Electrolysis

Question 1

suggest ONE way to remove ions of metal which are LESS REACTIVE than zinc from the zinc sulphate solution before electrolysis

Answer 1

Add Zn(s)/ Mg(s)/ Al(s) to DISPLACE ions of less reactive metals

Question 2

Why it is NOT necessary to remove ions of metals which are MORE REACTIVE than Zn from the solution

Answer 2

1. Zn2+ ions will be preferentially discharged
2. ions of more reactive metals will remain in the solution
   ( Zn > ↑reactive metal in E.C.S/ Zn is a stronger oxidizing agent)

Question 3

Y car bumpus coated with a layer of nickel prior to the chromium-planting process

Answer 3

1. As an undercoat
2. to protect the steel from corrosion
   after the chromium layer has been scratched off

Question 4

Y H2SO4(aq) is used in the chromium-plating bath

Answer 4

1. To increase electrical conductivity/

2. To provide mobile ions

Question 4

Y H2SO4(aq) is used in the chromium-plating bath

Answer 4

1. To increase electrical conductivity/

2. To provide mobile ions

Question 5

Why the volume of chlorine collected is significantly smaller than theoretical volume

Answer 5

1. Chlorine dissolves in water

Cl2(aq) + H2O(I)→ HOCl(aq) + HCl(aq)

Question 6

Why the volume of chlorine collected is significantly smaller than theoretical volume

Answer 6

1. Chlorine dissolves in water

Cl2(aq) + H2O(I)→ HOCl(aq) + HCl(aq)

Question 7

Advantages of carrying out experiments in MICROSCALE

Answer 7

1. Reduce chemical waste produced
2. Shorten the time required for conducting an experiment
3. Require less working space for carry out an experiment

Question 8

state what will happen to the bulb
when heating is stopped
and
the molten lead(ll) bromide is allowed to cool down gradually room temperature

Answer 8

The light bulb gradually goes out.
1. T°C↓,
→movement of ions slow down
→a smaller current flows through the external circuit
2. When PbBr2 becomes solid,
→Ions are no longer mobile
→no currents flow through the external circuit

Question 9

Is it possible to deuce the formula of water from the results of the electrolysis
(Cathode: 2H+(aq) +2e- → H2(g))
(Anode: 4OH-(aq)→ O2(g) + 2H2O(l)+ 4e-)

Answer 9

√
1. volume of H2(g) collected : volume of O2(g) collected
= 2:1
2. In water, H & O combine in mole ratio of 2:1
3. The ATOMICITY of H & O both =2
→ the formula of water = H2O

Question 10

Why is it necessary to coat the handles with copper before they are nickel-plated

Answer 10

To make the knobs a conductor of electricity for the nickel-plating process

Question 11

Nickel Electroplating
Why is it necessary to maintain the pH of the electrolytic bath within a small range
(Add Boric acid to maintain pH: 4~6)

Answer 11

1. At low pH,
   →H+(aq) instead of Ni2+(aq) will be discharged at the cathode.
   →the current efficiency will decrease
2. At high pH,
   →Ni2+(aq) will be precipitated as Ni(OH)2(s)
   ***It is necessary.

Question 12

How boric acid functions in the nikel-plating bath

Answer 12

H3BO3(aq) is a weak acid.
→the dissociation of H3BO3(aq) can
1. replenish the ions lost by discharge at the cathode
2. maintains the pH of the electrolytic bath

Question 13

How impure copper can be purified by electrolysis

Answer 13

1. The impure Cu(s) anode becomes Cu2+(aq)

2. Cu2+(aq) in solution discharge on the pure Cu(s) cathode

Question 14

Why is the concentration of Cu in CuSO4(aq) solution decreases during purification of Cu?

Answer 14

1. At anode,
   → Fe(s) & Zn(s) dissolves as ions(Fe2+(aq) & Zn(aq))
   → they lose e- more readily than Cu
   ∴Cu2+(aq) ions decreases gradually
2. At cathode, Cu2+(aq) ions are always preferentially discharged

Question 15

State TWO advantages of
building a factory in which
CONTACT PROCESS is carried out near the chemical plant (electrolysis: purification of Cu)

Answer 15

1. SO2(g) available as resources for Contact process
2. Prevent air pollution induced by SO2(g)
3. The cost of transportation of SO2(g) is minimized

Question 16

It is known that only cations can pass through the membrane (~porous device).
Y NaOH solution of higher concentration is eventually obtained
(at anode: ↑[NaCl(aq)])
(at cathode: ↓[NaCl(aq)])
(Higher lower)

Answer 16

1. Unreacted Na+(aq) ions the anode side
   → can pass through the membrane to the cathode side
2. ∵H+(aq) ions are discharged at the cathode,
   → OH-(aq) remain/ are excess

→→higher concentration of NaOH is obtained

Question 17

Why it is NOT recommended to use

a high current density in electroplating of Cu by electrolysis

Answer 17

1. H2(g) gas bubbles HINDER THE DEPOSITION of Cu(s) in the surface of the metallic object
2. → a loose spongy Cu deposit can be easily flaked off