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Section 4: Heat, Work, and Internal Energy
To keep track of how energy moves and changes in chemical reactions, it is important to strictly define the boundaries of the chemical system under study. In many lab experiments, the system is often a beaker or a flask in which chemical reactions occur. More generally, a system is the particular part of the universe under study. This can mean practically anything so long as the boundaries are well defined: a piston in a car engine, a human body, a nuclear reactor, or a star. The rest of the environment outside the system is called the surroundings.
Energy can enter and leave a chemical system as heat (represented by the variable q). Heat enters a system when the container contacts a hotter object such as a laboratory hot plate: Thermal energy conducts from the hotter object into the cooler one. Conversely, heat will leave the system when it contacts something at a lower temperature like an ice bath. When heat enters the system, the sign of q is positive; when heat leaves, q is negative.
Figure 7-6. Four-Stroke Cycle in a Gasoline Engine
In a four-stroke internal combustion engine, pistons move up and down, pulling fuel into combustion chambers, where it is compressed and ignited. Fuel ignition causes gases to expand, pushing the pistons down and transmitting power that moves the car.
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Energy can also enter and leave a chemical system as work (represented by the variable w). One type of work is pressure-volume work; this occurs when the volume of the system expands or contracts. The amount of work done is equal to the change in volume of the system times the exterior pressure. Compression adds energy to the system, and the sign of w is positive. Expansion removes energy, and w is negative. The cylinders in a gasoline engine release energy as pressure-volume work. In the cylinder, a spark plug ignites a mixture of gasoline and air. The burning gasoline inside the cylinder drives the piston out; the piston delivers this energy to the crankshaft to move the car. (Figure 7-6)
A battery is an example of a chemical system that releases energy as electrical work. Electrical work leaves the system when the battery powers a light or a motor, and energy enters the system when the battery recharges. (We will discuss electrochemistry in Unit 11.)
The energy inside a system is called the internal energy (which is given the variable U), and includes the thermal energy of the substances, the energy due to their phases (latent heat), and their chemical energy. Determining the sum total of all these energies is complex; it is more important to know how the internal energy of a system changes. For example, the amount of energy a lump of coal happens to contain is not a useful number, but it is important to know how much energy will be released when the coal burns. When writing an equation, the change in a value is represented by the uppercase Greek letter delta (Δ). The change in energy of a system (ΔU) is the amount of heat (q) and work (w) gained or lost by the system:
ΔU = q + w
Note: There are different conventions for the signs of q and w. In this text, all energy entering the system will have a positive sign, and all energy leaving the system will have a negative sign. (Table 7-1)
|System absorbs heat||q is positive|
|System releases heat||q is negative|
|System is compressed||w is positive|
|System expands||w is negative|