Enthalpy Formula

Enthalpy is the measurement of the energy of some chemical reactions in a thermodynamic system. The quantity of enthalpy equals to the total content of the heat in a system. It is also equivalent to the sum total of the system’s internal energy product of volume and pressure. Enthalpy describes the internal energy needed to generate a system. This article will help students to understand the enthalpy and enthalpy formula with examples. Let us learn it!

Enthalpy Formula

Concept of Enthalpy

While dealing with some chemical reactions, the knowledge of enthalpy and standard enthalpy both are important. The total internal energy of a system is not possible to calculate. But the changes involved in heat transfer will be calculated. Therefore the enthalpy of a reaction is noted as $\Delta H$ where the symbol $\Delta$ refers to the change.

The formula for enthalpy change:

When a process begins at some constant pressure, then heat will be evolved, either absorbed or released and it equals the change in enthalpy. Therefore Enthalpy change is the sum of internal energy denoted by E and product of volume and Pressure, denoted by P × V.

H = E+PV

Enthalpy is also described as the state function which is based on the state functions P, T and E. It is shown as the change in enthalpy $\Delta H$ of a process between the beginning and final states. The flow of heat q at the constant pressure in a process will be equal to the change in enthalpy based on the following equation,

$\Delta H$ = q

A relationship between q and $\Delta H$ can be defined by knowing whether q is endothermic or exothermic. An endothermic reaction absorbs heat and reveals that heat is consumed in the reaction from the surroundings, so q>0. Similarly, the heat is released in an exothermic reaction and given to the surroundings. So, q < 0.

Computation of $\Delta H$ of a chemical reaction:

We may calculate it in many ways:

Method-1: If the work done by or on a system is zero, the volume of the container does not change. The change in enthalpy will be equal to the heat transfer (q), where

q = $m \times s \times \Delta T$

In this equation m is the mass, s is the specific heat, and Δ T is the change in temperature.

Method-2: If the reaction is already known, then a table of heat change values $\Delta H_f$ can be used to calculate it. The $\Delta H_f$ is called the heat of formation. It refers to the heat to form the substance from its elements.

Thus $\Delta H_{reaction} = \sum \Delta H_{f(products)} – \sum \Delta H_{f(reactants)}$

Method-3:  Hess’s Law is useful here to calculate the enthalpy of a reaction.

Method-4: The $\Delta H_{reaction}$ can be calculated by using the bond energies of the reactants and products.

$\Delta H_{reaction} = \sum \Delta H_{bonds \;broken} – \sum \Delta H_{f(bonds \;formed}$

Solved Examples on Enthalpy Formula

Q.1: Calculate the heat of the following reaction using the table of values.

$C + O_2 \rightarrow CO_2$

Substance	$\Delta H_f (kj per mole)$
C	0
$O_2$	0
$CO_2$	-393.5

Solution: The $\Delta H_f$ is the heat of formation, and it refers to the heat it takes to form the substance from its elements. The $\Delta H_f for C and O_2$ have values of 0 because they are in elemental form.

Therefore,

$\Delta H_{reaction} = \sum \Delta H_{f(products)} – \sum \Delta H_{f(reactants)}$

$\Delta H_{reaction} = 9-393.5) – (0+0)$

$\Delta H_{reaction} = – 393.5 kj$