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

Kristienne Edrosolan
Posts: 19
Joined: Wed Nov 18, 2015 3:00 am

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

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
Posts: 18875
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 703 times

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

The problem asks for the change in internal energy, $\Delta U$, while the value given in the problem is the enthalpy of the reaction, $\Delta H$. So we need a way to relate the enthalpy of the reaction to the internal energy, which we can do using the following equation:
$\Delta H = \Delta U + p\Delta V = \Delta U + \Delta nRT$

Jeannie Huang 3B
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

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

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
Posts: 18
Joined: Fri Sep 25, 2015 3:00 am

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

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
Posts: 25
Joined: Fri Sep 25, 2015 3:00 am

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

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.

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

### Who is online

Users browsing this forum: No registered users and 3 guests