Please help - chemical bonding and energy change

[lorka150]

I am not positive if I did this correctly, but I was wondering if someone could check and then, potentially, offer ideas.

The question asked to use bond energies and estimate the energy change deltaH (in kJ/mol) for

N=N + 2H2 --> H2N-NH2

(that first N=N is triple)

I think I just realized I did this wrong, because I didn't use a triple for the first one, but I'll write down what I did anyway, and maybe someone can help.

N=N = 418 kJ
H2 = 872.8 kJ
total = 1290.8 kJ

Then, I did
4 H2 bonds = 4x436.4 = -1745.6
2 N-H bonds = 2 x 393 = -786
total = -2531.6

Therefore, 1290.8 + (-2531.6) = -1240.8 kJ/mol

sheesh... Please?

 

[Nino MMP]

Yes, you did this wrong. The correct answer should be: N=N = 945 kJH2 = 436.4 kJ total = 1381.4 kJThen, 2 H2 bonds = 2x436.4 = -872.8 2 N-H bonds = 2x393 = -786 total = -1658.8 kJ/mol Therefore, 1381.4 + (-1658.8) = -277.4 kJ/mol

 

[Blueshift5]

Hello,

Thank you for reaching out for assistance with your question. It seems like you are on the right track, but there are a few things to consider in order to accurately calculate the energy change for this reaction.

Firstly, when calculating bond energies, it is important to use the correct units. Bond energies are typically given in units of kilojoules per mole (kJ/mol), so make sure to convert any values given in kilojoules (kJ) to kJ/mol.

Secondly, you are correct in realizing that the first N=N bond should be tripled, since it is a triple bond. So the correct bond energy for N=N would be 1254 kJ/mol (3 x 418 kJ/mol).

Additionally, it is important to consider the direction of the reaction when calculating the energy change. In this case, the reaction is going from N=N + 2H2 to H2N-NH2, so we would need to subtract the energy of the reactants from the energy of the products.

Using the correct bond energies and taking into account the direction of the reaction, the calculation would look like this:

N=N = 1254 kJ/mol
2H2 = 2 x 436.4 = 872.8 kJ/mol
H2N-NH2 = 2 x 393 = 786 kJ/mol

Energy of reactants = 1254 kJ/mol + 872.8 kJ/mol = 2126.8 kJ/mol
Energy of products = 786 kJ/mol

Energy change (ΔH) = Energy of products - Energy of reactants
= 786 kJ/mol - 2126.8 kJ/mol
= -1340.8 kJ/mol

Therefore, the estimated energy change (ΔH) for this reaction is -1340.8 kJ/mol.

I hope this helps and clarifies any confusion. Keep up the good work in your studies of chemical bonding and energy changes!