CHEMISTRY COMMUNITY

Eugene Chung 3F

Sine it says pi comes from ATP, the two concentrations cancels and K depends on [ADP] only.

Eugene Chung 3F

Does this class have a curve at the end?

Eugene Chung 3F

On the final, #15 asked us to find \Delta H ionization for H20 --> OH- + H+. How were we supposed to find this? I googled the \Delta H formation of H2O and OH- and used \sum \Delta H products - \sum \Delta H reactants . Was this the right way to go about it or did we have to use the pH or \Delta H ...

Eugene Chung 3F

The amount of work possible is dependent on the cell potential of the electrochemical cell

Eugene Chung 3F

Vuong_2F wrote:
Eugene Chung 3F wrote:electrons will flow from area of high concentration to an area of low concentration. So, the concentration difference makes potential.

so once there is no concentration difference, there is no more potential?

Yup.

Eugene Chung 3F

If all of the reactions in the table represent reductions, for the oxidizing half reaction, it would be made negative, then would the sign in the Ecat-Eanode change to addition or would we subtract it? If you don't want to worry about changing the sign, it's easier to just do Ecat-Eanode and always...

Eugene Chung 3F

Cindy Adiwidjaja 1B wrote:I know that E is given, however, why do we need to flip the sign when we are trying to use the E for oxidation?

Honestly, for me, I feel like it's easier to do Cathode-anode, that way you don't have to worry about changing the E value given.

Eugene Chung 3F

We know that lower electronegativity and more negative E value means stronger reducing agent. Anode is oxidized and is the reducing agent. So, the one with more negative E value is conventionally the anode.

Eugene Chung 3F

Gabriel Ordonez 2K wrote:When are the instances that we have to add an inert solid to either end of the cell diagram? Is it always one or two ends of it?

When the other elements in the cell diagram is aq. state.

Eugene Chung 3F

using graphite (carbon) is also common, but I think Dr. Lavelle said we would be using platinum for chem 14b

Eugene Chung 3F

the deltaG for a cell is the amount of useful work obtainable from a system at constant T and P.

Eugene Chung 3F

I think the main thing to remember is deltaG = 1nFEcell. You can use any equation to find delta G then, apply this equation to find electrochemical variable values or vice versa.

Eugene Chung 3F

I just jot down all the equations that are relevant to the question and try to mix match them to figure out the variable I'm looking for.

Eugene Chung 3F

electrons will flow from area of high concentration to an area of low concentration. So, the concentration difference makes potential.

Eugene Chung 3F

n is for the moles of electrons being transferred.

Eugene Chung 3F

I think you just add hydrogen ion H+ where needed.

Eugene Chung 3F

Brandon Tao 1K wrote:Is a reaction within a galvanic cell, will it ever reach equilibrium and will we need to find the constants?

I think equilibrium is reached when Ecell = 0 V. This would mean that there is no net electron flow within the cell and the current has stopped.

Eugene Chung 3F

salt bridge maintains the charge balance b/c the electrons are moving from one half cell to the other half cell. the electrons will flow from one adnode to the cathode. So, the reaction reaching equilibrium slows down.

Eugene Chung 3F

Jasleen Kahlon wrote:The Van't Hoff Equation shows temperature dependence of K, but I'm still a little confused on what that means, and does the equation in any way connect to ∆H, ∆S, and ∆G?

deltaG = -RtlnK

and since deltaG = deltaH -T*deltaS, we can rearrange it to solve for deltaH and deltaS.

Eugene Chung 3F

I think Gibbs free energy is the maximum work at constant temperature and pressure if the system is reversible and the work is non-expansion work form a closed system.

Eugene Chung 3F

isocloric is constant volume. This means that there is no work being done. So the work is 0.

Eugene Chung 3F

It'll be from the second page of thermodynamics outline to what we learn next week.

Eugene Chung 3F

1. Go to LA sessions if you can and TA office hours. Those are really helpful.
2. Do the HW problems.

Eugene Chung 3F

I think it's okay to turn in any homework problems you did to study for the midterm.

Eugene Chung 3F

1. delta U = heat+work
2. For reversible system, deltaS of universe equals 0.
3. at 0K for a perfectly ordered molecule, the entropy change is 0

Eugene Chung 3F

heat is the energy transferred from high temperature to lower temperature whereas enthalpy is heat transfer at a constant pressure. The heat added or lost from system gives the change of enthalpy.

