Equations, balancing thermochemical

To develop the governing equations for thermochemical modeling, consider the material volume element in Figure 8.5. Performing an energy balance over this volume while neglecting convective processes yields... 

Note that two of the reactions (2) and (3) had to be reversed in order to produce the summation reaction. The use of the fractional coefficients is for convenience so that the chlorine and the oxygen will cancel, but also so that we are matching the equation we were given in the problem statement above. The use of fractions in the balancing of equations is common, especially when balancing thermochemical reactions and electrochemical reactions (Chapter 19). 

Sample The complete combustion of butane gas, C4H10, in oxygen gas, Oz, produces C02 and H20. It is a highly exothermic process releasing 2845 kj of heat per mole of butane. Write the balanced thermochemical equation, using all whole-number coefficients. Also, determine the enthalpy change in burning 50.0 g of butane gas. 

How can a balanced thermochemical equation tell you whether a chemical reaction is exothermic or endothermic ... 

The coefficients in a balanced thermochemical equation refer to the numbers of moles of reactants and products involved. In the thermodynamic interpretation of equations we never interpret the coefficients as numbers of molecules. Thus, it is acceptable to write coefficients as fractions rather than as integers, when necessary. 

Here a, b, c,. .. refer to balanced thermochemical equations that can be summed to give the equation for the desired reaction. 

Consider the following balanced thermochemical equation for a reaction sometimes used for H2S production ... 

When 1 mol of NO(g) forms from its elements, 90.29 kJ of heat is absorbed, (a) Write a balanced thermochemical equation for this reaction, (b) How much heat is involved when 1.50 g of NO decomposes to its elements ... 

The decomposition of slaked lime, Ca(OH)2(s), into lime, CaO(s), and H20(g) at constant pressure requires the addition of 109 kJ of heat per mole of Ca(OH)2. (a) Write a balanced thermochemical equation for the reaction, (b) Draw an enthalpy diagram for the reaction. 

For each of the following compounds, write a balanced thermochemical equation depicting the formation of one mole of the compound from its elements in their standard states and then look up for each substance in Appendix C. (a) N02(g), (b) S03(g), (c) NaBr(s), (d) Pb(N03)2(s). 

Solution Beginning with the balanced thermochemical equation and 74.6 g of SO2, the amount of heat evolved can be calculated from the following sequence of conversions ... 

The value of AH must be scaled to correspond to a particular amount of reaction, described by the balanced thermochemical equation. 

Write complete units, including substance formulas.Then remember to scale the heat change to the amount of reaction described in the balanced thermochemical equation. Heat is released, so this Is an exothermic reaction and has a negative value of AH . 

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