Aim 64

Hess's Law

Total heat equals sum of individual heats.

Heat of Formation (ΔHf)

Heat change when one mole forms from elements.

Standard Heat of Formation (ΔHfo)

ΔHf at 25°C and 101.3 kPa.

Exothermic Reaction

Releases heat, ΔH is negative.

Endothermic Reaction

Absorbs heat, ΔH is positive.

Stable Compound

Requires large energy to break apart.

Unstable Compound

Easily breaks apart, low energy required.

ΔHf of H2O(l)

-285.8 kJ/mol for formation from elements.

ΔH for breaking H2O

+285.8 kJ indicates endothermic process.

Large Negative ΔHf

Indicates exothermic and stable formation.

Small Negative or Positive ΔHf

Indicates unstable reaction, likely to break apart.

Example of Stable Reaction

C(s) + O2(g) ⇒ CO2(g), ΔH = -393.5 kJ.

Example of Unstable Reaction

N2(g) + O2(g) ⇒ 2NO(g), ΔH = +182.6 kJ.

ΔHrxn Calculation

Total heat change for a reaction.

ΔHf CuO(s)

-155.01 kJ/mol for copper(II) oxide.

Intermediate in Reactions

Substance formed in one step, used in another.

Mechanism of Reaction

Sequence of steps in a chemical reaction.

Step 1 of Reaction

CuO(s) ⇒ Cu(s) + ½ O2(g), ΔH = +155.01 kJ.

Step 2 of Reaction

H2(g) + ½ O2(g) ⇒ H2O(g), ΔH = -241.8 kJ.

Overall ΔHrxn

-86.79 kJ for CuO(s) + H2(g) reaction.

Practice Problem

Calculate ΔHrxn for Cl2(g) + 2HBr(g).

ΔHf HCl(g)

-92.38 kJ/mol for hydrochloric acid.

ΔHf HBr(g)

-36.57 kJ/mol for hydrobromic acid.