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Flashcards · 9202 · Topic 3

Chemical changes flashcards

20 cards on Topic 3, subtopics 3.3.1 to 3.3.2. Pick a subtopic or study them all.

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3.3.1 Metals

Why do metals conduct, and why can they be hammered into shape?
Conduct heat and electricity: delocalised electrons move through the structure. Hammered or bent: the layers of atoms slide over each other.
Alloy, and what steel is
Alloy: a mixture of at least two elements, at least one a metal (not a compound). Steel: iron mixed with carbon, sometimes with other metals too.
Why is copper used for wiring and plumbing?
Good conductor of heat and electricity; can be bent but is hard enough for pipes and tanks; does not react with water.
The 9202 reactivity series, most reactive first (with carbon)
Potassium, sodium, lithium, calcium, magnesium, (carbon), zinc, iron, copper.
Copper wire is placed in silver nitrate solution. Ionic equation, and what is oxidised and reduced?
Cu + 2Ag⁺ → Cu²⁺ + 2Ag Copper atoms are oxidised (lose electrons); silver ions are reduced (gain electrons). Nitrate ions are spectators.
How is a metal’s extraction method chosen?
Very unreactive (gold): found as the metal. Below carbon (zinc, iron): heat the oxide with carbon (reduction). Above carbon (aluminium): electrolysis of the molten compound — high energy, so expensive.
Blast furnace: raw materials and the reduction step
Iron ore, coke, limestone and hot air. CO₂ + C → 2CO, then Fe₂O₃ + 3CO → 2Fe + 3CO₂.
Phytomining vs bioleaching
Phytomining: plants absorb copper compounds, then are burned; the ash contains copper compounds. Bioleaching: bacteria produce a leachate solution containing copper compounds.
Three reasons to recycle metals
Extraction uses limited resources, costs a lot of energy and damages the environment.
Thermal decomposition of copper carbonate: equation and colour change. Which carbonates do not decompose in a Bunsen flame?
CuCO₃ → CuO + CO₂; green → black. Sodium and potassium carbonates do not decompose at Bunsen temperatures.
6.20 g of CuCO₃ is heated to constant mass. Mass of copper oxide left? (Ar: Cu = 64, C = 12, O = 16)
Mr CuCO₃ = 124, Mr CuO = 80. 6.20 × 80 ÷ 124 = 4.00 g

3.3.2 Electrolysis

Electrolysis and electrolyte
Electrolysis: breaking down an ionic substance, molten or in solution, by passing an electric current through it. Electrolyte: the molten or dissolved ionic substance that is broken down.
Why must an ionic compound be melted or dissolved for electrolysis?
In the solid lattice the ions are fixed in place. Melting or dissolving frees them to move and carry charge.
Cathode and anode: charge, which ions go there, and what happens
Cathode: negative; positive ions move to it; reduction (gain of electrons). Anode: positive; negative ions move to it; oxidation (loss of electrons).
Aqueous solution: rules for the cathode and anode products
Cathode: hydrogen if the metal is more reactive than hydrogen; otherwise the metal (copper, silver). Anode: the halogen from a concentrated halide; otherwise (dilute halide, sulfate, nitrate) oxygen.
Required practical: copper sulfate solution with carbon electrodes. What is seen at each electrode?
Cathode: pink-brown copper coating, mass increases (Cu²⁺ + 2e⁻ → Cu). Anode: bubbles of oxygen, which relight a glowing splint (4OH⁻ → O₂ + 2H₂O + 4e⁻). The blue colour fades.
Electroplating: the cathode, anode and electrolyte, and three reasons for it
Cathode: the object. Anode: the plating metal. Electrolyte: a solution of ions of the plating metal. Reasons: appearance, durability, prevention of corrosion.
Aluminium manufacture: why cryolite, and why are the anodes replaced?
Aluminium oxide is dissolved in molten cryolite, which is molten at a much lower temperature, saving energy. The carbon anodes react with the oxygen formed (C + O₂ → CO₂), so they wear away.
Electrode half equations in aluminium manufacture
Negative electrode: Al³⁺ + 3e⁻ → Al Positive electrode: 2O²⁻ → O₂ + 4e⁻
Electrolysis of sodium chloride solution: products and uses
Cathode: hydrogen. Anode: chlorine. Solution: sodium hydroxide. Sodium hydroxide → soap; chlorine → bleach and plastics.

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