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Chemistry > UNIT 6: electricity and chemistry > Flashcards

UNIT 6: electricity and chemistry Flashcards

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1
Q

define electrolysis

A

the decomposition of an ionic compound by the passage of electricity when it is either in a molten state or in an aqueous solution

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2
Q

why must they be in molten or aqueous solution?

A

ionic compounds only conduct electricity in these states, as the ions are free to move

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3
Q

electrolyte definition

A

the compound that conducts electricity when molten or in an aqueous solution and decomposes

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4
Q

what moves: the ions or the electrons?

A

the ions

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5
Q

electrode definition

A

rods that carry the electric current to and from the electrolyte. they are normally made out of an inert conductor (e.g. graphite or platinum) so that they don’t react with the electrolyte

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6
Q

state the positive electrode

A

anode

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7
Q

state the negative electrode

A

cathode

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8
Q

describe the products and observations in the electrolysis fo molten lead (II) bromide

A
  • bromine gas (reddish-brown gas) at anode
  • lead (grey molten beads) at the cathode
    PbBr₂ (l) → Pb (l) + Br₂ (g)
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9
Q

what is formed at the cathode and what is formed at the anode

A 
cathode= metal 
anode= non-metal
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10
Q

what would the products be of the electrolysis of molten sodium chloride?

A

sodium (cathode) and chlorine (anode)

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11
Q

products of electrolysis of aqueous sodium chloride

A

hydrogen at cathode

chlorine at anode

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12
Q

what extra ions are found in aqueous solutions?

A

H⁺ + OH⁻

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13
Q

what series is used in electrolysis?

A

discharge series

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14
Q

does the discharge series go most likely to least likely or least likely to most?

A

least likely to most (so when going down, the more likely an ion is of reacting)

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15
Q

what is the discharge series for positive ions?

A

Na⁺

Mg²⁺

Al³⁺

H⁺

Cu²⁺

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16
Q

what is the discharge series for negative ions?

A

SO₄²⁻

NO₃⁻

OH⁻

Cl⁻

Br⁻

I⁻

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17
Q

products of electrolysis of concentrated hydrochloric acid when using inert electrodes + what happens to acid

A

cathode= hydrogen
anode= chlorine gas
acid gets used up as concentration of hydrogen ions falls

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18
Q

what is brine

A

concentrated aqueous solution of sodium chloride

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19
Q

what is brine used to mass produce?

A

chlorine, hydrogen, sodium hydroxide

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20
Q

what is chlorine used for

A

make solvents, treat drinking water, make bleaches

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21
Q

what is hydrogen used for

A

make ammonia, make margarine, as a fuel

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22
Q

what is sodium hydroxide used for

A

make soap and in the extraction of aluminium

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23
Q

what cell is brine electrolysed in?

A

diaphragm cell

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24
Q

what are the ions in brine?

A

Na⁺, H⁺, Cl⁻, OH⁻

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25
Q

when electrolysing brine, what happens at anode and cathode?

A

cathode: hydrogen ions accept electrons and hydrogen gas is discharged (2H⁺ (aq) + 2e⁻ → H₂ (g))
anode: chlorine ions lose electrons and chlorine gas is discharged ( 2Cl⁻ (aq) → Cl₂ (g) + 2e⁻)

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26
Q

what is left remaining in the solution after electrolysing brine?

A

aqueous solution of sodium hydroxide

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27
Q

what is added to electrolyse water? why?

A

some sulfuric acid (H⁺ and OH⁻) as to improve conductivity

28
Q

product of electrolysing water

A

hydrogen at cathode and oxygen (from the decomposing of OH⁻ ions) at anode

29
Q

in aqueous solutions, what can be formed in the cathode and anode?

A 
cathode= metal or hydrogen
anode= halogen or oxygen
30
Q

name of positive electrons and what electrode they’re attracted to

A

cations, cathode

31
Q

name of negative electrons and what electrode they’re attracted to

A

anions, anodes

32
Q

oxidation and reduction: which is loss and which is gain of electrons

A

oxidation is loss, reduction is gain (OIL RIG)

33
Q

what do half equations show?

A

what is happening at one electrode

34
Q

what are both half equations for the electrolysis of molten lead (II) bromide?

A

cathode: Pb²⁺ (aq) + 2e⁻ → Pb (l)
anode: 2Br⁻ (aq) → Br₂ (g) + 2e⁻

35
Q

what is formed at anode when electrolysing nitrates or sulfates? why? equation?

A

oxygen

  • the OH⁻ ions in the water lose electrons as they are lower in the discharge series
  • 4OH⁻ (aq) → O₂ + 2H₂O (l) + 4e⁻
36
Q

half equation for what happens at cathode when electrolysing aqueous sodium chloride?

A

2H⁺ (aq) + 2e⁻ → H₂ (g)

37
Q

what is copper used for

A

wiring, electrical equipment

38
Q

what do we need to do after extracting it from its ore (SiMPLE) why?

A

refine it further

to remove any impurities that may reduce electrical conductivity

39
Q

what is copper refining?

