Constant pressure reaction

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) ... 

Most chemical reactions are carried out at constant (atmospheric) pressure rather than at constant volume. It is of interest to know then whether the heat absorbed in a constant-pressure reaction depends on the path—that is, on the method by which the reaction is carried out—or whether it too is a function only of the initial and final states. If the latter were true, it would be possible to tabulate heat quantities for given chemical reactions and to use known values to calculate heats for new reactions that can be expressed as sums of known reactions. 

Constant-Pressure Reaction Calorimeters. A constant-pressure calorimeter measures the change in enthalpy AH for a chemical reaction occurring in solution under constant atmospheric pressure a trivial example is the coffee-cup calorimeter, which is constructed from two nested polystyrene (Styrofoam ) cups having holes through which a thermometer and a stirring rod can be inserted. The inner cup holds the solution in which the reaction occurs, while the outer cup provides insulation. (A fancier version uses a Dewar181 vessel to approximate adiabatic conditions for the reaction.) Then... 

Hess s law also indicates that a forward reaction has exactly the opposite change in enthalpy as the reverse. If the enthalpy change is positive, the reaction is said to be endothermic if it is negative, it is said to be exothermic. If we consider a reaction where the change in enthalpy is equal to the heat (a constant pressure reaction), then... 

The decomposition of calcium carbonate (Eq. 13.2-3), or any other reaction in which the reaction products and reactants do not mix in the gas or liquid phase, represents a, fundamentally different situation from that just considered, and such a reaction may go to completion. To see why this occurs, consider the reaction of Eq. 13.2-3 in a constant temperature and constant pressure reaction vessel, and let Ncaco2,o md Nqo-,0 represent the number of moles of calcium carbonate and carbon dioxide, respectively, before the decomposition has started. Also, since none of the species in the reaction mixes with the others, we use pure component molar than partial molar Gibbs energies in the analysis. Then,... 

A particular constant-pressure reaction is spontaneous at 390 K. The enthalpy change for the reaction is -P23.7 kJ. What can you conclude about the sign and magnitude of AS for the... 

A certain constant-pressure reaction is nonspontaneous at 45 C. The entropy change for the reaction is 72 J/K. What can... 

Finally, we went into some detail on one special kind of energy transfer -the heat absorbed or released during a chemical reaction, where the initial and final states have the same pressure, generally referred to as a constant pressure reaction. This quantity of heat is the enthalpy, and it is one of the fundamental building blocks of our model. The fact that there are no absolute values for // is a decided nuisance, but a very simple way around this is by using the formation from the elements convention. This means that for every compound, we measure LH for the reaction in which a compound is formed from its elements, each in its most stable form, and these quantities are given the symbol where the subscript / stands for formation from the... 

Enthaipy (H) A thermodynamic quantity (function of state) that is equal to the internal energy of a system plus the product of the pressure and volume EI = E + PV. It is equal to the heat change in constant-pressure reactions, such as most reactions in biological systems. 

Experimenters have used great ingenuity in designing calorimeters to measure reaction enthalpies and to improve their precision. In addition to the constant-pressure reaction calorimeter and bomb calorimeter described above, three additional types will be briefly mentioned. 

Qp heat of transformation at constant pressure reaction quotient in terms of partial pressure... 

ILLUSTRATIVE EXAMPLE 5.15 Consider the following irreversible elementary isothermal constant pressure reaction to apply for the following gas phase reaction in Questions (1-3)... 

The reaction mixture is in the inner cup. The outer cup provides additional thermal insulation from the surrounding air. The cup is closed off with a cork stopper through which a thermometer and a stirrer are inserted and immersed into the reaction mixture. The reaction in the calorimeter occurs under the constant pressure of the atmosphere. We consider the difference between constant-volume and constant-pressure reactions in Section 7-6. 

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