Unit 1 Inorganic Chemistry: Key Area 5 - Reaction Feasibilty

Question 1

What is thermochemistry?

Answer 1

The study of changes in energy which occur during chemical reactions.

Question 2

What is the first law of thermodynamics?

Answer 2

• The total energy of the Universe (System and Surroundings) is constant.
• Energy can be converted from one form to another while the total energy remains the same.
• Energy can neither be created nor destroyed.

Question 3

What is the symbol for a standard reaction?

Answer 3

()°

Question 4

What is standard state?

Answer 4

Most stable state of the substance under standard conditions (usually 298K = 25⁰C and pressure of 1 atm).

Question 5

Usuwiw

Answer 5

The standard enthalpy of formation, ΔH°,is the enthalpy change when one mole of a f
substance is formed from its elements in their standard states.

Question 6

What does the standard enthalpy of formation of elements always equal?

Answer 6

0

Question 7

What is the equation for the standard enthalpy?

Answer 7

The standard enthalpy of formation(f) of products - reactants:

ΔH° = ΣΔH°f(products) - ΣΔH°f(reactants)

Question 8

What do elements in their standard state have?

Answer 8

ΔH°f = 0

Question 9

What is entropy?

Answer 9

The entropy (S) of a system is a measure of the degree of disorder of the system.

Question 10

What does a greater degree of entropy represent?

Answer 10

A greater degree of disorder

Question 11

What does the second law of thermodynamics state?

Answer 11

That the total entropy of a reaction system and its surroundings always increases for a spontaneous process.

Question 12

What is the third law of thermodynamics?

Answer 12

The Third Law of Thermodynamics provides a reference against which entropies can be measured:

• “the entropy of a perfect crystal at 0K is zero”
• The entropy then increases as heat energy is supplied.
• Entropy increases as temperature increases.

Question 13

How does temperature effect entropy?

Answer 13

Entropy increases as temperature increases.

Question 14

When is there a rapid increase in entropy and an even more rapid change in entropy?

Answer 14

There is a rapid increase in entropy at the melting point of a substance and an even more rapid and larger change in entropy at the boiling point.

Question 15

How does entropy increase from a solid ➡️ liquid ➡️ solid?

Answer 15

The disorder increases

Question 16

How does entropy change from smaller molecules to larger molecules?

Answer 16

Entropy increases (larger molecules have greater energy).

Question 17

How does heat energy released by the reaction system into the surroundings affect entropy?

Answer 17

Increases the entropy of the surroundings.

Question 18

How does heat energy absorbed by the reaction system from the surroundings affect entropy?

Answer 18

Decreases the entropy of the surroundings.

Question 19

What are the units for standard entropy?

Answer 19

JK-1 mol-1

Question 20

What is the standard entropy of a substance?

Answer 20

The standard entropy of a substance is the entropy value for the substance in its standard state.

Question 21

What is the calculation for standard entropy?

Answer 21

ΔS° = ΣΔS°(products) - ΣΔS°(reactants)

Question 22

How does entropy change when a substance is dissolved?

Answer 22

The substance has greater entropy when it is dissolved.

Question 23

How does molecular shape like linear to trigonal pyramidal affect entropy?

Answer 23

Greater vibrations in trigonal pyramidal compared to linear increases the entropy, therefore trigonal pyramidal has higher energy.

Question 24

How does mixtures affect entropy?

Answer 24

Mixtures have greater randomness and therefore increase entropy.

Question 25

What is a spontaneous reaction?

Answer 25

A reaction is said to be spontaneous if it occurs without being driven by some outside force.

Question 26

How do spontaneous reactions affect entropy?

Answer 26

Spontaneous reactions increase the total entropy of the universe.

Question 27

What are examples of spontaneous reactions?

Answer 27

- Rusting - Salt dissolving - Ice melting - Fruit ripening

Question 28

How does water freezing affect entropy?

Answer 28

- Water freezing leads to a decrease in entropy within the system. - Being exothermic, however, leads to an increase in entropy in the surroundings. - Water freezing is a spontaneous process whenever there is an overall increase in entropy.

Question 29

How does ice melting affect entropy?

Answer 29

- Being endothermic leads to a decrease in entropy in the surroundings. - There must then be an increase in entropy within the system. - Ice melting is a spontaneous process whenever there is an Overall Increase In Entropy.

Question 30

What is the Gibbs free energy change?

Answer 30

The change in Gibbs free energy (ΔG) combines entropy and enthalpy into a single equation to describe the spontaneity of a process at constant temperature and pressure.

Question 31

What is the equation of Gibbs free energy?

Answer 31

ΔG° = ΔH° - TΔS°

Question 32

What does a ΔG<0 represent?

Answer 32

The reaction is spontaneous - Feasible.

Question 33

What does a ΔG>0 represent?

Answer 33

The reaction is non-spontaneous - Not Feasible.

Question 34

What does the T stand for in the Gibbs free energy equation?

Answer 34

Temperature in kelvin

Question 35

What does a negative value of ΔG mean?

Answer 35

- That a reaction can happen but does not mean it will happen. - The reaction is thermodynamically feasible but could be kinetically unfavourable. - i.e. it is very slow due to a high activation energy.

Question 36

What are the 2 ways of Gibbs free energy for a feasible reaction?

Answer 36

ΔG(-) ΔH(-) TΔS(+) or ΔG(-) ΔH(+) TΔS(TΔS > ΔH)

Question 37

What are the 2 ways of Gibbs free energy for a non-feasible reaction?

Answer 37

ΔG(+) ΔH(+) TΔS(-) or ΔG(+) ΔH(-) TΔS(TΔS < ΔH)

Question 38

What is a feasible reaction?

Answer 38

A feasible reaction is one that tends towards the products rather than the reactants.

Question 39

What is the critical temperature of a reaction?

Answer 39

The temperature at which the reaction is feasible.

Question 40

What is the equation for the critical temperature of a reaction?

Answer 40

T = ΔH° / ΔS° as we take ΔG° = 0

Question 41

What is the equation for the critical temperature of a reaction?

Answer 41

T = ΔH° / ΔS°

Question 42

At what value of ΔG is equilibrium?

Answer 42

0 (when the Gibbs energy of the products is equal to the Gibbs energy of the reactants).

Question 43

What will happen to a reversible reaction until the composition is reached where ΔG = 0?

Answer 43

reversible reaction will proceed spontaneously until the composition is reached where ΔG = 0.