Constant-Volume Calorimetry

7 Key excerpts on "Constant-Volume Calorimetry"

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  eBook - PDF

  Thermal Analysis

  • Bernhard Wunderlich(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  C H A P T E R 5 CALORIMETRY Calorimetry represents the effort to measure heat (caloric, see Fig. 1.1) in any of its manifestations. 1 This attempt to measure heat directly distinguishes the present discussion from Chapters 3 and 4, in which the measurement of temperature did not lead to quantitative, caloric information. There is, however, no heat meter, meaning there is no instrument which allows one to find the heat content of a system directly, as has been mentioned already in Sect. 4.1. The measurement of heat must always be made in steps and summed from a chosen initial state. The most common reference state is that of the chemical elements, stable at 298.15 Κ [Δ/ff (298) = 0; see Fig. 2.14]. 5.1 Principles and History The three common ways of measuring heat are listed at the top of Fig. 5.1. First, the change of temperature in a known system can be observed and related to the flow of heat into the system. It is also possible, using the second method, to follow a change of state, such as the melting of a known system, and determine the accompanying flow of heat from the amount of material transformed in the known system. Finally, in method three, the conversion to heat of known amounts of chemical, electrical, or mechanical energy can be used to duplicate (or compensate) the flow of heat, and thus measure heat by comparison. The prime difficulty of all calorimetric measurements is the fact that heat cannot be contained. There is no known perfect insulator for heat. During the time one performs the measurement, there are continuous losses. All calorimetry is thus beset by efforts to make corrections for heat loss. Matter always contains thermal energy, and this thermal energy is constantly exchanged. Even if there were a perfect vacuum surrounding the system under investigation, heat would be lost and gained by radiation. 219 220 Thermal Analyste PRINCIPLES AND HISTORY In calorimetry, heat measurements involve: 1.

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  eBook - ePub

  Physics for O.N.C. Courses

  • R.A. Edwards(Author)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  7.2 The Experimental Determination of Specific Heats. Calorimetry

  Any vessel designed to contain materials for the purpose of measuring heat quantities developed or exchanged by these materials is called a calorimeter. In its simplest form a calorimeter consists of a cylindrical can made usually of copper, which is a good conductor of heat and so may be assumed to undergo the same change in temperature as the contents. A so-called “continuous flow” calorimeter was used by Callendar and Barnes to determine the specific heat of water with great accuracy over the range of temperature from 0° to 100°C. The essential features of their apparatus is illustrated in Fig. 7.1 . Water from a constant head flows at a uniform rate over an electrically heated element situated along the length of the inner glass tube of the calorimeter. The vacuum jacket and water jacket which surround the inner tube are also of glass and serve to minimise heat loss to the surroundings.

  FIG. 7.1

  A steady current is maintained in the heating element and after some time a steady state is set up in which the readings of the two thermometers no longer change. The temperature θ2 of the water as it leaves the calorimeter is higher than its initial temperature θ1 , these two temperatures being recorded by thermometers T 2 and T 1 respectively. Since, in the steady state, these temperatures remain constant with time, they may be measured very accurately (e.g. using platinum-resistance thermometers, as did Callendar and Barnes) and so θ2 may be quite close to θ1 without loss of accuracy. In the steady state, the energy supplied electrically is equal to the heat carried off by the flowing water in the same time, so that

  (7.5)

  where s is the specific heat of the water in J g−1 degC−1 or in J kg−1 degC−1 if m is the mass of water in grams or kilograms, respectively, flowing through in t sec, V is the p.d. in volts across the heater and I the current in amps through it (held constant throughout). The (small) loss of heat to the surroundings is represented by h

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  Science of Heat and Thermophysical Studies

  A Generalized Approach to Thermal Analysis

  • Jaroslav Sestak(Author)
  • 2005(Publication Date)
  • Elsevier Science

    (Publisher)

  Chapter 12

  THERMOMETRY AND CALORIMETRY

  12.1. Heat determination by calorimetry

  Various thermometric assessments have been in the center of retailored techniques used to detect a wide variety of heat effects and properties. The traditional operation aims to measure, for example, heat capacities, total enthalpy changes, transitions and phase change heats, heats of adsorption, solution, mixing, and chemical reactions. The measured data can be used in a variety of clever ways to determine other quantities. Special role was executed by methods associated with enough adequate temperature measurements, which reveals an extensive history coming back to the first use of the word ‘calorimeter’ introduced by the work Wilcke and later used by Laplace, Lavoisier as already discussed in the previous Chapter 4 .

