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Analyzing the Results of Calorimetry Experiments

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 about calorimeters and how they measure the energy of a compound. We will also learn how to use the data from a calorimeter to determine the enthalpy of the reaction.

Definition

While enjoying a nice campfire you might decide to roast a few marshmallows. Despite your careful attention, it slides off your stick and into the fire. You watch with sadness as your perfectly golden marshmallow burns away into ash.

Although this is a sad story, did you know that by burning food (and other compounds) you can determine how much energy is in that compound? You couldn't actually measure it in the campfire, but if you have a calorimeter then it can be done fairly easily.

A calorimeter is a device that burns compounds and measures the heat given off to determine the energy in that compound. This information can be used for determining calories of a food, how a reaction will occur, energy efficiency of a fuel, and many other uses. The data obtained from a calorimeter can be found in tables such as 'heat of formation' or 'heat of combustion'.

A simple form of a calorimeter is simply a Styrofoam cup, water, the compound of interest, a heat source, and a thermometer. The thermometer measures the change in temperature of the water it takes for the compound to be burned. Since an unknown amount of heat can be lost through the Styrofoam cup this version isn't as exact as a bomb calorimeter but it uses the same idea.

In a bomb calorimeter, a sample is placed into a center chamber, heated with an ignition source. A thermometer then records the change in temperature as the sample is burned.
sketch

Calorimeter Results

So the information you obtain from a calorimeter is the change in temperature of the water. We base the rest of the calculations on the assumption that all the heat (or energy) from the burning of the compound is transferred to the water. The pressure also needs to be noted as temperatures can be different depending on the atmospheric pressure.

For example, at high elevations water boils at a lower temperature than at sea level. So when comparing results on heat of formation or heat of combustion tables you need to make sure that the same pressure was used for each table.

In order to change the temperature information into units of energy you also need to know the mass of your compound and the specific heat of water. The Specific Heat Formula is then:


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where Q is heat energy, c is specific heat, m is mass, and delta t is change in temperature.

Example

So let's say you are measuring the energy found in sucrose (also known as table sugar, which is one of the main ingredients in the marshmallow that was burned earlier). You use a 0.5 gram sample of sucrose, with 100 g of water, put the sucrose into the sample cup and ignite it.

The specific heat of water is 4.18 J/g/degree Celsius. The original temperature of the water was 25.00 degrees Celsius. You watch it increase up to 26.97 degrees. So the change in temperature was 1.97 degree Celsius.

So Q = 100 g x 4.18 J/g/degree Celsius x 1.97 degrees Celsius = 823.46 J. This is the amount of energy given off by 0.5 grams of table sugar.

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