# Comparing Specific Heats of Metals

Two different metals, aluminum and lead, of equal mass are heated to the same temperature in a boiling water bath. The specific heat capacities of each metal is displayed  to students:   Al 0.903 J/g°C       Pb  0.160 J/g°C

The metals are added to two insulated cups or calorimeters, each containing the same amount of water initially at room temperature.  Students are asked to predict what will happen to the temperature of water and the temperature of the metals.   The temperature of the water changes by different amounts for each of the two metals.  This demonstration assess students' conceptual understanding of specific heat capacities of metals.  If the accompanying computer animation is displayed students can gain a conceptual understanding of heat transfer between a hot sample of metal and the cool water at the particle level (atom level).  For each expompare the heat gained by the cool water to the heat released by the hot metal.  Compare the heat gained by the water in Experiment 1 to the heat gained by the water in experiment 2.

Curriculum Notes

The Law of Conservation of Energy is the "big idea" governing this experiment.   A natural transfer of heat or heat flow from a region of higher temperature to a region of lower temperature until an equilibrium temperature is reached. In this demonstration, heat energy is transferred from a hot metal sample to a cool sample of water:  qlost + qgain = 0

Each different type of metal causes the temperature of the water to increase to a different final temperature. This indicates that each metal has a different ability to absorb heat energy and to transfer heat energy.

The ability of a substance to contain or absorb heat energy is called its heat capacity. Heat capacity is an extensive property—it depends on the amount or mass of the sample. Specific heat is a measure of the heat capacity of a substance. Specific heat is defined as the amount of heat required to increase the temperature of one gram of a substance by one degree Celsius.

Specific heat:   Al 0.903 J/g°C       Pb  0.160 J/g°C

calorimetry computer simulation can accompany this demonstration.

A computer animation depicting the interaction of hot metal atoms at the interface with cool water molecules can accompany this demonstration (see file posted on the side menu).

An in-class activity can accompany this demonstration (see file posted on the side menu).

With some planning all three representations can be explored (not simultaneously) FROM ALEX JOHNSTONE'S triangle: macroscopic, microscopic, symbolic.

Learning Objectives

1. Use experimental data to develop a conceptual understanding of specific heat capacities of metals.

2.  Given appropriate calorimetry data for two metals, predict which metal will increase the temperature of water the most.

3. Use experimental data to develop a relationship among the variables: heat, mass, specific heat, and change in temperature.

4. Apply the First Law of Thermodynamics to calorimetry experiments.

5.  Identify what gains heat and what loses heat in a calorimetry experiment.

6.  For a physical process explain how heat is transferred, released or absorbed, at the molecular level.

7.  Given appropriate calorimetry data for two metals, predict which metal will increase its temperature the quickest (shortest time) when each metal starts at room temperature and is uniformly heated.

Discussion

Common misconception

Some students reason "the metal that has the greatest temperature change, releases the most heat".  When in fact the meal with the smallest temperature change releases the greater amount of heat.

Materials

Metals:  Al and  Pb, (and Zn) 100 g each

Digital thermometers, LapTop/PC with digital thermometer display

Hot plate

Balance, centigram (0.01-g precision)        Insulated coffee cups, 6

Beaker, 600-mL     Beakers, 400-mL,

Stirring rods, 3   Ring stand and clamp

Boiling stones      Beaker tongs

1.0 L of Deionized Water ;    Graduated cylinder, 100-mL

Procedure

Because the density of aluminum is much lower than that of lead and zinc, an equal mass of Al occupies a much larger volume than Pb or Zn. Choose a large enough beaker such that both the aluminum metal and lead metal will be submerged in the boiling water bath.  Heat the metals for about 6 minutes in boiling water.  Place 50 mL of water in a calorimeter.  Measure and record the temperature of the water in the calorimeter.  Have students predict what will happen to the temperature of the water in the two calorimeters when hot lead is added to one and hot aluminum is added to the other.  See the attached clicker question. After students have answered the question, use the tongs and grab the hot lead metal and place it in 50 mL of room temperature water.  Stir it up (Bob Marley).  Record the temperature of the water.  Use the tongs and grab the hot aluminum metal and place it in the second calorimeter containing 50 mL of room temperature water.  Stir it up.  Record the temperature of the water.  Compare the final temperature of the water in the two calorimeters. Compare the heat gained by the cool water to the heat released by the hot metal.  Compare the heat gained by the water in Experiment 1 to the heat gained by the water in experiment 2.

Sample Data and Sample Calculations

 Mass Initial Temp . Final Temp. Temp. Change water 50.0 g 19.2°C 24.9°C +5.7°C lead 100.0 g 100.0°C 24.9°C -75.1°C

q = m x c x t

q lost Pb = 100. g x 0.160 J/g °C x (-70.0°C) = -1201 J

q gained water= 50.0  g x 4.18 J/g °C x (5.7°C) = +1191 J

 Mass Initial Temp . Final Temp. Temp. Change water 50.0 g 19.2°C 43.5°C +24.3°C aluminum 100.0 g 100.0°C 43.5°C -24.3°C

q gained water = 50.0  g x 4.18 J/g °C x (24.3°C) = +5078 J

q lost Al = 100.0  g x 0.900 J/g °C x (-56.5°C) = +5085 J

Footnotes

Specific Heat A Chemistry Demonstration. Flinn Scientific, Batavia, Illinois.  2016.  https://www.flinnsci.com

Johnstone, A. H. 1993. The development of chemistry teaching: A changing response to changing demand.  Journal of Chemical Education, 70(9), p. 701-705.

Harrington, D.G. 2011. The Heat is on: An inquiry-based investigation for specific heat.  Journal of Chemical Education, 88,1558-1561.

### Topics:

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group