@sabisahmed
There isn’t a section devoted entirely to writing chemical reactions anymore. If it is asked at all, it would probably be worth 1 or 2 points on any frq. Last year they only asked to write equations twice, and they were easy ones.
The faraday thing is just a stoich problem
Zn + 2e- -> Zn(s)
.2 faradays = .2 mol e-
.2 mol e- = .1mol Zn(s)
.1 mol Zn = 6.54g Zn
You would use I = q/t if they gave you current and time to find charge, and then divide that charge by 96500 (Faraday’s constant) to find moles e-
GUYS! any predictions as to what this years frq might be?? did your teachers see any patterns or was something emphasized at their summer orientation? i have a feeling kinetics will be a big one. any thoughts?
Alright everyone, simple question:
What specifics should i cram for this exam?
i was thinking of just taking a look at past FRQs since theres not much else i can do. any advice is helpful. 23 hours till exam
Solid sodium iodide is added to a saturated solution of lead iodide at 25°C. Assume lead’s charge is 2+. Assuming that the volume of the solution does not change, does the molar concentration of Pb 2+ in the solution increase, decrease, or remain the same?
I said
NaI + PbI2 —> NaI + PbI2
The molar [ ] of Pb2+ will decrease. According to le chateliers principle, increasing the amount of Na will decrease the amount of Pb.
Sodium iodide dissocates 100% to Na+ and I-. This increases [I-]. Q value for the reaction is now greater. To reestablish equilibrium, [Pb+2] decreases by the formation of more precipitate PbI2
I figured that the x should have 4 decimal places as it is based on +/- rule. So I used 0.0087. I ended up getting 0.0034 when the answer was 0.00333.
The bottom line is I used a slightly different method to arrive at the answer and my sig figs are used properly in the process. My final answer doesn’t match exactly however.
Would you recommend just using the sigfigs given in the problem?
Also, in the equation for bond energy (bonds broken - bonds formed), if you get a positive value, does that mean the reaction is endothermic or exothermic?
@Mathman97#28) They tel you the rate-=k [Xsub2]. Basically you have to look at the slow steps for each Mechanism 1 and Mechanism 2 and compare them to that. Since both have [Xsub2] as the only reactant they are indeed consistent with experimentally determined rate law. Which is choice C) I believe. Occasionally these questions will be a little bit more complex and you may have to incorporate aspects of the other, faster, steps in order to set them equal. In THIS case it was pretty straightforwards so hopefully we can get something like this tomorrow