The balanced equation for the reaction is CaCO 3 s CaO s CO 2 g rxn f f 2 f 3

# The balanced equation for the reaction is caco 3 s

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The balanced equation for the reaction is: CaCO 3 ( s )   CaO( s ) CO 2 ( g ) rxn f f 2 f 3 [ (CaO) (CO )] (CaCO )     H H H H o o o o 198
CHAPTER 6: THERMOCHEMISTRY rxn [(1)( 635.6 kJ/mol) (1)( 393.5 kJ/mol)] (1)( 1206.9 kJ/mol) 177.8 kJ/mol H o 199
CHAPTER 6: THERMOCHEMISTRY The enthalpy change calculated above is the enthalpy change if 1 mole of CO 2 is produced. The problem asks for the enthalpy change if 66.8 g of CO 2 are produced. We need to use the molar mass of CO 2 as a conversion factor. 2 2 2 2 1 mol CO 177.8 kJ 66.8 g CO 44.01 g CO 1 mol CO 2 2.70 10 kJ H 6.61 Reaction  H  (kJ/mol) S(rhombic) O 2 ( g ) SO 2 ( g ) 296.06 SO 2 ( g ) S(monoclinic) O 2 ( g ) 296.36 S(rhombic) S(monoclinic) rxn 0.30 kJ/mol H o Which is the more stable allotropic form of sulfur? 6.62 Strategy: Our goal is to calculate the enthalpy change for the formation of C 2 H 6 from is elements C and H 2 . This reaction does not occur directly, however, so we must use an indirect route using the information given in the three equations, which we will call equations (a), (b), and (c). Solution: Here is the equation for the formation of C 2 H 6 from its elements. 2C(graphite) 3H 2 ( g )   C 2 H 6 ( g ) rxn ? H o Looking at this reaction, we need two moles of graphite as a reactant. So, we multiply Equation (a) by two to obtain: (d) 2C(graphite) 2O 2 ( g )   2CO 2 ( g ) rxn 2( 393.5 kJ/mol) 787.0 kJ/mol   H o Next, we need three moles of H 2 as a reactant. So, we multiply Equation (b) by three to obtain: (e) 3H 2 ( g ) 3 2 O 2 ( g )   3H 2 O( l ) rxn 3( 285.8 kJ/mol) 857.4 kJ/mol   H o Last, we need one mole of C 2 H 6 as a product. Equation (c) has two moles of C 2 H 6 as a reactant, so we need to reverse the equation and divide it by 2. (f) 2CO 2 ( g ) 3H 2 O( l )   C 2 H 6 ( g ) 7 2 O 2 ( g ) 1 rxn 2 (3119.6 kJ/mol) 1559.8 kJ/mol H o Adding Equations (d), (e), and (f) together, we have: Reaction  H  (kJ/mol) (d) 2C(graphite) 2O 2 ( g )   2CO 2 ( g ) 787.0 (e) 3H 2 ( g ) 3 2 O 2 ( g )   3H 2 O( l ) 857.4 (f) 2CO 2 ( g ) 3H 2 O( l )   C 2 H 6 ( g ) 7 2 O 2 ( g ) 1559.8 2C(graphite) 3H 2 ( g )   C 2 H 6 ( g ) H 84.6 kJ/mol 200
CHAPTER 6: THERMOCHEMISTRY 6.63 Reaction  H  (kJ/mol) CO 2 ( g ) 2H 2 O( l ) CH 3 OH( l ) 3 2 O 2 ( g ) 726.4 C(graphite) O 2 ( g ) CO 2 ( g ) 393.5 2H 2 ( g ) O 2 ( g ) 2H 2 O( l ) 2( 285.8) C(graphite) 2H 2 ( g ) 1 2 O 2 ( g ) CH 3 OH( l ) rxn 238.7 kJ/mol H o We have just calculated an enthalpy at standard conditions, which we abbreviate rxn H o . In this case, the reaction in question was for the formation of one mole of CH 3 OH from its elements in their standard state. Therefore, the rxn H o that we calculated is also, by definition, the standard heat of formation f H o of CH 3 OH ( 238.7 kJ/mol ).

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