Electrolysis Flashcards
Electrolysis
Is the process which uses electricity to split up molten or aqueous ionic compounds.
Electrolyte
Is a substance which conducts electricity and can be split up by a current when molten or in a solution. It will contain ions which are able to move.
Ions
Are charged particles formed when atoms lose or gain electrons.
Cation
Positively charged ion (goes to cathode).
Anion
A negatively charged ion (goes to anode).
Oxidation
Is a type of reaction involving the gain of oxygen or the loss of electrons.
Reduction
A type of reaction involving the loss of oxygen or the gain of electrons.
Redox
A type of reaction in which oxidation and reduction take place at the same time.
Electrode
Is a solid conductor of electricity, which is used to make electrical contact with an electrolyte.
When does hydroxide (OH-) go to the anode?
Unless the other negatively charged ion is a halide, hydroxide will go to the anode and form oxygen gas.
E.G CL- or OH-, CL- will go.
What happens when the hydroxide goes to anode?
Hydroxide goes to the anode but forms oxygen gas.
When does hydrogen go to the cathode?
If the metal is more reactive than hydrogen, hydrogen will go to the cathode and form H2.
If the metal is less reactive than hydrogen, that metal ion will go to the cathode (copper, silver and gold).
What is an electric current?
A flow of electrons or ions
What is the anode and what is the cathode?
PANiC
The anode is positive - Oxidation always happens at the anode
The cathode is negative - Reduction always happens at the cathode
What are the electrodes usually made out of?
Graphite as it is cheap and doesn’t decay quickly
When molten lead bromide is electrolysed what goes to the anode?
Bromide ions as the negative bromide ions are attracted to the positive anode
-When the bromIDE ions get to the anode, the extra electron which makes the bromide ion negatively charged moves onto the electrode because this electrode is short of electrons
-The loss of the extra electron turns each bromIDE ions into a bromINE atom. These join in pair to make bromine molecules
2Br- –>Br2 + 2e-
When molten lead bromide is electrolysed what goes to the cathode?
Lead ions as the negatively charged lead ions are attracted to the negative cathode
-When the positive lead ions reach the cathode, they pick up two electrons from the electrode to form neutral lead atoms. Since they have lost their charge they fall to the bottom of the container as molten lead
Pb2+ + 2e- –> Pb
What happens when the power is switched on but the lead bromide is still solid?
- As soon as you connect the power source, it pumps mobile electrons away from the left-hand electrode to the right-hand electrode
- The excess of electrons on the right-had electrode makes it negatively charged. The left-hand electrode is positively charged because it is short of electrons. There is a limit to how many electrons the ‘pump’ can squeeze into the negative electrode because of the repulsion by the electrons already there
What happens when lead bromide is electrolysed?
When the lead bromide melts:
- The bulb lights up showing that electrons are flowing through it
- There is bubbling around the anode as bromine gas is given off
- Nothing seems to happen at the cathode but afterwards metallic lead is found underneath it
- When you stop heating lead (II) bromide solidifies again, everything stops - there is no more bubbling and the bulb goes out
Can all ionic compounds be electrolysed?
Some can’t even when they are molten as their melting point is too high e.g. sodium chloride (801℃) but it ca be done industrially
What happens during the electrolysis of sodium chloride solution (Brine)?
- Cl- goes to the anode (oxidised) (colour of litmus is white (bleached))
- Two chloride ions lose their electrons and become one chlorine molecule
- Chlorine gas is given off - H+ goes to the cathode (reduced) (colour of litmus is blue)
- Two hydrogen ions accept two electrons to become one hydrogen molecule
- Hydrogen gas is given off. - NaOH is left in solution
What is the equation for the electrolysis of sodium chloride solution (Brine)?
2NaCl + 2H2O –> H2 + Cl2 + 2NaOH
Describe a bit about the chlor-alkali industry
- In the modern method salt is pumped out of the ground as brine
- Beginning of 19th century salt was used to make soap, bleach and soda (domestic) and salt products to make textile, paper and glass (industry)
- Electrolysis is used to decompose sodium chloride solution
What are some uses for the products of the electrolysis of sodium chloride solution (Brine)?
- Chlorine: manufacture of bleach and HCL, PVC, disinfectant (so kills bacteria), sterilising water supplies (chlorination)
- Hydrogen: fuel cells, used in Haber process, to make ammonia (fertiliser), to change oils into fats for making margarine
- NaOH: manufacture of bleach, paper and soap
- Sodium hydroxide is a very strong base and is used widely in the chemical industry
In a diaphragm cell which particles are prevented from passing through the porous diaphragm?
- Chlorine (chloride ions and not chlorine gas) (and hydrogen??)
- If chlorine comes into contact with the sodium hydroxide solution, it reacts to make bleach
- If chlorine comes into contact with hydrogen, it produces a mixture which would explode violently on exposure to sunlight or heat to give hydrogen chloride
How has the NaOH become alkaline?
