1. Energy and Chemical Change
Chemistry – Chapter 16

2. Energy Energy – ability to do work or produce heat
Potential energy – energy due to composition or position of an object
Ex: Water stored behind a dam
Kinetic energy – energy of motion
Law of conservation of energy – in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed
Chemical potential energy – energy stored in a substance b/c of its composition
Ex: Octane in gasoline
Heat (q) – energy that is in the process of flowing from a warmer object to a cooler object

3. Measuring heat
Calorie (cal) – amount of heat required to raise the temp. of 1 g of pure water by 1 0C (metric system)
Nutritional Calorie is 1000 calories (1 kcal)
Joule (J) – SI unit of heat and energy
1 cal = J
Specific Heat – amount of heat required to raise the temp. of 1 g of that substance by 1 0C
Water has a high specific heat so it can absorb and release large quantities of heat

4. Heat absorbed or released by a substance during a temp
Heat absorbed or released by a substance during a temp. change depends on the specific heat, mass of the substance, and amount by which the temp. changes.
q = c x m x ΔT
q = heat absorbed/released
c = specific heat of substance
m = mass of sample in grams
ΔT = change in temp. in 0C

5. Heat in Chemical Reactions and Processes
Measuring Heat
Calorimeter – insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process
Calorimeter can be used to determine the specific heat of an unknown metal
Chemical Energy and the Universe
Thermochemistry – study of heat changes that accompany chemical reactions and phase changes
Heat pack – O reacts w/ Fe in an exothermic reaction
Cold pack – ammonium nitrate dissolves in water (endothermic reaction)

6. Universe = System + Surroundings
System – in thermochemistry, this is the specific part of the universe that contains the reaction or process you wish to study
Surroundings – everything in the universe other than the system
Universe – system plus the surroundings
Universe = System + Surroundings
Enthalpy (H) – heat content of a system at constant pressure
You can’t measure actual energy or enthalpy of a substance but you can measure the change in enthalpy, which is the heat absorbed or released in a chemical reaction
Heat of reaction: ΔHrxn
ΔHrxn = Hproducts - Hreactants

7. Sign of enthalpy of reaction:
Heat pack reaction is exothermic
4Fe + 3O Fe2O kJ
4Fe + 3O Fe2O ΔHrxn = -1625kJ
Cold pack reaction is endothermic
27kJ + NH4NO NH4+ +NO3-
NH4NO NH4+ +NO ΔHrxn = 27kJ

8. Thermochemical Equations
Thermochemical equation – balanced equation that includes the physical states of all reactants and products and the energy change, usually expressed as the change in enthalpy (ΔH)
Ex: Highly exothermic combustion of glucose occurs in the body as food is metabolized to produce energy
ΔHcomb = kJ
Energy released is enthalpy of combustion – enthalpy change for the complete burning of one mole of a substance

9. Changes of State
Molar enthalpy of vaporization (ΔHvap) – heat required to vaporize one mole of a liquid
Molar enthalpy of fusion (ΔHfus) – heat required to melt one mole of a solid substance
Both are endothermic, so ΔH is positive

10. Calculating Enthalpy Change
Can not always measure heat change, so there is a way to theoretically determine ΔH
Hess’s law – states that if you can add 2 or more thermochemical equations to produce a final equation for a reaction, the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction
Enables you to calculate enthalpy changes for an enormous number of chemical reactions by imagining that each reaction occurs through a series of steps for which the enthalpy changes are known

11. 2S + 3O2 2SO3 ΔH = ? a. S + O2 SO2 ΔH = -297 kJ
First, chemical equations are needed that contain the substances found in the desired equation and have known enthalpy changes
a. S + O SO2 ΔH = -297 kJ
b. 2SO SO2 + O2 ΔH = 198 kJ
The desired equation shows 2 moles of sulfur reacting, so Equation a must be rewritten for 2 moles of sulfur by multiplying the coefficients by 2. The enthalpy change (ΔH) must also be doubled b/c twice the energy will be released if 2 moles of sulfur react (Equation a becomes Equation c)
c. 2S + 2O SO2 ΔH = -594 kJ

12. d. 2SO2 + O2 2SO3 ΔH = -198 kJ 2S + 2O2 2SO2 ΔH = -594 kJ
B/c you want to determine ΔH for a reaction in which SO3 is a product rather than a reactant, Equation b must be reversed (sign of ΔH changes) and becomes Equation d.
d. 2SO2 + O SO3 ΔH = -198 kJ
Now add Equations c and d.
2S + 2O SO2 ΔH = -594 kJ
2SO2 + O SO3 ΔH = -198 kJ
2SO2 + 2S + 3O SO2 + 2SO3 ΔH = -792 kJ

13. Entropy
Entropy – measure of the disorder or randomness of the particles that make up a system
Law of disorder – states that spontaneous processes always proceed in such a way that the entropy of the universe increases
Entropy is always increasing
The more time that passes, the messier your room gets until you clean it again