Standard heat of combustion

Oirly afew formation reactions can actually be carried out, and therefore datafor tlrese reactions must usually be detenrrined indirectly. One kind of reaction that readily lends itself to experiment is tire combustion reaction, and nrairy standard heats of fonrration come from standard. 

Tins is tire value of the standard heat of fonuation of n-butane hsted in Table C.4. 

In the foregoing sections, standard heats of reaction are discussed for a reference temperature of 298.15 K (25°C). In tlris section we treat the calculation of standard heats of reaction at other temperatures from knowledge of the value at the reference temperature. 

The Vi with their accompanying signs are called stoichiometric numbers. For example, when the ammonia synthesis reaction is written  

Tliis sign convention allows the definition of a standard heat of reaction to be expressed mathematically by the equation  

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The standard heat of combustion (ziH") of a chemical substance (usually an organic compound) is the same as the standard heat of reaction for complete oxidation of 1 mole of the substance in pure oxygen to yield COj(g) and HjO(f) as products. A reference state of 25°C and 1 atm is assumed in quoting standard heats of combustion in cal/g-mole. The value of AH" is always negative because combustion is an exothermic reaction. Note that the standard heats of combustion for carbon and hydrogen are the same as the heats of formation for CO,(g) and HjO(f), respectively. 

To calculate the heat release from combustion and the temperature of the products of combustion, the thermodynamic path shown in Figure 15.20 can be followed18. The actual combustion process goes from reactants at temperature T to products at temperature T2. However, it is more convenient to follow the alternative path from reactants at temperature T that are initially cooled (or heated) to standard temperature of 298 K. The combustion reactions are then carried out at a constant temperature of 298 K. Standard heats of combustion are available for this. The products of combustion are then heated from 298 K to the final temperature of 7i. The actual heat of combustion is given by18 ... 

Standard heats of combustion are widely available. Table 15.11 provides a list of some combustion reactions. 

When using data for standard heats of combustion, care should be taken regarding the initial state of the reactants and the final state of the products. If these do not correspond with the conditions for the actual combustion, then errors can arise. Note in Table 15.11, the final state of water was... 

Methyl ferf-butyl ether (MTBE), C5H12O, a gasoline additive used to boost octane ratings, has AH°f = —313.6 kj/mol. Write a balanced equation for its combustion reaction, and calculate its standard heat of combustion in kilojoules. 

Calculate methane standard heat of combustion with water as liquid product CH4 + 202 > C02 +2H20... 

First calculate the standard heat of combustion of propane C3H8 + 502 = 3C02(g) + 4H20 (g)... 

Heat of combustion AHc (also called enthalpy of combustion) is the heat of reaction when a chemical burns in O2 to form completely oxidized products such as CO2 and H20. It is also the heat of reaction for nutritional molecules that are metabolized in the body. The standard heat of combustion AHc° takes place at 25 °C and 100 kPa. Combustion is always exothermic, so the negative sign for values of AHc is often omitted. If a combustion reaction is used in Hess s Law, the value must be negative. 

Compare the standard heat of combustion at 25°C of ACH3OH(s) with the standard heat of combustion at 25°C of C4H12(s), reaction products in both cases being C02(g) and H20(p). 

A light fuel oil with an average chemical composition of C<,H15 is burned with oxygen in a bomb calorimeter. The heat evolved is measured as 47,730 J g-1 for the reaction at 25°C. Calculate the standard heat of combustion of the fuel oil at 25°C with H20(s) and C02(fl) as products. Note that the reaction in the bomb occurs at constant volume, produces liquid water as a product, and goes to completion. 

Only a few formation reactions can actually be carried out, and therefore data for these reactions must usually be determined indirectly. One kind of reaction that readily lends itself to experiment is the combustion reaction, and many standard heats of formation come from standard heats of combustion, measured calorimetrically. A combustion reaction is defined as a reaction between an element or compound and oxygen to form specified combustion products. For organic compounds made up of carbon, hydrogen, and oxygen only, the products are carbon dioxide and water, but the state of the water may be either vapor or liquid. Data are always based on 1 mol of the substance burned. 

Obtain data on the heats of combustion, note the corresponding final combustion products, and select the appropriate equation in Table 1.20. From standard reference sources, the standard heats of combustion and the final products are as follows. The appropriate equation, A or B, is selected based on what the final combustion products are, i.e., where they are within col. 1 of Table 1.20. 

Change in enthalpy Heat transfer from cold utility Heat transfer in exchanger Heat transfer from hot utility Heat available in nth interval Heat of mixing at temperature t Heat of reaction at temperature t Standard heat of combustion Standard enthalpy of formation Standard heat of mixing Standard heat of reaction... 

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