Enthalpy Formula

Q: What is the Enthalpy formula?

A thermodynamic state function $H = U + pV$; at constant pressure the enthalpy change equals the heat exchanged, $\Delta H = q_p$, which is negative for exothermic and positive for endothermic processes.

The enthalpy formula Δ H = H_products - H_reactants measures the heat change in a reaction at constant pressure.

The Formula

Δ H = H_{products} - H_{reactants}

When to use: Enthalpy change tells you how much heat a reaction releases or absorbs at constant pressure: $\Delta H < 0$ means heat is released (exothermic, the surroundings warm up) and $\Delta H > 0$ means heat is absorbed (endothermic).

Quick Example

Burning methane releases 890 kJ/mol — its ΔH = −890 kJ/mol (negative = heat released).

Notation

\Delta H

is the change in enthalpy in kJ/mol. Negative

\Delta H

means exothermic (releases heat); positive means endothermic (absorbs heat).

What This Formula Means

A thermodynamic state function $H = U + pV$ ; at constant pressure the enthalpy change equals the heat exchanged, $\Delta H = q_p$ , which is negative for exothermic and positive for endothermic processes.

Enthalpy change tells you how much heat a reaction releases or absorbs at constant pressure: $\Delta H < 0$ means heat is released (exothermic, the surroundings warm up) and $\Delta H > 0$ means heat is absorbed (endothermic).

Formal View

Enthalpy

H

is a state function defined as

H = U + PV

, where

U

is internal energy,

P

is pressure, and

V

is volume. At constant pressure,

\Delta H = q_p

(heat exchanged). Hess's law states that

\Delta H

for a reaction is the sum of

\Delta H

values for any set of steps that lead from reactants to products.

Worked Examples

Example 1

medium

Given

\Delta H_f^\circ

values:

NO(g) = +90

NO_2(g) = +33

kJ/mol. Find

\Delta H

for

2NO + O_2 \rightarrow 2NO_2

Answer

Δ H = - 114 kJ

First step

\Delta H = \sum \Delta H_f^\circ \text{(products)} - \sum \Delta H_f^\circ \text{(reactants)}

See the full worked solution + why-it-works coaching

SetupKey insightWhy it worksCommon pitfallConnection

Unlock answer keys One Family plan — every worked solution, all subjects

Example 2

medium

Use bond enthalpies (kJ/mol):

H{-}H = 436

Cl{-}Cl = 243

H{-}Cl = 431

. Estimate

\Delta H

for

H_2 + Cl_2 \rightarrow 2HCl

Example 3

hard

Use Hess's law: (1)

2H_2 + O_2 \rightarrow 2H_2O

\Delta H = -572

kJ; (2)

C + O_2 \rightarrow CO_2

\Delta H = -394

kJ; (3)

C + 2H_2 \rightarrow CH_4

\Delta H = -75

kJ. Find

\Delta H

for

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

Common Mistakes

Confusing enthalpy ( $\Delta H$ ) with temperature — enthalpy is the total heat exchanged, while temperature is a measure of average kinetic energy - Fix this by naming the substances or sample, checking "Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?", and attaching units, formulas, states, or evidence to the final statement.
Forgetting to flip the sign of $\Delta H$ when reversing a reaction — if the forward reaction has $\Delta H = -890$ kJ, the reverse has $\Delta H = +890$ kJ - Fix this by naming the substances or sample, checking "Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?", and attaching units, formulas, states, or evidence to the final statement.
Not scaling $\Delta H$ when multiplying coefficients — if you double all coefficients in a balanced equation, $\Delta H$ also doubles - Fix this by naming the substances or sample, checking "Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?", and attaching units, formulas, states, or evidence to the final statement.
Using enthalpy from a keyword alone - Signal words like reaction, reactant, product only point to a possible model; the substances and evidence must match too. - Fix this by naming the substances or sample, checking "Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?", and attaching units, formulas, states, or evidence to the final statement.

Why This Formula Matters

Enthalpy is central because chemistry studies how substances transform while atoms are conserved. It makes symbolic equations, lab evidence, and particle rearrangements part of one explanation.

Frequently Asked Questions

What is the Enthalpy formula?

How do you use the Enthalpy formula?

What do the symbols mean in the Enthalpy formula?

$\Delta H$ is the change in enthalpy in kJ/mol. Negative $\Delta H$ means exothermic (releases heat); positive means endothermic (absorbs heat).

Why is the Enthalpy formula important in Chemistry?

Enthalpy is central because chemistry studies how substances transform while atoms are conserved. It makes symbolic equations, lab evidence, and particle rearrangements part of one explanation.

What do students get wrong about Enthalpy?

Students often know a formula related to enthalpy but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.

What should I learn before the Enthalpy formula?

Before studying the Enthalpy formula, you should understand: exothermic, endothermic, activation energy.

← Back to Enthalpy See All Examples Practice Problems

Related Concepts

Exothermic Reaction Endothermic Reaction Activation Energy