AP Chemistry - Thermochemistry Unit
THERMOCHEMISTRY notes and assignments
10/6/08 (Monday)
Essential Question: What are the driving forces?
Homework: Read page 165 – 168 (Stop at "Work" on page 168)
Read page 172 (Start endo/exothermic) – 173 Identify terms.
10/7/08 (Tuesday)
Essential Question: What can you learn from calorimetry?
Thermochemistry - the study of heat absorbed or evolved during a chemical or physical change.
enthalpy (H) - energy available to do work at constant pressure
change in enthalpy (DH = heat gained or lost at constant pressure)
temperature - a measure of the average molecular kinetic energy of a substance or system.
heat (q) - the form of energy that moves spontaneously from a warmer object to a colder one.
state function - depends only on what is material is present, not pathway to present condition
exothermic - a process that produces energy (heat/light is released)
endothermic - a process that absorbs energy
calorie (cal) – originally: energy necessary to raise the temperature of 1 g of water 1 C. This is not Calorie with a capitol C as in food labels. That is a kilocalorie.
joule (J) = 1 kg m2/s2 = 1 newton-meter = 4.184 cal
calorimeter - a devise use to measure the heat gained or lost during a chemical or physical change.
calorimetry - measuring the heat gained or lost during a chemical or physical change.
specific heat (c) {also called specific heat capacity} - the amount of heat required to raise 1 g of a substance 1 C (So what is the specific heat of water?)
heat capacity - C = DH / DT
extensive property - depends on the amount of the substance
intensive property - does not depend on the amount of the substance
bomb calorimeter - constant volume
Sign of DH: Sample 5.3 (mole to mass), page 176
Watch units - heat or cooling a substance
q = m c DT DT = Tfinal - Tinitial = Tf - Ti
Sample 5.5 on page 181
Homework: page 204, #51
Determine a calorimeter constant. Example on transparency.
10/8/08 (Wednesday)
Essential Question: What is Hess's law?
First Law of Thermodynamics: Energy can be converted from one form to another, but it cannot be created or destroyed.
heat/enthalpy of reaction - enthalpy change during a reaction
exothermic (negative, add to product side)
endothermic (positive, add to reactant side)
Samples 5.6 & 5.7
Homework: Page 204, #41, 57
Hess's law (page 186): If a reaction is carried out in a series of steps, DH for the overall reaction will equal the sum of the enthalpy changes for the individual steps. The energy change between a set of reactants and products is a constant, whether the reaction takes place in one or a series of steps. DH is a state function!
Rules: If a reaction is reversed, the sign of DH is also reversed.
DH is directly proportional to the quantities of reactants and products. If the coefficients in a balance reaction are multiplied by a number, the value of DH is multiplied by the same number.
Hint: Sometimes it is easier to work the problem backwards.
Sample 5.8 and 5.9 and practice.
Homework: Page 205, #61, 63
Thermodynamic standards: 1 bar = 0.9869 atm = 100 kPa, 25 oC
Elements: Form of the element at standard conditions
Compounds: gases at 1 bar pressure, pure liquids or solids, solutions of exactly 1M
enthalpy of formation: DHf page 188.
standard enthalpy of formation: The change in enthalpy to form one mole of the compound from it’s elements in their standard states (DHfo)
Standard enthalpy of formation of elements is therefore zero.
Table C starting on page 1112
Sample 5.10 on page 190
Homework: Page 206, #67
Heats of reaction: DHorxn = SDHfo products - SDHfo reactants
This is another statement of Hess's Law
Samples 5.11 & 5.12 on page 192 and practice
Homework: Page 206, #71, 73, 77
Homework: Prelab 6: Hess’s law.
10/9/08 (Thursday)
Essential Question: Can you demonstrate Hess's Law in the lab?
To finish in one period – you will have to know what you are doing.
You can do one or more of the reactions while waiting for the water to heat up.
Lab will be due on Monday, 10/13.
For Experiment 6 lab report: No discussion of theory. To find the mass of the water, you will have to look up the density of water at the temperature of your experiment. Be sure to include the source information.
10/10 (Friday)
Essential Question: What happens with enthalpy during a phase change?
See heating curve. Page 450.
a. Use q=mcDT for rising temperature sections (heating solid, liquid or gas)
b. DH fusion
melting point: liquid & solid have identical vapor pressures
normal melting point: solid and liquid have same vapor pressure and the total pressure is 1 atm
Crystalline - sharp m.p. (forces holding structure are equal)
Amorphous - gradual softening and liquefaction (forces are different at different part of the random structure)
c. DH vaporization
boiling point: equilibrium vapor pressure = prevailing atmosphere
(implications for cooking)
normal boiling point: vapor pressure = 1 atm
supercooled, superheated (boiling chips needed to prevent "bumping")
Sample 11.4 on page 451
Homework: page 474, #33, 35, 39a; Read 11.6
