Thermochemical equations

A chemical equation that shows the enthalpy relation between products and reactants is called a thermochemical equation. This type of equation contains, at the right of the balanced chemical equation, the appropriate value and sign for AH. 

To see where a thermochemical equation comes from, consider the process by which ammonium nitrate dissolves in water ... 

The thermochemical equation for this reaction (Figure 8.5) must then be... 

By an entirely analogous procedure, the thermochemical equation for the formation of HC1 from the elements (Example 8.3) is found to be... 

These thermochemical equations are typical of those used throughout this text It is important to realize that—... 

To make effective use of thermochemical equations, three basic rules of thermochemistry are applied. 

The thermochemical equation allows us to relate the enthalpy change to amounts of reactants and products, leading to conversion factors such as... 

The Bunsen burners in your labs are fueled by natural gas, which is mostly methane, CH The thermochemical equation for the combustion (burning in oxygen) of methane is... 

Strategy The trick here is to work with the given information until you arrive at two equations that will add to give the equation you want (C + 02 —> CO). To do this, focus on CO, which, unlike C02 and 02, appears in only one thermochemical equation. Notice that you want one mole (not two) of CO on the right side (not the left side) of the equation. 

Reality Check Notice that thermochemical equations can be added in exactly the same manner as algebraic equations in this case 1C02 and 02 canceled when the equations were added. 

We have now written several thermochemical equations. In each case, we have cited the corresponding value of AH. Literally thousands of such equations would be needed to list the AH values for all the reactions that have been studied. Clearly, there has to be some more concise way of recording data of this sort. These data should be in a form that can easily be used to calculate AH for any reaction. It turns out that there is a simple way to do this, using quantities known as enthalpies of formation. 

The standard enthalpy change, Aff°, for a given thermochemical equation is equal to the sum of the standard enthalpies of formation of the product compounds minus the sum of the standard enthalpies of formation of the reactant compounds. 

Write thermochemical equations for the decomposition of one mole of the following compounds into the elements in their stable states at 25°C and 1 atm. 

Using Table 8.2, write thermochemical equations for the following. 

When 0.113 g of benzene, C6H6, burns in excess oxygen in a calibrated constant-pressure calorimeter with a heat capacity of 551 J-(°C) I, the temperature of the calorimeter rises by 8.60°C. Write the thermochemical equation for... 

STRATEGY The heat released by the reaction at constant pressure is calculated from the temperature change multiplied by the heat capacity of the calorimeter. Use the molar mass of one species to convert the heat released into the reaction enthalpy corresponding to the thermochemical equation as written. If the temperature rises, the... 

Self-Test 6.11A When 0.231 g of phosphorus reacts with chlorine to form phosphorus trichloride, PC1 , in a constant-pressure calorimeter of heat capacity 216 J-(°C)1, the temperature of the calorimeter rises by 11.06°C. Write the thermochemical equation for the reaction. 

Self-Tfst 6.11B When 0.338 g of pentane, C5H12, burns in an excess of oxygen to form carbon dioxide and liquid water in the same calorimeter as that used in Self-Test 6.1 1A, the temperature rises by 76.7°C. Write the thermochemical equation for the reaction. 

Thermochemical equations for the individual steps of a reaction sequence may be combined to give the thermochemical equation for the overall reaction. 

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