Are you guys sure they test for sig figs on the frq’s? I looked at 2014’s scoring guideline and there were no points awarded for correct sig figs.
Also, did they remove the balancing equations section from the frq?
@dallascowboys1 - There used to be equation-writing FRQs, but I think they removed those and just integrated them into other problems. They’re not supposed to test solubility rules anymore, so… Not entirely sure, though.
@dallascowboys1 I’d bet my life that they will be assessing you on significant figures. And yes the ridiculous balancing equations is gone from the FRC
i searched for more info abut sig figs and I came across this:
"Review and consistently apply rules for significant figures, and avoid rounding off before the final answer. (Round calculations to appropriate significant figures at the end of the problem.)
https://apstudent.collegeboard.org/apcourse/ap-chemistry/exam-tips
not much help, but it’s like the only official thing collegeboard says about sig figs.
also, in previous years, in the sample responses, they would write that points were taken off for incorrect sig figs
wait so for #28, since x is the only thing in the rate expression that means its the slow one, and the y is not in the expression because its quick and its rate is negligible?
Please no titration curves. Especially weak acid- strong base. I hate those.
Each step in a reaction mechanism is a one-step reaction.
For one-step reactions, the exponents of the rate law matches the coefficients of the chemical equation.
In a reaction mechanism, the slowest step is the rate-determining step (even though all the other steps can be really quick, the slowest step limits the speed of the overall reaction since it has to occur before/after the other steps can occur).
The rate-law for an overall reaction is based on the slow step (it is the rate-determining step)
So since the exponent of the given rate law matches the coefficient of the slow step in both reaction mechanisms, they are both consistent.
you can’t decide whether or not the step is slow just by looking at its species. they have to tell you.
yeah Y is ignored since it occurs after the slow step.
look out for if the reactants in the slow step are formed from an earlier step. if this is the case, then in the rate law that reactant in the slow step is replaced by the reactants in its formation.
Hi guys, what is the difference between Q and K in Acid-Base topic? Thanks!
Ok yet another question.
I know when they tell you to find the solubility in M for some “insoluble” compound you set up an s expression. So if
Zn(OH)2 —> Zn2+ + 2OH-
4s^3 = ksp
I ran across a problem where they give you a pH and according the solution, this “s” method doesn’t work. Can I assume the s method is only possible in a neutral pH.
Using the zinc hydroxide equation, they tell you pH=9.35 and ksp=7.7e-17.
So from what I understand of the solution (1998 frq if you’re curious) is
pOH = 4.65
[OH-] = 2.2e-5
Ksp = [Zn2+][OH-]^2
Zn2+ is in a 1:1 ratio with ZnOH so by solving for Zn2+ you get the molar solubility of Zn(OH)2.
I understand the logic now but when do you use this versus
4s^3 = ksp?
So lads is AP Chemistry the hardest AP exam there is?
so if the reaction expression said rate=k*[x]^2 then none of them would be correct since they would need the coefficient of x to be 2?
Its considered one of the hardest. I would say that AP Physics C and AP Calc BC is up there as well. Depends on who you’re asking
just got 37/50 on the 2014 exam btw, is this a high 4?
Q is the reaction quotient, while K is the equilibrium constant.
if Q>K, there are more reactants than products.
Q<K, there are more products than reactants.
Q=K equilibrium.
K is always equilibrium, while Q is a reaction that is not necessarily at equilibrium. @spincrus
@Mathman97
yeah thats right.
I think on last year’s FRQ they asked about comparing Q and K. Basically, the problem went a little like “if a student changes the concentration from 1M to 0.50M, would Q be the same value?” And the answer was yes, because 1M/1M = 1 and 0.50M/0.50M = 1. That’s as hard as it gets I feel. @spincrus
also for #3 on 2014, why is the answer not because its valence shell is full?
So weird that 10% of people get a 5 on this compared to 6% on AP Bio. I thought Bio was a lot easier!
and for number 11, why is the ph not 7?
sorry im asking so many, i just took the test
Can someone explain empirical formulas? Are those even on the AP?