C6 - Electrolysis Flashcards
What is an anode?
the positive electrode in electrolysis
What is brine?
concentrated sodium chloride solution that can undergo electrolysis to produce chlorine gas, hydrogen gas, and sodium hydroxide solution
What is a cathode?
the negative electrode in electrolysis
What is an electrolyte?
a liquid, containing free-moving ions, which is broken down by electricity in the process of electrolysis
What is a half equation?
an equation that describes reduction or oxidation
What does inert mean?
unreactive
What are cations?
positive ions
What are anions?
negative ions
Where do cations move towards during electrolysis?
the cathode
Where do anions move towards during electrolysis?
the anode
What are the electrodes made out of?
carbon graphite because it is a conductor, but also unreactive and has a high melting point
What happens to the ions at the electrodes?
they are discharged and they form elements
What is electrolysis?
The decomposition of an ionic compound by electricity
Why can’t covalent substances be broken down by electrolysis?
For a substance to be broken down by electrolysis, it must contain ions as they have a charge. Covalent substances do not contain charged ions, only neutral atoms
What type of reaction happens at the anode and why?
Oxidation, this is because the negative ions lose electrons to the positive anode
What type of reaction happens at the cathode and why?
Reduction, Positive ions gain electrons from the negative cathode
Describe the movement of electrons during electrolysis
Electrons move from the positive anode towards the negative electrode through the circuit
What is the -ve electrode?
Cathode (negative)
What is the +ve electrode?
Anode (Positive)
Describe what would happen at the -ve electrode in the electrolysis of PbBr₂(l)
- Grey/black deposit
- Pb²⁺ ions attracted
- Half equation: Pb²⁺ + 2e⁻ -> Pb
- Gains electrons therefore reduction
Describe what would happen at the +ve electrode in the electrolysis of PbBr₂(l)
- Bubbles of red-brown vapour
- Br⁻ ions attracted
- half equation: 2Br⁻ -> Br₂ + 2e⁻
- Loses electron therefore oxidation
Describe what would happen at the -ve electrode in the electrolysis of NaCl(l)
- Grey/black deposit
- Na⁺ ions attracted
- half equation: Na⁺ + e⁻ -> Na
- Gain of electrons therefore reduction
Describe what would happen at the +ve electrode in the electrolysis of NaCl(l)
- bubbles of green vapour
- Cl⁻ ions attracted
- half equation: 2Cl⁻ -> Cl₂ + 2e⁻
- Loses electrons, therefore oxidation
Describe what would happen at the -ve electrode in the electrolysis of Al₂O₃(l)
- grey/black deposit
- Al³⁺ ions attracted
- half equation: Al³⁺ + 3e⁻ -> Al
- Gains electrons, therefore reduction
Describe what would happen at the +ve electrode in the electrolysis of Al₂O₃(l)
- Bubbles of colourless vapour
- O²⁻ ions attracted
- half equation: 2O²⁻ -> O₂ + 4e⁻
- Loses electrons, therefore oxidation
Why does electrolysis only work when the ionic substance is molten?
The ions are only free to move once molten or as an aqueous solution, as a solid they are fixed in a structure and cannot move
Which ions are attracted towards the positive electrode and why?
anions (negative ions) are attracted towards the positive electrode because they are oppositely charged
Which ions are attracted towards the negative electrode and why?
cations (positive ions) are attracted towards the negative electrode because they are oppositely charged
What is the product at the cathode during the electrolysis of an aqueous solution?
At the negative electrode, Hydrogen will be formed unless the metal is less reactive tha hydrogen
What is the product at the anode during the electrolysis of and aqueous solution?
At the positive electrode, oxygen is produced unless the solution contains halide ions
What metals are less reactive than hydrogen?
Copper, Silver, Gold, Platinum
What is the equation for the product at the cathode in the electrolysis of aqueous solutions
Metal less reactive than hydrogen: Xʸ⁺ + ye⁻ -> X
No metal less reactive than hydrogen: 2H⁺ +2e⁻ -> H₂
What is the equation for the product at the anode in the electrolysis of aqueous solutions
Halide: 2X⁻ - 2e⁻ -> X₂
No Halide: 4OH⁻ - 4e⁻ -> O₂ + 2H₂O
What is the product at the cathode in the electrolysis of NaCl(aq)?
H₂
What is the product at the anode in the electrolysis of NaCl(aq)?
Cl₂
What is the product left in the solution in the electrolysis of NaCl(aq)?
NaOH
What is the product at the cathode in the electrolysis of CuSO₄(aq)?
Cu
What is the product at the anode in the electrolysis of CuSO₄(aq)?
O₂
What is the product left in the solution in the electrolysis of CuSO₄(aq)?
H₂SO₄
What is the product at the cathode in the electrolysis of CuCl₂(aq)?
Cu
What is the product at the anode in the electrolysis of CuCl₂(aq)?
Cl₂
What is the product left in the solution in the electrolysis of CuCl₂(aq)?
H₂O
What is the product at the cathode in the electrolysis of Pb(NO₃)₂(aq)?
H₂
What is the product at the anode in the electrolysis of Pb(NO₃)₂(aq)?
O₂
What is the product left in the solution in the electrolysis of Pb(NO₃)₂(aq)?
Pb(NO₃)₂
What is the equation for the electrolysis of water?
H₂O -> H⁺ + OH⁻
What is the half equation for the negative electrode in the electrolysis of KCl(l)?
K⁺ + e⁻ -> K
What is the half equation for the positive electrode in the electrolysis of KCl(l)?
2Cl⁻ -> Cl₂ + 2e⁻
What is the half equation for the negative electrode in the electrolysis of KCl(aq)?
