Example data table
| Reaction (per stoichiometric equation) | Reactants ΔHf (kJ/mol) | Products ΔHf (kJ/mol) | Computed ΔHrxn (kJ/mol) |
|---|---|---|---|
| H2(g) + 1/2 O2(g) → H2O(l) | H2: 0, O2: 0 | H2O(l): −285.83 | −285.83 |
| CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) | CH4: −74.81, O2: 0 | CO2: −393.51, H2O(l): −285.83 | −890.36 |
Values are typical reference data; use a consistent dataset for accuracy.
Formula used
Formation method: ΔHrxn = Σ(ν·ΔHf)products − Σ(ν·ΔHf)reactants.
Bond-energy method (approx.): ΔHrxn ≈ Σ(n·E)broken − Σ(n·E)formed.
Scaling: Q = ΔHrxn × extent. Negative ΔH indicates heat released, positive indicates heat absorbed.
How to use this calculator
- Select a method that matches your available data.
- Enter reactants and products with stoichiometric coefficients.
- Provide ΔHf values (or bond counts and energies) in one unit set.
- Set the extent to scale total heat for your process.
- Press Calculate to view ΔH above the form and export it.
Professional guide to reaction enthalpy calculations
1) What reaction enthalpy represents
Reaction enthalpy (ΔHrxn) is the heat change at constant pressure for a balanced chemical equation, reported per “mole of reaction.” Negative values release heat (exothermic); positive values absorb heat (endothermic). Always tie the sign to the written reaction direction.
2) Formation-enthalpy method
The formation approach uses ΔHrxn = Σ(ν·ΔHf)products − Σ(ν·ΔHf)reactants. Accuracy depends on consistent states (g, l, s, aq) and using one reference table. Elements in their standard states have ΔHf=0. Benchmarks include ΔHf[H2O(l)] ≈ −285.83 kJ/mol and ΔHf[CO2(g)] ≈ −393.51 kJ/mol.
3) Bond-energy method (approximate)
Bond energies estimate ΔHrxn by summing energy to break bonds and subtracting energy released when new bonds form. It is useful for fast screening but can deviate because values are averages. Typical numbers include H–H ≈ 436 kJ/mol, O=O ≈ 498 kJ/mol, and O–H ≈ 463 kJ/mol.
4) Units and conversions
Keep units consistent across inputs. This tool supports J, kJ, and kcal, with 1 kcal ≈ 4.184 kJ. Report the sign with the unit, and interpret it as heat flow for the defined reaction direction.
5) Scaling by extent
Processes rarely run exactly one reaction-mole. The extent parameter scales heat: Q = ΔHrxn × extent, where extent represents how many times the balanced equation “happens.” Example: methane combustion is about −890.36 kJ per reaction-mole; at extent 0.25, Q ≈ −222.59 kJ. Use scaled Q to estimate heating or cooling duty in equipment.
6) Linking to calorimetry
Experimental heats can differ due to losses, heat capacity of the setup, side reactions, or incomplete conversion. Comparing measured Q with calculated Q helps validate assumptions. For large temperature changes, apply heat-capacity corrections (often via Kirchhoff’s law) and confirm that the same phases were used.
7) Typical magnitudes
Combustion reactions often fall between roughly −200 and −1200 kJ per reaction-mole, depending on fuel size and product phase. Many dissolution and mixing steps are smaller, sometimes within ±50 kJ/mol, yet still relevant for temperature control in concentrated systems.
8) Best practices for reproducible results
Balance the equation first, enter coefficients carefully, and keep phase labels explicit. Avoid mixing gas and liquid formation values unless intended. For bond-energy estimates, document the bond table used and list only bonds that change. Export results to preserve assumptions and units.
FAQs
1) What does “mole of reaction” mean?
It means the stoichiometric amounts in the balanced equation act as one unit. If you double every coefficient, you also double the reported heat for that rewritten reaction.
2) Why do different datasets give different ΔH values?
Reference tables may use different temperature conventions, rounding, or phase definitions. Mixing sources can introduce noticeable differences, so use one consistent dataset and specify species states.
3) Should I use H2O(l) or H2O(g) for combustion?
Use the phase that matches your reaction definition and conditions. Producing liquid water yields a more negative ΔH than producing water vapor because condensation releases additional heat.
4) When is the bond-energy method acceptable?
It is acceptable for quick estimates, trend comparisons, or early design screening. For reporting and high accuracy, prefer formation enthalpies or experimentally measured calorimetry data.
5) What does the extent input represent?
Extent is the number of reaction-moles executed. It scales ΔH to a total heat value for a batch or flow segment, matching your process quantity.
6) Can I compute ΔH for non-standard temperatures?
Yes, but you should apply temperature corrections using heat capacities over the temperature interval. This calculator is designed for standard-reference style calculations and quick scaling.
7) Why is my result positive when I expected negative?
Common causes are swapped reactant/product entries, incorrect signs in ΔHf values, or using an unintended phase. Recheck stoichiometry, units, and physical states.