Heat of Combustion: Magnesium In this experiment, you will use Hess\'s Law to de

Question

Heat of Combustion: Magnesium In this experiment, you will use Hess's Law to determine a heat of reaction that would be difficult to obtain by direct measurement - the heat of combustion of magnesium ribbon. The reaction is represented by the equation: (4) Mgis) 1/2 02)Mg0(s) This equation can be obtained by combining, in some way, equations (1), (2), and (3): (1) MgO(s) + 2 HCl(aq)MgCl2(ag) +H200 (2) Mg(s) 2 HClaq)MgCl2(aq) H2g) (3) H2(g) 1/2 02g) H200) The pre-lab exercise below requires you to combine equations (1). (2), and (3) to obtain equation (4) before you do the experiment. Do this and include it as a part of your experiment summary. Heats of reaction (AH) for equations (1) and (2) will be determined in this experiment. AH for reaction (3) is -285.8 kJ/mol of H,o. PRE-LAB EXERCISE In the space provided below, combine equations (1). (2), and (3) to obtain equation (4). PROCEDURE With the LabQuest on, choose New from the File menu, discard any unsaved data. On the Meter screen, tap Rate. Change the data- collection rate to 0.5 sample/second (interval of 2 seconds/sample) 1. h to 500 sconds. Select OK.

Explanation / Answer

MgO+2HCl --- > MgCl2 + H2O      ?H°= -151 kJ

MgO+2HCl --- > MgCl2 + H2O + 151 kJ    ----------------- > (1)

Mg(s) + 2HCl(aq) --> MgCl2 (s) +H2 (g)   ?H°= -501 kJ

Mg(s) + 2HCl(aq) --> MgCl2 (s) +H2 (g) + 501 kJ ----------------- > (2)

H2(g) + 1/2 O2(g) --> H2O(l)    ?H°= -285.8 kJ

H2(g) + 1/2 O2(g) --> H2O(l) + 285.8 kJ ----------------- > (3)

eq : (2)+ (3) -(1)

Mg(s) + 2HCl(aq) + H2(g) + 1/2 O2(g) -MgO- 2HCl ------- > MgCl2 (s) +H2 (g) + 501 kJ+ H2O(l) + 285.8 kJ

                                                                                    - MgCl2 - H2O - 151 kJ

Mg(s)+ 1/2 O2(g) -MgO   ---- > 635.8 KJ

Mg(s)+ 1/2 O2(g) ---- > MgO + 635.8 KJ

Mg(s)+ 1/2 O2(g) ---- > MgO    ?H°= - 635.8 KJ

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