Hydrogen halides are compounds formed when halogens (Group 17 elements) react with hydrogen. The bond polarity of the hydrogen halides decreases from HF to HI. Some relevant data are shown in the table. hydrogen halide | HF | HCl | HBr | HI ---|---|---|---|--- boiling point/°C | 19 | -85 | -67 | -35 H–X bond energy/kJ mol⁻¹ | 562 | 431 | 366 | 299

1. (a)(i)Explain the meaning of the term bond polarity.[1]
2. (a)(ii)Suggest why the boiling point of HF is much higher than the boiling points of the other hydrogen halides.[2]
3. (a)(iii)Describe and explain the relative thermal stabilities of the hydrogen halides.[3]
4. (b)(i)The equation for the preparation of hydrogen chloride using concentrated sulfuric acid is shown. H₂SO₄ + NaCl → NaHSO₄ + HCl Use the Brønsted-Lowry theory of acids and bases to identify the base and its conjugate acid in this reaction. Explain your answer. Brønsted-Lowry base (base-I) = conjugate acid (acid-II) =[2]
5. (b)(ii)Explain why the reaction of concentrated sulfuric acid and sodium iodide is not suitable for the preparation of hydrogen iodide.[2]
6. (c)(i)Hydrogen chloride undergoes a reversible reaction with oxygen. 4HCl(g) + O₂(g) ⇌ 2Cl₂(g) + 2H₂O(g) The reaction is carried out at 400 °C in the presence of a copper(II) chloride catalyst. Use the data in the table to calculate the overall enthalpy change of reaction. compound | enthalpy change of formation/kJmol⁻¹ ---|--- HCl(g) | -92 H₂O(g) | -242 enthalpy change of reaction = .................... kJ mol⁻¹[2]
7. (c)(ii)State the type of catalyst used in this reaction. Explain how a catalyst is able to increase the rate of a chemical reaction.[2]
8. (c)(iii)The reaction exists in dynamic equilibrium. The reaction was repeated at 1000 °C and the same pressure. State and explain the effect on the composition of the equilibrium mixture of the change in temperature.[2]
9. (c)(iv)When 1.60 mol of HCl are mixed in a sealed container with 0.500 mol of O₂ at 400°C, 0.600 mol of Cl₂ and 0.600 mol of H₂O are formed. The total pressure inside the container is 1.50 × 10⁵ Pa. • Calculate the amounts, in mol, of HCl and O₂ in the equilibrium mixture. HCl = .................... mol O₂ = .................... mol • Calculate the mole fraction of Cl₂ and hence the partial pressure of Cl₂ in the equilibrium mixture. mole fraction of Cl₂ = .................... Pcl₂= .................... Pa[3]
10. (c)(v)In a separate experiment, an equilibrium reaction mixture was found to contain the four gases at the partial pressures shown in the table. gas | HCl | O₂ | Cl₂ | H₂O ---|---|---|---|--- partial pressure/Pa | 4.8 × 10⁴ | 3.0 × 10⁴ | 3.6 × 10⁴ | 3.6 × 10⁴ Kₚ = (P_Cl₂)² × (P_H₂O)² / ((P_HCl)⁴ × P_O₂) Use this information and the expression given for Kₚ to calculate a value for Kₚ. State the units of Kₚ.[2]
11. (c)(vi)The reaction is repeated without a catalyst. State the effect of this on Kₚ.[1]