4 posts • Page 1 of 1
When we are given mean bond enthalpy values, those values do not directly represent a change in energy coming about as a result of a specific reaction, unlike when we use the standard enthalpy of formation of a substance, which is by definition the change in energy resulting from the formation of a substance from its constituent elements in their standard states. For bond enthalpy, we use reactants - products, because we say that breaking the bonds of the reactants requires energy (therefore change in energy is positive, since there must be some form of energy input), whereas forming the bonds of the producst releases energy (therefore change in energy is negative, since there must be some form of energy output). Hope this helps!
Instead of thinking of them as being products and reactants, it is much easier if you think of it as bonds broken (reactants) - bonds formed (products). Since all bond enthalpies are positive because heat must be supplied to break a bond, you will need to taken into consideration that bond breaking is always endothermic (positive) and bond formation is always exothermic (negative). Thus, it makes sense it it would be reactants-products.
This is so because bond enthalpy measures the change in energy when breaking bonds. On the reactant side, bonds are broken but on the product side bonds are formed; therefore, we use reactants-products to give the bond enthalpy on the product side a negative sign, which makes sense because forming bonds is exothermic.
Who is online
Users browsing this forum: No registered users and 2 guests