Enthalpy change for a chemical reaction

How do we determine the energy and enthalpy changes for a chemical reaction We could perform calorimetry experiments and analyze the results, but to do this for every chemical reaction would be an insurmountable task. Furthermore, it turns out to be unnecessary. Using the first law of thermodynamics and the idea of a state function, we can calculate enthalpy changes for almost any reaction using experimental values for one set of reactions, the formation reactions. 

Now let us consider how the standard enthalpies of formation can be used to calculate the standard enthalpy change for a chemical reaction. First, we calculate the total standard enthalpy of formation of all the products. We then calculate the total standard enthalpy of formation of all the reactants. The difference between the two totals is the... 

The enthalpy of formation, which we have discussed in the previous section, offers an easy way to overcome this difficulty. We now introduce Hess s Law. Recall that enthalpy is a state function, and hence the enthalpy change depends on only the initial and fmal states. Hess s law is basically the same as stated above, but expressed in a different way Hess s law states that the enthalpy change for a chemical reaction is the same whether it takes place in one or several stages. Consider the combustion of methane again. ... 

The enthalpy change for a chemical reaction in which all reactants and products are in their standard states and at a specified temperature is called the standard enthalpy (written AFf°) for that reaction. The standard enthalpy is the central tool in thermochemistry because it provides a systematic means for comparing the energy changes due to bond rearrangements in different reactions. Standard enthalpies can be calculated from tables of reference data. For this purpose, we need one additional concept. The standard enthalpy of formation AH° of a compound is defined to be the enthalpy change for the reaction that produces 1 mol of the compound from its elements in their stable states, all at 25°C and 1 atm pressure. For example, the standard enthalpy of formation of liquid water is the enthalpy change for the reaction... 

Can be used to estimate the enthalpy change for a chemical reaction... 

The following section gives an example of a laboratory determination of the enthalpy change for a chemical reaction in which the reactants are mixed in water. After the reaction has taken place the final mixture consists of products, unused reactants and water. The principles of such measurements are as follows. 

The standard free enthalpy change for a chemical reaction can be calculated from tabulated values of various thermodynamic functions (1- ) For aqueous chemical reactions at 25 C, the standard free enthalpy change is computed from... 

Analyze We are asked to estimate the enthalpy change for a chemical reaction by using average bond enthalpies for the bonds broken and formed... 

Figure 9.12 I This conceptual diagram shows how to use tabulated enthalpies of formation to calculate the enthalpy change for a chemical reaction. We imagine that first the reactants are converted to elements in their standard states, and then those elements recombine to form the products. Because enthalpy is a state function, we do not need to know anything about the actual pathway that the reaction follows.

The molar standard free energy change for a chemical reaction (AG ) is given by Eq. (2.34). The molar standard enthalpy change for a chemical reaction is given by Eq. (Z14). The change in the molar absolute... 

Enthalpy, you may recall, is a state function (Section 6.3). This means that the enthalpy change for a chemical reaction is independent of the path by which the products are obtained. In 1840, the Russian chemist Germain Henri Hess, a professor at the University of St. Petersburg, discovered this result by experiment. Hess s law of... 

Chemical (stoichiometric) equations may be combined by addition or subtraction. The standard heat (enthalpy) of reaction (Aff ) associated with each equation may likewise be combined to give the standard heat of reaction associated with the resulting chemical equation. This is possible, once again, because enthalpy is a point function, and these changes are independent of path. In particular, formation equations and standard heats of formation may always be combined by addition and subtraction to produce any desired equation and its accompanying standard heat of reaction. This desired equation, however, carmot itself be a formation equation. Thus, the enthalpy change for a chemical reaction is the same whether it takes place in one or several steps. This is referred to as the law of constant enthalpy summation and is a direct consequence of the first law of thermodynamics. 

The enthalpy change for a chemical reaction depends upon the number of moles undergoing chemical change, and AH is expressed in terms of kj/mol. To compare reactions under the same conditions, the following conventions are used. Since we are only interested in the difference in enthalpy between the initial state, the reactants, and the final state, the products, we need a reference point. We call this reference point the standard state. 

We can obtain an approximate value for the standard enthalpy change for a chemical reaction, AH°, by considering the energy of the bonds that are cleaved or formed in the process. The exact bond energies are not known for complex molecules, but average bond energies can be estimated based on simple, strucurally analogous compounds. Consider the substitution reaction of 2.2-dimethylpropane with chlorine. 

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. 

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