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chemistry igcse > chapter 8 - electricity and chemical change > Flashcards

chapter 8 - electricity and chemical change Flashcards

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1
Q

Describe the set up of an electric circuit

A

A battery, a bulb, and a rod of graphite joined or connected to each other by copper wires.
(Graphite is a form of carbon)

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2
Q

What is electricity?

A

electricity is a stream of electrons.

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3
Q

How do electrons move through the circuit?

A

The battery acts as an electron pump.
Electrons leave it through the negative terminal and enter through the battery again through the positive terminal.
(Current flows in the OPPOSITE direction as electrons,: out of positive, into negative)

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4
Q

Why are electricity cables made of iron and steel?

A

they are conductors.

Aluminum is a better conductor than steel

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5
Q

Why do pylons have ceramic discs?

A

To support the bare cables.

And ceramic is an insulator and so prevents the current from running down the pylon.

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6
Q

Why is copper used for wiring?

A

It is a very good conductor?

But the wires are sheathed in plastic and the plug cases are made of plastic for safety

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7
Q

How can you test if a substance conducts or not?

A

by connecting it into an electrical circuit in the place of the graphite rod.
if the bulb lights, then it is a conductor.
if it doesn’t, then its an insulator, or a poor conductor.

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8
Q

What results would you get from a conductivity test?

A

1- The only solids that conduct are metals and graphite.
2- Molecular substances are non-conductors.
3-Ionic substances don’t conduct when solid, but do conduct when melted or dissolved in water.

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9
Q

Why do metal and graphite solids conduct?

A

they have free/ mobile electrons.

Molten metals also conduct for the same reason (but it’s hard to test graphite because it sublimes at room temperature)

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10
Q

Why don’t molecular substances conduct?

A

They contain no free electron, or other charged particles, that can flow through them.
they never conduct, whether solid or molten or aqueous.

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11
Q

Why don’t ionic substances conduct when solid?

A

No electrons present.

And ions are not free to move.

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12
Q

Why do ionic substances conduct when molten or dissolved in water/

A

They don’t have electrons, but they do have ions.

The ions become free to move when the substance is melted or dissolved.

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13
Q

*define electrolysis

A

The breaking down of an ionic compound, when molten or in aqueous solution, by the passage of electricity.

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14
Q

What do we call a liquid that conducts electricity?

A

An electrolyte.

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15
Q

What is the apparatus used in electrolysis?

A

Electrodes.

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16
Q

What is the anode?

A

The electrode attached to the positive terminal.

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17
Q

What is the cathode?

A

The electrode attached to thee negative terminal.

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18
Q

Describe the electrolysis of molten ionic compounds

A

Electrolysis breaks the molten ionic compound down to its elements, giving the metal at the cathode, and the non-metal at the anode.

1- electrons flow along the wire, from the -tive terminal tot he cathode.
2- in the liquid, the ions carry the current. they move to the electrode of opposite charge.
3- cations (positive ion) move to the cathode (negative electrode), and accept electrons.
4- Anions (negative ions) move to the anode (positive electrode), and give up electrons.
5- electrons flow from the anode, along the wire, to the +tive terminal.

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19
Q

Describe the electrolysis of molten lead II bromide.

A

1- Electrodes are made of graphite.
2- the electrolyte is molten lead II bromide.
3- ions present in the electrolyte are Pb2+ and Br-.
4- at the cathode, Pb2+ ions accept e-. lead begins to appear below the cathode.
5-at the anode, Br- ions give up e-. Red-brow bromine vapor bubbles off.

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20
Q

Write the chemical equation for decomposition of lead II bromide.

A

(Lead II bromide decomposes when it is electrolyzed)

PbBr2 (l) —> Pb(l) + Br2(g)

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21
Q

What are inert electrodes?

A

Graphite and platinum.

they carry the current into the liquid, but remain unchanged (do not interfere with the reaction)

22
Q

Why does the electrolysis of molten ionic compounds differ from that of aqueous ionic solutions?

A

Because the water in the aqueous solution itself produces ions as well.
H2O (l) —> H+(aq) + OH-(aq)
these ions also take part in the electrolysis, so products may change.

23
Q

What are the rules for electrolysis of aqueous ionic solutions?

A

at the cathode :
1-if the metal is more reactive than hydrogen, its ions stay in the solution, and hydrogen bubbles off.
2- if the metal is less reactive than hydrogen, the metal forms.
at the anode:
1- if it is a concentrated solution of a halide, the chlorine, bromine or iodine form.
2- but if the halide solution is dilute, or there is no halide, oxygen forms.

24
Q

The order of reactivity, from most reactive to least reactive.

A 
Potassium
sodium
calcium
magnesium
aluminum 
zinc
iron
lead
hydrogen
copper
silver
25
Q

what are the products formed from the electrolysis of concentrated solution of potassium bromide, KBr?

A

at the cathode, hydrogen

at the anode, bromine

26
Q

what are the products formed from the electrolysis of concentrated solution of silver nitrate, AgNO3?

A

at the cathode, silver

at the anode, oxygen

27
Q

what are the products formed from the electrolysis of concentrated solution of hydrochloric acid, HCl?

A

at the cathode, hydrogen

at the anode, chlorine

28
Q

what are the products formed from the electrolysis of dilute solution of sodium chloride, NaCl?

A

at the cathode, hydrogen

at the anode, oxygen

29
Q

what are the products formed from the electrolysis of dilute solution of sulfuric acid, H2SO4?

