entropy

Question 1

Why does the reaction have a negative entropy change?

Answer 1

5 mol/molecules (of gas) forms 3 mol/molecules (of gas)

Question 2

The freezing of water.

Answer 2

(entropy) decreases
AND
(solid/ice has) less disorder/ more order/ fewer ways of
arranging energy/ less freedom/ less random molecules 

Question 3

The reaction of calcium carbonate with hydrochloric acid.

Answer 3

(entropy) increases
AND
(CO2) gas is formed 
Could be from equation with CO2(g)

Question 4

The formation of O3(g) from O2(g).

Answer 4

entropy decreases
AND
3 mol O2 form 2 mol O3
OR 3O2  2O3
OR 3 mol gas form 2 mol gas

Question 5

) The enthalpy and entropy changes of a reaction both have a negative sign.Discuss how the feasibility of this reaction will change as the temperature increases.

Answer 5

Feasibility AND ∆G
Reaction becomes/is less feasible/not feasible
AND
∆G increases
OR ∆G becomes/is less negative/more positive
OR ∆G > 0 OR ∆H – T∆S > 0
OR ∆H – T∆S becomes/is less negative/more positive
OR ∆H > T∆S 
OR T∆S becomes/is more negative than ∆H 
————————————————————————-
Effect on T∆S
T∆S becomes more negative OR T∆S decreases
OR –T∆S becomes more positive OR –T∆S increases
OR magnitude of T∆S increases
OR | T∆S | increases 
———————————————————————————

Question 6

Explain why this reaction is not feasible at any temperature.

Answer 6

∆
H is positive OR
∆
H > 0
AND
∆
S is negative OR
T∆
S is negative OR
∆
S < 0 OR
T∆
S < 0
AND
∆
G will always be positive OR
∆
G > 0


Question 7

The change that produces lattice enthalpy is spontaneous but has a negative entropy change.
Why is this change able to take place spontaneously?

Answer 7

∆H < T∆S OR ∆H – T∆S < 0
OR
∆H is more negative than T∆S
OR
Negative value of ∆H is more significant than negative
the value of T∆S 

Question 8

Explain why this reaction has a negative entropy change.

Answer 8

3 gaseous moles  2 gaseous moles 

Less randomness OR becomes more ordered 

Question 9

State and explain how the feasibility of this reaction will change with increasing temperature.

Answer 9

Feasibility with increasing temperature
Reaction becomes less feasible/not feasible
AND
∆G increases OR ∆G becomes less negative
OR ∆G = 0 OR ∆G > 0 OR ∆G is positive
OR ∆G approaches zero 
***IF a candidate makes a correct statement about the
link between ∆G and feasibility, IGNORE an incorrect
∆H and T∆S relationship
IF there is no ∆G statement, then mark any ∆H and
T∆S relationship in line with the mark scheme
———————
Effect on T∆S
T∆S becomes more negative OR T∆S decreases
OR –T∆S increases
OR magnitude of T∆S increases 

Question 10

The exothermic reaction below occurs spontaneously at low temperatures but does not occur at very high temperatures.
2SO2(g) + O2(g) 2SO3(g)
Explain why.

Answer 10

With increasing temperature
T∆
S is more negative OR
T∆
S decreases
OR –
T∆
S increases OR |
T∆
S| increases
OR magnitude of
T∆
S increases

At high temperature
T∆
S is more negative that
∆
H
OR
at high
T, T∆
S outweighs/is more significant than
∆
H
OR
At low temperature
∆H –
T∆
S < 0
OR
At high temperature
∆H –
T∆
S > 0

Question 11

Explain why ammonium nitrate in the cold pack dissolves spontaneously in water even though this process is endothermic.

Answer 11

(During dissolving,) entropy/disorder increases
disorder increases 
T∆S > ∆H
OR T∆S is more positive than ∆H
∆H – T∆S is negative 

Question 12

Explain, in terms of entropy, why this reaction is not feasible at very high temperatures.

Answer 12

As the temperature increases,
T∆
S becomes more negative
OR T∆
S becomes more negative than
∆
H
 OR
T∆
S becomes more significant

Eventually
∆
H –
T∆
S becomes positive


Question 13

Suggest why a temperature of 400–500 °C is used for ammonia production, despite the reaction being feasible at room temperature.

Answer 13

Activation energy is too high
OR reaction too slow
