Calorimetry constant-volume

To calculate we need to know the heat capacity of the calorimeter (Qai) and the change in temperature, that is. 

The heat capacity of the calorimeter (Qai) is determined by burning a substance with an accurately known heat of combustion. For example, it is known that the combustion of a 1.000-g sample of benzoic acid (CeHsCOOH) releases 26.38 kJ of heat. If the measured temperature increase is 4.673°C, then the heat capacity of the calorimeter is given by 

Once Ceai has been determined, the calorimeter can be used to measure the heat of combustion of other substances. Because a reaction in a bomb calorimeter occurs under constant-volume rather than constant-pressure conditions, the measured heat change corresponds to the internal energy change (At/) rather than to the enthalpy change (A//) (see Equations 5.6 and 5.11). It is possible to 

Sample Problem 5.6 shows how to use constant-volume calorimetry to determine the energy content per gram of a substance. 

A Famous Amos bite-sized chocolate chip cookie weighing 7.25 g is burned in a bomb calorimeter to determine its energy content. The heat capacity of the calorimeter is 39.97 kJ/°C. During the  

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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. 

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 ... 

Equation can also be used to calculate the standard enthalpy of formation of a substance whose formation reaction does not proceed cleanly and rapidly. The enthalpy change for some other chemical reaction involving the substance can be determined by calorimetric measurements. Then Equation can be used to calculate the unknown standard enthalpy of formation. Example shows how to do this using experimental data from a constant-volume calorimetry experiment combined with standard heats of formation. 

Constant-volume calorimetry Constant-volume calorimetry directly measures a change in internal energy (A , not A/ for a reaction because it monitors heat flow at constant volume. Often, A and A//are very similar values. 

A common variety of constant-volume calorimetry is bomb calorimetry, a technique in which a reaction (often, a combustion reaction) is triggered within a sealed vessel called a bomb. The vessel is immersed in a water bath of known volume. The temperature of the water is measured before and after the reaction. Because the heat capacity of the water and the calorimeter are known, you can calculate heat flow from the change in temperature. 

Bomb" combustion calorimetry or constant-volume calorimetry is a technique that dates back to Lavoisier178 (Fig. 11.76), is now mostly relegated to undergraduate teaching laboratories and is in bad need of a renaissance. It measures the internal energy of combustion AEc, which is easily converted to AHc, and then converted to standard enthalpies of formation AHf s.is- In a typical "macro" experiment, with commercially available equipment, a very carefully measured mass m (-2.0 g) of a sample of molar mass M g/mol and... 

Constant-Volume Calorimetry In the coffee-cup calorimeter, we assume all the heat is gained by the water, but some must be gained by the stirrer, thermometer, and so forth. For more precise work, as in constant-volume calorimetry, the heat capacity of the entire calorimeter must be known. One type of constant-volume apparatus is the bomb calorimeter, designed to measure very precisely the heat released in a combustion reaction. As Sample Problem 6.5 will show, this need for greater precision requires that we know (or determine) the heat capacity of the calorimeter. 

If a calorimetry experiment is carried out under a constant pressure, the heat transferred provides a direct measure of the enthalpy change of the reaction. Constant-volume calorimetry is carried out in a vessel of fixed volume called a bomb calorimeter. Bomb calorimeters are used to measure the heat evolved in combustion reactions. The heat transferred under constant-volume conditions is equal to A Corrections can be applied to A values to yield enthalpies of combustion. 

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

Measuring AEforChemical Reactions Constant-Volume Calorimetry 262... 

Measuring A for Chemical Reactions Constant-Volume Calorimetry... 

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