Theory
Calorimetry can't be used to measure the enthalpy of certain reactions. These reactions generally fall into two categories:
In order to measure the heats of these reaction (enthalpies) chemists have developed a theoretical method. The method is based on a simple idea.
- Reaction that can't be isolated from others and occur at the same time.
For Example, the formation of carbon monoxide from carbon and oxygen.When carbon burns in air, it forms both carbon dioxide and carbon monoxide. The formation of carbon monoxide can't be separated from that of carbon dioxide, how then can we measure the heat of formation of carbon monoxide.- Reactions that proceed so slowly to effectively measure with a calorimeter.
For example, the rusting of iron is a very slow process, and the temperature changes in the surroundings are to small to measure.
Key
Idea
Examples of Key Idea
What exactly does this mean? Some examples of everyday events may make this easier to understand.
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.Example #2: Final Scores in Sporting contests
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.
Their wealth is therefore independent of how their lives were lived.
a) In their first game 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.Example #3 : Displacement
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.
Score then is independent of how the game is played
a) A student travels from Weyburn to Moose Jaw to see a friend whom they then take to a concert in Regina. Their final displacement from Weyburn is 113 km NW .Scientists have determined that enthalpy change is independent of how the system changes from beginning to end.
b) Another student travels to Fort Qu'appelle to pick up their friend and they also go to Regina to the same concert. Their final displacement from Weyburn is 113 km NW.
Both students started in the same spot, and finish in the same spot but travel different "paths" , and distances to get there.
Displacement is independent of path taken. It depends only the beginning and end point.
This suggests two new ideas about chemical reactions.
1) First: that reactions do not occur in a single step but rather by a series of steps.Chemist term these steps the reaction mechanism for the reaction.
2) Second: that reactions can take various "paths to completion". These paths or steps may be different yet produce the same result. Several reaction pathways or mechanisms may produce the same result.
Key
Idea
The man who first suggested this idea (based on experimental evidence) was G.H. Hess (1840 AD).
Hess's
Law
Hess's discovery means that we can now determine enthalpy change for a reaction without having to perform any direct measurements through calorimetry. If we know the steps of a reaction mechanism and the heats of these steps we can determine the heat of the overall reaction.H overall reaction ( net ) =
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