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Untitled - Kelly Walsh High School

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98 CHEMISTRY FOR THE UTTERLY CONFUSED<br />

Get Started<br />

6-1 Energy and Reactions<br />

Quick Tip<br />

In this chapter, we will help you learn about the energy changes, especially heat,<br />

which occurs during both physical and chemical changes. You might need to<br />

review the Unit Conversion Method in Chapter 1 and the sections in Chapter 3<br />

on balancing chemical reactions and the mole concept if you are not comfortable<br />

with them already. And remember to Practice, Practice, Practice.<br />

Thermochemistry deals with changes in heat that take place during chemical<br />

reactions. Heat is an extensive property, that is, it depends on the amount of<br />

matter (or the amount of reactants that undergo change). Many times, we will<br />

be measuring the temperature (average kinetic energy) of the system.<br />

Temperature is an intensive property, one that is independent of the amount of<br />

matter present. We will be discussing the energy exchanges between the system<br />

that we are studying and the surroundings. The system is that part of the universe<br />

that we are studying. It may be a beaker or it may be the solar system. The<br />

surroundings are the rest of the universe that the change affects.<br />

The most common units of energy that we use in the study of thermodynamics<br />

are the joule and the calorie. The joule (J) is:<br />

1 J 1 kgm 2 /s 2<br />

The calorie (cal) is the amount of energy needed to raise the temperature of<br />

1 g of water by 1C and relates to the joule as:<br />

1 cal 4.184 J<br />

This is not the same calorie that is commonly associated with food and diets.<br />

That is the nutritional Calorie, which is really a kilocalorie (1 C = 1000 cal).<br />

Calorimetry is the laboratory technique used to measure the heat released or<br />

absorbed during a chemical or physical change. The quantity of heat absorbed

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