AS Unit 2 Revision AQA AS Chemistry Revision Chapter 10 Redox.

AS Unit 2 Revision

AQA AS Chemistry Revision

Chapter 10 Redox

AS Unit 2 Revision

AS Unit 2 Revision

Oxidation and Reduction

• Oxidation is loss of electrons.

• Reduction is gain of electrons.

• An oxidising agent accepts electrons and is itself reduced.

• A reducing agent donates electrons and is itself oxidised.

AS Unit 2 Revision

Oxidation states

1. The oxidation state of an atom in an element is always zero.

2. The sum of the oxidation states of all the atoms and ions in a compound is always zero.

3. The oxidation state of a simple ion is it’s charge.4. The sum of the oxidation states of all the atoms and

polyatomic ion is it’s charge.5. The oxidation state of Flourine is always -1 as it is the

most electronegative element.6. The oxidation state of oxygen is nearly always -2.

(Except in peroxides where it is -1)7. The oxidation state of chlorine in a compound is usually

-1 (unless it is combined with F or O).8. The oxidation state of hydrogen is +1 (except when it is

bonded to a metal ion in which case it is -1)

AS Unit 2 Revision

Oxidation and reduction of s and p block elements

S block - Group 1 and 2• Group 1 elements have one

electron in their outer shell so always makes +1 ions. Oxidation state is always +1. They are reducing agents.

• Group 2 elements have one electron in their outer shell so always makes +2 ions. Oxidation state is always +2. They are reducing agents.

AS Unit 2 Revision

Examples

For the following reactions

– Potassium reacts with oxygen to form K2O

– Calcium reacts with water to form Ca(OH)2

1. Write full balanced equations and the

2. Write two half equations.

3. Assign oxidation states to each element and determine which has been oxidised and which reduced.

AS Unit 2 Revision

Oxidation and reduction of s and p block elements

P block - Group 5, 6 and 7• Group 5, E.g. Nitrogen can form

compounds with metals or non-metals.

• They can have positive or negative oxidation states.

The more electronegative element in a compound is always taken as the negative element when assigning oxidation states.

AS Unit 2 Revision

Examples of Nitrogen

Assign oxidation states and determine which is oxidised, which is reduced and name the oxidising agents and reducing agents.

• 3Mg(s) + N2(g) → Mg3N2(s)

• N2(g) + 3H2(g) ↔ 2NH3(g)

• N2(g) + O2(g) → 2NO

AS Unit 2 Revision

Oxidation and reduction of s and p block elements

P block - Group 5, 6 and 7• Group 6, Sulfur can exist in several

oxidation states.

• Group 7, Halogens always take -1 oxidation state by gaining 1 electron.

• When they react with metals that can take on variable oxidation states, the metal will tend to adopt its higher oxidation state. E.g. passing chlorine gas over hot iron forms FeCl3 instead of FeCl2

• They can also react with non-metals to form covalent compounds but usually still retain the -1 oxidation state.

AS Unit 2 Revision

Redox equations

• Two good power points for this

• Balancing redox equations

• Redox equations

• Now try PPQs

AS Unit 2 Revision

AQA AS Chemistry Revision

Chapter 11 Group 7, the halogens

AS Unit 2 Revision

AS Unit 2 Revision

When warmed, iodine crystals sublime (turn directly to a gas), forming a purple vapour.

AS Unit 2 Revision

Going down the group, there are more filled energy levels between the nucleus and the outer electrons.

This results in the outer electrons being shielded more from the attraction of the nucleus.

Going down the group, there are more filled energy levels between the nucleus and the outer electrons.

This results in the outer electrons being shielded more from the attraction of the nucleus.

The atomic radius increases down Group 7.The atomic radius increases down Group 7.

Atomic Radius

AS Unit 2 Revision

The strength of the instantaneous dipole−induced dipole forces between the molecules increases as the size of the molecules increases.

The strength of the instantaneous dipole−induced dipole forces between the molecules increases as the size of the molecules increases.

The boiling point increases down Group 7.The boiling point increases down Group 7.

Boiling point

AS Unit 2 Revision

Fluorine is the most electronegative element in the periodic table.

Fluorine is the most electronegative element in the periodic table.

Electronegativity decreases down Group 7.Electronegativity decreases down Group 7.

Electronegativity

The atomic radius increases, the outer electrons are more shielded, so bonding electrons are less strongly attracted to the nucleus.

