Physical state of reactants and products
December 19, 2009Heat energy is involved for changing the physical state of a chemical substance. For example in the conversion of water into steam, heat is absorbed and heat is evolved when steam is condensed.
Example
Consider the following two reactions –
It is observed that there is difference in the value of ΔH if water is obtained in gaseous or liquid state. ΔH value in second case is higher because heat is evolved when steam condenses. Hence, physical state always affects the heat of reaction.
Allotropic forms of the elements
Heat energy is also involved when one allotropic form of an element is converted into another. Thus, the value of ΔH depends on the allotropic form used in the reaction.
Example:
The value of ΔH is different when carbon in the form of diamond or graphite is used.
The difference between two values is equal to the heat absorbed when 12 g of diamond is converted into 12 g of graphite
Reaction carried out at constant pressure or constant volume
When a chemical reaction occurs at constant volume, the heat change is called the internal energy. However, most of the reactions are carried out at constant pressure and the enthalpy change is termed as the energy of reaction at constant pressure.
The relation between ΔH (Enthalpy change) and ΔE (Internal energy change) is given as follows:
ΔE + ΔngRT = ΔH
Dng = (Total number of moles of products) – (total number of moles of reactants).
R = Gas Constant
T = Temperature (in Kelvin)
The difference between ΔH and ΔE value is negligible when solids and liquids are involved in a chemical change. But, in reactions which involve gases, the difference in two values is considerable.