C5 - Electricity and chemistry (Electrolysis) Flashcards
Electrolysis
A chemical process where electricity is used to make a chemical change happen that wouldn’t happen otherwise
- it is composed of splitting ionic compounds into its ions using electricity
Diatomic elements
Elements that in their pure form exist in pairs
Br, I, N, Cl, H, O, F
Electrode
A rod of metal or graphite through which an electric current flows into or out of an electrolyte
Electrolyte
The ionic compound in a molten or dissolved solution that conducts the electricity
Anode
The positive electrode of an electrolysis cell where oxidation happens and electrons are lost
Cathode
The negative electrode of an electrolysis cell where reduction happens and electrons are gained
Anion
The negatively charged ion which is attracted to the anode
Cation
The positively charged ion which is attracted to the cathode
Describe the process of electrolysis
- Current needs to flow around the circuit
- In order for this to occur, charge must be transferred around the circuit by charge carriers (electrons)
- The power supply provides the cathode with a supply of electrons, causing it to become negatively charged
- Positive ions (cations) in the electrolyte move towards the cathode where they gain electrons
- Negative ions (anions) in the electrolyte move towards the anode where they lose electrons
- The electrons move from the anode back towards the power supply
Molten solution
No water present- just the ions of the products used
- will only form elements of its ions
Aqueous solution
A solution in which water is the solvent
- will form H and OH as well as the elements of the ions
Why does the electrolyte have to be an ionic compound?
They are able to dissociate into charged ions needed for electrolysis
Why is graphite a good electrode?
Graphite is a great conductor of electrolysis, it is not an ionic compound and is inert
Inert
Electrodes used in electrolysis that do not react
Three rules for electrolysis in aqueous solutions
- If there is a reactive metal attracted to the cathode (C-) than Hydrogen will be reduced (react) if it is a non-reactive metal, it will be that that reacts
- The ionic compound will split into its positive and negative ions as well as H and OH
- OH will only oxidise if SO or NO salts are present in the compound
What are reactive metals
Those elements in group 1, 2 and some of 3
What is the formula for OH oxidisation at the anode?
4OH(-) - 4e(-) -› O(2) + 2H(2)O
Redox reactions
A chemical reaction involving the transfer of one or more electrons from one reactant to another; when reduction and oxidation happen at the same time
Reduction
Gaining electrons or losing oxygen
Oxidation
Losing electrons or gaining oxygen
Reducing agent
A substance that is oxidised/loses electrons
Oxidising agent
A substance that is reduced/gains electrons
What are the four rules for oxidation numbers?
- For elements on their own, the oxidation number is 0
- The group oxidation numbers are:
1- (+1)
2- (+2)
3- (+3)
5- (-3)
6- (-2)
7- (-1) - If the oxidation number increases, oxidation has occurred
- If the oxidation number decreases, reduction has occurred
What are the three ways electrolysis is used in the industry?
- Electrolysis of brine (concentrated NaCl)
- Aluminium extraction
- Electroplating
What is the method for electrolysis of molten lead(II) bromide?
- Add lead(II) bromide into a beaker and heat it so it will turn molten (ions are free to move and conduct an electric charge)
- Add two graphite rods as the electrodes and connect this to a power pack or battery
- Turn on the power pack or battery and allow electrolysis to take place
- Negative bromide ions move to the positive electrode (anode) and each loses one electron to form bromine molecules: bubbling at the anode as brown bromine gas is given off
- Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode
How does the electrolysis of aqueous sodium chloride take place?
- Brine is a concentrated solution of aqueous sodium chloride
- It can be electrolysed using inert electrodes made from platinum or carbon/graphite
- When electrolysed, it produces bubbles of gas at both electrodes as chlorine and hydrogen are produced, leaving behind sodium hydroxide solution
What is chlorine used for industrially?
Making bleach
What is hydrogen used for industrially?
Making margarine
What is sodium hydroxide used for industrially?
Make soap and detergents
After the electrolysis of aqueous sodium chloride what substance is found at the cathode?
- The H+ ions are discharged at the cathode as they are less reactive than sodium ions
- The H+ ions gain electrons to form hydrogen gas
After the electrolysis of aqueous sodium chloride what substance is found at the anode?
- The Cl- ions are discharged at the anode
- They lose electrons and chlorine gas forms
- The Na+ and OH- ions remain behind and form the NaOH solution
How does the electrolysis of dilute sulphuric take place?
- Dilute sulphuric acid can be electrolysed using inert electrodes made from platinum or carbon/graphite
- Bubbles of gas are seen at both electrodes
After the electrolysis of dilute sulphuric acid what substance is found at the cathode?
H+ ions are attracted to the cathode, gain electrons and form hydrogen gas
After the electrolysis of dilute sulphuric acid what substance is found at the anode?
OH- ions are attracted to the anode, lose electrons and form oxygen gas and water
How is aluminium extracted by electrolysis?
- Aluminium is higher in the reactivity series than carbon, so it cannot be extracted by reduction using carbon, instead it is extracted by electrolysis
- Bauxite (aluminium ore) is first purified to produce aluminium oxide, Al2O3
- Aluminium oxide is then dissolved in molten cryolite: because aluminium oxide has a melting point of over 2000°C which would use a lot of energy and be very expensive
- the resulting mixture has a lower melting point without interfering with the reaction - The mixture is placed in an electrolysis cell, made from steel, lined with graphite
- The graphite lining acts as the cathode, with several large graphite blocks as the positive electrodes
- Carbon blocks are placed above the mixture to act as the anode
After the electrolysis of aluminium oxide what substance is found at the cathode?
- Aluminium ions gain electrons (reduction)
- Molten aluminium forms at the bottom of the cell
- The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell
After the electrolysis of aluminium oxide what substance is found at the anode?
- Oxide ions lose electrons (oxidation)
- Oxygen is produced at the anode:
Why do carbon blocks regularly have to be changed when electrolysing aluminium oxide?
They will start to wear away due to the oxygen gas
Electroplating
A process where the surface of one metal is coated with a layer of a different metal
How does the process of electroplating occur?
- The anode is made from the pure metal you want to coat your object with
- this is so that more ions of the metal will form in the solution as otherwise the salt would run out of the substance - The cathode is the object to be electroplated
- The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode
