Entropy and Gibbs free energy

Question 1

Spontaneous reactions

Answer 1

Reactions that proceeds without need for outside assistance at given set of conditions. Cannot be brought back to its original state and requires external influence to do so

eg: Diffusion of gas particles is spontaneous but to gather back particles into a smaller volume is non spontaneous

Sodium reacts violently with water to give sodium hydroxide and hydrogen but by reacting sodium hydroxide and hydrogen will never get sodium metal and water

Question 2

Non spontaneous reactions

Answer 2

Has no natural tendency to occur

Question 3

Relationship between Spontaneous reactions and ΔH

Answer 3

Exothermic reactions are usually spontaneous whereas endothermic usually not. However they can be.

eg: dissolution of NaCl is water which is endothermic but spontaneous

and melting of ice

ΔH alone not accurate in predicting whether reaction is spontaneous formula : ΔG= ΔH - TΔS is used where T is in kelvins, ΔS is in joules ΔH is in kJ/mol

Question 4

Entropy , S

Answer 4

Measure of the number of ways in which matter can be arranged and energy can be distributed in the system
The measure of disorderliness in a system

Question 5

Entropy change, ΔS

Answer 5

Sfinal-Sinitial

If entropy increases ΔS is more than 0
if entropy decreases ΔS is less than 0

Question 6

Factors affecting entropy

Answer 6

1. change in no.of gaseous particles (must check this first! to determine sign of ΔS)
   WHY?
   Δngas= (ngas products -ngas reactants)>0, S>0 —> ΔS >0
   Δngas = (ngas products -ngas reactants ) <0, S<0 —-> ΔS<0
2. mixing of particles (no chem rxn)
   S>0 —> ΔS>0
   WHY?
3. Change in phase (physical change) at MP/BP
   there will be no change in chemical composition of the system
   -solid to liquid ΔS>0
   -liquid to gas ΔS>0
   -Magnitude of change in entropy greater for boiling than melting
   WHY?
4. Temperature
   w/o change in phase
   when temp increases ΔS>0
   WHY?
5. Precipitation
   opposite of dissolution
   from aq to s state hence decreased disorderliness…
   ΔS<0
6. Dissolution of ionic compd in water
   usually ΔS>0, due to increases disorderliness but it also depends as it is NOT always the case

Question 7

State symbols in sign of ΔS

Answer 7

Sg»Saq>Sl>Ss

Question 8

Gibbs free energy ΔG

Answer 8

GO HOME NOT TO SLEEP
ΔG = ΔH - TΔS
at standard state T= 298K

Question 9

uses of ΔG to predict spontaneity of reaction

Answer 9

ΔG< 0 spontaneous
ΔG >0 not spontaneous

if forward reaction ΔG<0
backward reaction will be ΔG>0

if forward reactions not spontaneous can be made spontaneous by…

Question 10

ΔG =0

Answer 10

Where both forward and backward reactions are spontaneous.
for chemical reactions: system said to have obtained dynamic equilibrium no net change in amount of reactants/ products
for physical changes : at MP, melting processes and freezing processes equally spontaneous
at BP, boiling processes and condensation processes equally spontaneous

during changeover of state

Question 11

Effect of temperature on ΔG

Answer 11

Case 1
for reaction to be spontaneous ΔG <0
ΔH is -ve , ΔS +ve T +ve
Reaction is always spontaneous as ΔG<0 at all temperatures
eg: decomposition of H2O2

Case 2
for reaction to be spontaneous ΔG <0
ΔH is +ve ΔS -ve T +ve
Reaction is never spontaneous as ΔG>0 at all temperatures

Case 3
for reaction to be spontaneous ΔG <0
ΔH is +ve ΔS +ve T +ve
Reaction only spontaneous at high temperatures as ΔG<0 only at high temperature

eg: melting/boiling

Case 4
for reaction to be spontaneous ΔG <0
ΔH -ve ΔS -ve T +ve
Reaction only spontaneous at low temperatures as ΔG  <0 at low temperatures
freezing/condensation/ppt

Question 12

Assumptions when calculating ΔG

Answer 12

That ΔH and ΔS remain constant at all temperatures

only true if there is no phase change

Question 13

Limitations of ΔG

Answer 13

Can only be used to predict spontaneity of reaction and not the rate of reaction

Even though ΔG might be <0 the reaction might still not occur under standard conditions as rate of reaction can be very slow that it has no or little yield
for instance combustion: though its ΔG value is <0 it is suggesting it is spontaneous, and thermodynamically but reaction does not occur at rtp under standard conditions
as combustion has high Ea without heat reaction is very slow. Combustion at rtp is energetically feasible but not kinetically feasible / graphite to diamond

Question 14

To determine whether reaction will actually take place under standard conditions

Answer 14

1. Reaction must have negative ΔG knot value

2. energy provided at standard conditions must be able to overcome Ea