OxfordAQA International GCSE Chemistry 3.3 Chemical Changes Exam Questions

About OxfordAQA International GCSE Chemistry 3.3 Chemical Changes

Chemical Changes in OxfordAQA International GCSE Chemistry focuses on metals and electrolysis. You learn why metals and alloys are useful, the reactions of potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper with water and dilute acids, displacement reactions as redox, and how metals are extracted by carbon reduction, electrolysis, phytomining and bioleaching. Metal carbonates decompose on heating and react with acids. The electrolysis section covers molten lead bromide, copper sulfate solution, aluminium oxide with cryolite, sodium chloride solution and electroplating. Examiners often ask for half equations, predictions of products at each electrode, and the reasoning behind each extraction method.

What you need to know

• Metals and alloys Metals conduct heat and electricity and can be bent because layers of atoms slide. An alloy is a mixture of at least two elements, at least one a metal; steel is iron with carbon. Copper is used for wiring and pipes because it conducts well, can be bent but is hard enough, and does not react with water.
• Reactivity series Order: potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper, from most to least reactive; learn their reactions with water and dilute acids. A more reactive metal displaces a less reactive one from solution, for example Fe + Cu2+ → Fe2+ + Cu, a redox reaction.
• Extraction Gold is found uncombined. Metals less reactive than carbon, such as iron, are extracted by reducing the oxide with carbon. Metals more reactive than carbon, such as aluminium, need electrolysis of the molten compound, which uses a lot of energy. Phytomining and bioleaching obtain copper from low grade ores.
• Metal carbonates Carbonates of magnesium, copper, zinc, calcium and lithium decompose on heating to the metal oxide and carbon dioxide. Carbonates react with acids to give a salt, water and carbon dioxide.
• Electrolysis basics Positive ions move to the cathode and gain electrons (reduction); negative ions move to the anode and lose electrons (oxidation). Half equations include Pb2+ + 2e- → Pb and 2Cl- → Cl2 + 2e-.
• Aqueous solutions At the cathode the less reactive of the metal and hydrogen is produced; at the anode the product depends on relative ion concentrations. Electrolysis of sodium chloride solution gives hydrogen, chlorine and sodium hydroxide.
• Aluminium Aluminium oxide is dissolved in molten cryolite to lower the operating temperature. Aluminium forms at the negative electrode (Al3+ + 3e- → Al) and oxygen at the positive carbon electrode, which burns away as carbon dioxide.
• Required practical Investigate the products at the anode and cathode in the electrolysis of copper sulfate solution: with inert electrodes, copper forms at the cathode and oxygen at the anode.

Common mistakes in 3.3 Chemical Changes

• Saying positive ions go to the anode. Positive ions (cations) are attracted to the negative cathode.
• Writing half equations with electrons on the wrong side. Reduction at the cathode adds electrons on the left; oxidation at the anode has electrons on the right.
• Saying cryolite is a catalyst. It lowers the melting point of the electrolyte, saving energy.
• Saying sodium is produced at the cathode in sodium chloride solution. Hydrogen is produced because sodium is more reactive than hydrogen.
• Explaining displacement only as a colour change. State that the more reactive metal loses electrons (oxidised) and the less reactive metal ion gains them (reduced).

Try three 3.3 Chemical Changes exam-style questions