11. Group 17
A section of Chemistry, 9701
Listing 10 of 617 questions
The hydrogen halides, HCl, HBr and HI, can undergo thermal decomposition. In a sealed container an equilibrium is established according to the equation shown. 2HXH2+ X2(where X = Cl, Br or Some bond energies are shown in the table. bond energy / kJ mol–1 H–Br H–H Br–Br Use these data to calculate a value for the enthalpy change, ΔH, for the thermal decomposition of hydrogen bromide, HBr, according to the equation shown. ΔH = kJ mol–1 At a temperature of 700 K a sample of HBr is approximately 10% decomposed. Changing the temperature affects both the rate of decomposition of HBr and the percentage that decomposes. The Boltzmann distribution for a sample of HBr at 700 K is shown. Ea represents the activation energy for the reaction. proportion of molecules with a given energy molecular energy Ea Using the same axes, sketch a second curve to indicate the Boltzmann distribution at a higher temperature. With reference to the curves, state and explain the effect of increasing temperature on the rate of decomposition of HBr. The decomposition of HBr is endothermic. State the effect of increasing temperature on the percentage of HBr that decomposes. Use Le Chatelier’s principle to explain your answer. At 700 K HBr is approximately 10% decomposed but hydrogen iodide, HI, is approximately 20% decomposed. Explain this difference with reference to bond strengths and the factors that affect them. At temperatures above 1500 K, HCl will decompose. A sample of 0.300 mol of HCl decomposed in a sealed container. The resulting equilibrium mixture was found to contain 1.50 × 10–2 mol of Cl 2. Calculate the amounts, in mol, of H2 and HCl present in the equilibrium mixture. H2 = mol HCl = mol Calculate the mole fraction of each gas in the equilibrium mixture. mole fraction of HCl = mole fraction of H2 = mole fraction of Cl 2 = In another experiment under different conditions, an equilibrium mixture was produced with mole fractions for each species as shown. species mole fraction HCl 0.88 H2 0.06 Cl 2 0.06 Write the expression for the equilibrium constant, Kp, for the decomposition of HCl. 2HCl H2+ Cl 2Kp = Explain why the total pressure of the system does not need to be known for Kp to be calculated for this experiment. Calculate the value of Kp for this experiment. Kp =
9701_s17_qp_22
THEORY
2017
Paper 2, Variant 2
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