Energetics
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Energetics
Feasibility of a Reaction
In order to predict whether a reaction will
occur or not (the feasibility of a reaction) there are some limitations to use H
as a guide to predict the feasibility of a reaction.
Although, it can be used as a rough guide to the likelihood of a reaction, if H
for a reaction is negative, energy is lost when the reaction occurs. The products
are more stable than reactants. Thus, exothermic reactions are more likely to
occur than endothermic reactions.
There are following limitations with H to predict feasibility of a reaction.
1. H says nothing about the kinetic stabilities of the products relative to
the reactants. It shows only energetic stabilities of the reactants products
for a reaction.
2. H is no guide to the rate of a reaction. It can’t tell whether a reaction is
fast or slow.
For example, a reaction may be enormously exothermic, yet nothing
happens like a mixture of hydrogen and oxygen at room temperature.
This is because the reaction rate is very slow and the reactants are
kinetically stable with respect to products.
Entropy
In order to make accurate predictions whether a reaction will occur or not it is
necessary to consider energy lost or gained by the reacting system, and also any
energy changes inside that system. For example, when a solid dissolves in a
liquid or when a gas is produced in a reaction, there us a marked increase in the
disorder of the system itself and this disorder increases in the number of ways
in which the energy is distributed in the system.
As a result, in order to predict the likelihood of any reaction we should take
into account the changes in order (or disorder) introduced into the system.
There are certain endothermic reactions (with positive enthalpy change) occur
spontaneously this can be explained using disorder concept which is called
entropy.
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Energetics
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Spontaneity of a reaction - Gibbs free energy
Reactions which release heat (and so increase stability) tend to occur. Reactions
which increase entropy (ΔS is positive) tend to occur, but neither can be used to
accurately predict spontaneity alone.
Gibbs free energy (G) is defined as a measure of the total entropy of the
universe. Hence the change in Gibbs free energy (ΔG) is the change in the total
entropy of the universe. The total entropy of the universe must increase for any
process to occur.
When heat is released in a reaction (exothermic change) this energy heats up the
universe and effectively increases its entropy (there are a greater number of
possible energy states that the particles in the universe can adopt).
The total entropy of the universe must increase and consequently exothermic
reactions are favourable.
If the entropy of a reaction mixture increases then this is also favourable as the
total entropy of the universe also increases.
Gibbs free energy change = ΔH - TΔS
If Gibbs free energy change is negative (convention) then the total entropy of
the universe increases and the reaction is spontaneous. Why is the sign
negative?
When ΔG is negative, the reaction is spontaneous, when it's positive, the
reaction is not.
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Energetics
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Gibbs free energy calculations
Enthalpy changes can be calculated indirectly by summing the enthalpy values
of related equations using Hess' law. Entropy changes can be calculated in the
same way. It follows then that Gibbs free energy changes can be calculated from
a knowledge of Gibbs free energy values in related equations.
Spontaneity of reaction
Determined by the relationship
ΔG = ΔH - Temperature(in kelvin) x ΔS
Enthalpy change
positive
negative
negative
positive
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Entropy change
positive
positive
negative
negative
Gibbs free energy
Spontaneity
depends on T, may be + or -
yes, if the
temperature is high
enough
always negative
always spontaneous
depends on T, may be + or -
yes, if the
temperature is low
enough
always positive
never spontaneous
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Energetics
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HESS’S Law of Constant Heat Summation
One of the important outcome of first law of thermodynamics is the Hess’s law. This law was
put forth by G.H. Hess and states that “the enthalpy change in a chemical or physical
process is same whether the process is carried out in one step or in several steps”.
This can be explained in the following way as well.
“The energy change in converting the reactants A+B, to products, X+Y, is the same,
regardless of the route by which the chemical change occurs, provided the initial and final
conditions are same”.
Path II
C
H2
A+B
H3
H1
X+Y
Path I
H1 = H2 + H3
It simply an application of more fundamental law of conservation of energy i.e.
1st law of thermodynamics.
It implies that the enthalpy change of a reaction depends on the initial and final
state and it is independent of the manner by which the change is brought about.
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