = kPa(and then you have to do some cancelling)
When Kc = 1, equilibrium is halfway between reactants Other species MUST be ignored… they are irrelevant
and products. (JUST GASES)
UNITS Cancel out what you have and don’t have etc.
Homogenous equilibria = contains equilibria species that all
have the same state or phase.
Heterogeneous equilibria = contains equilibria species that have
different states or phases (ignore solids)
