Unit 9 AP Chemistry ~ Thermodynamics and Electrochemistry

Increase in entropy
(5 Bulletpoints)

(Energy and matter more or less dispersed? More or less volume? Temperature increasing or decreasing? More total # of moles in products or reactants?)

Matter more dispersed

Energy more dispersed

More volume

Temperature increases

Total # of moles in the product > total # of moles in the reactants

Calculating absolute entropy change
(What must be consider?)

The # of moles from each substance MUST be considered when calculating entropy

Gibbs free energy change

Describes whether a reaction is thermodynamically favorable and unfavorable

What does it mean when a reaction is thermodynamically favorable?

Proceeds to equilibrium WITHOUT external intervention

∆G°

Gibbs free energy for a chemical process

Finding STANDARD gibbs free energy equation. When is it thermodynamically favorable? What can we neglect out of the equation?

Thermodynamically favorable~ ∆G° < 0

Neglect solids

Kinetic control

Some reactions are thermodynamically favorable reactions but DO NOT make products at a measurable rate

Do reactions under kinetic control have a small or large activation energy?

Large activation energy

What can kinetic control reactions use to speed up the reaction? Does this have an effect on the favorability of the reaction?

Enzymes

Does not have a effect on the favorability of a reaction

What do thermodynamically favorable (∆G° < 0) reactions favor at equilibrium?

They favorable the products at equilibrium. (K > 1)

Connections between K and ∆G° can be made qualitatively through estimation:
What happens when ∆G° is negative?
What happens when ∆G° is positive?
What happens when ∆G° is 0?

Negative: K < 1, reaction favors the products

Positive: K > 1, reaction favors reactants

Zero: Equilibrium

Why are some salts partially soluble and some salts fully soluble?

∆G° indicates if a dissolving process is thermodynamically favorable

What contributes to the value of ∆G°?

∆S° and ∆H° contribute to the value of ∆G°

What would happen to the solubility of a salt when ∆G° is positive?

Slightly soluble

What would happen to the solubility of a salt when ∆G° is negative?

Soluble

What happens when we break attractions? (∆H°)

Negative ∆H°

What happens when we form attractions? (∆H°)

Positive ∆H°

What happens if the ∆H° of a dissolution is positive?

(Endo or exo? Will enthalpy contribute to favorability?)

Endothermic

Enthalpy DOES NOT contribute to favorability

What happens if the ∆H° of a dissolution is negative?

(Endo or exo? Will enthalpy contribute to favorability?)

Exothermic

Enthalpy DOES contribute to favorability

What happens when we break attractions? (∆S°)

Dispersing particles therefore positive ∆S°

What happens when we form attractions? (∆S°)

Organizing particles, therefore negative ∆S°

What happens if the ∆S° of a dissolution is positive?

Increasing

Entropy change DOES contribute to favorability

What happens if the ∆S° of a dissolution is negative?

Decreasing

Entropy change DOES NOT contribute to favorability

∆G° can be determined or predicted if enthalpy and entropy values are available by using the equation…

∆G° can also be predicted for enthalpy and entropy signs

(Table)

What happens if the ∆G° of a dissolution is positive?

Dissolving is NOT thermodynamically favorable

What happens if the ∆G° of a dissolution is negative?

Dissolving IS thermodynamically favorable

What a unfavorable reaction do to BECOME favorable

Couple with a favorable reaction

If the sum of the ∆G° value is negative then the process is..

Thermodynamically favorable

Galvanic Cell: The Anode Side

(Reduction or oxidation? What happens when there’s a metal reactant? Describe the flow of electrons?)

Oxidation happens here

Metal reactant~ Mass decreases and converted into ions

Flow~ Electrons flow from anode through wire to cathode

Galvanic Cell: The Cathode Side

(Reduction or oxidation? What happens when there’s a solid? Describe the flow of electrons?)

Reduction

Solid~ Mass of cathode increases

Flow~ Electrons flow into cathode

Galvanic Cell: The Salt Bridge

(What is it? Describe movement in the bridge. What happens if no salt bridge?)

Allows for movement of ions between half-cells

Movement~ Movement of cations in reduction half-cell

Movement of anions in oxidation half-cell

NO SALT BRIDGE NO REACTION

Electrolytic cells

(Thermodynamically favorable or unfavorable? What is needed to power the cell? Single or double chamber? Electron flow? Where do the cations go and what type of reaction is it? Where do the anions go and what type of reaction is it?)

Thermodynamically favorable

Power source needed

Single chamber

Electrons flow~ Anode → Power Source → Cathode

Cations → Cathodes (Reduction)

Anions → Anodes (Oxidations)

What do we use when comparing reduction potential?

(When does it only work? What do the table show?)

Use standard reduction potentials

ONLY work at STP

Tables ONLY list reduction half - reactions

How do we find oxidation half - reaction?

Reverse reduction half - reaction and sign of voltage

What is the standard cell potential equation?

Positive overall cell potential
(Favorable or unfavorable? Which cell?)

Favorable

In voltaic cells

Negative overall cell potential
(Favorable or unfavorable? Which cell?)

Unfavorable

In electrolytic cells

What does cell potential depend on?

Concentrations or pressures of reactants and products

What happens at equilibrium?
(Think magnitude and cell potential)

Magnitude of cell potential decreases, reaching 0 at equilibrium.

Voltaic cell (K > 1): What happens when Q is increased above one?

System is closer to equilibrium and cell potential decreases

EX: Decreasing product concentration

Voltaic cell (K > 1): What happens when Q is decreased below one?

System is further from equilibrium and cell potential increases

EX: Increasing product concentration

Electrolytic cell (K < 1): What happens when Q is increased above one?

System is further from equilibrium and cell potential increases

EX: Increasing product concentration

Electrolytic cell (K < 1): What happens when Q is decreased below one?
(Provide example)

System is closer to equilibrium and cell potential decreases

EX: Decreasing product concentration

Charge flow in electrolysis equation

(What is q)

l = q / t

q~ related to changes of reactants and products in cell

Tips for solving charge from products
(Current, time, faraday’s constant, moles of electrons)

Reminder units:
-Current: Amperes (A), equivalent to coulombs / seconds (C/S)
-Time: needs to be in seconds when using Faradays law equation
-Faraday’s constant: charge per mol of electrons
-Moles of electrons: determined from chemical equation

Use dimensional analysis