chemistry - chemical changes: electrolytic processes (3.22 - 3.31)

3.22 electrolytes definition

ionic compounds molten/dissolved in water

3.23 electrolysis

energy from (2 electrodes connected to) d.c. supply decomposes electrolytes

3.24 movement of cations in electrolysis

+ cations attracted to & migrate to - cathode

3.24 movement of anions in electrolysis

- anions attracted to & migrate to + anode

3.25 rules for formation of products in electrolysis

anode:

• anions → anode: oxidation (e^- lost)

1. halide ions discharged as halogen gases (e.g. Cl^- → Cl₂)

2. hydroxide ions discharged as oxygen gas (OH^- → O₂)

cathode:

• cations → cathode: reduction (e^- gained)

• copper/silver/gold/platinum discharged as solid (e.g. Ag⁺ → Ag)

• hydrogen gas discharged

(anything less reactive than hydrogen discharged, otherwise hydrogen discharged)

3.25 formation of products in electrolysis (inert electrodes) of copper chloride solution

anions → anode: chlorine & hydroxide

• chlorine gas produced

cations → cathode: copper & hydrogen

• solid copper produced

3.25 formation of products in electrolysis (inert electrodes) of sodium chloride solution

anions → anode: chlorine & hydroxide

• chlorine gas produced

cations → cathode: sodium & hydrogen

• hydrogen gas produced

3.25 formation of products in electrolysis (inert electrodes) of sodium sulfate solution

anions → anode: sulfate & hydroxide

• oxygen gas produced

cations → cathode: sodium & hydrogen

• hydrogen gas produced

3.25 formation of products in electrolysis (inert electrodes) of water acidified with sulfuric acid

(acidified water: water that has had acid added to it)

anions → anode: sulfate & hydroxide

• oxygen gas produced

cations → cathode: hydrogen & hydrogen

• hydrogen gas produced

3.25 formation of products in electrolysis (inert electrodes) of molten lead bromide

anions → anode: bromine

• bromine gas produced

cations → cathode: lead

• solid lead produced

3.26 predict products of electrolysis of other binary, molten ionic compounds

identify elements in ionic compound

+ metal ion produced at cathode

- non-metal ion produced at anode

3.27 half equations for reactions at anode & cathode in electrolysis

e.g. electrolysis of molten zinc chloride

2Cl^- → Cl₂ + 2e^-

• Cl^- → anode: lose electrons, become chlorine molecules (oxidation)

Zn²⁺ + 2e^- → Zn

• Zn²⁺ → cathode: gain electrons, become zinc atoms (reduction)

3.28 oxidation & reduction

Oxidation Is Loss of electrons

Reduction Is Gain of electrons

OILRIG

3.29 which electrodes do oxidation & reduction occur at in electrolysis?

OXidation at ANode (oxidation = loss of e^-: - anions → + anode, lose e^-)

REDuction at CAThode (reduction = gain of e^-: + cations → - cathode, gain e^-)

RED CAT, AN OX

3.30 formation of products in electrolysis (copper electrodes) of copper sulfate solution - purifying copper

copper atoms in anode —lose e^-→ copper ions

dissolve in solution, migrate to cathode - deposited as pure copper

impurities from anode don’t form ions, collect below anode as ‘sludge’

3.31 core practical: investigate electrolysis of copper sulfate solution with inert electrodes & copper electrodes

1. select 2 clean pieces of copper foil, label one ‘anode’ & other ‘cathode’

2. measure & record masses of 2 electrodes

3. set up electrolysis circuit as in diagram

4. turn on power & adjust variable resistor to give current about 0.2A, record current & adjust variable resistor to keep it constant, leave power on for 20 mins

5. turn off power & remove electrodes from beaker

6. wash electrodes with distilled water, dip into propanone, lift out & shake off propanone, let rest of propanone evaporate

7. measure & record masses of dry electrodes

8. repeat experiment using currents 0.3A, 0.4A & 0.5A