A

purifying copper through electrolysis

40
Q

how to set up for copper refining

A
  • an impure strip of copper is connected to the positive end of a power supply (forming the anode)
  • a thin strip of pure copper is connected to the negative end of the power supply (forming the cathode)
  • the electrolyte is a solution of copper (II) ions, usually copper (II) sulfate solution
41
Q

copper refining products

A
  1. at the anode, copper atoms lose their valency electrons and form copper ions
    these go into the solution as part of the electrolyte
    Cu(s) → Cu²⁺ (aq) + 2e⁻
  2. at the cathode, copper ions from the electrolyte gain electrons and form copper atoms
    these are deposited on the strip of pure copper
    Cu²⁺ (aq) + 2e⁻ → Cu(s)
  3. as the electrolysis proceeds, the cathode becomes thicker (as it gains more and more copper)
  4. after a time, the cathode of pure copper is replaced by a new one
  5. the anode loses mass and and the impurities fall to the bottom of the electrolysis cell as ‘anode slime’
  6. other valuable metals can be extracted from this ‘anode sludge’ e.g. gold and platinum
42
Q

overall result of copper refining?

A

pure copper transferred from anode to cathode

43
Q

what would be different if we used inert electrodes for copper refining instead of copper ones?

A

anode: hydroxide ions discharged, oxygen bubbles off
4OH⁻ (aq) → O₂ + 2H₂O (l) + 4e⁻
cathode: copper ions discharged, copper metal deposited
Cu²⁺ (aq) + 2e⁻ → Cu (s)

44
Q

what happens to colour of solution when refining copper with inert electrodes?

A

solution gradually loses its colour as copper ions are being discharged but not replaced

45
Q

products of copper refining with copper electrodes?

A

anode: loses electrons and copper ions go into solution
it gets smaller,, Cu (s) → Cu²⁺ (aq) + 2e⁻

cathode: copper ions discharged
Cu²⁺ (aq) + 2e⁻ → Cu (s)

46
Q

what happens to colour of solution when refining copper with copper electrodes?

A

the electrolyte remains the same deep blue colour as copper ions removed from the solution at the cathode are replaced in solution by copper ions formed at the anode

47
Q

what is electroplating used for?

A

putting a thin layer of one metal on top of another metal through electrolysis

48
Q

is object you want to electroplate put in the negative or positive electrode?

A

negative

49
Q

what must the object you want to electroplate be? why?

A

very clean so that the metal that’s electroplated on top of that does not flake off

50
Q

is the plating metal put in the negative or positive electrode?

A

positive

51
Q

what are typical metals for plating?

A

silver, gold, tin, chromium

52
Q

what is the electrolyte when electroplating?

A

a solution of an ionic compound of the plating metal

53
Q

when plating, what happens to the anode?

A

it gets smaller as the metal is transferred to the cathode

54
Q

when electroplating with copper, what happens to the colour of the solution?

A

nothing, as the copper ions that are being discharged at the cathode are being replaced by those from the anode

55
Q

electroplating with silver– how?

A
  1. at the anode, the silver atoms lose electronsthey become silver ions which go into the solutionAg (s) → Ag⁺ (aq) + e⁻
  2. the silver ions move to the cathode, where they gain electrons to become silver atomsthey then form a thin layer of silver on the surface of the object to be platedAg⁺ (aq) + e⁻ → Ag (s)

electrolyte= silver cyanide

56
Q

two main reasons for electroplating

A

protect metals from corrosion and improve appearance

57
Q

example of electroplating for protection

A

steel cans are plated with tin, as it protects it from air and water (and so rusting)

chromium used for car parts, metal parts of furniture, etc as is hard and doesn’t scratch easily, and is shiny

58
Q

example of electroplating for appearance

A
  • chromium= shiny, doesn’t go dull
  • silver= jewellery, cutlery
  • gold= jewellery, electronic equipment
59
Q

why must we use electrolysis to extract aluminium?

A

it’s more reactive than carbon and so that cannot be used to extract it like iron

60
Q

name of aluminium ore

A

bauxite

61
Q

other name for aluminium oxide

A

alumina

62
Q

main impurities in bauxite

A

oxides of iron, silicon, titanium

63
Q

how to purify aluminium ore?

A

crush it and. mix with sodium hydroxide (which dissolves the aluminium oxide)

  • Al₂O₃ (s) + 2NaOH (aq) → 2NaAlO₂ (aq) + H₂0 (l)
  • impurities are insoluble and so are filtered off
64
Q

how to make aluminium molten. why?

A
  • dissolve in molten cryolite so aluminium’s melting point is lower. saves lots of energy and improves electrical conductivity of electrolyte.
65
Q

why is the electrolysis of aluminium so expensive?

A

lots of energy is needed to melt aluminium due to its high melting point and low conductivity

66
Q

what happens at the cathode when electrolysing aluminium ?

A

cathode:

  • is the carbon lining of the steel electrolysis cell
  • Al³⁺ ions gain 3 electrons (REDUCTION) and aluminium metal is discharged (falling to the bottom of the cell)
    • it is removed from time to time using a syphon tube

Al³⁺ + 2e⁻ → Al

67
Q

what happens at the anode when electrolysing aluminium?

A

anode:
- O²⁻ ions lose 2 electrons (OXIDATION) discharging oxygen.
- the oxygen reacts with the hot carbon anodes to form CO₂ gas.
- as the carbon anodes ‘burn away’, they need to be replaced from time to time

2O²⁻ → O₂ + 4e⁻

Chemistry (11 decks)

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