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17.3: Exothermic and Endothermic Processes

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Heat flows from a warm object to a cold object

How do you keep warm?

A camp fire is an example of basic thermochemistry. The reaction is initiated by the application of heat from a match. The reaction converting wood to carbon dioxide and water (among other things) continues, releasing heat energy in the process. This heat energy can then be used to cook food, roast marshmallows, or just keep warm when it’s cold outside.

Exothermic and Endothermic Processes

When physical or chemical changes occur, they are generally accompanied by a transfer of energy. The law of conservation of energy states that in any physical or chemical process, energy is neither created nor destroyed. In other words, the entire energy in the universe is conserved. In order to better understand the energy changes taking place during a reaction, we need to define two parts of the universe, called the system and the surroundings. The system is the specific portion of matter in a given space that is being studied during an experiment or an observation. The surroundings is everything in the universe that is not part of the system. In practical terms for a laboratory chemist, the system is the particular chemicals being reacted, while the surroundings is the immediate vicinity within the room. During most processes, energy is exchanged between the system and the surroundings. If the system loses a certain amount of energy, that same amount of energy is gained by the surroundings. If the system gains a certain amount of energy, that energy is supplied by the surroundings.

In the study of thermochemical processes, things are viewed from the point of view of the system. A chemical reaction or physical change is endothermic if heat is absorbed by the system from the surroundings. In the course of an endothermic process, the system gains heat from the surroundings and so the temperature of the surroundings decreases. The quantity of heat for a process is represented by the letter q . The sign of  q for an endothermic process is positive because the system is gaining heat. A chemical reaction or physical change is exothermic if heat is released by the system into the surroundings. Because the surroundings is gaining heat from the system, the temperature of the surroundings increases. The sign of  q for an exothermic process is negative because the system is losing heat.

Endothermic reactions absorb heat, while exothermic reactions release heat

(A) Endothermic reaction. (B) Exothermic reaction.

Units of Heat

Heat flow is measured in one of two common units: the calorie and the joule. The joule (J) is the SI unit of energy.  The calorie is familiar because it is commonly used when referring to the amount of energy contained within food. A calorie (cal) is the quantity of heat required to raise the temperature of 1 gram of water by 1°C. For example, raising the temperature of 100 g of water from 20°C to 22°C would require 100 × 2 = 200 cal.

Calories contained within food are actually kilocalories (kcal). In other words, if a certain snack contains 85 food calories, it actually contains 85 kcal or 85,000 cal. In order to make the distinction, the dietary calorie is written with a capital C.

1 \ \text{kilocalorie}=1 \ \text{Calorie} = 1000 \ \text{calories}

To say that the snack “contains” 85 Calories means that 85 kcal of energy are released when that snack is processed by your body.

Heat changes in chemical reactions are typically measured in joules rather than calories. The conversion between a joule and a calorie is shown below.

1 \text{ J} = 0.2390 \text{ cal  or 1 cal} = 4.184 \text{ J}

We can calculate the amount of heat released in kilojoules when a 400. Calorie hamburger is digested.

400 \cdot \text{Cal} = 400 \cdot \text{kcal} \times \frac{4.184 \text{ kJ}}{1 \text{ kcal}}=1.67 \times 10^3 \text{ kJ}

Summary

  • Processes of heat exchange between the system and surroundings are described.

Practice

Do the problems (as many as you have time for) at the link below:

http://www.chemteam.info/Thermochem/Thermochem-WS2.html

Review

Questions

  1. What does the law of conservation of energy say?
  2. What is a reaction called that gives off heat?
  3. A cake is made by mixing ingredients and placing the material in an oven for baking. What type of reaction is involved?

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Date Created:

May 01, 2013

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Aug 20, 2014
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