Combustion, enthalpy change reaction

Heat of formation (AH ) the enthalpy change for formation of a compound directly from the ele ments is one type of heat of reaction In cases such as the formation of CO2 or H2O from the combustion of carbon or hydrogen respectively the heat of forma tion of a substance can be measured directly In most... 

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... 

We have seen that a constant-pressure calorimeter and a constant-volume bomb calorimeter measure changes in different state functions at constant volume, the heat transfer is interpreted as A U at constant pressure, it is interpreted as AH. However, it is sometimes necessary to convert the measured value of AU into AH. For example, it is easy to measure the heat released by the combustion of glucose in a bomb calorimeter, but to use that information in assessing energy changes in metabolism, which take place at constant pressure, we need the enthalpy of reaction. 

It is difficult to measure the enthalpy change of this reaction. However, standard enthalpies of combustion reactions are easy to measure. Calculate the standard enthalpy of this reaction from the following experimental data ... 

There is considerable variation in the heat of reaction data employed in different articles in the literature that deals with this reaction. Cited values differ by more than an order of magnitude. If we utilize heat of combustion data for naphthalene and phthalic anhydride and correct for the fact that water will be a gas instead of a liquid at the conditions of interest, we find that for the first reaction (equation 13.2.3) the standard enthalpy change will be approximately — 429 kcal/g mole for the second reaction it will be approximately — 760 kcal/g mole. These values will be used as appropriate for the temperature range of interest. Any variation of these parameters with temperature may be neglected. 

The experiments are usually carried out at atmospheric pressure and the initial goal is the determination of the enthalpy change associated with the calorimetric process under isothermal conditions, AT/icp, usually at the reference temperature of 298.15 K. This involves (1) the determination of the corresponding adiabatic temperature change, ATad, from the temperature-time curve just mentioned, by using one of the methods discussed in section 7.1 (2) the determination of the energy equivalent of the calorimeter in a separate experiment. The obtained AT/icp value in conjunction with tabulated data or auxiliary calorimetric results is then used to calculate the enthalpy of an hypothetical reaction with all reactants and products in their standard states, Ar77°, at the chosen reference temperature. This is the equivalent of the Washburn corrections in combustion calorimetry... 

Chemists use different subscripts to represent enthalpy changes for specific kinds of reactions. For example, AWcomb represents the enthalpy change of a combustion reaction. 

Magnesium ribbon burns in air in a highly exothermic combustion reaction. (See equation (1).) A very bright flame accompanies the production of magnesium oxide, as shown in the photograph below. It is impractical and dangerous to use a coffee-cup calorimeter to determine the enthalpy change for this reaction. 

Instead, you will determine the enthalpy changes for two other reactions (equations (2) and (3) below). You will use these enthalpy changes, along with the known enthalpy change for another reaction (equation (4) below), to determine the enthalpy change for the combustion of magnesium. 

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... 

In some cases, the value given in the table depends on that calculated previously for some other bond. For example, to obtain ec-c, we combine the enthalpy of combustion of ethane, —1,588kJ mol , with the proper multiples of the AHm s in Equations (4.38)-(4.41) to obtain the enthalpy change for the reaction... 

This reaction equation describes the combustion of methane, a reaction you might expect to release heat. The enthalpy change listed for the reaction confirms this expectation For each mole of methane that combusts, 802 kJ of heat is released. The reaction is highly exothermic. Based on the stoichiometry of the equation, you can also say that 802 kJ of heat is released for every 2 mol of water produced. (Flip to Chapter 9 for the scoop on stoichiometry.)... 

Calorimeters are devices that measure heat of reactions (enthalpy change). In the adjoining figure, a bomb calorimeter is shown. It is so called because the reaction occurs in a steel container at the center of the calorimeter, that is known as a bomb . The bomb is inserted in another container filled with water and isolated. The compound is then inserted in a bomb and ignited by electricity. The heat released by the combustion of the compound in the bomb warms up the water. In other words, the heat produced by the combustion of the compound is absorbed by the bomb and the water. For this reason. 

The enthalpy is a state function therefore the value of AH is independent of the path between given initial and final states. We saw an application of this approach in Section 6.12, where we calculated the enthalpy change for an overall physical process as the sum of the enthalpy changes for a series of two individual steps. The same rule applies to chemical reactions. In this context, the rule is known as Hess s law the overall reaction enthalpy is the sum of the reaction enthalpies of the steps into which the reaction can be divided. Hess s law applies even if the intermediate reactions or the overall reaction cannot actually be carried out. Provided the equation for each step balances and the individual equations add up to the equation for the reaction of interest, a reaction enthalpy can be calculated from any convenient sequence of reactions (Fig. 6.28). As we shall see, Hess s law also lets us use readily obtainable combustion data to compile information on a wide variety of reactions. 

The value of the enthalpy change AH reported for a reaction is the amount of heat released or absorbed when reactants are converted to products at the same temperature and in the molar amounts represented by coefficients in the balanced equation. In the combustion reaction of propane discussed in the previous section, for instance, the reaction of 1 mol of propane gas with 5 mol of oxygen gas to give 3 mol of C02 gas and 4 mol of water vapor releases 2043 kj. The actual amount of heat released in a specific reaction, however, depends on the actual amounts of reactants. Thus, reaction of 0.5000 mol of propane with 2.500 mol of 02 releases 0.5000 X 2043 kj = 1022 kj. 

The enthalpy of formation, which we have discussed in the previous section, offers an easy way to overcome this difficulty. We now introduce Hess s Law. Recall that enthalpy is a state function, and hence the enthalpy change depends on only the initial and fmal states. Hess s law is basically the same as stated above, but expressed in a different way Hess s law states that the enthalpy change for a chemical reaction is the same whether it takes place in one or several stages. Consider the combustion of methane again. ... 

As previously stated for internal energy, the change in enthalpy is solely a reflection of the initial and final states of the system. It does not matter whether the change is the result of one reaction or many intermediate reactions. For instance, the test tube combustion of glucose to carbon dioxide and water has the same enthalpy of reaction in an isothermic, isobaric, closed system as the algebraic sum of the enthalpies for all the reactions in the respiratory degradation of the sugar by cells in a similar system. This is an example of Hess s Law. 

Now A mH, the molar enthalpy change of the combustion reaction, is given as ... 

Much like enthalpies of formation, enthalpies of combustion can also be used to determine enthalpy changes for reactions otherwise difficult to measure. Thus for a reaction between a solid and a gas, for example ... 

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