5. Electrochemistry

Question 1

Describe electrolysis and electrolyte

Answer 1

the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

A substance that conducts electricity when in the molten state or in aqueous state.

Question 2

transfer of charge during electrolysis

Answer 2

the movement of electrons in the external circuit (anode to cathode)
A- +ve electrode containing anion
C- -ve electrode containing cation

the loss or gain of electrons at the electrodes.
anode (oxidation)
cathode (reduction)

the movement of ions in the electrolyte

Question 3

products formed at the electrodes, observation, ionic eqn- molten lead(II) bromide using graphite or platinum electrodes

Answer 3

molten lead(II) bromide

cathode- lead- grey metal
anode- bromine gas- fizzing- brown vapour

Pb^2+ (l) + 2e = Pb(l)
2Br^- (l) = Br^2(l) + 2e

Question 4

products formed at the electrodes, observation, ionic eqn- concentrated aqueous sodium chloride- brine using graphite or platinum electrodes

Answer 4

concentrated aqueous sodium chloride

cathode- hydrogen (easily lose e-) fizzing
anode- Chlorine gas- pale green gas
anode(dilute)- oxygen- fizzing

2H^+ (aq) + 2e = H2(g)
2Cl^- = Cl2(g) + 2e

4OH^-(aq) = 4e- + 2H2O(l)+ O2(g)

Question 5

products formed at the electrodes, observation, ionic equation- dilute sulfuric acid using graphite or platinum electrodes

Answer 5

dilute sulfuric acid

(easily lose e-)
cathode- hydrogen- fizzing
anode- oxygen- fizzing

2H^+ (aq) + 2e = H2(g)
4OH^-(aq) = 4e- + 2H2O(l)+ O2(g)

Question 6

products formed at the electrodes, observation, ionic equation- aqueous copper(II) sulfate using graphite electrodes AND copper electrodes

Answer 6

Graphite electrode-
effervescence, red-brown deposits, blue electrolyte fades

Cu^2+ (aq) + 2e = Cu(s)
4OH^-(aq) = 4e- + 2H2O(l)+ O2(g)

Copper electrode-
red-brown deposits, no effervescence, blue electrolyte stays

Cu^2+ (aq) + 2e = Cu(s)
Cu(s) = Cu^2+(aq) + 2e-

Question 7

difference between copper and carbon electrodes for electrolysis of aqueous copper(II) sulfate.

mass, electrolyte color and replacement, product.

Answer 7

platinum/ graphite-

mass of C ^ mass of A stays same
blue colour of electrolyte fades
electrolyte is replaced
oxygen at cathode.

copper- impure Cu an & pure Cu ca

mass of C ^ mass of A decreases
blue colour of electrolyte stays
electrolyte not replaced
nothing at anode.

Question 8

why and how are metals electroplated

Answer 8

Metal objects are electroplated to
improve their appearance and resistance to corrosion.

cathode- metal to be plated (spoon)
anode- metal used for plating (silver)
electrolyte- ionic compound electrolyte (silver nitrate)

Question 9

Describe a hydrogen–oxygen fuel and describe the advantages and disadvantages of using hydrogen oxygen fuel cells in comparison with gasoline /petrol engines in vehicles.

Answer 9

A hydrogen–oxygen fuel cell uses
hydrogen and oxygen to produce electricity with water as the only chemical product.

A- makes non- polluting water as the only product, no external changing required, capable of producing electricity as long as reactants are supplied

D- gas at rtp so difficult to store in car. no infrastructure- eg- no. of refuelling stations. less durable so not as long lasting. expensive.