2. Chemistry 141 Friday, October 20 2017 Lecture 19
Chemistry 11 - Lecture 11
9/30/2009
Chemistry 141
Friday, October
Lecture 19
Enthalpy and Calorimetry

3. Why do we care about energy?
One factor that indicates whether a reaction is likely to happen
Tells us about the stability of substances/states of material
Tells us whether a reaction can be useful as a source of work or heat (energy)

4. Heat Exchange Endothermic Exothermic Thermite reaction:
Fe2O3 + 2 Al → 2 Fe + Al2O3

5. E, q, w, and Their Signs

6. State Functions
The final state does not depend on the path to get there.

7. Example
A balloon is cooled by removing kJ of heat. It shrinks on cooling, and the atmosphere does 382 J of work on the balloon. What is the change in internal energy? Is the process endothermic or exothermic?

8. Work

9. How much work, in Joules, is done when a container expands 2
How much work, in Joules, is done when a container expands 2.55 L against an external pressure of 1.50 atm? J = 1 L atm Include units in your answer.

10. Heat capacity defined
Heat capacity is the ability of a substance to absorb heat
Heat capacity (C) – heat required to change the temperature of a substance or system by 1 ˚C (units: J/˚C or J/K)
Molar heat capacity – heat required to change the temperature of 1 mole of a substance by 1 ˚C (units: J/(mol•K))
Specific heat (Cs)– heat required to change the temperature of 1 g of a substance by 1 ˚C (units: J/(g•K))

11. Example
1 calorie is the heat needed to raise the temperature of 1 g of water from 14.5 to 15.5 ˚C. What is the heat capacity of 34.6 ml of water in J/˚C? How much heat is needed to raise the temperature of 34.6 ml of water from 25.0 ˚C to 85.0 ˚C?

12. Calorimetry