Specific Heat of metals

Chapter 8

Specific Heat of Metals Objective To measure the specific heat capacity of various metals using the method of Calorimetry.

Apparatus 1. 2. 3.

Bunsen burner Boiler, boiler cup and boiler stand Two styrofoam cups

8.1

4. 5. 6.

Gas and steam hoses Metal specimens and weight balance Two thermometers

Introduction

Some substances require more heat than others to raise their temperature by the same amount. For example, the iron burner on an electric stove warms up a lot faster than the water contained in a pan, the sand on a beach gets hot a lot faster than the water in the ocean, etc. This behavior depends on the material and is characterized by a quantity called specific heat. If Q is the amount of heat required to raise the temperature of a substance with mass m by an amount ∆T , then the specific heat c of the substance is given by, c=

Q m∆T

(8.1)

Here, Q is in measured in calories (cal), m is in grams (g) and ∆T is in o C. One calorie (cal) is defined as the heat required to raise the temperature of 1 gram of water from 14.5 o C to 15.5 o C. The Calories you read on the nutrition labels of food packages are equivalent to 1000 cal. To distinguish them from lowercase calories we represent them with the capital C. The method used to determine experimentally the specific heat of a substance is called calorimetry. This method uses the conservation of energy principle. We heat a certain mass mx of substance to a known temperature Tx . We then place the substance in a sealed container that contains water of mass mw . The water is 71

Exp. 8: Specific heat.

PHYSICAL SCIENCE 101 LAB

initially at a cool temperature Ti . From our everyday life, we are familiar with the observation that when in contact, objects at different temperatures will exchange heat until they reach a common final temperature Tf . The high temperature object looses heat and the lower temperature one gains it. The process must occur in a thermally isolated device that prevents heat escaping to or entering from the surrounding environment. Such devices are called calorimeters. In this lab, we will use a calorimeter made of two styrofoam cups one inside the other. A styrofoam plate will be used as a top cover. The conservation of energy principle dictates that the heat lost from the hot substance, Qx , must be gained by the cool water, Qw . There is no mechanical energy involved in the process, and the styrofoam does not absorb any heat. We then write, Heat lost = Heat gained or, Qx = Qw

(8.2)

mx cx (Tx − Tf ) = mw cw (Tf − Ti )

(8.3)

From Eqs. 8.1, and 8.2 we get,

Thus the specific heat of a substance is given by, cx =

mw cw (Tf − Ti ) mx (Tx − Tf )

(8.4)

To simplify calculations further, we can choose the mass of the specimens and the mass of the water contained in the cup to be equal. Thus the above equation will read as, cx =

cw (Tf − Ti ) (Tx − Tf )

(8.5)

where cw = 1 cal/g o C

8.2

Experimental Procedure

1. Weigh the boiler cup empty. Add exactly 150 g of one type of metal shot. 2. Place the boiler cup inside the boiler (which should be three quarters full of water), place the boiler on the ring stand with the wire gauze, and light the bunsen burner under it. CAUTION: If you are not completely sure of how to light the burner; STOP. Call your instructor for help. Place one of the thermometers in the boiler cup so that the bottom part of it is buried in the shot, and let the water boil. 72

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Exp. 8: Specific heat.

3. While the shot is warming up, obtain two styrofoam cups and make a double walled calorimeter. Pour in exactly 150 g of water. Place the styrofoam lid on top of the calorimeter and slide the other thermometer through the opening at the center of the lid. 4. Monitor the temperature of the shot at the boiler cup. The temperature will stop rising between 92 o C and 98 o C. Wait for a minute after you notice the temperature stopped rising, and record this value as Tx . 5. Record the initial temperature of the water, Tw , in the calorimeter cup. Carefully remove the boiler cup from the boiler, and slowly pour the metal shot into the calorimeter cup so that there is no splashing. 6. Monitor closely the temperature of the metal shot-water system. They will reach equilibrium very shortly. At that point the temperature will not rise any further. Record this temperature as the common final temperature, Tf . 7. Substitute your values in Eq. 8.5 and compute the specific value of the metal. 8. Repeat the above steps with the second type of metal shot. 9. Read the accepted values of the the specific heat for each metal from the table in Appendix B. Calculate the percent error of your value from the accepted value.

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Exp. 8: Specific heat.

Data Table 8.1 Type of Specimen

Initial Temp. of metal, Tx

Initial Temp. of water, Tw

Final Temp. Tf

Experimental c (cal/g o C)

Show detailed calculations for the specific heat in the space below.

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Accepted c (cal/g o C)

Percent error

PHYSICAL SCIENCE 101 LAB

8.3

Exp. 8: Specific heat.

Questions

1. Calculate the amount of heat (in calories) needed to raise the temperature of 454 g of water from 20 o C (room temperature) to 100 o C (boiling temperature).

2. Calculate the amount of heat needed to raise the temperature of an aluminum pan and that of a copper pan, both of mass of 454 g, from 20 o C to 100 o C.

3. Which pan requires less heat? Explain why?

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