Calorimetry: Determining the energy in a nut

In this lab, you will use two methods to find the amount of energy stored in a nut. In one, using a bomb calorimeter, the system is kept at constant volume, and in the other the system is left open to the air, and so is at constant pressure. Each method measures a temperature change due to the energy released as heat from the combustion of the nut. Heat is transferred from the nut to the calorimeter, or -q_nut = q_calorimeter. Luckily we can measure the temperature change of the calorimeter and from this calculate the energy stored in the nut.

Equipment:

• Mixed nuts and cheesy puffs
• Nutstand consisting of a paperclip and cork
• Soda can
• Lighter
• Ringstand and clamp
• Thermometer or temperature probe
• Bomb calorimeter

Note: We only have 4 bomb calorimeters, so you’ll need to schedule your timing accordingly. It’s not necessary to do the two parts in order.

Procedure Part 1 (the quick and dirty method): Choose a nut, weigh it carefully, and arrange it on the nutstand. Fill a soda can with an accurately measured amount of water (high enough to immerse the thermometer bulb) and suspend the can above the nut (use your imagination!). Carefully light the nut on fire, and measure the temperature change of the water that results from the heat released from the burning nut. Be sure you’re recording the actual temperature of the water – don’t rest the thermometer on the bottom of the can and stir thoroughly to ensure an even temperature. Try this experiment with at least one type of nut and a cheesy puff. A slight warning: the ash left from the burnt food is very sooty, so try not to get it on your clothes.

After conducting the experiment once, speculate on the sources of error and how these errors would effect the measured temperature change. If time allows, refine your apparatus and procedure to try to eliminate or account for this error. Perform additional trials to check the precision of your measurements.

Procedure Part 2 (bomb calorimeter): Choose a nut (try to use the same type as you used for part 1 to compare results), weigh it carefully, and place it in the bomb calorimeter. Place 1.00mL of water into the bomb (not on the nut!). Place 1250mL of distilled water in the outer chamber of the calorimeter. Record the initial temperature of the water for a minute (You’ll want to increase the collection time on LoggerPro. You can do this under the data acquisition menu), then ignite the nut inside the chamber, recording the temperature as it increases due to the heat released from the nut. Eventually the temperature will begin taper off. Continue to record the temperature for another minute. Save your temperature vs. time data in your folder on masu.

Write up: Include your speculations on sources of error and how they effect the measured temperature change. Detail any changes you made to the procedure for part 1(why you made them and whether they were effective). Diagram the parts of the bomb calorimeter (consult figure in Silberberg for help). Be careful to label parts and briefly describe their function.

Analysis: Compare your results of the calorie content of your nut using the two methods.

Some help in using the bomb calorimeter data: (The concept is the same for the quick and dirty method, but less complicated.)

1. Note that both the bomb and the water in the calorimeter absorb heat. Calculate the heat absorbed by the water in the calorimeter (in Joules).

q_water = c_water DT

2. Calculate the heat absorbed by the bomb. In order to do so, you need the heat capacity of the bomb and the temperature change. (We’ll give you the heat capacity of the bomb.)

q_bomb = c_bomb DT

3. The heat absorbed by the bomb and water came from the combustion of the peanut. Assuming that there is no heat loss to the environment, calculate the heat given off by the peanut during its combustion (in calories).

q_calorimeter = q_water + q_bomb = -q_nut

4. Calculate the calories per gram of peanut.

Calculate the kilocalories per gram of peanut. Then express your answer in food calories. (A chemist’s calorie is given the symbol cal, a kilocalorie is given the symbol kcal and a food calorie is given the symbol Cal. Also 1000 cal = 1kcal = 1 Cal)

Look up the food industry’s determination of the calorie content of your nut and/or cheesy puff (by reading the label or a Google search). How does this compare with the values you determined? Calculate the difference based on your bomb calorimeter value:

Percent difference = (experimental value - industry value) x 100

industry value

Suggest possible reasons for this error.