Lecture VII

Physics 367

Chemical Energy and Energy Generation



Types of Chemical Reactions

The formation of a compound occurs when the compound is more stable than the constituents were when they were alone. There are two different sorts of chemical reactions

those that take in energy - endothermic
those that give off energy - exothermic

Exothermic reactions result when the chemical bonds between atoms in molecules are reformed or when the overall result of a chemical reaction is the making of bonds.

The reactants fall into their most stable available state by giving up energy. These bonds all involve atomic electrons. The nuclei of the constituents remain unchanged.

To understand this think of a golf ball falling into a hole in the ground. It is more stable than that same golf ball on the ground, because the one on the ground could fall into the hole. The one in the hole can fall no farther. It gave up gravitational energy to fall into the hole. To bring it back out off the hole involves payment of an energy price. The ball must gain as much energy as it lost going into the hole to get out again.


A golf ball in a hole.

In a similar way, chemicals before reacting have a certain amount of energy. They may react together and give up energy by making bonds among the atoms (or constituents). Thus, they "fall into a hole," so to speak. To restore them to their original states, energy must be supplied--the same amount they gave up to form the compound.


Formation of a chemical compound

The falling into the hole part involves the giving off of energy during the making of chemical bonds. It is exothermic. The getting out of the hole part involves the absorption of energy during the breaking of chemical bonds. It is endothermic.

Activitation Energy

To get a reaction to start, even one that produces useful energy, it is necessary to give some energy to the system.

The energy provided to start a reaction going is called activation energy.

Example: Water is formed from hydrogen gas and oxygen gas.

If we put them into a container and waited, there would be a very slow reaction. After thousands of years, molecules would break into atoms, atoms would hit appropriate other atoms as they jostle about with their thermal energy, and the compound would form.

However, supplying a small amount of activation energy [with a match, say] starts off a reaction. A large number of molecules break into atoms near the matchhead. The atoms combine to form water, which liberates a large amount of energy. This energy supplied breaks up molecules farther away, they combine, until the whole volume of gas is water. This sort of reaction is called a chain reaction.

When chemicals burn, that is, combine with oxygen in the air, they form bonds and release energy. Combustion involves liberation of energy (usually as thermal energy) - it is an exothermic process. The heat of combustion is the amount of energy given off per kilogram of the substance burned. Various heats of combustion are listed below:

Substance H of C (MJ/kg) H of C (kWh/kg)
Fuel - crude oil 45.0 12.5
        - gasoline 46.9 13.0
        - kerosene 46.7 13.0
        - Ohio coal 29.5 8.2
        - hydrogen 141.9 39.4
Foods - butter 38.5 10.7
        - animal fats 37.8 11.0
        - egg white 23.9 6.6
        - egg yolk 33.9 9.4
        - olive oil 39.3 10.9
Other - oak 16.7 4.6
        - pine 18.5 5.1
        - dynamite 5.4 1.5
        - iron 6.6 1.8

Example: the transformation of the organic chemical benzene, a liquid, into the solid explosive TNT.

Benzene is obtained from coal tar (it is also found in crude oil in very small amounts). Benzene is shown below:

The replacement of one branch from a carbon-to-a-hydrogen bond with a carbon-to-a-(carbon with three hydrogen) bond forms the liquid toluene.

If another carbon-to-hydrogen bond in toluene is replaced by a carbon-to-(nitrogen with two oxygens), nitrotoluene is formed. If this replacement is accomplished for three carbon-to-hydrogen bonds, trinitrotoluene (TNT) is formed

This replacement takes energy [toluene is added at 120°C to nitric acid, sulfuric acid, and SO3 to form TNT]. The resulting chemical is a pale yellow solid at room temperature.

TNT melts at 83°C, and explodes at 240°C. Supplying TNT with activation energy produces N2, CO, CO2, H2O, and lots of energy in a chemical chain reaction.

Catalysis

It is possible to change the rate at which reactions occur. This process is known as catalysis.

Catalysts are the materials which speed up the reaction but which are not used up or even affected in the reaction. In living things catalysts called enzymes make life possible.

Depletion of the ozone layer is made possible by catalysts containing chlorine and bromine:

ClO + BrO Cl + Br + O2

Cl + O3 ClO + O2

Br + O3 BrO + O2

Energy Generation

Carbon compounds play a vital role in the existence of life on Earth. Plants inhale carbon dioxide, exhale oxygen and store energy in glucose....animals reverse this process.

