Ek 1001 Chemistry Q 371

[99dmg]

Given: X + 6Y ---> 4Z..............ΔH = -1279kJ/mol

What is the enthalpy change for the following reaction?

3X + 18Y ---> 12Z

A) -3837kJ/mol
B) -1279kJ/mol
C) -426kJ/mol
D) 1279kJ/mol

I picked B since the same enthalpy change would occur per mol, but the answer key says A

"Admittedly, the kJ/mol notation used in such cases is a bit confusing, but it really means kJ per number of moles shown in the equation."

Is that statement above true? I always thought kJ/mol meant PER mol

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[sakabato93]

Given: X + 6Y ---> 4Z..............ΔH = -1279kJ/mol

What is the enthalpy change for the following reaction?

3X + 18Y ---> 12Z

A) -3837kJ/mol
B) -1279kJ/mol
C) -426kJ/mol
D) 1279kJ/mol

I picked B since the same enthalpy change would occur per mol, but the answer key says A

"Admittedly, the kJ/mol notation used in such cases is a bit confusing, but it really means kJ per number of moles shown in the equation."

Is that statement above true? I always thought kJ/mol meant PER mol

It does mean per mol. What the explanation is showing though is that the enthalpy is dependent upon how many moles of reactants and products are participating in the reaction. The explanation is poorly worded, but it means that if you have 3x the moles of some reaction, then the enthalpy of the reaction will also increase by a factor of 3.



Think about it this way: energy is additive, right? You are running a reaction with exactly 3 times as many reactants as usual, that means you are going to generate 3 times as much heat (or use up 3 times as much heat if endothermic).

 

[99dmg]

Yeah, I get that enthalpy is an extensive property, I was just wondering if the way they use their notation is correct.

Like on the MCAT or in sceintific literature would kJ/mol mean kJ per mol, or "kJ for the reaction shown" like the EK book says.

And since an extensive property/extensive property = intensive property, kJ/mol would be intensive

 

[sakabato93]

Yeah, I get that enthalpy is an extensive property, I was just wondering if the way they use their notation is correct.

Like on the MCAT or in sceintific literature would kJ/mol mean kJ per mol, or "kJ for the reaction shown" like the EK book says.

And since an extensive property/extensive property = intensive property, kJ/mol would be intensive

Well, its the same meaning. The notation kJ/mol means per mole of reaction, if that makes sense. It serves as a comparison the enthalpies of the products and the reactants, but then it also shows as you increase/decrease the molar amounts of species participating in the reaction, the enthalpy changes accordingly.

I apologize if I'm not making much sense; I'm pretty tired right now.

 

[99dmg]

I think I get what your saying.

They mean the same thing since in the real world you always divide the reaction into the lowest common denominator so they don't run into the confusion like this.

Like no one would write the rxn in terms of this: 3X + 18Y ---> 12Z, since they could reduce it

 

[benjaminl1nus]

The answer key is wrong. The units for the answers are also wrong, assuming A is the right answer. PER MOLE, your answer is correct. B is definitely the correct answer in this case, since they are dealing with an intensive property.

Now, if the units of the answer choices were in kJ only, then you'd know that the answer they're looking for is A.

As it stands, as the units are written, the answer is B, like you said.