# Chemistry Help: Bond Enthalpies

Question from OCR A Student Book: 9.3 Summary Questions

Use Table 1 and the following data:

2SO2(g) + O2(g) 2SO3(g) ΔH= -192kJmol-1

Assume that SO2 and SO3 contain only S=O bonds
a) Calculate the average bond enthalpy for the S=O bond.
b) The actual bond enthalpy for the S=O bond in SO2 is 531kJmol-1. Calculate the actual bond enthalpy for the S=O bond in SO3.

Table 1:
Original post by Cocomama123
Question from OCR A Student Book: 9.3 Summary Questions

Use Table 1 and the following data:

2SO2(g) + O2(g) 2SO3(g) ΔH= -192kJmol-1

Assume that SO2 and SO3 contain only S=O bonds
a) Calculate the average bond enthalpy for the S=O bond.
b) The actual bond enthalpy for the S=O bond in SO2 is 531kJmol-1. Calculate the actual bond enthalpy for the S=O bond in SO3.

Table 1:

What have you tried/what specifically is causing trouble in answering this question?
Original post by MexicanKeith
What have you tried/what specifically is causing trouble in answering this question?

By using the equation, I found that 2SO2 has 4(S=O) bonds, O2 has 1 (O=O) and 2SO3 has 6 (S=O),
Since (O=O) is 498 and the Enthalpy change is -192
I set up an equation:
4(S=O) + (O=O) —> 6(S=O)
4(S=O) + 498 6(S=O) = –192kJmol-1

By rearranging:
690 = 2(S=O)
345 = (S=O)

However the answer to the questions in the book is 469 which doesn't show working out. I know the book has many mistakes with answers so I'm not sure if I'm completely off the mark or if there's something wrong with the books answer.

Since I'm not sure about question a, I really don't know how to go about answering b)
Original post by Cocomama123
By using the equation, I found that 2SO2 has 4(S=O) bonds, O2 has 1 (O=O) and 2SO3 has 6 (S=O),
Since (O=O) is 498 and the Enthalpy change is -192
I set up an equation:
4(S=O) + (O=O) —> 6(S=O)
4(S=O) + 498 6(S=O) = –192kJmol-1

By rearranging:
690 = 2(S=O)
345 = (S=O)

However the answer to the questions in the book is 469 which doesn't show working out. I know the book has many mistakes with answers so I'm not sure if I'm completely off the mark or if there's something wrong with the books answer.

Since I'm not sure about question a, I really don't know how to go about answering b)

You have calculated it correctly.
Original post by charco
You have calculated it correctly.

Wow
Thank you so much,
Do you have any idea about question b)
Original post by Cocomama123
Wow
Thank you so much,
Do you have any idea about question b)

As @charco said your calculation is correct for part a

for part b you can use the equation you have written down, but now you have to treat the S=O bonds in SO2 and SO3 as different.

so

4(S=O)SO2 + 498 6(S=O)SO3 = –192kJmol-1

you know the value of (S=O)SO2 so you can rearrange
Original post by Cocomama123
By using the equation, I found that 2SO2 has 4(S=O) bonds, O2 has 1 (O=O) and 2SO3 has 6 (S=O),
Since (O=O) is 498 and the Enthalpy change is -192
I set up an equation:
4(S=O) + (O=O) —> 6(S=O)
4(S=O) + 498 6(S=O) = –192kJmol-1

By rearranging:
690 = 2(S=O)
345 = (S=O)

However the answer to the questions in the book is 469 which doesn't show working out. I know the book has many mistakes with answers so I'm not sure if I'm completely off the mark or if there's something wrong with the books answer.

Since I'm not sure about question a, I really don't know how to go about answering b)

U create an equation as well
Using the actual bond enthalpy of s-o in so2 which is 531kjmol find the total
4(o-s-o)= 2124
= 2124+498-6x=-192
=2124+498+192=6x
=2814=6x
=2814/6= 469kjmol
Original post by Udoc
U create an equation as well
Using the actual bond enthalpy of s-o in so2 which is 531kjmol find the total
4(o-s-o)= 2124
= 2124+498-6x=-192
=2124+498+192=6x
=2814=6x
=2814/6= 469kjmol

thank you! this is so helpful i actually understand the qs now🙂