Heat- of combustion

AHf co2) formation of CO2 from its elements in their most stable states. So, the reaction is  

1 mol C is 12 grams, it can simply be written as the ratio between mass of C and the amount of heat released. 

Combustion is a type of chemical reaction in which fuels react rapidly with oxygen to result in the production of light and heat. If the burning compound contains only C and H atoms, the products are always CO2 and H2O. 

The molar heat of combustion of the reaction is the amount of heat when 1 mol of propene (42 g/mol) is burnt. So, 

Matured gas used in kitchens contains mainly methane, ethane, propane, etc. 

The heat of combustion of a fuel is defined as the amount of heat released when unit quantity is oxidized completely to yield stable products. 

By the heat of combustion of a substance is meant the amount of heat evolved in the combustion of i gram-molecule of the substance. If m be the mass of substance burned, the molecular weight of which is M and if W and w represent the weight of water and the water-equivalent of the apparatus 

The heat of combustion is best determined by the method due to Berthelot, which consists in burning the substance in an atmosphere of compressed oxygen. The original design of the autoclave in which the combustion takes place (the Berthelot bomb) has been modified in various ways and the form to be described here is the modification due to Mahler and to Kroeker. 

A general equation linking the heat of combustion of vegetable oils to IV and S V (i.e. average fatty acid composition) has been developed by Bertram (Perkins 1995b) 

Therefore, the higher the degree of saturation and the longer the fatty acyl groups, the higher the energy content of the oil. 

The heat of formation of an organic compound is equal to the sum of the heats of formation of the products of combustion (CC 2, H2O, N2, SC 2 etc), minus the heat of combustion of the compound, as given in tables 

For instance, the heat of formation of methane may be calculated as follows  

The heat of combustion of methane is given in Ref 8, p 264 as 212 kcal/mole. The heats of formation of the products of reaction  

Note In view of the uncertainties introduced into the calculation of heats of formation from heats of combustion of all but the simplest organic molecules, it has been found simpler and more reliable to measure directly the heats of certain types of reactions of theoretical interest, instead of 

Values for heats of formation of explosives and propellants are given by Schmidt (Ref 8), Blatt (Ref 22b), Tomlinson Sheffield (Ref 4la) and in the Table located here following Heat of Explosion and Heat of Detonation  

The whole apparatus is contained in a water calorimeter, in which the rise in temperature produced by the combustion is measured. In calculating the heat of combustion, we must correct for the heat evolved in the combustion of the iron wire. The calorimeter may be calibrated with a substance of which we know the heat of combustion, or by reproducing with a known electric current the rise in temperature produced by the combustion. The heat of combustion is then equal to the electrical energy produced inside the cylinder. 

The following table contains the heats of combustion of a number of important organic compounds  

Cane sugar Glucose Phenol o-Nitrophenol Picric acid a-Naphthol Formaldehyde Acetaldehyde Benzaldehyde Acetone -Benzophenone Formic acid Acetic acid Propionic acid Benzoic acid Oxalic acid Succinic acid Tartaric acid Urea 

The heats of formation in the table are calculated for the states of matter (solid, liquid, or gaseous) to which the compound or elements in question belong at room temperature. The heats of combustion are given for constant pressure.  

For very accurate determinations E. Fischer and Wrede substitute a platinum resistance thermometer for the mercury thermometer, and emphasise the importance of efficient stirring of the water in the calorimeter. In this way they determined with the greatest care the heats of formation of cane sugar (3954 cal. per gram) and benzoic acid (6328 cal. per gram), and recommend these substances for the cahbration of calorimeters. 

---

The problem of explosion of a vapor cloud is not only that it is potentially very destructive but also that it may occur some distance from the point of vapor release and may thus threaten a considerable area. If the explosion occurs in an unconfined vapor cloud, the energy in the blast wave is generally only a small fraction of the energy theoretically available from the combustion of all the material that constitutes the cloud. The ratio of the actual energy released to that theoretically available from the heat of combustion is referred to as the explosion efficiency. Explosion efficiencies are typically in the range of 1 to 10 percent. A value of 3 percent is often assumed. 

Example 9.1 A process involves the use of benzene as a liquid under pressure. The temperature can be varied over a range. Compare the fire and explosion hazards of operating with a liquid process inventory of 1000 kmol at 100 and 150°C based on the theoretical combustion energy resulting from catastrophic failure of the equipment. The normal boiling point of benzene is 80°C, the latent heat of vaporization is 31,000 kJ kmol the specific heat capacity is 150 kJkmoh °C , and the heat of combustion is 3.2 x 10 kJkmok. ... 

The heat released on combustion of a substance is called its heat of combustion The heat of combustion is equal to —AH° for the reaction written m the direction shown By convention... 

Table 2 3 lists the heats of combustion of several alkanes Unbranched alkanes have slightly higher heats of combustion than their 2 methyl branched isomers but the most important factor is the number of carbons The unbranched alkanes and the 2 methyl branched alkanes constitute two separate homologous senes (see Section 2 9) m which there is a regular increase of about 653 kJ/mol (156 kcal/mol) m the heat of combustion for each additional CH2 group... 

Heats of combustion can be used to measure the relative stability of isomeric hydrocarbons They tell us not only which isomer is more stable than another but by how much Consider a group of C His alkanes... 

