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chemistry IGCSE > electrochemistry > Flashcards

electrochemistry Flashcards

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1
Q

electrolysis

A

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

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2
Q

electrolytic cell

A

an electrochemical cell in which electrical energy is transferred into chemical energy

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3
Q

What are electrodes?

A

the solid electric conductors through which an electric current enters or leaves an electrolytic cell

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4
Q

What are electrodes usually made of and why?

A

usually made of carbon or platinum due to their unreactivity

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5
Q

cathode

A

negative electrode (where cations (+) go)

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6
Q

anode

A

positive electrode (where anions (-) go)

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7
Q

electrolyte

A

aqueous or molten substance that conducts electricity

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8
Q

main stages in electrolysis

A
  • Electrolyte is made molten or dissolved in water
  • An electric current is passed through the cell
  • Cations move towards the cathode
  • Anions move towards the anode
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9
Q

where do cations come from?

A

metal ions

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10
Q

where do anions come from?

A

non-metal ions

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11
Q

where do metals form at?

A

at the cathode

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12
Q

where do nonmetals form at?

A

at the anode

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13
Q

Stages in electrolysis (oxidation and reduction):

A
  • Electrons flow from the negative terminal of the battery and charge the cathode with negative charge
  • This attracts the cations and makes them move towards the cathode to gain electrons (REDUCTION)
  • The anions move towards the anode with positive charge and lose electrons (OXIDATION)
  • Electrons flow from the anode to the cathode
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14
Q

ionic half-equations showing the reactions occurring at the cathode and anode:

A

At the cathode: cations/metal ions/hydrogen ions gain electrons and reduction happens:
Mn + + n e– → M reduction

At the anode: non-metal ions/anions lose electrons and oxidation happens:
2X– → X2 + 2e– oxidation

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15
Q

(for aqueous electrolytes) what is based on the reactivity series?

A

the product at the cathode. less reactive metals leave the solution first and more reactive metals will be the last to do so

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16
Q

half-equation of cathode (-) reaction for molten lead(II) bromide

A

Pb2+(l)+2e–→Pb(s)

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17
Q

molten lead(II) bromide: product at cathode, anode and observation

A

product at cathode:lead
product at anode: bromine
observation:silvery solid at the cathode
brown gas at the anode

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18
Q

concentrated aqueous sodium chloride (electrolysis of brine): product at cathode, anode and observation

A

product at cathode: hydrogen
product at anode: chlorine
observation:colourless gas at cathode, which makes a lighted splint go ‘pop’ (hydrogen gas)
pale yellow-green gas at anode, which turns universal indicator red
pale yellow-green gas at anode also bleaches red litmus paper as chlorine is an acidic gas

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19
Q

dilute sulfuric acid: product at cathode, anode and observation

A

product at cathode: hydrogen
product at anode: oxygen
obs: colourless gases at both electrodes
cathode gas makes a lighted splint go ‘pop’
anode gas relights a glowing splint

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20
Q

molten sodium chloride: product at cathode and anode

A

cathode: sodium
anode: chlorine

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21
Q

molten silver iodide: product at cathode and anode

A

cathode: silver
anode: iodine

22
Q

molten aluminium oxide: product at cathode and anode

A

cathode: aluminium
anode: oxygen

23
Q

molten lead sulfate: product at cathode and anode

A

cathode: lead
anode: sulfur

24
Q

for aqueous electrolytes, what happens if the cation is below hydrogen in the reactivity series?

A

this will reduce in preference to H+ ions and the metal will form

25
Q

for aqueous electrolytes, what happens if the cation is above hydrogen in the reactivity series?

A

H+ ions will reduce and hydrogen gas forms. The metal ion will stay in solution.

26
Q

aqueous sodium chloride: product at cathode and anode

A

cathode: hydrogen (sodium is above hydrogen in the reactivity series)
anode:chlorine

27
Q

aqueous copper chloride: product at cathode and anode

A

cathode: copper (copper is below hydrogen in the reactivity series)
anode:chlorine

28
Q

what is the product at the anode based on?

A

the electrochemical series

29
Q

what is the electrochemical series?

A

a list of half equations showing the tendency of a reaction to want to be oxidized or reduced

30
Q

if halide ions (like Cl– , Br– or I– ions) are present in the anode, what forms?

A

the element (for example, chlorine, bromine and iodine respectively)

31
Q

if ions such as sulfate (SO42– ) ions, or nitrate (NO3– ) ions are present in the anode, what will form? and why?

