HEAT CHANGES AND THERMOCHEMISTRY

Background Thermochemistry is the study of heat changes and transfers associated with chemical reactions. In this thermochemical laboratory study, you will determine the enthalpy change that occurs when a strong base, sodium hydroxide, reacts with a strong acid, hydrochloric acid. Other mixtures studied will include ammonium chloride mixed with sodium hydroxide and ammonia mixed with hydrochloric acid. These three reactions are represented as ... 

In this chapter, you learned about thermochemistry, the heat changes accompanying chemical reactions. You learned about calorimetry, the technique used to measure these heat changes, enthalpies, and the types of heat capacities that we can use in thermochemistry calculations. Finally, you learned about Hess s law and how we can use it to calculate the enthalpy change for a specific reaction. 

The first law of thermodynamics leads to a broad array of physical and chemical consequences. In the following Sections 3.6.1-3.6.8, we describe the formal theory of heat capacity and the enthalpy function, the measurements of heating effects that clarified the energy and enthalpy changes in real and ideal gases under isothermal or adiabatic conditions, and the general first-law principles that underlie the theory and practice of thermochemistry, the measurement of heat effects in chemical reactions. 

Thermochemistry is the study of the thermal (heat) changes that are associated with physical and chemical changes. Thermodynamics is much broader in scope because it includes the study of all forms of energy, including work. Some of these aspects are considered in later chapters. 

THERMOCHEMISTRY. That aspect of chemistry which deals with die heat changes which accompany chemical reactions and processes, the heal produced by them, and die influence of temperature and odier thermal quantities upon them. Tt is closely related to chemical thermodynamics. The heat of formation of a compound is the heat absorbed when it is formed from its elements in their standard states. An exothermic reaction evolves heat and endothermic reaction requires heat for initiation. 

The experimental study of heat produced or absorbed in chemical reactions is usually called thermochemistry. Such investigations are often best conducted by direct calorimetric measurements, but values for heat quantities and their time and temperature derivatives can also be obtained from other kinds of thermodynamic experiments. A heat quantity, Q, which is lost or gained in a process conducted under constant pressure, p (in chemistry and biology most frequently the atmospheric pressure), is defined as the enthalpy change, AH, accompanying the process. 

Virtually every chemical reaction and change of physical state either releases or absorbs heat. Recall that an exothermic reaction is one in which energy is released and an endothermic reaction is one in which energy is absorbed. What happens to the heat released by an exothermic chemical reaction What is the somce of the heat absorbed by an endothermic reaction Thermochemistry provides answers to these questions. Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes. 

Thermal pollution. The heating of the environment to temperatures that are harmful to its living inhabitants. (12.4) Thermochemical equation. An equation that shows both the mass and enthalpy relations. (6.3) Thermochemistry. The study of heat changes in chemical reactions. (6.2) Thermodynamics. The scientific study of the interconversion of heat and other forms of energy. (6.7)... 

We have seen that the heat supplied to a system at constant pressure is equal to the enthalpy increase. For many chemical reactions it is possible to make a direct determination of the heat change at constant pressure for other reactions indirect methods (to be discussed later) can be used. In these ways it has proved possible to build a considerable body of data on the enthalpy changes of chemical processes of all kinds and these data have been of great value in an understanding of molecular structure and in various other ways, The study of enthalpy changes in chemical systems is known as thermochemistry. 

The subject of thermochemistry deals with the heat changes resulting from chemical processes its laws are direct consequences of the first law of thermodynamics. As most reactions are carried out under constant pressure, our treatment will be restricted to a discussion of enthalpy changes. A corresponding set of equations could easily be obtained for the internal energy. In this section we discuss heat changes in chemical reactions and the thermochemistry of solutions. 

Hess s law states that the heat changes of successive processes are additive if they are carried out at the same temperature and at constant pressure. This property follows from the fact that the enthalpy is a function of state and thus is a function only of the initial and final states of a system. Hess s law is extremely useful in thermochemistry since it permits the calculation of the heat change of a reaction difficult to perform from a series of reactions which are more easily carried out. 

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