Chemistry of Natural Resources

Chapter 4 - Energy

Chapter Overview:

Energy as Heat

energy is defined as the capacity to do work

calorie - heat required to raise temperature of 1 g of water 1·C

Calorie (or kilocalorie) = 1000 calories (Calorie used for foods)

joule - energy required to raise 1 kg a height of 10 cm (about 4 joules = 1 calorie)

First Law of Thermodynamics - energy is neither created nor destroyed

(conservation of energy)

means the energy of the universe is constant

forms of energy are always interconverting, however

Energy Sources

individuals use energy each day for a variety of purposes

nations use huge amounts of energy (see Fig. 4.2)

the mix of energy sources varies and the total is increasing (see Figs. 4.4 & 4.5)

Chemical Energy

some chemical reactions generate heat - called exothermic reactions

other chemical reactions require heat input - called endothermic reactions

balanced reaction of methane combustion could also show the energy balance:

CH 4 + 2 O 2 ----> CO 2 + 2 H 2 O + 802.3 kJ/mole (heat released per mole of methane)

energy level diagrams show relative energy content of molecules (see Fig. 4.7)

Bond Energies - Calculating Heats of Reaction

compare bonds broken in a reaction (endothermic - require energy input)

with bonds made in a reaction (exothermic - release energy)

bond energies - characteristic energy required to break a particular type of bond

see Table 4.1 - shows strengths of most common types of bonds

note that H makes a stronger bond with O than with C

(means hydrogen is more stable as part of water than as part of methane)

heat of combustion is the balance of energies from all bonds broken and all bonds made

see Fig. 4.8 for a stepwise accounting of energies involved

on an energy diagram, up (positive energy) means higher energy content

and down (negative energy) means lower energy content

Activation Energy - Rates of Reactions

a negative heat of reaction does not necessarily mean that a reaction will proceed quickly

activation energy - typically some excess energy must be put in to get things started

see Fig. 4.9

activation energy is never as large as the sum of the bonds needed to be broken,

but in some cases it can be substantial, in other cases it is negligible

(depends on how the molecules need to reorganize themselves to make products)

Fossil Fuels

originally from photosynthesis long ago

CO 2 + H 2 O + sunlight -----> (CH 2 O) n + O 2

carbohydrates (e.g., glucose, cellulose) formed by plants

energy input from sunlight corresponds to 470 kJ per mole of CO 2 fixed

same photosynthetic reaction is going on today (carbon cycle - about 110 bmt/year)

besides fixing carbon, photosynthesis stores energy

energy is released by burning (exact reverse reaction)

(CH 2 O) n + O 2 -----> CO 2 + H 2 O + 470 kJ

fossil fuels are simply ancient biomass that has been converted to other forms

coal (see Table 4.2) is a variable mixture of many components

average composition about C 135 H 96 O 9 N S (mainly carbon by weight)energy content of coal about 30 kJ/g, but varies depending on quality (Table 4.2)

combustion of coal generates unburned particles (soot), SO 2 , and NO x

petroleum - liquid form is more convenient than coal

energy content about 48 kJ/g, higher than for coal

like coal, consists of a broad mixture of compounds, mostly hydrocarbons

Petroleum Refining

distillation (separation by boiling) - see Fig. 4.11

generates several "fractions" from petroleum based on different boiling points

lowest boiling (smallest molecules) are the gases

gasoline is taken over a broad range (40 - 200·C)

higher boiling fractions (larger molecules) are diesel fuels, lubricants, etc.

"cracking" of large molecules into smaller ones makes gasoline fraction larger

e.g., C 16 molecules broken into 2 C 8 molecules or n-octane converted to isooctane

isomers - different molecules with the same molecular formula

they have the same atoms in same numbers, but differ by the arrangement of atoms

e.g., C 8 H 18 : octane (8 carbons in a row), isooctane (7 carbons in a row plus one branch)

Alternative Fuels

water gas: mixture of CO and H 2 (both good fuels), made from coal plus steam

C + H 2 O ------> CO + H 2 ( once used widely in U. S. before natural gas)

biomass: plant materials used as fuel

wood is mainly carbohydrate, so combustion is the reverse of photosynthesis

(CH 2 O) n + O 2 -----> CO 2 + H 2 O + 470 kJ/mole (16 kJ/g)

note the lower energy output compared to coal or gasoline

in general, more highly oxygenated fuels produce less energy per weight

(some Cs and Hs already are bonded to O, plus some of the weight is already O)

ethanol: from fermentation of sugars

C 6 H 12 O 6 ---(yeast enzymes)---> 2 C 2 H 5 OH + 2 CO 2

C 2 H 5 OH + 3 O 2 ------> 2 CO 2 + 3 H 2 O + 1367 kJ/mole

combustion gives ~ 30 kJ/g (= 1367 / 46 , where 46 g/mole is the molar mass of ethanol)

garbage: typically contains large amounts of combustible materials

Energy Transformations

kinetic energy - energy of motion, like mechanical energy

heat is actually a form of kinetic energy (the molecules are moving)

potential energy - stored energy, like chemical energy

typical uses of energy involve conversions between different types of energy

e.g., a power plant burns fuel (potential energy --> heat) to generate steam, which

expands (heat --> work), turning a turbine (work --> kinetic or mechanical

energy), which generates electricity (mechanical --> electrical energy)

Energy Efficiency

every stage of energy conversion necessarily involves some losses of energy

typically lost as heat to the surroundings

overall efficiency is related to absolute temperatures at which the process runs

efficiency = ( T high - T low ) / T high

absolute (Kelvin scale) temperature - starts at absolute zero ( - 273·C)

·K = ·C + 273

only by running at absolute zero can a process be 100% efficient

at typical temperatures, efficiencies are limited to about 60%

Entropy - Order and Disorder

Second Law of Thermodynamics:

the entropy (disorder) of the universe is continually increasing

heat (disordered energy) can't be completely converted to work (ordered energy)

entropy - a statistical measure of disorder (in location or in energy)

try several examples to discover how changes always lead to more disorder

Energy Conservation

inherent limitation of energy supplies

fossil fuels are used for a variety of other (nonfuel) purposes in chemical industry

significant reductions in energy usage are possible, but require consumer education