HEAT, TEMPERATURE, AND CALORIMETRY

Temperature is a property of a system, and changes in temperature are able to cause other properties of a system to change. But what causes changes in temperature? By definition, a flow of heat into a system causes the temperature of the system to increase, and a flow of heat out of a system cause the temperature of the system to decrease. Heat is not a property of a system; it is a separate entity altogether.

But what is heat? An early theory assumed that heat was a fluid, like water, that existed between the particles of matter, and that heat fluid (or caloric, from the Latin word calor, meaning heat) could be absorbed by or squeezed out of matter, much like water and a sponge. However, an American-born Englishman, Count Rumford, showed that the behavior of matter did not match the predictions of caloric theory. However, James Prescot Joule, through careful experimentation in the mid-1800s, was able to demonstrate that work could be converted into heat, suggesting that heat was a form of energy.

A theory of heat existed long before its link to energy was discovered, so if heat really existed then it should be measurable. The science of heat measurement is called calorimetry. The amount of heat (q) needed to cause a measured change in the temperature of a body (ΔT) is proportional to the mass (m) of material being heated and also the nature of the material (c):

q = m c ΔT

The property of the material denoted by c is called the specific heat of the material, and has units of J/g-C. The unit of heat energy known as the calorie was originally defined as the amount of heat it took to raise one gram of water one degree Celsius; in other words, the specific heat of water was defined to be 1.00 unit. The result of Joule’s experiments was to demonstrate the conversion of mechanical energy into the form of heat, and he measured the mechanical equivalent of heat. In current units of energy, Joule showed that 1 calorie = 4.184 J, so the specific heat of water is 4.184 J/g-C.

MEASURING HEAT

Purpose

The purpose of this activity is to test the plausibility of the relationship, Q = m c ΔT.

Materials

Lab Pro system, temperature probe, hot plate, 400 mL beaker, water

Instructions

Set up the Lab Pro system with a temperature probe, and set the time collection to 200 seconds. Also set up and turn on a hot plate, setting the temperature dial to approximately a “medium high” heating rate. Measure out 150 grams (or 150 mL) of water in a 400 mL beaker, and put the temperature probe in the beaker of water. Start data collection, then immediately place the beaker on the hot plate. Record the temperature of the water sample for up to200 seconds, or until the water starts to boil.

1) Describe the curve produced on the graph. What kind of relationship between temperature and heating rate does the curve show?

We will assume that the heating rate of the hot plate is constant. If heating rate is constant, then the same amount of heat is delivered to the water over the same time frame.

2) If heating rate is constant, then what kind of relationship exists between added heat and change in temperature? Cite specific data points to prove this.

3) Record the total change in temperature over the 200 seconds of heating, or until the water starts to boil.

Repeat the experiment using 300 mL of water instead of 150 mL of water, or twice the amount. Do not adjust the heating rate of the hot plate.

4) Compare the total change in temperature over a particular change in time for 150 mL and 300 mL of water. Based on these numbers, what appears to be the relationship between heat added, change in temperature, and mass of the water sample?

[NOTE: we are assuming water has a constant density, so that volume is proportional to mass. Why do we choose to discuss the mass of the sample and not its volume?]

Heat 40 mL of water and 40 mL of vegetable oil simultaneously in separate 100 mL beakers for up to 200 seconds on the same hot plate. Use two temperature probes to record both sets of data at the same time.

5) How are the curves the same, and how are they different?

6) Given your observations, is the equation Q = m c ΔT plausible? Explain.

7) Calculate the value of “c” for vegetable oil, assuming the value of c for water is

4.184 J/g-C.