Electricity and chemistry

Question 1

Electrolysis

Answer 1

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

Question 2

electrode products and the observations made during the electrolysis of:
– molten lead(II) bromide
– concentrated hydrochloric acid
– concentrated aqueous sodium chloride
– dilute sulfuric acid
between inert electrodes (platinum or carbon)

Answer 2

ionic​ ​solution will​ ​contain the​ ​ions​ ​that​ ​make​ ​up​ ​the​ ​ionic​ ​compound,​ ​and​ ​the​ ​ions​ ​in​ ​water (OH-​ ​​ ​and​ ​H​+)​
● at​ ​the​ ​cathode​ ​(-):
○ hydrogen​ ​(from​ ​H+​ ​​ ​in​ ​water)​ ​is​ ​produced​ ​UNLESS​ ​the​ ​+​ ​ions​ ​in​ ​the​ ​ionic compound​ ​are​ ​from​ ​a​ ​metal​ ​less​ ​reactive​ ​than​ ​hydrogen
○ if​ ​the​ ​metal​ ​is​ ​less​ ​reactive,​ ​it​ ​will​ ​be​ ​produced​ ​instead
● at​ ​the​ ​anode​ ​(+):
○ oxygen​ ​(from​ ​OH​-​​ ​in​ ​water)​ ​will​ ​be​ ​produced​ ​UNLESS​ ​the​ ​ionic​ ​compound contains​ ​halide​ ​ions​ ​(Cl-​ ,​ ​ ​Br​-,​ ​ ​I​-)​
○ if​ ​there​ ​are​ ​halide​ ​ions,​ ​the​ ​halogen​ ​will​ ​be​ ​produced​ ​instead​ ​(e.g.​ ​Cl​2)​
● Electrolysis​ ​of:
○ Molten​ ​lead​ ​(II)​ ​bromide: Pb​2+​​ ​to​ ​cathode​,​ ​Pb​ ​(s)​ ​is​ ​produced​, Br-​ ​​ ​to​ ​anode​,​ ​Br​2​​ ​(l)​ ​is​ ​produced​.
○ Concentrated​ ​hydrochloric​ ​acid: H​+​​ ​ions​ ​go​ ​to​ ​cathode​,​ ​H2​ ​​ ​(g)​ ​is​ ​produced​, Cl-​ ​​ ​ions​ ​go​ ​to​ ​anode​,​ ​Cl​2​​ ​(g)​ ​is​ ​produced​.
○ Sodium​ ​chloride​ ​solution: H​+​​ ​ions​ ​go​ ​to​ ​cathode​,​ ​H2​ ​​ ​(g)​ ​is​ ​produced​ ​(Na​ ​is​ ​more​ ​reactive​ ​than
hydrogen), Cl-​ ​​ ​ions​ ​go​ ​to​ ​anode​,​ ​Cl​2​​ ​(g)​ ​is​ ​produced​ ​(Cl-​ ​​ ​are​ ​halide​ ​ions)
○ Dilute​ ​sulfuric​ ​acid: H​+​​ ​to​ ​cathode​,​ ​H2​ ​​ ​(g)​ ​is​ ​produced​ ​(these​ ​are​ ​the​ ​other​ ​ions​ ​present in​​ sulfuric ​​acid​​ H​2S​O​4), OH-​ ​​ ​to​ ​anode​,​ ​O​ ​ ​(g)​ ​is​ ​produced​ ​(SO​ 2-​​ ​ions​ ​are​ ​not​ ​halide​ ​ions)

