Thermodynamics, Free Energy, and Electrochemistry

Thermodynamically favorable Reaction

• A reaction that occurs under specific conditions

• always move a reaction mixture toward equilibrium.

• Often known as Exergonic

Thermodynamically Unfavorable reaction

• A reaction that does not occur under specific conditions

• Often known as Endergonic

Entropy

• Amount of molecular disorder in a system

• Chaos

Methods of Changing:

• Altering temperature

  • increased movement → increase reacting

• Moles created/used

  • If products have more moles → more chaos

• Increase Gas volume

  • more space → more methods of configuration → increase

• Atom size

  • Larger atoms → more chaos (Usually)

• Molecule complexity

  • More complex → more states of formation/reaction → increased chaos

• Forming mixture

  • Pure substance to mixture → increased randomness → increased chaos

Second Law of Thermodynaics

• Reactions proceed in the direction that increases the entropy of the Universe.

Meaning:

• High Chaos/entropy → Favorable

• Low Chaps/entropy → Unfavorable

Third Law of Thermodyanmics

Entropy of a perfectly ordered crystalline substance at 0 K is zero - Absolute Entropy!

Standard Molar Entropy (S°)

• Absolute entropy of one mole of a pure substance at 1 atm & specified Temperature

General Rules:

• S_gas>S_liquid>S_solid

• S_{Large Molecule}>S_{small molecule}

Standard Molar Entropy of Reaction (ΔS°_rxn)

• the difference in entropy of the products and the reactants

Gibbs Free Energy (ΔG)

• Looks at enthalpy and entropy of the system to determine if overall reaction is favorable.

• Represents energy left/ Excess energy available to make reaction favored

Meanings of Signs:

• if ΔG is -, Forward reaction favored & reaction is favorable

• if ΔG is +, reverse reaction favored & reaction is unfavorable

• if ΔG is 0, both reactions equal & reaction is at equilibrium

ΔG if ΔS>0 and ΔH<0

ΔG is -

ΔG if ΔS<0 and ΔH<0

• Temperature Dependent

  • if High enough T, ΔG is +

  • If Low T, ΔG is -

ΔG if ΔS >0 and ΔH>0

• Temperature dependent

• When T is high enough ΔG is -

• When T is low ΔG is +

ΔG when ΔS<0 and ΔH>0

• ΔG is +

Standard Gibbs Free Energy (ΔG°_rxn)

• ΔG of products -reactants

• Free Energy for a reaction under standard conditions]

  • 1 atm

  • 1 M concentration

  • Specified temp (usually 25 Celcius)

Kinetic vs Thermodynamic control

• Kinetic Control favors lower activation energy

• Thermodynamic control favors most negative ΔG value

How to interpret ΔG° graphs

• Where line comes from is starting point, have line straight from there to end

• Like a ball on a curve

  • As the curve goes down, ball can easily roll so favorable.

  • Can’t go back up on its own so not-favorable

  • Direction doesn’t matter

  • Equilibrium is where ball would eventually stop, at the complete bottom

• ΔG is distance from line of initial to the point on curve

• On X axis is formation of reactants and products

  • Left side is more reactants, right side more products

How to find Normal ΔG (Not on Reference Sheet MUST MEMORIZE)

Thermodynamic Coupling

• Method to overcome lack of favorability

• Use external source of energy in reaction to overcome positive ΔG

Main methods:

• Addition of energy

• Couple non-favorable reaction with favorable

  • Hess’s Law

    • For ΔG it is plus minus and multiplication, not the K ones

  • Ensures overall reaction is favorable

Importance of Electrons

• Are reasons for Bond formation

  • Transferred in ionic

  • Shared in Covalent

• Atoms use Intramolecular forces

Redox Reactions

• Often known as Oxidation- Reduction Reactions

• Process in which one or more electrons are transferred between reaction partners.

• Both Oxidation and Reduction MUST occur together

Oxidation Number

• Number telling us whether atom is neutral or electron rich or lacking

• Compare old and new number to determine if atom has gained or lost electrons

• NOT ALWAYS EQUAL TO CHARGE ON ION

How to Assign Oxidation Numbers

1. Atom in elemental state → Oxidation number of 0

2. Atom in monatomic ion state → Oxidation number = to charge

3. Atom in polyatomic ion or molecular compound → same number if monoatomic ion

4. Sum of oxidation numbers in neutral compound =0

5. Sum of oxidation numbers in charged polyatomic = charge

6. When one atom loses an e^- another must gain an e^-

IMPORTANT MEMORIZES FOR OXIDATION NUMBERS

Hydrogen:

• Is +1 when bound to a non-metal

• Is -1 when bound to a metal

Oxygen:

• Usually has oxidation number of -2

• In peroxides (O-O bonds) Oxygen has -1

Halogen:

• Usually oxidation number of -1

• When with Oxygen, Cl, Br, and I, have positive oxidation numbers

LEO the Lion goes GER

LEO

• Lose Electrons Oxidation

GER

• Gain Electrons Reduction

Reduction Agent

• a substance that loses electrons

  • is oxidized

• Increases oxidation number

Oxidizing agent

• Substance that gains electrons

  • Is reduced

• Oxidation number decreases

Half Reactions

• Method of breaking apart a redox reaction

• Creates 1 oxidation reaction and 1 reduction reaction

Balancing Redox Reaction

• Write balanced net ionic equation

• Split it into ½ reactions

• Add e^- to each half reaction

• Multiple by whole numbers to get equal e^-s in both equations

• Add together reactions

Electrical Potential

• The strength of the movement of electrons (push and pull) within a redox reaction

  • Higher Potential → increased work capability

• Oxidation reactions cause “push” of electrons

• Reduction reactions cause → pull of electrons

E°

• Overall, just Standard Potential

• Both Standard Half-cell potential and cell potential

standard cell potential (E°_cell)

• Sum of standard half reaction potentials for oxidation & reduction half reactions

  • Do not have to multiply by coefficient of your equation, just what’s on the sheet

• Must be positive for favorable reaction

Voltaic Cells

• Also known as Galvanic Cells

•  a favorable chemical reaction generates an electric current that can do work

Parts of Voltaic Cell

• Electrode —  metal strips/wires connected by an electrically conducting wire

  • Anode — Where oxidation occurs, anions migrate towards,-

  • Cathode — Where reduction occurs, cations migrate towards, +

• Salt Bridge — Tube with a ionic salt. Ions don’t react with each other. Neutralizes growing charge, necessary for current, anions towards anode, cations towards cathode

• Load — part of circuit which utilizes the flow of electrons to perform a function

Shorthand notation of Voltaic Cell