Question

What is meant by enthalpy and entropy please explain in a very detailed way?

Answer 1

Enthalpy:
Enthalpy is a form of energy with unit Joules.
We cannot measure enthalpy of a system (H). But a change in entha of the system can be measured.
Consider the combustion of carbon. During this reaction lot of heat energy is raised. This heat energy increases the temperature of surroundings (water or glass etc). So the temperature of surroundings increases from T1 to T2 (say 30oC to 45oC). Now the heat evolved in the combustion reaction is calculated as
Q= m*s*(T2-T1). Here "m" is mass of the system and "s" is specific heat capacity of the system.
So we are having heat evolved in the reaction in Joules. If the reaction takes place in open container the reaction is taking place at constant pressure. In such cases the enthalpy of the reaction is equal to heat evolved during the reaction.

If the reaction takes place at constant volume (or the container of the reaction is closed) then the enthalpy change is the sum of heat evolved and pressure volume work

So in simple words enthalpy is the heat evolved in the reaction.
Entropy on the other hand is the measure of irregularity.
If the system is ordered and regular then the entropy is low
For example entropy of ice is low because the particles are orderly packed in it.
But entropy of water is more because the particles are irregular and moving at random. So when ice is melting to water the entropy increases (low to high) and when water is changing to ice entropy decreases