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02.03 Heat Capacity of Gases Thermochemistry/Calorimetry

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02.03 Heat Capacity of Gases Thermochemistry/Calorimetry Thermochemistry/Calorimetry LEC 02 02.03 Heat capacity of gases What you can learn about 1st law of thermodynamics Universal gas constant Isobars Isotherms Isochors and adiabatic changes of state Principle and tasks Heat is added to a gas in a glass ves- sel by an electric heater which is switched on briefly. The temperature increase results in a pressure in- crease which is measured with a manometer. Under isobaric condi- tions a temperature increase results in a volume dilatation that can be read from a gas syringe. The molar heat capacities Cv and Cp are calcu- lated from pressure and volume change. What you need: Experiment P3020311 with Cobra3 Basic-Unit Experiment P3020301 with digital counter Precision manometer 03091.00 1 1 Barometer/Manometer, hand.held 07136.00 1 1 Cobra3 Basic-Unit, USB 12150.50 1 Power supply12 VDC/2 A 12151.99 1 Cobra3 current probe 6A 12126.00 1 Software Cobra3 Universal writer software 14504.61 1 Digital counter, 4 decades 13600.93 1 Digital multimeter 07128.00 2 Mariotte flask, 10 l 02629.00 1 1 Gas syringe, 100 ml 02614.00 2 2 Stopcock, 1.way, straight 36705.00 1 1 Stopcock, 3.way, T.shaped, capillary 36732.00 1 1 Pressure change ⌬p as a func tion of the heat-up time ⌬t. U = 4.59 V, Rubber Stopper 26/32, 3 holes 39258.14 1 1 I = 0.43 A. Rubber Stopper 50.5/59.5, 1 hole 39268.01 1 1 Rubber tubing, d = 6 mm 39282.00 2 2 Nickelel ectrode, d = 3 mm, with socket 45231.00 2 2 Nickelel ectrode, 76 mm x 40 mm 45218.00 1 1 Chrome-nickel wire, d = 0,1 mm 06109.00 1 1 Scissors, straight, blunt, l = 140 mm 64625.00 1 1 Two-way switch, single pole 06030.00 1 Push-button switch 06039.00 1 Connecting cord, 32 A, 500 07361.01 1 1 Connecting cord, 32 A, 500 07361.02 2 Connecting cord, 32 A, 750 07362.01 1 1 Universal clamp 37715.00 2 2 Connecting cord, 32 A, 500 07361.04 3 4 Right angle clamp 37697.00 2 2 Connecting cord, 32 A, 750 07362.04 1 PC, Windows® XP or higher Tripod base -PASS- 02002.55 1 1 Retord stand, 210 x 130 mm, h 37694.00 2 2 Heat capacity of gases P3020301/11 24 Laboratory Experiments Chemistry PHYWE Systeme GmbH & Co. KG · D-37070 Göttingen LEC Heat capacity of gases 02.03 Related topics Scissors, straight, blunt, l = 140 mm 64625.00 1 Equation of state for ideal gases, 1st law of thermodynamics, Two-way switch, single pole 06030.00 * 1 universal gas constant, degree of freedom, mole volumes, iso- Connecting cord, l = 750 mm, blue 07362.04 * 1 bars, isotherms, isochors and adiabatic changes of state. Connecting cord, l = 500 mm, red 07361.01 1 Connecting cord, l = 500 mm, yellow 07361.02 * 2 Principle Connecting cord, l = 750 mm, red 07362.01 1 Heat is added to a gas in a glass vessel by an electric heater Connecting cord, l = 500 mm, blue 07361.04 * 3 which is switched on briefly. The temperature increase results in Tripod base -PASS- 02002.55 1 a pressure increase, which is measured with a manometer. Retort stand, h = 750 mm 37694.00 2 Under isobaric conditions, a temperature increase results in a Universal clamp 37715.00 2 volume dilatation which can be read from a gas syringe. The Right angle clamp 37697.00 2 molar heat capacities CV and Cp are calculated from the pres- sure or volume change. Changes in the equipment required for use of the Basic- Unit: Equipment (instead of * above mentioned) Precision manometer 03091.00 1 Cobra3 Basic-Unit, USB 12150.50 1 Barometer/Manometer, hand-held 07136.00 1 Power supply 12 V/ 2 A 12151.99 1 Digital counter, 4 decades 13600.93 * 1 Cobra3 current probe 6 A 12126.00 1 Digital multimeter 07128.00 * 2 Push-button switch 06039.00 1 Mariotte flask, 10 l 02629.00 1 Connecting cord, l = 500 mm, blue 07361.04 4 Gas syringe, 100 ml 02614.00 2 Cobra3 Universal writer software 14504.61 1 Stopcock, 1-way, straight 36705.00 1 PC, Windows® 95 or higher Stopcock, 3-way, T-shaped, capillary 36732.00 1 Rubber stopper, d = 26 / 32 mm, 3 holes 39258.14 1 Tasks Rubber stopper, d = 50.5 / 59.5 mm, 1 hole 39268.01 1 Determine the molar heat capacities of air at constant volume Rubber tubing, di = 6 mm 39282.00 2 CV and at constant pressure Cp. Nickel electrode, d = 3 mm, with socket 45231.00 2 Nickel electrode, 76 x 40 mm 45218.00 1 Set-up and procedure Chrome-nickel wire, d = 0.1 mm, l = 100 m 06109.00 1 Perform the experimental set-up according