Energetics extra

Question 1

Define the term standard enthalpy of formation,ΔHΘf

Answer 1

The enthalpy of formation, , is the enthalpy change when one mole of compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states.

Question 2

Define the term standard enthalpy of combustion,ΔHΘc

Answer 2

The enthalpy change of combustion,ΔHΘc , is the enthalpy change when one mole of substance is completely burned in oxygen at 298 K and 100 kPa or standard conditions.

Question 3

State why the standard enthalpy of formation,ΔHΘf , of oxygen is zero.

Answer 3

By definition,ΔHΘf for ANY element is zero.

Question 4

Explain the term mean bond enthalpy

Answer 4

We have to put in energy to break a covalent bond; this is an endothermic change. Bond Dissociation enthalpy is defined as the enthalpy change required to break a covalent bond with all species in the gaseous state. The same amount of energy is given out when the bond is formed, this is an exothermic change. However, the same bond, for example C-H, may have slightly different bond enthalpies in different molecules, but we usually use the average value. This value is called the Mean Bond Enthalpy.