Enthalpy, entropy, free energy: The fundamentals you often look for answers

Yesterday, I wrote several short posts on the aforementioned topics on LN to clear up any confusion and answer any questions. This is a consolidated note for the benefit of students, in particular.

Why in nature heat does not flow from low temperature to high temperature?

What matters is entropy. Entropy always increases. What the second law says should be correctly interpreted. No book answers

When the second law says heat moves from high to low energy it means Gibbs free energy. Heat always moves in the direction of lower Gibbs free energy dG = dH - TdS, that is higher entropy.

?Why is higher entropy more stable? Any idea?

The larger the entropy the nearer the system is to a stable state of equilibrium.

Are there two types of energies [1] Total energy and [2] Gibbs free energy

No. Energy is only one type [thermal]

Explanation

Enthalpy

Energy is only one type. Energy is enthalpy, H. H is the sum of internal energy, U and work energy, W. H = U + W. This is conserved. This is 1st law of thermodynamics.

Question:

Why do we separately account for U and W?

Answer:

Because, U and W are both nonadditive types of energies.

U is a state function. It depends on the state of the system. Water and vapor do not have the same internal energy U. Internal energy is a point function. Internal energy does not depend on how water converts to vapor.

W is a path function. There are several thermodynamic paths for work and each requires a different amount of energy. Example: isothermal, adiabatic, etc. The type of situation of a system decides the path the thermodynamic work PV would follow.

Therefore, two types of energies cannot be collapsed into one between two points.

Entropy

When heat Q converts to work at temperature T, Q/ T is entropy generation or the generation of disordered heat which is not useful. This is entropy, S

So you have available energy = H - TS. At constant temperature and pressure, this is Gibbs free energy.

Gibbs free energy

It is Gibbs free energy which is a measure of any useful work, we write equation dG = dH -TdS. At equilibrium dH/ T = dS (reversible)

Why can't heat stay in one place?

Heat Q is being chased by entropy and does not let heat stay in one location. Entropy Q/T wants to increase all the time. Heat Q is being chased all the time to lower a temperature T to make Q/T larger.

This explains (1) why heat flows always to a lower temperature and (2) why heat is always in transit.

What is the final destination of entropy?

One definition of entropy is " how far is your system from equilibrium" Everything on earth needs a stable state of equilibrium and entropy is no exception.

There are three types of thermodynamic systems, (1) open (2) close, and (3) isolated

Isolated system

An isolated system is a special case. It has no surroundings. It cannot share energy with its surroundings. Example: Our universe

Entropy keeps building in the system until there is an equilibrium or all useful energies have been converted to entropy. There are many references in books on " heat death of the universe "

Open and closed systems

In our daily routine life, we deal with open and closed systems. Let us focus on that.

Everything we do where heat Q is involved at temperature T we generate entropy Q/T.

Mostly our processes are exothermic. Our surrounding temperature if less than the system

T (surrounding)< T (system) we generate more entropy in the surrounding because Q/ T for the surrounding is larger than the system.

This goes on until the surroundings can absorb entropy creation.

When the system has no space to absorb entropy, the system changes its phase and goes to a state where it has more capacity for entropy.

Example: Boiling of water. Water - vapor reaches equilibrium as water is boiled until there is an equilibrium. When the water has no space for entropy water converts to vapor.

What is specific heat’s designation?

The eventual destination of specific heat is entropy. Specific heat = dQ / dT = entropy

What is temperature? Is it a property of a substance?

Q = m x s x t

t = Q / m x s

For a fixed Q and fixed m, the temperature t is inversely proportional to the specific heat s of a substance. This means the temperature is an intensive property of a substance ( does not depend on the mass) which is purely a function of the atomicity of molecules

To note, I repeat, the temperature is an intensive property of a substance