1. Thermochemistry
Chapter 10

2. Energy Chemical potential energy (PE) Measuring energy
Energy stored in bonds
Unstable compounds have lots of PE
More PE more reactive
Measuring energy
Experimentally using q=mcΔT and q=mΔH
Theoretically using bond energy, heat of formation, energy diagrams

3. Heat vs. Temperature Heat ≠ Temperature
Temperature: average KE of molecule
Heat: amount energy exchanged due to temperature difference
Heat depends on AMOUNT of substance
Heat either given off or absorbed (lose or gain heat due to temp difference)
Heat is directional (moves from area of hot to cold)
ENERGY LOST = ENERGY GAINED

4. Calorimetry
q = mcΔT
q = heat in Joules (J)
m = mass in grams (g)
c = specific heat capacity J/g˚C
ΔT= change in temp in ˚C or K
At constant pressure, heat = change in enthalpy (ΔH)
Enthalpy: total energy of system (KE + PE)
Heat = Energy = Enthalpy (FYI: Used interchangeably)

5. Energy Measurement and conversions
A joule is the SI unit of heat and energy
A calorie (cal) is the quantity of heat that raises the temperature of 1 gram of pure water 1 degree Celsius.
A food Calorie (Cal) is a kilocalorie (kcal) calories = 1 Calorie
1 Calorie = 1000 calories = 1 kcal = 4186 Joules
Specific heat capacity of water: 4.18 J/g◦C or 1.00 cal/g◦C

6. Enthalpy Change in energy = change in enthalpy (at constant pressure)
Enthalpy: measure of energy content of substance
q = ΔH
Endothermic ΔH > 0
Exothermic ΔH < 0
Reactants
Products
Energy
Reaction Coordinate
Activated Complex

7. Energy Diagram Change in energy is change in enthalpy
Reaction Coordinate
Activation Energy
H < 0
Change in energy is change in enthalpy
Reaction is exothermic
Activation energy
ALL reactions require activation energy!

8. Endothermic Reaction
Energy
Reaction Coordinate
Activation Energy
H > 0
Reactants
ENDOTHERMIC REACTION
Likelyhood of endothermic rxn taking place not likely (why?)
Non-spontaneous
Substances like to be in a lower energy state
Exothermic rxns usually spontaneous

9. Summary/ Review Thermochemistry
How does a reaction happen?
Energy is added
Activation energy!
Used for BREAKING bonds between reactants
Activated complex or intermediate formed
New bonds formed that RELEASE energy
Change in energy is enthalpy (ΔH)

10. Energy of Phase Changes
A few definitions
Melting point: temperature at which all energy absorbed causes change of state from solid to liquid
Freezing point: same temperature as melting point causes change from liquid to solid
Pure substances freezing and melting points are at the same temp, not true always for mixtures
Boiling point: temperature at which all energy absorbed causes change of state from liquid to gas or gas to liquid (condensation point)

11. Heating Curve 1. Solid 2.Melting point 3.Liquid 4.Boiling point 5. Gas
time
Heating Curve 1 2 3 4 5

12. Phase Changes Changing temperature, calculating heat exchanged use:
Q = mCΔt
Remember C is dependent on phase (state of matter)
Melting/ freezing/ boiling/etc, calculating heat exchanged use:
Q = mΔH
Units J/mol or J/gram
M = mass or moles (depending on ΔH)

13. Other Vocabulary:
Heat of Reaction: Hrxn heat produced in a chemical reaction
Heat of Combustion: Hcomb heat produced by a combustion reaction
Heat of Neutralization: neutral heat produced in a neutralization reaction (when an acid and base are mixed to get water, pH = 7)
Heat of solution: Hsol heat produced by when something dissolves
Heat of Fusion: Hfus heat produced when something melts
Heat of Vaporization: Hvap heat produced when something evaporates
Heat of Sublimation: Hsub heat produced when something sublimes
Heat of formation: Hf change in enthalpy that accompanies the formation of 1 mole of compound from it’s elements