[jzaid11]

Hey guys,

The problem is about a coffee cup calorimetry passage in which room temp HCL and 0.5 M NaOH were mixed together in the calorimeter and the temp was plotted against time. It goes up sharply then slowly falls.

Here is the question:
"104. If 0.5 M NH4OH (a weaker base) were used instead of NaOH, how would this affect the results of the experiment?"

A. The temperature change would be greater because more energy is required to dissociate NH40H.

B. The temperature change would be less because more energy is required to dissociate NH40H.

C. The temperature change would be greater because less energy is required to dissociate NH40H.

D. It would not change thc results bccause both bases are ionic compounds and the energy required to separate equal charges is always the same.

I thought the answer would be C. It was B. My thinking was that since NH4OH is weaker, it would take less energy to dissociate. Therefore less energy would be used to dissociate the NH4OH (weaker) as opposed to more energy being used to dissociate NaOH (stronger). According to their answer the overall change in temp would go down as more energy would be used to break the weaker bond. How is this possible?

Thanks in advance!

[Bublik]

My thinking was that since NH4OH is weaker, it would take less energy to dissociate. Therefore less energy would be used to dissociate the NH4OH (weaker) as opposed to more energy being used to dissociate NaOH (stronger). According to their answer the overall change in temp would go down as more energy would be used to break the weaker bond. How is this possible?

Since NH4OH is weak because it requires more energy to dissociate fully. Strong acids take the least energy to dissociate (which is WHY they dissociate!).

At least that's my train of thought.

[Borek]

My thinking was that since NH4OH is weaker, it would take less energy to dissociate.

Quite the opposite. Bond in a weak acid is stronger, which is why it is dissociated less than the strong base.

[jzaid11]

Then what does "strong" and "weak" refer to? How is a molecule that disintegrates so easily considered strong?

[Bublik]

The "strong" and "weak" terms that are used have nothing to do with the actual strength of the bonds. A "strong" acid is an acid that ionizes fully, like HCl or HNO3. A weak acid has stronger hold on the proton and does not let it ionize completely.

Just realize that in your question, NH4OH is a weak base BECAUSE the molecule is "happy" the way it is and does not want to release too much OH^-. As a result, this suggests that more energy needs to be inputted in order to make a weak base or a weak acid to ionize more.