WJEC Chemistry for AS Level: Revision Workbook

Rebecca’s answer (a) (i) Ca 3 (PO 4 ) 2 ض (ii) 2H 3 PO 4 + 3CaCO 3 — Ca 3 (PO 4 ) 2 +3CO 2 + 2H 2 O غض (b) (i) 4 H 3 PO 3 — 3 H 3 PO 4 + PH 3 ض (ii) Hydrogen is reduced as its oxidation state goes from +1 to a mixture of +1 in H 3 PO 4 and −1 in PH 3 . Phosphorus is +3 in H 3 PO 3 and stays as this in PH 3 . غغ MARKER COMMENTARY (a) (i) The correct formula gains the mark. (ii) Rebecca has remembered that acids form a salt, water and carbon dioxide in their reactions with metal carbonates and so gains a mark for identifying the products. Her equation is not correctly balanced so she cannot gain the second mark. (b) (i) She has correctly balanced the equation and so she gains 1 mark. (ii) The oxidation states given for phosphorus and hydrogen at the start are correct; however, those in the products are incorrect. She does not show that the redox reaction includes both reduction and oxidation and so cannot gain the second mark. Overall Rebecca gains 3 out of 6 marks as she can write correct formulae and balance some equations but cannot use oxidation states for unfamiliar compounds and reactions. Q & A 2 Silver bromide, AgBr, was used in early photography due to this compound decomposing in the presence of light. (a) Silver bromide is a precipitate formed when silver nitrate solution is added to a solution containing bromide ions. (i) Write the ionic equation for this reaction. Include state symbols. [2] (ii) Give the colour of the precipitate of silver bromide. [1] (b) The decomposition of silver bromide in the presence of light produces the elements silver and bromine. (i) Write an equation for this process. [1] (ii) Show that this is a redox reaction whilst the precipitation reaction in part (a) is not a redox reaction. [3] (c) Many unreactive metals such as silver do not react easily with acids. (i) Silver can react with nitric acid in the presence of air to give silver nitrate, water and nitric oxide, NO. Balance the equation for this reaction. [1] ............ Ag + ............ HNO 3 r ............ AgNO 3 + ............ NO + ............ H 2 O (ii) Other unreactive metals only dissolve in aqua regia, a mixture of concentrated nitric and hydrochloric acid that react to produce products including Cl 2 and NOCl. Complete the equation for this process. [2] ............ HNO 3 + ............ HCl r .................................................................................... Interpretation At A Level you are expected to build upon the work you have studied previously, which includes common reactions, such as those of acids and bases and simple chemical tests, and this question requires you to recall these. All equations at A Level are symbol equations but state symbols are only required where stated, such as in part (a). You can be expected to show oxidation and reduction in different ways, and in part (b) (ii) you are free to use any appropriate method as the question does not limit you. In most cases oxidation states are the most straightforward but gain or loss of electrons is also acceptable in this example. 1.1: Formulae and equations | 15 Unit 1 Practice questions