Chm Exam 3 Concepts

chm2046

When E_cell > E°_cell

• Q < 1 < K

• log(Q) < 0

When E_cell < E°_cell

• K > Q > 1

• log(Q) > 0

The smaller the Q, the ____ work we can do

more

Concentration cell

• electrodes and solutions are the same substance, just at different concentrations

• E°_cell = 0

• Q = dilute/concentrated

Half reactions of the electrolysis of water at pH = 0

• cathode: 2H⁺ + 2e^- → H₂

  • E°_cell = 0

• anode: 2H₂O → O₂ + 4H⁺ + 4e^-

  • E°_cell = 1.23

Half reactions of the electrolysis of water at pH = 14

• cathode: 2H₂O + 2e^- → H₂ +2OH^-

  • E°_cell = -0.83

• anode: 4OH^- → O₂ + 2H₂O (l) + 4e^-

  • E°_cell = 0.40

If we use the same half-rxns on both sides but with different concentrations, we can get a ____

current

E_cell > 0 as long as…

the half reactions have different concentrations. once they are of equal concentrations, the cell is no longer able to do work

What conditions increase corrosion?

• when Fe behaves as the anode

• in acidic conditions

What do you do when both atm and M are present?

• Use the Nernst equation

• Q = products/reactants → atm and M can both be used for products and reactants

primary batteries

• cannot be recharged

• exist as voltaic cells

Secondary batteries

• rechargeable → when it runs down, electrical energy is supplied to reverse the rxn and form more reactant

• discharge: voltaic cell to generate energy

• recharge: overall rxns are reversed → nonspontaneous so energy must be supplied

Cell potential measures…

voltage between two electrodes in a voltaic cell

A battery dies when…

Q = K

“Equilibrium constant”

K

“Standard reference half cell”

2H⁺ + 2e^- → H₂

When choosing products that form first during electrolysis in aqueous solution…

• consider pH: if it is anything but 0 and 14, you must use the Nernst equation

• cathode: choose the least negative E_cell value

• anode: choose the most negative E_cell value

Half reactions of the electrolysis of water at pH = 7

• anode: 2H₂O → O₂ + 4H⁺ + 4e^-

  • E_cell = 0.82

• cathode: 2H₂O + 2e^- → H₂ + 2OH^-

  • E_cell = -0.42

What drives a concentration cell?

differing concentrations in the cathode and anode

Which side of the concentration cell will have higher vs. lower concentration?

The cathode will always have higher concentration

G1/2 hydrides are basic or acidic?

They are basic

Hydrogen has a lot of unique qualities because of its…

very small size

Hydrogen reacts with nonmetals to form…

covalent hydrides → covalent = sharing of e^-

Describe interstitial hydrides

H₂ molecules occupy the holes in the metal’s crystal lattice

Are alkali metals hard or soft?

soft

Alkali metals have ___ MP and BP compared to other groups of metals

lower

Lithium and oxygen form…

lithium oxide → Li₂O

Sodium and oxygen form…

sodium peroxide → Na₂O₂

All other alkali metals (not Na or Li) form…

superoxides → MO₂

The alkali metals reduce hydrogen to form…

ionic hydrides → saltlike

The alkali metals are ___ reactive metals

extremely

Alkali metals and water form…

2E (s) + 2H₂O (l) → 2E⁺ (aq) + 2OH^- (aq) + H₂ (g)

Alkali metals and hydrogen form…

ionic hydrides: 2E (s) + H₂ (g) → 2EH (s)

Why are G2 metals called alkaline earth metals?

their oxides give basic solns and melt at high T

Be forms what kind of bonds?

polar covalent

The G2 metals react with…

sulfites and carbonates → MCO₃ (s)

G2 metals are reactive because…

they have high lattice energy and charge density

Ba reacts with oxygen to form

barium peroxide → BaO₂

The alkaline earth metals react with oxygen to form…

oxides → MO

G2 metals react with halogens to form…

ionic halides: E (s) + X₂ → EX₂ (s)

As elements become more nonmetallic and EN…

the oxides shift from basic to acidic

Covalent vs. ionic bonding

• covalent = sharing electrons

• ionic = transferring electrons

Hydride

when hydrogen has a negative 1 charge

What are the ways in which hydrogen can be synthesized?

• electrolysis of water

• steam-methane reforming rxn

• water-gas shift rxn

Which G1 metal reacts with N? Why?

Lithium → high charge-to-size ratio

Be compounds are mostly covalent. Why?

it has a very high charge density → it shares electrons to decrease that charge density

BeO is…

amphiprotic → can serve as an acid or a base

G2 react with N to make…

metal oxides

G2 will react with CO₂ to make…

metal oxides

G2 oxides will react with CO₂ to form…

carbonates

formation of limestone

CaO + CO₂ → CaCO₃

All G2 metals decompose with…

heat

Sources of G2 metals

sea water, beryl, sulfates