Electrolysis Flashcards
What is electrolysis?
- passing electric current through electrolyte
- causes ions to move to electrodes
- positively charged ions -> cathode
- negatively charged ions -> anode
- ions are discharged at electrodes
- produces elements
What is an electrolyte?
- liquid or solution that can conduct electricity
- contains ions that can move
What is the charge of the non metal ion in electrolysis?
- negative
What is the charge of a metal ion in electrolysis?
positive
Give a half equation for the formation of Br2 at the positive electrode
2Br- -> Br2 + 2e-
Give a half equation of the formation of Mg at the negative electrode
Mg2+ + 2e- -> Mg
Why does aluminium need to be extracted from electrolysis?
- it is too reactive to be extracted by reduction with carbon
Why is cryolite added to aluminium oxide?
- to lower the melting point
- reduces energy costs
Why does the carbon anode need to be replaced frequently?
- carbon reacts with oxygen produced to form carbon dioxide
What is an aqueous compound?
- dissolved in water
What is the rule for the product of the negative electrode in the electrolysis of aqueous solutions?
- hydrogen is produced if metal is more reactive than hydrogen
What is the rule for the product of the positive electrode in the electrolysis of aqueous solutions?
- oxygen is produced unless solution contains halide ions (group 7)
Describe the test and expected observations for the hydrogen
- put lit splint near the gas
- products squeaky “pop” sound = hydrogen is present
Describe the test and expected observations for oxygen
- put glowing splint near gas
- if relights = oxygen is present
Describe the test and expected observations for chlorine
- bleaches litmus paper
What is the difference between a molten substance and an aqueous solution?
- molten substance only contains 2 types of ions
- aqueous solutions contains hydrogen and oxygen ions
What does discharged mean?
ion loses its charge and becomes neutral
Describe what happens at the electrodes in terms of reduction and oxidation
- cathode - positively charged ions gain electrons, reduced
- anode - negatively charged ions lose electrons, oxidised
What uses the most energy during the electrolysis of aluminium?
- melting the compounds
- producing electrical current
What is the half equation at the cathode during electrolysis of aluminium oxide?
Al 3+ + 3e- –> Al
What is the half equation at the anode during electrolysis of aluminium oxide?
2O2- –> O2 + 4e-
Reactivity series for electrolysis of aqueous solutions
potassium
sodium
lithium
calcium
magnesium
zinc
iron
hydrogen
copper
silver
hydrogen and up = hydrogen at cathode
hydrogen down = metal at cathode
What do water molecules break down into?
- hydrogen ions H+
- hydroxide ions OH-
What is the half equation for copper sulfate solution?
Cu2+ + 2e- –> H2
4OH- –> O2 + H2O + 4e-
metal less reactive than hydrogen
no halide ion in compound = oxygen
What is the half equation for sodium chloride solution?
2H+ + 2e- –> H2
2Cl- –> Cl2 + 2e-
sodium more reactive than hydrogen
halogen present
Describe the test and expected observations for the carbon dioxide
- turns limewater cloudy when bubbled into it