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Posted by on Sunday, January 22, 2012 at 10:53pm.

Predict whether the following reactions will be exothermic or endothermic

A) N2(g)+ 3H2(g) --> 2NH3(g)
B) S(g) +O2(g) ---> SO2(g)
C) 2H2O(g) ---->2H2(g) + O2(g)
D) 2F(g) +F2(g)

Bond
H-H 432 kj/mol
N-N(triple bond) 942 kj/mol
O=O 494 KJ/Mol
F-F 155 KJ/mol
H-N 386 KJ/mol
H-O 459 kj/mol
S=O 522 KJ/mol

  • Chemistry - , Sunday, January 22, 2012 at 11:00pm

    BE = bond energy
    BE reaction = BE for N2 + BE for 3*H2 - BE for 2*3(N-H)
    If BE rxn is -, exothermic
    If BE rxn is +, endothermic
    The others are done in a similar fashion.

  • Chemistry - , Sunday, January 22, 2012 at 11:12pm

    I got A) Exo
    B) Exo
    C) Endo
    D) Endo

    But its wrong :/ i have no idea what i am doing wrong.

    A) 942+3(432)-6(386)=-78
    B)494-522=-28
    C)4(459)-2(432)+494=478
    D)2(155)-155=155

  • Chemistry - , Monday, January 23, 2012 at 12:18am

    I know that A is an exothermic reaction.

  • Chemistry - , Monday, January 23, 2012 at 12:22am

    http://antoine.frostburg.edu/chem/senese/101/thermo/faq/enthalpy-from-bond-energies.shtml

  • Chemistry - , Monday, January 23, 2012 at 8:14am

    Yes, from general knowledge
    A is exothermic
    B is exothermic
    C is endothermic (the reverse reaction is exothermic!)
    D is the formation of F2(g) from its elements in their standard states. All elements in their standard states have a standard enthalpy of formation of zero, as there is no change involved in their formation. So D is neither exothermic or endothermic as
    deltaH(f)=0

  • Chemistry - , Friday, March 29, 2013 at 5:00pm

    A B & C are correct
    D.Should say: 2F---Fsub2. Exothermic. One bond is formed F-F
    0-155= -155kJ/mol.

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