Enthalpy of Reaction Calculator
From enthalpies of formation.
Formula first
Overview
The standard enthalpy of reaction represents the change in heat content that occurs when substances are transformed during a chemical process under standard state conditions. It is determined by the difference between the total energy stored in the chemical bonds of the products and the total energy stored in the bonds of the reactants.
Symbols
Variables
= Reaction Enthalpy, (prod) = Sum Prod Formation, (react) = Sum React Formation
Apply it well
When To Use
When to use: Apply this equation when calculating the net heat exchange of a chemical reaction at constant pressure using tabulated standard formation data. It assumes the reaction occurs at 298.15 K and 1 atm, and that all reactants and products are in their standard states.
Why it matters: Understanding reaction enthalpy is vital for industrial safety, as it allows engineers to predict if a reaction will release dangerous amounts of heat. It is also fundamental for calculating the fuel efficiency of combustible materials and the metabolic energy provided by food.
Avoid these traps
Common Mistakes
- Subtracting in wrong order.
- Forgetting to multiply by coefficients.
One free problem
Practice Problem
Calculate the standard enthalpy of reaction (Dr) for the combustion of methane. The total enthalpy of formation for the products (P) is -965.1 kJ/mol and the total enthalpy of formation for the reactants (R) is -74.8 kJ/mol.
Solve for: Dr
Hint: Subtract the reactant sum from the product sum using the formula Dr = P - R.
The full worked solution stays in the interactive walkthrough.
References
Sources
- Atkins' Physical Chemistry
- IUPAC Gold Book: Standard enthalpy of reaction
- Wikipedia: Hess's Law
- IUPAC Gold Book: Standard molar enthalpy of formation
- NIST Chemistry WebBook
- Atkins' Physical Chemistry, 11th Edition
- IUPAC Gold Book (Compendium of Chemical Terminology)
- AQA A-Level Chemistry — Energetics