Electrochemistry - Chp 20

CNAP

Cathode Negative

Anode Positive

OIL RIG

Oxidation is loss

Reduction is gain

OARC

Oxidation Anode

Reduction Cathode

Who discovered electrolysis + how did they use it?

Humphry Davy

- used it to extract reactive group I and group II elements

Uses of electrolysis:

- producing reactive metals, eg Na, K, Al

- producing hydrogen and oxygen

- purifying copper

- electrolplating, silver-plating nickel cutlery

Electrolysis

the use of electricity to bring about a chemical reaction in a electrolyte, ie a chemical reaction is produced when an electric current is passed through an electrolyte

Electrolyte

is a compound which when molten or dissolved in water will conduct an electric current

What are electrodes + what are they usually made from?

- the rods that dip into the electrolyte and make electrical contact with it

- carbon or platinum as they are unreactive

Inert electrode

do not react with the electrolyte

Active electrode

electrodes that react with the electrolyte

Electrolysis of aqueous copper(II) sulphate using copper electrodes:

Anode: the solid copper electrode is oxidised

Cu - 2e⁻ → Cu²⁺

Cathode: copper ions from the copper sulphate solution are reduced

Cu²⁺ + 2e⁻ → Cu

Result: cathode increases in size as copper ions change into solid copper atoms + build up on the electrode (impurities fall to the bottom)

Electrolysis of acidified water using inert electrodes:

Anode: water is oxidised

H₂O - 2e⁻ → 2H⁺ + ½O₂

Cathode: hydrogen ions are reduced

2H⁺ + 2e⁻ → H₂

Result: twice as much gas is collected at the cathode

Electrolysis of aqueous sodium sulphate solution using universal indicator and inert electrodes:

Anode: water oxidised (since the SO₄²⁻ ions are too stable to be discharged)

H₂O - 2e⁻ → 2H⁺ + ½O₂

Cathode: water molecules are reduced (since the Na⁺ ion are too stable to be discharged)

2H₂O + 2e⁻ → H₂ +2OH⁻

Result: in addition to the collection of oxygen gas

Electrolysis of aqueous potassium iodide using phenolphthalein indicator and inert electrodes:

Anode: iodide ions are oxidised

2I⁻ - 2e⁻ → I₂

Cathode: molecules of water are reduced (since the K⁺ ions are too stable to be discharged)

2H₂O + 2e^- → H₂ + 2OH^-

Result: solid brown iodine will be observed forming at the anode

Electroplating

a process where electrolysis is used to put a layer of one metal on the surface of another

Galvanic/Voltaic Cell

a cell in which a chemical reaction results in production of an electric current

What is the function of a salt bridge in a galvanic/voltaic cell?

to maintain a balance of ions in each part of the cell

Electrochemical series

is a list of elements in order of their standard electrode potentials

What is the order of the electrochemical series from most to least reactivity?

K
Ca
Na
Mg
Al
Zn
Fe
Pb
H
Cu
Hg
Ag
Au