The AP Chemistry Study Thread

[whylion]

yep -- methane, ethane, propane butane pretty much gaseous, pentane and up are liquid

[whylion]

Its because the longer chains get tangled up like spaghetti
For isomers -- the more branched the chain, the lower the boiling point
so hexane will be higher than say 2methylpentane

[sristi]

scareya~ the answer is +1. Formal charge isn't really hard to do but I forgot how to do them . need to look it up

[shanianwang]

Formal charge:

Total electrons on atom - lone pair electrons - (1/2)*bonded pairs = formal charge of atom

[tentash2002]

AS the number of oxygen atoms increases in any series of oxygen acids, such as HXO, HXO2, HXO3...is it true that the acid strength increases..or is it only if X is a metal?

[vu_preuss_06]

Yes the more oxygen atoms in an oxyacid, the stronger the acid is.

[tentash2002]

regardless if X is a metal or not???

[Evilbooyaa]

well let's check it out.
HClO less soluble than HClO4...... so it works w/ non-metals

never heard of a acid w/ metal in it.

looked it up- H2CrO4- chromic acid - strong acid
H2CrO3- chromous acid - weak acid.

so yea. i think it's regardless of whether X is a metal or not.

[xorwat]

The rules on strong and weak acids are as follows
The only strong binary acids are HCl, HBr and HI
In all other strong acids, the number of oxygens is 2 greater than the number of hydrogens e.g. H2SO3 is weak while H2SO4 is strong
All other acids are weak

[smder99]

I have a question about titration curves.

What causes the "rapid-rise" portion of the curve as the solution is close to approaching the equivalent point?? Why is the curve not smooth or a linear model?

[fabrizio]

pH is defined logarithmically; it's not linear. It is exponential.

[Evilbooyaa]

smder, as the titration curve approaches the equivalence point, the concentration of OH-/H+ is decreasing.

think of it this way.

say you start w/ HCl w/ a Ph of 0 (1.0M HCl) and like 100mL of it.
you start adding NaOH..... you add 10mL of 1.0M NaOH

titration curves usually have Ph on the y-axis, volume of NaOH (in this case) on the x-axis.
what is the Ph after the 10mL of NaOH? .1moles HCl- .01moles NaOH
= .09moles HCl into 110mL of NaOH

what is the molarity of that-------- .09/.110 = .8181M
-log(.8181) = .08715 which is very close to 0. so adding 10mL of NaOH to 100mL of HCl barely changes the Ph.

go up to 100mL 1.0M HCl w/ 50mL 1.0M NaOH
.1mol HCl + .05mol NaOH --> .05 mol HCl left
150mL of solution
.05/.150 =.333M
Ph of that = .47756 --> still not even at 1.

now go to 80mL
.1mol HCl + .08mol NaOH --> .02mol HCl left.
180mL
.02/.180=.11111111M
Ph of that = .9542--> STILL not even at 1!
80mL of NaOH and not even at 1!

that's why it isn't linear......... if you continued at this rate using just up to 80mL at extrapolating to future Ph, it wouldn't go above like 2 or 3.

let's look at 95mL NaOH now.
.005 mol HCl left
195mL--> M = .0256M
ph of that = 1.5911--> finally above 1, but still very low (remember that 7 is neutral), that's only 5mL away from equivalence point!

look at 99mL NaOH.
.001 mol HCl left
199mL -->.005025M
Ph = 2.298864--> not even 7, and this is ONE mL away from equivalence point!

at 100mL, Ph = 7..... think of the jump between that! take the slop assuming it's linear.
7-2.298864/(.1-.099) = 4.7011/.001 = 4701.1--> slope of the line.... very VERY high.

that's why there's such a straight line straight up near Ph.

[high on life 76]

what colors do we have to know. ex: calcium=orange, sodium-yellow, etc

[shanianwang]

The following reaction is nonspontaneous at 25C:

Cu2O(s) --> 2Cu(s) + .5 O2(g) change in G = 141 kJ

If change in S (entropy) is 75.8 J/K, above what temperature will the reaction become spontaneous?

[shanianwang]

You guys will have to look up numbers yourselves.