by Alexius Tan

Created on: March 18, 2010

Enthalpy and entropy are terms commonly encountered in the field of thermodynamics.

In a very simplified model of explanation, both the terms enthalpy and entropy are measure of energy.

Before I continue with explaining each of them, I feel it is necessary for me to introduce some concepts of thermodynamics.

The first Law of thermodynamics- Energy is conserved.

This is an essential law, for calculations involving energy to be possible. If we were to define a closed boundary (more comprehensive if I say “control volume”) with absolute insulation from the environment, energy within this system is constant. It can also be said that the internal energy of the system is constant.

This internal energy, commonly denoted by U in thermodynamic context, can be increased or decreased when there is a source of energy supplied to it. Such sources of energy can come from a change in gravitational elevation, a change in system pressure, heat added to the system, and etc.

When we consider energy, it is reasonable to use the concept of enthalpy, which is essentially has the same meaning as internal energy, subjected to isobaric (constant pressure) conditions. It is used more often because what we encounter is usually subjected to the constant atmospheric pressure.

In thermodynamics enthalpy is denoted by H, and H = U + pV, whereby p is the system pressure and V is the system volume. Imagine a balloon filled with water and boils on its own (without heating). This balloon (assuming it doesn’t burst) will increase in volume due to larger volume occupied by gaseous water vapour. To increase the volume, the contents in the balloon have to overcome the pressure exerted on it by the atmosphere. Hence the pV term.

But generally, enthalpy has no appreciable meaning unless we measure the change in enthalpy. Suppose now, the water boils in the same balloon with a heater heating the water, there obviously is a change in the internal energy U (as heat energy is supplied). And the change in enthalpy of the system is the amount of heat supplied to heat the water.

In Chemistry, the enthalpy of a reaction (to determine endothermic or exothermic) is determined by measuring the temperature of the starting mixture and the temperature of the ending mixture, assuming no change in pressure and volume, the pV term is ignored. Using the final and initial temperatures, as well as the (usually average) heat capacity of the mix, the enthalpy may be calculated.

So, if we can program a

• 1
• 2
• Next Page >>