Ethalphy and Entrophy explained

 Entropy and enthalpy are both ways of measuring heat and its motion in various circumstances. In the case where hot systems are cooling or giving up their energy, or heating and absorbing energy, systems such as boilers, car engines, air conditioners, or all kinds of machinery these two thermodynamic “forces” come into play. Also at work is the idea of temperature which relates them.

Enthalpy is defined as H= U +PV, where U is the internal energy of the system and P is constant pressure, and V is volume. We are most interested in the change in enthalpy so usually we use ΔH = ΔU +PΔV. This means that a change in enthalpy results in a change in total energy and volume of a gas or liquid, and vice versa. A change of this sort involves also a change in temperature, and when we measure these together we encounter the concept of entropy.

Entropy is defined as ΔS= ΔH/T where T is the temperature in °K or °C. This is for a reversible transfer of heat between the system and its surroundings. Entropy measures the amount of disorder a system undergoes while transferring heat.

Rather than analyze complex systems I will illustrate with a simple instrument: A mercury thermometer. This device when inserted into a hot water bath captures some of the kinetic energy of the water molecules in the system and transfers it to the mercury in the thermometer. The mercury expands and shows the temperature on the markings on the glass. In thermometer A there is a small amount of air whose volume contracts when the mercury expands, but it has little effect on the reading itself as the mercury is not greatly affected by the air pressure here. So the differential enthalpy increases as the heat is absorbed and the volume expands, but once you take the thermometer out of the bath it cools and the enthalpy also goes down. Some of the heat has also increased the entropy inside the thermometer, as the molecules of air and mercury have increased in random activity due to heating, but once the thermometer cools down the entropy is returned to the air and mercury, so there is no net gain in entropy.

In thermometer B , however which has been broken at the top, the hot air escapes and the entropy of the system increases, because, in a large room the air molecules have escaped and increased their disorder, in a way that is not reversible easily, so the entropy of the system has changed and increased.