Hess's Law and Enthalpy?

Question

My chemistry teacher had to leave for a few weeks while her husband is having back surgery. She left my class notes and an assignment. She gave 10 people in my class a section of the current chapter to present/teach to the class. My section deals with Hess's law and enthalpy. I read the chapter, the notes, and the definitions, so I do not need those to be repeated. I need someone to please explain to me what enthalpy and Hess's law are, essentially. I kind of understand it, but not well enough to present. If someone could help I would be extremely greatful. Thanks!

Answer 1

Hess's Law, which can be applied either to internal energy E or to enthalpy H, is really a consequence of the law of conservation of energy, or (in more technical language) of the fact that E and H are state functions, independent of the path.

What they mean, is that if you go from A to C by way of B, the total change in going from A to C is the sum of the changes from A to B and from B to C. This may not sound like very much, but it is extremely useful. It is why we can work out the total enthalpy change of a reaction by subtracting the enthalpies of formation of all the reactants from the enthalpies of formation of all the products. What this means is, you imagine doing the reaction by unmaking the reactants, and then rearranging the atoms of the elements to make the products.

Enthalpy: I'm sure you know the definition

H = E + PV

E is the internal energy, and you can think of PV as the extra potential energy that the material has, simply because it is occupying space. Remember that if you expand through a volume V at constant pressure P, then PV is the amount of work you have to do.

When you are studying processes at constant volume, there is no PV work involved, and what you use is Delta E. When you're studying processes are constant pressure, the volume will change considerably if you are changing the number of moles of gas in the system, and using Delta H will take the resulting PV work into account.

In the last resort, you have to generate your own take on this stuff, and having to teach it is the toughest test there is on how well you understand it. I hope what I have given you (my own take) is of some help you.

Answer 2

Enthalpy's textbook defenition is the heat change of the systen when 1 mole of the substance reacts with oxygen to form products under STP (298 K and 1 atm.) It really measures "heat", which is basically a type of energy.

Hess's law is changing around enthalpy reaction formations to find out the total enthalpy change over the course of the reaction. Basically, taking some smaller reactions and combining them in a simple way.

Answer 3

Sum of Products minus Sum of reactants