Question

For the reaction
Al(s) + 3 HCl(aq)->AlCl3(s) +3/2 H2(g) it was observed that when 4.00g of aluminum reacted with excess HCl, exactly 30,0 kJ of heat was released. What is the delta H for this reaction?

I do not understand how to get the delta H value.

Answer 1

Delta H for the reaction is the heat released when 1 mol Al reacts with 3 mol HCl. You have 30 kJ from 4.00 g Al. You want to convert that to 1 mol Al.
So 30 kJ x molar mass/4.00 = ? kJ per mol.

Answer 2

I did 30kJ X 26.9815/4.00 = 202.36

One of the anser choices that is given is -202 kJ. Is this incorrect or did I do the calculation wrong?

Answer 3

No, you did it correctly. We don't know how accurately the problem maker wants or exactly what molar mass s/he used for the molar mass Al. Perhaps 27 instead of 26.98. And, too, sometimes those answers are rounded and you are expected to pick the most reasonable answer. In this case, I think the the reason is rounding. You are given 4.00 which has three significant figures so three is what we can use in the answer; therefore, your answer of 202.36 would be rounded to 202 to 3 s.f. and you would be expected to look for an answer of 202.

Answer 4

Ok thank you for your help!