Figure 2 14 compares the heats of combustion of these isomers on a potential... 

FIGURE 2 14 Energy dia gram comparing heats of combustion of isomeric CsHig alkanes... 

Equations (1) and (2) are the heats of formation of carbon dioxide and water respectively Equation (3) is the reverse of the combustion of methane and so the heat of reaction is equal to the heat of combustion but opposite in sign The molar heat of formation of a substance is the enthalpy change for formation of one mole of the substance from the elements For methane AH = —75 kJ/mol... 

The heats of formation of most organic com pounds are derived from heats of reaction by arith metic manipulations similar to that shown Chemists find a table of AH values to be convenient because it replaces many separate tables of AH° values for indi vidual reaction types and permits AH° to be calcu lated for any reaction real or imaginary for which the heats of formation of reactants and products are available It is more appropriate for our purposes however to connect thermochemical data to chemi cal processes as directly as possible and therefore we will cite heats of particular reactions such as heats of combustion and heats of hydrogenation rather than heats of formation... 

The heat evolved on burning an alkane increases with the number of car bon atoms The relative stability of isomers may be determined by com paring their respective heats of combustion The more stable of two iso mers has the lower heat of combustion... 

The heats of combustion of methane and butane are 890 kj/mol (212 8 kcal/mol) and 2876 kJ/mol (687 4 kcal/mol) respectively When used as a fuel would methane or butane generate more heat for the same mass of gas" Which would generate more heat for the same volume of gas" ... 

In each of the following groups of compounds identify the one with the largest heat of combustion and the one with the smallest (Try to do this problem without consulting Table 2 3)... 

Cycloalkane Number of CH2 groups Heat of combustion per CH2 group... 

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

The CIS and trans forms of 1 2 dimethylcyclopropane are stereoisomers Stereoisomers are isomers that have their atoms bonded m the same order—that is they have the same constitution but they differ m the arrangement of atoms m space Stereoiso mers of the cis-trans type are sometimes referred to as geometric isomers You learned m Section 2 18 that constitutional isomers could differ m stability What about stereoisomers We can measure the energy difference between as and trans 1 2 dimethylcyclo propane by comparing their heats of combustion As illustrated m Figure 3 20 the two compounds are isomers and so the difference m their heats of combustion is a direct measure of the difference m their energies Because the heat of combustion of trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) less than that of its cis stereoisomer it follows that trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) more stable than as 1 2 dimethylcyclopropane... 

FIGURE 3 20 The enthalpy difference between as- and trans 1 2 dimethylcyclopropane can be determined from their heats of combustion Van der Waals strain between methyl groups on the same side of the ring make the cis isomer less stable than the trans... 

Their heats of combustion (Table 3 2) reveal that trans 1 4 dimethylcyclohexane is 7 kJ/mol (17 kcal/mol) more stable than the cis stereoisomer It is unrealistic to believe that van der Waals strain between cis substituents is responsible because the methyl groups are too far away from each other To understand why trans 1 4 dimethylcyclo hexane is more stable than cis 1 4 dimethylcyclohexane we need to examine each stereoisomer m its most stable conformation... 

Orientation of methyl groups m most stable conformation Heat of combustion heat of combustion More stable stereoisomer... 

At one time all cycloalkanes were believed to be planar It was expected that cyclopentane would be the least strained cycloalkane because the angles of a regular pentagon (108°) are closest to the tetrahedral angle of 109 5° Heats of combustion established that this is not so With the exception of cyclopropane the rings of all cycloalkanes are nonplanar... 

The heats of combustion of the more and less stable stereoisomers of the 12 13 and... 

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

By assuming that the heat of combustion of the cis isomer was larger than the trans structural assignments were made many years ago for the stereoisomenc 2 3 and 4 methylcyclohexanols This assumption is valid for two of the stereoisomenc pairs but is incorrect for the other For which pair of stereoisomers is the assumption incorrect Why" ... 

Earlier (Sections 218 3 11) we saw how to use heats of combustion to compare the sta bilities of isomeric alkanes We can do the same thing with isomeric alkenes Consider the heats of combustion of the four isomenc alkenes of molecular formula C4Hj All undergo combustion according to the equation... 

When the heats of combustion of the isomers are plotted on a common scale as m Fig ure 5 4 we see that the isomer of highest energy (the least stable one) is 1 butene H2C=CHCH2CH3 The isomer of lowest energy (most stable) is 2 methylpropene (CH3)2C = CH2... 

From the heats of combustion of the 4 alkenes m Figure 5 4 we see that each... 

The difference m stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond A particularly striking example compares as and trans 22 5 5 tetramethyl 3 hexene m which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10 5 kcal/mol) higher than that of the trans The cis isomer IS destabilized by the large van der Waals strain between the bulky tert butyl groups on the same side of the double bond... 

Match each alkene with the appropriate heat of combustion Heats of combustion (kilmoX) 5293 4658 4650 4638 4632 Heats of combustion (kc lmoX) 1264 9 1113 4 11114 1108 6 1107 1... 

The pattern of alkene stability determined from heats of hydrogenation parallels exactly the pattern deduced from heats of combustion... 

---