2H⁺ + 2e⁻ -> H₂
What is the half equation for the positive electrode in the electrolysis of KCl(aq)?
2Cl⁻ -> Cl₂ + 2e⁻
What is the half equation for the negative electrode in the electrolysis of ZnCl₂(l)?
Zn²⁺ + 2e⁻ -> Zn
What is the half equation for the positive electrode in the electrolysis of ZnCl₂(l)?
2Cl⁻ -> Cl₂ + 2e⁻
What is the half equation for the negative electrode in the electrolysis of CaSO4(aq)?
2H⁺ + 2e⁻ -> H₂
What is the half equation for the positive electrode in the electrolysis of CaSO4(aq)?
2O²⁻ - 4e⁻ -> O₂
Describe the method for the electrolysis of aqueous solutions
1) Half fill a beaker with the solution to be electrolysed
2) Submerge the ends of the two electrodes in the solution, and using the crocodile clips and
wires connect one to the low voltage power supply and the other to a light bulb. Connect the other end of the light bulb to the low voltage power supply.
3) Make sure that the electrodes are not touching
4) Plug in the power supply then turn on and observe the solution
5) Look for bubbling at each electrode, test using litmus paper to find out which gas it is
6) If there is not bubbling at both electrodes, leave the cell connected for 5 minutes and then disconnect and study the electrodes for evidence of metal coating
What is the test for Hydrogen?
Hold a lit splint near the gas being tested, if you hear a ‘pop’ sound, the gas is hydrogen
What is the test for Oxygen?
Hold a glowing splint near the gas being tested, if it relights, the gas is oxygen.
What is the test for Chlorine gas?
Hold a piece of damp blue litmus paper near the gas being tested. If the litmus paper turns red and then bleaches, the gas is chlorine.
What is the observation of sodium during electrolysis?
grey coating on the electrode
What is the observation of copper as a product during electrolysis?
brown coating on the electrode
Explain why hydrogen will form at the cathode in the electrolysis of some aqueous solutions rather than a product from the discharge of ions in the compound dissolved in the solution.
Hydrogen will form at the cathode because it is less reactive than the product formed. This means that hydrogen will remain as an ion and be attracted to the cathode whereas the more reactive product will react with other substances and not be attracted to the cathode.
Explain why oxygen will form at the anode in the electrolysis of some aqueous solutions rather than a product from the discharge of ions in the compound dissolved in the solution.
Oxygen will form at the anode because it is less reactive than the product formed. This means that oxygen will remain as an ion and be attracted to the anode whereas the more reactive product will react with other substances and not be attracted to the anode.
What is the substance broken down in electrolysis called?
Electrolyte
What does the mnemonic PANIC stand for?
Positive Anode, Negative Is Cathode
Why are the electrodes made of inert substances?
So they do not react with the products
Which ore do you get aluminium oxide from?
bauxite
Why can aluminium not be extracted form aluminium oxide by a displacement reaction with carbon?
Aluminium is more reactive than carbon meaning it cannot be displaced by it
Why is aluminium oxide mixed with cryolite during the electrolysis process?
The electrolysis of aluminium oxide requires large amounts amounts of energy. Aluminium oxide melts at over 2000C. Aluminium oxide is mixed with cryolite, which lowers the melting point of the mixture. The molten mixture can then be electrolysed at 850C. This saves money as not as much energy is required
What is the chemical formula of aluminium oxide?
Al₂O₃
What is the overall equation for the electrolysis of Aluminium Oxide?
2Al₂O₃(l) -> 4Al(l) + 3O₂(g)
What is the half equation at the cathode for the electrolysis of Aluminium Oxide?
Al³⁺ + 3e⁻ -> Al
What is the half equation at the anode for the electrolysis of Aluminium Oxide?
2O²⁻ -> O₂ + 4e⁻
Why do the carbon anodes have to be replaced regularly in the electrolysis of Aluminium Oxide?
The oxygen that forms at the carbon anodes reacts to produce CO₂ (C + O₂ -> CO₂)
How and why does the cell used for the electrolysis of aluminium oxide differ from normal electrolysis cells?
The anodes are suspended within the solution but the cathode is the lining of the cell. This is so the molten aluminium collected at the cathode can be tapped off from the cell
Describe the process of extracting aluminium from the earth
1) The ore bauxite is mined from the ground
2) The bauxite is purified and aluminium oxide is collected
3) The pure aluminium is extracted from the aluminium oxide by electrolysis
Describe how oxygen is formed during the electrolysis of water
Water is broken down into H+ and OH- ions. OH- ions are attracted to the anode, lose electrons and form oxygen gas.
What is the half equation for the formation of oxygen during electrolysis of water?
4OH–(aq) - 4e- → 2H2O(l) + O2(g)
What is the half equation for the formation of hydrogen during electrolysis?
4H+(aq) + 4e– → 2H2(g)
Describe what forms at the anode in the electrolysis of Sodium sulfate solution and why
At the anode, hydroxide ions lose electrons to form oxygen gas and water - this is given by the half equation “4OH- - 4e- -> 2H2O + O2”. oxygen atoms form as hydroxide ions are less reactive than sulfate ions.
Describe what forms at the cathode in the electrolysis of Sodium sulfate solution and why
At the cathode hydrogen ions gain electrons to form hydrogen ions - this is given by the half equation “2H+ + 2e- -> H2”. Hydrogen forms as the hydrogen ions are less reactive than the sodium ions
Describe how ions are attracted to the electrodes during electrolysis.
When a current is passed through the electrolyte, the electrodes become charged and attract oppositely charged ions.
Why may the volume of chlorine produced in the electrolysis of sodium chloride solution not reflect the amount of moles produced?
Chlorine may dissolve in the solution