to Fig. 1. Fig. 1: Experimental set-up (CV). PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen P3020301 1 LEC Heat capacity of gases 02.03 Insert the two nickel electrodes into two holes in the three-hole the measuring series. To achieve this, connect one of the digi- rubber stopper and fix the terminal screws to the lower ends of tal multimeters in series as an ammeter and the other in parallel the electrodes. Screw two pieces of chrome nickel wire, which as a voltmeter. are each about 15 cm long, into the clamps between these two Perform at least 10 measurements. After each measurement, electrodes so that they are electrically connected in parallel. The perform a pressure equalisation with the ambient atmospheric wires must not touch each other. Insert the one-way stopcock pressure by opening the three-way cock. The electrical current into the third hole of the stopper and insert the thus-prepared which flows during the measurements must not be too strong, stopper in the lower opening of the bottle. i.e. it must be sufficiently weak to limit the pressure increase due The 5 V output of the electrical 4-decade digital counter serves to the heating of the gas to a maximum of 1 hPa. as the power source. The electrical circuit is illustrated in Fig. 3. To determine CV, connect the precision manometer to the bot- tle with a piece of tubing. To do this, insert the second stopper, Fig. 3: Connecting the counter-timer. which has been equipped with the three-way stopcock, into the upper opening of the bottle. The manometer must be posi- tioned exactly horizontally. Read the pressure increase immedi- ately after cessation of the heating process. The manometer must be filled with the oil which is supplied with the device. The scale is now calibrated in hPa. The riser tube of the manometer must be well wetted before each measurement. Start the measuring procedure by activating the push-button switch. The measuring period should be as short as possible (less than one second). Determine the current which flows dur- ing the measurement and the voltage separately at the end of Fig. 2: Experimental set-up (Cp) 2 P3020301 PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen LEC Heat capacity of gases 02.03 For this reason it may be necessary to use only one heating Set-up and procedure with Cobra3 wire. Perform the experimental set-up according to Fig. 4. Insert the two nickel electrodes into two holes in the three- In order to be able to determine Cp, replace the manometer with two gas syringes which are connected to the bottle via the hole rubber stopper and fix the terminal screws to the lower three-way stopcock (see Fig. 2). One of the gas syringes is ends of the electrodes. Screw two pieces of chrome nickel mounted horizontally and the other is positioned vertically with wire, which are each about 15 cm long, into the clamps its plunger oriented downwards. While making measurements, between these two electrodes so that they are electrically the three-way cock must be positioned in such a manner that it connected in parallel. The wires must not touch each other. only connects the vertical syringe with the bottle. To increase Insert the one-way stopcock into the third hole of the stopper the plunger's mass, a nickel sheet metal electrode is attached and insert the thus-prepared stopper in the lower opening of to it with two-sided tape. Start the plunger rotating manually the bottle. before the measurement so that it rotates throughout measure- The 5 V output of the Cobra3 Basic-Unit serves as the power ment. In this manner the static friction between the plunger and source. the body of the syringe is minimised and the measured values To determine CV, connect the precision manometer to the bot- are sufficiently exact. If the plunger stops prematurely, the vol- tle with a piece of tubing. To do this, insert the second stop- ume increase (⌬V) read on the vertically mounted syringe is too per, which has been equipped with the three-way stopcock, low. Determine the air pressure, which is required for the calcu- into the upper opening of the bottle (Fig. 4). The manometer lations, with the aid of a digital barometer. For the pressure in must be positioned exactly horizontally. Read the pressure the gas container use a value which lies 14 hPa below the increase immediately after cessation of the heating process. atmospheric pressure due to the weight of the syringe’s plunger.
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