You are Here: Home

# Bond Enthalpy Calculations

1. Ok Hess cycles next lesson - Want to consolidate this first though, so the answer will be either exothermic or endothermic. This process should show what exactly... it doesnt show how much energy the reactants have, just the energy required to break their bonds.

Excuse the equals, ok so this reaction is exothermic, how much so?
Method,

Energy in (to break bonds)

(Theres a 0.5 here, how is that bonded and calculated?)

Energy out (create bonds)

Questions
I guess its then Energy out - Energy in for your answer?
I just want to make sure i know all the main points about this, does it show how much energy a molecule has, or just how much is released/sucked in when it reacts?
Does this method of calculating enthalpy change have name?
Answer is always negative for exothermic reaction?

Any other related theory welcome, AS level. Thanks
2. energy required (endo, +)
1 x C-C
6 x C-H
3.5 x O=O

energy released (exo, -)
4 x C=O
6 x O-H

sum

(1 x C-C) + (6 x C-H) - (4 x C=O) - (6 x O-H)

as straightforward as that.
3. Can someone answer the questions
4. (Original post by LearningMath)

Questions
I guess its then Energy out - Energy in for your answer?
1. I just want to make sure i know all the main points about this, does it show how much energy a molecule has, or just how much is released/sucked in when it reacts?
2. Does this method of calculating enthalpy change have name?
Answer is always negative for exothermic reaction?

Any other related theory welcome, AS level. Thanks
1. This calculation shows the energy change when 1 mole of ethane is burned in excess air or oxygen.
A combustion is always exothermic therefore it's always a negative value.

2. No specific name. Reaction enthalpy by the bond energy method.

3. By definition the enthalpy change of an exothermic process is negative.
5. Thank you... Had an idea it was more complicated.... Meh oh well, cheers

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: September 24, 2008
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### A-level exams coming up?

Find everything you need here.

### Cupping competition aftermath

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.