What are Reversible Processes? A thermodynamic process (state i → state f ) is said to be reversible if the process can be turned back such that both the system and the surroundings return to their original states, with no other change anywhere else in the universe.

In a reversible process the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process. Irreversible processes cannot be undone by exactly reversing the change to the system. Spontaneous processes are irreversible.

We can conclude that a spontaneous process has a direction. A process that is spontaneous in one direction is not spontaneous in the opposite direction. The direction of a spontaneous process can depend on temperature. Ice turning to water is spontaneous at T > 0°C.

Entropy is the loss of energy available to do work. Another form of the second law of thermodynamics states that the total entropy of a system either increases or remains constant; it never decreases. Entropy is zero in a reversible process; it increases in an irreversible process.

This gives events an irreversible direction. In each transformation, the total entropy increases, as does the total disorder. Since we cannot decrease the total amount of entropy, we cannot reverse the arrow of time either.

This concept is fundamental to physics and chemistry, and is used in the Second law of thermodynamics, which states that the entropy of a closed system (meaning it doesn’t exchange matter or energy with its surroundings) may never decrease.

In a closed system, available energy can never increase, so (because energy is conserved) its complement, entropy, can never decrease. During this process, the entropy of the system increases.

The total entropy of a closed system is always increasing is another way of stating the second law of thermodynamics. A closed system is a system that does not interact in any way with its surroundings.

If a reaction is exothermic ( H is negative) and the entropy S is positive (more disorder), the free energy change is always negative and the reaction is always spontaneous….

At temperatures above the boiling point, 100oC, the process of liquid water changing to vapour is spontaneous as the process leads to increase in entropy. Below the boiling point, the process in non-spontaneous as it requires energy in form of heat for the process to occur.

For a temperature above 100 °C, the boiling of water is spontaneous. According to the thermodynamic interpretation, at 100 °C, both liquid and gas coexist in equilibrium, and water is not committed to vapor until the temperature exceeds 100 °C.

Thus ΔG = 0, and the liquid and vapor are in equilibrium, as is true of any liquid at its boiling point under standard conditions. At 110°C, ΔG < 0, and vaporization is predicted to occur spontaneously and irreversibly.

Melting of kryptonite at room temperature is not a spontaneous process. Melting of kryptonite at room temperature is a spontaneous process. Liquid kryptonite will spontaneously evaporate at room temperature.

Recall that the condensation of water vapor occurs spontaneously at temperature below 100°C but not above. Condensation is an exothermic process; to see this, consider that the reverse process, evaporation, obviously requires heat input. Therefore, condensation only occurs at lower temperatures.

A spontaneous process is capable of proceeding in a given direction without needing to be driven by an outside source of energy. An endergonic reaction (also called a nonspontaneous reaction) is a chemical reaction in which the standard change in free energy is positive and energy is absorbed.

When NaCl (table salt) dissolves in water, the reaction is endothermic. Yet, when added to water, it dissolves easily (spontaneously) without added energy.

When ice melts, the change is endothermic ( H is positive), and entropy increases ( S is positive) as the water molecules lose ordered arrangement of ice crystals.

Melting ice makes a perfect example of entropy. As ice the individual molecules are fixed and ordered. As ice melts the molecules become free to move therefore becoming disordered. As the water is then heated to become gas, the molecules are then free to move independently through space.

It is a spontaneous reaction. The entropy change ΔS of the reaction is negative. So, entropy change is positive. So, clearly, a spontaneous reaction is possible with both when entropy change is negative and also when entropy change is positive.

So the change of entropy of the entire system is zero. That’s because there is no change of temperature.

However freezing is also a process that reduces the system entropy. When water molecules are constrained, as in ice, their positional entropy is reduced. So water freezing is a process that is favored by the change in enthalpy and disfavored by the change in entropy.

The entropy increases whenever heat flows from a hot object to a cold object. It increases when ice melts, water is heated, water boils, water evaporates. The entropy increases when a gas flows from a container under high pressure into a region of lower pressure.

Entropy is a measure of the disorder or randomness of the system. The greater the randomness in a system, greater is its entropy. The randomness is greater in liquid state as compared to solid state so the entropy increases when ice melts into water.

Water has a greater entropy than ice and so entropy favours melting. But ice has a lower energy than water and so energy favours freezing. Therefore, as the surroundings get hotter, they are gaining more energy and thus the entropy of the surroundings is increasing.

Negative entropy means that something is becoming less disordered. In order for something to become less disordered, energy must be used. This will not occur spontaneously. A messy, or disordered, room will not become clean, or less disordered, on its own.

Basically, melting ice is an endothermic reaction because the ice absorbs (heat) energy, which causes a change to occur.