Homework Question 8.49 - Why can you not use a mole ratio?

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Kristienne Edrosolan
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Joined: Wed Nov 18, 2015 3:00 am

Homework Question 8.49 - Why can you not use a mole ratio?

Postby Kristienne Edrosolan » Sun Jan 10, 2016 3:16 pm

Hi, I'm looking for some help with homework question 8.49, which asks,

"Oxygen difluoride is a colorless, very poisonous gas that reacts rapidly with water vapor to produce O2, HF, and heat.

It gives the formula

OF2 (g) +H20 (g) --> O2 (g) + 2HF (g) with delta H being -318 kJ.

What is the change in internal energy for the reaction of
1.00 mol OF2?"

I'm confused why you can't just use the mole ratio that for every 1 mol of OF2 being produced, the energy released is 318 kJ according to the equation. I checked the solutions manual and it talks about a net production of 1 mole of gas, but OF2 is a reactant and produces 3 moles of gas. Can anyone please clarify this for me?

Thanks!

Chem_Mod
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Re: Homework Question 8.49 - Why can you not use a mole rati

Postby Chem_Mod » Sun Jan 10, 2016 4:19 pm

The problem asks for the change in internal energy, , while the value given in the problem is the enthalpy of the reaction, . So we need a way to relate the enthalpy of the reaction to the internal energy, which we can do using the following equation:

Jeannie Huang 3B
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Joined: Fri Sep 25, 2015 3:00 am

Re: Homework Question 8.49 - Why can you not use a mole rati

Postby Jeannie Huang 3B » Fri Jan 22, 2016 7:01 pm

I believe that the "net production of 1 mole of gas" refers to the fact that 1 mole of OF2 (part of the 2 moles of total reactant) produces 3 moles of product. Thus, the net difference is 3-2 moles = 1 mole of gas difference between reactants and products.

Emily Beck 3A
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Re: Homework Question 8.49 - Why can you not use a mole rati

Postby Emily Beck 3A » Sat Jan 23, 2016 6:08 pm

I have an additional question regarding this problem.

The solution manual plugged in (298 K) for the temperature but this wasn't stated in the problem. How do we know the temperature is assumed to be 298 K?

Jana Sandhu 3J
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Joined: Fri Sep 25, 2015 3:00 am

Re: Homework Question 8.49 - Why can you not use a mole rati

Postby Jana Sandhu 3J » Sun Jan 24, 2016 4:29 pm

Whenever temperature is not given in a situation where it is needed for calculations, always assume it is 298K (25 degrees Celcius) because that is part of standard conditions.


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