Question 3

Relate the products of electrolysis to the electrolyte and electrodes used

Answer 3

●Electrolysis​ ​of​ ​copper​ ​(II)​ ​sulfate​ ​solution
o Cu+​ ​​ ​ions​ ​go​ ​to​ ​cathode​,​ ​Cu​ ​(s)​ ​is​ ​produced​ ​(Cu​ ​is​ ​less​ ​reactive​ ​than
hydrogen)
o OH-​ ​​ ​ions​ ​go​ ​to​ ​anode​,​ ​O​ ​ ​(g)​ ​is​ ​produced​ ​(SO​4 2-​​ ​ions​ ​are​ ​not​ ​halide​ ​ions)
Copper​ ​electrodes:
● set​ ​up:
o anode​ ​is​ ​made​ ​of​ ​impure​ ​copper​ ​(that​ ​you​ ​are​ ​purifying)
o cathode​ ​is​ ​made​ ​of​ ​pure​ ​copper
o the​ ​solution​ ​is​ ​copper​ ​sulfate
● what​ ​happens:
o Cu2​+​​​ions​​ from ​​the ​​anode ​​move​​ to​​ the​​ cathode,​​where​​ they ​​gain​​ electrons and​ ​are​ ​discharged​ ​as​ ​pure​ ​copper
o impurities​ ​form​ ​as​ ​sludge​ ​below​ ​the​ ​anode
● the​ ​cathode​ ​will​ ​increase​ ​in​ ​mass​ ​as​ ​it​ ​gains​ ​pure​ ​copper,​ ​whilst​ ​the​ ​anode​ ​will lose​ ​mass​ ​as​ ​copper​ ​ions​ ​are​ ​lost​ ​(they​ ​replace​ ​the​ ​ones​ ​from​ ​the​ ​CuSO​4​​ ​solution that​ ​go​ ​to​ ​the​ ​cathode)​ ​and​ ​so​ ​are​ ​impurities
Carbon​ ​electrodes​ ​:
● at​ ​the​ ​cathode​ ​Cu​ ​(s)​ ​is​ ​produced​ ​(Cu​ ​is​ ​less​ ​reactive​ ​than​ ​hydrogen)
● at​ ​the​ ​anode​ ​O​ ​ ​is​ ​produced​ ​(SO​ 2-​​ ​ions​ ​are​ ​not​ ​halide​ ​ions) 2​ 4​
● this​ ​leaves​ ​H​+​​ ​and​ ​SO​ 2-​​ ​ions​ ​in​ ​the​ ​solution,​ ​which​ ​will​ ​react​ ​to​ ​form​ ​H​2SO4​-
sulfuric​ ​acid

Question 4

electrolysis in terms of the ions present and reactions at the electrodes in the examples given

Answer 4

● During​ ​electrolysis,​ ​​positively​ ​charged​​ ​​ions​ ​(cations)​ ​​move​ ​to​ ​the​ ​​negative electrode​ ​(cathode)​,​ ​and​ ​​negatively​ ​charged​ ​ions​ ​(anions)​ ​​move​ ​to​ ​the​ ​​positive electrode​ ​(anode).
● Ions​ ​are​ ​discharged​ ​at​ ​the​ ​electrodes​ ​producing​ ​elements.

Question 5

State the general principle that

Answer 5

metals or hydrogen are formed at the negative electrode (cathode), and that non-metals (other than hydrogen) are formed at the positive electrode (anode)

Question 6

electroplating of metals

Answer 6

● electroplating​ ​is​ ​a​ ​method​ ​of​ ​coating​ ​objects​ ​with​ ​a​ ​metal,​ ​using​ ​electrolysis
● Negative​ ​electrode​ ​=​ ​object​ ​to​ ​be​ ​electroplated
● Positive​ ​electrode​ ​=​ ​metal​ ​to​ ​coat​ ​the​ ​object​ ​with
● Electrolyte​ ​=​ ​solution​ ​of​ ​the​ ​coating​ ​metal​ ​(with​ ​ions​ ​of​ ​the​ ​coating​ ​metal​ ​in)
● the​ ​metal​ ​ions​ ​from​ ​solution​ ​are​ ​deposited​ ​on​ ​the​ ​negative​ ​electrode,​ ​and​ ​metal ions​ ​are​ ​released​ ​from​ ​the​ ​positive​ ​electrode​ ​to​ ​replace​ ​those​ ​lost​ ​in​ ​solution

Question 7

uses of electroplating

Answer 7

● Coating​ ​a​ ​cheaper​ ​metal​ ​with​ ​a​ ​more​ ​expensive​ ​one, silver​ ​or​ ​copper
● Silver:Cutlery
● Copper:Metal​ ​pan

Question 8

constructing ionic half-equations

Answer 8

○ negative​ ​electrode:​ ​X​+​​ ​->​ ​X,​ ​so​ ​ionic​ ​equation​ ​must​ ​be:
X​+​​ ​+​ ​e-​ ​​ ​->​ ​X,​ ​electrons​ ​gained,​ ​so​ ​positive​ ​ions​ ​are​ ​reduced
○ positive​ ​electrode:​ ​X​-​​ ​->​ ​X,​ ​so​ ​ionic​ ​equation​ ​must​ ​be:
X​-​​ ​->​ ​e-​ ​​ ​+​ ​X,​ ​electrons​ ​are​ ​lost,​ ​so​ ​negative​ ​ions​ ​are​ ​oxidised

• Oxidation​ ​Is​ ​Loss​ ​(of​ ​electrons)​ ​and​ ​Reduction​ ​Is​ ​Gain​ ​(of​ ​electrons)​ ​-​ ​OIL​ ​RIG
• ANODE​ :loss​ ​of​ ​electrons,​ ​oxidation​ ​(positive​ ​electrode)​ ​anions​ ​come​ ​to​ ​it
• CATHODE​ :gain​ ​of​ ​electrons,​ ​reduction​ ​(negative​ ​electrode)​ ​cations​ ​come​ ​to​ ​it
• “Red Cat An Ox”, oxidation occurs at the anode and reduction occurs at the cathode.

