AP CHEM UNIT 9

Entropy

Entropy

measure of the energy lost to molecular disorder or randomness in a sample

units: J per Kelvin

• + value means system increaded in entropy

• - value indicates that the entropy of the system decreased

Third Law of Thermodynamics

entropy of a perfect crystal at 0K is zero

microstates

invisible possible sections

More spread out particles

higher disorder/entropy

Gases have the highest what

entropy

products have more moles

entropy increase, thetaS is positive

Higher Temp

Greater KE, greater Entropy

Entropy increases

• Decomposotion reaction

• Increase in temp

• Dissociation of ions or dissolution of a solute to form a solution

• Endothermic phase changes

• increase in the number of moles of gas

• Gas expansion(decreasing p, increasing V)

Entropy decreases

• Composition reaction

• decrease in temp

• precipitation or formation of a solid from aqueous solutions(aq → s)

• exothermic phase changes

• decrease in the number of moles of gas

• Gas compression

Second Law of thermodynamics

Entropy of the universe increases in for a spontaneous process and remains unchanged when a system is at equilibrium

Equation for Entropy of a reaction

theta S = sum of thetaS (pdts) - sum of thetaS (rcts)

At equalibrium

theta S = 0

spontaneous process

theta S > 0

Gibbs free energy(G)

energy available in a system that can be used to do work

-G = favorable

Reactions that take place

thermodynamically favorable or spontaneous

Kinetic Control

favorable reaction that are very slow, look at the kinetics of the reaction to determine the rate of reaction

Enthalpy

energy required or released in a chemial process

exothermic = favorable ( negative H)

endo= unfavorable(Positive H)

Favorable G (non temp dependant)

-H and +S

Unfavorable G (non temp dependent)

+H and -S

temperature dependant favorable G

-H and -H

+ at high temp

-at low temp

+S and +S

+ at low temp

-at high Temp

G = thetaH - T thetaS

equation

if entropy is unfavorable at high temps

it determines

if enthalpy is unfavorable at low temps

it determines

theta G(reaction) = sum of thetaG(pdts) - sum of thetaG(reactants)

standard G of formation of reaction

What happens when a reaction is unfavorable what can you do

force it by supplying sufficent energy

light and electrical energy

or to Couple it into a reaction that is thermodynamically favorable

coupled system

the negative G from the thermodynamically favorable reaction provides the energy needed by the unfavorable reaction and drives the unfavorable reaction to proceed

G = -nFE

gibbs free energy equation

intermediates

in pdts and then in reactants in another step

oxidation-reduction reaction

electrons are transfered from one species to another

can be measured by an electrochemical cell

simple/Galvanic cell

two half cells

electrodes

solids(gas systems) that are placed into each of the solutions

cathode

reduction occurs because it attracts the cations from the salt bridge and the other solution

red-cat

Au3+ + 3e- →Au(s)

mass increases

the one with the higher reduction potential should stay as reduced and the other oxidized so flip

anode

attracts anions from the salt bridge and the solution

an-ox

Ag(s) →Ag+ + e-

mass decreases

FAT CAT

from anode to cathode

E cell equation

Ecell = Ereduced(anode) - Eoxidation(cathode)

positive Ecell

favorable

Galvanic or voltaic cells

-Ecell

unfavorable - requires external source

Electrolytic cell

magnitude of voltage doesn’t change when..

multiply one or more of the reactions in order to balance

1 volt

1 joule/columbs

reached equilibrium

zero for cell potential

Nernst Equation

shows the relationship between the cell potentials at non-standard states and standard states

Q

pdts/reactants

E - E(standard) - (RT/nF)lnQ

nernst equation

always use the ions when molarity for finding Q

Q

tells us which way the equation shifts

reaction quotient

current

defined as the amounts of charge transferred over time

(I) is measured in amperes

1 ampere = 1 coulomb/second

Faradays constant F

way to relate coulombs to moles of electrons

current equation

I = q/t

Q>1

“Too manhy products”

E cell will be less than Standard Ecell

Products are favored so more products are formed

Q<1

Ecell will be greater than standrad Ecell

reactants are favored so less products are formed

if Q=K

at equilibrium