Bomb Calorimeter: Definition, Equation & Example Video

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  • 0:04 What Are Calories?
  • 0:40 Why Does Food Have Calories?
  • 1:05 What Is a Bomb Calorimeter?
  • 3:15 Example
  • 5:24 Lesson Summary
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Lesson Transcript
Instructor: Stephanie Bryan

Stephanie has a master's degree in Physical Chemistry and teaches college level chemistry and physics.

In this lesson we will describe a bomb calorimeter and understand how it is used to determine the amount of heat given off or taken in during a chemical reaction.

What Are Calories?

Have you ever wondered how the number of calories on the back of food packages is determined? This is done by bomb calorimetry, but before we can fully understand this, we need to define a calorie. A calorie is the scientific unit that is defined as the amount of energy required to raise the temperature of one gram of water by one degree Celsius. In the United States, food packaging lists calories, but these food calories are actually kilocalories, or 1000 calories. In most other countries, you will see food packaging with kcals listed. Kcal is the abbreviation for kilocalorie.

Why Does Food Have Calories?

Food has calories because our bodies process the food in a chemical reaction, called combustion, that breaks down the food and gives off heat. It's this heat energy, or calories, that our body uses or stores as energy to keep us going. Many other chemical reactions give off heat as well. These are exothermic reactions. Reactions that take in heat from the surroundings are called endothermic reactions.

What Is a Bomb Calorimeter?

A bomb calorimeter is a device that is designed to measure the amount of heat that is given off or taken in by a reaction. It's designed to be isolated from the surroundings, meaning that no heat can leave or enter the device. In order to accomplish this, the outside wall of the calorimeter is a vacuum-sealed wall similar to an insulated mug you might carry coffee in. Inside the calorimeter is a vessel in which the reaction occurs surrounded by a water bath. The temperature of the water bath can be monitored via thermometer.

When a reaction takes place in the calorimeter, the temperature of the water bath changes. If an exothermic reaction occurs, the water bath temperature goes up. If an endothermic reaction occurs, the water bath temperature goes down.

In an ideal calorimeter, all the heat exchange would take place with the water. However, in real life, the materials from the calorimeter itself, the stainless steel, thermometer, etc., exchange heat with the system as well. We can correct for this by using a heat capacity for the calorimeter in our calculations.

Because all the heat is exchanged inside the calorimeter, we can set the total heat to zero. We have three terms for the three parts of our calorimeter: the calorimeter itself, the water bath, and the chemical reaction, which you can see labeled in the equation below:

q total equals zero

Substituting the equations for the heat of the water and the heat of the calorimeter, we get this much more detailed equation:

substituting for q values

Looking at this equation, you should note that the term for qcal has no mass. This is because it is assumed that the mass of the calorimeter is constant and therefore can be included in Ccal .

Also note that ΔT should always be calculated as Tf (final temp) - Ti (initial temp). This means that if the temperature of that object goes down, ΔT will be negative. Cp is the specific heat capacity for water and is 1 cal/g°C.

Looking at the equation below, you can see how we solve for qrxn :

qrxn solved


Let's take a look at an example. First, here's our problem:

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