Unit 11- Redox and Electrochemistry

• Anode• Cathode• Electrochemical cell• Electrode• Electrolysis• Electrolyte• Electrolytic cell• Half-reaction• Oxidation• Oxidation number

• Redox• Reduction• Salt bridge• Voltaic cell

What’s the point ?

• Electrical production (batteries, fuel cells)

REDOX reactions are important in …

• Purifying metals (e.g. Al, Na, Li)

• Producing gases (e.g. Cl2, O2, H2)

• Electroplating metals

• Protecting metals from corrosion• Balancing complex chemical equations• Sensors and machines (e.g. pH meter)

C3H8O + CrO3 + H2SO4 Cr2(SO4)3 + C3H6O + H2O

What is redox?• Oxidation- loss of electrons by an atom or ion• Reduction- gain of electrons by an atom or ion• **since one can’t occur without the other

– Combine terms to Redox– Mnemonic: LEO the lion says GER

• Lose Electrons Oxidation• Gain Electrons Reduction

Oxidation numbers• On periodic table• Determines what is oxidized and reduced in a

reaction• If they change it’s a redox reaction

What type of reaction is this (besides redox)???

Assigning Oxidation numbers• Identify the formula• If element is free (uncombined) its ox # is 0• Monotomic ions- ox # is same as ion charge• Metals in Groups 1,2 and 3 have ox #’s of +1, +2 and +3

respectively• Fluorine is always -1 in a compound• Hydrogen is always +1 unless it’s combined with a metal then

it’s -1• Oxygen is usually -2, except when combined with a more

electronegative element then it’s +2• *sum of oxidation #’s in a compound must be 0• *sum of oxidation #’s in a polyatomic ion must equal its charge

Try these:

• HNO3

• CO2

• K2PtCl6

• PCl5

• H2SO4

Redox reactions• Once you determine oxidation numbers you

can see what element was oxidized and what was reduced

• Oxidizing agent- substance that was reduced (gained electrons)

• Reducing agent- substance that was oxidized (lost electrons)

Half-reactions

• Show oxidation or reduction of redox rx• Ex:

• Shows conservation of mass and charge – Charge does not have to be 0

Balancing redox rx’s• Assign oxidation numbers to determine what

is oxidized and what is reduced.• Write the oxidation and reduction half-

reactions.• Balance each half-reaction.

– Balance charge by adding electrons.• Multiply the half-reactions by integers so

that the electrons gained and lost are the same

Example: Cu + AgNO3 Cu(NO3)2 + Ag

• Add the half-reactions, subtracting things that appear on both sides.

• Make sure the equation is balanced according to mass.

• Make sure the equation is balanced according to charge.

Practical use for redox reactions

• Electrochemical cells– Involves a chemical reaction and flow of electrons– 2 types:

• Voltaic- spontaneous• Electrolytic- requires electric current (nonspontaneous)• Each have 2 electrodes- site of oxidation and reduction

– Oxidation occurs at the anode– Reduction occurs at the cathode– An Ox Red Cat– Anode- oxidation, reduction-cathode

Voltaic cells

• Once even one electron flows from the anode to the cathode, the charges in each beaker would not be balanced and the flow of electrons would stop

Voltaic cells• Therefore, we use a

salt bridge, usually a U-shaped tube that contains a salt solution, to keep the charges balanced. (completes the circuit)– Cations move

toward the cathode.– Anions move toward

the anode.

Voltaic Cells• In the cell, then,

electrons leave the anode and flow through the wire to the cathode.

• As the electrons leave the anode, the cations formed dissolve into the solution in the anode compartment.

Voltaic Cells• As the electrons reach

the cathode, cations in the cathode solution are attracted to the now negative cathode.

• The electrons are taken by the cation, and the neutral metal is deposited on the cathode.

• Activity series helps identify anode and cathode– Metal higher on

chart- oxized (anode)– Metal lower on

chart- site of reduction (cathode)

Determining electric potential• Voltmeter is used• Voltage is compared to the reduction of H

which is 0 volts• The more “+” the reading; reduction is more

likely

• Reduction potentials for many electrodes has already been measured

Cell potentials

• At standard conditions can be determined using this equation:

• The strongest oxidizers have the most positive reduction potentials.

• The strongest reducers have the most negative reduction potentials.

Ecell = Ered (cathode) − Ered (anode)

Cell Potentials• For the oxidation in this cell,

• For the reduction,

Ered = −0.76 V

Ered = +0.34 V

Cell Potentials

Ecell = Ered (cathode) − Ered (anode)

= +0.34 V − (−0.76 V)= +1.10 V

Dry Cells

• Dry cells use two electrodes and a “paste” as an electrolyte.

• Some pastes are acidic and others are alkaline.

• Carbon is generally used as the cathode and zinc as the anode.

Examples of Voltaic Cells:

Lead-Acid Batteries

• Lead-Acid batteries usually contain six cells.(2 V each)

• The battery contains lead plates, lead oxide plates, dividers, and a sulfuric acid electrolyte.

• The lead plate is the anode and the lead oxide plate is the cathode.

• Each cell is connected to form one cathode and one anode on the top or side of the battery.

Fuel Cells

• Fuel cells bring in the oxidizing and reducing agents as gases

• Graphite is typically the anode and cathode for the reaction which produced electricity.

• Fuel cells are clean and efficient.

Corrosion

• Corrosion is defined as the disintegration of metals.

• Corrosion is typically caused by oxygen (O2).

• A familiar example of corrosion is iron rusting.

• Corrosion is a result of a redox reaction involving a metal.

Iron oxide (rust)

Corrosion con’t…

Corrosion Prevention

• Typical corrosion protection involves plating the iron with another metal.

• The production of steel (iron and carbon) reduces the rate of corrosion of the iron.

• Aluminum, zinc, titanium are some metals which corrode slowly, or have different properties used to protect iron.

…Corrosion Prevention

Electrolytic Cells• Electricity is used to force a chemical reaction

– Electrolysis• Used to obtain metals from molten salts

• Starting/keeping a car running• Plating metals

Electroplating

• Item to be plated is cathode

• Metal that will plate is anode

• Put in solution containing ions- electrolyte

Electroplating con’t

• Benefits– Resists corrosion– Improves appearance– Cheaper

• Drawbacks– Plating isn’t always even– Can wear off– Solutions are toxic