Thermodynamics Flashcards
Spontaneous change
one that occurs without a continuous input of energy from outside the system (though activation energy may be required to initiate it)
If a change is spontaneous in one direction
it will be non-spontaneous in the reverse direction
how may non-spontaneous reactions and processes be driven?
with continual input of energy
is enthalpy change a predictor of spontaneity?
many exothermic processes are spontaneous (eg combustion reactions)
examples of spontaneous endothermic transformations
- melting of ice (at higher temps)
- dissolution (of some solids at some concentrations and temps)
all show an increase in the freedom of motion of particles in the system
relationship between freedom of motion and spontaneity
an increase in freedom of motion (dispersal of energy) favours spontaneity
entropy, S
measure of energy dispersal, or freedom of motion, in a system
positive value of ΔS indicates
negative value of ΔS indicates
increased dispersal of energy
decreased dispersal of energy
is entropy a state function?
yes
ΔS = S(final) - S(initial)
units of entropy
J/K
S’
standard molar entropy: entropy of 1 mole of the pure substance in its standard state
entropy trends
- effect of physical state
- effect of particle numbers
- effect of molecular complexity
- effect of temperature
effect of physical state
S of solids < S of liquids «_space;S of gas
solids - less energy dispersed, lower entropy
effect of particle numbers
more molecules have higher entropy than fewer molecules
effect of molecular complexity
entropy increases with chemical complexity and flexibility
- this only holds for substances in the same physical state
- the effect of physical state dominates the effect of molecular complexity
effect of temperature
as temperature increases, entropy increases
- higher temperature means more freedom of molecular motion
draw graph for temperature vs entropy
discontinuous jumps at phase changes
entropy change upon dissolution
- salt gains entropy as it is dispersed
- water loses entropy as it is ordered around the ions
- net entropy change depends on the relative magnitudes of entropy changes in both solute and solvent entropy
why does increased entropy favour spontaneous change?
high-entropy configurations can be achieved in more ways than low-entropy configurations. they are therefore more likely to occur
draw a diagram for spontaneous expansion of a gas and explain it
- arrangement B has higher entropy (is entropically favoured)
- gas expands when stopcock is opened because there are more ways to achieve the configuration on the right (B) than on the left (A)
absolute entropies can be calculated from
the number of micro states (W) a system may occupy
2nd law of thermodynamics
spontaneous reactions proceed in the direction that increases the entropy of the universe (system + surroundings)
ΔS(Universe) = ΔS(sys) + ΔS(surr) > 0
thus, any decrease int he entropy of the system must be offset by a larger increase in the entropy of the surroundings for that process to be spontaneous
2nd law has profound implications:
- isolated systems always evolve toward higher entropy states
- entropy of the universe is always increasing
how are temperature, heat flow, and entropy linked?
- if heat flows into a system from the surroundings, the entropy of the system increases. the surroundings lose entropy
- the amount by which a given amount of heat flow changes entropy depends on temperature. if it is low, the effect on entropy can be enormous
3rd law of thermodynamics
a perfect crystal has zero entropy at absolute zero
S(sys) = 0 at 0K
has flawless alignment of all its particles. at absolute zero, particles have minimum energy so there is only one micro state
contrast between S and H
- entropy scale is anchored to an absolute value
- enthalpy does not have an absolute 0
standard entropy of a reaction ΔS’(rxn)
entropy change that occurs when all reactants and products are in their standard states
ΔS’(rxn) = S’(products) - S’(reactants)
remember to include coefficients
Gibbs free energy change ΔG
evaluates spontaneity as a function of enthalpy and entropy of the system alone
ΔG(sys) = ΔH(sys) - TΔS(sys)
ΔG < 0 process is spontaneous
ΔG = 0 process is at equilibrium
ΔG > 0 process is non-spontaneous
- lowering free energy is the driving force of chemical reactions
- negative ΔH(sys) and positive ΔS(sys) favour spontaneity
- entropic contribution to free energy change (-TΔS) is increasingly important at higher temperatures
give another way to define ΔG
the maximum useful work that can be done by a system as it undergoes a spontaneous process at constant temperature and pressure
ΔG = w(max)
also the minimum work that must be done on a system to drive the occurrence of a non-spontaneous process
is ΔG a state function?
yes
describe the extensive property of ΔG
scales linearly with amount
ΔG’f
standard free energy of formation of a compound from its constituent elements in their standard states
ΔG’f of an element in its standard state is
0
thermodynamics vs kinetics
ΔG tells us whether a reaction will/won’t proceed
the free energy of activation (including Ea) tells us how fast a reaction proceeds
ΔG and spontaneity
reaction is spontaneous when ΔG(rxn)<0
how do we make a non-spontaneous reaction happen?
must be driven by coupling the non-spontaneous reaction with a spontaneous reaction of sufficiently favourable ΔG
how are free energies and equilibrium position linked?
ΔG depends on how much product and reactant is present at that instant (Q0 compared to their equilibrium values (K), the temperature and the gas constant:
ΔG = RTln(Q/K)
how does the magnitude of ΔG tell us how far out of equilibrium the mixture is?
ΔG < 0 ; Q < K ; ln(Q/K) < 0 - process proceeds (forward spontaneously)
ΔG = 0 ; Q = K ; ln(Q/K) = 0 - process is at equilibrium
ΔG > 0 ; Q > K ; ln(Q/K) > 0 - reverse process proceeds spontaneously
if Q and K are very different
ΔG has a very large value (negative or positive). The reaction releases or absorbs a large amount of free energy as it proceeds to equilibrium
if Q and K are nearly the same,
ΔG has a very small value (negative or positive). The reaction releases or absorbs very little free energy as it proceeds to equilibrium.
how are thermodynamic Q and K different to Qc and Kc
in the thermodynamic Q and K, all substances are referenced to their own standard states, So, Q and K may include mixed states
equation linking ΔG, gas constant, temp, and K
-RTln(K)
another formula for ΔG
= ΔG’ + RTln(Q)
draw the two free energy hills with ΔG
features of free energy hills
- slope at any given point tells us the value of ΔG for that mixture
- as the system approaches equilibrium, ΔG approach 0
- at equilibrium, the free energy is at a minimum.
