Heat and Enthalpy

Heat Q=mass x specific heat x temperature change Heat transfers from warmer to cooler objects If the system is under constant pressure, heat is equal to Enthalpy

Heat transfer is a physical change Heat due to a chemical reaction is a chemical change. This comes from breaking and reforming bonds

Where does energy in the chemical reaction come from? Bonds breaking and forming Can calculate energy by calculating the energy of the bond breaking and forming.

We take the energy of the bonds of the reactants that have been broken, and subtract from it the energy of the bonds of the products which have been made. Energy Bonds broken – Energy Bonds made

Enthalpy Enthalpy refers to the total energy of a system. We don’t measure Enthalpy, just the change in enthalpy ΔH So for now, ΔH =q

It takes energy to break a bond It takes energy to break a bond. So you are adding in energy to the reactants to break the bonds. Its like taking lego bricks apart before you rebuild. Energy is released when a bond forms, so you get “a refund” when you make new lego brick formation

Energy diagram--exothermic

Energy diagram--endothermic

ΔH = reactants - products The reactant bonds break, taking in energy, + ΔH The product bonds form, giving energy off. - ΔH To find ΔH of reaction, ΔH= Σreactant bond energies–Σproduct bond energies All units are kJ/mol

Enthalpy change for a reaction at standard pressure and Temperature Enthalpy = products - reactants (-393.5 + -635.1 ) – (-1207.6)

Now you know how to calculate heat of formation using bond energies, and how to use heats of formation to calculate the heat of reaction for a reaction. Lets add some stoichiometry!

You can calculate the enthalpy of a reaction using bond energies, and now you can do it using entalpy of formations What if you don’t know an enthalpy of formation??? Or what if the reaction occurs in steps?

Hess’s Law says that the sum of reactions will add up to the total enthalpy of reaction Example Problem:  Given the following chemical equations, calculate the energy change for the reaction that produces SO3.  (A)    S   +    O2  SO2    ∆H  =  - 297 kJ (B)    2SO3   2SO2  +  O2  ∆H  =  198 kJ  The desired chemical equation is:  2S +   3O2    2SO3 

Its like combining like terms (A)    S   +    O2  SO2    ∆H  =  - 297 kJ (B)    2SO3   2SO2  +  O2  ∆H  =  198 kJ  The desired chemical equation is:   2S +   3O2    2SO3