A

oxygen will form from the dissociation of OH– ions as nitrate and sulfate ions are more reactive than the hydroxide ions

32
Q

aqueous copper sulfate: product at cathode and anode

A

cathode: copper
anode: oxygen

33
Q

aqueous silver nitrate: product at cathode and anode

A

cathode: silver
anode:oxygen

34
Q

half equation of anode (+) reaction for molten lead bromide

A

2Br–(aq)→Br2(g)+2e–

35
Q

half equation of anode (+) reaction for concentrated aqueous sodium chloride

A

2Cl–(aq)→Cl2(g)+2e–

36
Q

half equation of anode (+) reaction for dilute sulfuric acid

A

4OH–(aq)→2H2O(l)+O2(g)+4e–

37
Q

half equation of cathode (-) reaction for concentrated aqueous sodium chloride

A

2H+(aq)+2e–→H2(g)

38
Q

half equation of cathode (-) reaction for dilute sulfuric acid

A

2H+(aq)+2e–→H2(g)

39
Q

half equation of cathode for aqueous copper(II) sulfate (graphite electrode used)

A

Cu2+(aq)+2e–→Cu(s)

40
Q

half equation of cathode for aqueous copper(II) sulfate (copper electrode used)

A

Cu2+(aq)+2e–→Cu(s)

41
Q

half equation of anode for aqueous copper(II) sulfate (copper electrode used)

A

Cu(s)→Cu2+(aq)+2e–

42
Q

half equation of anode for aqueous copper(II) sulfate (graphite electrode used)

A

4OH–(aq)→2H2O(l)+O2(g)+4e–

43
Q

what happens in electrolysis for dilute solutions?

A

water molecules are present in excess, so both hydrogen and hydroxide ions are discharged preferentially

44
Q

what are the three main applications of electrolysis?

A
  1. electroplating
  2. purification of copper/refining copper
  3. electrolysis of concentrated sodium chloride
45
Q

electroplating

A
  • Used to coat one metal with another
  • Usually to improve the appearance of an object or to protect it from corrosion
  • The object to be electroplated is made the cathode (replacing the cathode by connecting the object to the negative terminal of the power supply)
  • The anode is made from the metal that will provide the coating layer
  • The electrolyte is a solution containing metal ions of the metal providing the electroplating layer
46
Q

copper refining or purification

A
  • Copper metal is extracted from its ore by smelting but it’s not pure enough, so it must be purified using electrolysis
  • Electrolyte- solution of copper sulfate in contact with a pure copper sheet and an impure copper sheet
  • Pure copper sheet = cathode
  • Impure copper sheet = anode
  • Copper sulfate electrolyte consists of:
    • Cuˆ2+ ions
    • SO4ˆ2- ions
  • Cuˆ2+ ions move two the cathode (REDUCTION)
    • Cuˆ2+(aq) + 2eˆ– → Cu(s)
    • Size of the cathode increases because pure copper metal is being deposited at the cathode
  • SO4ˆ2- ions move to the anode
    • Cu(s) → Cu2+(aq) + 2e–
    • Anode gradually disappears and a sludge is left at the bottom of the reactive vessel
      • Anode sludge contains less reactive metals than copper (such as silver or gold). More reactive metals than copper will form ions and dissolve into the solution
47
Q

electrolysis of concentrated aqueous sodium chloride solution

A
  • Brine: aqueous concentrated sodium chloride
  • 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g) 
 brine → sodium hydroxide + chlorine + hydrogen
  • At the cathode: 2Hˆ+ + 2eˆ– → H2
  • At the anode: 2Cl ˆ–(aq) → CI2(g) + 2e-
48
Q

Products of the electrolysis of sodium chloride solution (brine):

A
  • Hydrogen
  • Chlorine
  • Sodium hydroxide
49
Q

Uses of hydrogen:

A
  • Fuel cells
    • Manufacture of margarine and nylon
    • Hydrogen peroxide in the manufacture of ammonia
50
Q

uses of chlorine

A
  • Plastic
    • Solvents
    • Water purification
    • Medical drugs
    • Weed killers
    • Pesticides
51
Q

uses of sodium hydroxide

A
  • Soap and detergents
    • Textiles
    • Papers
    • Dyes
    • Medical drugs

chemistry IGCSE (10 decks)

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