The atomic radius increases, the outer electrons are more shielded, so bonding electrons are less strongly attracted to the nucleus.

AS Unit 2 Revision

Oxidation ability

17

What is Oxidation? Oxidation is the loss of electrons. Oxidation is the loss of electrons.

What is an oxidizing agent? An oxidizing agent is an electron acceptor, the agent is reduced during the course of the reaction.

This forms a redox reaction.

An oxidizing agent is an electron acceptor, the agent is reduced during the course of the reaction.

This forms a redox reaction.

AS Unit 2 Revision

Oxidising power trend: Cl2 > Br2 > I2

When a halogen acts as an oxidising agent, it gains electrons (taken from the oxidised species).

X2 + 2 e- → 2 X-

Going down the group it becomes harder to gain an electron because:

atoms are larger & there is more shielding (due to extra electron shell)

Going down the group it becomes harder to gain an electron because:

atoms are larger & there is more shielding (due to extra electron shell)

Cl

Br

I

AS Unit 2 Revision

Reducing agents

19

2 X– → X2 + 2 e–

When a halide ion reduces another substance, the halide is oxidised to a halogen.

2 X– → X2 + 2 e–

When a halide ion reduces another substance, the halide is oxidised to a halogen.

What is reduction? Reduction is the gain of electronsReduction is the gain of electrons

What happens when a Halide is used as a reducing agent?

Give the half equation for the reaction

AS Unit 2 Revision

Reducing power trend: Cl– < Br– < I–

When a halide ion acts as a reducing agent, it loses electrons (given to the reduced species).

2 X– → X2 + 2 e–

Down the group it becomes easier to lose an electron because:

ions are larger & there is more shielding (due to extra electron shell)

Cl–

Br–

I–

AS Unit 2 Revision

Chloride

IodideBromide

The samples to be tested are first acidified using dilute nitric acid, then silver nitrate solution is added.

White precipitate

of silver chloride

Cream precipitate

of silver bromide

Yellow precipitate

of silver iodide

Ag+(aq) + Cl−(aq) → AgCl(s)

Ag+(aq) + Br−

(aq) → AgBr(s) Ag+(aq) + I−

(aq) → AgI(s)

Nitric acid is needed to get rid of carbonates or hydroxides.

•These would form silver carbonates or silver hydroxides which are soluble.

AS Unit 2 Revision

Uses of Chlorine and Chlorate• Chlorine and water

– Cl2(g) + H2O(l) ↔ HClO(aq) + HCl(aq)

– This is disproportionation.

• Chlorine and alkali

– Cl2(g) + 2NaOH(l) → NaClO(aq) + NaCl(aq) + H20(l)

Oxidising agent that kills bacteria.

NaClO is an oxidising agent and the active ingredient in bleach

AS Unit 2 Revision

AQA AS Chemistry Revision

Chapter 11 Group 2, the alkaline earth metals

AS Unit 2 Revision

AS Unit 2 Revision

Physical Properties of Group 2• All have 2 electrons in outer s orbital. As

you go down the group there is more shielding and outer electron is farther from the nucleus.

• Atomic radius increases down the group.

• Melting point decreases down the group.

• Ionisation energy decreases down the group.

0

200

400

600

800

1000

Be Mg Ca Sr Ba

1st

ion

isat

ion

en

erg

y

0

0.05

0.1

0.15

0.2

0.25

Be Mg Ca Sr Ba

ato

mic

rad

ius

(nm

)

0

500

1000

1500

2000

Be Mg Ca Sr Ba

mel

tin

g p

oin

t (K

)

AS Unit 2 Revision

Reactions of Group 2 Elements with Water

With water With steam

Mg

Ca

Sr

Ba

Insignificant reaction

Moderately fast reaction

alkaline Ca(OH)2 (some

white ppt) + colourless H2 gas

Fast reaction alkaline colourless Sr(OH)2

solution + colourless H2 gas

Very fast reaction

alkaline colourless Ba(OH)2

solution + colourless H2 gas

Burns vigorously

white MgO solid + colourless H2 gas

Burns very vigorously

white CaO solid + colourless H2 gas

Burns explosively white SrO solid + colourless H2 gas

Burns explosively

white BaO solid + colourless H2 gas

concentrate on these

AS Unit 2 Revision

Test for Sulphates

• Reacting the sulphate with barium chloride forms an insoluble salt Barium sulfate, this is seen as a precipitate.

