3.7 Entropy and Feasibility 2/3

Question 1

what is entropy?

Answer 1

entropy, S of a system is measure of disorder or randomness, higher the entropy the more freedom particles possess so the more disordered the system

Question 2

what is the general relation bet natural processes and entropy?

Answer 2

all natural processes will always tend towards increasing entropy as they are more energetically stable when there is more disorder, there is an increase in entropy towards a max for all natural changes

Question 3

how does physical state affect entropy?

Answer 3

particles in a solid have much less freedom and disorder than those in a gas, entropy increases s<l<g

Question 4

define the standard entropy of a substance, S^theta

Answer 4

the entropy of one mole of substance under standard conditions

Question 5

eqn for calculating the overall entropy change from absolute entropy values

Answer 5

deltaS= products - rctnts

Question 6

units of entropy

Answer 6

JK^-1mol^-1

Question 7

what does is mean if a rctn is ‘spontaneous’

Answer 7

a spontaneous process is capable of taking place w/o needing to be driven by an outside source of energy

Question 8

is a rctn w +ve or -ve entropy change more likely to be spontaneous?

Answer 8

a rctn with +ve entropy change is more likely to occur spontaneously as rctns will naturally try to increase the amnt of disorder

Question 9

is a rctn w +ve or -ve enthalpy change more likely to be spontaneous?

Answer 9

-ve enthalpy change exothermic rctns more likely to be spontaneous than endothermic rctns

Question 10

what does it mean for a rctn to be ‘feasible’?

Answer 10

for a rctn to be feasible at a given temp, it must occur spontaneouly so no external energy input is req for rctn to take place

Question 11

explain why rctn may be feasible despite being endothermic

Answer 11

there is an increase in entropy as rctn produces gases and liquids which have a higher entropy than solids since they are more disordered. rctn is also favoured as there is an increase in number of moles increasing entropy, increase in entropy overcomes the fact that it is an endothermic rctn making rctn feasible

Question 12

what is Gibbs free energy change?

Answer 12

deltaG is a measure used to predict whether a rctn is feasible

Question 13

eqn for Gibbs free energy change

Answer 13

deltaG = deltaH - T x deltaS

Question 14

how dos the feasibility of a rctn relate to Gibbs free energy change?

Answer 14

for a rctn to be feasible, deltaG must be negative or 0

Question 15

consider an endothermic rctn and explain effect of T and deltaS on the feasibility of the rctn

Answer 15

deltaG = deltaH - TxdeltaS
for an endothermic rctn deltaH is +ve
- if deltaS is -ve then deltaG is always +ve so rctn would not be feasible at any temp
- if delta S is +ve then rctn will only be feasible at a high enough temp when TdeltaS>deltaH

Question 16

consider an exothermic rctn and explain effect of T and deltaS on the feasibility of the rctn

Answer 16

deltaG = deltaH - TxdeltaS
for an exothermic rctn deltaH is -ve
-if deltaS is +ve then deltaG is always -ve so rctn would be feasible at any temp
-if deltaS is -ve the rctn will only be feasible at lower temps when TdeltaS<deltaH

Question 17

key factors which tell u the sign of entropy change of a rctn?

Answer 17

no of mols in rctn, more mols made in prod means increase in entropy
change of state, where particles have higher KE leads to increase in entropy

Question 18

limitations of using Gibbs free energy change as an indication whether a rctn will occur??

Answer 18

Gibbs free energy only indicates if a rctn is feasible doesnt take into acc rate of rctn or Ea, it might still req a very high Ea or be so slow that effectively no rctn occurs

Question 19

how dyou calc temp at which a rctn becomes feasible

Answer 19

a rctn becomes feasible when deltaG=0
deltaH - TdeltaS = 0 —-> T=deltaH/deltaS