Pressure calorimetry

Propose a reason for why two Styrofoam cups are often used instead of just one. 

This simple apparatus is used to measure temperature changes of reactions at constant pressure. 

For an exothermic reaction, heat is lost by the reaction and gained by the water in the solution, so the temperature of the solution rises. The opposite occurs for an endothermic reaction Heat is gained by the reaction and lost by the water in the solution, and the temperature of the solution decreases. The heat gained or lost by the solution, gsoin is therefore equal in magnitude but opposite in sign to the heat absorbed or released by the reaction, The value of q oin is readily calculated from 

5soin (specific heat of solution) X (grams of solution) X AT = [5.23] 

For dilute aqueous solutions we usually assume that the specific heat of the solution is the same as that of water, 4.18 J/g-K. 

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VDW terms (C , Pure solvent properties [56] Vapor pressure, calorimetry,... 

Volume, pressure, temperature, and amounts of substances may change during a chemical reaction. When scientists make experimental measurements, however, they prefer to control as many variables as possible, to simplify the interpretation of their results. In general, it is possible to hold volume or pressure constant, but not both. In constant-volume calorimetry, the volume of the system is fixed, whereas in constant-pressure calorimetry, the pressure of the system is fixed. Constant-volume calorimetry is most often used to study reactions that involve gases, while constant-pressure calorimetry is particularly convenient for studying reactions in liquid solutions. Whichever type of calorimetry is used, temperature changes are used to calculate q. 

Constant-pressure calorimetry requires only a thermally insulated container and a thermometer. A simple, inexpensive constant-pressure calorimeter can be made using two nested Styrofoam cups. Figure 6-16 shows an example. The inner cup holds the water bath, a magnetic stir bar, and the reactants. The thermometer is inserted through the cover. The outer cup provides extra thermal insulation. 

Example illustrates an application of constant-pressure calorimetry. Our Box (see page 234) describes uses of constant-pressure calorimetry in studies of biological systems. 

To determine A E using measured values of q, we also must know w. Because heat and work are path functions, however, we proceed differently for constant volume than for constant pressure. To distinguish between these different paths, we use a subscript v for constant-volume calorimetry and a subscript p for constant-pressure calorimetry. This gives different expressions for the two t q)es of calorimeters ... 

In our world, most chemical processes occur in contact with the Earth s atmosphere at a virtually constant pressure. For example, plants convert carbon dioxide and water into complex molecules animals digest food water heaters and stoves bum fiiel and mnning water dissolves minerals from the soil. All these processes involve energy changes at constant pressure. Nearly all aqueous-solution chemistry also occurs at constant pressure. Thus, the heat flow measured using constant-pressure calorimetry, gp, closely approximates heat flows in many real-world processes. As we saw in the previous section, we cannot equate this heat flow to A because work may be involved. We can, however, identify a new thermod mamic function that we can use without having to calculate work. Before doing this, we need to describe one type of work involved in constant-pressure processes. 

Constant-pressure calorimetry Constant-pressure calorimetry directly measures an enthalpy change (A/ ) for a reaction because it monitors heat flow at constant pressure AH=qp. 

HP he study of the behavior of electrolytes in mixed solvents is currently arousing considerable interest because of its practical and fundamental implications (1). Among the simpler binary solvent mixtures, those where water is one component are obviously of primary importance. We have recently compared the effects of small quantities of water on the thermodynamic properties of selected 1 1 electrolytes in sulfolane, acetonitrile, propylene carbonate, and dimethylsulfoxide (DMSO). These four compounds belong to the dipolar aprotic (DPA) class of solvents that has received a great deal of attention (2) because of their wide use as media for physical separations and chemical and electrochemical reactions. We interpreted our vapor pressure, calorimetry, and NMR results in terms of preferential solvation of small cations and anions by water and obtained... 

The measurement of heat using a simple calorimeter such as that shown in Fig. 9.7 is an example of constant-pressure calorimetry, since the pressure (atmospheric pressure) remains constant during the process. Constant-pressure calorimetry is used in determining the changes in enthalpy occurring in solution. Recall that under these conditions the change in enthalpy equals the heat. 

In the examples of constant-pressure calorimetry we have considered so far, the reactions have occurred in solution, where no appreciable volume changes occur (that is, the total volume of the reactant solution is the sum of the volumes of the solutions that are mixed and remains constant as the reaction proceeds). Under these conditions no work occurs (since AV = 0, PAV = 0, and w = 0). Thus, since AH = qp (constant pressure) and w = 0,... 

The enthalpy change, dH = T dS + V dp, can be described as dH = dq - -V dp, and for a constant-pressure process, c/p = 0, we have dH = dqp. For a finite state change at constant pressure, qp = AH, that is, the heat transferred is equal to the enthalpy change of the system. This relation is the basis of constant pressure calorimetry, the constant-pressure heat capacity being Cp = dqldT)p. The relationship qp = AH is valid only in the absence of external work, w. When the system does external work, the first law must include dw. Then, the heat transferred to the system under constant-pressure conditions is qp = AH -f w. Thus, if a given chemical reaction has an enthalpy change of -50 kJ mol and does 100 kJ mol" of electrical work, the heat transferred to the system is —50 + 100 = 50 kJ mol". ... 

Constant-Pressure Calorimetry A coffee-cup calorimeter (Figure 6.7) is often used to measure the heat transferred (gp) in processes open to the atmosphere. One common use is to find the specific heat capacity of a solid that does not react with or dissolve in water. The solid (system) is weighed, heated to some known temperature, and added to a sample of water (surroundings) of known temperature and mass in the calorimeter. With stirring, the final water temperature, which is also the final temperature of the solid, is measured. 

The measurement of heat nsing a simple calorimeter such as that shown in Fig. 9.7 is an example of constant-pressure calorimetry, since the pressure... 

In this Chapter we have made an attempt to describe mixing and heat and mass transfer in supercritical fluids. Using Laser Doppler Anemometry, Computational Fluid Dynamics and High-Pressure Calorimetry, some basic guidelines have been derived. When compared to the behavior of ordinary liquids, the behavior of SCCO2 is quite different, especially near the critical point. However, when the thermodynamic behavior of CO2 is taken into account (in terms of constant-pressure heat capacity, viscosity, and thermal conductivity), its behavior is consistent with that of other liquids. 

Specific Heat and Heat Capacity Constant-Volume Calorimetry Constant-Pressure Calorimetry... 

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