3 Energetics - Measuring enthalpy change

Question 1

Define standard enthalpy change of formation

Answer 1

Enthalpy change where one mole of a suubstance is formed from it’s elements with reactants and products in their standard states under standard conditions

Example
2Fe(s) + 1½O₂(g) → Fe₂O₃(s)

Question 1

Define standard enthalpy change of a reaction

Answer 1

Enthalpy change when a reaction occurs in the molar quantities shown in the balanced equation under standard conditions.

Example
2FeO₃(g) + 3C(s) → 4Fe(s) + 3CO₂(g)

Question 2

Define standard enthalpy change of combustion.

Answer 2

Enthalpy cange when 1 mole of a substance is completely burnt in oxygen under standard conditions in standard states.

Question 3

Equation calculating temperature change

Answer 3

Q = mcΔT

Q - Energy transferred J
m - mass of substance (g)
c - specific heat capacity
ΔT - temp change (K)

Question 4

Calculating enthalpy change steps

Answer 4

1 - calculate heat change (Q=mcΔT), convert J to KJ
2 - calculate number of moles (mass=mrmol or mol=concvol)
3 - calculate ΔHr in KJmol⁻¹ using actual heat change (q in KJ) and the number of moles
EQUATION - ΔHr = (Q/ mol)
((Exothermic reactions need a -))

Question 5

What kind of experiments measure enthalpy changes?

Answer 5

Calorimetry