Quiz Question:Calculating delta H--amino acid bonds

Moderators: Chem_Mod, Chem_Admin

Imani Johnson 1H
Posts: 14
Joined: Sat Jul 23, 2016 3:00 am

Quiz Question:Calculating delta H--amino acid bonds

Postby Imani Johnson 1H » Thu Feb 09, 2017 11:20 am

Okay, so when given amino acid bonds, are we supposed to cancel out bonds on the products and reactants side in order to work with the bond enthalpies given in the chart? Can someone just explain how to solve this problem... >.<

Natalie_Boyd_1C
Posts: 14
Joined: Wed Nov 09, 2016 3:00 am

Re: Quiz Question:Calculating delta H--amino acid bonds

Postby Natalie_Boyd_1C » Fri Feb 10, 2017 12:30 am

The easiest way to do this problem is to cancel out bonds on the two sides of the reactions. What you're looking to do is to look only at what is new: which bonds are being broken, and which ones are being formed. All bonds that are formed release energy. This is because the molecule goes to a lower energy state; and if something goes from a higher energy state to a lower energy state it releases energy. Bonds that are being broken require energy because they go from a lower energy state to a higher energy state. The reaction is favorable if overall energy is released.

This is how I do these problems... I'm going to make up a reaction because I don't know what your quiz looks like specifically.
let's say you have carbon dioxide and water forming carbonic acid.
CO2 + H2O --> H2CO3

1. Draw the molecules and write all of the bonds present
Reactants:
2 C=O
2 O-H
Products:
2 O-H
2 C-O
1 C=O

If you were going to write this out:
2 C=O + 2O-H ---> 2O-H + 2C-O + 1C=O
and as you can see, the two O-H bonds on each side of the equation cancel; so you don't need to consider them when you calculate delta H and 1 C=O bond cancels, so you don't need to consider it.

thus you have: C=O ---> 2 C-O

so if you looked at a table you'd see this:
C=O 1072 kJ/mol
C-O 358 kJ/mol but forming bonds releases energy so you'd want this number to be a negative 358 kJ/mol

delta H= bonds broken + bonds formed
=(1mol)1072 kJ/mol +(2mols)(-358 kJ/mol) = 356kJ

Imani Johnson 1H
Posts: 14
Joined: Sat Jul 23, 2016 3:00 am

Re: Quiz Question:Calculating delta H--amino acid bonds

Postby Imani Johnson 1H » Sat Feb 11, 2017 6:28 pm

Thank you!


Return to “Calculating Standard Reaction Entropies (e.g. , Using Standard Molar Entropies)”

Who is online

Users browsing this forum: No registered users and 1 guest