Lecture 18.4- Free Energy

Bellwork- Common ion

A 1x10-3 M solution of PbCl2 does notprecipitate solid lead chloride becausethe (concentration)2 is less than the Ksp,which is 1.6x10-5.

Which of the following salts would causeprecipitation of lead chloride if added tothe solution described above? ExplainWHY.

KNO3 Pb(NO3)2 NaCl NaBr

A spontaneous reaction occursnaturally and favors the formationof products at the specifiedconditions.

18.4

A nonspontaneous reaction is areaction that does not favor theformation of products at the specifiedconditions.

Photosynthesis is anonspontaneousreaction that requiresan input of energy.

18.4

Spontaneous reactions producesubstantial amounts ofproducts at equilibrium andrelease free energy.

Free energy is energy that isavailable to do work.

Entropy is a measure of thedisorder of a system.

•Physical and chemical systemsattain the lowest possible energy.

•The law of disorder states that thenatural tendency is for systems tomove in the direction of maximumdisorder or randomness.

2nd law of thermodynamics

Spontaneous processes ofteninvolve an increase in entropy.

18.4

For a given substance, the entropy of the gasis greater than the entropy of the liquid or thesolid. Similarly, the entropy of the liquid isgreater than that of the solid.

Less entropy solid ⇒ liquid ⇒ gas more

Entropy increases when asubstance is divided into parts.

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Entropy tends to increase inchemical reactions in which thetotal number of product moleculesis greater than the total number ofreactant molecules.

Entropy tends to increasewhen temperature increases.As the temperature increases, themolecules move faster and faster,which increases the disorder.

The Gibbs free-energy change isthe maximum amount of energythat can be coupled to anotherprocess to do useful work.

ΔG is negative in spontaneousprocesses because the systemloses free energy.

Change inEnthalpy

Change inEntropy

Temp inKelvins

Gibbs Free-Energy

Gibbs Free-Energy

1. Free energy from a reaction is theamount of energy that is

a. absorbed by an entropy decrease.

b. equal to the enthalpy change.

c. wasted as heat.

d. available to do work.

18.4 Section Quiz.

2. Free energy is always available fromreactions that are

a. endothermic.

b. nonspontaneous.

c. at equilibrium.

d. spontaneous.

18.4 Section Quiz.3. Choose the correct words for the

spaces: Spontaneous reactionsproduce ________ and substantialamounts of _________ at equilibrium.

a. free energy, products

b. no free energy, reactants

c. free energy, reactants

d. no free energy, products

18.4 Section Quiz.4. Which of the following involves a

decrease in entropy?

a. Natural gas burns.

b. A liquid freezes.

c. Dry ice sublimes.

d. Water evaporates.

5. A reaction is spontaneous if

a. enthalpy decreases and entropyincreases.

b. enthalpy increases and entropyincreases.

c. enthalpy decreases and entropydecreases.

d. enthalpy increases and entropydecreases.

18.4 Section Quiz.

6. Choose the correct words for thespaces: Gibbs free-energy change isthe _________ amount of energy thatcan be ___________ another processto do useful work.

a. maximum, coupled to

b. maximum, duplicated by

c. spontaneous, coupled to

d. minimum, duplicated by

18.4 Section Quiz.