Thermodynamics: thermochemistry

Question 1

Define exothermic reactions

Answer 1

Exothermic reactions release heat. ΔH < 0

Question 2

Define endothermic reactions

Answer 2

Endothermic reactions absorb heat. ΔH > 0

Question 3

Define the standard state of a substance at a specific temperature

Answer 3

The standard state of a substance at temperature T is its pure and most stable form at 1 bar

Question 4

Define the standard reaction enthalpy, Δ_RH⁰

Answer 4

This is the reaction enthalpy of a given reaction. Note that it is dependent on the stoichiometric numbers, so the value for enthalpy must be quoted with the associated reaction equation.

Question 5

Define the standard combustion enthalpy, Δ_CH⁰

Answer 5

The standard combustion enthalpy is the energy required for the complete oxidation of organic compounds to H₂O and CO₂

Question 6

Define standard enthalpy of formation, Δ_FH⁰

Answer 6

The standard enthalpy of formation is the enthalpy required to form a substance from its elements in their standard states.

Typically, the stoichiometric number of the product is 1

Δ_FH⁰ of elements in their standard states = 0

Question 7

Describe how a bomb calorimetry experiment is carried out

Answer 7

• A sample of known mass is burnt in oxygen
• The amount of heat released during the reaction can be determined using the temperature increase of the water.
• The calorimeter is isochoric, so we have measured ΔU

Question 8

Describe Hess’ law

Answer 8

The total enthalpy of a reaction is independent of the path taken.

Δ_RH⁰ is the sum of the standard enthalpies.

Question 9

State the equation needed to work out reaction enthalpy from enthalpies of formation

Answer 9

Question 10

Give Kirchoff’s law in relation to working out the reaction enthalpy

Answer 10

Note that this only works if there are no phase transitions between T₁ and T₂

Question 11

Describe how to calculate various reaction enthalpies at different temperatures

Answer 11

Unknown Δ_RH⁰ can be calculated using a known reaction enthalpy at a different temperature and the heat capacities of the reactants and products. We use the difference of molar heat capacities of products and reactants and then apply Kirchoff’s law

Question 12

Describe phase transitions

Answer 12

Pure substances transform from one phase to another at precisely defined temperatures. The enthalpies are endothermic going from low-temperature to high-temperature phases

Question 13

Describe, give the notation for and give an example of the solid-liquid phase transition

Answer 13

Fusion. Δ_fusH⁰

Eg Ice melting

Question 14

Describe, give the notation for and give an example of the solid-gas phase transition

Answer 14

Δ_SubH⁰

Eg dry ice

Question 15

Describe, give the notation for and give an example of the liquid -gas phase transition

Answer 15

Vapourisation. Δ_VapH⁰

Eg water boiling

Question 16

Describe, give the notation for and give an example of the solid-solid phase transition

Answer 16

Δ_TrsH⁰

Eg graphite-diamond

Question 17

Describe the Debye approximation

Answer 17

a = T³

The approximation is used to estimate C_p in the 0 - 10K range (as this is hard to access experimentally).

Question 18

Describe the total heat required to heat materials

Answer 18

Phase transitions need to be considered plus the heat required to heat the material