chapter 8 - electricity and chemical change

Question 1

Describe the set up of an electric circuit

Answer 1

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

Question 2

What is electricity?

Answer 2

electricity is a stream of electrons.

Question 3

How do electrons move through the circuit?

Answer 3

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)

Question 4

Why are electricity cables made of iron and steel?

Answer 4

they are conductors.

Aluminum is a better conductor than steel

Question 5

Why do pylons have ceramic discs?

Answer 5

To support the bare cables.

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

Question 6

Why is copper used for wiring?

Answer 6

It is a very good conductor?

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

Question 7

How can you test if a substance conducts or not?

Answer 7

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.

Question 8

What results would you get from a conductivity test?

Answer 8

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.

Question 9

Why do metal and graphite solids conduct?

Answer 9

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)

Question 10

Why don’t molecular substances conduct?

Answer 10

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

Question 11

Why don’t ionic substances conduct when solid?

Answer 11

No electrons present.

And ions are not free to move.

Question 12

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

Answer 12

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

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

Question 13

*define electrolysis

Answer 13

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

Question 14

What do we call a liquid that conducts electricity?

Answer 14

An electrolyte.

Question 15

What is the apparatus used in electrolysis?

Answer 15

Electrodes.

Question 16

What is the anode?

Answer 16

The electrode attached to the positive terminal.

Question 17

What is the cathode?

Answer 17

The electrode attached to thee negative terminal.

Question 18

Describe the electrolysis of molten ionic compounds

Answer 18

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.

Question 19

Describe the electrolysis of molten lead II bromide.

Answer 19

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.

Question 20

Write the chemical equation for decomposition of lead II bromide.

Answer 20

(Lead II bromide decomposes when it is electrolyzed)

PbBr2 (l) —> Pb(l) + Br2(g)

Question 21

What are inert electrodes?

Answer 21

Graphite and platinum.

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

Question 22

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

Answer 22

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.

Question 23

What are the rules for electrolysis of aqueous ionic solutions?

Answer 23

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.

Question 24

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

Answer 24

Potassium
sodium
calcium
magnesium
aluminum 
zinc
iron
lead
hydrogen
copper
silver

Question 25

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

Answer 25

at the cathode, hydrogen | at the anode, bromine

Question 26

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

Answer 26

at the cathode, silver | at the anode, oxygen

Question 27

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

Answer 27

at the cathode, hydrogen | at the anode, chlorine

Question 28

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

Answer 28

at the cathode, hydrogen | at the anode, oxygen

Question 29

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

Answer 29

at the cathode, hydrogen | at the anode, oxygen

Question 30

What happens to ions in molten lead II bromide?

Answer 30

``` 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- ```

Question 31

What can be said about electrolysis reactions?

Answer 31

they are decomposition reactions. | they are redox reactions

Question 32

What does OILRIG stand for?

Answer 32

Oxidation Is Loss of electrons. | Reduction Is Gain of electrons.

Question 33

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

Answer 33

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)

Question 34

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

Answer 34

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-

Question 35

What is brine?

Answer 35

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

Question 36

How can brine be obtained?

Answer 36

by pumping water into salt mines to dissolve the salt. | by evaporating seawater.

Question 37

write the word equation for electrolysis of brine

Answer 37

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

Question 38

write the chemical equation for electrolysis of brine

Answer 38

2NaCl + 2H2O ---> 2NaOH + Cl2 + H2

Question 39

what is the chlorine produced used for?

Answer 39

``` (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 ```

Question 40

what is the hydrogen produce used for?

Answer 40

``` (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 ```

Question 41

what is the sodium hydroxide solution produced used for?

Answer 41

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

Question 42

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

Answer 42

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

Question 43

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

Answer 43

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-

Question 44

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

Answer 44

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

Question 45

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

Answer 45

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-

Question 46

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

Answer 46

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

Question 47

describe how electrolysis can be used to refine copper

Answer 47

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.

Question 48

what impurities could that sludge contain?

Answer 48

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

Question 49

Define electroplating

Answer 49

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

Question 50

describe the process of electroplating

Answer 50

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

Question 51

How would we electroplate a steel jug with silver?

Answer 51

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)