Enthalpy, Entropy, and Gibbs Free Energy

AP Chemistry Unit 9 - Gibbs Free Energy

By Nicole Woltschlaeger

First Law of Thermodynamics

Energy in the universe is finite and constant.

ΔE_universe = ΔE_surroundings + ΔE_system = 0

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Energy = heat (q) + work (w).

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It is possible to convert energy between heat and work.​

Second Law of Thermodynamics

For any spontaneous process the entropy of the universe increases.

While the 1st law indicates that energy can be converted from heat and work and vise versa, the 2nd law indicates that this cannot be done with 100% efficiency. In other words, energy lost to the surroundings increases the entropy of the surroudings.

Third Law of Thermodynamics

The entropy of a perfect crystal at absolute zero is zero.

Thus, evaluating the change in entropy as 1 mole of a substance at 1 atm and 298 K gives us the standard molar entropy, ΔS°.

Entropy changes can be calculated using:

ΔS_reaction = ∑Δ​S_(products) - ∑ΔS_(reactants)

Multiple Choice

The First Law of Thermodynamics indicates that

• Work is force exerted over a distance.

• Energy in a universe is finite.

• An increase in disorder is part of any spontaneous process.

• Work is a state function.

Multiple Choice

The Third Law of Thermodynamics indicates that the

• enthalpy of the universe is constant.

• entropy of the universe increases.

• sum of the entropies of the system plus surroundings is zero.

• entropy of a perfectly ordered pure crystal is zero at absolute zero.

Indicators of ΔS > 0

• A move to a more disordered phase (S_solid < S_liquid << S_gas)​.

• A solution formed when a solid or liquid solute dissociates into a liquid solvent shows increased entropy.

• Increasing the number of electrons or atoms shows a greater number of possible positions so entropy increases.

More indicators of ΔS > 0

• Increasing delocalization of electrons means greater entropy. Entropy increases from ionic to covalent to metallic bonds as a result.

• Weaker bonds imply greater entropy.

• Increased chemical complexity means increasing entropy.​

Multiple Choice

A process which causes a decrease in entropy is

• condensation of steam to form liquid water

• vaporization of liquid water to form steam

• dissociation of solid sodium chloride into aqueous sodium cations and chloride anions

• sublimation of snow to water vapor

Multiple Choice

The substance with the smallest value of S^o is

• AlCl₃

• MgCl₂

• NaCl

• SnCl₄

Multiple Choice

Which of the following forms would have the greatest standard molar entropy?

• H₂(g)

• H₂(l)

• H₂O(g)

• H₂O(l)

Gibbs Free Energy

• A change in a system that will occur without any outside influence is called a spontaneous change (thermodynamically favored) favored by increased entropy and an exothermic process.

• ΔG = ​ΔH -TΔS

• You can use ΔG°f to calculate ΔG° in a Hess' Law like process (ΔG_reaction = ∑Δ​G_(products) - ∑ΔG_(reactants)) or the equation above with standard entropy and enthalpy values.

Some text here about the topic of discussion

Multiple Choice

In a commercial chemical cold pack, an inner pouch containing water is broken and the water is allowed to mix with a sample of solid ammonium nitrate. The signs of the values for deltaH and deltaS for the dissociation reaction must be, repectively

• positive, positive

• positive, negative

• negative, positive

• negative, negative