Hess’ Law

1. Mr. Chapman Chemistry 30 Hess’ Law

2. Enthalpy Change Can Be Tough • Sometimes it is impossible or completely impractical to measure the enthalpy change of a reaction by standard means. • One example of a reaction that occurs incredibly slowly is the conversion of diamond (carbon) to graphite (another form of carbon).

3. Diamond to Graphite • This reaction occurs so slowly that measuring the enthalpy change by conventional means would be impossible.

4. Other Problems Also Arise • Sometimes, chemical reactions that chemists would like to know the energy change in cannot be isolated. • For example, when you combine carbon and oxygen, you always get two products: Carbon monoxide AND carbon dioxide • What does a chemist do if they want to know the energy change in the formation of carbon monoxide only?

5. Some aspects of a system are independent of the way the system changes from start to finish. MAIN IDEA:

6. Example 1: Personal Wealth a) A man works diligently his whole life and retires at age 55 , with a personal wealth of 2 million dollars. b) Another man makes and loses a fortune of 2 million dollars several times through his life and eventually ends up at 55 , with a personal wealth of 2 million dollars. c) Another man lives in poverty until the day he retires at 55 , and wins a lottery worth 2 million dollars. Each man lived a lifetime, and in each case traveled a different path, but all ended up with 2 million dollars.

7. Ex 2: Final Scores in Sporting Contests a) In their first game the Toronto Maple Leafs score 3 goals in the first period and did not score again, while the Boston Bruins score one goal in each period. The game ends up tied at 3-3. b) In another game, Toronto scores 1 goal in the first period, none the second and two in the third, while Boston scores all three goals in the third. The game ends up tied at 3-3. In both cases the score ends up tied at 3-3 even though the games were played very differently.

8. Reactions do not occur in one step, but rather in a series of steps. That series of steps is called a reaction mechanism. Reactions can take various paths to completion, but attain the same result. Two New Ideas About Chemical Reactions

9. The sum of the enthalpies of the steps of a reaction will equal the enthalpy of the overall reaction. Hess’ Law This allows us to deal with situations in which the reaction occurs too slowly to measure enthalpy, or multiple products are formed..

10. Hess’ Law • Hess’ Law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reaction is the enthalpy change for the final reaction. • Recall that a thermochemical equation is a balanced chemical equation that includes the states of all the chemicals, as well as the associated energy change. H overall reaction ( net ) = H step 1 + H step 2 + H step 3 etc.

11. Hess’ Law • An example of a thermochemical equation: 4 Fe (s) + 3 O2 (g) 2 Fe2O3 (s)ΔH = -1625 kJ In order to apply Hess’ Law, we need to have chemical equations that contain the desired substances and also have known enthalpy changes.