5.2.2: Enthalpy and entropy

Question 1

Define entropy and give its units

Answer 1

Quantitative measure of the degree of disorder of a system
JK-1mol-1

Question 2

What does a higher energy dispersel and a more disordered system mean in terms of entropy ?

Answer 2

A higher entropy

Question 3

Describe the entropy of solid

Answer 3

Low entropy due to ordered, regular arrangement of particles

Question 4

Describe the entropy of a gas

Answer 4

High entropy due to disordered, random arrangements of particles

Question 5

What causes entropy to increase if reaction is in the same state ?

Answer 5

More moles of product are produced or a higher number of particles produced

Question 6

Higher temperature = ??

Answer 6

Higher entropy

Question 7

How do you calculate the standard entropy change of a system ? Give the units

Answer 7

Products - reactants
JK-1mol-1

Question 8

What are the 3 conditions for enthalpy change of a system ?

Answer 8

1 mole
298K
1atm

Question 9

Is standard entropy change in general always negative or positive ?

Answer 9

Positive

Question 10

Give 3 cases where enthalpy changes are negative

Answer 10

-Precipitation reactions
-Some lattice enthalpies
-If there are more gas moles on the left

Question 11

What does free energy explain ?

Answer 11

How endothermic reactions can take place spontaneously when entropy changes overcome enthalpy changes

Question 12

What does feasibility depend on ?

Answer 12

-Entropy change in a system and temperature TS
-Enthalpy change of a system H

Question 13

What are the 2 conditions for a reaction to be feasible ?

Answer 13

-Total entropy change must be positive
-Delta G must be negative

Question 14

Give Gibb’s equation and units

Answer 14

Delta G = Delta H / T Delta S

Delta G = KJmol
Delta H = Jmol-1
Temperature = K
T Delta S = JK-1mol-1

Question 15

Give 2 reasons why a reaction may be feasible but not observed

Answer 15

1) Activation energy is to high so hasn’t been overcome
2) Rate of reaction/kinetics are too slow

Question 16

Give an example of a reaction which is only feasible at high temperatures

Answer 16

Thermal decomposition of CaCO3

Question 17

What are 2 conditions needed for an endothermic reaction to occur spontaneously?

Answer 17

Delta G must be negative and temperature must be high so that T Delta S is larger than Delta H

Question 18

How do you calculate the temperature a reaction becomes feasible ?

Answer 18

When Delta G = 0
T = Delta H / Delta S

Question 19

Why might the entropy of water be zero

Answer 19

Particles are at a maximum state of order/no disorder in system

Question 20

What is assumed if enthalpy change of reaction is seen in a question ?

Answer 20

Standard conditions apply 298K

Question 21

Ba(NO3)2(aq) + Na2SO4(aq) → BaSO4(s)
+ 2NaNO3(aq). Explain the entropy changes

Answer 21

Entropy decreases as BaSO4 solid has less disorder

Question 22

Why does this reaction have negative entropy ?4H2 + CS2 –> CH4 + 2H2S

Answer 22

5 moles of gas forms 3 moles of gas so reaction has formed fewer moles (less moles of gas = less disorder)

Question 23

If enthalpy and entropy changes are both negative, how does feasibility of reaction change as temperature increases ?

Answer 23

Reaction becomes less feasible as deltaG becomes more positive. TdeltaS becomes more negative than deltaH

Question 24

In terms of free energy change explain the meaning of the term feasible

Answer 24

Where deltaG is less than or equal to 0

Question 25

How will feasibility of a reaction change with increasing temperature ?

Answer 25

Feasibility decreases as DeltaG will increase meaning TdeltaS will become more negative