SME
Grade 10 Science · Problem type
ΔH from heats of formation
Use this for ΔH of this reaction, CH₄ + 2O₂ → CO₂ + 2H₂O, from the heats of formation.
Diagram
Reactants: Elements → CH₄ + 2 O₂, −74.8 kJ
Products: Elements → CO₂ + 2 H₂O, −965.1 kJ
ΔH = CO₂ + 2 H₂O − (CH₄ + 2 O₂)
ΔH = −965.1 − (−74.8) = −890.3 kJ
Down the ladder: heat is given off (exothermic).
This is an example. Type your own values to start a new problem.
ΔHf(CH₄)
Heat of formation of CH₄
example · kJ/mol
ΔHf(CO₂)
Heat of formation of CO₂
example · kJ/mol
ΔHf(H₂O)
Heat of formation of H₂O
example · kJ/mol
Hᵣ
Reactants’ heats of formation, added
worked out (not typed) · kJ
Hₚ
Products’ heats of formation, added
worked out (not typed) · kJ
ΔH
Enthalpy change of the reaction
worked out (not typed) · kJ
Formulas
Hᵣ = ΔHf(CH₄) + 2 × 0
−74.8 = −74.8 + 2 × 0
Hₚ = ΔHf(CO₂) + 2 × ΔHf(H₂O)
−965.1 = −393.5 + 2 × (−285.8)
ΔH = Hₚ − Hᵣ
−890.3 = −965.1 − (−74.8)
Assumptions
•
An element in its standard state has a heat of formation of 0, so O₂ adds nothing.
•
Each heat of formation is per mole, so multiply it by the coefficient.
•
The water is liquid.
Step-by-step
WE KNOW
ΔHf(CH₄) = −74.8 kJ/mol, ΔHf(CO₂) = −393.5 kJ/mol, ΔHf(H₂O) = −285.8 kJ/mol
FIND
reactants’ heats of formation, added (Hᵣ), products’ heats of formation, added (Hₚ), enthalpy change of the reaction (ΔH)
Step 1 · Find reactants’ heats of formation, added (Hᵣ)
Use Hᵣ = ΔHf(CH₄) + 2 × 0
One CH₄, and O₂ is an element: 0.
Hᵣ = −74.8 + 2 × 0
Hᵣ = −74.8 kJ
Step 2 · Find products’ heats of formation, added (Hₚ)
Use Hₚ = ΔHf(CO₂) + 2 × ΔHf(H₂O)
One CO₂ and two H₂O, each times its coefficient.
Hₚ = −393.5 + 2 × (−285.8)
Hₚ = −393.5 − 571.6
Hₚ = −965.1 kJ
Step 3 · Find enthalpy change of the reaction (ΔH)
Use ΔH = Hₚ − Hᵣ
Products minus reactants: both are measured from the same elements.
ΔH = −965.1 − (−74.8)
ΔH = −890.3 kJ
Check
Put the numbers back in. Both sides should match.
−74.8 = −74.8 + 2 × 0 ✓
−965.1 = −393.5 + 2 × (−285.8) ✓
−890.3 = −965.1 − (−74.8) ✓
Topic check
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Related lessons
Thermochemistry: heat, enthalpy and calorimetry
Main lesson
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Use this for ΔH of this reaction, C + O₂ → CO₂, from C + ½O₂ → CO and CO₂ → CO + ½O₂.