CALORIMETRY

A) Theoretical
	Heats of chemical and physical processes and specific heats of materials are determined in this field of activity.
	There are many different kinds of calorimeters and they could be classified in many different ways.. According to the basic idea of their operation it is as follows:
	Adiabatic calorimeters:	Heat Q generated or absorbed by System I to be measured is completely transferred to/from  System II whose heat capacity C is known and temperature-change T can be measured. A simple energy balance 

				Q = - C  T						1)

can be written for this heat transfer if there is not any other heat flux, or, at least, its extent is negligible. The adiabatic calorimeter containing Systems I and II must be thermally isolated from the environment or it must be guarded by a secondary environment.
			An automatic control of the secondary environment in a guarded calorimeter 			assures it to have the same temperature the calorimeter has, consequently, 			there is no driving force for a heat flux to/from the calorimeter.
	Adiabatic calorimetry has a compromise: since heat capacity is a function of temperature, T should be kept at the lowest level allowed by the needed accuracy.
	Isotherm calorimeters are free of the previous problem. Here heat Q of System I is compensated by the (usually electric) energy-absorbtion or release of System II in such a way that temperature of the calorimeter remains constant. Amount of the compensating energy is measured in this method. Of course, heat fluxes to/from environment must be excluded in the ways used with adiabatic calorimeters.
	Heat transfer calorimeters can be applied for measurement of specific heat. Contrarily to the previous two types, heat transfer to/from environment is not tried to be reduced. A temperature-equalisation takes place between the material in the calorimeter and the environment, and the rate of this process is measured. Heat transfer  usually obeys an ohmic law:

						2)

		where	C is the heat capacity of the calorimeter,
			k is the heat transfer coefficient
			c is the specific heat,
			V is the volume and 
			 the density of the material under investigation.
If temperature of the environment Tenvironment is constant

									3)

and Eq. 2) can be integrated:

							4)

If temperature-dependences of k and (C+cV) balance each other, the plot of logarithm of temperature-difference between material and environment vs. time must be linear. It has two parameters: C and k, (constancy of the latter must be ensured by the construction of the calorimeter), so calibration can be carried out with two liquids of known specific heat and density.

B) Equipment
	We use an adiabatic and a heat transfer calorimeter. 
	The adiabatic calorimeter (Figure 1:) is composed of a Dewar vessel 1 whose lid 2 is thermally isolated by styrofoam 3. The lid holds a thermometer 4, a calorifer 5 and a metering pipette 6. Magnetic stirrer 7 ensures homogeneity in the calorimeter. There is a power supply for the calorifer which has voltage and amperometers, too.  Amounts of liquids can be measured with burettes and measuring cylinders. Time can be measured by our own watch.

		Figure 1				Figure 2

	The heat transfer calorimeter (Figure 2) is a plastic cylinder 1 circumvented by a thermostated vessel 2. Silicon rings 3 ensure the constant distance between the cylinder and the vessel. The rate limiting step of the heat transfer is that through the air being between, consequently, heat transfer coefficient k is independent of the material we measure. Magnetic stirrer 4 provides homogeneity inside of the plastic cylinder. Temperature difference is measured by a thermocouple. One of its junctions 5 is immersed in the thermostating liquid while the other junction 6 is immersed in the liquid to be measured. The thermoelectric voltage is amplified by amplifier 7 and measured by  digital multimeter 8. The measured value is registered vs. time by means of a laptop computer connected to the digital multimeter via a serial interface.

C) Experimental
	1. Let the thermostate of the heat transfer calorimeter get 50C. 
	2. Measure 15.0cm3 distilled water into the plastic cylinder. Assemble the apparatus, start stirring and register T for ca. 8 minutes.
	3. Clean the plastic cylinder and repeat procedure 2. with ethanol and 1-propanol.
	4. Draw the plots ln T - t for all the three measurements. Read slope p's.
	5. As densities and specific heats of water and ethanol are given, apply them as calibrating materials. Calculate k and C of the heat transfer calorimeter with Eq. 4)..
	6. Calculate specific heat of 1-propanol from your data with Eq. 4).

		,  kg/m3			c,  J/kgK		
T, C	    20	    35	     50	     20	      35	      50	
water	998.3	994.2	988.0	4182	4178	4181	
ethanol	789	776	763	2395	2558	2801	
1-propanol	804	792	779				

	7. Measure the molar enthalpy change of reaction

				5)

Add 300cm3 distilled water into the adiabatic calorimeter and 30cm3 2M NaOH to it. (Their volume may be measured with measuring cylinders.) Measure 25.00cm3 CH3COOH into the metering pipette (by means of a burette). Assemble the adabatic calorimeter, start stirring and read temperature in each minute.
	8. When 10 minutes have gone, start the chemical reaction by blowing the content of metering pipette into the calorimeter. Continue reading temperatures for another 10 minutes.
	9. In order to measure the heat capacity of the calorimeter C, turn on the calorifer for 2-4 minutes (time of heating must be measured precisely!). Read voltage U and current I of the calorifer.Continue reading temperatures for another 10 minutes after turning off heating.
	10. Plot measured temperatures vs. time. Read temperature changes due to the reaction (T1) and that due to heating with the calorifer (T2). As heat isolation of this calorimeter is not perfect, temperature gradually changes even in absence of any process. Reading temperature changes needs lengthening the straight lines of the  pre and after periods and measuring T in the middle (see Figure 3.).

				Figure 3

	11. Since the calorimeter works open to the atmosphere, the measurable heat is a change of enthalpy. The molar change of enthalpy of  reaction 5) is:

								6)
Number of reacting moles n has to be calculated from the amount being in the metering pipette as this is smaller then the amount of NaOH in the calorimeter.