How to Determine a Calorimeter Constant

By John Brennan; Updated April 24, 2017
Calorimeters help determine the energy released or absorbed by a reaction.

Calorimeters measure the heat of a chemical reaction or a physical change like ice melting to liquid water. The heat of reaction is important for understanding the thermodynamics of chemical reactions and predicting what kinds of reactions will take place spontaneously. A basic calorimeter is very easy to construct -- all you need is a couple of Styrofoam coffee cups, a lid and a thermometer. Before using your calorimeter, however, you need to calibrate it and determine its calorimeter constant. To find the calorimeter constant for your device, follow the steps outlined below.

Put on lab coat, goggles and gloves.

Assemble the coffee cup calorimeter by inserting one Styrofoam coffee cup into the other and attaching the lid. It may seem simple, but if it's properly calibrated, this coffee-cup calorimeter can be surprisingly useful for finding heats of reaction.

Measure out approximately 50 mL of cold water using the graduated cylinder. There's no need to be exact at this stage.

Measure the weight of your empty coffee-cup calorimeter to the nearest 0.01 grams (or as close as you can get). Now, add the 50 mL of cold water, replace the lid and re-weigh the calorimeter. The difference between the empty and full weights is the weight of the cold water. Record this value (to the nearest 0.01 grams).

Weigh the beaker and record its weight (to the nearest 0.01 grams). Add approximately 50 mL of water and re-weigh the beaker. The difference between the empty and full weights is the weight of the hot water. Record this value (to the nearest 0.01 grams).

Using the ringstand and clamp, secure the beaker so that it stands atop the wire gauze mesh over the bunsen burner. The wire gauze mesh prevents the flame from coming in direct contact with the glass. Position one of the two thermometers in the beaker and secure it using a clamp so that it is suspended in the water, but does not touch the bottom of the beaker.

Light the bunsen burner and gently heat the hot water to about 80 degrees C. It is better to heat it slowly rather than heating it too rapidly and bringing it to boil.

Insert the second thermometer into the calorimeter through the lid. Stir the water inside the calorimeter for four minutes and record its temperature at one-minute intervals to the nearest 0.1 degrees C. The temperature should remain more or less constant; if it is not, allow the cold water to sit for at least two minutes more.

Just before the fifth minute, turn off the Bunsen burner if you have not done so already, and record the temperature of the hot water and the cold water. Quickly and carefully pour all the hot water into the calorimeter, then replace the lid and resume stirring with the thermometer.

Measure and record the temperature in the calorimeter at 30-second intervals until a total of five minutes has elapsed.

Open Excel or another spreadsheet program. Enter the time as the X-values and the temperatures as the y-values and graph your data. Use the spreadsheet program to find a line of best fit for the data after adding the hot water. Do not include the data points from before adding the hot water in your line of best fit. The trend line should be linear.

Write down the line of best fit from your graph. Plug in 5 minutes for x and calculate y (the extrapolated temperature at 5 minutes). We'll call this extrapolated temperature Tf.

Subtract Tf from the temperature of the hot water just before you added it to the calorimeter. This will give you the change in the temperature of the hot water, Th. Multiply Th by 4.184 and the mass of the hot water to find out how much energy the hot water lost in joules.

Subtract the temperature of the cold water from Tf; this will give you Tc, the temperature change of the cold water. Multiply by the mass of the cold water and 4.184 to find the amount of energy gained by the cold water in joules.

Subtract the energy gained by the cold water from the energy lost by the hot water. This will give you the amount of energy gained by the calorimeter.

Divide the energy gained by the calorimeter by Tc (the temperature change of the cold water). This final answer is your calorimeter constant.

Tip

The calorimeter constant can never be negative -- if it is, you have made a mistake... Try performing multiple trials and averaging out the results of those trials to reduce your error. The uncertainty in your final average will be plus/minus 2x the standard deviation.

Warning

ALWAYS be extremely careful when working with an open flame. Never allow your hair, clothing or any flammable materials to come close to the fire. Extinguish the burner when it's no longer in use. Be extremely careful when working with hot water; water at 80 degrees C can cause nasty burns or scalds if you spill it on your skin.

About the Author

Based in San Diego, John Brennan has been writing about science and the environment since 2006. His articles have appeared in "Plenty," "San Diego Reader," "Santa Barbara Independent" and "East Bay Monthly." Brennan holds a Bachelor of Science in biology from the University of California, San Diego.