Enthalpy Change Calculator

Calculate enthalpy change and heat of reaction using bond energies and thermochemical data

Calculation Mode

From Formation Enthalpies

Use ΔH°f values (Hess's Law)

From Bond Energies

ΔH = Bonds broken - Bonds formed

Calorimetry

q = mcΔT

Simple ΔH Input

Enter ΔH directly for analysis

Reactants (Left Side of Equation)

Products (Right Side of Equation)

What is Enthalpy Change?

Enthalpy change (ΔH) is the amount of heat absorbed or released by a system during a chemical reaction at constant pressure. It is a fundamental thermodynamic property that indicates whether a reaction is exothermic (releases heat, ΔH < 0) or endothermic (absorbs heat, ΔH > 0). Enthalpy is a state function, meaning the enthalpy change depends only on the initial and final states, not on the path taken (Hess's Law).

Key Equations:

ΔH°rxn = Σ(n × ΔH°f products) - Σ(n × ΔH°f reactants)

Hess's Law using standard enthalpies of formation

ΔH = Σ(Bond energies broken) - Σ(Bond energies formed)

Bond enthalpy method

q = mcΔT

Calorimetry equation (heat transfer)

• ΔH = Enthalpy change (kJ or kJ/mol)
• ΔH°f = Standard enthalpy of formation (kJ/mol)
• n = Stoichiometric coefficient
• q = Heat energy (J or kJ)
• m = Mass (g)
• c = Specific heat capacity (J/g·°C)
• ΔT = Temperature change (°C or K)

Exothermic vs. Endothermic

Exothermic (ΔH < 0)

• Releases heat to surroundings
• Products have less energy than reactants
• Temperature increases
• Examples: Combustion, crystallization

Endothermic (ΔH > 0)

• Absorbs heat from surroundings
• Products have more energy than reactants
• Temperature decreases
• Examples: Melting, evaporation, photosynthesis

Hess's Law

Hess's Law states that the enthalpy change for a reaction is the same whether it occurs in one step or multiple steps. This is because enthalpy is a state function. This principle allows us to calculate enthalpy changes for reactions that are difficult to measure directly by using known enthalpies of formation or combining equations.

Using Standard Enthalpies of Formation (ΔH°f)

Standard enthalpy of formation is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states at 25°C (298 K) and 1 atm.

For reaction: aA + bB → cC + dD

ΔH°rxn = [c×ΔH°f(C) + d×ΔH°f(D)] - [a×ΔH°f(A) + b×ΔH°f(B)]

Note: ΔH°f for elements in their standard state is defined as zero (e.g., O₂(g), H₂(g), C(graphite) all have ΔH°f = 0).

Worked Examples

Example 1: Formation Enthalpies (Hess's Law)

Problem: Calculate ΔH°rxn for:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

Given ΔH°f values:
CH₄(g) = -74.8 kJ/mol
CO₂(g) = -393.5 kJ/mol
H₂O(l) = -285.8 kJ/mol
O₂(g) = 0 kJ/mol (element)

Step 1: Products enthalpy
Σ(products) = [1(-393.5) + 2(-285.8)] = -965.1 kJ

Step 2: Reactants enthalpy
Σ(reactants) = [1(-74.8) + 2(0)] = -74.8 kJ

Step 3: Calculate ΔH°rxn
ΔH°rxn = -965.1 - (-74.8) = -890.3 kJ

Answer: ΔH°rxn = -890.3 kJ (Exothermic - combustion releases heat)

Example 2: Bond Energies

Problem: Calculate ΔH for: H₂(g) + Cl₂(g) → 2HCl(g)

Bond energies:
H-H: 436 kJ/mol
Cl-Cl: 243 kJ/mol
H-Cl: 432 kJ/mol

Step 1: Bonds broken (energy input, +)
1 H-H + 1 Cl-Cl = 436 + 243 = 679 kJ

Step 2: Bonds formed (energy released, -)
2 H-Cl = 2(432) = 864 kJ

Step 3: Calculate ΔH
ΔH = 679 - 864 = -185 kJ

Answer: ΔH = -185 kJ (Exothermic)

Example 3: Calorimetry

Problem: 100 g of water is heated from 25°C to 35°C. How much heat is absorbed?
(Specific heat of water = 4.184 J/g·°C)

Step 1: Calculate ΔT
ΔT = 35 - 25 = 10°C

Step 2: Apply q = mcΔT
q = 100 g × 4.184 J/g·°C × 10°C

Step 3: Calculate heat
q = 4184 J = 4.184 kJ

Answer: q = 4.184 kJ (Heat absorbed - endothermic process)

Common Standard Enthalpies of Formation (ΔH°f at 25°C)

Compound	Formula	State	ΔH°f (kJ/mol)
Water	H₂O	liquid	-285.8
Water	H₂O	gas	-241.8
Carbon dioxide	CO₂	gas	-393.5
Methane	CH₄	gas	-74.8
Ammonia	NH₃	gas	-46.1
Sulfur dioxide	SO₂	gas	-296.8
Sodium chloride	NaCl	solid	-411.2
Ethanol	C₂H₅OH	liquid	-277.7
Glucose	C₆H₁₂O₆	solid	-1273.3
Elements (standard state)	O₂, H₂, N₂, etc.	varies	0

Applications of Enthalpy Calculations

Energy Production

Calculating heat released from fuel combustion for power generation, heating systems, and engine design. Optimizing energy efficiency in industrial processes.

Chemical Manufacturing

Designing reactors and cooling/heating systems for industrial synthesis. Determining if reactions need external heating or cooling to control temperature.

Biochemistry & Nutrition

Calculating caloric content of food, energy release in metabolic pathways (like glucose oxidation), and understanding ATP hydrolysis in cells.

Materials Science

Predicting heat of formation for new compounds, understanding crystal formation, and designing endothermic/exothermic phase change materials.

Environmental Science

Assessing carbon footprint of reactions, understanding greenhouse gas formation energetics, and evaluating alternative energy sources.

Safety & Hazard Assessment

Identifying highly exothermic reactions that pose explosion or fire risks. Designing safe storage and handling procedures for reactive chemicals.

References

Enthalpy calculations are based on fundamental thermochemical principles:

Note: This calculator uses standard thermochemical conventions. Standard enthalpy of formation (ΔH°f) values are referenced to 25°C (298 K) and 1 atm pressure. Elements in their standard states have ΔH°f = 0 by definition. Bond energy calculations give approximate values since bond strengths vary slightly depending on molecular environment. For precise research calculations, consult authoritative databases like NIST or CRC Handbook. Always verify that temperature and pressure conditions match standard state when using tabulated values.

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