Experiment 10

Experiment 10: Calorimetry

Purpose

(1)  To study heat transfer processes, including those involving phase changes.

(2)  To measure the heat of fusion of water (latent heat).

Apparatus

calorimeter; dial balance; a beaker with some water at room temperature; a spare beaker; supply of ice; paper towels; and more water at room temperature on the side.

Theoretical Summary

(A) When heat is absorbed or emitted by a certain mass of a substance, without a change in phase (solid, liquid, or gas), the amount of heat Q is given by formula (1),

where:

M = mass
Tf = final temperature
Ti = initial temperature
C = specific heat for the particular substance / (1) / BASIC FORMULA
Q = M · C (Tf – Ti)

Note: If Tf > Ti the heat is absorbed; if Tf < Ti the heat is emitted.

Remark: the specific heat C is, in general, different for different phases of a substance,

in particular, it is different for ice and water.

(B) When the phase of a substance changes,
this process takes place at a constant temperature. The amount of heat Q absorbed or emitted in this process is given by formula (2) where:
M = mass
L = latent heat / (2) / BASIC FORMULA
Q = M · L

Note: If the change of phase is between the solid and a liquid (as in ice/water) L is known as the latent heat of fusion.

The Calorimeter: This is an apparatus designed to provide thermal insulation (ie no exchange of heat) between its CORE (the interior cup with its contents plus the stirrer) and the SURROUNDINGS (mostly the air in the room). The insulation consists of the jacket, the lid, the thermometer, and the rubber ring.

Procedure Part I.

(a) Set up the Table I on your data sheet. Measure all temperatures to 0.1º C.

TABLE I
/ Mcsw / Mip / Mpap / Mf / / Tice / Tf

(b) Remove the interior cup of the calorimeter, together with the stirrer, (but without the rubber ring) and record its mass (cup + stirrer) as Mcs.

Put approximately 200 gm of water (should be available in a beaker) into the cup and record the total mass (cup, stirrer and water) as Mcsw.

Close the calorimeter and record the initial temperature of the water as T0. (This

should be the room temperature - if in doubt, check with your instructor).

(d) Record the initial temperature of the ice as Tice. (Remember the freezing point of

water is 0˚C.)

Take about 50 grams of ice, put it on a piece of paper towel and record the total

mass as Mip. Immediately after this, put the ice into the calorimeter along with the

water and close it.

Measure the mass of the paper and record it as Mpap. Also,

(e) Keep watching the thermometer for several minutes, and move the stirrer once in a while, until the temperature ceases to fall. This will be the final equilibrium temperature (which should remain steady for another 5 minutes).

When you believe this temperature is reached, open (and close) the calorimeter quickly to check if all ice has melted (if it didn’t, you should wait a bit longer).

Record the final equilibrium temperature as Tf.

(f) Remove the cup, with the stirrer and the water contained in it, and record this mass as Mf. Check whether Mf + Mpap equals Mcsw + Mip accurately to within 0.5 grams,

(If not, check all your measurements before you proceed further.)

Procedure Part II.

(g) Wipe all parts of the calorimeter with paper towel and let them dry out completely at room temperature. In the meantime, set up Table II on your data sheet:

TABLE II
/ Mcsw´ / Mip´ / Mpap´ / Mf´ / / Tice / T´

(h) After all parts of the calorimeter are completely dry, put approximately 100 grams of water at room temperature into the cup and record the total mass as Mcsw´. Close

the calorimeter and record its initial temperature as T0´

(i) Record the initial temperature of the ice as Tice.

Take about 80-100 gm of ice, put it on a piece of paper towel and record the total mass as Mip´. Immediately after this, put the ice into the calorimeter and close it.

Measure the mass of the paper and record it as Mpap´.

(j) Keep watching the thermometer, and move the stirrer once in a while, until the temperature reaches its lowest point. (This may take up to half an hour.) Record

this temperature as T´ (it should be definitely less than 2ºC).

Open the calorimeter and quickly pour out all water, leaving only the unmelted ice.

Quickly weigh the cup with the stirrer and unmelted ice in it and record as Mf’.

BEFORE YOU LEAVE THE LAB: WIPE OUT YOUR CALORIMETER WITH PAPER TOWELS AND LEAVE IT OPEN TO DRY OUT.


Lab Report

Part I.

(1) On the basis of formulae (1) and (2), the following equation represents the heat exchanges involved in Part I:

Mcs · C0 · (T0 – Tf) + Mw · Cw · (T0 – Tf)

Heat emitted by the Heat emitted by the

Cal. Cup & Stirrer original water

= Mice · Cice · (0 – Tice) + Mice · L + Mice · Cw · (Tf – 0)

Heat Absorbed by Ice Heat needed Heat Absorbed by

Until it is Heated to 0ºC for Melting Molten Ice

(2) Solve this equation for L (heat of fusion of water) using the specific heats

C0 = 0.22 cal/gm. ºC (calorimeter); Cice = 0.50cal/gm. ºC (ice);

Cw = 1.00 cal/gm. ºC(water).

(3) Display your % error by comparing to the correct value of 80 cal/gm.

Part II.

(4) Write an equation similar to Equation (3) that represents the heat exchanges

in Part II. Your equation should account for the fact that not all the ice has melted.

Explain each term as has been done above.

Denote this as Equation (4).

(5) Using L = 80 cal/gm, calculate Mmelted-calc from Equation (4). Mmelted-calc = the

calculated amount of melted ice. Compare it to the measured value Mmelted-meas:

Mmelted-meas = Mcs + Mip΄ - Mpap΄ - Mf΄. Display the % discrepancy:

% Discrepancy = Mmelted-calc - Mmelted-meas x 100%

Mmelted-meas

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