Ri Y4 Chem 2022 FE Revision Metals and Electrolysis Ans

1 Reactivity Series of Metals Metals can be ranked in a series based on their reactivity with water, steam and dilute acid. This series also indicates whether a metal can be extracted by reduction of its ore using carbon or hydrogen, and whether the compound (carbonate) of a metal is stable to heat. The table below ranks 12 (out of about 100) metals, from most reactive to least reactive. The more reactive metal can reduce. the ion or compound of a less reactive metal to form that less reactive metal. Alternatively, we say, the more reactive metal can displace the less reactive metal from its salt solution or oxide. Examples: 1 Zn (s) + CuSO4 (aq)  Cu (s) + ZnSO4 (aq) or Zn (s) + Cu2+ (aq)  Cu (s) + Zn2+ (aq) Observations: the blue solution fades in colour and a pink or brown solid is formed Explanation: the blue of the solution is due to Cu 2+ (aq) and as its concentration falls the blue colour fades off. The solid copper metal is formed which appears as pink or brown or red-brown in colour. 2 2Al (s) + Fe2O3 (s)  Al2O3 (s) + 2Fe (s) Extraction of Metals Many metals are extracted from their oxides (ores) by reduction with carbon or carbon monoxide. Those that cannot be extracted by such a method (e.g. aluminium) would usually be extracted by electrolysis. Iron is extracted in a blast furnace at above 1000 oC. The inputs into the furnace are haematite (Fe 2O3), coke (C), limestone (CaCO3) and hot air. The outputs are molten iron, molten slag (CaSiO3) and waste gases such as CO2. The main reaction is Fe2O3 (s) + 3CO (g)  2Fe (l) + 3CO2 (g) The side reactions are 1. Generate CO (the reducing agent): C + O2  CO2, then CO2 + C  2CO 2. Remove sand (acidic impurity): CaCO3  CaO + CO2, then CaO + SiO2  CaSiO3 (slag) Alloying Metals are soft and malleable due to the regular layers of atoms that can slide over each other easily. So they are usually mixed with other metals or with carbon to form alloys which are harder and stronger – since the atoms are of different sizes they no longer form regular layers and thus cannot slide over each other. Iron is alloyed with carbon to make steel. Mild steel ( or low-carbon steel, with ~0.25% carbon) is the most common steel, used to make car bodies, bridges and railway lines. Hard steel (or high-carbon steel, with ~1% carbon) is harder and less malleable or more brittle than mild steel, and is used to make tools. Stainless steel has additives other than carbon; it is used to make cutlery and surgical instruments. Corrosion of Metal The corrosion (or rusting) of iron requires air(oxygen) and water Rusting can be prevented by 1. Surface protection: cover the iron with a barrier (paint or oil or plastic or metal such as tin or chromium) to block air and water from reaching the iron underneath. 2. Sacrificial protection: coat the iron with zinc (or have the iron in contact with magnesi um or zinc). The more reactive metal reacts with oxygen and water in place of the iron thus protecting it. The advantage is that even if the layer of magnesium or zinc is scratched to expose the iron underneath the iron is still protected (until all the magnesium or zinc is corroded off). 3. Stainless steel: the iron i s mixed with carbon and chromium and nickel. The chromium and nickel prevents the iron from reacting with water and oxygen. Aluminium, even though it is more reactive than iron, does not corrode easily. This is because it is covered with a thin layer of non-porous aluminium oxide that protects the metal from reacting. Raffles Institution Raffles Programme Year Four Chemistry Name: _____________________________ ( ) Class: _______ Date: _____________ 2022 FE Revision – Metals and Electrolysis ANSWERS

2 Recycling Metals Benefits Disadvantages 1. Helps conserve metals and ores 2. Saves cost and reduces pollution during extraction of new metals 3. Reduces use of unsightly landfills and open mines 1. Recycling involves collecting, transporting, washing, sorting, etc, thus recycling may be more expensive than extracting 2. Smelting during recycling can produce pollutants Practice Questions 1 (a) Choosing your answer from the list only, name (i) two metals which have to be stored under oil barium and sodium (ii) two metals which will not react with steam copper and silver (iii) two metals that react with water to give hydrogen and an alkali barium and sodium (iv) two metals that could be produced by the reduction of their oxides with carbon zinc and iron (v) two metals that do not react with dilute acids copper and silver (vi) one metal whose carbonate does not decompose on heating sodium (b) Aluminium is near the top of the reactivity series, but tends to react slowly. State the reason for this. Aluminium has a tenacious non-porous layer of aluminium oxide on its surface. This layer protects the metal underneath from reacting with other reactants such as water and oxygen, thus aluminium reacts slowly. 2 (a) chromium and iron (b) platinum (c) aluminium, zinc, chromium and iron (d)(i) Aluminum has low density so more aluminium cans can be packaged per ton, for delivery (ii) Steel is very strong so the cans can withstand knocks and rough handling during delivery (iii) 2Al + 6H+  2Al3+ + 3H2 or Fe + 2H+  Fe2+ + H2 3(a) Silver Barium Copper Chromium chromium(III) nitrate solution (green) no reaction grey deposit in colourless solution no reaction barium nitrate solution (colourless) no reaction no reaction no reaction silver nitrate solution (colourless) silvery deposit in colourless solution silvery deposit in blue solution silvery deposit in green solution copper(II) nitrate solution (blue) no reaction reddish-brown deposit in colourless solution reddish-brown deposit in green solution (b) 2Cr + 3Cu2+  2Cr3+ + 3Cu (c) barium (d) Cr2(CO3)3  Cr2O3 + 3CO2

3 Electric Cell vs Electrolytic Cell Factors affecting Electrolysis 1 Electro-chemical Series 2 Concentration Effect – our focus is only between the halides and hydroxide ions. Example: Electrolysis of dilute NaCl produces O2 at the anode (4OH−  4e− + O2 + 2H2O) Electrolysis of concentrated NaCl produces Cl2 at the anode (2Cl−  2e− + Cl2) 3 Nature of electrode Inert electrodes are graphite (carbon) and platinum. They do not interfere in the electrolysis (although they may undergo side reactions, e.g. graphite may react with the oxygen produced at the anode) Active electrodes are metals (other than platinum). An active anode would take part in the electrolysis and be preferentially oxidized over OH − and other anions. Thus the active anode dissolves away. Electric Cell Electrolytic Setup Key Ideas 1. The 2 electrodes are 2 different metals 2. Electrons flow from the more reactive metal to the less reactive metal, and are then ‘captured’ by the cations in the electrolyte. 3. The greater the difference in the reactivity of the metals the greater the voltage produced. 1. During electrolysis, using inert electrodes, the electrolyte decomposes. 2. If the electrolyte is molten and contains 1 type of cations and 1 type of anions, then the cations would be attracted to the negative electrode and become reduced, while the anions would be attracted to the positive electrode and become oxidized 3. If the electrolyte is an aqueous solution, it would contain H+ and OH− as well as the dissolved substance’s cation and anion. At the electrodes, competition will occur and the result is based on the electro-chemical series. Energy Conversion Spontaneous Reaction: chemical energy converted to electrical energy System does work on load/surroundings. Non-spontaneous Reaction: electrical energy converted to chemica