OCR A Level Chemistry B Elements from the Sea (ES) Exam Questions

About OCR A Level Chemistry B Elements from the Sea (ES)

Elements from the Sea is a Year 1 storyline of OCR A Level Chemistry B (Salters) about extracting useful elements, especially the halogens, from seawater. It develops redox chemistry using oxidation states and half-equations, applies it to electrolysis and the reactions of halogens and halide ions, and uses iodine-thiosulfate titrations to analyse oxidising agents such as bleach. It also introduces equilibrium and le Chatelier's principle in the context of industrial processes. Examiners test oxidation state changes, half-equations, observations of halogens in solvents and with silver nitrate, and titration calculations with multiple steps.

What you need to know

• Redox and oxidation states Oxidation is loss of electrons or increase in oxidation state. Assign oxidation states using standard rules and use Roman numerals in names such as chlorate(I) and chlorate(V).
• Half-equations and electrolysis Write half-equations for reactions at electrodes, such as 2Cl- gives Cl2 + 2e- at the anode. Electrolysis of brine produces chlorine, hydrogen and sodium hydroxide.
• Halogens Physical properties and colours in water and in organic solvents; oxidising power decreases down the group, shown by displacement reactions. Disproportionation of chlorine in alkali to make bleach.
• Halide ions Silver nitrate gives white AgCl, cream AgBr and yellow AgI precipitates with different solubilities in ammonia. Halide ions become stronger reducing agents down the group, as shown with concentrated sulfuric acid.
• Iodine-thiosulfate titrations An oxidising agent liberates iodine from excess iodide; the iodine is titrated with sodium thiosulfate using starch near the end point: I2 + 2S2O3 2- gives 2I- + S4O6 2-.
• Equilibrium Dynamic equilibrium and le Chatelier's principle explain how changing conditions affects the position of equilibrium in processes such as making hydrogen halides or chlorine.

Common mistakes in Elements from the Sea (ES)

• Writing halogen colours in organic solvents the same as in water. Iodine is brown in water but violet or purple in a hydrocarbon solvent.
• Adding starch at the start of the titration. It is added when the solution is pale yellow, near the end point.
• Forgetting the 2 : 1 ratio of thiosulfate to iodine in calculations.
• Not balancing charge in half-equations.
• Saying chloride ions reduce concentrated sulfuric acid. Only bromide and iodide are strong enough reducing agents.

Try three Elements from the Sea (ES) exam-style questions