ACSBR 18E Energy Changes (Extensions) - Assignment ANS

Anglo-Chinese School (Barker Road) A Methodist Institution Founded in 1886 CHEMISTRY DEPARTMENT OF SCIENCE Name: ANSWERS ( ) Class: SEC 4 ENERGY CHANGES (EXTENSIONS) – ASSIGNMENT ALEX LEE & SHERRI TAN/2010 PAGE 1 OF 8 PURE Multiple-Choice Questions [20 Marks] TOTAL SCORE / 30 Write in your selected answer for the multiple-choice questions in the boxes provided. 1 2 3 4 5 6 7 8 9 10 C A C D D D B B B A 11 12 13 14 15 16 17 18 19 20 D B C A A D A A B A 1. What is meant by an ‘endothermic’ reaction? A A catalyst is needed for the reaction to take place. B Light energy is absorbed during the reaction. C The reactants have less energy than the products of the reaction. D They are chemical reactions which require heat to start. 2. A pupil added 0.06 mol of sodium nitrate to 100 cm3 of water at 20.0 °C. The enthalpy change of solution of sodium nitrate is +20.5 kJ/mol. After dissolving the solute, the temperature of the solution will be about A 18.0 °C B 20.0 °C C 23.0 °C D 26.0 °C 3. Early photographers used the oxidation of magnesium powder to produce flashes of light in the presence of oxygen from the air. Magnesium reacts according to the equation: 2 Mg (s) + O2 (g) ⎯→ 2 MgO (s) ΔH = – 2400 kJ/mol When 0.24 g of magnesium reacts with excess oxygen, the energy released would be A 3.00 kJ B 6.00 kJ C 12.0 kJ D 24.0 kJ 4. The combustion of octane, C8H18, can be expressed in the equation below. 2 C8H18 (l) + 25 O2 (g) ⎯→ 16 CO2 (g) + 18 H2O (g) Given that combusting 57 grams of octane results in x kJ of energy being evolved, what is the enthalpy change for the above reaction? A + x kJ B + 2x kJ C – 0.5x kJ D – 4x kJ

ALEX LEE & SHERRI TAN/2010 PAGE 2 OF 8 5. The hydration of quicklime to become slaked lime is an exothermic reaction, and can be expressed in the following equation: CaO (s) + H2O (l) ⎯→ Ca(OH)2 (s) ΔH= – 65.2 kJ/mol An unknown mass of quicklime was reacted as above. Given that the mass of the quicklime increased by 37.3 grams in the reaction, calculate the energy evolved. A 32.9 kJ B 43.4 kJ C 65.2 kJ D 135 kJ 6. Hydrogen reacts with chlorine. H2 (g) + Cl2 (g) ⎯→ 2 HCl (g) ΔH= – 184 kJ/mol Why does the enthalpy change for this reaction have a negative sign? A Hydrogen and chlorine are covalent, but hydrogen chloride is ionic. B More bonds are formed than are broken. C The speed of the reaction increases as temperature increases. D The total energy of bond breaking is less than that of bond forming. 7. The energy level diagram for the reaction between aqueous sodium hydroxide and dilute hydrochloric acid is shown. What can be deduced from the diagram? A The OH- ions have more energy than the H+ ions. B The products contain less energy than the reactants. C The reaction has a high activation energy. D The reaction occurs readily at room temperature. 8. A series of chemical r