| what is electrolysis | breakdown of an ionic compound when molten or in aqueous solution by the passage of electricity |
| what is an inert electrode | electrode that doesn’t take part in the reaction |
| what is an electrolyte | ionic compound that is either in the molten state (heated so they become liquids), or dissolved in water. |
| what is an anode | positively charged electrode in electrolysis. |
| what is a cathode | negatively charged electrode in electrolysis |
| what happens when an electrie current is applied to an electrolyte through electrodes | the positive ions (cations) move towards the negative electrode, the cathode and the negative ions (anions) move towards the positive electrode, the anode. |
| when ions reach the electodes they either.... | gain or lose electrons |
| at the cathode | cations need to gain electrons to become atoms therefore reduction occurs at the cathode |
| at the anode | need to lose electrons to become atoms therefore oxidation occurs at the anode. |
| Electrolysis of molten lead (II) bromide: | -The electrolyte in this reaction is PbBr₂. This will become Br- and Pb²+ ions.
-The Pb will move towards the cathode since it is a negative ion where it will gain electrons -to form lead atoms. Half equation for the reaction at the cathode:
-Pb²+(aq) + 2e- -> Pb(s)
-The Br- will move towards the anode since it is a positive ion where it will lose electrons to -form bromide atoms. Half equation for the reaction at the anode:
-2Br- -> Br₂ + 2e-
-The overall reaction looks like this:
-Pb²+ + 2Br- -> Pb +Br₂ |
| Electrolysis of concentrated aqueous sodium chloride (brine) | -The electrolyte in this reaction is a solution of NaCl and H₂O. This will become hydrogen (2H+), chloride (2Cl-), hydroxide and sodium ions.
-The chloride will move towards the anode since it is a positive ion where it will lose electrons to form chlorine gas. Half equation for the reaction at the anode:
-2Cl- -> Cl₂ + 2e- (oxidation)
-However, at the cathode sodium is not formed but hydrogen is formed. This is because the sodium is more reactive than hydrogen (explained further below). Half equation for the reaction at the anode:
-2H+ +2e- -> H₂ (reduction)
-In this electrolysis, sodium hydroxide is left behind which is useful just like the rest of the substances made. This electrolysis is done at a large scale with sea water to make these products for industrial use. |
| Electrolysis of dilute sulfuric acid | -The electrolyte in this reaction is a (dilute) solution of H₂SO₄ and H₂O. This will become H+, SO₄²- and OH-.
-Since there is only one positive ion that can be formed at the cathode, hydrogen is formed. Half equation for the reaction at the cathode:
-2H+ +2e- -> H₂ (reduction)
-At the anode, OH- is formed as it is less reactive than the sulphate (explained below). OH will undergo oxidation and become water and oxygen. Half equation for the reaction at the anode:
-4OH²- -4e- -> 2H₂O + O₂
-Both gases are colourless but can be tested for by using the squeaky pop test and the relighting a glowing splint test for hydrogen and oxygen, respectively. |
| generally, where do hydrogen and metals usually form | at the cathode |
| where do non metals usually form | at the anode |
| what can electrolysis do | used to purify metals, such as in the purification of copper where the copper is used as the electrodes (not inert electrodes). |
