A quick question. When we use the bond energies to find delta H, should we use the sum of the bond energies of the reactants minus the sum of the bond energies of the products, or vice versa?
For example, H2 + Br2 --> 2HBr
Given that
H-H bond is 436 KJ/mol
Br-Br is 193 KJ/ mol
H-Br bond is 366 KJ/mol
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So sum of bond energies for reactants is 436+193=629 KJ
and the sum of bond energies for the products is 2*366=732KJ
And my question is: Is delta H 629-732 or 732-629?
Thanks.
The sum of bond energies reactants - sum bond energies products.
629-732=-103
When using bond energies to find ΔH (the enthalpy change) for a reaction, you need to use the sum of the bond energies of the reactants minus the sum of the bond energies of the products. This is because breaking bonds requires energy (endothermic process), and forming bonds releases energy (exothermic process).
In your example, the sum of bond energies for the reactants is 629 KJ (436 KJ for H-H bond + 193 KJ for Br-Br bond). The sum of bond energies for the products is 732 KJ (2 * 366 KJ for H-Br bonds).
To calculate ΔH, subtract the sum of bond energies of the products from the sum of bond energies of the reactants: ΔH = 629 KJ - 732 KJ = -103 KJ.
The negative sign indicates that the reaction is exothermic, meaning it releases energy.