<p>I'm struggling to do an AP chem lab on Hess's law right now. Any help would be appreciated. I'm stuck on one of the calculations.</p>
<p>Calculate the value of enthalpy change (delta H) in terms of kJ/mol for each of the reactions.</p>
<p>First reaction: reaction between HCl and NaOH
heat of reaction (Q rxn) = -841.18 J.</p>
<p>Thanks:-)</p>
<p>Well first you have to balance the formula
HCL + NaOH > H20 + NaCl therefore you kno that there is one mole of each reactants in this reaction and from that convert you -841.18 J into KJ ie -.84118 and since you have one mole shouldnt it just be -.84118KJ/mol </p>
<p>some one back me up on this...its been a while</p>
<p>that's what I thought but I wasn't sure. Thanks for the help.</p>
<p>If Qrxn=-841.18J is based on lab measurements, daman11 is missing a step. 0.841 kjoules were released by however much chemical was mixed together - probably less than 1 mole. Look at your lab directions, figure out how much chemical was responsible for the release of 0.841 kjoules, then use a ratio to determine how much kj would be released by a full mole.</p>
<p>q calculations from lab measurements are always in joules per amount actually used.</p>
<p>there was 50 mL of HCl and 50 mL of NaOH. So that's 103 grams total. Its just stoichiometry from here, right?</p>
<p>No, there was 50 mL of each <em>solution</em>, not 50 mL of HCl. You also need the molarity of the solution. For example, if HCL was 0.1 M, then MxV = 0.1 x .050 = .005 mol HCl. Then use proportions</p>
<p>-.841 kJ/0.005mol = X kJ/1 mol</p>
<p>deltaH = -168 kJ/mol HCl</p>
<p>Since the balanced reaction has 1 mol HCl, then deltaH(rxn) is also -168 kJ/mol</p>