OCR A Jun 2013 Paper 1 Q3

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The hydrides of Group 5 elements all exist as gases at room temperature. (a) Phosphine gas, PH3, can be prepared by adding phosphorus, P4, to warm concentrated aqueous sodium hydroxide as shown in the equation below. P4(s) + 3NaOH(aq) + 3H2O(l) PH3(g) + 3NaH2PO2(aq) (i) Using oxidation numbers, explain why this is a disproportionation reaction.[3] (ii) A chemist reacts 1.86 g of P4 with excess NaOH(aq). Calculate the volume of phosphine gas, in cm3, produced at room temperature and pressure, RTP. volume of phosphine gas =cm3 [2] (b) Phosphine gas burns in air to form an oxide of phosphorus, P4O10, and water. Write the equation for this reaction.[1]OCR 2013<br />
 (c) Phosphoric acid, H3PO4, can be made by reacting P4O10 with water. Sodium phosphate, Na3PO4, is a salt that can be prepared by reacting H3PO4 with sodium hydroxide, NaOH. A student prepared a solution of Na3PO4 by reacting 15.0 cm3 of 0.100 mol dm3 H3PO4 with 0.200 mol dm3 NaOH. (i) Why is Na3PO4 described as a salt of H3PO4?[1] (ii) Calculate the amount, in moles, of H3PO4 in 15.0 cm3 of 0.100 mol dm3 H3PO4. amount =mol [1] (iii) The equation for the preparation of Na3PO4 from NaOH and H3PO4 is shown below. 3NaOH(aq) + H3PO4(aq) Na3PO4(aq) + 3H2O(l) Calculate the volume of 0.200 mol dm3 NaOH that reacts exactly with 15.0 cm3 of 0.100 mol dm3 H3PO4. volume =cm3 [1]OCR 2013 Turn over<br />
 (d) Ammonia, NH3, is another gaseous Group 5 hydride. NH3 and PH3 are both simple molecules. The boiling points of NH3 and PH3 are shown in the table below. Group 5 hydride Boiling point / C NH3 PH3 33 88 (i) Complete the table below to show the main intermolecular forces present in NH3 and PH3. Group 5 hydride Main intermolecular force NH3 PH3 [2] (ii) Suggest why PH3 has a lower boiling point than NH3.[1] (e) NH3 reacts with molecules of BF3 to form H3NBF3, shown below. One of the bonds in H3NBF3 is a dative covalent bond. (i) A covalent bond is a shared pair of electrons. What is a dative covalent bond?[1]OCR 2013<br />
 (ii) Draw a dot-and-cross diagram to show the bonding in H3NBF3. Label the dative covalent bond in your diagram. Show outer electrons only. (iii) The FBF bond angle in BF3 is different from the FBF bond angle in H3NBF3. Complete the table to predict the FBF bond angles in BF3 and in H3NBF3. FBF bond angle /Molecule BF3 H3NBF3 [2] [2] (iv) The HNH bond angle in NH3 is 107. A student predicted that the HNH bond angle in H3NBF3 is larger. Explain why the student might expect the HNH bond angle to be larger in H3NBF3 than in NH3.[3]OCR 2013 [Total: 20] Turn over<br />

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