Using standard enthalpies of formation

Glucose and fructose (Atlas S3) are simple carbohydrates with the molecular formula C6H12O6. Sucrose (Atlas S5), or table sugar, is a complex carbohydrate with molecular formula CijHjjOn that consists of a glucose unit covalently linked to a fructose unit (a water molecule is released as a result of the reaction between glucose and fructose to form sucrose). Estimate the standard enthalpy of combustion of sucrose from the standard enthalpies of formation of the reactants and products. 

The standard enthalpy of formation of an element in its reference state (oxygen gas in this example) is written 0, not 0 kj mol , because it is zero whatever units we happen to be using. 

Strategy We write the chemical equation, identify the stoichiometric numbers of the reactants and products, and then use eqn 1.23. Note that the expression has the form products - reactants Numerical values of standard enthalpies of formation are given in the Resource section. The standard enthalpy of combustion is the enthalpy change per mole of substance, so we need to interpret the enthcdpy chcmge accordingly. 

Inspection of the chemiccd equation shows that, in this instance, the per mole is per mole of sucrose, which is exactly what we need for an enthalpy of combustion. It follows that the estimate for the standard enthalpy of combustion of sucrose is -5644 kJ moh. The experimental value is -5645 kJ mol .  

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Enthalpy changes for reactions in solution can be determined using standard enthalpies of formation of aqueous ions, applying the general relation... 

EXAMPLE e.ll Using standard enthalpies of formation to calculate a standard enthalpy of reaction... 

Use standard enthalpies of formation to calculate the standard enthalpy of a reaction, and vice versa (Examples 6.11 and 6.12). 

Using standard enthalpies of formation from Appendix 2A, calculate the standard reaction enthalpy for each of the following reactions ... 

For example, let s use standard enthalpies of formation to calculate AHan for ... 

Calculate the enthalpy change at 25°C for the following reaction using standard enthalpies of formation ... 

A major difference with desired reactions is that the stoichiometry is often unknown, that is, the decomposition products are unknown. The reason is that decomposition reactions are often affected by the triggering conditions and thus often run along different reaction paths. This is a major difference compared to a total combustion, for example. The consequence is that the decomposition enthalpy cannot be predicted using standard enthalpies of formation AHjj taken from, for example, tables or estimated by group increment methods, such as Benson groups [3, 4] ... 

In principle, it should be possible to calculate the heat of hydration from the quantitative phase compositions of the unreacted mix and of the paste, using standard enthalpies of formation. The sensitivity of such calculations to small errors in the latter data probably renders this approach unsatisfactory with existing data. 

The value of AH for this process cannot be obtained readily by measurement in a calorimeter. We will show next how to calculate AH for chemical reactions and physical changes by using standard enthalpies of formation. 

Compare your answers from parts a and b of Exercise 31 with AH values calculated for each reaction using standard enthalpies of formation in Appendix 4. Do enthalpy changes calculated from bond energies give a reasonable estimate of the actual values ... 

Use tabulated values of bond energies (Table 13.6) to estimate AH for this reaction. Compare this result to the AH value calculated using standard enthalpies of formation in Appendix 4. Explain any discrepancies. 

When using standard enthalpies of formation to determine the enthalpy change of a chemical reaction, remember the following equation. 

Always be sure to check the states of matter when you use standard enthalpy of formation data. H20(g) and H20(Z) have different values. 

Using standard enthalpies of formation What is the significance of standard enthalpies of formation How are they used Standard enthalpies of formation provide useful data for calculating the enthalpies of reactions under standard conditions (A//°x ) using Hess s law. Suppose you want to calculate A// for this reaction, in which sulfur hexafluoride is produced. Sulfur hexafluoride is a stable, unreactive gas with some interesting applications, one of which is shown in Figure 16-15. 

Recall that Hess s law allows you to combine equations and their Mi values to produce the desired equation and its AH value. To apply Hess s law using standard enthalpies of formation data, you must have one equation for the formation of each compound in the desired equation. You can find these in Table 16-7. Three compounds are in the equation, HF, SFg, and H2S, so the three equations are these. 

For more practice using standard enthalpies of formation, go to Supplemental Practice Problems in Appendix A. 

Use standard enthalpies of formation from Table C-13 in Appendix C to calculate AH°x for each of these reactions. 

Applying Concepts Write the thermochemical equation for the decomposition of liquid hydrogen peroxide (H2O2) to water vapor and oxygen gas. Calculate AH j for the reaction using standard enthalpies of formation. Analyze the reaction and explain why NASA found this reaction suitable for providing thrust in the control jets of some space vehicles. 

The value of A/f for this process cannot be obtained by direct measurement in a calorimeter because the process is much too slow under normal conditions. However, as we saw in Example 6.7, AH for this process can be calculated from heats of combustion. This is only one example of how useful it is to be able to calculate A77 values for chemical reactions. We will next show how to do this using standard enthalpies of formation. 

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