Eugene Chung 3F

reversible processes have delta S universe = 0. Since deltaS(sys) + deltaS(surr) = deltaS(universe) = 0, deltaS(sys) = -deltaS(surr).

Eugene Chung 3F

HW question 4.19 says to calculate the molar kinetic energy of Kr at different temperatures. What is the formula for average kinetic energy ? Should we know this for the midterm?

Eugene Chung 3F

If you mean question 4a, you would flip the sign if u flip the chemical equation.

Eugene Chung 3F

I think we have to know what they are at least for calculations. I'm not too sure about concepts. 1st law: delta U = q+w 2nd law: for irreversible process (all real processes), delta S(uni) > 0. (total entropy of surroundings and system increases) 3rd law: for perfect crystal at 0K, deltaS= 0 --> 1 ...

Eugene Chung 3F

205154661_Dis2J wrote:For 4D7, why are we using the equation DeltaH= DeltaU + DeltaNRT? I do not quite understand why we are including DeltaNRT.

We know that delta U = heat + w and that in this case, delta H= heat. We also know that w=-P deltaV. The ideal gas law states PV=nRT. So, -P delta V = -delta nRT.

Eugene Chung 3F

y-axis is pressure. Since irreversible systems are isobaric, pressure is constant.

Eugene Chung 3F

I think the main difference is whether or not C is given per mol/g or just per T. For example, to calculate q of calorimeter, the question would say that the calorimeter has total heat capacity of 80 j/C, in this case it would just be q= CdeltaT

Eugene Chung 3F

residual entropy is the left-over entropy at 0K. (perfect crystal). The entropy in this case would only depend on the ways in which molecule can position/orient themselves.

Eugene Chung 3F

I'm guessing it's because heat capacity is the measure of enthalpy (heat required to be transferred in rising T by 1C) and enthalpy is a state function. Also, heat capacity does not depend on how heat was added or how the temperature changed.

Eugene Chung 3F

I'm not sure but it wouldn't hurt to know! It's pretty easy because irreversible one would just have a straight horizontal line for y-axis because pressure would be constant. Reversible graph has larger surface area below the curve. The surface area represents work. Less work is done for irreversibl...

Eugene Chung 3F

AlyssaYeh_1B wrote:From the data the question gives, I did the calculation: -1500 - (-1300 + 2(-286)) = 312kJ/mol. However, the answer key shows that it's -312kJ/mol. What did I do wrong?

If you add the standard enthalpies up, it should be -1300 +1560 +2(-286) which comes out to -312

Eugene Chung 3F

Dr.Lavelle posted the constants and formulas on 14b class website so you can look at it and fill in formulas that are not on there. I noticed it didn't have W=X^n. Most equations are from another equation so I read through the formula sheet so that I don't have to memorize all of them

Eugene Chung 3F

All spontaneous processes have an increase in entropy. In other words, heat will always flow from hot to cold and never cold to hot spontaneously. So, S(universe) >0 for spontaneous process (2nd law of thermo.). We also know that delta H is negative for exothermic reactions and that delta G = delta ...

Eugene Chung 3F

In reality, all natural (real) processes are irreversible because biochemical systems often use highly irreversible rxns to speed up processes. Also, all spontaneous processes are irreversible. (like hot going to cold) Reversible reactions are the "ideal" and "perfect" system. (e...

Eugene Chung 3F

Hi,

for 4D #7, the problem doesn't give you the temperature but the solutions suggest that it its assumed as 298K (25C).
Do we assume that T is 25C for certain circumstances?

Eugene Chung 3F

Generally, reactions are favorable when they result in a decreased enthalpy and increased entropy of the system. When the reaction both decrease enthalpy and increase entropy, it occurs natually.

Eugene Chung 3F

what exactly is expansion in terms of thermodynamics?

Eugene Chung 3F

In class, we went over calculating w right before it ended. Could someone explain what the microstates are?

Eugene Chung 3F

Could someone explain what the first law of thermodynamics states conceptually? I get how to do the calculations but don't quite get what exactly the law is.

Eugene Chung 3F

How do you know which is the system and which is the surrounding?
For example, for 4A 1, for mercury in a thermometer or gasoline burning in an automobile engine, could someone tell me what is the system and surrounding in these two cases?

Eugene Chung 3F

I'm pretty sure the solution is wrong (I have the same problem). Since [H30][OH]=1*10^-14, the answer would not be what it says in the solutions.

