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Enthalpy Formula

The changes accompanied by the heat content of the materials which react, where the correct term for heat content is enthalpy(H). Usually this change in the heat content or enthalpy is shown by a change in temperature.

Every chemical reaction involves a change in energy – in the form of heat. The chemical reaction may be between elements and/or compounds. Either a reaction proceeds by supplying this energy or the reactions are spontaneous, with release of energy.
Chemical reactions are generally run on constant pressure of 1 atmosphere and not on constant volume and hence the heat absorbed or evolved is not equal to the intrinsic energy change (ΔE).

The difference of this energy from the start of a reaction to the completion of the same is called enthalpy change (Δ H). H can be thus related to E + PV.

During the reaction there is change in the heat of the reaction. The enthalpy of products is H2 and is less than the heat content of reactants H1. The enthalpy change for the reaction is denoted by $\Delta$H (sometimes called the heat of reaction).

Enthalpy Formula is given by

Since the enthalpy change magnifies itself as heat the term "heat of reaction" is often used in place of enthalpy change of reaction.

The key relation between enthalpy change and heat of reaction.

The enthalpy of reaction equals the heat of reaction at constant pressure.

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Enthalpy Problems

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Solved problems based on enthalpy are given below.

Solved Examples

Question 1: Calculate the enthalpies of formation for the thermite reaction.

2Al + Fe2O3 $\rightarrow$ Al2O3 + 2Fe

This reaction occurs when a mixture of powders aluminium and iron(III) oxide is ignited with a magnesium fuse.


$\Delta$H for Fe2O3 = -826kJ/mol
$\Delta$H for Al2O3 = -1676 kJ/mol
$\Delta$H for Al = $\Delta$H for Fe = 0

We use the equation.

Enthalpy Formula

$\Delta$H (reaction) = $\Delta$H for Al2O3 - $\Delta$H for Fe2O

$\Delta$H (reaction) = -1676kJ - (-826kJ)
$\Delta$H (reaction) = -850kj


Question 2: For a combustion reaction.

2C8H18 + 25CO2 $\rightarrow$ 16CO2 + 18H2O

What is the change in enthalpy of the reaction?

$\Delta$H for CO2 = -394kJ
$\Delta$H for H2O = -286kJ
$\Delta$H for C8H18 = -269kJ

$\Delta$H = 16 $\times$ $\Delta$H for CO2 + 18 $\times$ $\Delta$H for H2O - 2 $\times$ $\Delta$H for C8H18

$\Delta$H = 16 $\times$ (-394kJ) + 18 $\times$ (-286kJ) - 2 $\times$ (-269kJ)

$\Delta$H = -1.09 $\times$ 104kJ

More topics in Enthalpy Formula
Henry's Law Formula Calorimetry Formula
Specific Heat Capacity Formula Heat of Fusion Formula
Heat of Vaporization Formula Heat Index Formula
Heat Load Formula Heat Loss Formula
Heat of Solution Formula Heat Gain Formula
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