C2 5.4 Electrolysis and Aluminium Flashcards
Why do ionic compounds have to be in solution or molten before they can undergo electrolysis?
This is because there are strong forces of electrostatic attraction between the oppositely charged ions. As a result the ions aren’t free to move.
What is electrolysis?
Electrolysis is the breaking down of a substance using electricity.
What happens in electrolysis? (Brief)
Positive ions move to the negative electrode (cathode) while negative ions move to the positive electrode (anode).
Half Equations (detail)
Half equations show what is happening at EACH electrode. Make sure on a half equation, that the CHARGES and the RATIO are equal and the ions cancel each other out.
+ -
Eg: Pb 2 + 2e —> Pb
As you can see, there is one lead (Pb) on the left side and one lead on the right so the ratio is the same. There are 2+ ions on the left and 2- on the left. 2-2=0 so they cancel each other out. The charges and the ratio balance- so as a result this half equation is balanced
Common half equation problem
One common problem is the perception of the charges and the ions
2-
2O
The above oxygen ion shows that there are 2 Oxygen ions and 4 negative charges as 2 x -2 = -4 which means
-
A 4e And an O2 is needed to balance this half equation.
Remember the acronyms PANIC and OILRIG
Positive Oxidation
Anode Is
Negative Loss
Is Reduction
Cathode Is
Gain (of electrons)
What happens during electrolysis in water?
The less reactive element between hydrogen and the metal is usually produced at the negative electrode. At the positive electrode we often get oxygen gas given off from discharged hydroxide ions.
Describe the aluminium extraction process
Aluminium is often found in its ore bauxite. The aluminium oxide is mixed with molten cryolite to lower the melting point of the aluminium and to save energy. Aluminium is formed at the negative electrode and oxygen is formed at the positive electrode
Why does aluminium oxide have to be melted before electrolysis?
So that the ions are able to move to the electrodes.
What happens at the negative electrode (cathode) during aluminium electrolysis?
Each aluminium ion Al 3+ gains 3 electrons. The ions turn into aluminium atoms. We say the Al 3+ ions are reduced to form Al atoms as 3 electrons are added to remove the +3 charge.
The aluminium metal formed is molten at the temperature of the cell and it collects at the bottom. It is then siphoned or tapped off.
What happens at the positive electrode (anode) during aluminium electrolysis?
Each oxide ion O2- loses two electrons and the ions turn into oxygen atoms. These bond in pairs to form molecules of oxygen gas O2.
The oxygen then reacts with the hot, positive carbon electrodes to form carbon dioxide gas. The electrodes need to be replaced regularly.
What three products do we get by the electrolysis of brine?
We get:
Chlorine gas produced at the positive electrode.
Hydrogen gas produced at the negative electrode.
Sodium hydroxide solution is also formed.
Summed up by:
Sodium chloride solution -> hydrogen + chlorine + sodium hydroxide solution
What happens at the positive electrode in the electrolysis of brine?
The negative Cl- ions are attracted to the positive electrode. When they get there, they lose one electron so they are oxidised. The chlorine atoms bond in pairs as Cl2.
What happens at the negative electrode in the electrolysis of brine?
There are H+ ions in brine formed when water breaks down. These positive ions are attracted to the negative electrode but sodium is also attracted to the negative electrode.
The LESS reactive element gets discharged and the sodium ions stay in solution. The H+ ions gain one electron so they are reduced, they also bond in pairs as H2.
Half equations for the electrolysis of brine:
At the positive electrode:
2Cl- (aq) -> Cl2 (g) + 2e-
At the negative electrode:
2H+ (aq) + 2e- -> H2 (g)
What is chlorine used for?
We can react chlorine with the sodium hydroxide produced in the electrolysis of brine to make a solution of bleach. Bleach is good for killing bacteria. Chlorine can also be used for other disinfectants and plastics such as PVC.
What is hydrogen used for?
The hydrogen produced on the electrolysis of brine can be used in the food industry. Margarine is made by reacting hydrogen with vegetable oils.
What is sodium hydroxide used for?
The sodium hydroxide produced is used to make soap and paper. It can also make bleach.
What is an electrolyte?
An electrolyte is a liquid which conducts electricity.
What is an electroplated object?
An electroplated object is an object coated with a thin layer of metal by electrolysis.
Why do we electroplate objects?
We electroplate objects to:
•To protect the metal beneath from corroding.
•To make the object look more attractive.
•To increase the hardness of a surface and its resistance to scratching.
•To save money by using a layer of a precious metal instead of the pure expensive metal.
What is the negative electrode in electroplating?
This is the thing that we want plated.
In the case of copper and nickel. The nickel ions Ni 2+ from the solution are reduced. They gain two electrons and form nickel atoms which are deposited on the copper electrode.
What is the positive electrode in electroplating? And what happens?
This is the thing made from the plating metal.
In the case of nickel. The nickel atoms become oxidised. They lose two electrons each and form nickel ions Ni 2+ which go into solution.
ZnBr2 (l) half equations
Cathode: Zn2+ + 2e- —> Zn
Anode: 2Br- —> Br2 + 2e-
CuCl2 (aq) half equations
Cathode: Cu2+ + 2e- —> Cu
Anode: 2Cl- —> Cl2 + 2e-
H2SO4 (aq) half equations
Cathode: 2H+ + 2e- —> H2
Anode: 4OH- —> O2 + 4e- + H2O
