Enthalpy Thursday

Today was a pretty easy day. We started off class by going over some new notes.

We learned that because most chemistry reactions take place at constant pressure, we can say the a change in enthalpy is equal to the heat supplied. The symbol for enthalpy is "H" so we can write: (the change in)H=q. This is good to know becasue it is not possible to measure the the change of H in a lab but we can measure q which is the same as H. Yay!

We also leanred about how exothermic and endothermic reactions relate to enthalpy. For an exothermic reaction, the heat released corresponds to a decrease in enthalpy, sooo...(the change in)H<0.>

For an endothermic process, an input of heat corresponds to an increase in enthalpy, which means (the change in)H>0. An example of this is photosynthesis.

We also learned that all chemical reactions either release or absorb heat. For exothermic reactions, the heat is written on the right side of the arrow, like it is a product: reactants --> products + energy as heat (H).

For endothermic reactions, the heat is written like a reactant and written on the left side of the arrow: reactants + energy as heat (H) --> products.

Mr. Lieberman also taught us how to use stoich in an equation to find the change in H.

We solved the sample in our notes.

When 1 mole of methane (CH4) is burned,

890 kJ of energy is released. Calculate the

(change in)H for a process that uses 5.8

grams of methane.

First, we need to make sure that it s a therodynamic reaction. The problem tells us that energy is released which means it is an exothermic reaction.

Then we write the chemical equation with the energy written as a product in this case:

CH4 + O2 --> CO2 + 2H20 +890kJ

Finally, we use stoich to solve the equation:

5.8 grams CH4 x 1mole CH4/12 grams x 890kJ/1 mole= 322.63kJ

The last ratio is the ratio of the amout of kJ used per mole which they give us in the

problem. You can write the amount of energy as a negative because it is an

exothermic reaction, but you don't need the negative because the problem already

says that the energy is "released."

So that is pretty much the notes we went over in class today. Mr. Lieberman gave us the rest of the class period to work on the labs from Tuesday and Wednesday which were due at the end of the period. For homework we had a WebAssign, a pre-lab, and a worksheet.

The next scribe will be Mollie M.