Combustion, enthalpy change fuels

This reaction, having equal number of mols of gas reactants and products, has a negligible change in entropy and thus a negligible heat effect if carried out reversibly at constant temperature. The maximum work available from a fuel cell under these circumstances would then be approximately the enthalpy change of the reaction, i.e., the heat of combustion of the... 

Since AF = AH - T S. it follows ihat. depending upon ihe value of A5 the electrical energy to be derived from the cell, can be larger or smaller than the energy AW obtained by direct combustion ol the fuel. AH = reaction enthalpy for the current-generating reaction AS = entropy change and T = absolute temperature. 

Suppose f(mol/s) of a fuel species with heat of combustion A/ ° is burned completely with pure oxygen or air in a continuous adiabatic reactor. If the reference states of the molecular feed and product species are those used to determine the enthalpy change from inlet to... 

Enthalpies of combustion and physiological fuel values. The heat of combustion -AH,) of an organic substance is usually determined from AE,., which is measured in a bomb calorimeter. Since AE and AEp are nearly identical, it follows that AHp = AEy + P AV. Here AV is the volume change which would have occurred if the reaction were carried out at constant pressure P thus, AHp can be estimated by calculation. Since AH is desired for combustion to carbon dioxide, water, elemental nitrogen (N2), and sulfur, correction must be made for the amounts of the latter elements converted into oxides. By these procedures, it has been possible to obtain highly accurate values of AH both for biochemical compounds and for mixed foodstuffs. In nutrition, -AH, is sometimes referred to as the gross energy. Values are usually expressed in kilocalories (kcal) by chemists but often as Cal (with a capital C) in the nutritional literature. 

Note that because reactions in a bomb calorimeter occur under constant-volume rather than constant-pressure conditions, the heat changes do not correspond to the enthalpy change A// (see Section 6.3). It is possible to correct the measured heat changes so that they correspond to A// values, but the corrections usually are quite small, so we will not concern ourselves with the details of the correction procedure. Finally, it is interesting to note that the energy contents of food and fuel (usually expressed in calories where 1 cal = 4.184 J) are measured with constant-volume calorimeters (see Chemistry in Action essay on p. 215.) Example 6.3 illustrates the determination of the heat of combustion of an organic compound. 

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. 

The determination of AH is, in principle, similar to that for other enthalpy changes. In practice, the fuel (or food) is mixed with an oxidizing agent (such as solid sodium peroxide (Na202) or pure oxygen gas) and sealed in a steel-walled vessel called a bomb calorimeter. The mixture is ignited by an electric current and the temperature rise recorded. The combustion reaction in a bomb calorimeter is not taking place at constant pressure, but this is easily allowed for in the final calculations. 

The enthalpy change that occurs on the combustion of a material in a reactive atmosphere, usually oxygen under pressure, is a most important method of obtaining thermochemical data, both for the stability of materials and also for the characterisation of the energy content of foods and fuels. Complete combustion of organic compounds to C02(g) and H20(l) is essential, and corrections may be needed for elements such as nitrogen and sulfur. 

Ethanol (C2H5OH) is currently blended with gasoline as an automobile fuel, (a) Write a balanced equation for the combustion of liquid ethanol in air. (b) Calculate the standard enthalpy change for the reaction, assuming H20(g) as a product (c) Calculate the heat produced per liter of ethanol by combustion of ethanol under constant pressure. Ethanol has a density of 0.789 g/mL. (d) Calculate the mass of CO2 produced per k) of heat emitted. 

One can, in principle, calculate the heat energy (enthalpy) change in a chemical reaction if the balanced chemical equation is known and the bond energies of all the bonds broken and made are known. For our purposes, we have only included a very abbreviated table (Table 12.1) of the types of bonds that might be involved in the combustion of a hydrocarbon or related fuel. 

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-... 

Measuring the enthalpy change of combustion of a liquid fuel using a flame combustion calorimeter... 

The experimental literature values for standard enthalpy changes of combustion are obtained using a more accurate bomb calorimeter. These can be obtained for fuels as well as energy contents of dried foodstuffs. 

Theoretically all the combustible materials can serve as a fuel to the SOFC via the direct electrochemical oxidation on the anode. Table 1 lists a number of fuel reactions with AG° (i.e., the change in the Gibbs free energy) and AH° (the change in the enthalpy), E° (theoretically reversible potential or voltage), and the fuel cell efficiency. Fuel cell efficiency is defined as ... 

The enthalpy of combustion and the standard enthalpy of formation of a fuel can be determined by measuring the temperature change in a calorimeter when a weighed amount of the fuel is burned in oxygen. 

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