Electrolysis Flashcards
How do you know what method to use when extracting a metal?
If below carbon in reactivity series, it can be displaced from its ore by carbon.
If above carbon it can’t, so use electrolysis
What are some of the uses for iron and aluminium?
⇒ iron is used in electromagnets as soft magnetic material, e.g. Doorbell
⇒ aluminium is low density and doesn’t corrode so used for aeroplane bodies
What is electrolysis?
Break down of a substance by electricity
How does electrolysis work?
- the ions in a compound separate
- positive ions go to the cathode (negative electrode) to gain electrons
- negative ions go to anode (positive electrode) to lose electrons
- ∴ become atoms
- no electrons actually pass via the electrolyte, instead they are given in at the anode, transported around the circuit, and given out at the cathode
- because ions in compound are separated, new products are formed
Why does electrolysis only work in molten or in solution ionic compounds
- to conduct must have charged particles, i.e. Electrons or ions, that are free to move
- covalent molecules don’t conduct as haven’t lost/gained electrons- Shared ∴ no electrons free to move
- ionic compounds only conduct when molten/in solution as the pos and neg ions separate ∴ free to move
What happens in molten compounds?
• metals ions go to cathode ∴ gain electrons
• non-metal ions go to anode ∴ lose electrons
∴ no longer charged so become atoms
What happens in aqueous compounds?
- if metal more reactive than hydrogen, H2 formed at cathode. If not (Cu, Ag, Au, Pt), then metal formed
- if non-metal complex, then oxygen formed at the anode. If not (like a halogen) that that is formed
What is faraday’s rule?
• amount of substance produced is proportional to the quantity of charge used
Charge = moles x faradays
• 1 faraday = 96,000 coloumbs
Are the moles of the substance made and the moles of electrons needed the same?
NO
Use the mole ratios to find out the number of mols of electrons from the mols of the substance or vice versa
What are the equations of the electrolysis of brine?
Overall: 2NaCl + 2H2O ⇒ Cl2 + H2 + 2NaOH
Anode: 2Cl- ⇒ 2e- + Cl2
Cathode: 2H+ + 2e- ⇒ H2
Why do they use a membrane cell in the electrolysis of brine?
This allows the hydroxide ions to travel from the anode to the cathode to form NaOH, but doesn’t allow things to go the other way ∴ the hydroxide ions can’t react with the chlorine, which would be explosive
What are the uses of the products of the electrolysis of brine?
Hydrogen- margarine
Chlorine- water treatment, disinfectants
Sodium hydroxide- oven cleaner, soap, rayon, dyes
How do you work out if something is an electrolyte or not?
- set up circuit with bulb and two wires
- out wires into molten/aqueous substance
- if bulbs light up, electrolyte. If not, non-electrolyte
How does the extracting of aluminium from its ore work?
• Al more reactive than C ∴ use electrolysis
cathode: Al3+ + 3e- ➯ Al
Anode: 2O2- ➯ 2O2 + 2e-
Overall: 2Al2O3 ➯ Al + 3O2
• ore (aluminium oxide) must be melted and purified before electrolysis. This needs lots of energy ∴ expensive
• instead use catalyst: molten cryolite, so it melts at 850°C, reducing the operating temperature ∴ the costs
• positive electrodes must be replaced regularly as at high temps the O2 reacts w the C of the electrode to form CO2
• cryolite remains in cell, and fresh Al2O3 is added
What happens in the blast furnace?
• haematite (iron ore) becomes iron
• put in iron ore(often has sand in it), limestone, coke and heat
1. Oxygen in air reacts w the coke (mainly C) to form CO2
2. Limestone (calcium carbonate) decomposes to form CO2 and CaO
3. CO2 reacts w more carbon to for CO
4. CO reduces iron oxide to give molten iron 3CO + Fe2O3 ⇒ 2Fe + 3CO2
5. Limestone from 2 reacts w sand to form slag CaO + SiO ⇒ CaSiO
6. Slag and iron drained from bottom of furnace. Slag ➯ roads. Molten iron ➯ left to solidify, called cast iron, ie. railings. Rest of iron used for steel. Waste gases escape at top
