Constant-pressure calorimeter

Calculate the heat transfer, Q, from a bomb calorimeter (constant volume) or a steady flow calorimeter (constant pressure), Qp, from theory or experimental data. 

Thermochemistry Most chemical reactions involve the absorption or release of heat. At constant pressure, the heat change is equal to the enthalpy change. The heat change is measured by a calorimeter. Constant-pressure and constant-volume calorimeters are devices for measuring heat changes under the stated conditions. 

This is the working equation for a constant volume calorimeter. Alternatively, a calorimeter can be maintained at constant pressure p equal to the external pressure p in which case... 

The heat capacity of a gas at constant pressure is nonually detenuined in a flow calorimeter. The temperature rise is detenuined for a known power supplied to a gas flowing at a known rate. For gases at pressures greater than about 5 MPa Magee et al [13] have recently described a twin-bomb adiabatic calorimeter to measure Cy. 

Thermochemistry is concerned with the study of thermal effects associated with phase changes, formation of chemical compouncls or solutions, and chemical reactions in general. The amount of heat (Q) liberated (or absorbed) is usually measured either in a batch-type bomb calorimeter at fixed volume or in a steady-flow calorimeter at constant pressure. Under these operating conditions, Q= Q, = AU (net change in the internal energy of the system) for the bomb calorimeter, while Q Qp = AH (net change in the enthalpy of the system) for the flow calorimeter. For a pure substance. 

As noted earlier, for a reaction at constant pressure, such as that taking place in an open coffee-cup calorimeter, the heat flow is equal to the change in enthalpy. If a reaction is carried out at constant volume (as is the case in a sealed bomb calorimeter) and there is no mechanical or electrical work involved, no work is done. Under these conditions, with w = 0, the heat flow is equal to the change in energy, AE. Hence we have... 

In the combustion reaction as carried out in the calorimeter of Figure 7-2, the volume of the system is kept constant and pressure may change because the reaction chamber is sealed. In the laboratory experiments you have conducted, you kept the pressure constant by leaving the system open to the surroundings. In such an experiment, the volume may change. There is a small difference between these two types of measurements. The difference arises from the energy used when a system expands against the pressure of the atmosphere. In a constant volume calorimeter, there is no such expansion hence, this contribution to the reaction heat is not present. Experiments show that this difference is usually small. However, the symbol AH represents the heat effect that accompanies a chemical reaction carried out at constant pressure—the condition we usually have when the reaction occurs in an open beaker. 

Since these mixing processes occur at constant pressure, // is the heat evolved or absorbed upon mixing. It is usually measured in a mixing calorimeter. The excess Gibbs free energy, is usually obtained from phase equilibria measurements that yield the activity of each component in the mixtureb and S is then obtained from equation (7.17). The excess volumes are usually obtained... 

FIGURE 6.11 The energy released or absorbed as heat by a reaction at constant pressure can be measured in this simple calorimeter. The outer polystyrene cup acts as an extra layer of insulation to ensure that no heat enters or leaves the inner cup. The quantity of energy released or absorbed as heat is proportional to the change in temperature of the calorimeter. 

The implication of this equation is that, because chemical reactions typically take place at constant pressure in vessels open to the atmosphere, the heat that they release or require can be equated to the change in enthalpy of the system. It follows that if we study a reaction in a calorimeter that is open to the atmosphere (such as that depicted in Fig. 6.11), then the measurement of its temperature rise gives us the enthalpy change that accompanies the reaction. For instance, if a reaction releases 1.25 kj of heat in this kind of calorimeter, then we can write AH = q — —1.25 kj. 

When 0.113 g of benzene, C6H6, burns in excess oxygen in a calibrated constant-pressure calorimeter with a heat capacity of 551 J-(°C) I, the temperature of the calorimeter rises by 8.60°C. Write the thermochemical equation for... 

STRATEGY The heat released by the reaction at constant pressure is calculated from the temperature change multiplied by the heat capacity of the calorimeter. Use the molar mass of one species to convert the heat released into the reaction enthalpy corresponding to the thermochemical equation as written. If the temperature rises, the... 

Self-Test 6.11A When 0.231 g of phosphorus reacts with chlorine to form phosphorus trichloride, PC1 , in a constant-pressure calorimeter of heat capacity 216 J-(°C)1, the temperature of the calorimeter rises by 11.06°C. Write the thermochemical equation for the reaction. 

We have seen that a constant-pressure calorimeter and a constant-volume bomb calorimeter measure changes in different state functions at constant volume, the heat transfer is interpreted as A U at constant pressure, it is interpreted as AH. However, it is sometimes necessary to convert the measured value of AU into AH. For example, it is easy to measure the heat released by the combustion of glucose in a bomb calorimeter, but to use that information in assessing energy changes in metabolism, which take place at constant pressure, we need the enthalpy of reaction. 

