Summary
Summary ALEVEL CHEMISTRY - Entropy and Gibbs Free Energy Notes
- Module
- Unit 3.1.8 - Thermodynamics
- Institution
- AQA
Entropy and Gibbs Free Energy Notes
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Entropy and Gibbs Free EnergyEntropy is a measure of the amount of chaos in a system. A measure of ways particles can be
rearranged and number of ways energy can be shared out by the particles. Endothermic
reactions occur because of entropy. An increase in entropy makes a system more ordered.
Entropy is the reaction that endothermic reactions occur.
Factors that affect entropy:
Physical State - solid particles have the lowest entropy while gaseous particles have the
highest entropy
Dissolution - dissolving a substance increases its entropy as dissolved particles move
more freely and are no longer in one place
Number of particles - more particles = more entropy as more particles increases the
number of ways they can be rearranged and number of collisions. More moles = increased
entropy.
As molecules become more complexed entropy increases - more ways of arranging the energy
of the molecule in a disordered way over bigger molecules than smaller ones.
A system becomes more stable when its energy is spread out in a more disordered state.
What happens when an ionic solid dissolves in water?
The ionic solid is highly ordered, and so has a relatively low entropy. Pure liquid water also has a
certain amount of order as explained above. But when the solid dissolves in water, the whole
system becomes highly disordered as the crystal breaks up and the ions find their way between
the water molecules. Entropy increases.
Example 1:
This is just a crystalline solid going into solution. The solid is highly ordered; the solution is
disordered. There is an increase in entropy.
Example 2:
The water is changing from the highly disordered gas into a more ordered liquid. The entropy
will fall.
Example 3:
, There are three moles of gas on the left-hand side of the equation, but only one on the right.
The starting materials are more disordered than the products, and so there is a decrease in
entropy.
Notice that if the water had been formed as steam, you couldn't easily predict whether there
was an increase or a decrease in entropy, because there would be three moles of gas on each
side.
Example 4:
The presence of the five moles of liquid water on the left-hand side means that there will be far
more disorder before the change than there is in the products. The copper(II) sulphate crystals
formed will be very ordered. Entropy will decrease.
ΔS systemꝋ = ƩΔS productsꝋ - ƩΔS reactantsꝋ
ΔS is the difference between the reactants and products.
Most reactions will not occur unless the entropy change is positive.
Units are J/mol
However using this calculation leaves out the important factor of heat. If a reaction is
exothermic heat is given out to the surroundings and as heat energy is added the number of
different possibilities for rearranging energy over the molecules increases. In an endothermic
reaction the surroundings are cooled and the entropy of the surroundings decreases:
The units for entropy change and enthalpy change are different. Therefore X1000 the enthalpy
change.
For a reaction to be feasible, the total entropy has to increase - in other words the sign of the
total entropy change must be positive.
Entropy change must be positive for a reaction to occur but Gibbs Free Energy must be
negative for a reaction to be feasible.
Gibbs Free Energy Change
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