Thermodynamics

Question 1

Spontaneous change

Answer 1

one that occurs without a continuous input of energy from outside the system (though activation energy may be required to initiate it)

Question 2

If a change is spontaneous in one direction

Answer 2

it will be non-spontaneous in the reverse direction

Question 3

how may non-spontaneous reactions and processes be driven?

Answer 3

with continual input of energy

Question 4

is enthalpy change a predictor of spontaneity?

Answer 4

many exothermic processes are spontaneous (eg combustion reactions)

Question 5

examples of spontaneous endothermic transformations

Answer 5

• 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

Question 6

relationship between freedom of motion and spontaneity

Answer 6

an increase in freedom of motion (dispersal of energy) favours spontaneity

Question 7

entropy, S

Answer 7

measure of energy dispersal, or freedom of motion, in a system

Question 8

positive value of ΔS indicates
negative value of ΔS indicates

Answer 8

increased dispersal of energy
decreased dispersal of energy

Question 9

is entropy a state function?

Answer 9

yes
ΔS = S(final) - S(initial)

Question 10

units of entropy

Answer 10

J/K

Question 11

S’

Answer 11

standard molar entropy: entropy of 1 mole of the pure substance in its standard state

Question 12

entropy trends

Answer 12

• effect of physical state
• effect of particle numbers
• effect of molecular complexity
• effect of temperature

Question 13

effect of physical state

Answer 13

S of solids < S of liquids &laquo_space;S of gas
solids - less energy dispersed, lower entropy

Question 14

effect of particle numbers

Answer 14

more molecules have higher entropy than fewer molecules

Question 15

effect of molecular complexity

Answer 15

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

Question 16

effect of temperature

Answer 16

as temperature increases, entropy increases
- higher temperature means more freedom of molecular motion

Question 17

draw graph for temperature vs entropy

Answer 17

discontinuous jumps at phase changes

Question 18

entropy change upon dissolution

Answer 18

• 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

Question 19

why does increased entropy favour spontaneous change?

Answer 19

high-entropy configurations can be achieved in more ways than low-entropy configurations. they are therefore more likely to occur

Question 20

draw a diagram for spontaneous expansion of a gas and explain it

Answer 20

• 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)

Question 21

absolute entropies can be calculated from

Answer 21

the number of micro states (W) a system may occupy

Question 22

2nd law of thermodynamics

Answer 22

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

Question 23

2nd law has profound implications:

Answer 23

• isolated systems always evolve toward higher entropy states
• entropy of the universe is always increasing

Question 24

how are temperature, heat flow, and entropy linked?

Answer 24

• 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

Question 25

3rd law of thermodynamics

Answer 25

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

Question 26

contrast between S and H

Answer 26

• entropy scale is anchored to an absolute value
• enthalpy does not have an absolute 0

Question 27

standard entropy of a reaction ΔS’(rxn)

Answer 27

entropy change that occurs when all reactants and products are in their standard states

ΔS’(rxn) = S’(products) - S’(reactants)

remember to include coefficients

Question 28

Gibbs free energy change ΔG

Answer 28

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

Question 29

give another way to define ΔG

Answer 29

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

Question 30

is ΔG a state function?

Answer 30

yes

Question 31

describe the extensive property of ΔG

Answer 31

scales linearly with amount

Question 32

ΔG’f

Answer 32

standard free energy of formation of a compound from its constituent elements in their standard states

Question 33

ΔG’f of an element in its standard state is

Answer 33

0

Question 34

thermodynamics vs kinetics

Answer 34

ΔG tells us whether a reaction will/won’t proceed
the free energy of activation (including Ea) tells us how fast a reaction proceeds

Question 35

ΔG and spontaneity

Answer 35

reaction is spontaneous when ΔG(rxn)<0

Question 36

how do we make a non-spontaneous reaction happen?

Answer 36

must be driven by coupling the non-spontaneous reaction with a spontaneous reaction of sufficiently favourable ΔG

Question 37

how are free energies and equilibrium position linked?

Answer 37

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)

Question 38

how does the magnitude of ΔG tell us how far out of equilibrium the mixture is?

Answer 38

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

Question 39

if Q and K are very different

Answer 39

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

Question 40

if Q and K are nearly the same,

Answer 40

ΔG has a very small value (negative or positive). The reaction releases or absorbs very little free energy as it proceeds to equilibrium.

Question 41

how are thermodynamic Q and K different to Qc and Kc

Answer 41

in the thermodynamic Q and K, all substances are referenced to their own standard states, So, Q and K may include mixed states

Question 42

equation linking ΔG, gas constant, temp, and K

Answer 42

-RTln(K)

Question 43

another formula for ΔG

Answer 43

= ΔG’ + RTln(Q)

Question 44

draw the two free energy hills with ΔG

Question 45

features of free energy hills

Answer 45

• 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.