Enthalpy changes per mole of reaction

AH Enthalpy change per mole of reaction as written J/kmol... 

In the weighted sums in the equations above for A H°, the terms are formed by multiplying the standard molar enthalpies of formation by the corresponding stoichiometric coefficients or stoichiometric numbers, both of which are simply numbers (without units). As a result, AjH° has units of kj mol. The basis for these equations is shown in Figure 7-21 and is applied in Example 7-11. At this point, you should also recognize that A H° is the enthalpy change per mole of reaction for the following process. 

The standard enthalpy of reaction, ArH°, is the enthalpy change per mole of reaction for a reaction in which all reactants and products are in their standard states. 

The modem process for manufacturing nitric acid depends on the catalytic oxidation of NH3 over heated Pt to give NO in preference to other thermodynamically more favour products (p. 423). The reaction was first systematically studied in 1901 by W. Ostwald (Nobel Prize 1909) and by 1908 a commercial plant near Bochum. Germany, was producing 3 tonnes/day. However, significant expansion in production depended on the economical availability of synthetic ammonia by the Haber-Bosch process (p. 421). The reactions occurring, and the enthalpy changes per mole of N atoms at 25 C are ... 

The per mole in the unit for AH means that this is the enthalpy change per mole of the reaction (or process) as it is written that is, when 1 mole of ice is converted to 1 mole of liquid water. 

The bond enthalpy is the enthalpy change, per mole of gaseous molecules, required to break a particular bond in a molecule. Exact and average bond enthalpies can be used to estimate the standard enthalpy change for reactions. 

Polyethylene is formed by linking many ethylene molecules into long chains. Estimate the enthalpy change per mole of ethylene for this reaction (shown below), using bond energies. 

The reaction is shown with an exotherm of 92 kJ. Expressed as the exotherm per mole of ammonia, it is 46 kJ. This is at standard temperature and pressure. Although this is fine for most academic discussions, one should realize that enthalpy changes are influenced by both temperature and pressure. Ammonia is not commercially made at standard temperature and pressure (a temperature of 273.15 K, 0 C and a pressure of 1 atmosphere 1 atmosphere = 1.01 bar = 0.101 MPa) so heats need to be either calculated or experimentally determined at the conditions of the synthesis. For example, at atmospheric pressure with an increase in temperature from 300 K to 600 K to 900 °K, the enthalpy change per mole of ammonia is increased from 46.35 kJ to 52.04 kJ to 55.06 kJ [21]. The enthalpy change increases with increasing pressure. For example, at 400 °C, it is 53.09 kJ/mol NH3 at 10 MPa. At the same temperature and 40 MPa, it increases to 57.18 kJ. At 100 MPa, it is 65.37 kJ [22]. 

The rate of enthalpy change per mole of a reaction is given by... 

A// Enthalpy change per mole (Heat of reaction = - AH) J/kmol MN- L2T-2... 

We use units of kJ/mol for the heat of formation of a substance. But in writing the enthalpy change of a chemical reaction, we will use kJ as our preferred unit, not kj/mol. The reaction in this example illustrates why we do this. The value we calculated, AH° = -2219.9 kJ, is for a reaction in which one mole of propane reacts with five moles of oxygen to form three moles of carbon dioxide and four moles of water. So if we were to say -2219.9 kJ/mol, we would need to specify carefully which substance that mol refers to. We choose to write the AH value in kJ, with the understanding that it refers to the reaction as written. This is also dimension-ally consistent with Equation 9.12, provided that we treat the stoichiometric coefficients as carrying units of moles. You may see other texts that refer to values as per mole of reaction. ... 

We use dimensionless stoichiometric coefficients. One can think of the units of v, as moles of substance i per mole of reaction. The enthalpy change in Eq. (2.7-11) has the units of J mol (meaning joules per mole of the reaction as written). If all stoichiometric coefficients are doubled, AH for the reaction doubles. When we give a value of Afor a reaction, it is always for 1 mol of the reaction as the reaction equation is written. 

A H = -566 kj mol indicates that the enthalpy of the system decreases by 566 kJ per mole of reaction. On the other hand, AH is the enthalpy change of the system, expressed in J or kJ. To determine the value of AH, we must know the extent of reaction. A H and AH also differ in that A H is an intensive quantity whereas AH is an extensive quantity. 

The standard enthalpy of combustion, AH°, is the change in enthalpy per mole of a substance that is burned in a combustion reaction under standard conditions. The products of the combustion of an organic compound are carbon dioxide... 

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