Enthalpy change, for combustion

The Chemistry Put to Work box on pipelines in Section 10.5 mentions that the total deliverability of natural gas (methane, CH4) to the various re ons of the United States is on the order of 2.7 X 10 L per day, measured at STP. Calculate the total enthalpy change for combustion of this quantity of methane. (Note Less than this amount of methane is actually combusted daily. Some of the delivered gas is passed through to other regions.)... 

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-... 

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... 

The stoichiometric flame temperature ( Tg ) is used to characterize the burning gas surrounding the droplets because combustion naturally predominates at a distance where the fastest burning mixture is produced. This mixture approximates to the stoichiometric composition. The selection of the droplet surface temperature BP is discussed below. The enthalpy change for vaporization AH is given by... 

Heat of combustion the enthalpy change for the complete oxidation of a compound => for a hydrocarbon means converting it to C02 and water. 

Example Calculate the standard enthalpy change for the oxidation of ethanol (C2H5OH) to ethanal (CH3CHO), given the following standard enthalpies of combustion ... 

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. 

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... 

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 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. ... 

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 ... 

Calculate the enthalpy change for the combustion of 1.00 mol of butane (bottled gas), C4H10, to give CO and H2O. Use the following equation ... 

Given the following enthalpies of combustion, calculate the enthalpy change for the reaction of 25.0 g of C2H2 according to the equation... 

The H values for some common substances are shown in Table 9.4. More values are found in Appendix 4. The importance of tabulated AH values is that enthalpies for many reactions can be calculated using these numbers. To see how this is done, we will calculate the standard enthalpy change for the combustion of methane ... 

The energy released (-2808 kJ) is the enthalpy of combustion. The enthalpy (heat) of combustion (A//j,omb) of a substance is the enthalpy change for the complete burning of one mole of the substance. Standard enthalpies of combustion for several common substances are given in Table 16-5. Standard enthalpy changes have the symbol A//°. The zero superscript tells you that the reactions were carried out under standard conditions. Standard conditions are one atmosphere pressure and 298 K (25 °C) and should not be confused with standard temperature and pressure (STP). 

Cyclopropane possesses both significant angle and torsion strain. Strain in saturated cyclic hydrocarbons is determined from the heat of combustion, and comparisons are made per CH2 group in a molecule. For cyclopropane the molar enthalpy change of combustion is 2090 kJ mol-1, i.e. 696.5 kJ mol-1 per CH2 group. The corresponding figure for cyclohexane is 3948 kJ mol-1, i.e. 658 kJ mol-1 per CH2. If cyclohexane is taken to be strain free (which is a reasonable approximation), one can conclude that the strain in cyclopropane is (696.5 - 658) = 38.5 kJ mol-1 per CH2. It should be noted that these experimentally deter-... 

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