Reaction feasibility
AH Chemistry, Unit 2(d)
Thermodynamics
• Helps understand and predict the
behaviour of substances and their
reactions based on energy changes.
• Does not rely on knowing about atoms,
molecules, ions etc.
Questions chemists are interested in…
1. Will a reaction go “spontaneously” in the
direction written?
2. Will the composition of the reaction
mixture contain enough product at
equilibrium?
3. Will the reaction occur at an
reasonable speed?
• Thermodynamics predicts if a reaction
can happen, given enough time
(sometimes a very long time e.g. billions
of years).
• Kinetics predicts if a reaction happens at
a reasonable rate.
• Thermodynamically favoured
• But not kinetically favoured
Why do chemical reactions happen?
To make the Universe more disordered.
Important terms: system and surroundings
Important terms: spontaneous process
Hydrogen and oxygen
• Is the reaction of hydrogen and oxygen
spontaneous?
• YES. It is a processes that will happen
without any outside intervention.
True or false?
• A spontaneous process is
one which moves to
minimise it’s energy?
• All exothermic reactions
are spontaneous?
• No endothermic
reaction is spontaneous?
Ice
• Ice forms liquid water
spontaneously at
temperatures above 0C (an
endothermic process) but
the reverse is spontaneous –
water to ice (an exothermic
process) – at temperatures
below 0C.
• There must be another factor
(other than energy) which
plays a part in determining
whether changes or
reactions happen.
Barium hydroxide and ammonium chloride
Entropy
• This factor is ENTROPY, S.
• A measure of disorder.
• The larger the entropy, the greater the
disorder.
• ∆S = S(products) – S(reactants)
• To define the entropy of a substance in
absolute terms, a reference point is needed.
• A perfectly ordered system would have zero
entropy.
• The Third Law of thermodynamics states that:
“ the entropy of a perfect crystal at zero kelvin
is zero”.
Standard Entropy
• The standard entropy of a substance, Sº,
is the molar entropy at 298K and 1
atmosphere pressure.
• The units are J K-1 mol-1
Standard entropy change of a reaction
• ∆rSº = ∑Sº(products) – ∑Sº(reactants)
Practise
Calculate the standard entropy change of
reaction at 298K for:
1. The reaction of hydrogen and oxygen
Why do chemical reactions happen?
• To make the Universe more disordered.
Important terms: system and surroundings
Second Law of Thermodynamics
• Spontaneous processes are those that
increase the total entropy of the
Universe.
• ∆S(total) = ∆ S(system) + ∆ S(surroundings)
Gibbs energy change
• Gibbs energy change, ∆G, (free energy
change) combines changes in entropy
and enthalpy into a single equation to
describe the spontaneity of a process at
constant temperature and pressure,
using only information from the reaction
system.
• ∆G = ∆H - T∆S
If ∆G < 0
the reaction is spontaneous
If ∆G > 0
The reaction is non-spontaneous
(to any significant extent)
A negative ∆G means that a reaction CAN happen but does NOT
mean that it WILL happen – the reaction is thermodynamically
feasible but could be kinetically unfavourable i.e. it is very slow due to
a high activation energy.
Practise
Gibbs energy and equilibrium
• No chemical reaction proceeds in only
one direction.
• The reaction of hydrogen and oxygen at
298K nearly does (∆Gº = -237.1 kJ mol-1).
Gibbs energy and equilibrium
• The reaction of dinitrogen tetroxide to form
nitrogen dioxide (∆Gº = -5.8 kJ mol-1) is
different.
N2O4(g)
2NO2(g)
The reaction is spontaneous
but does not go to completion.
Gibbs energy and equilibrium
• When the Gibbs energy of the reactants
has become the same as the Gibbs
energy of the products, the mixture is at
equilibrium.
• Therefore, at a point in a reaction when
∆G = 0, the reaction is at equilibrium.
Coupling reactions
• A positive ∆G does not mean that a
reaction can never happen, just that it
will not occur spontaneously to any
significant extent.
• If it is coupled to a reaction with a larger,
negative ∆G, the reaction can be
made to occur.
• This is common in biochemical processes.
The blast furnace
Ellingham diagrams
2Fe2O3(s) → 4Fe(s) + 3O2(g)
G = +1487 kJ mol-1
• The opposite of iron rusting.
• A +ve G doesn’t mean that it can’t happen,
just that work must be done to make it happen.
• It can be coupled to a reaction which has a
more negative G in order to make it happen.
2CO(g) + O2(g)
2Fe2O3(s)
2CO2(g)
G = -514.4 kJ mol-1
x3
4Fe(s) + 3O2(g) G = +1487 kJ mol-1
2Fe2O3(s) + 6CO(g)
4Fe(s) + 6CO2(g) G = -56 kJ mol-1
Overall, the reduction of iron(III) oxide has become
thermodynamically feasible.