Calculate Delta H degree for (i) the complete hydrogenation of 1 mol of acetylen

Question

Calculate Delta H degree for (i) the complete hydrogenation of 1 mol of acetylene, (ii) the complete combustion of 1 mol of acetylene. (You may use the NIST Chemistry Web-book to get enthalpies of formation. http://weblook.nist.gov/chemistry) Use the following thermo chemical equations: 0.5Cl_2(g)+O_2 rightarrow ClO_2(g) Delta H degree=102.5 kJ 0.5Cl_(g)+1.5O_2 rightarrow ClO_3(g) Delta H degree=155.0 kJ Cl_2(g)+3.5O_2 rightarrow Cl_2O_7(g) Delta H degree=272.0 kJ to determine Delta H degree for the reaction 3 ClO_3(g) rightarrow Cl_2O_7(g)+ClO_2(g) Which oxidizing agent, O_2, O_3, or H_2O_2, will generate the greatest amount of energy (i) per mol of H_2, (ii) on a total mass basis of reactants? (This is of concern to those who care about rocket propulsion.) 2H_2(g)+O_2(g) rightarrow 2H_2O_(g) Delta H degree_rxn= -483.6 kJ 3H_2(g)+O_3(g) rightarrow 3H_2O_(g) Delta H degree_rxn= -868.2 kJ H_2(g)+H_2O_2(g) rightarrow 2H_2O(g) Delta H degree_rxn= -347.3 kJ

Explanation / Answer

a) C2H2+2H2----> C2H6

ond enthalpies (in kJ/mol): CC (347); CC (839); CH (413); HH (432)

Enthalpy change =energy required to break bonds- energy required to make bonds

C , C tripple bond = 839 Kj,ther are 2 C-H bonds= 2*413= 826 Kj and ther are 2 H-H bonds= 2*432=864 Kj

energy required to break bonds= 839+826+864=2529 Kj

Products contain 1 C-C whose energy is 347 Kj/mol and 6 C-H bonds whose energy is 6*413=2478

Enthallpy change= 2529-(2478+347)=-296 Kj/mol

b) The combustion of acetylene can be represented by C2H2+2.5 O2---> 2CO2+H2O(l)

Enthalpy change= enthallpy change of products- Enthalpy change of reactants

Enthalpy change , CO2=-393.5 Kj/mol, H2O = -286 Kj/mol               C2H2= 226.73 Kj/mol O2=0

Enthalpy change= 2*-393.5+(-286)- 226.73 =-1299.73 Kj/mol

b) enthalpy change= 272+102.5- 3*155=-90.5 Kj

c) for reaction 1, 2 moles of Hydrogen is generating -483.6 Kj. One mole generates -483.6/2=-241.8 Kj/mol

for the second reaction -868.2/3=-289.4 Kj/mol and for the last reactino is -347.3 Kj/mol. SO H2O2 produces great amount of energy

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