Eugene Chung 3F

If partial pressure of reactant is increased, then, the reaction will favor products to make up for the change and vice versa. If overall pressure is increased, meaning that the volume decreased, the side with less gases will be favored (check the stoichiometric coefficients of the gases) and vice v...

Eugene Chung 3F

I think of it like I do with spectator ion (ex.cl-). ions like Na+, K+, Ca2+ dissociate so in making ICE table, etc, we don't take them into account in terms of calculations

Eugene Chung 3F

I understand a but i don’t understand b.
Do i just approximate the partial pressure for each elements?
Also, how to i figure out the volume?

Eugene Chung 3F

Le Chatelier's principle tells us that chemical reactions adjust to minimize the effect of changes. Therefore, if CO (reactant) partial pressure increases, the reaction will want to minimize the effect of that change and shift left to produce more CO (kinda like compensating). The result of that is ...

Eugene Chung 3F

ICE table can be used to find K, pH, equilibrium concentrations, etc. It gives information about the initial concentrations and the final equilibrium concentration. We can also calculate how much is remaining after the equilibrium reaction also.

Eugene Chung 3F

To give an example, When HCl (strong acid) dissociate in aqueous solution, than the conjugate base is Cl-, an ion produced as a result of HCl's dissociation. It can technically accept protons to produce acid in reverse direction but because HCl is strong, and dissociates completely, the reaction onl...

Eugene Chung 3F

It's okay to make the ICE table with just mol (as opposed to mol/L) and then divide it by volume later. But, to be safe, yes, it is better to convert them all beforehand. So, for mmol, you would just multiply 10^-3 and divide by the volume

Eugene Chung 3F

Since NH4HS is a solid, u can basically ignore it

Eugene Chung 3F

A reaction is exothermic if the products have more energy than the reactants, and it is endothermic if the reactants have more energy than the products. Usually, the reaction would give you a deltaH value if it is asking about temperature. DeltaH would be negative if the reaction is exothermic and ...

Eugene Chung 3F

for example, N2 + 3H2 - 2NH3 equilibrium reaction we went over in lecture is homogenous since the reactants and products are all in gas phase.

Eugene Chung 3F

the c in Kc represents concentration and p partial pressure.

Eugene Chung 3F

8 strong acids: HCl, HBr, HClO4, HI, HNO3, H2SO4
everything else is considered weak acid

Strong bases: basically group 1 and 2 hydroxides

common weak bases: NH3 (ammonia), CH3NH2 (Methylamine), C2H5NH2 (ethylamine), diethylamine, pyridine C5H5N

Eugene Chung 3F

K+ is in group 1 element which is neutral and we can also know this since the K+ is a conjugate acid of a strong base, KOH
and Br- is conjugate base of strong acid, HBr, so we know that it is also neutral.

Eugene Chung 3F

yes, they are used interchangeably

Eugene Chung 3F

We know that BrO- is stronger base since it has a negative charge. This means that when it accepts a proton H+, it will become neutral whereas for C17H19O3N, it will become positively charged. Neutral charge is more favorable.

Eugene Chung 3F

please quote my question so i get a notification, thanks in advance! i know there is something wrong with my thought process, but i cannot figure it out. my thought process is that if a base is weaker, less oh- will be dissociated out. taking the -log of that smaller number will result in a smaller...

Eugene Chung 3F

1. Water can't act as a polydentate because it is too small to form multiple bonds to a transition metal central atom. The lone pairs are both on the same O atom. 2. Carbonate can act as both mono and bidentate. Since it has two O atoms with - charge (lone pairs), it can interact in two places. It d...

Eugene Chung 3F

If you mean how do we rank radicals with stability, I believe having resonance increases stability by delocalizing charge.

Eugene Chung 3F

You would look for ligands that act as polydentate. For example, if the coordination compound has ethylenediamine or en, you would know that it is chelating since en is a bidentate.

Eugene Chung 3F

Which is stronger acid and why? (f) H2CO3 or H2GeO3. I understand how higher electronegativity of C will result in it being the stronger acid, but I thought the acidity also depends on the bond length and that weaker (larger) bonds are stronger (Example in lecture was with H-F, H-Cl, H-Br with H-F b...

Eugene Chung 3F

since the test is cumulative, i assume that calculators are allowed.

Eugene Chung 3F

ligands are electron pair donors.

Eugene Chung 3F

I heard the same. The final distribution can alter your grade by a bit.

Eugene Chung 3F

I heard from past students that the final grade might not be as bad as you think! You should not take the final if you want to retake it however.

