Chapter 4: Electrolysis

Question 1

What is electrolysis?

Answer 1

the breakdown of an ionic compound, molten or in an aqueous solution, by the passage of electricity.

Question 2

What is an important feature of the electrolysis substance?

Answer 2

the substance needs to be molten or dissolved in something.

Question 3

What are the two electrodes in electrolysis called?

Answer 3

anode (+)
cathode (-)

Question 4

What is the molten or dissolved substance called and what is it’s purpose?

Answer 4

It’s called the electrolyte and it’s purpose is to conduct the electricity which is why it needs to be molten so that we have mobile ions moving around.

Question 5

When can we electrolyze molten compounds?

Answer 5

only if they are ionic.

Question 6

What happens at the cathode during the electrolysis of molten lead (ii) bromide?

Answer 6

Lead cations move to cathode
they pick up electron and discharge to form molten lead
Pb2+ (l) +2e- —> Pb (s)
Reduction at cathode

Question 7

What happens at the anode during the electrolysis of molten lead (ii) bromide?

Answer 7

bromide ions move to anode
they loose electrons and discharge becoming neutral bromine gas
2Br- (l) —> Br2 (g) + 2e-
oxidation at anode

Question 8

What happens at the anode during the electrolysis of concentrated sodium chloride solution?

Answer 8

anions (Cl- and OH-) move to anode
Cl- ions oxidize
OH- ions remain in solution

2Cl- —> Cl2 + 2e-

Question 9

What happens at the cathode during the electrolysis of concentrated sodium chloride solution?

Answer 9

cations (Na+ and H2) move to cathodes
H+ ions reduce by discharging
Na+ ions stay on the solution

2H+ + 2e- —> H2

Question 10

How do you tell which type of ion will be discharged at the cathode on a dissolved substance?

Answer 10

-ions of reactive metals will stay in solutions
-less reactive metals discharge and reduce at cathode to form a metal solid

Question 11

How do you tell which type of ion will be discharged at the anode on a dissolved substance?

Answer 11

-usually OH- ions discharge and oxidize to form oxygen gas
-UNLESS… the ion from the compound is a halide then the halide discharges and oxidizes.

Question 11

Electrolysis of hydrochloric acid:

Answer 11

at cathode: 2H+ +2e- —> H2
at anode: 2Cl- —> Cl2 +2e-

Question 12

Electrolysis of dilute sulfuric acid:

Answer 12

at cathode: 2H+ + 2e- —> H2
at anode: 4OH- —> 2H2O + O2 + 4e-