Chem Energy of Rxn Ws2

Secondary 4 Express Pure Chemistry – Energy of Reactions Wksheet 2 Page 1 of 3 Name: ______________________( ) Class: _________ Date: __________ Section A: Multiple Choice Questions 1 Which of the following is an endothermic reaction? A the reaction of nitrogen with oxygen in a car engine B the oxidation of carbon to carbon dioxide C the combustion of ethanol in air D the reaction of magnesium with dilute sulphuric acid 2 The formation of hydrogen bromide from hydrogen and bromine is an exothermic reaction. H H + Br Br H Br + H Br What may be deduced from this information? A The number of bonds broken is less than the number of bonds formed. B Energy is given out in breaking H H and Br Br bonds. C The H Br bonds are weaker than the H H and Br Br bonds. D The total energy change in bond formation is greater than that in bond breaking. 3 The reaction scheme shows how paraffin wax (C40H82) is converted into carbon dioxide and water in four stages. Which stages are exothermic? A I and II B I, II and III C III and IV D I and IV 4 When solid potassium iodide is added to water, the temperature of the liquid decreases. What deduction can be made from this observation? A The change is endothermic. B New bonds are made in the change. C The change is exothermic. D The solid forms ions in water. Section B: Structured Questions C40H82(s) C40H82(l) C40H82(g) 40CO2(g) +41H2O(g) Stage I Stage II Stage III Stage IV 40CO2(g) +41H2O(l)

Secondary 4 Express Pure Chemistry – Energy of Reactions Wksheet 2 Page 2 of 3 1 Samples of two compounds were dissolved in water at 28oC. The temperature of each solution was measured immediately after the solid had dissolved. name of compound formula of compound temperature / oC water solution temperature change on dissolving solid potassium carbonate 28 35 ammonium chloride 28 16 (a) Complete the table. [4] (b) State the type of energy change which takes place when ammonium chloride dissolves in water. Give a reason for your answer. type of energy change: .............................................................................................. reason: ................................................................................................................................ [2] 2 When liquid oxygen (O2) is mixed with liquid ethanol (C2H5OH), a very fast exothermic reaction takes place, which has been used to propel rockets. The reaction products are steam and carbon dioxide gas. (a) (i) Construct the equation, with state symbols, for the reaction. ............................................................................................................................... (ii) Is H for this equation, positive or negative? Explain your answer. ................................................................................................................................ [2] (b) One of the bonds in ethanol can be represented as C-C. Show in a similar way (i) two other bonds which are broken in this reaction. ..................................................................................................................... (ii) two bonds which are formed in this reaction. ..................................................................................................................... [4] (c) If the rocket is loaded with 23 kg of ethanol, calculate the mass of liquid oxygen needed to react with it.

Secondary 4 Express Pure Chemistry – Energy of Reactions Wksheet 2 Page 3 of 3 ........................................................................................................................ ......................................................................................................................... .......................................................................................................................... [3] Section C: Essay Questions 1 Ammonia decomposes according to the equation: 2NH3  N2 + 3H2; H = +92 kJ/mol (a) What is the total number of bonds in two molecules of ammonia? (b) Explain why this reaction is endothermic. [3] 2 Hydrogen chloride gas can be manufactured by reacting hydrogen and chlorine together, in an exothermic reaction. The equation for this reaction is shown below. H H + Cl Cl H Cl + H Cl (a) (i) What is meant by the term exothermic? (ii) Explain in terms of the bonds being made and broken, why this reaction is exothermic. [3] (b) Calculate the mass of hydrogen chloride that can be made from 9600 dm3 of chlorine gas, measured at r.t.p. [2]