Standard state combustion reaction compounds

The usual form in which reaction-heat data are available in the literature is a tabulation of either the heat of formation or the heat of combustion of the individual compounds in their standard states. For inorganic compounds the heat of formation is usually tabulated, while for organic compounds the data given are heats of combustion. Sources of data are listed at the end of this chapter. 

Tables of standard entlialpies of formation, combustion and reaction are available in the literature for a wide variety of compounds. It is important to note that these are valueless imlcss tlie stoicliiomctric equation and tlie standard state of reactants and products are included.

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. 

In the bomb process, reactants at the initial pressure pi and temperature 7 are converted to products at the final pressure pf and temperature Tf. The primary goal of a combustion calorimetric experiment, however, is to obtain the change of internal energy, Ac//°(7r), associated with the reaction under study, with all reactants and products in their standard states pi = pf = O.IMPa) and under isothermal conditions at a reference temperature 7r (usually 298.15 K). Once AC//°(298.15K) is known, it is possible to derive the standard enthalpy of combustion, AC77°(298.15K), and subsequently calculate the standard enthalpy of formation of the compound of interest from the known standard enthalpies of formation of the products and other reactants. 

In equations 7.27 and 7.28 m(BA), m(cot), m(crbl), and m(wr) are the masses of benzoic acid sample, cotton thread fuse, platinum crucible, and platinum fuse wire initially placed inside the bomb, respectively n(02) is the amount of substance of oxygen inside the bomb n(C02) is the amount of substance of carbon dioxide formed in the reaction Am(H20) is the difference between the mass of water initially present inside the calorimeter proper and that of the standard initial calorimetric system and cy (BA), cy(Pt),cy (cot), Cy(02), and Cy(C02)are the heat capacities at constant volume of benzoic acid, platinum, cotton, oxygen, and carbon dioxide, respectively. The terms e (H20) and f(sin) represent the effective heat capacities of the two-phase systems present inside the bomb in the initial state (liquid water+water vapor) and in the final state (final bomb solution + water vapor), respectively. In the case of the combustion of compounds containing the elements C, H, O, and N, at 298.15 K, these terms are given by [44]... 

The combustion of organochlorine or -bromine compounds is commonly referred to the following standard state reaction (normally n = 600) ... 

To our knowledge, the question of the standard state corrections in DSC experiments has never been addressed. These corrections may in general be negligible, because most studies only involve condensed phases and are performed at pressures not too far from atmospheric. This may not be the case if, for example, a decomposition reaction of a solid compound that generates a gas is studied in a hermetically closed crucible, or high pressures are applied to the sample and reference cells. The strategies for the calculation of standard state corrections in calorimetric experiments have been illustrated in chapter 7 for combustion calorimetry. 

If we want to determine the heat of reaction, where do we even begin The easiest place is to look at a measurement known as the standard enthalpy of formation, A H°f This is based on two different units, the enthalpy of formation, AHfi which represents the enthalpy change that occurs when a compound is formed from its constituent elements, and the standard enthalpy of reaction, AH0, which is the enthalpy for a reaction when all reactants and products are in their standard state (the state they exist in at 25°C and 1 atm). The standard enthalpy of formation is 1 mole of a compound from its constituent elements in their standard states. Enthalpies of formation can be found in many different reference books. Let s take a look at how we can use enthalpies of formation to determine the enthalpy of reaction for the combustion of ethanol. 

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. 

When dealing with the combustion of standard polymeric materials it is usual to consider it as a reaction between the combustible polymeric compound and atmospheric oxygen. In this case, the polymer is in the condensed state and the oxidant is a gas. 

Phenol-formaldehyde (PF) resins have been used as model compounds for the study of pyrolysis and combustion reactions that occur in solid fuels [10]. Utilising these resins it is possible to incorporate a wide range of heteroatomic and hydrocarbon moieties to simulate compounds that arise naturally in the solid fuels. A series of phenol resins crosslinked with thiophene, dibenzo-thiophene, diphenylsulfide, benzyl phenyl sulfide, thioanisole, 8-hydroxyquin-oline and 2-hydroxycarbazole were synthesised. These samples were then cured at 200°C (Fig. 15.2.1) and the resulting resins examined by solid-state NMR spectroscopy. The C CP/MAS spectra of a standard PF resin is shown... 

In certain instances, such as in the study of large organic compounds that require a complicated synthesis procedure or of biochemical molecules, it is not possible to measure the heat of formation directly. A substitute procedure in these cases is to measure the energy change for some other reaction, usually the heat of combustion, that is, the energy liberated when the compound is completely oxidized (all the carbon is oxidized to carbon dioxide, all the hydrogen to water, etc.). The standard heat of combustion AcH°(7 ) is defined to be the heat of combustion with both the reactants and the prod- ucts in their standard states, at the temperature T. The standard heat of combustion at... 

