AP CHEM Unit 9

Thermodynamics & Electrochemistry

Spontaneous Processes

Processes that proceed without any outside assistance!

• If they are spontaneous in one direction, they are nonspontaneous in the other (ex. a gas entering an empty chamber is spontaneous, but for all of it to go back on its own is nonspontaneous)

What impacts spontaneity?

Temperature & Pressure (ex. ice melting or freezing)

Second law of Thermodynamics

The entropy (randomness) of the universe increases in any spontaneous reaction (hence, spontaneous reactions decrease “order”)

• S is positive for spontaneous reactions (or irreversible ones)

• S is 0 for reactions at equilibrium (or reversible ones)

Entropy increases when the following increase

• temperature

• volume

• number of independently moving molecules

  • when solids or liquids turn to gas

  • when solids turn to liquids or solutions

  • the number of GAS molecules increases during a chemical reaction

Third Law of Thermodynamics

the entropy of a pure, ideal, crystalline substance at absolute zero IS 0. (ex. all atoms in a perfect lattice at 0K will have no entropy). At this state, there will be no movement or freedom, and there will be absolute order!

Standard Molar Entropies

• generally larger for gases than for solids or liquids

• increase with molar mass

• increase with the number of atoms in a formula

• the values for elements ARE NOT 0 j/mol K @ 298K, because the reference for entropy is 0K

Calculating Entropy Changes (units and formula)

• usually joules, not kilojoules

• dependent on number of moles (when taking sum of products - sum of reactants)

• every substance has a nonzero value for entropy

Gibbs Free Energy (deltaG)

• if it is greater than 0, it is nonspontaneous

• if it is equal to 0, the system is at equilibrium

• if it is less than 0, the system IS SPONTANEOUS

• 3 different formula definitions for it (look on equation sheet based on given values)

• standard Gibbs free energy of formations are (sum of products - sum of reactants)

When are reactions entropy or enthalpy driven?

• If both deltaH and deltaS are negative, it is enthalpy driven

• If both deltaH and deltaS are positive, it is entropy driven

Kinetic Control

reactions that are thermodynamically favored but don’t appear to make products are considered “under kinetic control”

• they usually have a very high activation energy, which accounts for the lack of product formation

• ***just because a reaction doesn’t appear to occur doesn’t mean its at equilibrium or not thermodynamically favorable***

Coupled Reactions

• two reactions that share an intermediate (a product of one is a reactant in the other)

• the sum of the reactions have a negative deltaG to make the whole process favorable

• hess’s law is used to determine the whole deltaG

Oxidizing/Reducing Agent

• if something causes something else to be oxidized, its the oxidizing agent

• if something causes something else to be reduced, its the reducing agent

Assigning oxidation numbers

oxygen = -2

hydrogen = +1

F = -1

monatomic ion = charge

pure elements = 0

sum of oxidation numbers = overall charge of an ion or zero in a compound

balancing redox reactions (method)

1. make 2 half reactions

2. balance atoms other than O and H, keeping the side of the reaction they’re on the same (using H+, OH-, H2O)

3. add electrons to balance charges

4. multiply by common factor to make electrons in half-reactions equal

5. add the half reactions

6. simplify by dividing by common factor or converting H+ to OH- if basic

7. double check atoms and charges balance!

Voltaic Cells

• oxidation at the anode (anions from the salt bridge flow here)

• reduction at the cathode (cations from the salt bridge flow here)

• requires a salt bridge to operate

• anode and cathode are in separate containers

• cathode has an electrode with a larger mass

• anode has an electrode with a smaller mass

• all voltaic cells have positive Ecells because they CREATE energy

Cell Potential

electrons flow in a redox reaction from high to low potential energy

• measured in volts

• difference between anode and cathode in a cell

Standard Reduction Potentials

• reduction potential is 0V for 2H+ + 2e- —> H2 (this is the base for all the other ones)

• they’re at 25 degree C and 1M

• they must be reversed if you’re looking at standard oxidation potential

• Ecell = Ered (cathode) - Ered(anode)

• not dependent on concentrations

How to know which one is the anode and cathode

Based on standard reduction potential

• half reaction with more postiive value is the cathode

• half reaction with more negative value is the anode

Nernst Equation and implications

Used for nonstandard conditions

• –nFE = nFE° + RT ln Q

• E = E° – (RT/nF) lnQ

where Q is the reaction quotient (products/reactants)

If Q decreases, Q<K, so the reaction moves forward, and E increases

If Q increases, Q>K, so the reaction moves towards reactants, and E decreases

SOLIDS/LIQUIDS AREN’T PART OF THE Q EQUATION

Concentration Cells

• cell potential depends on concentration

so a voltaic cell can be constructed with the same substance in both anode and cathode as long as concentrations are different (in this case, Ecell = 0 because the anode and cathode Ered are exact opposites)

Electrolytic Cells

• require outside electricity to drive the reaction (called electrolysis)

• Ecell is always negative (because it consumes energy)

• anode and cathode are in the same chamber

• no salt bridge needed

• require ion flow (even if its in the same chamber) for the reaction to occur

Electrolysis of Molten Salts

the anode always moves from liquid (ion) to pure form, and the cathode always moves from pure form to liquid (ion) because the SALT IS MOLTEN.

• the movement of ions is what constitutes the current

• we have to oxidize molten instead of aqueous salt if we want the same outcome because H20 could be oxidized if its more favorable (has a lower energy (or more positive Ecell) required to run)

  • when looking at possible ox/red, you should always start the water with H20

Electroplating

Uses electrolysis to deposit a thin layer of metal onto that same metal for beauty/anticorrosiveness

• since its the same metal, Ecell is 0

• only a small voltage is needed to transfer atoms from one electrode to another

Faraday’s Law

the amount of a substance produced or consumed in an electrolytic cell is directly proportionate to the amount of electricity (charge) that passes through the cell.

Electrolysis and Stoichometry

q = It = nF

I = q/t

n = moles of e-

q = charge (C)

I = current (amperes)

F = faraday’s constant (96,485)

t = time in seconds