1. Enthalpy
Videodisk Unit 4
Demos
The Purple Cow
Hot and Cold Packs

2. Thermochemistry
Study of energy changes that occur during chemical reactions and changes in state.
Heat (q) is energy that transfers between objects due to temperature differences.
Heat always flows from a warmer object to a cooler object.
Most reactions involve energy. Either the reaction needs energy to work, or it gives off energy as it happens.

3. Endothermic & Exothermic
Heat flows into system Heat flows out of system q q

4. Enthalpy
Enthalpy is a measure of the total energy in a thermodynamic system at a constant pressure.
The heat released or absorbed by a reaction or substance is a change in enthalpy (DH) or heat content.
The enthalpy change for a reaction is called the Heat of Reaction (DHrxn)
Units for enthalpy are kilojoules (kJ).

5. Reactions that release heat energy.
Exothermic Reactions
Reactions that release heat energy.
C3H8 + 5O2  3CO2 + 4H2O kJ
Heat is one of the products
The temperature of the surrounding area increases. (The reaction feels warm)
Enthalpy is negative
DHrxn = kJ Meaning 2043 kJ of heat is released
Joules is the metric unit of heat. Kilojoules is used most often. Calories is also a common unit of heat.
Purple Cow Demo
Paper cone-cup
Potassium permaganate
0.5 mL glycerine
Half-fill a paper cone-cup with potassium permaganate. Suspend it in a small ring a few centimeters above an asbestos pad. Add 0.5 mL glycerine. Wait until it burns with a purple fire. The time varies from a few seconds to a few minutes.

6. Exothermic Energy Diagram
Activation Energy DH
Reactants have more bond energy than the products so heat is released from the reaction (negative DH)

7. Endothermic Reactions
Reactions that absorb heat energy.
C(s) + H2O(g) kJ  CO2(g) + H2(g)
Heat is one of the reactants
The temperature of the surrounding area decreases. (feels cold)
Enthalpy is positive
DHrxn = +113 kJ Meaning 113kJ are absorbed
Hot and Cold Packs
192 g NH4NO3
2 ziploc bags
Cold Pack: Place NH4NO3 in a large ziploc and place a second ziploc inside with about 100 mL of water. Mix.
30 g CaCl2
Hot Pack: Place CaCl2 in a large ziploc and place a second ziploc inside with about 100 mL of water. Mix.

8. Endothermic Energy Diagram
Activation Energy DH
Reactants have less bond energy than the products so heat is absorbed by the reaction (positive DH)

9. *x and y are from a balanced chemical equation
Using Enthalpy
Enthalpy is proportional to the equation coefficients, conversion factor is:
x mol = y kJ
*x and y are from a balanced chemical equation
Do calculations the same as stoichiometry problems

10. Proportions to Conversions
2 H2O2(l)  2 H2O(l) + O2(g)
Hrxn = -190 kJ
What proportions (conversions) can we get from this info?
If you react exactly 2 moles of H2O2 , exactly 190 kJ of energy is released (-190kJ) so:
2 mole H2O2 = -190 kJ which can be represented as a conversion

11. Proportions to Conversions
2 H2O2(l)  2 H2O(l) + O2(g) , Hrxn = -190 kJ
2 mole H2O2 = -190 kJ
2 mole H2O = -190kJ
1 mole O2 = -190 kJ Or Or Or

12. Example 2 H2O2(l)  2 H2O(l) + O2(g) Hrxn = -190 kJ
How much heat will be released if 1.0 g of hydrogen peroxide (H2O2) decomposes in a bombardier beetle to produce a steam spray?
2 H2O2(l)  2 H2O(l) + O2(g)
Hrxn = -190 kJ

13. -2.8 kJ 2 H2O2(l)  2 H2O(l) + O2(g) DH= -190 kJ
1.0 g H2O2  mole H2O2  kJ
1.0 g H2O2 x 1 mol H2O2 x kJ =
34.02 g H2O mol H2O2
-2.8 kJ

14. C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(l)
Sample Problem
How much heat is transferred when 9.22 g of glucose (C6H12O6) in your body reacts with O2 according to the following equation?
C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(l)
Hrxn = kJ
-144 kJ

15. H and Changes of State
Change in state always involves a change in heat energy
As more heat is added the temperature increases, meaning the particles are moving faster.
During a change of state, however, the temperature remains constant because all of the heat energy is being used to break or form bonds

16. Energy for Freezing
To make something freeze, you continuously remove heat
to decrease the speed of molecules
to increase the attractions between molecules causing them to clump

17. Energy for Melting To make something melt, you continuously add heat
to increase the speed of the molecules
to break the attractions between molecules causing them to separate

18. Heating Curve of water
Heat of Fusion
Heat of Vaporization

19. Freezing & Melting
Freezing point (or melting point) is the temperature at which both the solid and liquid forms exist.
The Heat of Fusion is the amount of heat needed for the phase change from solid to liquid at standard pressure.
Heat of Fusion (Hf) of water is 334 J/g.
Found on pg 1 of ref. pack

20. Vaporization (Boiling) & Condensation
Boiling point is the temperature at which both the liquid and gas forms exist.
The Heat of Vaporization is the amount of heat needed for the phase change from liquid to gas at standard pressure.
Heat of Vaporization(Hv) of water is 2260 J/g
Found on pg 1 of ref. pack

21. Calculations
How much heat is absorbed when 24.8 g H2O at 100 oC is converted to steam at 100 oC? (DHvap= 2260 J/g)

22. Calculations
How much heat is removed when 15.7 moles of water freezes? Water’s heat of fusion is 334 J/g.