Catalog

Electrochemical:

A system which produces electrical energy.

Example: 2Ag⁺ + Cu à 2Ag _(s) + Cu²⁺

Half cell:

Each half of the electrochemical cell is called a half cell.

Example: Ag⁺ + e^-à Ag _(s)

Oxidation reaction:

Half reaction in which a specie loses electrons.

Example: Ag _(s) à Ag⁺ + e^-

Reduction reaction:

Half reaction in which a specie gains electrons

Example: Ag⁺ + e^-à Ag _(s)

Reduction – Oxidation reaction:

a reaction involving the loss and gain or electrons.

Example: 2Ag⁺ + Cu à 2Ag _(s) + Cu²⁺

Note:

The oxidizing agent is reduced during the reaction.

Oxidation number:

Charge of atoms possess if the species containing ions is made up of ions.

Example: O^2-O.N= -2

Note:

The sum of positive charges and negative charges equal the overall charge of the species.

Alkali metals O.N is +1

Alkali earth metals O.N is +2

Oxygen is -2 and halogens are -1.

Spontaneous reaction:

If two half cell are joined reduction half reaction is higher on the table than the reactant to be oxidized.

Example: 2Ag⁺ + Cu à 2Ag _(s) + Cu²⁺

Non spontaneous:

The reactant being reduced lower on the table than the reactant to be oxidized.

Example: Zn²⁺ + Cu_(s) à Zn_(s) + Cu²⁺

Note:

If both reactants are on the same side of the table. No possible reaction can occur.

Electrochemical cell :

A system of chemical which produce electrical energy.

Consists of the following:

Electrode: a conductor at which half reaction occurs.

Anode: electrode at which oxidation occurs.

Example: Ag _(s) à Ag⁺ + e^-

Cathode: electrode at which reduction reaction occurs

Example: Ag⁺ + e^-à Ag _(s)

Note:

Electrons flow from anode to cathode in the wire.

No electrons flow in solution only ions.

Number of electrons in oxidation reaction must equal the number of electrons in reduction reaction.

Voltage or Electrical potential:

Work done per electron transferred.

Example: Ag⁺ + e^-à Ag _(s)E^o = 0.80

Arbitrarily:

A zero point on the voltage scale

Example:

Hydrogen half cell

2H+ + 2e^-àH₂

Note:

· E^o= standard reduction potential.

· If a half reaction is reversed than the signs are reversed as well for E^o.

· If two half reactions can be added together to give a redox equation the voltages associated with the half reaction can also be added.

_·E^o_cell= E^o_red- E^o_ox

· If E^ois positive than the reaction is spontaneous.

_·If E^ois negative than the reaction is non spontaneous.

_·If E^ocell is large it doesn’t tell the rate of reaction.

_·The surface area of electrode doesn’t effect on the cell potential.

_·E^o= 0.00 when the cell reaches equilibrium.

_·When several different reduction half reaction can occur the highest tendency to accept electrons will occur preferentially.

· When several oxidation half reaction can occur the half reaction having the highest tendency to lose electrons will occur preferentially.

Lead acid storage battery:

Electrodes are made of Pb and PbO₂
It is immersed in diluted H₂SO₄
When external source of electrical energy is applied to the battery to recharge it. The spontaneous discharge reaction is driven backwards.

Zinc carbon battery:

Cathode is made of carbon.
Zn liner is anode
Paste of NH₄Cl, MnO₂ and graphite is inside the cell.

Alkaline Battery:

Similar to zinc battery. Only difference is it operates in basic condition.
It delivers much greater current.

Fuel cells:

Carbon electrode impregnated with catalyst.
Electrolyte is KOH.
Use H₂and O₂ to produce H₂O and electricity.

Corrosion of metals:

Metal oxidize when H2O or a gas makes contact with it.

Prevent Corrosion:

Apply protective layer such as paint or plastic to the surface.
Cathodic protection, Zn, Mg or another metal which oxidizes before the metal being protected oxidizes
Change the conditions in the chemical surrounding so as to lower the tendency of the surrounding to reduce.

Electrolysis:

Process of supplying electrical energy to a molten ionic compound or a solution ion so as to produce a chemical change.

Example: Na⁺ +2Cl^- à 2Na + Cl₂

Electrolytic cell:

An apparatus in which electrolyses can occur.

Example: Na⁺ +2Cl^- à 2Na + Cl₂

Molten binary cell:

Anode and cathode are made up of inert Pt or carbon.
Electrolysis of aqueous solution:
- NaI the cell includes water.
- The electrodes are made up of inert Pt or carbon.
Note:
- During the electrolysis of aqueous solutions you must always consider the possibility that H₂O may oxidize and or reduce.
- The preferred reaction will be the one requiring the least voltage input.
- The half reactions having the greatest tendency to reduce and greatest tendency to oxidize are preferred.

Electroplating:

The cathode is made out of the material which a metal is reduced or plated out at cathode.
The electroplating solution contains the ions of the metal which is to be plated onto the cathode.
The anode may be made of the same metal which is to be plated out onto the cathode.