Hi Albert, the value of ΔH given with the balanced equation for the reaction tells you how much heat is released or absorbed when the moles of substances given in the equation react. The moles of substances in the equation are simply the coefficients in the balanced equation.
So, in each of these problems, you need to calculate the moles of substances reacted. If the amount of a reactant is not given, you can assume that it is present in sufficient amount that the reactant whose amount is given will react completely. That is, the reactant in given amount is the limiting reactant.
In the first problem, first calculate the moles of HCl and the moles of NaOH. Which is the limiting reactant? Once you determine that, you can calculate the amount of heat released. For example, if HCl is the limiting reactant, you would have,
moles of HCl X 56.2 kJ =
1 1 mol HCl
If NaOH is the limiting reactant, you would have,
moles of NaOH X 56.2 kJ =
1 1 mol NaOH
In the second problem, you need to calculate the moles of aluminum. The amount of Fe2O3 is not given, so we have to assume that aluminum is the limiting reactant. Then,
moles of Al X 849 kJ =
1 2 mol Al
Same thing with the third problem. Ethanol is the limiting reactant since the amount of oxygen was not given (and will be in excess amount if the ethanol is burned in the open air).
Steve
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