The first Law of thermodynamics
Energy can neither be created nor destroyed, although it may be
changed from one form into another. The total energy of an
isolated system remains constant, although it may change form
one form to another.
Mathematical Derivation
Suppose a system changes form state A to state B with the
energy changes from UA to UB. The system, while undergoing
change from state A to B, absorbs heat q from the surrounding
and also performs some work W. On the other hand, the
performance of work by the system tends to lower the energy of
the system because performance of work requires expenditure of
energy.
Thermochemistry
Chemical reactions are accompanied by evolution or absorption
of heat. If a reaction is accompanied by the evolution of heat, it
is called an exothermic reaction, and if it is accompanied by the
absorption of heat, it is called an endothermic reaction. The
science of thermochemistry is that branch of physical chemistry
that deals with the thermal changes accompanying chemical and
physical transformations. It deals with transformation of
chemical energy into heat energy and vice versa. The energy
change that occurs in a chemical reaction is largely due to a
change in bond energy , that is change in potential energy that
results from the breaking of bonds in the reactants and formation
of new bonds in the products. Some thermochemical definitions
are given here:
Heat of reaction: the amount of heat evolved or absorbed
during a chemical reaction is called the heat of reaction (
this is the difference between the heat contents (or enthalpies) of
the products and the reactants; when molar quantities of the
reactants indicated by the balance chemical equation, react
completely, or mathematically, the heat of reaction,
= Enthalpy of product – Enthalpy of reactant
will be positive when heat content of the products is
greater than heat content of reactants. Heat will be absorbed
during the reaction and the reaction will be endothermic.
will be negative when heat content of the reactants is greater
than heat content of products. Heat will be evolved during the
reaction and the reaction will be exothermic.
Heat of combustion: heat of combustion of a compound
or an element is the change in heat content when 1 mole of the
substance is completely burnt in excess of oxygen because
combustion reactions are always exothermic ,the heat of
combustion is always negative. For example:
CH4 (g) + 2O2 (g)
CO2 (g) + 2H2O (l);
= - 890.2 k J