Unit 2-Reaction Feasibility

Question 1

What is meant by the standard enthalpy of formation, ΔH^o_f ?

Answer 1

The enthalpy (energy) change when one mole of a substance is formed from its elements in their standard states. Standard conditions of 1 atmosphere of pressure and temperature of 298 K

Question 2

How can the standard enthalpy be calculated?

Answer 2

ΔH^o = ΣΔH^o_f (products) - ΣΔH^o_f (reactants)

Question 3

What is entropy?

Answer 3

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

The greater the degree of disorder, the greater the entropy.

Question 4

Which state of matter has the lowest disorder?

Answer 4

Solids have low disorder and gases have high disorder.

Question 5

What is the relationship between entropy and temperature?

Answer 5

Entropy increases as temperature increases.

Question 6

What is meant by a feasible reaction?

Answer 6

A reaction which always tends towards the products rather than the reactants.

Question 7

What meant by free energy?

Answer 7

Free energy is the part of the total energy of a system that can do work at constant temperature and pressure.

Question 8

How do we calculate the standard free energy change if given enthalpy and entropy?

Answer 8

ΔG° = ΔH° -TΔS°

Question 9

How can the standard free energy change of a reaction be calculated from the standard free energies of formation of the reactants and products

Answer 9

ΔG° = ∑ΔG°f(products) - ∑ΔG°f(reactants)

Question 10

What must ΔG° be if a reaction is to be feasible?

Answer 10

Negative

Question 11

How can the temperature at which an equation becomes feasible be calculated?

Answer 11

T = ΔH° /ΔS°

Question 12

If ΔH°(reaction) = - 91.8 kJ mol^-1 and ΔS°(reaction) = -197.3 J K^-1mol^-1, calculate the temperature at which the reaction just becomes feasible.

Answer 12

T = ΔH° /ΔS°

T = - 91800 /- 197.3

T = 465K or 192°C