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Unit 7: The Energy in Chemical Reactions—Thermodynamics and Enthalpy

Section 5: Energy Conversions and Conservation

When a star burns out or a car runs out of gas, it may seem that energy has been used up. In fact, this energy did not disappear, but was converted to another form. In the same way, energy cannot be created. "Generators" that produce electricity actually convert chemical or thermal energy into electrical energy. Because energy cannot be created or destroyed, energy is conserved, just as matter is conserved. This principle is expressed in the First Law of Thermodynamics, which states that energy cannot be created or destroyed. In an isolated system, energy never increases or decreases. Though many people throughout history have tried, no one has ever invented a machine that produces more energy than it consumes.

The world is full of energy transformations. Rubbing two sticks together warms them up. Chemical energy in oil is converted to thermal energy to heat our homes. Plants convert radiant energy from the sun into chemical energy via photosynthesis. Electricity is converted to thermal energy in toasters, radiant energy in light bulbs, and kinetic energy in the electric motors that power washing machines, power tools, fans, and disk drives.

Joule's Apparatus

Figure 7-7. Joule's Apparatus

© Wikimedia Commons. Public Domain.

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Joule's Apparatus

Figure 7-7. Joule's Apparatus

Joule’s water friction apparatus demonstrated that kinetic energy could be converted into heat energy in water.

Though we take these energy transformations for granted, it wasn't always so. Building on the work of Sir Benjamin Thompson and his cannons, English physicist James Prescott Joule solidified and quantified the relationship between mechanical movement and thermal energy. To show this conversion, Joule reportedly spent time during his honeymoon in France measuring the difference in water temperature at the top and bottom of a waterfall. He theorized that water at the bottom of the falls should be warmer than at the top, because upon impact the kinetic energy of the water was transformed into thermal energy.

To show this conversion in the lab, Joule built the water friction apparatus shown in Figure 7-7. It consisted of a weight hung from a length of twine wrapped around a vertical rod. Paddles projected from the bottom of the rod, which was submerged in an insulated tub of water. When the weight fell, the rod and paddles would spin, churning the water and raising its temperature.

By measuring the temperature change and the energy lost by the descending weight, Joule demonstrated that the energy gained by the water was proportional to the energy lost by the falling weight. His apparatus converted the potential energy of the weight into the thermal energy of the water.

Manufacturing Snow

Figure 7-8. Manufacturing Snow

© Wikimedia Commons, Creative Commons License 3.0. Author: Calyponte, 7 November 2006.

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Manufacturing Snow

Figure 7-8. Manufacturing Snow

Snow cannon at the Canmore Nordic Centre, Alberta, Canada.

Joule showed that when work was put into a system, the temperature increased. A fire-starting device called a "fire piston" puts this directly to practical use. The user places a small piece of paper in a narrow cylinder made out of wood or metal. Rapidly driving the piston down heats up the air inside enough to start the tinder smoldering. A diesel engine uses the same technology; the mixture of gas and air heats up and ignites due to compression (without a spark plug).

Rapid expansion has the opposite effect. When a gas quickly expands, its temperature drops because energy leaves the system as work. Snow machines at ski slopes use this principle when nature hasn't been cooperative. A snow machine shoots out a mixture of cold water and compressed air. As the air expands, its temperature drops enough to freeze the water into snow. (Figure 7-8)

Glossary

Diesel engine

An internal-combustion engine in which the compression of fuel, not a spark plug, causes the fuel to ignite.

Fire piston

A fire-starting device that burns tinder by rapidly compressing a gas.

First Law of Thermodynamics

Energy can never be created or destroyed. The total energy in the universe is constant.

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