Using bond energies to calculate energy changes

[bjoyful]

Homework Statement

Use the bond energies (in Table 4.2) to calculate the energy changes associated with each of these reactions. In each case indicate whether the overall reaction is endothermic or exothermic. Be sure to show your calculation work.

2. Homework Equations [/b

The Attempt at a Solution

A. H2(g) + O2(g) -----> H2O2(g) Answer: H2 is H-H (+436) and O2 is O=O (+498). Total energy in breaking bonds is 934 kJ. H2O2 is H-O=O-H (-467,-498,-467). Total in making bonds is -1432 kJ. Therefore, the net energy change is -498kJ, with the overall reaction being exothermic.

B. 2H2(g) + O2(g) -----> 2H2O(g) Answer: 2H2 is H-H (+436), H-H (+436). O2 is O=O (+498). Total energy in breaking bonds is +1370kJ. 2H20 is H-O-H (-467x2), H-O-H (-467x2). Total energy in making bonds is -1868kJ. Therefore, the net energy change is
-498kJ, with the overall reaction being exothermic.

C. 2H2(g) + CO(g) -----> CH3OH(g). Answer: H-H (+436), H-H (+436). CO is C=O (+1073). Total in breaking bonds is +1208. CH3OH is C-H (-416x3), C=O (-803), O-H (-467)

I find it strange that I came up with exothermic for all three of these...

 

[Mapes]

Why?

 

[Kushal]

many more reactions are exothermic than endothermic. exothermic reaction tends to form more stable products, and hence are favoured by nature.

 

[bjoyful]

okay, that makes sense. I thought I had done my work right, but it seems usually there are examples of both things in such problems. Thanks for explaining that to me.

 

[bjoyful]

another question on this...did I do this correctly as far as how I broke it down: CH3OH is C-H (-416x3), C=O (-803), O-H (-467). I am doubing guessing myself!

 

[Kushal]

draw the lewis structure. you will see that there are 3 C - H bonds (correct), 1 O - H bond (correct) but a C - O bond instead of C = O. the C - O bond is about 360 kJ.

 

[bjoyful]

I guess I totally drew that wrong:(Thanx for showing me the right way...