How to Calculate Enthalpy of Combustion with Formula and Examples
The Enthalpy of combustion is a fundamental concept in thermochemistry, critical for JEE Main Chemistry as it connects energy, chemical reactions, and practical fuel comparisons. It measures the heat change when one mole of a compound reacts completely with oxygen under standard conditions, and applies to both organic and inorganic compounds. Recognizing the sign, units, and applying the enthalpy of combustion formula enables students to master numerical questions across hydrocarbons, alcohols, and other common substances in the exam.
What is Enthalpy of Combustion?
The enthalpy of combustion (ΔHc) is defined as the energy released as heat when one mole of a substance is burnt completely in oxygen at standard temperature and pressure (298 K, 1 atm). It is always expressed as a negative value (exothermic process) and typically measures in kilojoules per mole (kJ mol–1). For a typical hydrocarbon such as methane or propane, the enthalpy of combustion represents the energy output when using them as fuels.
- Unit: kJ mol–1
- Sign: Always negative (exothermic)
- Notation: ΔHc (or ΔHcombustion)
Enthalpy of Combustion vs. Enthalpy of Formation
| Aspect | Enthalpy of Combustion (ΔHc) | Enthalpy of Formation (ΔHf) |
|---|---|---|
| Definition | Heat released when 1 mole of compound burns in O2 | Heat change when 1 mole of compound forms from elements |
| Typical Sign | Negative (exothermic) | Can be positive or negative |
| Common Symbol | ΔHc | ΔHf |
| Example | CH4 + 2O2 → CO2 + 2H2O | C (graphite) + 2H2 + ½O2 → CH3OH(l) |
Remember, the Hess’s law often relates these concepts in energetic calculations.
Enthalpy of Combustion Formula for JEE
In JEE, calculate the enthalpy of combustion using:
- Direct calorimetry: ΔHc = – q / n, with q as total heat released and n as moles burnt.
- Formation enthalpies (using Hess’s Law):
ΔHc = ΣΔHf (products) – ΣΔHf (reactants)
A stepwise approach is crucial for JEE Main numericals. For example, calculate the enthalpy of combustion of ethanol (C2H5OH):
- Write the equation: C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
- Find ΔHf values: CO2(g) = –393.5, H2O(l) = –285.8, C2H5OH(l) = –277, O2(g) = 0 (all in kJ mol–1)
- ΔHc = [2×(–393.5) + 3×(–285.8)] – [–277 + 0]
- ΔHc = [–787 + (–857.4)] – [–277] = –1644.4 + 277 = –1367.4 kJ mol–1
Always apply the sign convention: answer is negative for exothermic combustion.
Sign Convention in Enthalpy of Combustion
Enthalpy change for combustion reactions is always negative. This is because energy is released, not absorbed, when fuel is burned. Remember, if you get a positive value in your calculation, there’s likely a sign error—this is a common JEE trap!
Standard Enthalpy of Combustion Data (ΔHc⊖)
Standard values (at 298 K, 1 atm) for key compounds help in quick answers:
| Compound | ΔHc⊖ (kJ mol–1) |
|---|---|
| Methane (CH4) | –890.3 |
| Ethane (C2H6) | –1560 |
| Propane (C3H8) | –2220 |
| Butane (C4H10) | –2877 |
| Ethanol (C2H5OH) | –1367 |
| Hydrogen (H2) | –286 |
| Carbon (C, graphite) | –394 |
Refer to these standard enthalpy values in quick calculations for JEEs.
Experimental Determination: Enthalpy of Combustion
JEE questions sometimes describe calorimetry experiments for finding the enthalpy of combustion of fuels like alcohols. The typical apparatus includes a spirit burner, a metal can, water, thermometer, and balance.
- Weigh the burner (with known mass of fuel) before and after burning.
- Burn the fuel beneath a fixed volume of water and measure final temperature rise (ΔT).
- Calculate heat absorbed by water: q = mwater × c × ΔT (with c = 4.18 J g–1 K–1).
- Enthalpy per mole: ΔHc = – q / nfuel.
Some heat is always lost (e.g. to air, can), so experimental values are usually less exothermic than theoretical data.
JEE Numericals: Enthalpy of Combustion Sample
Example: Calculate the enthalpy of combustion for 1 mole of benzene (C6H6), given: ΔHf[CO2] = –393.5, ΔHf[H2O(l)] = –285.8, ΔHf[C6H6(l)] = 49.0 (all kJ mol–1).
- Equation: C6H6 + 7.5O2 → 6CO2 + 3H2O
- ΔHc = [6 × (–393.5) + 3 × (–285.8)] – [49.0 + 0]
- ΔHc = [–2361 + (–857.4)] – 49.0 = –3218.4 – 49.0 = –3267.4 kJ mol–1
Analysis of such stepwise calculations can boost your speed for JEE Main Chemistry.
Applications and JEE Main Tips
- Use enthalpy of combustion to compare energy efficiency of fuels in Chemistry MCQs.
- Apply standard enthalpy of combustion when answering questions on energy cycles.
- Combustion is a classic example of exothermic reactions—important in energy change identification.
- Remember the sign convention: negative for any combustion (energy released).
- Double check unit conversions (J to kJ, per mole basis).
- Link to calorimetry in practical and numerical sections.
Common Errors and Revision List
- Neglecting to write negative sign for ΔHc (common loss of marks).
