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AS Unit 2 Revision AQA AS Chemistry Revision Chapter 10 Redox
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 its charge. 4.The sum of the oxidation states of all the atoms and polyatomic ion is its 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 K 2 O –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) + N 2(g) Mg 3 N 2(s) N 2(g) + 3H 2(g) 2NH 3(g) N 2(g) + O 2(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 FeCl 3 instead of FeCl 2 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
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. Atomic Radius
AS Unit 2 Revision The strength of the instantaneous dipoleinduced dipole forces between the molecules increases as the size of the molecules increases. The boiling point increases down Group 7. Boiling point
AS Unit 2 Revision Fluorine is the most electronegative element in the periodic table. 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.
AS Unit 2 Revision Oxidation ability 17 What is Oxidation? 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: Cl 2 > Br 2 > I 2 When a halogen acts as an oxidising agent, it gains electrons (taken from the oxidised species). X 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 – X e – When a halide ion reduces another substance, the halide is oxidised to a halogen. 2 X – X e – When a halide ion reduces another substance, the halide is oxidised to a halogen. What is reduction? Reduction 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 – X 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–I–
AS Unit 2 Revision ChlorideIodideBromide 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 –Cl 2(g) + H 2 O (l) HClO (aq) + HCl (aq) –This is disproportionation. Chlorine and alkali –Cl 2(g) + 2NaOH (l) NaClO (aq) + NaCl (aq) + H 2 0 (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
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.
AS Unit 2 Revision Reactions of Group 2 Elements with Water With waterWith steam Mg Ca Sr Ba Insignificant reaction Moderately fast reaction alkaline Ca(OH) 2 (some white ppt) + colourless H 2 gas Fast reaction alkaline colourless Sr(OH) 2 solution + colourless H 2 gas Very fast reaction alkaline colourless Ba(OH) 2 solution + colourless H 2 gas Burns vigorously white MgO solid + colourless H 2 gas Burns very vigorously white CaO solid + colourless H 2 gas Burns explosively white SrO solid + colourless H 2 gas Burns explosively white BaO solid + colourless H 2 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. Ba 2+ (aq) + SO4 2- (aq) BaSO 4(s)
AS Unit 2 Revision Remember Opposite trends Ca(OH) 2 & CaSO 4 sparingly soluble BaSO 4 insoluble MgSO 4 Mg(OH) 2 CaSO 4 Ca(OH) 2 SrSO 4 Sr(OH) 2 BaSO 4 Ba(OH) 2 more soluble
AS Unit 2 Revision Uses of Group 2 compounds Barium meal BaSO 4 Milk of magnesia Mg(OH) 2 Lime Ca(OH) 2
AS Unit 2 Revision AQA AS Chemistry Revision Chapter 13 Extraction of metals
AS Unit 2 Revision
Converting sulfide ores to oxides Roasting in oxygen producing SO 2(g) 2CuS + 3O 2(g) 2CuO (s) + 2SO 2(g) Problem of acid rain. SO 2 + H 2 O H 2 SO 3 and SO 3 + H 2 O H 2 SO 4 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 Furnace 1. 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 equations At the negative electrode: Al e - Al (reduction) aluminium oxide aluminium + oxygen 2 Al 2 O 3 (l) 4 Al (l) + 3 O 2 (g) At the positive electrode: 2O 2- O 2 + 4e - (oxidation) Cryolyte needed to form a solution so Al can melt at a lower temperature. Carbon electrodes burn away to CO 2 and must be replaced.
AS Unit 2 Revision Extraction of Titanium Titanium ore is mainly the oxide TiO 2, which is converted into titanium tetrachloride TiCl 4 by heating with carbon and chlorine. Titanium chloride is a gas and must be separated using fractional distillation under an inert atmosphere. TiO 2(s) + 2C (s) + 2Cl 2(g) TiCl 4(l) +2CO (g)
AS Unit 2 Revision Extraction of titanium from its ore The titanium tetrachloride is then reacted with sodium or magnesium TiCl 4 + 2Mg Ti + 2MgCl 2 or TiCl 4 + 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. MgCl 2. 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. TiCl 4 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 H 2 is so flammable °C WO 3(S) + 3H 2(g) W (s) + 3H 2 O (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 CO 2 than extraction from ore. Aluminium –Avoids pollution –Uses about 5% energy needed to extract from ore.