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. 

A constant-pressure calorimeter can be constructed from two St3Tofoam cups, a cover, a stirrer, and a thermometer. 

C06-0015. When 10.00 mL of 1.00 M HCl solution is mixed with 115 mL of 0.100 M NaOH solution in a constant-pressure calorimeter, the temperature rises from 22.45 °C to 23.25 °C. Assuming that the heat capacity of the calorimeter is the same as that of 125 g of water, calculate q for this reaction. 

Figure 6-17 illustrates a constant-volume calorimeter of a type that is often used to measure q for combustion reactions. A sample of the substance to be burned is placed inside the sealed calorimeter in the presence of excess oxygen gas. When the sample bums, energy flows from the chemicals to the calorimeter. As in a constant-pressure calorimeter, the calorimeter is well insulated from its surroundings, so all the heat released by the chemicals is absorbed by the calorimeter. The temperature change of the calorimeter, with the calorimeter s heat capacity, gives the amount of heat released in the reaction. 

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 ... 

A — (jv(Constaiit — volume process) For a constant-pressure calorimeter, the volume of the reacting chemicals may change, so Wp 0 and must be evaluated. We do this in Section 6-1. 

C06-0018. Adding 1.530X 10 Jof electrical energy to a constant-pressure calorimeter changes the water temperature from 20.50 °C to 21.85 °C. When 1.75 g of a solid salt is dissolved in the water, the temperature falls from 21.85 °C to 21.44 °C. Find the value of gp for the solution process. 

Whenever a chemical process occurs at constant pressure, the volume can change, particularly when gases are involved. In a constant-pressure calorimeter, for instance, the chemical system may expand or contract. In this change of volume, the system moves against the force exerted by the constant pressure. Because work is force times displacement, w = F d, this means that work is done whenever a volume change occurs at constant pressure. 

When a reaction occurs in a constant-pressure calorimeter, the external pressure is fixed but the volume of the chemical system can change, so work is done. 

As Example shows, 20.0 g of NH4 NO3 absorbs 5.28 kJ of energy when the compound dissolves in a constant-pressure calorimeter. This information lets us calculate the molar enthalpy change for the dissolving... 

C06-0031. Write a paragraph describing what happens to the energy released during a chemical reaction that occurs in a constant-pressure calorimeter. 

C06-0069. Constant-pressure calorimeters can be calibrated by electrical heating. When a calorimeter... 

CO6-OIIO. A 44.0-g sample of an unknown metal at 100.0 °C is placed in a constant-pressure calorimeter... 

The coffee-cup calorimeter can be used to measure the heat changes in reactions that are open to the atmosphere, qp, constant pressure reactions. We use this type of calorimeter to measure the specific heats of solids. We heat a known mass of a substance to a certain temperature and then add it to the calorimeter containing a known mass of water at a known temperature. The final temperature is then measured. We know that the heat lost by the added substance (the system) is equal to the heat gained by the surroundings (the water and calorimeter, although for simple coffee-cup calorimetry the heat gained by the calorimeter is small and often ignored) ... 

Many of the reactions that chemists study are reactions that occur at constant pressure. During the discussion of the coffee-cup calorimeter, the heat change at constant temperature was defined as qp. Because this constant-pressure situation is so common in chemistry, a special thermodynamic term is used to describe this energy enthalpy. The enthalpy change, AH, is equal to the heat gained or lost by the system under constant-pressure conditions. The following sign conventions apply ... 

F. D. Rossini. Calibrations of Calorimeters for Reactions in a Flame at Constant Pressure. In Experimental Thermochemistry, vol. 1 F. D. Rossini, Ed. Interscience New York, 1956 chapter 4. 

If the heats of reaction at a given temperature are known for two separate reactions, the heat of reaction of a third reaction at the same temperature may be determined by simple algebraic addition. This statement is the Law of Heat Summation. For example, reactions (1.6) and (1.7) can be carried out conveniently in a calorimeter at constant pressure ... 

In section 5.1, however, you learned that an enthalpy change represents the heat change between products and reactants at a constant pressure. Therefore, the calorimeter you use to determine an enthalpy change should allow the reaction to be carried out at a constant pressure. In other words, it should be open to the atmosphere. 

To determine enthalpy changes in high school laboratories, a coffee-cup calorimeter provides fairly accurate results. A coffee-cup calorimeter is composed of two nested polystyrene cups ( coffee cups ). They can be placed in a 250 mL beaker for added stability. Since a coffee-cup calorimeter is open to the atmosphere, it is also called a constant-pressure calorimeter. 

Q VSUM What properties of polystyrene make it a suitable material for a constant-pressure calorimeter Why are polystyrene coffee cups not suitable for a constant-volume calorimeter ... 

Explain why two nested polystyrene coffee cups, with a lid, make a good constant-pressure calorimeter. 

---