N2H4(g) + H2(g)--> 2 NH3(g) H1 = –1876kJ
3 H2(g) + N2(g)--> 2 NH3(g) H2 = –922 kJ
The H.f for the formation of hydrazine: 2 H2(g) + N2(g)--> N2H4(g) will be______kj/mol
I am very confused here. I thought I would just add H1 and H2 and divide by the molar mass of N2H4, but that is not correct. Please Help. Thank You!
I got the answer to be 95.4 kj/mol
I reversed the 1st equation to get an overall 187.6 kj = h1
then I just added that to H2 and I got the answer.
But I still don't really understand the processes. If someone could explain that to me I would truly appreciate it.
You have to reverse the direction of one reaction and change the sign of H.
3 H2+ N2--> 2 NH3 H2 = -922 kJ/mole
2NH3 -> N2H4 + H2 H = +1876 kJ/mole
NOW add, and cancel out terms that appear on both sides.
N2 + 2H2 -> N2H4 H = -954 kJ
There is another problem. Your heat of formation of NH3 does not agree with accepted data. It should be -10.97 kJ/per mole of NH3, or twice that much for the reaction as written (which forms two moles). Also, the standard form of N2H4 at room temperature is a liquid, and your reaction involves the gas.
For what it is worth, the correct heat of formation of N2H4 (liq.) is +12.10 kcal/mole and for N2H4(g) it is +22.79 kcal/mole. Those values are at 298 K. They are a few kcal/mole different at 0 K. Multiply by 4.18 for kJ/mole. That gives +95.3 kJ/mole for the N2H4(g) heat of formation, which agrees well with your second answer. You seem to have misplaced decimal points in your first version of the question.
Those values come from the JANAF Thermochemcial Tables of the U.S National Bureau of Standards, 2nd edition.
To determine the standard enthalpy change for the formation of hydrazine (N2H4), you cannot simply add H1 and H2 and divide by the molar mass of N2H4. Instead, you need to use the enthalpy of formation values and apply the Hess's Law of constant heat summation.
Hess's Law states that the total enthalpy change of a chemical reaction is independent of the pathway between the initial and final states. In other words, if you can express the desired reaction as a combination of known reactions, you can sum up their enthalpy changes to obtain the enthalpy change for the desired reaction.
In this case, we have two given reactions:
Reaction 1: N2(g) + 3 H2(g) --> 2 NH3(g) ΔH = -922 kJ/mol
Reaction 2: 2 H2(g) + N2(g) --> N2H4(g) ΔH = ?
To use Hess's Law, we need to manipulate these reactions so that they can be added together to give us the desired reaction. By reversing Reaction 1 and multiplying it by 2, we can cancel out N2H4 and obtain the desired reaction:
2 (Reaction 1): 2 NH3(g) --> N2(g) + 6 H2(g)
Now, we have:
-2 (Reaction 1): -2 NH3(g) --> -N2(g) - 6 H2(g) ΔH = 2 * -922 kJ/mol
Adding Reaction 2 to -2 (Reaction 1), we get:
-2 (Reaction 1) + Reaction 2: N2H4(g) --> N2(g) + 5 H2(g)
Now, we have:
-2 (Reaction 1) + Reaction 2: N2(g) + 5 H2(g) --> N2H4(g) ΔH = -2 * -922 kJ/mol + ΔH for Reaction 2
Finally, we can calculate the enthalpy change for the formation of hydrazine:
ΔH for the formation of N2H4 = ΔH for Reaction 2 - 2 * ΔH for Reaction 1
= -1876 kJ/mol - 2 * (-922 kJ/mol)
= -1876 kJ/mol + 1844 kJ/mol
= -32 kJ/mol
Therefore, the standard enthalpy change for the formation of hydrazine is -32 kJ/mol.