CHEMISTRY COMMUNITY

Clarisse Wikstrom 1H

a catalyst is used up (as a reactant) and then produced again (as a product).

Clarisse Wikstrom 1H

FYI If you derive the Arrhenius equation yourself, you may also get ln(k2/k1) = (Ea/R)(1/T1- 1/T2), and it works as well. As long as whatever k you have on the top, you have the corresponding T second.

Clarisse Wikstrom 1H

The problem may ask you to determine which is the rate-limiting step, which we know is the slow step. If so, you have to analyze each step to see if it's individual rate law matches the overall rate law. Remember, you may have to use the pre-equilibrium approach in order to do so. Whichver step's ra...

Clarisse Wikstrom 1H

Hey girl. Are you made out of salt? Because your SO-DIUM fine. ;)

Clarisse Wikstrom 1H

Why is k' used in the pseudo-first-order rate law? Doesn't k' mean the rate constant for the reverse reaction? Thanks!

Clarisse Wikstrom 1H

For example, the rxn for #85a is CH3CHO --> CH3 + CHO You know that in this reaction, the bond between the two carbon atoms is breaking. Therefore, the structure for the activation complex would be [H3C ---- CHO] (and put the little symbol for activation complex) Basically, you put a line to represe...

Clarisse Wikstrom 1H

I have the same question!!

Clarisse Wikstrom 1H

It would seem easier to just add up the E values for each half reaction, correct? However, you can only do this when solving for the potential of an entire reaction, not a half reaction. To solve for the potential of a HALF-REACTION, you must find the delta G values of each one, add them together, t...

Clarisse Wikstrom 1H

It would make sense that as time increases, ln[A] should decrease as well. However, the rate constant, k, is always a positive number, therefore a negative sign must be put in front to make the slope of the line negative.

Clarisse Wikstrom 1H

Because HCl is a strong acid and likely to dissociate into it's ions.

Clarisse Wikstrom 1H

A higher (positive) reduction potential means that it is more likely to be reduced. Therefore if the standard reduction potential is negative, the couple is more liekly to be oxidized and act as a reducing agent ( higher reducing strength).

Clarisse Wikstrom 1H

If the mediums you're working with do not include a solid as a conductor. For example, if one ion gets oxidized or reduced to another ion instead of a solid. Therefore it needs an inert conductor like platinum

Clarisse Wikstrom 1H

K= [H+]/[OH-]

@pH 1: 1=-log[H+] --> [H+] = 1x10^-1 M
therefore, pOH 13: [OH-]= 1x10^-13 M

@pH 14: [H+]= 1x10^-14 M
therefore, pOH 0: [OH-]= 1x10^0= 1.0 M

There is an error in the solution manual for this problem, so I'd check on Lavelle's website.

Clarisse Wikstrom 1H

How do we know how many sig figs to include when solving for K for our final answer? Is K usually just 1 sig fig?
Thanks

Clarisse Wikstrom 1H

Same, I thought it would have been Au+(aq) --> Au3+(aq) + 2e- , with Pt(s) as the conducting solid....

Clarisse Wikstrom 1H

Is a reaction at STP (standard Temp and Pressure) @ 0*C or 25*C? I believe I've seen both used so I just want to clarify.

Clarisse Wikstrom 1H

When you draw out the cis molecule (X2 on same side) versus the trans molecule (X2 on opposite sides), and proceed to rotate them, the cis molecule has many more orientations than the other molecule.

Clarisse Wikstrom 1H

Irreversible is spontaneous in one direction, and is highly unlikely to proceed in reverse. It does less work than reversible expansion because it is typically expanding on a lower pressure. On the other hand, you can think of a reversible expansion as equally proceeding in both directions because t...

Clarisse Wikstrom 1H

When discussing entropy, what's the difference between positional and thermal disorder? (Question 9.85)

Clarisse Wikstrom 1H

I think the "inital deltaH" that you are thinking of is the deltaH(fus) of water, which is needed for the phase change of the ice cube to liquid water. The other steps only require q=mCdeltaT

Clarisse Wikstrom 1H

Some people believe that the universe is always expanding, therefore entropy must be increasing.
Other reasons are because an increase in entropy is favorable, and the universe will always go toward the more favorable outcome.

Clarisse Wikstrom 1H

Can think of a reversible reaction as being at equilibrium?

