Enthalpy Heats of Reaction and Chemical Change

By allowing compounds to react in a calorime ter It IS possible to measure the heat evolved in an exothermic reaction or the heat absorbed in an en dothermic reaction Thousands of reactions have been studied to produce a rich library of thermo chemical data These data take the form of heats of reaction and correspond to the value of the enthalpy change AH° for a particular reaction of a particular substance... 

Earlier, it was stated that absolute enthalpy could not be determined for a substance, and therefore we can deal only with changes or differences in this quantity. To simplify calculations of heat of reaction and to make them consistent, we must therefore arbitrarily define a standard state to which we reference all changes in enthalpy for chemical reactions. The standard state used for most engineering calculations is defined as 25°C (298 K) and 1 atm pressure. 

But why define a property if you can t know its absolute value Although you can t measure the actual energy or enthalpy of a substance, you can measure the change in enthalpy, which is the heat absorbed or released in a chemical reaction. The change in enthalpy for a reaction is called the enthalpy (heat) of reaction (AT/ J. You have already learned that a symbol preceded by the Greek letter A means a change in the property. Thus, AT/j n is the difference between the enthalpy of the substances that exist at the end of the reaction and the enthalpy of the substances present at the start. 

The heat content of a system at constant pressure is called the enthalpy H) of the system. The heat absorbed or released during a change in a system at constant pressure is the change in enthalpy (A//). The enthalpy change for a chemical reaction is called the enthalpy of reaction or heat of reaction and is defined by this equation. 

The heal of reaction (see Section 4.4) is defined as tlie enthalpy change of a system undergoing chemical reaction. If the retictants and products are at tlie same temperature and in their standard states, tlie heat of reaction is temied tlie standard lieat of reaction. For engineering purposes, the standard state of a chemical may be taken as tlie pure chemical at I atm pressure. Heat of reaction data for many reactions is available in tlie literature. ... 

Standard Heat of Reaction. This is the standard enthalpy change accompanying a chemical reaction under the assumptions that the reactants and products exist in their standard states of aggregation at the same T and P, and stoichiometric amounts of reactants take part in the reaction to completion at constant P. With P = 1 atm and T = 25°C as the standard state, AH (T,P) can be written as... 

The heat absorbed in a process at constant pressure is equal to AH, the increase in the enthalpy of the system. It can thus be said that the heat change accompanying a chemical reaction is equal to the difference between the total heat content of the products and that of the reactants, at constant pressure and temperature conditions. This quantity is called the heat of reaction, AH, and can be expressed as follows... 

The reaction is exothermic and the enthalpy change AH° is therefore negative. The heat of reaction —AH° is positive. The superscript ° denotes a value at standard conditions and the subscript r implies that a chemical reaction is involved. 

Heat effects accompanying chemical reaction influence equilibrium constants and compositions as well as rates of reaction. The enthalpy change of reaction, AHr, is the difference between the enthalpies of formation of the participants. It is positive for endothermic reactions and negative for exothermic ones. This convention is the opposite of that for heats of reaction, so care should be exercised in applications of this quantity. Enthalpies of formation are empirical data, most often known at a standard temperature, frequently at 298 K. The Gibbs energies of formation, AGfl likewise are empirical data. 

The enthalpy change of a chemical reaction is known as the enthalpy of reaction, AHrxn- The enthalpy of reaction is dependent on conditions such as temperature and pressure. Therefore, chemists often talk about the standard enthalpy of reaction, AH°rxn - the enthalpy change of a chemical reaction that occurs at SATP (25 C and 100 kPa). Often, Alf n is written simply as AW°, The symbol is called nought. It refers to a property of a substance at a standard state or under standard conditions. You may see the enthalpy of reaction referred to as the heat of reaction in other chemistry books. 

All chemical reactions involve heat exchange. Reactions that release heat are called exothermic, and those that consume heat are called endothermic. Heat exchange is measured as the enthalpy change AH (the heat of reaction). This corresponds to the heat exchange at constant pressure. In exothermic reactions, the system loses heat, and AH is negative. When the reaction is endothermic, the system gains heat, and AH becomes positive. 

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. 

Those heal effects can be easily calculated when the enthalpies of formation and the enthalpy-temperature relations are available for the substances considered. Usually, the heat of reaction is defined as the heat evolved by the process, and it is equal to the enthalpy change but opposite in sign, while heats of fusion or vaporization always refer to ihe heat adsorbed, and for heals of solution the usage varies. In order to avoid any confusion, it is recommended to express heat effects of chemical process by reporting the enthalpy change. AH. 

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