2. Chemistry 141 Monday, October 23 2017 Lecture 20
Chemistry 11 - Lecture 11
9/30/2009
Chemistry 141
Monday, October
Lecture 20
Calorimetry and Hess’s Law

3. Notes about Enthalpy
Enthalpy is an extensive property - depends on amount of material. ΔHrxn is for the moles in the balanced equation
Enthalpy of the reverse reaction is equal magnitude but opposite sign of the forward reaction
Enthalpy of reaction depends on the states of reactants and products
Enthalpy is one indicator of whether a reaction is likely to occur spontaneously (large, negative ΔH = likely spontaneous; large positive ΔH = not likely)

4. 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))

5. 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?

6. Calorimetry

7. Hess’s Law
If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.

8. Enthalpies of formation
ΔHf = Enthalpy of formation
= Enthalpy to make compound from its elements
Standard state = most stable form of pure substance at 25 ˚C and 1 atm
ΔHf˚ = Standard enthalpy of formation
= Enthalpy to make 1 mole of compound from its elements, all at standard conditions
ΔHf˚ for elements in their standard states is ZERO

9. Example
Calculate ΔH˚rxn for the following: CaO (s) + 2 HCl (g)  CaCl2 (s) + H2O (g)