chem 1312 exam 3

exothermic

heat is lost by system

Hess’s Law

delta H= (sum of delta H products)-(sum of delta H reactants)

delta S= entropy. entropy is:

dispersal of a system’s energy/measure of disorder

cases of spontaneity

1. solids or liquids form gases

2. solids form liquids or solutions

3. # of gas moles increase during a reaction

4. S increases with temp

5. S increases with molar complexity (more atoms = more accessible microstates)

microstates

possible distributions

S=

(k)(lk(W)) ; W= # of microstates ; W= 2^N (N= number of gas molecules)

delta Ssurr =

-delta H / T

delta Suniverse=

delta Ssurr + delta Ssys

delta S_universe=

delta S_sys + -deltaH_sys/T > 0 ← spontaneous

delta G < 0

spontaneous

delta G = 0

equilibrium

delta G > 0

nonspontaneous

standard state gibbs free energy formula

nonstandard state gibbs free energy formula

or K= e^ -(deltaG/ RT)

if Q<K, the delta G is

negative, spontaneous in forward direction

if Q>K, delta G is

positive, spontaneous in reverse

always do what for calculations!!

convert J to kJ

“boiling point” →

equilibrium, delta G=0

delta S_surr=

-delta H / T

delta S_universe=

-delta G / T

given solubility, what is delta G?

1. solubility/molar mass= M

2. sub M for k

3. delta G= -RTln(k)

Q=

products over reactants raised to stoichiometric coefficients

rules for assigning ON:

1. An element in pure state is 0 (Cl2 -> 0)

2. A monoatomic ion equals its charge number (Na+ ->1)

3. ONs are assigned in order of priority: (G1, G2, H, F, O, 7A, 6A, 5A)

4. Sum of ONs of all atoms in a neutral formula = 0 

5. Sum of all oxidation states of all atoms in a polyatomic ion = charge of ion

   1. ClO3- so O=-2 and Cl=+5 gives 3(2-)+5= -1

Increase in oxidation number :

indicates oxidation (loss of e-)

Decrease in oxidation number:

indicates reduction (gain of e-)

reducing agent is

oxidized (loses e-)

oxidizing agent is

reduced (gains e-)

balancing a half reaction in acid steps

1. determine ON

2. separate into half reactions (oxidation and reduction)

3. balance atoms of each rxn

   1. atoms not O or H

   2. balance O by adding H2O to side that lacks O

   3. balance H by adding H+ to side that lacks H

4. balance charge

5. match e- between reactions with LCM

6. add rxns, cancelling out e- and common chemical species

7. check

indicators of change in ON

• elemental → compound

• something that changes to count of O2

balancing a half reaction in a basic solution

same as acidic but two more steps:

1. Identify acid that needs to be neutralized

   1. add OH to both sides to neutralize acids

   2. cancel H2O on both sides

2. check that elements and charges are balanced

other notes on balancing acids

• products that are not acids dont have to be dissociated and can be written as normal

• H+ and another ion (like Cl-) can be combined if theyre on the same side and fit together to make HCl/ HA

electrochemical cell

• seperates ½ rxns into separate physical cells

• uses external circuit for e- transfer

• generates electric current (1 Amp = C/s) ⭐

• uses a spontaneous reaction

oxidation half cell

• contains Zn strip in ZnSO4

• Zn= anode

anode

where oxidation happens

reduction ½ cell

• contains Cu strip in CuSO4

• copper is cathode

cathode

where reduction happens

0 voltage =

equilibrium

positive voltage =

spontaneous reaction (has energy)

measures of spontaneity

• entropy

• gibbs free energy

• Q/K

• voltage

delta E=

charge (q) x delta Volt

1 volt =

1J/C or 1C*1V=1J

positive standard reduction potential

spontaneous

Ecell=

Ered(cathode)-Ered(anode)

favorable reductions are

positive (e- is a reactant)

reductions are on the

right

cathode is the one with

bigger E0

more negative →

stronger reducing agent

cell movements

• electrons go toward (+)

• positive spectator ions go RIGHT

• as e- flows mass might decrease

line notation conventions

• single vertical bar at phase boundaries

• double || used at salt bridge frit

• commas seperate components in same place

• parentheses indicate phase, conc., and pressure

line notation format

anode (unless both reactants are metal) | gas | anode ions || cathode ions | cathode

(cell potential) delta G=

-nFEcell

• n= # of e- transferred in ½ rxns

• F= faradays constant

delta G big equation that describes all relationships

-RTln(K/Q)=-nFEcell if Q=1

note about Q

Q often hides concentrations → batteries cab change conc. as rxn goes forward.