Introductory Laboratory EST - Experiment 9 Calorimetry Introductory Laboratory Energy Science and Technology. Experiment 9. Physical Chemistry

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1 Introductory Laboratory EST - Experiment 9 Calorimetry Introductory Laboratory Energy Science and Technology Experiment 9 Physical Chemistry Calorimetry Abstract The heat of combustion of Naphthalene shall be determined by means of a bomb calorimeter. The temperature increase, which is a result of combustion of Naphthalene in an Oxygen atmosphere, reveals information about the released heat. The heat of combustion is calculated using basic thermodynamic equations. Experimentally gained values for heat of combustion shall be compared with enthalpy of reaction taken into account theoretical values of enthalpies of formation. Version 1.1, January 2010, M. Roos, G. Kucerova, S. Brimaud

2 Introductory Laboratory EST - Experiment 9 Calorimetry Task Determine heat of combustion of Naphthalene by burning a Naphthalene pill in an Oxygen rich atmosphere. 2. Fundamentals The heat of combustion (hereinafter referred to H 0 c ) of a substance is defined as the thermal effect that accompanies a combustion process. Upon reaction of a given substance with gaseous oxygen heat is released and specific combustion products such as CO 2, H 2 O, SO 2, N 2 are formed. It is defined only sharp when the chemical and physical properties specify the substance before and after combustion. These include in addition to the chemical formula and modification, aggregation state, temperature and pressure in the initial and final states. Numerical values for the H 0 c therefore relate mostly to standard conditions, i.e. Temperature T = 25 C, pressure p = 760 torr in the initial and the final state in these conditions, the corresponding states of aggregation. The unit of heat of combustion ( H 0 c ) is usually expressed with the quantities J/g or kj/mol. The heat of combustion is traditionally measured with a bomb calorimeter, see the following figure. This bomb is a pressure-tight container of acid-resistant steel, which contains the burning material under a pressure of 30 bar of oxygen. It should also be calculated as the difference between the heat of formation ( f H 0 ) of the products and reactants (see the Hess s law). Getting the value of the heat of combustion requires the measurements of the following values: a) increasing of temperature in calorimeter b) the amount of material which is incinerated c) heat capacity of calorimeter Such kind of experiments will be used as an illustration of the First Law of thermodynamics. The energy of an isolated system is constant That means energy can neither be created nor destroyed. It can only change forms. The evidence for the law in this form is the impossibility of constructing perpetual motion machines, for if energy could arise spontaneously an engine could

3 Introductory Laboratory EST - Experiment 9 Calorimetry constructed that would run without fuel. Experience has shown this to be unrealizable, and the First Law is based on the gloomy acceptance of its impossibility. This law is mathematically expressed as follow: U = dq +dw (1) where U equals the internal energy change for the system, dq equals energy transfert to the system by heat flow and dw, the work done by the system. If we assume that only pressure (p ext ) - volume (V) work is done, i.e. the work coming from stirring can be neglected, and the volume is constant (e.g. V = 0), we have: dw = - p ext V = 0 (2) So, the First Law becomes: U = dq (3) Since the calorimeter is isolated from the rest of the universe (dewar, bucket, insulating jacket), we can define the reactants (sample and oxygen) to be the system and the rest of the calorimeter (bomb and water) to be the surroundings. The change in internal energy of the reactants upon combustion can be calculated from: U total = U syst + U surround = 0 (4) U syst. = - U surround (5) Thus U is the quantity that an adiabatic bomb calorimeter determines directly through the measurement of dq surround. The volume is constant (calorimeter bomb), and if the heat capacity C cal of the calorimeter is known, the measurement of the water bath temperature change ( T) provide directly a measurement of the internal change for the system: U system = -C cal T (6) Note that since the temperature change is small, it is usually valid to consider C to be constant.

4 Introductory Laboratory EST - Experiment 9 Calorimetry By definition of enthalpy, this in the present case is enthalpy of combustion: H = U + (pv) (7) H and U would be identical only if the pressure in the bomb remains constant. However, for combustion reactions, the molar amount of gases changes. Assuming that the gaseous components in the bomb behave according to the ideal gas law, Eq. (7) becomes: H = U + RT n gas (8) By means of the extent of reaction d dυi = = dξ dn i equation (8) can be rewritten to H = Q + RT υ i (9) i With ν i = the stoichiometric numbers of the gaseous products (+) and reactants (-). 3. Experimental Section A Preparation of calorimeter Be careful not to scratch or dent mirrored areas of the calorimetric vessel and in calorimeter. All parts of calorimeter and high-pressure bomb have to be kept clean. Calorimeter consists of a high-pressure cell (so called bomb) which is immersed in a calorimeter vessel. See Figure 1 for details. Fill the calorimeter vessel with demineralised water (m water = 2800 g). Water mass is determined by measuring the calorimetric vessel (with stirrer) empty and filled with water on the dial-balance with accuracy of 1 g. Put the calorimetric vessel back to the calorimeter and switch the calorimeter (Switch: net) as well as magnetic stirrer on (Switch: pump).

