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Flashcards in Chemical Changes.5 Deck (46):
1

Properties of acids

pH of aqueous solutions is less than 7

Source of hydrogen ions
H+ (aq) in solution

Hydrochloride acid releases H+ ions:
HCL (aq) -> H+ (aq). + Cl- (aq)

Ethanoic

2

What is electrolysis used for

To decompose ionic compounds in the molten state or dissolved in water

3

Electrolyte

Ionic compound in the molten state (liquid) or dissolved in water

4

Electrolysis is

A process in which electrical energy from a direct current (dc) supply, decomposes an electrolyte

5

Anions are

Negatively charged ions that move to the positive electrode (anode)

6

Cations are

Positively charged ions that life to the negative electrode (cathode)

7

Moving charges

The ions in an electrolyte are charged particles. An electric current will pass through the electrolyte only if the ions are free to move from place to place

8

Remember that ions :

Can move about in liquids and solutions

Cannot move about in solids

9

Ions in a solution

Water is a covalent compound some of its molecules naturally form ions

H2O (l) reversible H+(aq) + OH-(aq)

10

Presence of these ions means that a solution of an ionic compound contains

Cations and anions from the dissolved ionic compound and
H+ and OH- ions from the water

11

Competing ions

During electrolysis of an aqueous solution all the ions in the electrolyte compete to be discharged and from products

1- hydrogen gas is produced if H+ ions are discharged: 2H+ (aq) + 2e- -> H2 (g)

2- oxygen gas is produced if OH- ions are discharged: 4OH- (aq) -> 2H2O (l) + O2 (g) + 4e -

12

What happens st the cathode

Negative electrode:
Hydrogen is produced unless the compound contains ions from a metal less reactive than hydrogen
In that case
The metal is produced instead

13

What happens at the anode

The positive electrode:

Oxygen is produced from OH- ions unless
The compound contains halide ions (Cl-, Br- or I- )
In that case:
Chloride produce chlorine, Cl2
Bromides produce bromine, Br2
Iodides produce iodine, I2

14

Metal + water ->

Metal hydroxide + hydrogen

15

Hydrogen is produced if a metal reacts with

Dilute acid

16

Metal + acid ->

Salt + hydrogen

17

Rates of reaction is greater in warm or cold acid?

WArm

18

In general the more reactive the metal the greater the rate of

Bubbling

19

Some reactive metals react unexpectedly slowly with water or acids

1 aluminium has a layer of aluminium oxide that stops water reaching the metal below

2 in the reaction of calcium with dilute sulfuric acid a layer of insoluble calcium surface forms slowing the reaction

3 in the reaction of magnesium with water a layer of sparingly soluble magnesium hydroxide forms slowing the reaction

20

What does magnesium react vigorously with?

Steam

21

What is the reactivity series

Potassium
Sodium
Calcium
Magnesium
Aluminium
Sin
Iron
—— HYDROGEN——
Copper
Silver
Gold

22

A more reactive metal will ——— a less reactive metal from its salts in solution

Displace

23

Magnesium is more reactive than copper thederfore what happens in a copper surface solution?

It can displace the copper

24

Magnesium displacing copper from copper sulfate solution, in this reaction you observe:

1 the colour of the solution fading as blue copper sulfate is replaced by colourless magnesium sulfate

2 an orange-brown coating of copper forming on the surface of the magnesium

3 an increase in temperature because reaction is exothermic

25

Method for deducing a reactivity series

You can work out a reactivity series for metals by observing what happens when you mix different combinations of metals and their salt solutions to do this:

Add a powdered metal to a test tube or beaker of a metal salt solution (as above right) or

Dip a small piece of metal into a metal salt solution on a sporting tile

Then look for evidence o a reaction such as a change in colour or temperature

26

How are cations formed

Metal atoms lose electrons

27

OIL RIG

Oxidation is loss
Reduction is gain

28

Metal atoms form cations when metals react with

Water or dilute acids

29

Metals react with water to form

Metal hydroxides

30

Metals react with dilute acids to form

Salts

31

Displacement reactions are known as

Redox reactions

32

Shat happens in redox atoms

The atoms of the more reactive metal are oxidised - they lose electrons

The metal cations of the less reactive metals are reduced - they gain electrons

33

Most metals are extracted by the

Reduction of ores found in the earths crust

34

What’s an ore

Is a rock that contains enough of a metal to make its extraction economical

35

Rocks may contain too little metal to make extraction worthwhile so over time metal prices may

Rude and these low-grade ores may become useful

36

Unreactive metals

Such as gold are placed at the bottom of reactivity series

Found in earths crust

In their ‘native state’ unconvinced with other elements:
They are not found as compounds but
They may occur naturally as alloys (mixtures of metals)

37

Oxidation is

The gain of oxygen by a substance eg magnesium is oxidised to magnesium oxide in air
2Mg + O2 -> 2MgO

The loss of electrons by a substance eg
Mg -> Mg2+ + 2e-

38

Reduction is

The loss of oxygen by a substance eg zinc oxide is reduced to zinc when it is heated with carbon :

ZnO + C -> Zn + CO

the gain of electrons by a substance eg
Zn2+ + 2e- -> Zn

39

Resistance to corrosion

Metals corrode when they react with substances around them such as air and water. How easily a metal corrodes depends on how reactive it is

40

What metal does not corrod

Gold

41

Easily corrodes is at the ——— of the reactivity series

Top

42

The method used to extract a metal from its ore is related to

Cost of extraction process

Posting in reactivity series

In principle, all metals can be extracted from their compounds using electrolysis. But:
Electricity is needed, which is expensive
Reduction by heating with carbon can be used if a metal is less reactive than carbon
Chemical reactions may be needed to separate silver and gold from other metals

43

Iron extraction

Iron is less reactive than carbon
So it’s produced by reducing iron oxide using carbon
This happens in industrial equipment called a blast furnace

44

Iron extraction

At the high temperatures inside the blast furnace :

Iron oxide is reduced by carbon

Iron oxide + carbon -> iron + carbon monoxide

Also it’s reduced by carbon monoxide

Iron oxide + carbon monoxide -> iron + carbon dioxide

Molten iron is produced

45

Aluminium extraction

Aluminium is more reactive than carbon it is produced by reducing aluminium oxide in and electrolytic cell

46

Aluminium extraction steps

1 aluminium oxide is dissolved in molten cryolite. This reduces the temperature needed for electrolysis to happen when an electric current passes through the mixture
1 at the cathode aluminium ions gain electrons and are reduced to aluminium atoms:
Al3+ + 3e- -> Al
3 at anode oxide ions lose electrons and form oxygen gas:
2O2- -> O2 + 4e-
Oxygen reacts with the graphite anodes so these must be replaced every few weeks