Using bond energies to calculate energy changes

In summary: I was just confused because I had done this before and the C-O bond was in the wrong place.)In summary, the three reactions are exothermic, with the overall reaction being favored by nature.
  • #1
bjoyful
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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...
 
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  • #2
Why?
 
  • #3
many more reactions are exothermic than endothermic. exothermic reaction tends to form more stable products, and hence are favoured by nature.
 
  • #4
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.
 
  • #5
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!
 
  • #6
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.
 
  • #7
I guess I totally drew that wrong:(Thanx for showing me the right way...
 

Related to Using bond energies to calculate energy changes

What is the concept of bond energy?

Bond energy is the amount of energy required to break a chemical bond between two atoms in a molecule.

How is bond energy measured?

Bond energy is measured in units of kilojoules per mole (kJ/mol) using various experimental techniques such as calorimetry and spectroscopy.

What is the relevance of bond energy in calculating energy changes?

Bond energy is essential in calculating energy changes because it determines the stability of a molecule and the amount of energy needed to break or form bonds during a chemical reaction.

How can bond energies be used to calculate energy changes in a reaction?

The overall energy change in a reaction can be calculated by subtracting the total bond energies of the reactants from the total bond energies of the products. If the result is a negative value, the reaction is exothermic, and if it is positive, the reaction is endothermic.

What are some limitations of using bond energies to calculate energy changes?

Using bond energies to calculate energy changes assumes that all bonds in a molecule are broken or formed simultaneously, which may not always be the case in a real chemical reaction. Additionally, bond energies can vary depending on the environment and other factors, making the calculations less accurate.

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