Muddiest points on part 2C 2C.I Why is there 3.76 N2? This is to represent the components other than oxygen that are in air. From SB&vw, page 525, "The assumption that air is 21% oxygen and 79% nitrogen by volume leads to he conclusion that for each mole of oxygen, 79/21=3.76 moles of nitrogen are 2C. 2 What is the most effective way to solve for the number of moles in the reactions? What we are doing is basically counting atoms on both sides of the reactants -products statement. See Section 14.2 of SB&vw for a description of the combustion process and for going from ratios in terms of moles to ratios in terms of mass 2C. 3 Do we always assume 100% complete combustion? How good an approximation is this? In the problems we do, we will only consider 100% combustion. It is a good approximation for the range of problems that we address 2C. 4 Is the enthalpy offormation equal to the heat transfer out of the combustion during the formation reaction? As defined the enthalpy of formation relates to a process in which the initial and final states are at the same temperature. If there is combustion in between, heat will have to be removed for this condition to occur. The enthalpy of formation is equal to the negative of the magnitude of the heat outflow. If we consider the combustion as occurring in a control volume then per kmole hf--goa where gods the heat transfer out of the control volume per kmole. This is not in accord with our convention, and if you please feel free to transform it back into the notation we have used before. (I find that if i do this there are too many minus signs to keep track of easily. 2C.5 Are the enthalpies of H, and H(monoatomic hydrogen) both zero at 298K? The enthalpies of the elements are taken as zero at 298 and 0. 1 MPa. In some cases there are more than one form of the element In that case the form chosen to have the value of zero is that which is chemically stable at the reference state. The other forms then have an enthalpy which is consistent with the reaction that produces this form of the element For hydrogen H2rhas zero enthalpy at the reference conditions and h has an enthalpy of 217,999 kJ/kmole(see Table A. 8 in SB&vw), consistent with the idea that energy has to be supplied to break the molecule apart.Muddiest points on part 2C 2C.1 Why is there 3.76 N2? This is to represent the components other than oxygen that are in air. From SB&VW, page 525, “The assumption that air is 21% oxygen and 79% nitrogen by volume leads to the conclusion that for each mole of oxygen, 79/21 =3.76 moles of nitrogen are involved.” 2C.2 What is the most effective way to solve for the number of moles in the reactions? What we are doing is basically counting atoms on both sides of the reactants�products statement. See Section 14.2 of SB&VW for a description of the combustion process and for going from ratios in terms of moles to ratios in terms of mass. 2C.3 Do we always assume 100% complete combustion? How good an approximation is this? In the problems we do, we will only consider 100% combustion. It is a good approximation for the range of problems that we address. 2C.4 Is the enthalpy of formation equal to the heat transfer out of the combustion during the formation reaction? As defined, the enthalpy of formation relates to a process in which the initial and final states are at the same temperature. If there is combustion in between, heat will have to be removed for this condition to occur. The enthalpy of formation is equal to the negative of the magnitude of the heat outflow. If we consider the combustion as occurring in a control volume, then per kmole hf o = −Qcv , where Qcv is the heat transfer out of the control volume per kmole. This is not in accord with our convention, and if you please feel free to transform it back into the notation we have used before. (I find that if I do this there are too many minus signs to keep track of easily.) 2C.5 Are the enthalpies of H2 and H (monoatomic hydrogen) both zero at 298K? The enthalpies of the elements are taken as zero at 298 and 0.1 MPa. In some cases there are more than one form of the element. In that case the form chosen to have the value of zero is that which is chemically stable at the reference state. The other forms then have an enthalpy which is consistent with the reaction that produces this form of the element. For hydrogen H2 has zero enthalpy at the reference conditions and H has an enthalpy of 217,999 kJ/kmole (see Table A.8 in SB&VW), consistent with the idea that energy has to be supplied to break the molecule apart