Chemical Energy

Recall that chemical energy arises when the electron clouds of atoms, molecules, or compounds (= combinations of atoms or molecules) are reorganized spatially in a chemical reaction. The changed locations alter the binding energy of the electrons, acting as a source (exothermic reaction) or sink (endothermic reaction) for other forms of energy. The number and types of atoms are conserved in a chemical reaction. For example, hydrogen oxidation is an exothermic reaction that produces water but also liberates binding energy:

2H2 + O2 → 2H2O + 68kcal/mole

Another is oxidation of sugar in our bodies

C6H12O6 + 6O2 → 6CO2 + 6H2O + 690kcal/mole

A mole is 1 gram (g) molecular weight, the mass equivalent of the sum of atomic weights of the reactant, expressed in grams. For example, 1 mole of H gas has mass 2 g because the H molecule consists of 2 H atoms each with atomic weight 1. The atomic weight is always the number of nucleons (protons and neutrons) in the atomic nucleus. C has atomic weight 12, so 1 mole masses 12 g. 1 mole of an element always contains 6.02 × 1023 atoms of the element (Avogradro’s number), and 1 mole of any molecule always contains Avogadro’s number of molecules. Using these concepts, we can work out e.g. how many calories of heat are released upon burning say 0.36 kg of sugar, the 2nd reaction above. Solution: The molar weight of sugar from the above is 12 × 6 + 1 × 12 + 16 × 6 = 180 g per mole. So, 360 g is 2 moles and we would produce 690 × 2 kcal. In this combustion, we would produce 6 × (12 + 16 × 2) = 88 g of CO2 per mole or 176 g, and 6 × (1 × 2 + 16) = 108 g per mole of water or 216 g. Other examples are given in the notes. Note: you will need to do such calculations automatically to pass this course! Here are some problems to turn in that give you practice in this and with energy conversions:

1. The methane in natural gas oxidizes (burns) as shown:

CH4 + 2O2 → CO2 + 2H2O

How many tons of CO2 are produced for each ton of methane that is burned? 2. Per capita annual energy consumption in the US averages to 3.3 × 108 BTU. Convert this to the energy equivalent of a) barrels of oil, and b) tons of coal. c) At what rate does a person use this energy (i.e. what is his/her average power consumption) in horsepower (hp) and kilowatts (kW)? Convert the latter to typical 100 w lightbulbs. 3. In the US, the average person eats about 3000 food calories daily. To what power in watts for the person to work and otherwise dissipate this energy does this correspond? 4. If a person lifts a 20 lb barbell upward a distance of 2 ft in 1 sec, at what rate is s/he working (i.e. power) in ft lb/s, kW, and hp? If s/he does this continuously for 40 sec, how much energy is expended in calories?

5. A car of mass 1000 kg travels at 30 m/sec. What is its kinetic energy in joules? 6. If the mass of your pencil is 10 g, what is its equivalent mass in kW hr? 7. In one standard human lifetime (70 years) a city of 100,000 people that enjoys (?) a constant 5% annual growth rate in population will attain how large a population? 8. How many g of candy bars (pure sugar) does a 150 lb person need to eat to climb a 3000 ft hill? 9. How much energy, in foot pounds, is needed to raise one barrel of oil 25,000 ft? How much is this energy in terms of the equivalent barrels of oil?

Do these independently and hand them in 1 wk from today. No excuses and delays tolerated!

1