3.1.4 Energetics

Question 1

Standard enthalpy formation

Answer 1

Heat energy change when 1 mol of a substance is formed from its constituent elements
Under standard conditions

Question 2

Standard combustion enthalpy

Answer 2

The enthalpy change when 1 mole of a substance burns
Under standard conditions

Question 3

Explain Hess’s Law

Answer 3

The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same

Question 4

Define mean bond enthalpy

Answer 4

The enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules

Question 5

explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law

Answer 5

Mean bond enthalpies are mean values calculated across a range of compounds containing that bond where as Hess’s law find the actual values of the bond enthalpies in that reaction