Enthalpy question

[lia sepanek]

When solving the enthalpy of a reaction using bond enthalpies, why do we perform reactants minus product while for all other enthalpy calculations we use products minus reactants?

[Niharika Reddy 1D]

The enthalpy of a reaction is the sum of the bond enthalpy changes. Bond enthalpies are all positive because by definition, a bond enthalpy is the heat required to break a bond, which is an endothermic process. For the reactants, bonds are broken and energy is needed for this to happen (endothermic, positive enthalpy change). For the products, bonds are formed, leading to a more stable state, so energy is released (exothermic) and the changes in bond enthalpies are negative. This is why it appears the overall enthalpy of a reaction is calculated by doing reactants minus products, but it is actually just the sum of the bond enthalpy changes and these changes are always the negative of the bond enthalpies for the products and positive bond enthalpies for the reactants.

[Tiffany Ju]

Hi,
Sorry but just to double check...For bond enthalpies the equation is (bonds broken) - (bonds formed), and since the reactants' bonds are broken, and the products' bonds are formed, the equation can also be written as (reactants) - (products)...is that correct? Thanks!

[Chem_Mod]

Yes, you can rewrite the enthalpy equation as (bonds broken) - (bonds formed) or (reactants) - (products) as long as you keep all energies positive. You have already accounted for the negative sign with these expressions.