11.2. The chemical properties of the halogen elements and the hydrogen halides
A subsection of Chemistry, 9701, through 11. Group 17
Listing 10 of 102 questions
The halogens and their compounds have a wide variety of uses and the chemical and physical properties of the elements show regular patterns related to their positions in Group VII. Chlorine, bromine and iodine all react with hydrogen. State the trend in the reactivities of the halogens with hydrogen. Explain this trend in terms of bond energies. In the laboratory it is not very convenient to prepare hydrogen halides from their elements. Hydrogen halides can be prepared from their salts. Write an equation for the reaction of calcium chloride, CaCl 2, with concentrated sulfuric acid. Explain why hydrogen iodide is not prepared in this way. When potassium bromide, KBr, reacts with concentrated sulfuric acid, sulfur dioxide, SO2, is produced. State what you would see and write an equation for this reaction. Give the structures of the four structural isomers of C4H9Br and identify each as primary, secondary or tertiary. Name the isomer of C4H9Br that contains a chiral centre and draw the three-dimensional structures of the two optical isomers. name structures Aqueous silver nitrate solution was added to separate tubes containing chloroethane, bromoethane and iodoethane. The tubes were heated in a water bath. A yellow precipitate appeared fi rst in the tube containing iodoethane, followed by a cream precipitate in the tube containing bromoethane and fi nally a white precipitate appeared in the tube containing chloroethane. Explain these observations. Give the full name of the mechanism for the reaction between aqueous sodium hydroxide and bromoethane. Complete the diagram below to represent this mechanism. Include all necessary curly arrows, partial charges and lone pairs. C H Br H H C H HO– H C H OH + Br – H H C H H In the past, CFCs such as CF3Cl were widely used as refrigerants. State a property of CFCs which makes them suitable for use as refrigerants. State the damaging effect of CFCs in the upper atmosphere. Explain your answer.
9701_s14_qp_23
THEORY
2014
Paper 2, Variant 3
View
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
View
The halogens, chlorine, bromine and iodine, and their compounds, show a variety of similarities and trends in their physical and chemical properties. Give the colours and states of chlorine, bromine and iodine at room temperature and pressure. halogen colour state chlorine bromine iodine The halogens become less volatile down the group. Explain this trend in volatility. The halogens are oxidising agents. State and explain the trend in oxidising power of the halogens. Concentrated sulfuric acid reacts with solid sodium halides. State any observations that would be made on addition of concentrated sulfuric acid to ● solid sodium chloride, ● solid sodium iodide. Give reasons for the difference in the observations in . The addition of concentrated sulfuric acid to solid sodium bromide, NaBr, produces brown fumes and an acidic gas that decolourises acidified potassium manganate(solution. This acidic gas is a significant contributor to acid rain. Write the equation for the reaction of concentrated sulfuricacid with sodiumbromide. An aqueous solution, Z, contains a mixture of sodium chloride and sodium iodide. Excess aqueous silver nitrate is added to Z in a test-tube. A yellow precipitate forms. Explain the colour of this precipitate. Aqueous ammonia is then added to the test-tube in . The mass of precipitate decreases. Explain this observation.
9701_s17_qp_23
THEORY
2017
Paper 2, Variant 3
View
Questions Discovered
102