Apply Hess’s Law to find enthalpy changes.
Draw and interpret energy level diagrams.
Use standard enthalpies of formation (∆fH°) to calculate the standard enthalpy change (∆rH°) for a reaction.
If a chemical reaction is the sum of two or more other reactions, then:
∆rH° (overall) = ∆rH°₁ + ∆rH°₂ + ...
Used when: the enthalpy change of a reaction is not directly measurable.
How: Manipulate and add known reactions to match the target reaction.
Reverse a reaction → change sign of ∆H°.
Multiply coefficients → multiply ∆H° by same factor.
Example:
To find the enthalpy change for:
C(s) + ½ O₂(g) → CO(g)
Use:
CO(g) + ½ O₂(g) → CO₂(g) ∆H° = −283.0
C(s) + O₂(g) → CO₂(g) ∆H° = −393.5
Rearrange and add to find ∆H° = −110.5 kJ/mol.
Visual tool for showing enthalpy changes.
Y-axis = energy; reactants/products placed according to their ∆H°.
Direction of arrow shows exo or endo thermic nature.
Key Takeaway:
State function: Enthalpy change depends only on initial and final states, not on the path.
∆fH° = enthalpy change when 1 mol of a compound forms from its elements in their standard states at 25°C and 1 bar.
Elements in standard state (e.g., O₂(g), C(s), H₂(g)) → ∆fH° = 0
Most ∆fH° values are negative → compound formation is usually exothermic.
Some ∆fH° values are positive → unstable compounds (e.g., NO(g)).
NaCl(s):
Na(s) + ½ Cl₂(g) → NaCl(s) ∆fH° = −411.12 kJ/mol
C₂H₅OH(ℓ):
2 C(s) + 3 H₂(g) + ½ O₂(g) → C₂H₅OH(ℓ) ∆fH° = −277.0 kJ/mol
Formula:
ΔrH∘=∑nΔfH∘(products)−∑nΔfH∘(reactants)\Delta_rH^\circ = \sum n \Delta_fH^\circ(\text{products}) - \sum n \Delta_fH^\circ(\text{reactants})ΔrH∘=∑nΔfH∘(products)−∑nΔfH∘(reactants)
Where:
n = coefficient in balanced equation
All values must be in kJ/mol
CaCO₃(s) → CaO(s) + CO₂(g)
Given:
∆fH°[CaCO₃] = −1207.6
∆fH°[CaO] = −635.1
∆fH°[CO₂] = −393.5
ΔrH∘=[(−635.1)+(−393.5)]−(−1207.6)=−1028.6+1207.6=+179.0 kJ/mol\Delta_rH^\circ = [(-635.1) + (-393.5)] - (-1207.6) = -1028.6 + 1207.6 = +179.0\ \text{kJ/mol}ΔrH∘=[(−635.1)+(−393.5)]−(−1207.6)=−1028.6+1207.6=+179.0 kJ/mol
✅ Endothermic reaction (requires heat input).
∆rH°: Standard enthalpy change of a reaction.
∆fH°: Standard enthalpy of formation.
Standard state: Most stable form of a substance at 25°C and 1 bar.
Exothermic: Releases heat (∆H < 0).
Endothermic: Absorbs heat (∆H > 0).
State function: Path-independent property.
Always balance the chemical equation.
Label units carefully (kJ/mol-rxn).
Double-check if you reverse or scale equations in Hess’s Law.
Use Appendix L or your provided table for ∆fH° values.