# Hess's Law Calculator

Calculate reaction enthalpy using Hess's Law and standard formation enthalpies. Enter up to 4 products and 4 reactants with coefficients.

## What this calculates

Calculate the standard enthalpy of reaction using Hess's Law. Enter the stoichiometric coefficients and standard formation enthalpies for up to four products and four reactants. The calculator applies the formula: delta-H(rxn) = sum of (delta-Hf products) minus sum of (delta-Hf reactants).

## Inputs

- **Product 1 Coefficient** — min 0, max 20 — Stoichiometric coefficient for product 1.
- **Product 1 ΔHf° (kJ/mol)** (kJ/mol) — Standard enthalpy of formation for product 1.
- **Product 2 Coefficient** — min 0, max 20 — Stoichiometric coefficient for product 2. Set to 0 if unused.
- **Product 2 ΔHf° (kJ/mol)** (kJ/mol) — Standard enthalpy of formation for product 2.
- **Product 3 Coefficient** — min 0, max 20 — Stoichiometric coefficient for product 3. Set to 0 if unused.
- **Product 3 ΔHf° (kJ/mol)** (kJ/mol) — Standard enthalpy of formation for product 3.
- **Product 4 Coefficient** — min 0, max 20 — Stoichiometric coefficient for product 4. Set to 0 if unused.
- **Product 4 ΔHf° (kJ/mol)** (kJ/mol) — Standard enthalpy of formation for product 4.
- **Reactant 1 Coefficient** — min 0, max 20 — Stoichiometric coefficient for reactant 1.
- **Reactant 1 ΔHf° (kJ/mol)** (kJ/mol) — Standard enthalpy of formation for reactant 1.
- **Reactant 2 Coefficient** — min 0, max 20 — Stoichiometric coefficient for reactant 2. Set to 0 if unused.
- **Reactant 2 ΔHf° (kJ/mol)** (kJ/mol) — Standard enthalpy of formation for reactant 2. Elements in standard state = 0.
- **Reactant 3 Coefficient** — min 0, max 20 — Stoichiometric coefficient for reactant 3. Set to 0 if unused.
- **Reactant 3 ΔHf° (kJ/mol)** (kJ/mol) — Standard enthalpy of formation for reactant 3.
- **Reactant 4 Coefficient** — min 0, max 20 — Stoichiometric coefficient for reactant 4. Set to 0 if unused.
- **Reactant 4 ΔHf° (kJ/mol)** (kJ/mol) — Standard enthalpy of formation for reactant 4.

## Outputs

- **ΔH°rxn** (kJ/mol) — Standard enthalpy of reaction.
- **Σ(ΔHf° Products)** (kJ/mol) — Weighted sum of product formation enthalpies.
- **Σ(ΔHf° Reactants)** (kJ/mol) — Weighted sum of reactant formation enthalpies.
- **Reaction Type** — formatted as text — Whether the reaction is exothermic or endothermic.
- **Calculation Steps** — formatted as text — Step-by-step breakdown.

## Details

Hess's Law states that the total enthalpy change for a reaction depends only on the initial and final states, not on the path taken. This lets you calculate reaction enthalpies from tabulated formation enthalpies without running the reaction.

**The Formula**

ΔH°rxn = Σ[n × ΔHf°(products)] - Σ[n × ΔHf°(reactants)]

Where n is the stoichiometric coefficient of each species.

**Worked Example: Combustion of Methane**

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)

Formation enthalpies:
- CH4(g): -74.8 kJ/mol
- O2(g): 0 kJ/mol (element in standard state)
- CO2(g): -393.5 kJ/mol
- H2O(l): -285.8 kJ/mol

Calculation:
- Products: 1(-393.5) + 2(-285.8) = -393.5 + (-571.6) = -965.1 kJ/mol
- Reactants: 1(-74.8) + 2(0) = -74.8 kJ/mol
- ΔH°rxn = -965.1 - (-74.8) = **-890.3 kJ/mol** (exothermic)

**Important Rules**

- Elements in their standard state (O2 gas, Fe solid, C graphite) have ΔHf° = 0 by definition
- A negative ΔH°rxn means the reaction releases heat (exothermic)
- A positive ΔH°rxn means the reaction absorbs heat (endothermic)
- Formation enthalpies are always for forming 1 mole of compound from its elements

**Common Formation Enthalpies (kJ/mol)**

| Compound | ΔHf° |
|----------|-------|
| H2O(l) | -285.8 |
| H2O(g) | -241.8 |
| CO2(g) | -393.5 |
| CH4(g) | -74.8 |
| NH3(g) | -45.9 |
| C2H5OH(l) | -277.7 |
| NaCl(s) | -411.2 |

## Frequently Asked Questions

**Q: What is Hess's Law?**

A: Hess's Law says the enthalpy change of an overall reaction equals the sum of enthalpy changes for each step, regardless of the route taken. Because enthalpy is a state function, the total change depends only on the starting and ending states. This lets you combine known reactions to find the enthalpy of a reaction that is hard to measure directly.

**Q: Why do elements in their standard state have zero formation enthalpy?**

A: The standard enthalpy of formation is defined as the enthalpy change when 1 mole of a compound forms from its elements in their most stable forms at standard conditions (25 degrees C, 1 atm). Forming an element from itself requires no change, so its formation enthalpy is zero by convention.

**Q: How do I know if a reaction is exothermic or endothermic?**

A: If delta-H is negative, the reaction is exothermic and releases energy as heat. If delta-H is positive, the reaction is endothermic and absorbs energy. For example, combustion reactions are almost always exothermic, while photosynthesis is endothermic.

**Q: Where do I find standard formation enthalpy values?**

A: Formation enthalpies are listed in chemistry textbooks, the CRC Handbook of Chemistry and Physics, and NIST databases. The most commonly used values include water (-285.8 kJ/mol for liquid), carbon dioxide (-393.5 kJ/mol), and methane (-74.8 kJ/mol).

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Source: https://vastcalc.com/calculators/chemistry/hess-law
Category: Chemistry
Last updated: 2026-04-08