AP Chemistry: Unit 9

Entropy

Entropy

the dispersal of matter of energy → changes in entropy measure how dispersed the matter or energy is in a system.

Increased disorder, increased dispersion, and increased temperature will yield

a higher entropy value

Entropy of a gas increases as

the number of gaseous particles increases and the volume increases

Unit of entropy

J/mol*K

Unit for enthalpy

kJ/mol

ΔS°=

Σ(S° of the products)-Σ(S°reactants)

Gibbs Free Energy Change

describes whether a reaction is thermodynamically favorable or unfavorable depending on enthalpy and entropy

ΔG°=

Σ(ΔG° of the products)-Σ(ΔG°reactants)

Unit for ΔG°

kJ/mol

Positive entropy value

Favorable

Negative entropy value

Unfavorable

Positive free energy value

Unfavorable

Negative free energy value

Favorable

ΔG°=

ΔH°-T(ΔS°)

If entropy (+) and enthalpy (-)

ΔG° is favorable at all temperatures

If entropy (-) and enthalpy (+)

ΔG° is favorable at no temperatures

If entropy (+) and enthalpy (+)

ΔG° is favorable at high temperatures - Driven by entropy

If entropy (-) and enthalpy (-)

ΔG° is favorable at low temperatures - Driven by enthalpy

Kinetic control

describe specific kinetic controls, such as high activation energy, and its effect on the rate of a thermodynamically favorable reaction. (Slow rate of a thermodynamically favorable reaction)

What effect does thermodynamic favorability have on the equilibrium constant?

It will yield a higher equilibrium constant since the products are favored

If ΔG°=0

System is at equilibrium

If ΔG°>0

K<1

If ΔG°<0

K>1

k=

e^ΔG/RT

ΔG°=

-RTI

How can we make a thermodynamically unfavorable process occur?

Through a coupled reaction (combining it with another favorable reaction with a common intermediate)

How do we find the ΔG° of a coupled reaction mechanism?

Using Hess’s Law principles to make sure the overall ΔG° for the reaction will be favorable.

Galvanic cells (Voltaic)

they will convert the energy released by a thermodynamically favorable redox reaction to electrical energy

Electrolytic Cells

they will use electrical energy to drive a thermodynamically unfavorable redox reaction

Anode

where the oxidation reaction takes place

Cathode

where the reduction reaction takes place

Salt bridge

allows for the movement of ions between the half cells in galvanic cells. It contains an inert ionic solution.

Cations from the salt bridge flow into the

cathode

Anions from the salt bridge flow into the

anode

The anode will experience what change in mass?

Lower mass (s)→(aq)

The cathode will experience which change in mass?

Higher mass (aq)→(s)

Galvanic cells will have what type of voltage?

Positive voltage

Electrolytic cells will have which type of voltage?

Negative voltage

Electrolytic cells require what?

A power source and an aqueous solution/molten solution of salt

Reduction potential (E=volts)

describes the electrical potential changes between the oxidation and reduction processes occurring in the half-cells

To find the oxidation potential we…

change the sign of the voltage found in the reduction potential value.

In tables that show reduction potentials, the higher row ones will be

the most favorable.

E°cell=

E°cathode-E°anode

Coefficients are negligible when we are

FINDING THE REDUCTION AND OXIDATION POTENTIALS TO FIND THE E°cell

Thermodynamically favorable cells will have a ___ E°cell

Positive (Galvanic)

Thermodynamically unfavorable cells will have a ___ E°cell

Negative (Electrolytic)

ΔG°=

-(moles)(faraday’s constant)(E°cell)

As we approach the formation of more products, the cell potential

decreases

Charge flow in electrolysis

Current

Amperes - Coulombs/Seconds

Time

Seconds

Faraday’s Constant

Coulombs/mole e-

mol e-

refers to the ratio between the reactant/product with the number of e- involved in oxidation or reduction