(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

This is for a chemistry report. This is the task:

"Determine the molar enthalpy change for the decomposition of sodium hydrogen carbonate into sodium carbonate, CO(2) and water.

2NaHCO(3) -> Na(2)CO(3) + CO(2) + H(2)O

This enthalpy change cannot be measured directly".

Instead, we reacted NaHCO(3) and Na(2)CO(3) in separate containers with HCL (in excess (100 mL), about 1M).

We measured the temperature changes, and the masses of each solid/liquid.

For the NaHCO(3) reaction:

Mass(NaHCO(3)): 0.001021 kg ± 0.000001 kg

Mass(HCL): 0.100 ± 0.001 kg

Change in T = 1 K

For the Na(2)CO(3) reaction:

Mass(Na(2)CO(3)): 0.000905 kg ± 0.000001 kg

Mass(HCL): 0.100 ± 0.001 kg

Change in T = -0.3 K

2. Relevant equations

Nr 1: 2NaHCO(3) -> Na(2)CO(3) + CO(2) + H(2)O

Nr 2: NaHCO(3) + HCl -> NaCL + CO(2) + H(2)O

Nr 3: Na(2)CO(3) + HCl -> 2NaCL + CO(2) + H(2)O

Note: By multiplying equation 2 by two, reversing equation 3, and then adding them together, we get equation 1. This has to be done to the enthalpies I calculate in Part 3, too.

3. The attempt at a solution

My biggest problem is figuring out how to find theMolarDelta H in kJ/mol. By doing the following, I can calculate the approximate Delta H (only in kJ) for reaction 1:

Following the equation of Heat = Mass * Specific Heat (c) * Delta T, we get:

(Note, here I add the two masses of HCl and the solids, and then use the specific heat of water. I understand this holds quite a large uncertainty. If anybody has a better idea, please say so. I am also really unsure on what I do after calculating the heat, ie. when I invert the sign of the heat form + to - and vica versa. Is this correct?)

For NaHCO(3):

Mass = 0.101021 kg

Specific Heat (water) 4186 J/(kg * K)

Delta T = 1 K

Heat = 422.87 J. As this is an exothermic reaction, it becomes -422.87 J

For Na(2)CO(3):

Mass = 0.100905 kg

Delta T = -0.3 K

Heat = -126.71 J. As this is an endothermic reaction, it becomes +126.71 J.

Then, by doing the same to these numbers as is mentioned in Part 2 (the reactions), I end up with (-422.87 * 2) + (126.71*-1) = -970.71 J = -0.971 kJ!

But this however, is just the enthalpy change for the reaction. Not themolarenthalpy change...

Any comments on this would be absolutely wonderful, and greatly appreciated!

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# Hess Law: Molar Enthalpy Change for Decomposition

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