Silver jewellery tarnishes because a black layer of silver sulfide, Ag₂S, forms on its surface.
One way to remove the tarnish is to put the jewellery on aluminium foil in a hot solution of sodium hydrogencarbonate. The silver sulfide turns back into silver.
The overall reaction can be written as:
3Ag2S(s)+2Al(s)→6Ag(s)+Al2S3(s) \mathrm{3Ag_{2}S(s) + 2Al(s) \rightarrow 6Ag(s) + Al_{2}S_{3}(s)} 3Ag2S(s)+2Al(s)→6Ag(s)+Al2S3(s)Silver sulfide contains Ag⁺ ions and S²⁻ ions. Aluminium sulfide contains Al³⁺ ions and S²⁻ ions.
A student says:
"This is not a redox reaction, because no oxygen is gained or lost."
Evaluate the student's statement.
Write the half equations for the change of silver ions to silver atoms and for the change of aluminium atoms to aluminium ions.
Use your half equations to explain why 2 aluminium atoms react with 6 silver ions.
The sulfide ions are not oxidised or reduced in this reaction.
Explain how you can tell this from the information given.
Aluminium also reacts with iron oxide in the thermite reaction.
2Al+Fe2O3→Al2O3+2Fe \mathrm{2Al + Fe_{2}O_{3} \rightarrow Al_{2}O_{3} + 2Fe} 2Al+Fe2O3→Al2O3+2FeIdentify the substance that is oxidised and the substance that is reduced. Explain your answer in terms of oxygen.
53 exam-style questions on OCR GCSE Chemistry 3.3 Types of chemical reactions, covering 3.3.1 Oxidation and reduction in terms of oxygen, 3.3.2 Acids, alkalis and neutralisation, 3.3.3 The pH scale and measuring pH, 3.3.4 Oxidation and reduction in terms of electrons, 3.3.5 Dilute and concentrated, weak and strong acids, and 3.3.6 Hydrogen ion concentration and pH. Each one has a worked solution and a mark scheme showing where the marks go.