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
A B & C are correct
D.Should say: 2F---Fsub2. Exothermic. One bond is formed F-F
0-155= -155kJ/mol.
I know that A is an exothermic reaction.
To predict whether a reaction is exothermic or endothermic, we need to consider the bond energies of the reactants and the products.
The general rule is that if the bond energy of the reactants is greater than the bond energy of the products, the reaction will be exothermic (energy is released). On the other hand, if the bond energy of the products is greater than the bond energy of the reactants, the reaction will be endothermic (energy is absorbed).
Let's analyze each reaction:
A) N2(g) + 3H2(g) --> 2NH3(g)
In this reaction, we have a nitrogen-nitrogen triple bond breaking and six hydrogen-hydrogen bonds forming. Since the triple bond in N2 is much stronger (942 kJ/mol) than the single bond in NH3 (386 kJ/mol), more energy is required to break the bonds than is released when the new bonds are formed. Therefore, this reaction is endothermic.
B) S(g) + O2(g) --> SO2(g)
Here, we have a sulfur-oxygen double bond forming. The bond energy of the sulfur-oxygen double bond in SO2 is 522 kJ/mol, while the bond energy of the oxygen-oxygen double bond in O2 is 494 kJ/mol. Since more energy is released when the new bond is formed than is required to break the initial bonds, this reaction is exothermic.
C) 2H2O(g) --> 2H2(g) + O2(g)
In this reaction, we have two hydrogen-oxygen bonds breaking and four hydrogen-hydrogen bonds forming. The bond energy of the hydrogen-oxygen bond in H2O is 459 kJ/mol, and the bond energy of the hydrogen-hydrogen bond is 432 kJ/mol. Since more energy is required to break the initial bonds than is released when the new bonds are formed, this reaction is endothermic.
D) 2F(g) + F2(g)
For this reaction, we have three fluorine-fluorine bonds breaking. Since the bond energy of the F-F bond is 155 kJ/mol, and no new bonds are formed, this reaction will require energy to break the bonds. Therefore, it is endothermic.
To summarize:
A) Endothermic
B) Exothermic
C) Endothermic
D) Endothermic
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.
http://antoine.frostburg.edu/chem/senese/101/thermo/faq/enthalpy-from-bond-energies.shtml
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
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