Phosphoric(V) acid, H3PO4, is used in both inorganic and organic reactions.

1. (a)(i)H3PO4 is made in a two-step process from phosphorus. step 1 Phosphorus reacts with an excess of oxygen to form a white solid. step 2 The white solid then reacts with water to form H3PO4. Write an equation for each step.[2]
2. (a)(ii)H3PO4 is a weak Brønsted-Lowry acid. Define weak Brønsted-Lowry acid.[2]
3. (b)(i)H3PO4 is also formed in the process shown in reaction 1. reaction 1 4H3PO3 → 3H3PO4 + PH3 Table 3.1 shows some relevant thermodynamic data. [Table 3.1] compound | enthalpy change of formation, ΔHf / kJ mol-1 ---|--- H3PO3 | -972 H3PO4 | -1281 PH3 | +9 Define enthalpy change of formation.[2]
4. (b)(ii)Use the data in Table 3.1 to calculate the enthalpy change, ΔHᵣ, of reaction 1.[2]
5. (b)(iii)Explain why reaction 1 is a disproportionation reaction. Explain your reasoning with reference to relevant oxidation numbers.[2]
6. (c)(i)Fig. 3.1 shows a reaction scheme that involves H3PO4 in several reactions. [Figure 3.1] Identify A, which reacts with propene in the presence of H3PO4 in reaction 2.[1]
7. (c)(ii)Draw the structure of B.[1]
8. (c)(iii)Name the type of reaction that occurs in reaction 3.[1]
9. (c)(iv)Reaction 3 is monitored using infrared spectroscopy. It is not possible to use the O—H absorption frequency to monitor the reaction. Use Table 3.2 to identify a suitable bond whose absorption frequency can be used to monitor the progress of reaction 3. State the change you would see in the infrared spectrum during reaction 3. [Table 3.2] bond | functional groups containing the bond | characteristic infrared absorption range (in wavenumbers)/cm-1 ---|---|--- C–O | hydroxy, ester | 1040–1300 C=C | aromatic compound, alkene | 1500–1680 C=O | amide | 1640–1690 | carbonyl, carboxyl | 1670–1740 | ester | 1710–1750 C–H | alkane | 2850–2950[2]
10. (d)(i)H3PO4 also reacts with alcohols to form organophosphates. Organophosphates are compounds similar to esters. They have the general structure shown in Fig. 3.2. [Figure 3.2] Complete the equation to suggest the products of the reaction of H3PO4 with methanol, CH3OH. H3PO4 + 3CH3OH →[1]
11. (d)(ii)Compound T is a simple organophosphate. The mass spectrum of T shows a molecular ion peak at m/e = 182. This peak has a relative intensity of 12.7. The relative intensity of the M +1 peak is 0.84. Deduce the number of carbon atoms in T. Hence suggest the molecular formula of T. Assume that phosphorus and oxygen exist as single isotopes. Show your working.[3]

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