  Calorimetry is a direct and often the only way of obtaining the data in the area of thermal properties of materials, today especially aimed to higher temperatures. Detailed descriptions are available in various books [3 , 9 , 590 –596 ] and reviews [597 –599 ]. Although the measurements of heat changes is common to all calorimeters, they defer in how heat exchanges are actually detected, how the temperature changes during the process of making a measurement are determined, how the changes that cause heat effects to occur are initiated, what materials of construction are used, what temperature and pressure ranges of operation are employed, and so on. We are not going to describe herewith the individual peculiarities of instrumentation as we merely focus our attention to a brief methodical classification.

  If the calorimeter is viewed as a certain ‘black box’ [3] , whose input information are thermal processes and the output figures are the changes in temperature or functions derived from these changes. The result of the measurement is an integral change whose temperature dependence is complicated by the specific character of the given calorimeter and of the measuring conditions. The dependence between the studied and measured quantity is given by a set of differential equations, which are difficult to solve in the general case. For this reason, most calorimetric measurements are based on calibration. A purely mathematical solution is the domain of a few theoretical schools [3 , 594 , 596

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  Theory of Heat

  • (Author)
  • 2012(Publication Date)
  • Dover Publications

    (Publisher)

  CHAPTER III.

  CALORIMETRY.

  HAVING explained the principles of Thermometry, or the method of ascertaining temperatures, we are able to understand what we may call Calorimetry, or the method of measuring quantities of heat.

  When heat is applied to a body it produces effects of various kinds. In most cases it raises the temperature of the body; it generally alters its volume or its pressure, and in certain cases it changes the state of the body from solid to liquid or from liquid to gaseous.

  Any effect of heat may be used as a means of measuring quantities of heat by applying the principle that when two equal portions of the same substance in the same state are acted on by heat in the same way so as to produce the same effect, then the quantities of heat are equal.

  We begin by choosing a standard body, and defining the standard effect of heat upon it. Thus we may choose a pound of ice at the freezing point as the standard body, and we may define as the unit of heat that quantity of heat which must be applied to this pound of ice to convert it into a pound of water still at the freezing point. This is an example of a certain change of state being used to define what is meant by a quantity of heat. This unit of heat was brought into actual use in the experiments of Lavoisier and Laplace.

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  Chemical Thermodynamics

  Volume 1

  • M L McGlashan(Author)
  • 2007(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  Such reactions constitute the most widely studied group. In its essentials, the calorimeter consists of a reaction vessel of about 50 to 200 cm3capacity which contains one liquid reactant and is equipped with a thermometer, a stirrer, and the means of introducing the second (solid or liquid) reactant with minimum disturbance of the system. Commonly the second substance is contained in a frangible ampoule which is attached to the stirrer and broken by being depressed against a spike. Sometimes the la4 V. P. Kolesov, 0. G. Talakin, and S . M. Skuratov, Russ. J. Phys. Chem., 1968, 42, 1617. 122 Chemical Thermodynamics reaction vessel is immersed in a stirred-water calorimeter,125but more commonly it also serves as the calorimeter. In this case, the reaction vessel contains an electrical heater and is surrounded by an isothermal jacket, often a submarine vessel immersed in a water thermostat. Much useful calorimetry has been carried out using Dewar flasks as calorimeters (e.g. Vanderzee has reported results of good precision126) but in general they are slow to reach temperature equilibrium. Sunner and Wadso have investigated the efficiency of various designs of reaction calorimeter, including Dewar vessels, and describe one with an equilibration time of less than 2 min which yields results having accuracy of k 0.1 per cent.127 Accuracy of this order or better is obtainable from the best design of re- action calorimeter, but it must always be borne in mind that considerably less reliable results may arise from the chemical reaction being ill-defined or not proceeding to completion. Because enthalpies of reaction are in general much less than enthalpies of combustion, since fewer chemical bonds are broken, they need not be known as accurately to yield enthalpies of formation of accuracy comparable with those obtained by combustion calorimetry.