Question 9

reasons for the use of copper and (steel-cored) aluminium in cables, and why plastics and ceramics are used as insulators

Answer 9

• Aluminium​ ​–​ ​good​ ​conductor​ ​of​ ​electricity,​ ​therefore​ ​is​ ​used​ ​in​ ​overhead​ ​power cables
• Copper​ ​–good​ ​conductor​ ​of​ ​electricity,​ ​therefore​ ​is​ ​used​ ​for​ ​electrical​ ​wiring
• Plastics​ ​&​ ​ceramics​ ​do​ ​not​ ​conduct​ ​electricity​ ​–​ ​therefore​ ​they​ ​are​ ​used​ ​as insulators

Question 10

transfer of charge during electrolysis to include:
– the movement of electrons in the metallic conductor
– the removal or addition of electrons from the external circuit at the electrodes
– the movement of ions in the electrolyte

Answer 10

-Movement​ ​of​ ​electrons​ ​in​ ​the​ ​metallic​ ​conductor
o Electrons​ ​move​ ​in​ ​the​ ​wire
o Move​ ​from​ ​positive​ ​electrode​ ​to​ ​negative​ ​electrode

-Movement​ ​of​ ​ions​ ​in​ ​the​ ​electrolyte
o Positive​ ​ions​ ​move​ ​to​ ​negative​ ​electrode​ ​to​ ​receive​ ​electrons​ ​and​ ​are
reduced
o Negative​ ​ions​ ​move​ ​to​ ​positive​ ​electrode​ ​to​ ​lose​ ​electrons​ ​and​ ​are
oxidised
​​ ​​​​ ​​

Question 11

the production of electrical energy from simple cells

Answer 11

• you​ ​have​ ​2​ ​metal​ ​electrodes​ ​in​ ​an​ ​electrolyte​ ​solution​ ​and​ ​connect​ ​the​ ​2 electrodes​ ​by​ ​a​ ​wire​ ​(outside​ ​of​ ​the​ ​solution),​ ​this​ ​creates​ ​a​ ​current​ ​of​ ​electrons and​ ​so​ ​creates​ ​electrical​ ​energy
• More​ ​reactive​ ​metal​ ​will​ ​lose​ ​electrons,​ ​least​ ​reactive​ ​will​ ​gain​ ​them

Question 12

manufacture of:
– aluminium from pure aluminium oxide in
molten cryolite
– chlorine, hydrogen and sodium hydroxide from concentrated aqueous sodium chloride

Answer 12

-Aluminium​ ​is​ ​manufactured​ ​by​ ​the​ ​electrolysis​ ​of​ ​a​ ​molten​ ​mixture​ ​of​ ​aluminium oxide​ ​and​ ​cryolite​ ​using​ ​carbon​ ​as​ ​the​ ​positive​ ​electrode​ ​(anode).
o Aluminium​ ​oxide​ ​has​ ​a​ ​very​ ​high​ ​melting​ ​point,​ ​so​ ​it​ ​would​ ​be​ ​too expensive​ ​to​ ​melt​ ​it,​ ​which​ ​is​ ​why​ ​it​ ​is​ ​mixed​ ​with​ ​cryolite
o the​ ​positive​ ​electrodes​ ​need​ ​to​ ​be​ ​continually​ ​replaced​ ​because​ ​oxygen​ ​is formed,​ ​which​ ​reacts​ ​with​ ​the​ ​carbon​ ​of​ ​the​ ​positive​ ​electrodes,​ ​forming carbon​ ​dioxide,​ ​and​ ​they​ ​gradually​ ​burn​ ​away

-Sodium​ ​chloride
o Chlorine​ ​gas​ ​forms​ ​at​ ​the​ ​positive​ ​electrode
o Hydrogen​ ​gas​ ​forms​ ​at​ ​the​ ​negative​ ​electrode
o Solution​ ​of​ ​sodium​ ​hydroxide​ ​forms
o Must​ ​use​ ​inert​ ​electrodes​ ​since​ ​the​ ​products​ ​are​ ​reactive