Why do we put a minus sing in front of the bond enthalpies of the bonds that are being formed?
And why are the enthalpies of formation negative in general?
I also don't understand why we subtract the enthalpy of formation of the reactants from the enthalpy of formation of the products?
Maybe someone could help me understand this.
Thanks!
The definition of bond energy is the amount of energy released when one mole of constituent gaseous atoms form. Generally it is an exothermic process and someone decided years ago that the release of energy would be counted as negative. Delta H for the reaction is the delta H products - delta H reactants. If the products are more negative than the reactants the delta H rxn is negative and that means it is spontaneous. Subtracting the other way would not give the proper sign. Said another way, the products are more stable than the reactants.
To understand why a minus sign is put in front of the bond enthalpies of the bonds being formed, we need to first establish the concept of enthalpy and its significance in chemical reactions.
Enthalpy (H) is a measure of the total energy content of a system. In a chemical reaction, the difference in enthalpy between the products and the reactants (denoted as ΔH) determines whether the reaction is exothermic (releasing heat) or endothermic (absorbing heat). A negative ΔH indicates an exothermic reaction where energy is released, while a positive ΔH indicates an endothermic reaction where energy is absorbed.
When a bond is broken, energy is required to overcome the attractive forces holding atoms together. This energy is called bond enthalpy, and it is always positive because breaking a bond requires input of energy. Conversely, when a bond is formed, energy is released because the atoms involved are now more stable. The bond enthalpy in this case is negative to signify the energy being released.
Now, coming to the enthalpies of formation, they represent the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states. These values are reported as ΔHf, with a negative sign. The negative sign indicates that the formation of the compound is an exothermic process, resulting in the release of energy.
The reason why the enthalpy of formation values are negative in general is because, when compounds form, the new interactions between atoms lead to a more stable state, reducing the overall energy content. This decrease in energy is reflected by the negative ΔHf value.
Lastly, when calculating the enthalpy change of a reaction (ΔHrxn), you subtract the enthalpy of formation of the reactants from the enthalpy of formation of the products. This is based on the principle of energy conservation. To determine the overall change in energy, you consider the difference between the amount of energy required to break the reactant bonds and the amount of energy released when the product bonds are formed. Subtracting the enthalpy of formation of the reactants accounts for the energy input needed for bond breaking, while adding the enthalpy of formation for the products accounts for the energy released during bond formation.
I hope this explanation helps clarify the concepts. If you have any more questions, feel free to ask!