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Molar Heat of Combustion: Definition & Calculations

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  • 0:04 Measuring Energy
  • 0:31 Calorimeter
  • 1:32 Bomb Calorimeter
  • 2:04 Ethanol Example
  • 3:19 Other Examples
  • 6:21 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we will learn what molar heat of combustion is. We will walk through several examples using experimentally obtained data to calculate the molar heat of combustion.

Measuring Energy

We often talk about how much energy is in a compound or in a bond, but how do we know this? One way that energy is determined is through calculating the molar heat of combustion.

The molar heat of combustion is how much heat is released when we burn 1 mole of a compound. Typically, this is set at a specific pressure of 1 atmosphere. The reason this works is because one form of energy is heat, so we assume that all of the energy is converted into heat.

Calorimeter

To measure the heat of combustion, a calorimeter is used. A calorimeter is an isolated system where all of the heat transfer can be monitored. When you start a bonfire, the wood is burning, and the energy from the bonds is changed into heat. But because the heat escapes into the environment around us, it's really difficult to measure all of the heat change. So a calorimeter uses an enclosed system in order to measure the change in temperature.

Typically, the calorimeter is filled with water, and the change of temperature in the water is measured. Water is used because we know the specific heat of water. In other words, we know how much energy it takes to increase water by one degree Celsius. And it is a cheap, easily accessible medium.

The heat of combustion is calculated by multiplying the mass of the water times the specific heat of the water times the change in temperature. This entire equation is multiplied by -1, because heat of combustion is negative because heat is being lost or released.


The formula for molar heat of combustion
Formula


Bomb Calorimeter

To measure the change in temperature, a bomb calorimeter is used. A bomb calorimeter insulates the water so that all heat from the burning sample can be accounted for. A bomb calorimeter has a water bath with a stirrer and thermometer inserted. In the center of the water bath is the bomb cell, which contains the sample and an ignition source. The initial temperature of the water is recorded, then the sample is ignited. The final temperature of the water is then recorded. Then we can use the heat of combustion equation to determine the energy in the sample.

Ethanol Example

Let's look at an example using Ethanol. First, it's set up with 100 grams of water in the water bath. We record the initial temperature as 25 degrees Celsius.

We then add 1 gram of ethanol to the bomb cell and ignite it. The temperature of the water increases up to 95.91 degrees Celsius.

95.91 - 25 = 70.91

So the change in temperature is 70.91 degrees Celsius. The specific heat of water is 4.2 J/g/C. Here is the heat of combustion equation for Ethanol.


Ethanol equation


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