October 1, 2001

class began with return of exams and discussion of grading

return to Class Web site for more details

there are a few topics left from Friday 9/28 notes that will be covered on Wed 10/3

Energy and Bookkeeping

• In an exothermic chemical reaction
  • heat is produced
  • reaction products : hotter than reactants
  • we can pass that heat to other objects
  • fires are one (extreme) example
  • metabolism is another
    • (warm blooded animals, cold blooded animals often shiver to warm muscles-- some even shiver to develop heat to help eggs develop)
• In an endothermic reaction
  • heat is required and absorbed
  • from surroundings usually
  • example: water evaporating
    • (O.K., not a chemical reaction)
    • water cools, your body cools
  • chemical reactions of this type are not very common
    • without an energy source (heat) reactants stay unreacted

• alternative energy sources make endothermic reactions more common
  • photosynthesis
  • electrical decomposition of water
  • reducing ores (spend energy in C)
  • creating fuels (pump in energy)

Enthalpy and DH of a reaction

• Enthalpy = heat
  • under our usual conditions
  • specifically, open container
    • (if reaction produces gas, some energy used to work against the atmosphere)

Since Energy is Conserved

• A reaction that produces heat (energy) must
  • get it from elsewhere
• Stored chemical energy
• Can make tables-- how much stored energy
  • for each species
• Tables: we'll look at enthalpy specifically
  • table will be in chemist's units
    • (Joules) / (mole)
    • bookkeeping easier in moles
    • (buy fuel by mass or volume)
  • called Enthalpy of Formation DH_fo

• The Enthalpy (Heat) of reaction depends only on the difference in enthalpy of
  • Products, all of them
  • Reactants, all of them
• So we need a balanced equation
  • CH₄ + 2 O₂ ----> CO₂ + 2 H₂O

In the following DH really is delta-H or DH
It's just very tedious to get a delta entered into Web html lingo

• Enthalpy of products = [DH_fo (CO₂) + 2 DH_fo (H₂O) ]
  • Enthalpy of reactants = [DH_fo (CH₄) + 2 DH_fo (O₂)]
  • DH (reaction) =[DH_fo (CO₂) + 2 DH_fo (H₂O) ] - [DH_fo (CH₄) + 2 DH_fo (O₂)]

always Products - Reactants

• two values for water
  • if water (product) is a gas, vapor, steam
    • (usually in a flame)
  • if product is a liquid
    • (metabolism, cooler combustion with condensation)
• Furnaces, condensing
  • if the furnace plenum (heat box)
  • stays cool (<100 F)
    • then the water condenses on it
    • gives off additional heat
    • more efficient
    • needs a water drain
• Usual Furnace
  • keep plenum warm, avoid condensation
  • (normal steel plenum would rust)
  • less expensive, less efficient

Table of Energy / Enthalpy

Alternative is table of Bond Energy

• Part of the Goal-- find simple rules
• tolerate some loss of accuracy and precision
  • Is molecule just the sum of all its bonds?
  • Approximately so

• C-H 411 (kJ/mole)
• C-C 346
• C=O 799
• H-O 459
• O₂ 494
  • If we react a hydrocarbon like C₂H₆
    • 1 C-C [346]
    • 6 C-H [411 x 6 = 2466]
    • 5 O2 [494 x 2 = 988]..... sum = 3800 kJ
  • We get 2CO₂ and 3 H₂O
    • 4 C=O [799 x 4= 3196]
    • 6 H-O [459 x 6 = 2744] .... sum 5950kJ
  • Net change .... 2150 kJ
    • (available as heat and other energy)

Digression-- From Where Do Things Come?

Materials (Stuff)

• a. Dig a hole, extract it
  • mineral deposits
  • coal (strip mining or deep mining/tunnels )
  • oil (small deep hole, pump it out)
    • both as fuel and chemical starting material
  • water (wells)
  • natural gas and helium
    • (deep well hole, self pressurized)
  • All of these are non-renewable resources
• b. Atmosphere and Surface Water
  • Nitrogen, oxygen, Argon
  • Water
  • A few minerals in water (salt, Mg)
  • Generally renewable or appear renewable

• c. Agriculture and Silvan-culture (trees; sp?)
  • Food, fibers, structural material like wood
  • Fuels, Paper, Soaps, rubber
  • Vegetable based industrial oils
  • Ethanol
  • Generally renewable
    • As a rule, processes consumes non-renewables

Energy

• a. Renewable (assuming the sun cooperates)
  • water power
  • wind power
  • burning wood
  • solar power
• b. Indirectly Renewable
  • Hydrogen from water, solar power
  • Ethanol, from agriculture
• c. Nonrenewable, Finite Resources
  • Fossil fuels
  • Limited resources, especially easily removed $ d. Nuclear
  • Nonrenewable, but potentially massive resources
  • Fission-- dig up Uranium
  • Fusion-- from water (Deuterium), not developed