1. Heat & Hess’ LAW

2. Goals To Accomplish Today 1.22.13
Enthalpy and Hess’ Law
(A) Hess Law Practice #1
BRING LAB MATERIALS TOMORROW!!!

3. Thermodynamics Study of energy relationships in a chemical system
We will look at energy in terms of three concepts
Temperature
Heat
Enthalpy (H)

4. Key Terms that you should know/recognize
Temperature
- measurement of avg. KE of particles in an object
Heat
- heat is the transfer of energy btw 2 objects due to a ∆ temperature
Enthalpy ( H)
- total energy in a chemical system with constant pressure
- we will be concerned with the TRANSFER of enthalpy (∆ H)

5. Measuring All these “things”
Temperature ( Celsius and Kelvin)
Energy (joules or calories)
Ability of a system to do work or supply ( or produce) heat
Heat (CANNOT MEASURE)
We can only measure through changes in temperature

6. Tro's "Introductory Chemistry", Chapter 3
Units of Energy
Calorie (cal) is the amount of energy needed to raise one gram of water by 1 °C.
kcal = energy needed to raise 1000 g of water 1 °C.
food calories = kcals.
Energy Conversion Factors
1 calorie (cal) =
4.184 joules (J)
1 Calorie (Cal)
1000 calories (cal)
1 kilowatt-hour (kWh)
3.60 x 106 joules (J)
Tro's "Introductory Chemistry", Chapter 3

7. → Change in Heat Example Energy always flows in the same direction
Window in the winter time
Energy always flows in the same direction →
When does the energy flow stop?

8. Enthalpy (H)
Energy is stored in chemicals, found in bonds that hold atoms together
As chemical rxn takes place, bonds break, new bonds are created, energy is exchanged
Change in energy is change in enthalpy

9. Exothermic vs. Endothermic
Energy can be released or absorbed
Heat releasing (Exothermic)
Reactants --> Products + E H = (-)
Heat Absorbed (Endothermic)
Reactants + E --> Products H = (+)

10. S + O2  SO ∆H= -296 kJ/mol
a. How much heat is released when 275 grams of sulfur is burned?

11. S + O2  SO ∆H= -296 kJ/mol
b. How much heat is released when 25 mol of sulfur is burned in excess oxygen?

12. Hess’ Law
Some chemical reactions we cannot carry out in a calorimeter they release or absorb tons of energy
We must indirectly calculate the change in energy for these reactions
We use Hess’ Law

13. Hess’ Law Before we discuss Hess’ Law we must first talk about Energy
Energy is a state function
This means that the pathway one uses to get from one energy level to another is not important

14. Hess’ Law
Change in enthalpy for any equation can be calculated without actually carrying out the reaction
Just simply add up the H of other related experiments

15. 2NO2 (g) --> N2O4 (g)
N2(g) + 2O2(g) --> 2NO2 (g) H = 67.8 kJ N2(g) + 2O2 (g) --> N2O4 (g) H = 9.67 kJ