Page 34 In a chemical reaction, bonds in the reactants are broken and the atoms rearrange to form new bonds in the products. Flames or heat associated with the combustion reaction indicate that the reaction is exothermic. Where does the energy in an exothermic reaction come from? (Do you think it comes from bond breaking or bond forming? What evidence do you have for your answer?)

Bond Breaking and Bond Making

Potential energy Heat

Average Bond Energies (per mole of bonds) Bond H-HC-HC-CC=CO-HC-OC=OO=OO-O Bond Energy (KJ/Mol)

Direction Every energy measurement has three parts. 1. A unit ( Joules or kiloJoules). 2. A number how many. 3. and a sign to tell direction. negative - exothermic positive- endothermic

One more time..... Consider the combustion of methane: CH O 2 → CO H 2 O (all gases)

In this example, the bonds to break are: 4(C-H) and 2 (O=O) The bonds to make are 2(C=O) and 4(H-O)

That translates into 4(414) + 2(498.7) = kJ And 2(745) + 4(460) = kJ So  H = kJ/mol

H 2 + F 2  2HF How much energy is associated with this reaction? Does it require energy or release energy? Is it exothermic or endothermic?

So: to break H-H bond requires kJ/mol to break F-F bond requires kJ/mol to make 2 H-F bonds releases 2(568.2 kJ/mol)

But which energy is going in and which is coming out? The first two steps are endothermic; energy is required; we give them + signs. The last step is exothermic; energy is released; we give it a - sign. To get the overall change in enthalpy,  H, we add the values to get kJ/mol. The actual value as measured experimentally is kJ/mol!!! (per mol of what?)

Potential energy Heat Try this problem