Homework Help: Chemistry

Posted by Kara on Sunday, March 27, 2011 at 11:45pm.

In the classroom, we reacted
Mg(s) + 2HCl -> MgCl2(aq) + H2 (g)
(0.31 g of Mg(s), and 99.46 g of HCl)
Initial temp. = 22.5 celsius, max. temp. 36.0 celsius

and

MgO(s) + 2HCl -> MgCl2(aq) + H2(g)
(1.00 g of MgO(s) and 97.71 g of HCl)
Initial temp. 22.0 celsius, max. temp. 28.5 celsius.

Please note that the temperatures were crudely taken, so a wide range of experimental error is likely.

When calculating the deltaH for the first equation, this is what I got

Q=mcDeltaT
Q=(99.46)(4.18)(13.5)
Q=5612.5 J
Q=5.6 kJ

and the second reaction;

Q=mcDeltaT
Q=(97.71)(4.18)(6.5)
Q=2654 J
Q=2.7 kJ

My question is, how do those numbers make any sense? Or am I just confused as to what those numbers represent? Thanks in advance!

Chemistry - DrBob222, Monday, March 28, 2011 at 12:22am
The first one is
q = mass H2O x specific heat H2O x (Tfinal-Tinitial)
The water is what absorbed the heat generated by the reaction.
Your value of 5.6 kJ is the heat generated by 0.31 g or by (0.31/24.3) moles Mg. The usual reporting is done in kJ/mol; therefore, 5.6 kJ x (24.3/0.31) = kJ/mol.
I note that the second does NOT produce H2 gas. The reaction is
MgO + 2HCl ==> MgCl2 + H2O.

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