A

at the cathode, hydrogen

at the anode, oxygen

30
Q

What happens to ions in molten lead II bromide?

A 
1- the ions move because opposite charges attract. 
\+ lead ions move to - electrode.
- Bromide ions move to + electrode.
the moving ions carry the current.
2- at the cathode, reduction happens. RIG.
Pb2+ (l) + 2e- ----> Pb (l)
3- at the anode, oxidation happens. OIL.
2Br-(l) ----> Br2(b) + 2e-
31
Q

What can be said about electrolysis reactions?

A

they are decomposition reactions.

they are redox reactions

32
Q

What does OILRIG stand for?

A

Oxidation Is Loss of electrons.

Reduction Is Gain of electrons.

33
Q

What happens to ions in a concentrated solution of sodium chloride?

A

1- Solution contains 4 ions: H+, Na+, Cl-, OH-.
sodium is more reactive than hydrogen, so it stays in the solution, while H+ ions move to the cathode.
The solution is a conc halide, so Cl- moves to the anode.
2- At the cathode, hydrogen ions are reduced to hydrogen.
2H+ (aq) + 2e- —-> H2 (g)
3- at the anode, chlorine ions are oxidized.
2Cl- (aq) —> Cl2(aq) + 2e-
(hydrogen and chlorine bubble off, solution of sodium hydroxide left behind)

34
Q

What happens to ions in a dilute solution of sodium chloride?

A

1- solution still contains the same ions, but sodium chloride is at a lower conc.
2- at the cathode, hydrogen ions are reduced to hydrogen.
4H+ (aq) + 4e- —-> 2H2 (g)
3- at the anode, since conc of chlorine is low, OH- will be oxidized instead.
4OH- (aq) —-> O2 (g) + 2H2O (l) +4e-

35
Q

What is brine?

A

brine is a concentrated solution of sodium chloride (common salt).

36
Q

How can brine be obtained?

A

by pumping water into salt mines to dissolve the salt.

by evaporating seawater.

37
Q

write the word equation for electrolysis of brine

A

sodium chloride(aq) + water(l) —-> sodium hydroxide(aq) + chlorine(g) + hydrogen(g)

38
Q

write the chemical equation for electrolysis of brine

A

2NaCl + 2H2O —> 2NaOH + Cl2 + H2

39
Q

what is the chlorine produced used for?

A 
(poisonous yellow-green gas) -- the most widely used
used in making...
the plastic PVC
solvents for degreasing and dry cleaning
medical drugs
weed killers and pesticides
paints and dyestuffs
bleaches
hydrogen chloride and hydrochloric acid
40
Q

what is the hydrogen produce used for?

A 
(colorless, flammable gas)
used...
in making nylon
to make hydrogen peroxide
to 'harden' vegetable oils to make margarine
as a fuel in hydrogen fuel cells
41
Q

what is the sodium hydroxide solution produced used for?

A 
(alkaline and corrosive)
used in making...
soaps
detergents
viscose (rayon) and other textiles
paper
ceramic (tiles, furnace bricks)
dyes
medical drugs
42
Q

what ions are present in a solution of copper II sulfate?

A

Cu2+, (SO4)2-, H+ and OH-.

43
Q

Describe the electrolysis of aqueous copper II sulfate, using INERT electrodes

A

electrodes: carbon or platinum
rxn at the cathode: copper ions are reduced to copper
2Cu2+(aq) + 4e- —> 2Cu(s)
copper coats the electrode
rxn at the anode: oxygen ions are oxidized
4OH-(aq) —> 2H2O(l) + O2(g) + 4e-

44
Q

what can be observed during the the electrolysis of aqueous copper II sulfate, using INERT electrodes?

A

blue color pf the solution fades
bubbles at the anode
reddish brown color at the cathode

45
Q

Describe the electrolysis of aqueous copper II sulfate, using COPPER electrodes

A

rxn at the cathode: copper ions are reduced to copper
Cu2+(aq) + 2e- —> Cu(s)
at the anode: the anode dissolves, giving copper ions in solution
Cu(s) —> Cu2+(aq) + 2e-

46
Q

what can be observed during the the electrolysis of aqueous copper II sulfate, using COPPER electrodes?

A

(color of solution persists)
cathode increases in mass
anode dissolves and decreases in mass

47
Q

describe how electrolysis can be used to refine copper

A

anode is made of impure copper
cathode is pure copper
electrolyte is a dilute solution of copper II sulfate
1- the copper in the anode dissolves, but the impurities don’t, and they drop to the floor as a sludge.
2- a layer of pure copper builds up on the cathode.

48
Q

what impurities could that sludge contain?

A

valuable metals such as platinum, gold, silver and selenium.

49
Q

Define electroplating

A

using electricity to coat one metal with another, to make it look better, or to prevent corrosion.

50
Q

describe the process of electroplating

A

cathode is the object to be electroplated
anode is the metal X
electrolyte is a solution of a soluble compound of X

51
Q

How would we electroplate a steel jug with silver?

A

jug is the cathode
silver is the anode
solution is silver nitrate
at the anode, the silver dissolves, forming ions in solution.
Ag(s) —-> Ag+(aq) + e-
at the cathode, the silver ions are attracted to the cathode, where they are reduced and form a coat of silver on the jug.
Ag+ (aq) + e- —-> Ag(s)

chemistry igcse (6 decks)

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