- Mixing up formation and combustion enthalpy in equation direction.
- Using incorrect standard states (e.g. H2O(l), not H2O(g)).
- Overlooking "per mole" in unit conversions—always relate to one mole.
- Forgetting to multiply formation enthalpy by actual moles in the equation.
- Using non-standard temperatures or pressures without converting values.
Linking to concepts like hydrocarbons and periodic properties can help contextualize these errors during revision. Vedantu highlights these common mistakes and their fixes in expert JEE Main sessions.
Further Exploration and Related Concepts
- Dive into enthalpy of combustion values versus hydrocarbon structure for energy density analysis.
- Boost problem-solving by mastering Hess’s law energy cycles using both thermodynamics and formation/combustion data.
- Connect heat of combustion to chemical bonding and oxidation reactions.
- Review calorimetry principles for lab-based MCQs or assertion-reason questions.
Thorough understanding of enthalpy of combustion helps you solve energy-based MCQs, analyze fuel choices, and master thermal analysis in both organic and inorganic chemistry. For structured practice, refer to other Vedantu Chemistry resources and topic-wise mock tests on thermochemistry, hydrocarbons, and calorimetry.
FAQs on Enthalpy of Combustion Explained for Chemistry Students
1. What is enthalpy of combustion?
Enthalpy of combustion is the heat change that occurs when one mole of a substance burns completely in oxygen under standard conditions.
Key points include:
- Represented as ΔHc (standard enthalpy: ΔHc°)
- Unit: kJ mol–1
- Always negative (exothermic reaction)
- Important for JEE/NEET and CBSE board exams in Chemistry
2. How do you calculate the enthalpy of combustion?
To calculate enthalpy of combustion (ΔHc), use the enthalpy change formula based on standard enthalpies of formation:
ΔHcombustion = Σ ΔHf (products) – Σ ΔHf (reactants)
Steps to solve:
- Write the balanced combustion equation for 1 mole of substance
- Insert standard enthalpy of formation (ΔHf) values for all reactants and products
- Apply the formula and compute the value
- Result will be negative for exothermic reactions
3. What is the standard enthalpy of combustion?
The standard enthalpy of combustion (ΔHc°) is the heat released when 1 mole of a substance completely combusts in oxygen under standard conditions (298 K, 1 atm, 1 M).
Key points:
- Measured in kJ/mol
- Values are tabulated for common fuels (e.g., methane, propane)
- Used for comparison and calculations in chemical thermodynamics
4. Is enthalpy of combustion always negative?
Yes, the enthalpy of combustion is always negative because combustion is an exothermic reaction—
- Heat is released to the surroundings
- The enthalpy of products is lower than that of reactants
- A negative sign indicates energy loss from the system
5. What is the difference between enthalpy of combustion and enthalpy of formation?
The main difference lies in what each enthalpy represents:
- Enthalpy of combustion (ΔHc): Heat released when 1 mole of a substance is completely burned in oxygen.
- Enthalpy of formation (ΔHf): Heat change when 1 mole of a compound forms from its elements in their standard states.
6. Why is a higher enthalpy of combustion better for fuels?
A higher (more negative) enthalpy of combustion means the fuel releases more energy per mole when burned.
Benefits include:
- More efficient energy output
- Better fuel economy and heating value
- Preferred for industrial and automotive applications
7. Can the enthalpy of combustion ever be positive?
No, the enthalpy of combustion is never positive.
- Combustion always involves release of heat (exothermic)
- If calculated as positive, it usually indicates a sign mistake in the calculation
8. What is the correct symbol and unit for enthalpy of combustion?
The standard symbol for enthalpy of combustion is ΔHc (or ΔHc° for standard conditions).
- Unit: kilojoules per mole (kJ/mol)
- It always refers to the amount of energy released during the complete combustion of 1 mole of substance
9. How do experimental errors affect the measured enthalpy of combustion?
Experimental errors can cause differences between measured and theoretical enthalpy of combustion values.
Common sources of error:
- Heat loss to surroundings (not all heat heats the calorimeter water)
- Incomplete combustion
- Evaporation or splashing of water
- Inaccurate measurement of mass change or temperature rise
10. Why are standard conditions used for enthalpy of combustion values?
Standard conditions (298 K, 1 atm, and 1 M concentrations) are used for enthalpy values to allow meaningful comparison between different reactions.
Standardizing conditions ensures:
- Data is reliable and comparable across chemistry sources
- Reduces variables affecting the enthalpy value
- Matches values in data tables for JEE/NEET, CBSE/NCERT use
11. How do you choose between formation and combustion enthalpies in JEE numericals?
Choose formation enthalpy (ΔHf) when asked about heat change in forming compounds from elements, and combustion enthalpy (ΔHc) when calculating energy released on burning substances.
In JEE/Boards:
- Read the question's wording carefully
- Look for keywords like 'combustion', 'formation', 'burning', or 'synthesis'
12. What are some common mistakes in calculating enthalpy change?
Common mistakes when solving enthalpy of combustion problems include:
- Switching the sign—come back and check if the value should be negative (exothermic)
- Using equation coefficients incorrectly—those must match the enthalpy definition (per mole)
- Ignoring state symbols (g, l, s) in enthalpy tables
- Mixing up ΔHc and ΔHf values
- Missing standard conditions
