# Chemistry

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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 -

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 -

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 -

I know that A is an exothermic reaction.

• Chemistry -

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

• Chemistry -

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 -

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