3.1.4 Energetics

Question 1

what is an enthalpy change ?

Answer 1

Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
Standard enthalpy changes refer to standard conditions ie 100 kPa and a stated temperature (eg ∆H298 Ɵ).

Question 2

whats the enthalpy of formation

Answer 2

enthalpy change when 1 mole of a compound is formed from its constituent elements under standard states & standard condition

Question 3

What is the enthalpy of combustion

Answer 3

the enthalpy change when 1 mole of a compound is completely burned in oxygen under standard states & standard condition

Question 4

why does H2 not have an enthalpy of formation

Answer 4

because its already in its standard elemental state

Question 5

what is hesses law

Answer 5

hesses law states that the enthalpy change for a reaction is the same no matter what route is taken from reactants to products

Question 6

what is a mean bond enthalpy ?

Answer 6

the enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules,
- an average enthalpy change over multiple compounds with that same bond
- bond energies of formation is more accurate

differred value either by heat loss or incomplete combustion

Question 7

where do the arrows point in a hesses law thermochemical cycle ?

Answer 7

( given formation values) - one arrow points up to reactants and one arrow points up to the products from the intermediate step

(given combustion values) - all arrows point down to the intermediate step