C and 1 bar is listed in Appendix A. V for a number of compounds. (Note that there dse. two entries in this table, one corresponding to the liquid phase and the other to the vapor phase, being the standard state for water.) Given the standard heat of combustion, the standard heat of reaction is computed as indicated in Illustration 8.5-1. 

Standard states are chosen conditions for substances. When 1 moi of a compound forms from its eiements with aii substances in their standard states, the enthalpy change is A/-/ . Hess s iaw aiiows us to picture a reaction as the decomposition of reactants to their eiements, foliowed by the formation of products from their elements. We use tabulated A/-/° values to find AH°xn or use known A/-/°xn and AH° values to find an unknown A/-/°. As a result of increased fossil-fuel combustion, the amount of atmospheric CO2 is climbing, which is seriously affecting Earth s climate. 

When reactants and products are in their standard states, the enthalpy change is the standard heat of reaction. If the reaction is a combustion reaction, the enthalpy change is the standard heat of combustion AHc. If the reactants are the elements in their standard state, the product is a compound in its standard state, the enthalpy change is the standard heat of formation AH/. 

A single calorimetric experiment consists of three different parts a) a calorimetric part where the energy developed in the combustion of the compound under the experimental conditions of the combustion bomb is determined b) a chemical part, where the initial and final states of the reaction of combustion are characterized witii high precision, and c) a third part in which the energy of combustion in the standard state at 298.15 K is determined from the results obtained in the other parts. From this value, using Hess s law, the standard enthalpy of formation in the crystalline state can be calculated. 

Taking by convention the heat of formation of all elements in the standard state (explained below) as zero, the heat of formation of a compound can be calculated from heat of combustion data. Again, the pyrotechnician will use established values from previously quoted sources but must be careful in the use of such figures to observe their positive or negative character An exothermic compound has, for the scientisty a negative heat of formation because the system loses energy, and the same applies to an exothermic reaction. 

In a preceding paragraph we have described the heat of a reaction as the amount of heat evolved or absorbed when the reaction takes place at constant temperature and pressure. Two mutually contradictory definitions of heat of reactions are used at the present time in textbooks and reference books. For over a century it has been customary to define the heat of a reaction (heat of combustion, heat of formation, heat of solution) as the heat evolved in the process that is, as —AH°. On the other hand, heats of fusion and vaporization have been defined as the heat absorbed during fusion or vaporization. During the last few years many chemists have adopted the definition of heat of reaction as the heat absorbed in the process. This usage is to be found, for example, in the valuable reference book Selected Values of Chemical Thermodynamic Properties, Circular of the U.S. Bureau of Standards No. 500 (1952), in which values of heats of formation of compounds from elements in their standard states and some other properties of substances are given. 

When hydrocarbons or other organic compounds are burnt in a flame calorimeter it is often necessary to pre-mix the gas with oxygen to increase flame stability and obtain complete reaction. However, if the optimum proportion of oxygen is exceeded the flame temperature becomes too high and thermal decomposition with deposition of carbon occurs below the jet. Since the combustion takes place at constant pressure close to 1 bar, the enthalpy of combustion is measured under conditions near to those of the standard states. Flame calorimetry has the advantage that enthalpies of combustion are obtained for the gaseous state without the necessity of measuring enthalpies of vaporization in separate experiments and, moreover, completeness of combustion can be established by determination of both the water and the carbon dioxide produced. 

A combined application of direct calorimetric measurements and thermochemical investigations has made possible to obtain a number of important thermochemical quantities characterizing the interaction of the N—H bond of the amine with the epoxy ring 53). Combustion and evaporation enthalpies of phenylglycidyl ether and its condensation products with aniline and butylamine have been determined. Standard enthalpies of the formation of these compounds, strain energies of the epoxy ring in the phenylglycidyl ether molecule and — AH values for the three-phase states, which are most important for the determination of the true thermodynamic reaction characteristics, have been estimated. 

E° = E°(cathode) — °(anode). standard enthalpy of combustion AHc° The change of enthalpy per mole of substance when it bums (reacts with oxygen) completely under standard conditions, standard enthalpy of formation AH° The standard reaction enthalpy per mole of compound for the compound s synthesis from its elements in their most stable form at 1 atm and the specified temperature, standard entropy of fusion ASfus° The standard entropy change per mole accompanying fusion (the conversion of a substance from the solid state to the liquid state), standard entropy of vaporization ASvap° The standard entropy change per mole accompanying vaporization (the conversion of a substance from the liquid state to the vapor state). 

Usually all of the enthalpies in reaction 2.2 refer to the reactants and products in the ideal gaseous state, and appropriate corrections must be made for a change of state if some of the components are in the liquid or solid state. To appreciate this fact fully, let us consider the combustion of a typical organic compound. Depending on the atoms in the molecule, the final products of combustion would be H2O, CO2, SO2, N2, and HX (where X is a halogen). The heat evolved in such a reaction is called the heat of combustion. Standard heats of combustion are often listed with H2O in the liquid state (i.e., as water) thus a suitable correction must be made to get the values with all of the products in the gaseous state. This is illustrated in Example 2.1. 

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