8 Energetics

Question 1

Change in Enthalpy

Answer 1

Represented by ∆H. A negative ∆H value indicates an exothermic reaction, an positive ∆H indicates an endothermic reaction

Question 2

Standard Enthalpy of Combustion

Answer 2

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants being in their standard states

Question 3

Standard Enthalpy Change of a Reaction

Answer 3

The enthalpy change when the number of moles of a reaction specified in the balanced equation, react with one another under standard conditions

OR

Change in enthalpy under constant pressure and standard conditions for a reaction.

Question 4

Energy Required Per Mole

Answer 4

q = m c ∆T
m = mass
c = specific heat capacity
∆T = change in temperature

Question 5

State what is meant by the term Hess’s law

Answer 5

the enthalpy change of a reaction is independent of the route taken

Question 6

state what is meant by the term mean bond enthalpy

Answer 6

the energy required to break the covalent bond into gaseous atoms averaged over different molecules

Question 7

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

Answer 7

Mean bond enthalpies used are average values across a range of compounds containing that bond (so value may not be exact) due to different atoms and arrangements surrounding the bonds