Untitled - Kelly Walsh High School

98 CHEMISTRY FOR THE UTTERLY CONFUSED
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6-1 Energy and Reactions
Quick Tip
In this chapter, we will help you learn about the energy changes, especially heat,
which occurs during both physical and chemical changes. You might need to
review the Unit Conversion Method in Chapter 1 and the sections in Chapter 3
on balancing chemical reactions and the mole concept if you are not comfortable
with them already. And remember to Practice, Practice, Practice.
Thermochemistry deals with changes in heat that take place during chemical
reactions. Heat is an extensive property, that is, it depends on the amount of
matter (or the amount of reactants that undergo change). Many times, we will
be measuring the temperature (average kinetic energy) of the system.
Temperature is an intensive property, one that is independent of the amount of
matter present. We will be discussing the energy exchanges between the system
that we are studying and the surroundings. The system is that part of the universe
that we are studying. It may be a beaker or it may be the solar system. The
surroundings are the rest of the universe that the change affects.
The most common units of energy that we use in the study of thermodynamics
are the joule and the calorie. The joule (J) is:
1 J 1 kgm 2 /s 2
The calorie (cal) is the amount of energy needed to raise the temperature of
1 g of water by 1C and relates to the joule as:
1 cal 4.184 J
This is not the same calorie that is commonly associated with food and diets.
That is the nutritional Calorie, which is really a kilocalorie (1 C = 1000 cal).
Calorimetry is the laboratory technique used to measure the heat released or
absorbed during a chemical or physical change. The quantity of heat absorbed