Eugene Chung 3F

The final will be cumulative but probably focusing on the latter topics

Eugene Chung 3F

Sigma bond is the initial, or first bond, so basically all the single bonds and one of the double/triple bonds. Pi bond is the other one/ones in the double/ triple bond, or the bond that comes after sigma bond.

Eugene Chung 3F

Bent and angular are use synonymously

Eugene Chung 3F

Most diatomic molecules like o2 are nonpolar.

Eugene Chung 3F

Basically, the VSEPR model is the 3D model of a molecule where all ‘things’ are as far as they can be.

Eugene Chung 3F

I would first draw the lewis structure, identify the vsepr shape to find out if the molecule is symmetrical or not. If it is symmetrical then it is most likely nonpolar since the dipole moments will cancel out. But if it isn’t symmetrical, draw the dipole moment arrows (towards the more electronegat...

Eugene Chung 3F

higher polarizability means there will be stronger LDF. So, the molecules will attract one another more strongly and the melting/ boiling points of these covalent substances will increase with size because bigger the size= higher the polarizability.

Eugene Chung 3F

I think what he was trying emphasize is that stability is preferred and we can observe some patterns for different elements to reach that stability by sharing e-, etc

Eugene Chung 3F

I believe it is a polar covalent bond with ionic character

Eugene Chung 3F

I would focus on Intermolecular forces and VSEPR theory.

Eugene Chung 3F

bond between non-metal and non-metal is usually a covalent bond and between non-metal and metal is ionic bond. Ion-Ion interaction is between ions, the particles with unequal # of protons and electrons (look at charge). Ion-dipole interaction is between ion and someting with partial charge. dipole d...

Eugene Chung 3F

Writing out the electron configuration for Cu, there is one valence electron: [Ar] 3d10 4s1

Eugene Chung 3F

There are a video module and assessment questions available on the class website. I find it helpful to work out the problems on there because they deal with both conceptual and calculation problems.

Eugene Chung 3F

I believe it is 100nm to 400nm.

Eugene Chung 3F

Should we be able to know the formula (and the charge) based on the names?

Eugene Chung 3F

Since the general guideline for determining the central atom is to choose the one with lowest ionization energy, I expected O to be the central atom. However, the solution provides N as central. Could anyone explain to me why?

Eugene Chung 3F

During test, they will give you all the constants you need in the front page. I would try to use the ones that are given by the class on the front page.

Eugene Chung 3F

I tried using this formula first,
1.14 x 10^14 = 3.29x10^15((1/n^2)- (1/16)
But i ended up getting n=3.

So i just tried with n=5 and n=6 and chose 6.
Is the above formula correct? Or am i missing something?

Eugene Chung 3F

I tried using this formula first,
1.14 x 10^14 = 3.29x10^15((1/n^2)- (1/16)
But i ended up getting n=3.

So i just tried with n=5 and n=6 and chose 6.
Is the above formula correct? Or am i missing something?

Eugene Chung 3F

Formal charge of an atom indicates gain/loss of electrons while forming covalent bond. So, to assign formal charge of an atom, first, determine the number of valence electrons for each bound atom, then compare the number of bounding (shared, S) electron and the number of lone pairs (L) of free atom ...

Eugene Chung 3F

okay thanks! One more thing, if the answer was 10^-5, would it still be reasonable?

Eugene Chung 3F

I got 1.00 x 10^-5 m/s and thought it was reasonable since it is smaller than the speed of light. Could anyone explain why this is wrong?

Eugene Chung 3F

is the answer D? I feel like A, B, C are right because they explain the main photoelectric effect equation showing conservation of energy

Eugene Chung 3F

thank you so much!

Eugene Chung 3F

Since the work function is 150.6 kJ, I just converted in from KJ to J by multiplying10^3 and chose B.
Can someone help me understand how to solve this problem?

Eugene Chung 3F

This image should help!

Eugene Chung 3F

i noticed in some HW questions, it includes more than just first row of d-block. So, it might be helpful to make sure you understand how to do it.

Eugene Chung 3F

in terms of calculations in this class, I believe it doesn't matter. But, it would help a lot in the future to know that they are two different concepts for Physics.

Eugene Chung 3F

So, from what I understand, the Bohr Frequency Condition and equation for calculating energy of different n levels are specific to H atoms. Professor Lavelle mentioned C-atom during the lecture and I was wondering if we know the condition for C-atoms? Just to make sure, are these condition and equat...

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