1. Heat of the reaction
Thermochemistry III

2. What is Heat? Q = mass X Csp X ΔT Units: J = g X J/(g X °C) X °C
What substance did you measure the ΔT?
What specific heat (Csp) did you use?
Therefore, what mass do you use in this calculation?
All three items in the calculation are of the same substance!!

3. Why can we do that?
Water is the surroundings to the reaction, which is the system.
In all heat measurements, the heat leaving the system is equal to the heat entering the surrounding.
Δ H system = - Δ H surroundings

4. What do we do with the data on the system?
The weight of the reactants can be used to divide the heat of the reaction to get heat per gram of reactant.
Heat in kJ/grams of reactant = kJ/g
The weight can be divided by the molar mass and used to divide the heat of the reaction to find the molar heat.
Heat in kJ / number of moles= kJ/mol

5. Enter your data CLASS RESULTS TABLE
Marshmallows Peanuts Cashews Popcorn ________ kJ/g________ kJ/g________ kJ/g________ kJ/g ________ kJ/g________ kJ/g________ kJ/g________ kJ/g ________ kJ/g________ kJ/g________ kJ/g________ kJ/g ________ kJ/g________ kJ/g________ kJ/g________ kJ/g ________ kJ/g________ kJ/g________ kJ/g________ kJ/g Average for each food type:
________ kJ/g________ kJ/g________ kJ/g________ kJ/g

6. Calorimetry The precise and accurate measurements of heat changes.
By insulating the reaction, it reduces the loss of heat.
This insures that all heat from the system is transferred to the surroundings.
Take a measurement of the surroundings and determine the heat of the system.
Δ H system = - Δ H surroundings

7. Heat of the reaction
Any equation the includes the heat of the reaction is a thermochemical equation.
Looking at the heat of vaporization of water.
H2O (l)  H2O (g) ΔH= +44kJ/mol
Endothermic or exothermic?
When heat is consumed by the reaction, the ΔH is +
When heat is a product of the reaction the ΔH is -

8. Heats of reaction Heat of combustion: ΔHcomb
Heat of vaporization: ΔHvap
Heat of condensation: ΔHcond
Heat of solidification: ΔHsolid
Heat of fusion: ΔHfus
Heat of solution: ΔHsoln

9. Review homework Remember: What is the equation for heat?
What are the units for energy?
Which mass do you use in the calculation?