Ba2+(aq) + SO42-

(aq) → BaSO4(s)

AS Unit 2 Revision

• RememberOpposite trendsCa(OH)2 & CaSO4 sparingly solubleBaSO4 insoluble

MgSO4

Mg(OH)2

CaSO4 Ca(OH)2

SrSO4 Sr(OH)2

BaSO4 Ba(OH)2

more solubl

e

more solubl

e

AS Unit 2 Revision

Uses of Group 2 compounds

Barium mealBaSO4

Milk of magnesiaMg(OH)2

LimeCa(OH)2

AS Unit 2 Revision

AQA AS Chemistry Revision

Chapter 13 Extraction of metals

AS Unit 2 Revision

AS Unit 2 Revision

Converting sulfide ores to oxides

• Roasting in oxygen producing SO2(g)

• 2CuS + 3O2(g) 2CuO(s) + 2SO2(g)

• Problem of acid rain.• SO2 + H2O H2SO3 and SO3 + H2O H2SO4

Reducing oxides• Possible reductants

– Coke – impure form a carbon. Pros – cheap, cons – requires high temp and can form brittle metal carbides.

– Hydrogen – used to reduce W tungsten– Electrolysis – Pros – gives pure product, cons – expensive, lots of energy

required.– More reactive metals as reductants. Con – expensive.

AS Unit 2 Revision

The Blast Furnace1. A mixture of limestone,

coke and haematite is added at the top of the blast furnace.

2. Hot air is ‘blasted’ into the blast furnace.

3. The carbon reacts with the hot air to form carbon dioxide.

5. The carbon monoxide reduces the iron ore to form iron and carbon dioxide.

7. Impurities collect at the bottom in a layer, ‘slag’, and can be tapped off.

6. The iron, pig iron, collects at

the bottom of the blast furnace to be

tapped off.

8. Waste gases are removed from the blast furnace.

4. The coke reacts with the carbon dioxide to form carbon

monoxide.

1900°C

AS Unit 2 Revision

Extracting aluminium – redox equationsAt the negative electrode:

Al3+ + 3e- Al (reduction)

aluminium oxide aluminium + oxygen

2 Al2O3 (l) 4 Al (l) + 3 O2 (g)

At the positive electrode:

2O2- O2 + 4e- (oxidation)

Cryolyte needed to form a solution so Al can melt at a lower temperature.

Carbon electrodes burn away to CO2 and must be replaced.

AS Unit 2 Revision

Extraction of Titanium

Titanium ore is mainly the oxide TiO2, which is converted into titanium tetrachloride TiCl4 by heating with carbon and chlorine.

Titanium chloride is a gas and must be separated using fractional distillation under an inert atmosphere.

TiO2(s) + 2C(s) + 2Cl2(g) → TiCl4(l) +2CO(g)

AS Unit 2 Revision

Extraction of titanium from its ore

• The titanium tetrachloride is then reacted with sodium or magnesium

TiCl4 + 2Mg Ti + 2MgCl2

or

TiCl4 + 4Na Ti + 4NaCl

• This reaction is carried out in an atmosphere of inert argon gas so none of the metals involved becomes oxidised by atmospheric oxygen.

Na is used in UK

AS Unit 2 Revision

• Batch process• Cooled in

between and Ti sponge removed. (Takes days to cool)

• Sponge has air spaces and contains 30% impurities, e.g. MgCl2.

• Wash with HCl to remove MgCl

• Removed by evaporation at high T.

AS Unit 2 Revision

Disadvantages of this process

• Cl, Na and Mg are expensive, must be produced by electrolysis.

• High temperatures are needed.

• TiCl4 is highly reactive with water and must be handled with care.

• An inert atmosphere is required to avoid reactions with oxygen.

AS Unit 2 Revision

Tungsten• Also known as wolfram, symbol W,

from the ore wolframite.• High MP (3410°C) therefore used in

light bulbs.• Cannot use carbon as tungsten

carbide would form.• Must be reacted with Hydrogen at

high temperatures to be reduced.

• Draw back is that H2 is so flammable.

700-1000°C

• WO3(S) + 3H2(g) W(s) + 3H2O(l)

AS Unit 2 Revision

• Scrap metals can be used instead of ores.• Iron

– Less land fill– All ready extracted from ore, easily separated as it is

magnetic.

– Produces less CO2 than extraction from ore.

• Aluminium– Avoids pollution– Uses about 5% energy needed to extract from ore.