5 Introductory Laboratory EST - Experiment 9 Calorimetry Figure 1: Schematic representation of a bomb calorimeter B Preparation of benzoic acid (C 6 H 5 COOH) and naphthalene (C 10 H 8 ) pellet Safety: Before touching any chemicals in the laboratory (benzoic acid, naphthalene), wear gloves! Make sure everything (screw press, press mold, mortar) is clean. Cut off about 60 cm of fuse wire, measure its length accurately. Fold the wire in the middle two times, hang a heavy item in the formed loop and twirl it repeatedly. Finally, weight the wire on analytical balances. Pull both wire ends through the boreholes in the closing plate of the mould so that a small loop stays on the upper part of it. Bend the wire on the other side so that it is lying in the channel. Put the plate under the mould and fill the substance with a spatula in the press mould. Please make sure you are in the right weight category (the mass of pellet of benzoic acid and naphthalene should be around and 0.5 g, respectively) weight it before the filling. While assembling the press mould into the press take care that the wire is not squeezed because it will break during the compression moulding. After compression, wait for about 30 sec., then put the screw of the press ½ turn back and fold out the bottom table. Press the pellet out by turning the screw of the press down. Remove the pellet from the and weight it on an analytical balance. To avoid hydration of your substances please work fast and always close the storage container of the chemicals.

6 Introductory Laboratory EST - Experiment 9 Calorimetry C Filling the high-pressure cell of calorimeter Connect the wires of your pill with the electrodes of the bomb in a way that the pill is placed in quartz vessel. Make sure your electrodes/wires have no contacts with the outer walls of your bomb. Afterwards, close the high-pressure vessel and fill it with 30 bar of O 2 under assistance of your assistant. After the high-pressure cell is filled, put it into the calorimeter vessel and contact the electrodes. Check the leak-tightness of your bomb (bubbles). After the measurement, the pressure of the bomb has to be released in the hood. Weight the remaining unburned wire. Make sure to refill the water to the EXACT amount as before the measurement otherwise you will always have to calculate a new heat capacity of your calorimeter. D Recording the data Use the thermometer to determine the change of the temperature. Display range covers ± 4.999K. In case, this region is over crossed, the signal starts to blink. Before measurement pay attention to following: 1. Sensor has to be immersed always in the same place of calorimeter. 2. First/starting value/temperature should be chosen so that the display range is sufficient for the expected increase in the temperature. You can start the measurement as soon as the temperature sensor is immersed in the calorimetric vessel. After ~ 8min the benzoic acid is ignited. The temperatures are recorded via a measuring computer. Please bring an USB stick with you to exchange the data. To provide accuracy of your result you have to repeat each measurement. This means you will first do E Measurement step by step 1. Fill the calorimetric vessel with about 2.8 kg demineralised water. 2. Start stirring. 3. Measure and cut a piece of wire ~ 60 cm. 4. Fold, twirl and weight the wire. 5. Press pellet of chemical substance and wire.

7 Introductory Laboratory EST - Experiment 9 Calorimetry Weight pellet + wire. 7. Attach pill to bomb. 8. Close high-pressure cell. 9. Fill it with 30 bar O Put high-pressure cell in the calorimetric vessel. 11. Contact high-pressure cell. 12. Start measurement. 13. After 8 min, ignite. 14. Wait another 10 min 15. Finish measurement don t forget to save your data. 16. Take out the high-pressure cell. 17. Release the pressure in the hood. 18. Open the high-pressure cell and weight the rest of wire. Repeat 1-18 two times with Benzoic acid and two times with naphthalene. III Evaluation of the data The aim of experiment is to determine the enthalpy of combustion of organic bonds. The calibration measurements with benzoic acid serve for the measurement of heat capacity of the used calorimeter. Following values are given: Caloric (heating) value benzoic acid (BA): f BA = J/g Caloric (heating) value wire (W): F W = 4 J/cm The temperature differences should be determined using the Tangent-method putting the values for measured temperature (T) against time (t). To have higher accuracy, print the diagram in high format. For determination of calorimetric constant K take in account followings: Q in = Q out Q in = C. cal T Q out = Σf. m = f. BA m BA + f. W m W

8 Introductory Laboratory EST - Experiment 9 Calorimetry Where: C - heat capacity m mass f caloric (heating) value BA Benzoic acid w wire Evaluation of caloric (heating) value and specific heat of combustion of naphthalene: Q in = Q out Q in = C. cal T Q out = Σf. m Corrections in bomb calorimetry 1. Combustion of fuse 2. Nonadiabacity of calorimeter A bomb calorimeter is only approximately adiabatic. In reality, there is a small heat leak through the dewar and the stirrer does work on the calorimeter (w calorimeter 0). Nonadiabaticity is corrected for with an empirical radiative correction, RC. Figure 2: Evaluation of data and radiative correction Error calculation: Calculate the errors for: heat capacity of calorimeter, caloric value of Naphthalene and the heat of combustion of Naphthalene. m analytical balances ± 0.1 mg m technical balances ± 0.1 g ( T) ± 0.1 K l wire ± 1 mm

9 Introductory Laboratory EST - Experiment 9 Calorimetry Measurement Protocol Experiment 9 Date: Team: Names: Results: measurement substance m water / g T water / K l wire / cm m wire / g m wire+pill / g m pill / g m unburned wire / g m burned wire / g T / K supervisor s signature:

10 Introductory Laboratory EST - Experiment 9 Calorimetry Keywords to guide you through the preparation for the colloquium Topics for colloquium: Basics in thermodynamic Definition of systems, intensive and extensive variable, state function, state variable Chemical potential Laws of thermodynamic Description of reaction by reaction degree Chemical thermodynamic Calorimetric equation of state Definition of enthalpy and inner energy Heat capacity C, relation between c p and c v Temperature dependence c p and c v Relation between enthalpy and inner energy Temperature dependence of enthalpy, Kirchhoff s law Hess s law Kinetic theory of gas and gas thermodynamic Equation of state for ideal and real gas