Calorimetry constant-pressure

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. 

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... 

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

Constant-pressure calorimetry can also be used to determine the heat capacity of an object or the specific heat of a substance. Suppose, for example, that we have a lead pellet with a mass of 26.47 g originally at 89.98°C. We drop the pellet into a constant-pressure calorimeter containing 100.0 g of water at 22.50°C. The temperature of the water increases to 23.17°C. In this case, we consider the pellet to be the system and the water to be the surroundings. Because it is the temperature of the surroundings that we measure and because q = we can rewrite Equation... 

Sample Problem 5.5 shows how to use constant-pressure calorimetry to calculate the heat capacity (O of a substance. 

In a constant-pressure calorimetry experiment, a reaction gives off 21.8 kJ of heat. The calorimeter contains 150 g of water, initially at 23.4°C. What is the final temperature of the water The heat capacity of the calorimeter is negligibly... 

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