C6 - Electrolysis Flashcards
What is an electrolyte?
-when an ionic compound is melted/dissolved in water, the individual ions can move about in the liquid/solution
-they can now conduct electricity, and are called electrolytes
In short, a liquid containing ions that are free to move about
What is an anode?
the electrode where oxidation takes place (applies to both electrolysis cells and electrochemical cells, including fuel cells)
Not necessarily always positively charged
For electrolysis, what charges do the cathode and anode have?
-cathode is negative
-anode is positive
It is the other way around for fuel cells
Describe what happens when an electric current is passed through an electrolyte between 2 electrodes:
-ions move to the electrodes and discharge, forming elements (electrolysis)
-cations move to cathodes (negative electrode)
-anions move to anodes (positive electrode)
Why do cations move to the cathode during electrolysis?
-cations have a positive charge
-cathode has a negative charge
-cation is attracted to opposite electric charge
-cation is free to move about in the electrolyte, so it moves to the cathode
Explain what happens when a molten ionic compound undergoes electrolysis:
-metal produced at cathode
-non-metal produced at anode
How can electrolysis be used in the extraction of metals?
-metals can be extracted from an ionic compound with electrolysis
-only used if the metal is more reactive than carbon, or if it reacts with carbon
Describe an example of where electrolysis is used to extract a particular metal:
-aluminium is extracted from a molten mixture of aluminium oxide and cryolite using carbon as the anode
-the cryolite lowers the melting point of aluminium oxide to ~1000°C
What is the half equation for the reaction at the positive electrode during the electrolysis of molten aluminium oxide and cryolite?
the oxygen formed here then reacts with the electrode to form carbon dioxide
State some drawbacks of using electrolysis to extract aluminium from aluminium oxide:
-very expensive (lots of energy used to supply current, and increase temperature to melt the compounds)
-anode must be continually replaced (reacts with the oxygen from the oxide to produce CO₂)
When aluminium is extracted from the electrolysis of aluminium oxide and cryolite, why does it come out as a liquid?
-high temperatures used
-aluminium has a melting point below that temperature
Does pure water only contain H₂O molecules?
-no
-it also contains equal amounts of H⁺ and OH⁻ (to keep it neutral)
-this happens because sometimes the thermal energy of the water causes them to split up, even though it is more stable as a single molecule
Only about 1 in 10⁷ molecules of water will split up though
What happens when an aqueous solution is electrolysed?
-the ions discharged at the electrodes depend on their relative reactivity
cathode - the least reactive element is formed here (could be hydrogen, or the metal)
anode - oxygen is produced, unless the solution contains halide ions, which would mean the halogen gets produced
Explain why the element produced at the cathode during the electrolysis of an aqueous solution depends on its reactivity:
-the cathode has an excess of e⁻
-the ions that get discharged at the cathode must have the strongest attraction to the e⁻
-the least reactive element will have the strongest attraction to the e⁻ because the less reactive a metal is, the stronger its electrostatic force on its outer electrons is
-therefore, the least reactive element will be produced at the cathode
Describe, using chemical equations, what happens at the electrodes during electrolysis:
-at cathode, cations are reduced (eg 2H⁺ + 2e⁻ → H₂)
-at anode, anions are oxidised (eg 4OH⁻ → O₂ + 2H₂O + 4e⁻)
Don’t confuse this with the overall reaction for fuel cells which produces water. The overall reaction for electrolysis doesn’t do anything important apart from turn the ions into elements.
Explain why electrolysis would not take place using the apparatus above:
-solid zinc chloride does not conduct electricity
-because its ions cannot move
This apparatus is used to see how the volume of gases produced changes with time during electrolysis. How should it be changed?
-use measuring cylinders instead of test tubes
-because test tubes can’t measure volume because they have no scale on the side
The same amount in moles of gas is produced at each electrode, and no gas has escaped the reaction container. Suggest one reason for the difference in volume of gases collected:
some oxygen gas dissolved in the water
Can happen with other gases like chlorine too
