Chemguide: Support for CIE A level Chemistry
Learning outcomes 5.1.4, 5.1.5 and 5.1.6
These statements talk about bond energies (bond enthalpies) and looks at simple calculations involving them.
Before you go on, you should find and read the statements in your copy of the syllabus.
You will find bond enthalpies on the Chemguide page bond enthalpy (bond energy).
Two points before you start.
In the second half of the page, two of the three calculations use what are known as Hess's Law cycles. These are dealt with in statement 5.2. It would be a good idea to leave the two calculations involving these cycles until you have read and understood statement 5.2. It is pointless reading them before that.
There is one small addition you need here, although it is mentioned throughout the page. Just to be clear:
To break a bond needs energy. When you make a bond, energy is released. If it takes, say +200 kJ/mol to break a bond, you will get an enthalpy change of -200 kJ/mol when you make it. You get the same amount of energy out when you make a bond as you put in to break it.
The overall enthalpy change of any reaction is simply the net efect of all the bonds being broken and made.
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© Jim Clark 2019