18 (A410U10-1) Examiner only 14. Materials are often classified according to their physical properties and chemists use their knowledge of their structures to explain these properties. (a) One way to classify materials is according to their physical state. The halogens chlorine, bromine and iodine have different physical states at room temperature. Give the physical state for each of these halogens and explain why they have different physical states. [3] © WJEC CBAC Ltd. Substance Solubility in water at 20 °C / g dm–3 CaCl2 (anhydrous) 745 CaCl2.4H2O (hydrated) 908 butan-1-ol (CH3CH2CH2CH2OH) 73 octan-1-ol (CH3CH2CH2CH2CH2CH2CH2CH2OH) 0.46 (A410U10-1) Turn over. 19 Examiner only (i) A student says that this shows that the concentration of calcium ions in a saturated solution of calcium chloride is the same for solutions formed by dissolving anhydrous and hydrated forms of CaCl2. Is he correct? Give a reason for your answer. [2] (ii) Explain why the alcohols butan-1-ol and octan-1-ol can dissolve in water, giving a reason why the solubility of octan-1-ol is lower than that of butan-1-ol. [3] © WJEC CBAC Ltd. (b) Another way to classify materials is according to their solubility in water. The solubilities of four compounds were found in an online database. 20 (A410U10-1) Examiner only (c) The physical properties of materials can be modified by using additives. AIBN is an additive used to modify the properties of rubber. AIBN decomposes in solution in the solvent dioxane, shown as (sol) below, under standard conditions. NC—C(CH3)2—N=N—C(CH3)2—CN (sol) 2NC—C(CH3)2• (sol) + N2(g) AIBN (i) Give the temperature and pressure used as standard conditions. [1] (ii) The reaction can be followed by measuring the absorbance of the reactant in the solution at a wavelength of light of 350 nm. The dioxane solvent also absorbs a certain amount of light of this wavelength. The graph below shows the results of this experiment undertaken by two students, Anna and Megan. © WJEC CBAC Ltd. 0 10 20 30 40 50 60 70 80 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 Time / s Absorbance / % (A410U10-1) Turn over. 21 Examiner only I. State the percentage absorbance due to the solvent. Explain how you reached your conclusion. [2] II. Describe and explain fully the shape of the graph. Use the graph to prove that the reaction is first order with respect to AIBN. [6 QER] © WJEC CBAC Ltd. 22 (A410U10-1) Examiner only (iii) Upon heating, AIBN decomposes extremely rapidly. The rate equation for this process is as follows. rate = k[AIBN] The value of the rate constant k, can be found using the Arrhenius equation. Anna incorrectly writes the Arrhenius equation as I. State the correct Arrhenius equation. [2] II. Anna uses the correct temperature, frequency factor and activation energy in her incorrect Arrhenius equation. The values of two of these are given below. frequency factor, A = 6.92 × 109 s–1 temperature = 600 K Anna calculates that the rate constant is 4.89 × 1082 s–1. Megan says the true value should be much smaller. Find the value of the activation energy then use the correct Arrhenius equation to find the true value of the rate constant. State whether Megan is correct. [4] Ea = ........................................................ kJ mol–1 k = ........................................................ s–1 © WJEC CBAC Ltd. 23 k = A e Ea T (A410U10-1) Turn over. © WJEC CBAC Ltd. 23 Examiner only 15. Ethyne, H—C C—H, is commonly known as acetylene and is burned as a fuel in oxy-acetylene welding torches. It is stored in cylinders where the gas is dissolved in propanone and this is adsorbed onto an inert substance and kept under pressure. (a) Propanone is used to dissolve the ethyne as this gas is only slightly soluble in water. State why the solubility of ethyne in water is low. [1] (b) (i) When used in an oxy-acetylene torch the ethyne is released at a pressure of 135 kPa at 20 °C. Find the number of moles of ethyne in 1 cm3 and hence calculate the density of ethyne gas, in g cm–3, at this pressure at 20 °C. [4] density = mass ÷ volume Density = ........................................................ g cm–3 (ii) The density of dry air at 0 °C and 135 kPa is 1.27 × 10–3 g cm–3. A student says that this shows that a vessel of negligible mass filled with ethyne will float in air. Calculate the density of dry air at 20 °C and 135 kPa and show whether the student is correct. [2] Density = ........................................................ g cm–3 — — — 24 (A410U10-1) Examiner only © WJEC CBAC Ltd. (c) A student wrote the equation below for the standard enthalpy of combustion (ΔcHθ) of ethyne. 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g) (i) Identify two errors that the student has made. [2] (A410U10-1) Turn over. 25 Examiner only (ii) The student found the following data in a reference source. He wanted to calculate the most accurate value for the enthalpy change for his reaction. © WJEC CBAC Ltd. Standard enthalpy values / kJ mol–1 ΔfHθ [C2H2(g)] 227 ΔfHθ [CO2(g)] –394 ΔfHθ [H2O(l)] –242 ΔvaporisationHθ [H2O(l)] 41 Bond Bond energy / kJ mol–1 C—H 412 C C 837 C O 743 O—H 463 O O 496 — — — Select appropriate data to calculate the most accurate value you can for the enthalpy change for the reaction below. Explain your choice of method. [4] 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g) Enthalpy change of reaction = ........................................................ kJ mol–1 — — — — 26 (A410U10-1) (iii) The student decided to compare his value with one obtained experimentally. He used the apparatus and method given below. © WJEC CBAC Ltd. thermometer conical flask appropriate volume of water gas burner containing ethyne Method Measure an appropriate volume of water into the conical flask. Measure the mass of the gas burner containing the ethyne on a 3 decimal place balance. Select a thermometer that has 0.2 °C as its smallest division and place this in the conical flask. Record the temperature of the water and then immediately light the gas burner. Heat the water for 2 minutes. Extinguish the gas burner and record the temperature of the water and the mass of the gas burner. (A410U10-1) 27 Examiner only I. The difference between the initial and final temperature readings is 37.4 °C. Calculate the percentage error in this value. [1] Percentage error = ........................................................ % II. An appropriate volume of water was selected for the experiment. Explain why a volume which was much smaller or much greater than this would give results which were of a lower accuracy. [2] Much smaller volume of water ........................................................................................................................... Much greater volume of water ........................................................................................................................... III. Suggest two improvements to the method and explain how these would lead to improved results. [2] END OF PAPER © WJEC CBAC Ltd. 18 (A410U10-1) 28 Examiner only © WJEC CBAC Ltd. For continuation only.