Heat of Combustion Calculator

Calculate heat of combustion from standard enthalpies of formation or calorimetry data. Find energy released per gram and per mole for fuels and organic compounds.

🔥 Thermochemistry📐 ΔH°comb = ΣΔH°f(products) - ΣΔH°f(reactants)🧪 Chemistry
Sum n*dHf products (kJ)
Sum n*dHf reactants (kJ)
Molar mass of fuel (g/mol)
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Heat of Combustion CalculatorSee formulaΔH°comb = ΣΔH°f(products) - ΣΔH°f(reactants)kJ/mol

Step-by-Step Examples

Example 1
Methane CH4

CH4+2O2 to CO2+2H2O(l). dHf: CO2=-393.5, H2O(l)=-285.8, CH4=-74.8 kJ/mol.

  • Products: -393.5+2(-285.8)=-965.1
  • Reactants: -74.8
  • dH=-965.1-(-74.8)=-890.3 kJ/mol
  • Per gram: -890.3/16.04=-55.5 kJ/g
✓ delta-H(combustion CH4) = -890.3 kJ/mol (-55.5 kJ/g)
Example 2
Ethanol C2H5OH

dHf(C2H5OH)=-277.7. Products: 2CO2+3H2O(l).

  • Products: 2(-393.5)+3(-285.8)=-1644.4
  • Reactants: -277.7
  • dH=-1644.4-(-277.7)=-1366.7 kJ/mol
  • Per gram: -1366.7/46.07=-29.7 kJ/g
✓ delta-H(combustion ethanol) = -1367 kJ/mol (-29.7 kJ/g)
Example 3
From Calorimetry

0.500 g sucrose heats 1500 g water by 4.20 C.

  • q(water) = 1500x4.184x4.20 = 26359 J = 26.36 kJ per 0.500 g
  • Per gram: 52.7 kJ/g
  • Per mol (M=342.3): 52.7x342.3 = 18050 kJ/mol
✓ Heat of combustion of sucrose ~18050 kJ/mol

Real-World Applications

⛽
Fuel Engineering
Gasoline: -47 kJ/g, diesel: -45 kJ/g, hydrogen: -143 kJ/g. Fuel selection by energy density.
🍿
Food Science
Food calorimetry. Fat: -37 kJ/g (9 Cal/g), carbs: -17 kJ/g (4 Cal/g).
🌍
Environmental Impact
CO2 released per kJ varies by fuel; natural gas is the cleanest fossil fuel.
🚀
Rocket Propellants
H2/O2 combustion: highest specific impulse due to high energy and low product molar mass.

Common Mistakes to Avoid

⚠️
Products use liquid water for standard value

Complete combustion: products are CO2(g) and H2O(l) at 25 C standard conditions.

⚠️
Per gram vs per mole

kJ/mol is standard. For energy density comparisons: divide kJ/mol by molar mass to get kJ/g.

⚠️
Sign should be negative for combustion

Combustion always releases heat: dH < 0. If your answer is positive, check which subtraction direction you used.

Frequently Asked Questions

What are heats of combustion for common fuels? ▾
H2: -286 kJ/mol (-143 kJ/g). CH4: -890 kJ/mol (-55.5 kJ/g). Ethanol: -1367 kJ/mol (-29.7 kJ/g). Glucose: -2803 kJ/mol (-15.6 kJ/g).
What is the difference between HHV and LHV? ▾
Higher heating value (HHV): water as liquid product. Lower heating value (LHV): water as gas. HHV = LHV + heat of vaporization of water formed.
Why does hydrogen have the highest energy density by mass? ▾
H2 is lightest molecule. Each gram contains maximum H atoms which form strong O-H bonds when burned.
How many Calories in 1 mol of glucose? ▾
Combustion dH = -2803 kJ/mol. 2803/4.184 = 670 Cal per mol; 670/180 = 3.72 Cal/g of glucose.
What is a food Calorie? ▾
1 food Cal = 1 kcal = 4184 J. Fat: 9 Cal/g, carbs: 4 Cal/g, protein: 4 Cal/g, alcohol: 7 Cal/g.

Related Chemistry Calculators

🌡️ Calorimetry🔥 Enthalpy🔥 Hess Law⚡ Gibbs Free Energy🔥 Standard Enthalpy

Formula Explorer connections

Interpretation: This relationship tracks energy transfer, state-function change or the balance between enthalpy and entropy in a chemical process. Assumption: Keep energy units compatible, use kelvin for absolute temperature, and match standard states and reaction stoichiometry. Thermodynamic favorability does not determine reaction speed.

Kirchhoff's Law ΔH(T) Calculator →Heat of Solution Calculator →Hess's Law Calculator →Chemistry Formula Explorer →