Calculating Energy Changes

AIM To learn to calculate the quantities of energy required for various processes. 

Cases will be discussed fully in We have seen that the motions of molecules in a substance increase as we 

Calorie the energy (heat) required to raise the temperature of 1 g of water by 1 °C. 

Note that the 1 in the denominator is an exact number by definition and so does not limit the number of significant figures. 

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Calculate energy changes from calorimetry data and write a thermochemical equation (Examples 6.4 and 6.7). 

Correlating configurations with periodicity Predicting periodic variations in properties 0 Calculating energy changes for ion formation... 

The calculated energy change for this reaction gives the difference in olefinic strain between the alkene and cyclohexene, and the olefinic strain of the latter is negligible. With tri- and tetrasubstituted alkenes, it is more appropriate to use methylcyclohexane or 1,2-dimethylcyclohexane as the unstrained reactant since it is known that alkyl substitution stabilizes double bonds. The values of olefinic strain for a number of alkenes are summarized in Figure 15.2. Compounds with high OS are expected to be relatively reactive. 

Fig. 7. Schematic showing DFT-calculated energy changes for structural variations of a model Ga-trishydroxamate complex which are then used to develop FF parameters.

Figure 25 The calculated energy change in the [2 + 2] cyclodimerization of chalcogen diimides E(NMe)2 (E = S, Se, Te). The calculations have been performed at LDA as well as at MP2/ECP, CCSD/ECP, and CCSD(T)/ECP levels of theory ( 2005 from Ref. 165 Reproduced by permission of Taylor Francis, Inc. http //www.taylorandfrancis.com)...

As in the miniLAB above, you can use calorimetry to measure the energy released or absorbed in a chemical reaction or change of state. However, sometimes carrying out an experiment is difficult or even impossible. In the next section, you ll see that there are ways to calculate energy changes. 

Table 7 Calculated energy changes for isodesmic reactions for histamine (I) and derivatives (2-8), and amthamine (9) and derivatives (10-12) ...

Skill 26.2 Evaluate the thermodynamic feasibility of various reactions and calculate energy changes during chemical reactions. 

Hess s law provides a useful means of calculating energy changes that are difficult to measure directly. For instance, it is impossible to measure directly the enthalpy for the combustion of carbon to form carbon monoxide. Combustion of 1 mol of carbon with 0.5 mol of O2 produces both CO and CO2, leaving some carbon unreacted. However, solid carbon and carbon monoxide can both be completely burned in O2 to produce CO2. We can therefore use the enthalpy changes of these reactions to calculate the heat of combustion of carbon. 

Association reaction with H For calculated energy changes, see pp. 141, 160. 

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