Clarisse Wikstrom 1H

You use the one with R when you have V1/V2. You use the one with C when you have T1/T2.

Clarisse Wikstrom 1H

What exactly is q(REV) and how does it differ from q? Why is the (REV) significant, since when solving for deltaS we tend to replace it with deltaH anyways?

thanks!

Clarisse Wikstrom 1H

The change in enthalpy of a phase change will often be given to you, as deltaHfus, deltaHvap. You could also calculate these if you know the amount of heat released or needed (q) and the number of moles, by using the equation q=ndeltaHfus/vap.

Clarisse Wikstrom 1H

In an isothermal system, deltaT is zero vecause the temperature is constant. For an ideal gas, deltaU=(3/2)nRdeltaT. Therefore, if deltaT=0, then deltaU=0. I believe this is only for ideal gases...

Clarisse Wikstrom 1H

Example... Irreversible: 4 atm --> 2 atm Reversible (@ equilibrium): 3 atm --> 3 atm Which one does more work, the one pushing on 2 atm or the one pushing on 3 atm? @ equilibrium, the system is theoretically doing the max work it can. That's why reversible reactions do not often occur in biological ...

Clarisse Wikstrom 1H

When putting our final answer, how do we know whether the units for deltaH are kJ or kJ/mol? I've seen both so I was wondering if we would get marked off on the test if we wrote the wrong one. Thanks!

Clarisse Wikstrom 1H

It is also important to note that limiting reactants may still apply to a problem. For example in question 101, you have to determine the limiting reactant and then the amount of moles in excess. You would also the have to add the moles of excess to the moles of product you calculated using the bala...

Clarisse Wikstrom 1H

In addition, you must do this because it would take much more energy to break the bonds for liquid or solid compounds... the bond enthalpies would extremely high.

Clarisse Wikstrom 1H

Cv is the molar heat capacity of a gas @ CONSTANT VOLUME, whereas Cp is the molar heat capacity of a gas at CONSTANT PRESSURE. In one example (#31), you would use it when doing q=mCdeltaT if you're given an ideal gas but no molar heat capacity. The Cp and Cv for ideal gases can be found on Lavelle's...

Clarisse Wikstrom 1H

In class we discussed that the Internal Heat, U, of an isolated system has no change since it not affected by q or w, since it is isolated. Therefore, deltaU = 0.

Clarisse Wikstrom 1H

Enthalpy of the total reaction.

Clarisse Wikstrom 1H

This is why sometimes Lavelle will write qp instead of q, indicating it is at constant pressure and is interchangeable with deltaH.

Clarisse Wikstrom 1H

If you're doing the hw problems, I would say the "enthalpy" section, which starts with problem 8.29.

Clarisse Wikstrom 1H

This is as opposed to Cs, which would be specific heat capacity with units J/(g*C)

Clarisse Wikstrom 1H

Just remember that the goal is to get the desired reaction. Sometimes, you have to flip the internal reactions in order to get the correct reactants and products for the desired reaction.

Clarisse Wikstrom 1H

First you have to calculate the number of moles needed to produce that amount of heat absorbed. Since you know that q=n*deltaH, therefore n=(q)/(deltaH). this will give you moles per reaction, but you have to remember that there are 4 moles of CS2 produced per every reaction, so multiply what you go...

Clarisse Wikstrom 1H

The work is done TO the system, since the outside pressure pushes the piston down causing the compression. Because the system doesn't have to do any work, therefore the work on the interior air is positive. IF the piston moved out to increase the volume inside, then the system would be DOING the wor...

Clarisse Wikstrom 1H

This explains why oxoacids are strong acids. They are more willing to lose their H+ because after losing the H+, they have more options for resonance, which makes the compound more stable.

Clarisse Wikstrom 1H

They will either give you the chemical equation or tell you what the reactants and products are. However, you could sometimes assume the direction if one of the compounds is a strong acid because you know it will completely dissociate and is not likely to reform.

Clarisse Wikstrom 1H

pH and pOH take the -log[ H+ or OH-]. They are not the same as their respective Ka or Kb, which shows the relationship between all concentrations in the rxn, not just [H+] or [OH-].

Clarisse Wikstrom 1H

The question is not super clear but if you are referring to writing net ionic equations then you know which species dissociates based on solubility rules and strong acids and bases. There are 3 basic steps to writing net ionic equations: (1) write a balanced equation with the states of matter inclu...

