# What Is Enthalpy

Enthalpy of a system can be defined as the total energy present within a thermodynamic system. One has to understand that internal energy and enthalpy are two different concepts. The international system of units makes use of the unit Joule to measure enthalpy. However, the older units like the calorie and British thermal unit are still in use. It is worth noting that the term, ‘enthalpy’ has been derived from the Greek word, ‘enthalpos’, meaning put heat into. The enthalpy of system can be got by adding up the internal energy of the system and the energy required in establishing the system. In most cases, the measurement of enthalpy of system is a very difficult task and the change in enthalpy is measured. The symbol, ‘H’ is used to refer to the enthalpy of the system.

Basically, reactions are of two kinds namely, exothermic reactions and endothermic reactions. In an exothermic reaction, the change in enthalpy is negative, while it is positive in the case of endothermic reactions. In any reaction that takes place in the universe, there is always a change in enthalpy of the entire system in question. When reactions take place at constant pressure, the change in enthalpy is equal to the sum of the change in internal energy of the system and the work done by the system on its surroundings.

Another term that requires attention when it is a discussion about enthalpy is ‘specific enthalpy’. Specific enthalpy refers to an important property of a working mass when it is to be studied under thermodynamics. Mathematically, specific enthalpy can be given as the sum of the internal energy of the mass and the product of pressure and specific volume of the working mass. It is written as h= u + p*v, where h=H/m. It is to be noted here that specific enthalpy is represented by ‘h’, whereas total enthalpy is represented by ‘H’, and ‘m’ stands for the mass of the system.

The three main factors that affect the enthalpy of a reaction are as mentioned below. Temperature of the system, partial pressures of the gases involved and the concentration of reactants and products are the main factors that affect changes in enthalpy. An important law with regard to enthalpy is the Hess’s law formulated by German scientist Henry Hess in the year 1940.

The main types of enthalpy that one comes across in terms of physical properties of a system are enthalpy of enthalpy of vaporization, enthalpy of fusion, enthalpy of sublimation and lattice enthalpy, whereas enthalpy of combustion, enthalpy of atomization etc are enthalpies corresponding to certain chemical reactions.