CHEM 101-Introductory Chemistry I-Lab 2 - Thermochemistry

Thermochemistry – Specific Heat

Purpose: To study the transfer of heat by monitoring temperature changes; to use laboratory measurements and mathematical formulas to determine specific heat.

Introduction:

In this experiment, you will follow heat as it is exchanged from one component to another. The basic principle in this experiment is that the heat, q, lost by one component is equal to the heat, q, gained by the other.

q(lost) = - q(gained)

Note the negative sign that indicates that heat flows in the opposite direction for each component.

We can describe the heat lost or gained when a substance changes temperature by the following equation:

q = m x C x  T

Where

q = heat lost or gained in joules (J)

m = mass in grams of sample (g)

C = specific heat (J/g oC)

 T = the change in temperature (oC) = Tfinal - Tinitial

In this lab, you will be adding hot metal to a container of water. Before the experiment, you will measure the mass of water used, the mass of metal used, the initial temperature of the water and the initial temperature of the metal. You will also use the specific heat of the water

(a constant, 4.1840 J/g/ oC). When the experiment is complete, you will have the final temperature of the metal and the water (it is the same temperature). Your task is to solve for the specific heat of the metal.

When you add the hot metal to the water the water will gain the heat lost by cooling the metal. Thus:

(m x C x  T)metal = -(m x C x T)water

NOTE: T of the metal is NOT equal to T of the water since they have different initial temperatures. Their final temperatures are the same. It is the equilibrium temperature.

You will determine the specific heat for each of three metals.

Procedure:

1) Prepare a calorimeter by nesting 2 Styrofoam cups together. Determine the mass of the two Styrofoam cups. Place 50.0 mL of water in the calorimeter. Determine the mass of the two cups plus the water. Determine the mass of the water.

2) Place about 35 g of metal into a large test tube. (Record the exact mass!) Suspend the test tube into a beaker of boiling water, using a hotplate as your heat source. Leave the test tube in the boiling water for at least 10 minutes. Measure the temperature of the boiling water. We will assume that the metal is at the temperature of the boiling water at this point.

NOTE: Do not allow any water to get into the inside of the tube where the metal is.

3) Using the same thermometer record the temperature of the water in the calorimeter. This is the initial temperature of the water.

4) Carefully add the metal to the calorimeter, replacing the cover immediately. Measure the temperature every thirty seconds for four minutes. The maximum temperature will be recorded as the final temperature of the mixture.

5) Pour off the water and place the metal in the appropriate container for reuse.

6) Repeat steps 1-5 for two additional different metal samples.

EXAMPLE PROBLEM:

A 50.05 g sample of zinc metal at 99.5 oC was dropped into a Styrofoam cup containing 100.0 g of water at 21.0 oC. The water in the cup reached a temperature of 24.5 oC. Calculate the specific heat of the metal.

Solution: First calculate the heat gained by the water. q = m x C x (T2 – T1) where m is the mass and C is the specific heat. For water C = 4.184 J/g/oC.

H = 100.0 g * 4.184 J/g/oC * (24.5 – 21.0) oC = 1464 J

The heat gained by the water is thus 1464 J. Since the assumption is that this is the same as the heat lost by the metal we can calculate the specific heat of the metal.

-1464 J = = 50.05 g * (24.5 – 99.5) oC _____________________ _____________________ Heat =

mass * Temperature change Specific Heat =

Specific Heat = 0.390 J/g/oC.

Data and calculations

Metal 1 Metal 2 Metal 3

Mass of calorimeter cups __7.333___ g 7.333__ g ____7.333_ g

Mass of calorimeter cups plus water __57.835__ g ___58.056__ g ___57.556_ g

Mass of water 50.502 g 50.723 g 50.223g

Mass of metal ___35.000_ g __35.137___ g __35.075__ g

Initial temperature of water ___21.8___ oC ___22.0___ oC ___22.0__ oC

Initial temperature of metal ___99.9___ oC ____99.7__ oC ___99.5__ oC

Final temperature ___32.0___ oC ___26.3___ oC __27.7___ oC

 Twater ___10.2 oC ____4.3___ oC ____5.7__ oC

Tmetal ___-67.9 oC ___-73.4__ oC ___-71.8_ oC

Heat gained by water 2155.264 J 912.568____ J 1197.758J

Heat lost by metal -2155.264 J -912.568____ J -1197.758J

Specific heat of the metal

(J/g/ oC) 0.907 J/g/oC 0.354 J/g/oC 0.476 J/g/oC