Heats of Reaction for Some Specific Reactions

Some classes of chemical reactions are sufficiently common or use l that they have heen assigned their own label for heats of reaction. The example reaction for methane we have already used falls into one such category, combustion. Because combustion is a common part of the energy economy, enthalpy changes for combustion reactions are used to compare various fuels. Sometimes these heats of combustion are designated as Similarly, the neutralization reactions be- 

Another class of reactions, formation reactions, has specially designated heats of reaction known as heats of formation, AH°, even though many of them are never carried out in practice. Their importance is due to the computational utility derived from the following definition. A formation reaction is the chemical reaction by which one mole of a compound is formed from its elements in their standard states. The standard state is the most stable form of the element at room temperature (25°C) and pressure (1 atm). The formation reaction for carbon monoxide is 

Note that the definition of the formation reaction immediately impHes that the heat of formation for any element in its standard state must always be zero. To see this, imagine writing the formation reaction for an element in its standard state, such as 02(g). We would need one mole of 02(g) as the product, and oxygen in its standard state as the reactant. But because the standard state of oxygen is 02(g), our formation reaction is really no reaction at all. 

Because both sides of this equation are the same, there can be no change in enthalpy. So AH° must be zero. This will be true for any element in its standard state. 

Because we require just one mole of CO on the right-hand side, we must use a fractional coefficient on the O2 to balance the equation, even though this may look odd. Two very common errors when writing formation reactions are including elements that are not in their standard forms or having more than one mole of product compound. For carbon monoxide, these mistakes might produce either of the following incorrect formation reactions. 

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Production Heat of Reaction Bonds and Energy Heats of Reaction for Some Specific Reactions... 

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