Enthalpy 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. 

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

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. 

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... 

Standard enthalpies of combustion are listed in Table 6.4 and Appendix 2A. We have seen in Toolbox 6.1 how to use enthalpies of combustion to obtain the standard enthalpies of reactions. Here we consider another practical application— the choice of a fuel. For example, suppose we want to know the heat output from the combustion of 150. g of methane. The thermochemical equation allows us to write the following relation... 

Whereas a lattice enthalpy is equal to the heat required (at constant pressure) to break up an ionic substance, a bond enthalpy is the heat required to break a specific type of bond at constant pressure. For example, the bond enthalpy of H2 is derived from the thermochemical equation... 

The overall enthalpy change is the same as that indicated earlier. An useful consequence of Hess s law is that thermochemical equations can be added and subtracted just like algebraic equations. This facilitates the calculation of enthalpy changes for reactions which cannot be studied experimentally. 

The values of enthalpies of peroxyl radical formation (A//r°) calculated from the enthalpies of hydroperoxide formation according to the thermochemical equation ... 

Table 8.3 Thermochemical equations and data used for the derivation of the enthalpy of reaction 8.9. Data from [143].

There are three different ways to represent the enthalpy change of an exothermic reaction. The simplest way is to use a thermochemical equation a balanced chemical equation that indicates the amount of heat that is absorbed or released by the reaction it represents. For example, consider the exothermic reaction of one mole of hydrogen gas with half a mole of oxygen gas to produce liquid water. For each mole of hydrogen gas that reacts, 285.8 kj of heat is produced. Notice that the heat term is included with the products because heat is produced. 

In section 5.1, you learned about the energy changes that accompany physical changes, chemical reactions, and nuclear reactions. You learned how to represent energy changes using thermochemical equations and diagrams. In the next section, you will determine the enthalpy of a reaction by experiment. 

A student uses a coffee-cup calorimeter to determine the enthalpy of reaction for hydrobromic acid and potassium hydroxide. The student mixes 100.0 mL of 0.50 mol/L HBpaq) at 21.0°C with 100.0 mL of 0.50 mol/L KOH(aq), also at 21.0°C. The highest temperature that is reached is 24.4°C. Write a thermochemical equation for the reaction. 

Use your results to determine the enthalpy change of the neutralization reaction, in kj/mol of NaOH. Write the thermochemical equation for the neutralization reaction. 

For example, you can combine thermochemical equations (1) and (2) below to find the enthalpy change for the decomposition of hydrogen peroxide, equation (3). 

Determine the enthalpy change of this reaction, given the following thermochemical equations. 

Write a thermochemical equation for the formation of each substance. Be sure to include the physical state of all the elements and compounds in the equation. You can find the standard enthalpy of formation of each substance in Appendix E. 

What is the relationship between the initial quantity of reactants for a reaction and the enthalpy change for a reaction Use a thermochemical equation in your answer. 

This entire expression is a thermochemical equation, a chemical equation together with the corresponding enthalpy change. The stoichiometric coefficients in a thermochemical equation are interpreted as the number of moles that react to give the reported change in enthalpy. In this case, the stoichiometric coefficient of CH4 is 1 and that of 02 is 2, so the... 

The enthalpy of reaction, AHr (or reaction enthalpy ), is closely related to the quantity AH that appears in a thermochemical equation, but its units are kilojoules per mole (kj-mol-1) rather than kilojoules. We interpret the per mole to mean per number of moles of each substance as indicated by its stoichiometric coefficient in the chemical equation. For example, the oxidation of methane described by reaction A signifies that 890 kj of heat is released per mole of CFI4 molecules consumed or per... 

The standard enthalpy of formation, AHf°, of a substance is the standard reaction enthalpy for the formation of a substance from its elements in their most stable form. (Phosphorus is an exception white phosphorus is used because it is much easier to obtain pure than the other, more stable allotropes.) Standard enthalpies of formation are expressed in kilojoules per mole of the substance (kj-mol-1). We obtain AHf for ethanol, for instance, from the thermochemical equation for its formation from graphite (the most stable form of carbon) and gaseous hydrogen and oxygen ... 

Notice that the thermochemical equation is written for the formation of 1 mol C2H5OH(l) so a fractional coefficient is necessary for 02. We report that AHf°(C2H5OH, 1) = —277.69 kj-mol1. Note also how the substance and its state are used to label the enthalpy change, so that we know which species and which form of that species we are talking about. 

Calculate enthalpy changes from calorimetry data and write a thermochemical equation, Examples 6.3, 6.5, and 6.7. 

Write the thermochemical equations that give the values of the standard enthalpies of formation for... 

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