Clarisse Wikstrom 1H

Sodium is left out because it is a cation completely unrelated to the proton transfer reaction. this is because it gets cancelled out when you do the net ionic equation for the reaction.

Clarisse Wikstrom 1H

think of resonance!!

Clarisse Wikstrom 1H

I forgot to mention that OH2 would be correct IF the transition metal was written on the LEFT side of water... if it was written on the RIGHT, it should be H2O :)

Clarisse Wikstrom 1H

Hi Mike, How water is written does not matter, so long as it is correctly written with two hydrogens and one oxygen. H2O and OH2 are equally correct. For part d., because oxalate has the prefix bis-, this has higher priority than "diaqua" as b precedes d. Hope this helps. But I thought th...

Clarisse Wikstrom 1H

What do you mean by "ratios of the change in equilibrium"?

Clarisse Wikstrom 1H

and a polydentate is when the ligand can bind at multiple sites the transition metal. However, although N2 has 2 pairs of lone pairs on each nitrogent, it is not a polydentate because it would be impossible for both nitrogens to bind to the T.M. at the same time.

Clarisse Wikstrom 1H

Sometimes if works if you count the number of Nitrogens because you know that they usually have a lone pair, however you also have to remember that if the nitrogens are right next to each other, they wont both bind at the same time... ultimately I would just draw the Lewis structure because then you...

Clarisse Wikstrom 1H

Yep so for example if the molecule has 2 single bonds and a double bond in resonance, then the bond order would be (1+1+2)/(3 total regions)= 1.33

Clarisse Wikstrom 1H

Hybridization occurs because it allows atoms to form more bonds than their stated number of unpaired e-. For example, Carbon is 2s^2 2p^2, so we know that the ones in the s- orbital are spin paired and the two in the p-orbital are not. By this logic, that should mean that carbon can only form two bo...

Clarisse Wikstrom 1H

To determine which one has more ionic character, compare the electronegativities of each. You don't have to know the exact numbers, but know that Cl is more electronegative than I, F is more electronegative than H, etc. The species with the higher difference in EN in each pair is the one with more i...

Clarisse Wikstrom 1H

It may also help to draw out the lewis structures for each one. If an atom in one has an unpaired e-, then the species is a radical.

Clarisse Wikstrom 1H

Ionic bonds join two nonmetal atoms, covalent bonds join two ions. If the electronegativity difference is >2 then it is considered an ionic bond, if electronegativity difference is <1.5 then it is considered a covalent bond. This first part of this answer is a bit mixed up: ionic bonds join ions (t...

Clarisse Wikstrom 1H

Monoatomic ions only have one atom, so since it's the only atom, it must be the middle atom.

Clarisse Wikstrom 1H

If you haven't memorized the periodic trends already, it's also important to know HOW they exists if you are not given the electronegativities. EN increases as you go to the right of the periodic table because as more valence e- are added, they become closer to an octet. You can think of EN as how m...

Clarisse Wikstrom 1H

The most ideal situation is...
1. Formal charge closest to zero
2. central atom w/o charge
3. charges (if any) distributed as evenly as possible among atoms (2- vs 1- and 1-)
Hope this helps!

Clarisse Wikstrom 1H

Also, it wouldn't be an equal sign because since it is "uncertain", it can never equal a certain number-- there must be a range of uncertainty where the exact position and velocity of an object lie at a certain time.

Clarisse Wikstrom 1H

This is because as the electron is moving farther away from the nucleus, eventually it will go to infinity , which in that case it's ejected and the Energy=0.

Clarisse Wikstrom 1H

Gabriela Carrillo 1B wrote:Is the module "Chemical Equilibrium Part 1" part of this week's lectures??

Nope, it's just next on the modules because it's the one other topic that he feels is hardest. After the Quantum World we're probably gonna do Chemical Bonds.

Clarisse Wikstrom 1H

On the test on 10/6/17, I had trouble allotting enough time to answer all the questions and I believe it was mostly as a result of me taking my time thinking about what significant figures I should be using throughout my intermittent calculations. Are significant figures a crucial part to the test ...

Clarisse Wikstrom 1H

I would definitely get or review an old AP Chem test prep book, in addition to office hours, etc.

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