WJEC Chemistry for AS Level: Revision Workbook

Question and mock answer analysis Q & A 1 Phosphoric acid, H 3 PO 4 , is used to make a range of phosphate salts. These are mixed with other substances to make agricultural fertilisers. (a) Some of the phosphate ions in fertilisers are trapped as insoluble calcium phosphate when the fertiliser is spread. (i) State the formula of the compound calcium phosphate. [1] (ii) Write an equation for the formation of calcium phosphate from phosphoric acid, H 3 PO 4 , and calcium carbonate, CaCO 3 . [2] (b) Phosphorous acid, H 3 PO 3 , is a different acid that decomposes on heating to form phosphoric acid and phosphine, PH 3 . (i) Write the equation for this reaction. [1] (ii) Use oxidation states to show that this is a redox reaction. [2] Interpretation At A Level you are expected to recall a range of common ions and deduce less familiar ions from given information. Here you need to recall the calcium ion and work out the formula and charge of the phosphate ion from the formula of the phosphoric acid. All equations at A Level are symbol equations and are often more challenging than many you have seen previously, so don’t be surprised to need numbers greater than 1, 2 or 3. You can be expected to show oxidation and reduction in different ways, but if the question refers to oxidation states then you will not gain credit for other methods. Dewi’s answer (a) (i) Ca 3 (PO 4 ) 2 ض (ii) 2H 3 PO 4 + 3CaCO 3 — Ca 3 (PO 4 ) 2 + 3CO 2 +3H 2 O ضض (b) (i) 4 H 3 PO 3 — 3 H 3 PO 4 + PH 3 ض (ii) The oxidation state of oxygen is −2 and hydrogen is +1 on both sides of the equation. At the start the phosphorus is +3 but it changes to −3 in PH 3 and to +5 in H 3 PO 4 . This shows that both oxidation and reduction have occurred. غض MARKER COMMENTARY (a) (i) This gains the mark. (ii) Dewi has recalled the products of a metal carbonate with an acid correctly from his previous studies. He has also balanced the equation correctly. (b) (i) The mark here is solely for balancing, as the formulae of all the compounds have been given. (ii) The oxidation states given for phosphorus are correct and gain a mark. Dewi states that oxidation and reduction have occurred but does not link this to the oxidation state values so does not gain the second mark. Overall Dewi gains 5 out of 6 marks as all the content is correct but he does not give enough detail to gain the final mark – references to oxidation and reduction need to link to oxidation states becoming more positive or more negative. 14 | WJEC Chemistry for AS Level: Revision Workbook | Unit 1: The Language of Chemistry, Structure of Matter and Simple Reactions