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Unformatted text preview: Thermochemistry Chap 6 Sec 1-3 notes F-09c.doc Page 1 Terms - Chapter 6.1 & 6.2 Tons of terms and it is necessary to understand them!!! System- Surroundings- Heat of Reaction = Enthalpy, ∆ H We can only find the change of the energy content of the chemicals which occur during a chemical reaction. Old way: Ca + 1/2 O 2-----> CaO + 653.1 kJ Gives off 653.1 kJ of heat as a product! New way: The heat content of the system is lowered by 653.1 kJ Here we are saying that the heat content of the products is less than that of the reactants and heat is to the surroundings. This is called an exothermic reaction. (Exo, exit, to go out from). ∆ H is negative for the chemicals. A reaction which heat like H 2 O(s) -----> H 2 O(l) or H 2 O(l) -----> H 2 O(g) is endothermic . It is absorbing heat from the surroundings. ∆ H is positive for the chemicals. ( When we directly heat these up it is obvious that heat is being picked up but when you put your hand by some melting ice you can feel the air (the surroundings) is colder. When you get out of swimming pool on a cold day your skin is contributing the heat and that can make you very cold.) A Burning gas CH 4 gives off heat B Melting Ice requires heat Notice that in Thermochemical equations that we often do balance with fractions. This is allowed and often used for reasons we will not go into this quarter. Thermochemistry Chap 6 Sec 1-3 notes F-09c.doc Page 2 Important rule: ∆ H is a state function . That means its value • An example is that if you burn sucrose C 12 H 22 O 11 in the air to make CO 2 + H 2 O, the ∆ H is –16.2 kJ/g or –5.55 x 10 3 kJ/mole. • Your body metabolizes sucrose in a very different manner by sending it through reactions controlled by enzymes but the products CO 2 + H 2 O are the same. The ∆ H for that reaction is also –16.2 kJ/mole –5.55 x 10 3 kJ/mole. Only the Energy Units: SI joule(J) often kJ are used Old calorie(cal) kcal can also be used 1 cal = 4.184 and will be given to you if you need it. 1 Nutritional calorie actually is Internal energy ( ∆ E) in a reaction which involves both heat & work. q = heat w = work ∆ E = q + w Easy math but watch your signs!!!! Heat given off by the system and work done by the system are Heat absorbed by the system and work done on the system are Example: A system gives off 15 kcal of heat and does 10 kcal of work....
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