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The change in Gibbs energy, ΔG, relates the energy that can be obtained from a chemical reaction to the change in enthalpy, ΔH, change in entropy, ΔS, and absolute temperature, T
At constant pressure, a change is spontaneous if the change in Gibbs energy, ΔG, is negative.
as a reaction approaches equilibrium, AGr becomes less negative and finally reaches zero and at equilibrium ΔG = -RTln K

Guiding Question:

What determines the direction of chemical change?

By the end of this presentation, you should know that:

Entropy, S, is a measure of the dispersal or distribution of matter and/or energy in a system
The more ways energy can be distributed, the higher the entropy
Under the same conditions, the entropy of a gas is greater than that of a liquid, which in turn is greater than that of a solid

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2 types of processes

Processes

Spontaneous

Non - spontaneous

Requires external energy to occur. (e.g boiling)

ΔG = +ve

=＞0

Occurs naturally without external energy. (e.g melting)

ΔG = -ve

= <0

Spontaneous processes are reactions or changes that occur naturally without the need for external energy. A classic example is melting ice. When ice melts, it absorbs heat from its surroundings, but no additional energy is required to initiate the melting itself.And, spontaneous processes are characterized by a negative Gibbs free energy change, This indicates that the process is energetically favorable/feasible and will proceed on its own.

On the other hand, non-spontaneous processes require an input of external energy to occur. For example, boiling water. Water will not boil at room temperature without the application of heat. It needs energy supplies to overcome intermolecular forces and turn liquid water into steam. Non-spontaneous processes are associated with a positive Gibbs free energy change,This means that the process is not naturally favorable/feasible and needs external energy to proceed.

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Entropy

Entropy and Disorder

The entropy of a system is a measure of the dispersal or distribution of matter and energy in the system. A system that has its particles dispersed throughout the system has a higher entropy than if the particles were concentrated in one area. Likewise, a system that has its energy distributed amongst all the particles has a higher entropy than if the same amount of energy was concentrated in only a few of the particles.

When a drop of ink is added to a beaker of water it will diffuse through the solution. The ink molecules spread out and become more disordered. Because the molecules are spread throughout the system after diffusion, the entropy of the system has increased. The amount of disorder in the system is an indicator of the entropy, and because of this, entropy and disorder are often associated together. More disorder is a sign of higher entropy.

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Calculating Standard

Entropy Change (ΔS^ꝋ)

-The standard molar enthalpy values, S^ꝋ, is the entropy of one mole of a substance at 298K and 100kPa.

-ΔS^ꝋ(reaction) = ΣS^ꝋ(products) - ΣS^ꝋ(reactants)

-When calculating ΔS^ꝋ, the equation must be balanced.

-Unit of ΔS^ꝋ: JK^-1Mol^-1

- ΔS^ꝋdepends on the level of disorder in a system. (increase in disorder = + ΔS^ꝋ)

- Increase disorder means the products have more ways of arranging the particles and distributing the energy in the system.

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Second Law of Thermodynamics

Spontaneous processes are those that increase the total entropy of the universe.
To determine the spontaneity of the process, both entropy change of the system and the surroundings must be considered together.

A full thermodynamic analysis tells us that the entropy change of the surroundings of a closed system at constant pressure and temperature is given by:

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Gibbs Energy Change

The Gibbs energy change represents the maximum amount of work that can be obtained from the reaction.

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Gibbs Energy change and temperature

If ΔH is greater than −TΔS and the reaction is spontaneous (ΔG < 0). However, if the temperature has been raised and the –TΔS term now dominates, making the reaction nonspontaneous (ΔG > 0)

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Gibbs Energy change and equilibrium

This graph indicates that it is spontaneous for reactions to form an equilibrium mixture from either direction.
The minimum Gibbs energy is when the reaction is at equilibrium and this is established closer to the pure products.The ΔGr, is the difference between the Gibbs energy of products and reactants in a particular ratio of reactants to products, they will have the value zero at equilibrium.