Hess's Law

Topics: Standard enthalpy change of formation, Enthalpy, Temperature Pages: 2 (340 words) Published: June 22, 2011
Hey =) I'm gonna do my best to help you and this is what I came up with.... I found out how to get the formula of Mg + 1/2 O2 ---> MgO...

The only thing is that I didn't find the (delta) H for the target formula. I believe you can find it using the appendix of your book ( assuming they want you to find the Standard Enthalpies of Formation for the other formulas ).... For my rference to them in this answer, I substituted the values with variables....

To find Mg + 1/2 O2 --> MgO,

(1) Mg + 2HCl --> H2 + MgCl2 (delta) H = X
(2) MgO + 2HCl --> H2O + MgCl2 (delta) H = Y
(3) H2 + 1/2 O2 --> H2O (delta) H = -285.8 KJ

You know you already need the Mg and the 1/2 O2 on the left side and the MgO on the right side of the equasion.... I saw quickly that the Mg and 1/2 O2 are already on the left but the MgO in equasion (2) was on the left. Therefore, I switched it around. I ended with

(1) Mg + 2HCl --> H2 + MgCl2 (delta) H = X
(2) H2O + MgCl2 --> MgO + 2HCl (delta) H = -Y
(3) H2 + 1/2 O2 --> H2O (delta) H = -285.8 KJ

Basically, only the second equasion changed and the Y is now negative cause u switched the products and reactants....

I tried working it here and combining them. Everything proceeded to cancel out except for the Mg, 1/2O2 and MgO =) Hope you understood all that!

OH and I'm sure you know how to find the Standard Enthalpy of Formation for an equasion... But in case you don't it's .... [Sum Products] - [ Sum Reactants] -- Like I said, all enthalpies should be in the back of the book. Be sure you're careful to mind the state of the chemical. Certain substances like Water exist as a liquid and a gas at room temperature. Therefore, make sure you look at the GAS Enthalpy for gas and the LIQUID enthalpy for liquid
Continue Reading

Please join StudyMode to read the full document

You May Also Find These Documents Helpful

  • Hess Law Lab Report Essay
  • Hess Law Essay
  • Indirect determination of ∆H via Hess's law Research Paper
  • Hess S Law Lab Essay
  • Essay on Rule of law
  • Us Law Essay
  • Muslim Law Term Paper
  • Role and Functions of Law Essay

Become a StudyMode Member

Sign Up - It's Free