Heat of Combustion: Magnesium In this experiment, you will use Hess\'s Law to de
ID: 1056669 • Letter: H
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) Mg(s) + 1/2 O2(g) ÷ MgO(s) This equation can be obtained by combining, in some way, equations (1), (2), and (3):
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 (?H) for equations (1) and (2) will be determined in this experiment. ?H for reaction (3) is S285.8 kJ/mol of H2O.
(1) Mg0(s) +2 HCI(aq) MgCl2(aq) +H2O(1) H2(g) Mg(s)+2 HCl(aq)MgC12(a) (2) (3) H2(g) +1 /2O2(g) H2O(1)Explanation / Answer
(4) Mg(s) + 1/2 O2(g) ÷ MgO(s)
we need Mg(s) in the lef,t so eqution 2 remains the same... we need MgO to the right so equation (1) must be inverted
so...
MgCl2 = MgO + 2HCl HRxn = -HRxn1 (must be inverted, so negativ esign)
Mg + 2HCl = MgCl2 + H2 HRxn = HRxn2
H2 + 1/2O2 = H2O HRxn = HRxn3
add all
MgCl2 + Mg + 2HCl + H2 + 1/2O2= MgO + 2HCl +MgCl2 + H2 +H2O
cancel common terms
Mg + 1/2O2= MgO +H2O
which is what we wanted
so HRxn 4 = - HRxn1 + HRxn2 + HRxn3
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