Free-Energy Change and Entropy Change Flashcards
What is entropy?
Entropy is a measure of the amount of disorder in a system and a system becomes energetically more stable when it is more disordered.
When are substances more energetically stable?
When there is more disorder, so particles move to increase entropy.
How does physical state affect entropy?
Solid particles just wobble about a fixed point- there’s hardly any randomness, so they have the lowest entropy.
How does dissolving affect entropy?
Dissolving solid also increases its entropy- dissolved particles can move freely as they’re no longer held in one place.
Why does more particles mean more entropy?
The more particles you have, the more ways they and their energy can be arranged.
What is the total entropy change?
The sum of the entropy changes of the system and the surroundings.
The tendency of a process to take place is dependent on three things…
The entropy, the enthalpy and the temperature
Put all these together and you get the free energy change
ΔG
= ΔH – TΔS
ΔS =
(enthalpy of products) - (enthalpy of reactants).
If ΔG is negative
the reaction occurs spontaneously
If ΔG is positive
the reaction is not spontaneous.
If ΔG is zero or close to it, i.e. between +20kJmol–1 and –20kJmol–1
the reaction will reach equilibrium, with both reactants and products present.
If ΔG is more negative than approximately –20kJmol–1,
the reaction will be spontaneous and will go to completion.
What happens when a system becomes more or less ordered?
If a change makes a system more random, ΔS is positive i.e. entropy increases. Conversely if a change makes a system more ordered, ΔS is negative and the entropy decreases and the system becomes more ordered.
Explain why some endothermic processes occur spontaneously. Refer to ΔG and ΔS
ΔG = ΔH - TΔS has to be negative.
Endothermic reactions are -ΔH so TΔS has to be very positive.