As the NaCl contains a halide, that goes to the cathode to form chlorine, leaving OH- in the solution which is alkaline, the H+ ions have gone to the cathode and been removed
The OH- ions join with the Na+ ions left, to form NaOH which is alkaline
In general aqueous solutions what goes to the cathode? (m+ or H+)
- If the metal is more reactive than hydrogen (the only ones which are less are copper, gold and silver), the H+ ions will go to the cathode to form H2: 2H+ + 2e- –> H2
- If the metal is less reactive than hydrogen the metal ion goes to the cathode, as since it is below hydrogen int he reactivity series its ion is easier to discharge: Cu2+ (aq) + 2e- –> Cu (s)
In general aqueous solutions what goes to the anode? (OH- or x-)
- If x- is a halide (F-, Cl-, Br-, I-) than the halide forms at the anode: 2F- –> 2e- + Cl2
- In all other cases (since they take more time to get to the anode as they are bigger and bulkier than OH-), OH- ions go to the anode and from oxygen gas (O2): 4OH- (aq) –> O2 (g) + 2H2O (l) + 4e-
What are the typical equation in general aqueous solutions?
H2O –> H+ + OH -
mx (aq) –> m+ + x-
Describe what happens in the electrolysis of Dilute Sulphuric acid?
- Twice as much hydrogen is produced than oxygen
- Every time one O2 is formed, you can get two H2.
- For every four electrons that flow around the circuit you can get one molecule of oxygen, but you can also can instead two molecules of hydrogen
- There is twice as much hydrogen produced as oxygen because each oxygen releases four electrons and each hydrogen only needs two electrons. therefore twice as much hydrogen is produced
- You get twice the number of molecules of hydrogen as of oxygen, and twice the number of molecules occupy twice the volume
- The concentration of sulphuric acid is increasing as the water is electrolysed and oxygen and hydrogen are lost from the solution
- You would not be able to use HCL as it has a halogen so chlorine could get mixed
Describe what happens in the electrolysis of Dilute Sulphuric acid?
- Product at the cathode: the carbon electrode is coated in copper
- Product at the anode: oxygen
- Left in the solution: hydrogen and sulphate ions, which will turn into dilute sulphuric acid, the electrolysis will then continue as for dilute sulphuric acid
- Eventually after a very long time, all the copper (II) ions will be used up and so the colour of the solution will fade from blue to colourless
What happens at the anode during the purification of copper?
At the anode the Cu atoms are pulled from the anode to become positively charged Cu2+ ions.
The Cu2+ ions are repelled from the anode (as it is positively charged) and flows through the solution and is attracted to the cathode (as it is negatively charged).
The Cu2+ ions are then deposited as copper atoms and stick to the cathode due to a metallic bond
What happens at the cathode during the purification of copper?
At the cathode the Cu2+ ions gain electrons to become copper atoms.
The copper atoms ‘stick’ to the copper cathode forming a new layer
What happens to the size of the electrodes during the purification of copper?
The pure copper cathode, increases in size, whilst the anode gets smaller. The impurities left behind at the anode form a silt layer/mud (usually gold Au/silver Ag) beneath it
What actually happened experimentally?
(The mass lost by the anode should equal the mass gained by the negative electrode?)
However the mass gained by the cathode was less that the mass lost by the anode why:
-The anode may have lost impurities
-Some of the copper may have feel into the solution (weak attachment) and even more when handling
-Cathode was still wet (we didn’t use propane) (wouldn’t the results be the other way round?) but more could have been lost in the drying process (like above second statement)
-Some copper ions could have come from the copper sulphate solution (wouldn’t the results be the other way round?)
ALWAYS assume that there are some impurities
What does electrolysis involve?
- The formation of new substances when ionic compounds conduct electricity
1. If you pass an electric current through an ionic substance that is molten or in solution it breaks down into new substances, this is called electrolysis
2. It requires a liquid to conduct the electricity called the electrolyte
3. Electrolytes are made by meting or dissolving ionic compounds
4. In either case it is the free ions which conduct the electricity
5. For the circuit to be complete, there has got to be a flow of electrons. Electrons are taken away from the ions at the anode and given to other ions at the cathode
6. As ions gain or lose electrons they become atoms or molecules
How do you distinguish between electrolytes and non-electrolytes?
- Electrolytes are liquids that conduct electricity
1. When you place a conductivity probe in an electrolyte, current flows through the circuit, so you can measure its conductivity
2. When you place a conductivity probe in a non-electrolyte, no current flows, so you will get a reading of zero conductivity
3. Another way of determining whether a substance is an electrolyte or not is to set up an electrolytic cell and if the substance undergoes electrolysis the it is an electrolyte
Describe the electrolysis of copper (II) sulphate
- A solution of copper (II) sulphate (CuSO4) contained four different ions, Cu 2+, SO4 2-, H+ and OH-
- Cooper ions accept electrons more easily than hydrogen ions, so at the cathode, copper metal is produced
- Hydroxide ions lose electrons more easily than sulphate ions, so at the anode oxygen and water are produced
How is sodium hydroxide and chlorine manufactured?
By the electrolysis of concentrated sodium chloride solution (brine) in a diaphragm cell
