Enthalpy Change Calculator

The Heat of Reaction: A Guide to the Enthalpy Change Calculator

Enthalpy (H) is a thermodynamic property of a system that represents the total heat content. While the absolute enthalpy of a system cannot be measured, the change in enthalpy (ΔH) for a chemical reaction can be calculated, and it is one of the most important values in thermochemistry. The enthalpy of reaction (ΔH°ᵣₓₙ) tells us whether a reaction releases heat into its surroundings (an exothermic reaction) or absorbs heat from its surroundings (an endothermic reaction). This is crucial information for understanding the energy dynamics of a chemical process and is essential for chemical engineers designing and controlling industrial reactors, for chemists studying reaction mechanisms, and for anyone working with chemical energy.

This calculator is designed to compute the standard enthalpy change of a reaction using a fundamental principle known as Hess's Law. This law states that the total enthalpy change for a reaction is the same regardless of the path taken. A practical application of this law allows us to calculate the enthalpy of reaction by using the tabulated 'standard enthalpies of formation' (ΔH°ᶠ) for each of the reactants and products. By inputting the stoichiometric coefficients from the balanced chemical equation and the standard enthalpies of formation for each substance, this tool will quickly calculate the overall ΔH°ᵣₓₙ for the reaction, providing a clear indication of its thermal nature.

The Formula: An Application of Hess's Law

The calculation for the standard enthalpy of reaction is based on summing the enthalpies of the products and subtracting the sum of the enthalpies of the reactants, with each being multiplied by its stoichiometric coefficient from the balanced equation.

ΔH°ᵣₓₙ = ΣnΔH°ᶠ(products) - ΣmΔH°ᶠ(reactants)

Where:

• ΔH°ᵣₓₙ is the standard enthalpy change of the reaction.
• Σ (sigma) means "the sum of".
• n and m are the stoichiometric coefficients of the products and reactants, respectively.
• ΔH°ᶠ is the standard enthalpy of formation for each substance.

Interpreting the Sign of ΔH

The sign of the calculated ΔH°ᵣₓₙ value is highly significant:

• If ΔH < 0 (negative), the reaction is exothermic. It releases heat into the surroundings. Combustion reactions, like burning wood, are classic examples of exothermic processes.
• If ΔH > 0 (positive), the reaction is endothermic. It must absorb heat from the surroundings to proceed. An instant cold pack, which gets cold when activated, is an example of an endothermic process.

What is Standard Enthalpy of Formation (ΔH°ᶠ)?

The standard enthalpy of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their most stable form under standard state conditions (298.15 K and 1 atm pressure). By definition, the standard enthalpy of formation of any element in its most stable form (like O₂(g), C(graphite), H₂(g)) is zero. These ΔH°ᶠ values are determined experimentally and are compiled in extensive reference tables for thousands of compounds.