Enthalpy, enthalpy change in chemical reactions

The change in internal energy gives the heat change accompanying a chemical reaction at constant volume. However, most of the chemical reactions carried out in laboratories, are open to normal atmospheric conditions. A chemical reaction in a laboratory may incur change in volume but the pressure remains constant i.e. atmospheric pressure.

To study the heat changes for reactions at constant pressure and at constant temperature, a new term called enthalpy has been introduced.

The enthalpy of a system may be defined as the sum of the internal energy and the product of its pressure and volume. It is denoted by the symbol H and is given by H = E + PV

where, E = internal energy, P = pressure and V = the volume of the system.

Enthalpy is also called heat content.Enthalpy-Change">

Enthalpy Change

Every substance has a definite value of enthalpy in a particular state. Like internal energy, the absolute value of enthalpy cannot be measured. However, the change in enthalpy accompanying a process can be determined as the difference between the enthalpies of the products and the reactants, i.e.,

DH = H_products - H_reactants = H_p - H_r

where, 'H_p' is the enthalpy of the products, 'H_r' is the enthalpy of the reactants and DH is the enthalpy change.

If a reaction is carried out at constant temperature and pressure the heat exchanged (evolved or absorbed) by the system with the surroundings (i.e., heat change - DH) is equal to change in enthalpy.

Origin of enthalpy change in a reaction

We know that in chemical reactions, energy is required to break old bonds of the reactants. Energy is also released to form new bonds, which give the end products. The net energy change (released or absorbed) in a reaction will be equal to energy required to break all the bonds in reactants minus the energy released during the formation of bonds in the products.

If energy required is greater than energy released, the net result will be the absorption of energy and the reaction will be endothermic, i.e. DH = + ve. If energy released is greater than energy required, the net result is the release of energy and the reaction will be exothermic (DH = - ve).

This can be illustrated by considering a chemical reaction between hydrogen gas and chlorine gas to form hydrochloric acid gas:

• The energy required in breaking one mole of bonds in hydrogen and chlorine molecules are 437 and 244 kJ respectively.

• It is observed that for the formation of one mole of HCl, 433 kJ of energy is released. Therefore, the energy released during the formation of 2 moles of HCl is 2 x 433 = 866 kJ.

• Thus, Enthalpy change = (437+244) - (2 x 433) = -185 kJ

DH = - 185kJ

Therefore, 185 kJ of energy is released during the formation of 2 moles of gaseous HCl from one mole each of gaseous hydrogen and gaseous chlorine.

The amount of heat exchanged with the surroundings for a reaction at constant pressure (DH) is different from that exchanged at constant volume (DE) and temperature. The energy changes for reactions at constant pressure, includes energy contributions due to expansion or contraction against atmospheric pressure i.e. the volume of the reacting system changes. If the volume increases, the system expands against the atmospheric pressure and energy is required for this expansion. Therefore, a part of energy will be used for the expansion. Thus the amount of heat exchanged at constant pressure (DH) would be less than the amount of heat exchanged at constant volume (DE).

Alternatively, if the system contracts at constant pressure, work is done on the system and the system absorbs some energy from the surroundings. Therefore, the amount of heat exchanged at constant pressure is greater than that exchanged at constant volume.DH-and-DE">

Relationship between DH and DE

Consider a reaction

at constant pressure 'P'. Let H_A be the enthalpy of the reactants and H_B be the enthalpy of products so that change in enthalpy, DH may be

DH = H_B - H_A

But H = E + PV.

Let E_A and V_A be the internal energy and volume of the reactants and E_B and V_B corresponding values for the products. Therefore,

H_A = E_{A +} PV_A and H_B = E_B + PV_B

DH = (E_B + PV_B) - (E_A + PV_A)

or DH = (E_B - E_A) + P(V_B - V_A)

or DH = E+ PDV

where DE is the change in internal energy and DV is the change in volume of the system.