How do you work out bond energy changes?
In a chemical reaction, how do you work out the energy change?
Is it:
Energy of Bonds made – Energy of Bonds broken or
Energy of Bonds broken – Energy of Bond made?
If the answer is negative, is the reaction endothermic?
Firstly, if it is negative it is exothermic as the negative sign shows that the heat is on the products side of the equation, so it is given out, rather than taken in.
And it is Bonds Made – Bonds Broken.
So for example, if we look at the nucleophilic substitution of Br – Br, into CH4. The products will be CH3Br and HBR.
So add up the Bond Enthalpy of the new bonds, which you will have to trust me equals 1889 kJ per mol. (The values are in your data booklet.) Then add up the Bond Enthalpy of the old bonds, which, again you’ll have to trust me, is 1850 kJ per mol.
So Bond Energy Change is 1889 – 1850 = 39 kJ per mol.
So for this example, the reaction is endothermic.
Hope that helps.
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