Topic 8: Thermodynamics

Question 1

Differentiate enthalpy and internal energy

Answer 1

• Enthalpy: includes heat absorbed and released by a system under standard conditions
• Internal energy: sum of all energy forces within a system

Question 2

State First Law of Thermodynamics

Answer 2

ΔU = q + w
w = -PΔV
ΔU = q - PΔV

Question 3

Why is there a negative sign in the equation of work done on a system?

Answer 3

When there is work done on the system, the system gains energy, making
w > 0. Also, work done on the system means the volume is compressed so ΔV <0, so the negative sign is there to ensure the relation.

Question 4

What is heat capacity and what the types of it?

Answer 4

• Heat capacity: the amount of energy required to heat 1 gram or mole of a substance by 1K
  + specific heat capacity: J g-1 K-1
  + molar heat capacity: J mol-1 K-1

Question 5

Identify the relationship between the amount of energy required to heat up 1 gram or mole of a substance by 1k and its specific/molar heat capacity

Answer 5

q = m c ΔT or
q = n C ΔT

Question 6

Identify the equation of change in enthalpy of reaction

Answer 6

ΔH = q (at constant pressure) = ΔU + PΔV

Question 7

Determine the sign of change in enthalpy of exothermic and endothermic reactions, of fusion, combustion, atomization and formation.

Answer 7

• Exothermic: (-)
• Endothermic: (+)
• Fusion: (+)
• Combustion: (-)
• Atomization: (+)
• Formation: (-)

Question 8

Identify the relationship of bond enthalpies in Hess’s Law

Answer 8

The overall change in enthalpy of a reaction can be calculated by adding all the bond enthalpies or enthalpies of atomisation of the reactants and products.

Question 9

Identify the relationship between the change in enthalpy of atomization and formation of a substance at standard conditions

Answer 9

ΔfH = - ΔaH

So, technically
ΔfH(rxn) = ΔaH(reactants) + ΔfH(products)

but ΔaH(reactants) = - ΔfH(reactants)

so, ΔfH(rxn) = ΔfH(products) - ΔfH(reactants)

Question 10

Identify the overall equation for change in enthalpy of reaction

Answer 10

ΔfH(rxn) = ΔfH(products) - ΔfH(reactants)

All at standard conditions

Question 11

What conditions need to be met for a reaction to occur?

Answer 11

• Have to overcome the activation energy level
• Have to be oriented in the right way
• The temperature has to be high enough