crayons U4 exam questions ~

[crayolé]

Thanks Vexx

For the first MC, the solutions says its C but I got D? I'm confused, in their '10 paper, they included the solids in the K value but they disregard it in this instance?
The full reaction is this
AgCl(s) + 2NH₃(aq) ----> Ag(NH₃)₂(aq) + Cl^-(aq)

For the second MC, the solutions say its A but would C also be a viable answer?
It would donate its proton to the NH₃ ions thus causing a net back reaction and increasing the amount of Cl^- and Ag⁺ thus less AgCl(s) would need be dissolved?

For the third MC - this sounds like a pretty stupid question (it probably is) but why cant it be assumed that ? Similar to how it is assumed in acid reactions

[physics]

would u like to add my name in the thread too i have a loota questions too

[Mao]

For the dissolution of silver chloride, technically the second equation refers to the formation of a silver-amine complex, not the dissolution of silver chloride. The mechanism is when amine is added, it removes some Ag+, thus driving the first equilibrium to the right, increasing Cl-. The solubility constant for AgCl will be maintained (Ag+ decreases and Cl- increases). The 'most' correct answer is A, though technically [AgCl] isn't a real quantity, and should be 1 and omitted from the equation.

For the second question, I don't see how A can work. An increased volume generally mean more salt can be dissolved, not an 'increase' in undissolved salt. Addition of Cl- might work for some cases, but generally when you have a low Ag+ concentration and there's little free Ag+, adding Cl- won't do much at all. (The competing equation with ammonia is quite strong, unless you add VERY concentrated NaCl, you won't really get the AgCl back.) I don't think any of the options are really correct.

For the third question, you can only assume that for weak acids because initially there's only water, no conjugate salts or H+ present. For most system however, you cannot assume the system is 'clean' initially, and there may be some products already present.

[crayolé]

Cheers Mao, much appreciated ;]

Question 2
A student calibrated a solution calorimeter containing 100 mL of water using electrical apparatus. The specific heat capacity of water is 4.18 J °C-1 g-1. Assuming that the student’s measurements were reliable, which calibration factor is most likely for a well-insulated calorimeter?
 
A.   240 J K-1
B.   480 J K-1
C.   960 J K-1
D.   1418 J K-1

Also, are we supposed to learn fusion/fission reaction formulas and derive it's properties? (refer to attachment)

[Mao]

You would assume the calibration factor to be a tiny bit bigger than the amount of energy required to heat all the water by 1 degree, considering the calorimeter works by the water absorbing heat. Thus B.

As for fusion/fission, yeah, you are meant to know how to balance those equations. You are also meant to know fusion is exothermic up until Fe, and is endothermic after that. Fission is endothermic up until Fe, and is exothermic after that.