Given the heats of reaction below: Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) △H=−143kJ MgO(s)+2HCI(aq)→MgCI2(aq)+H2O(g) △H=−216kJ H2(g)+1/2O2→H2O(I) △H=−285kJ Calculate the heat for the overall reaction from the above data. Mg(s)+1/2O2→MgO(s)
A
−644kJ
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B
−212kJ
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C
+644kJ
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D
−74kJ
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E
+74kJ
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Solution
The correct option is A−212kJ For the net reaction Mg(s)+1/2O2→MgO(s) Reaction 1 and reaction 3 need to progress in forward direction and reaction 2 need to progress in backward direction. Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) △H=−143kJ ...... (1) MgCI2(aq)+H2O(g)→MgO(s)+2HCI(aq) △H=+216kJ ...... (2) H2(g)+1/2O2→H2O(I) △H=−285kJ ...... (3) -------------------------------------------- Mg(s)+2HCl(aq)+H2(g)+1/2O2+MgCl2(aq)+H2O(g)→MgO(s)+2HCl(aq)+MgCl2(aq)+H2(g)+H2O(l) Hence, the net reaction is Mg(s)+1/2O2→MgO(s) The overall heat would be then: Heat of Reaction 1 - heat of Reaction 2 + heat of Reaction 3 ΔH=−143kJ−(−216kJ)+(−285kJ)=−212kJ