6CO2 + 6H20 + radiant energy C6H12O6 + 6O2

Carbon atoms also form most fuels known:

natural gas
coal
oil.

The branch of chemistry studying carbon is known as organic chemistry because carbon is also the atom of life. Carbon can form more complicated compounds than most other atoms.

Chemical change by burning is involved in most of the methods we use to generate electricity. Fossil fuel contains carbon, and it is burned by combining with oxygen in the air. Air is about 20% oxygen and 80% nitrogen. The combustion converts chemical energy stored in the fuel to kinetic energy [thermal energy]. Since fossil fuels contain hydrogen in addition to carbon, combustion also produces water.

But the process is not that simple !

What are the problems associated with burning fossil fuels?

Problems

Sulfur
Nitrogen
Heat
Particulates

Most power plants are cooled by water, which is heated and then returned hotter to the source of the water. This may cause deleterious effects on fish which can be injured or killed if the temperature is too high. They may also fail to reproduce, even at relatively low temperatures.

This exhaust of this waste heat is another pollutant emitted by power plants.

The chemicals that burn are made of other things. We call these molecules, or collections of atoms that are bound together. Every time we burn a chemical we produce byproducts in addition to heat. Every breath you take you breathe in several liters of air. There are about 1023 air molecules in that amount of air. Even a small contamination of certain particulates can destroy your lungs.

This exhaust of this particulates is another pollutant emitted by power plants.

Absolute temperatures

All particles--atoms, molecules, and so on--jitter about at room temperature because they have thermal energy. Now we have to think about what temperature means.

We are used to the Celsius temperature scale. That scale is defined by:

0°C = the temperature at which ice freezes
100°C = the temperature at which water boils [at sea level]

Using this scale a day with below zero temperatures is cold. When you drink a soft drink full of ice, the temperature is about 0°C. A day with 10°C temperatures would be brisk--you would wear a jacket. This is also the temperature of a cold beer. Room temperature is about 20°C. Your skin temperature is about 35°C and your internal body temperature is 37°C. A day with temperatures in the 30s would be hot. You eat hot foods at temperatures in the 60s.

That scale is fine for human affairs, but fails to be truly universal because it's based on Earthly conditions [sea level pressure].

Also, clearly, objects at temperatures below zero have thermal energy.

We would like a scale that would register zero when objects have zero thermal energy. The absolute temperature scale is one that has its zero at this temperature. The absolute zero of temperature is 273° below zero Celsius. The size of the unit of temperature is set to be the same as 1°C. The unit is called the kelvin(K) in honor of Lord Kelvin. Thus, Table 1 shows comparisons.

Celsius Temperature Absolute Temperature
-273°C     0 K
    0°C 273 K
  10°C 283 K
  20°C 293 K
  37°C 310 K
100°C 373 K

Temperature is a measure of an objects thermal energy [kinetic energy]. The distribution referred as the speeds of gas particles leads to the connection of the thermal energy to temperature.

Problem of the day:

A stable and highly soluble pollutant is dumped into a lake at the rate of 0.16 tonnes per day (0.16x103 kg per day). The lake volume is 4x107 m3 and the average water flow through the lake is 8x104 m3/day. Ignore evaporation (we will discuss this later) from the lake surface and assume the pollutant is uniformly mixed in the lake. What eventual steady-state concentration will the pollutant reach?

Once again we will use a steady-state model....so what do we expect?

And what do we need?

The amount of water in the lake, Mw is: 4x107 m3
The average water flow through the lake, Fw is: 8x104 m3/day

Then the residence time of the water in the lake is:

Tw = Mw/Fw

Tw = 4x107 m3/ (8x104 m3/day) = 0.5x103 day

Tw = 500 days

How do we calculate the residence time of the pollutant?

Because the pollutant is uniformly mixed in the lake, the residence time of the pollutant will equal the residence time for the lake water.

Tw = Tp = 500 days

Then it follows that the steady state stock of pollutant is simply the input rate times the residence time:

Mp = FpTp = 0.16 tonnes/day x 500 days

Mp = 80 tonnes

To get the concentration we need to know how much water there is in the lake in tonnes. If we multiply the volume of water by the density of water (1g/cm3 = 1kg/1000cm3 = 1000 kg/m3 we find that 1 cubic meter weighs exactly 1 metric ton (1000 kg) which is equal to 1 tonne. The the steady state concentration of the pollutant is:

Cp = 80 tonnes/4x107 tonnes = 2x10-6

Cp = 2 parts per million by weight