thermodynamics2 - free energy Flashcards
(19 cards)
free energy definition
the ability to do work - this only applies to spontaneous reactions, as non spontaneous reactions only occur as a result of work being done on them
is the ΔS of system the same as ΔS of surroundings?
no - for a reversible process, the sum of the entropy change of both the system and surroundings is 0
for an irreversible process, the total change of entropy of the combined system and surroundings is greater than 0
ΔStotal = ΔSsystem + ΔSsurroundings
how can ΔStotal be found fpr a process that occurs at constant pressure + temperature?
ΔStotal = ΔS - ΔH/T
how does gibbs free energy relate entropy and enthalpy?
gibbs free energy defines a balance between the entropy of the system and enthalpy of the surroundings, giving the equation ΔG = ΔH - TΔS for a system at constant pressure
what is the helmholtz free energy?
ΔA = ΔU - TΔS
this is equivalent to the gibbs free energy, same thing but at constant volume
what is the role of gibbs free energy in reactions?
gibbs free energy is the driving force of the reaction
what is ΔG in terms of work?
ΔG for a process is the maximum amount of non-expansion work that can be extracted from a process at constant pressure and temp
what is necessary for a reaction to do work?
any reaction that is spontaneous/can be spontaneous will do work
why do endothermic reactions go?
reactions can be either enthalpically driven or entropically driven - even if a reaction has a high enthalpic cost, if there is enough entropic benefit and the process proceeds at its most favourable temperature, it can go
how can gibbs free energy be visualised graphically?
ΔG = ΔH - TΔS => y = c -xm
this equation can be plotted into a linear graph
- any temperature at which the line is below y=0 is the conditions when the reaction is favourable
what is the relationship between ΔG and ΔStotal?
they are opposite/have opposite signs
- this can be seen when plotted on a reaction profile, the curves will mirror each other
- consider, for a process to be spontaneous, ΔG must decrease, ΔStotal must increase
explain equilibrium using ΔG
a reaction mixture will continually adjust its composition until a minimum value of gibbs energy is achieved, this point = equilibrium
when ΔG = 0
how does driving force change over the course of a reaction?
ΔG° = pure reactants/products in standard states, not true during a reaction
ΔG° = 0 predicts that at equilibirum reactants and products are equally favoured
ΔG = 0 means the system is in equilbirum
how are ΔG° and equilibrium constant k related?
if k>1, then ΔG°<0, the reaction is spontaneous
if k<1 then ΔG°>0, and the reaction is not spontaneous
if k=1 so lnk=0, then ΔG°=0 and reactants and products are equally favoured
how does internal energy of a system impact entropy + enthalpy at equilibrium?
when a reaction can occur, the boltzmann distribution is shared over both sets of energy levels (products + reactants), increasing entropy
enthalpy is goverened by the bottom/lowest possible state related to U
how are ΔH and equilibrium constant k related at equilibrium?
if ΔH = approx 0 and energy spacings are similar, K = approx 0
if either products or reactants are favoured these molecules will be dominantly populating the lower energy levels, this depends on whether a reaction is endothermic/exothermic:
if reactants are favoured, reaction = endothermic and k<1, the reaction is favoured at low temperatures and equilibrium moves towards reactants
if products are favoured, reaction = exothermic and k>1, the reaction is spontaneous and favoured at high temperatures and equilibirum moves towards products
how does the driving force of the reaction change if product energy levels are much closer than reactant energy levels?
this would mean product molecules are spread out more over a wider range and so have a higher entropy, meaning that entropy drives the reaction towards products as there are more product macrostates
what affects equilibrium constant k?
only temperature
what is the third law of thermodynamics?
‘no finite sequence of cyclic processes can succeed in cooling a body to absolute 0’ - essentially means nothing can be cooled to absolute 0
- this links 0th and 2nd law
could rephrase as ‘the entropy of all perfectly crystalline substances = 0 when T = 0’