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  Pharmaceutical Isothermal Calorimetry

  • Simon Gaisford, Michael A. A. O'Neill(Authors)
  • 2006(Publication Date)
  • CRC Press

    (Publisher)

  Each compartment can be separately charged with sample (either a solid or a liquid). Once thermal equilibrium has been attained, the contents of the two compartments are mixed, usually by rotating the vessel, and the heat of interaction is measured. Batch calorimeters are also known as mixing calorimeters. Flow Calorimetry In flow calorimetry, as is implied by its title, a liquid flows through the calorimetric vessel. Solutions are held in a reservoir external to the instrument and a peristaltic pump is used to circulate the liquid. Flow calorimeters operate in two modes, flow-through and flow-mix. In flow-through operation, a solution is pumped from an external reservoir, passes through the calorimetric vessel, and is returned either to the initial reservoir or to waste. If two or more samples are to be mixed, then Principles of Calorimetry 33 mixing takes place in the external reservoir. In flow-mix operation, two (usually) liquids are pumped into the calorimetric vessel where they are mixed and then pumped to waste. Thus, in flow-through operation the calorimeter measures the power output from a system after mixing and in flow-mix operation the calorimeter measures the power associated with the mixing process itself. Titration Calorimetry In isothermal titration calorimetry (ITC), small aliquots of a titrant solution (held in a reservoir external to the instrument) are added in sequential aliquots to a solution held within the calorimetric vessel and the heat change per injection is recorded. In a typical experiment, up to 30 injections ( 10–15 m L each) are made into the liquid reservoir. Usually, titration calorimetry is used to study the binding interaction between a ligand (a drug or potential drug candi-date) and a substrate (typically, a protein, enzyme, or some other biological target), although of course the technique is not limited to this area.

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  Enthalpy and Internal Energy

  Liquids, Solutions and Vapours

  • Emmerich Wilhelm, Trevor Letcher(Authors)
  • 2017(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  2

  5.3 Calorimetric Determination of the Enthalpy of Vaporization

  At near-equilibrium conditions, vaporization proceeds without hysteresis in any property. If vaporization occurs at constant pressure, the enthalpy of vaporization can be determined either as the heat absorbed during evaporation of liquid or as the heat released during condensation of the gas. The calorimeters used for determination of the enthalpy of vaporization are divided into condensation apparatuses and vaporization calorimeters.

  5.3.1 Condensation Apparatuses

  5.3.1.1 Condensation Calorimeters

  Calorimeters of this group are used to determine the heat released during condensation of vapor. An experimental setup implementing this technique involves a heat exchanger, in which the heat released during condensation is transferred to a coolant fluid whose heat capacity is known accurately. The enthalpy of vaporization is evaluated from the mass and energy balances using the inlet and outlet temperatures of the coolant, its flow rate, and the condensation rate of the studied compound.

  A schematic diagram of a condensation calorimeter by Svoboda et al. 11 is shown in Figure 5.2 . The left-hand side of the apparatus is used for control and monitoring of the coolant flow rate and temperature. The right-hand side provides cyclic flow of the studied liquid.

  Figure 5.2 Schematic of the condensation calorimeter by Svoboda. Here: A is the equilibration part of calorimeter; B is the countercurrent section; C is the condenser calorimeter; D is the external quartz boiling tube with heating coil (300 W); C′ is the pump for coolant; E is a thermally insulated vessel; F is the 15 m long heat exchange coil; J is the measured liquid level defining vessel; H is the connection for initial sampling; K is the studied liquid level control vessel; L is the sampling vessel for gravimetric determination of the mass flow; M is a Liebig condenser; R is the flow meter for fast checking of the water flow rate; T1 , T2 , T3 , are the thermistors for measuring and checking the equilibrium and isothermal conditions; T4 is the thermistor for determining the outlet temperature of the studied liquid; T5 and T6 are the pair of thermistors for measuring the difference between the temperatures of cooling water at the inlet and outlet of calorimeter; T7 is the liquid level sensor; 1 and 2 are the thermostats for coolant water; 3 is the slag wool thermal insulation; 4 is the asbestos layer thermal insulation between sections; 5 are the regulation valves for constant feed of liquid into the boiling tube; 6 is the valve for sample collecting for gravimetry; 7 is the valve for coolant water collecting; 8 is the glass ball electromagnetic valve for sample level control. Figure is adapted with permission from V. Svoboda, V. Hynek and J. Pick. Liquid-vapor equilibrium. XXXVIII. Simultaneous determination of vapor-liquid equilibrium and integral isobaric heat of mixture. Collect Czech. Chem. Commun. , 1968, 33

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