Posted by **Jess** on Thursday, February 1, 2007 at 10:41pm.

A few classmates and I were arguing about a problem we had to do for homework.

The problem states:

Limestone, CaCO3, when subjected to a temperature of 900 degrees C in a kiln, decomposes to calcium oxide and carbon dioxide . How much heat is evolved/absorbed when one gram of limestone decomposes?

Some of the students thought that 900 degrees C was not important to the calculation of the heat, so they did their work like this:

CaCO3 -> CaO + CO2

635.1 kj/mol + -393.5 kj/mol - (-1206.9 kj/mol)= 178.3 = delta H

1.00 g x 1mol/100.05 g = .009995 mol CaCO3

178.3 x .009995 mol = 1.78 kJ

Another student believed that the 900 degrees C, and explained his approach to the problem like this:

CaCO3 -> CaO + CO2

635.1 kj/mol + -393.5 kj/mol - (-1206.9 kj/mol)= 178.3 = delta H at 25 degrees C and 1 atm

n=PV/RT

1=1 atm x V/.08206 x 298.2

v= 24.47

n= 1 atm x 24.47 / .08206 x 1173.2

n= .254

178.3 x .254 = 45.28 kj/mol x .009995 mol CaCO3 = .45 [although he claimed to get 7. as an answer, so maybe I missed a step.]

For the most part, which idea is correct? And what changes [if any] need to be made to the set-up of the problem?

I don't have the delta H

_{f} values for these materials except for CO2. Assuming those values are correct, then the first method is the correct one to use. We COULD calculate the volume at 25 C if we wanted to do so and that is about 0.244 L or at 900 C it would be about 0.962 L; however, there is nothing about this being a closed system and we don't need to know the volume. But the real clinker here is that the second method then takes the volume and uses that to calculate the mols as about 0.254. But you KNOW the mols of CO2 will be 0.009995. The 1.0 g CaCO3 (0.009995 mol) taken initally sets the CO2 that is evolved and no other calculation can change that. There will be 0.009995 mol CO2 and 0.009995 mol CaO.

## Answer this Question

## Related Questions

- chemistry - How much heat is evolved or absorbed in this problem? Question ...
- chemistry - How much heat is evolved or absorbed in this problem? Question ...
- Chemistry - a sample of limestone and other solid materials is heated and the ...
- Chem - Caculate the volume of CO2 gas given off at 756 mm Hg and 23C when 150 kg...
- algebra - A piece of pottery is removed from a kiln and allowed to cool in a ...
- Materials Transport - You have a kiln where theinterior is a cube with an edge ...
- college algebra - A piece of pottery is removed from a kiln and allowed to cool ...
- college algebra - A piece of pottery is removed from a kiln and allowed to cool ...
- college algebra - A piece of pottery is removed from a kiln and allowed to cool ...
- Chemistry - OK I just need help with two final problems that I am stuck on: The ...

More Related Questions