CHEMISTRY COMMUNITY

Tiffany Chao 2H

Thank you Dr. Lavelle! I can't say that this class was easy or didn't make me stay up until ungodly hours of the night studying, but it's been a satisfying and enriching experience. I learned more than I ever thought I could within 2 quarters. Your jokes, music outros, smiley faces, and thoughtful p...

Tiffany Chao 2H

Thank you so much for all your hard work Lyndon! You have helped us tremendously and I can't thank you enough. Best of luck to you!

Tiffany Chao 2H

What is going to be done in regards to lecture, discussion, and our final?

Tiffany Chao 2H

Never mind... I figured out that it's because they switch between ln([A0]/[A]) or ln([A]/[A0]).

Tiffany Chao 2H

I'm doing 7B and I've noticed that for first order integrated law, the solutions manual will use the equation ln([A]/[A0]) = kt and other times, ln([A]/[A0]) = -kt. I've been consistently using ln([A]/[A0]) = -kt and will sometimes get a negative value (depending on if I'm calculating k or t). When ...

Tiffany Chao 2H

If you are provided delta G, R, T, and Q, then you can find standard delta G. However, if you are given K, it means that the reaction has reached equilibrium and delta G = 0, therefore you would get the equation standard delta G = -RTlnK.

Tiffany Chao 2H

Typically, ions are usually aqueous. We can't have an electrode that is aqueous, it needs to be a solid. This is when we would use an inert electrode such as platinum or graphite.

Tiffany Chao 2H

Next week in your discussion section.

Tiffany Chao 2H

Congratulations Rachana!

Tiffany Chao 2H

When you have ions (which are aqueous). We can't have an electrode that aqueous so we can use an inert metal solid such as platinum or graphite. For example, if you have Fe 2+ and Fe, you can have Fe (s) to be the electrode. But if you have Fe 2+ and Fe 3+, you need platinum. There's no difference b...

Tiffany Chao 2H

Can someone explain to me what ion-selective electrodes are and how they work?

Thanks

Tiffany Chao 2H

this is the half-reaction for the cathode. We know this because there are electrons being added to the reactants, reducing the Fluorine. Sorry I meant F2 (g) + 2 e- --> 2F- is the cathode and F2(g) + 2 H+ (aq) + 2 e- --> 2HF (aq) is the anode according to the solutions manual. It says the E*cell = ...

Tiffany Chao 2H

6.57: 6.57 Use the data in Appendix 2B and the fact that, for the half- reaction F2(g) + 2 H+ (aq) + 2 e- --> 2HF (aq), E* = + 3.03 V, to calculate the value of Ka for HF.

Based on F2 (g) + 2 e- --> 2F-, E*= +2.87V, how am I suppose to know which one is the cathode and anode?

Thanks

Tiffany Chao 2H

Whenever the problem asks for a calculation that requires the Nernst's equation, the solution manual never shows n (the number of moles) to be in the units of moles. For example, if the redox reaction used 2 moles, it would be 0.0257 V/2 instead of 0.0257 V/2 moles. Is there a reason why for this? W...

Tiffany Chao 2H

6N.1.b) Calculate the equilibrium constants for the following reactions: In^3+ (aq) + U^3+ (aq) <--> In^2+ (aq) + U^4+ (aq). My answer differs from the solutions manual. I got 107 instead of the textbook that got 1000 for K. However, I don't understand how they got 2 electrons for the Indium half re...

Tiffany Chao 2H

I was talking to my TA in section today and he said that there are a lot of errors in the solutions manual in this section along with 6L. He said that if some things just don't make sense at all, be wary of it.

Tiffany Chao 2H

6L.7.a) Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions: AgBr(s) <--> Ag+ (aq) + Br- (aq). The solutions manual says the half reactions are Ag+(aq) + e- --> Ag(s) and AgBr(s) + e- --> Ag(s) + Br-(aq) but I'm not sure how they were a...

Tiffany Chao 2H

6K.3.d) Balance each of the following skeletal equations by using oxidation and reduction half-reactions. All the reactions take place in acidic solution. Identify the oxidizing agent and reducing agent in each reaction. Reaction of chlorine in water: Cl2(g) --> HClO(aq) + Cl2(g) How should I procee...

Tiffany Chao 2H

Is there a difference between the two? In addition, the oxidizing agent versus oxidized species.

Thanks,

Tiffany Chao

Tiffany Chao 2H

The end of thermodynamics (like more in-depth on Gibbs Free Energy) and electrochemistry.

Tiffany Chao 2H

You're likely referring to R. In that case, you would use the constant that cancels out your units.

Tiffany Chao 2H

We calculate delta G by doing G of products - G of reactants. If at equilibrium, the G between products and reactants doesn't change. Therefore, delta G = 0

Tiffany Chao 2H

Room temperature - 298 K

Tiffany Chao 2H

I keep getting -2.7 kJ/mol as my answer. However, the book says -27 kJ/mol. Is anyone else getting the same answer as me or did I just calculate something wrong.

Thanks

Tiffany Chao 2H

I think you may be referring to the physics version of the equation. It's technically the same equation, but depends on the perspective of the system for work. I don't think you should worry about that; just focus on the one we have which is delta U = q+w !

Hope this helped

Tiffany Chao 2H

Makes sense! Thank you!

Tiffany Chao 2H

I get that exothermic means a -delta H but idk how to interpret the delta S part.

Thanks

Tiffany Chao 2H

Why does temperature change along an irreversible pathway? Why does it decrease then increase?

Thanks

Tiffany Chao 2H

In my notes, it says that it's because delta U = 3/2(nR*delta T) and if delta T = 0, then delta U will be 0. I can understand that logic but where did that delta U = 3/2(nR*delta T) equation come from? And why is it 0?

Thanks

Tiffany Chao 2H

The mass should typically be in moles too but just double check it. For example, the enthalpy of fusion for water is known as 6.01 kJ/mol (I believe) so you would need to multiply it by moles.

Tiffany Chao 2H

I don't think we'll be expected to know how to derive it.

Tiffany Chao 2H

Constant pressure may also be called isobaric and constant volume may be called isochoric.

Tiffany Chao 2H

For 4.19, we're supposed to use average kinetic energy = 3/2*RT. I did 3/2(8.314 J/mol.K)(328.85K) and got 4101.09 J/moles. However, the answer key says 4103.2 J/mol. I'm off by a pretty significant amount and I'm not sure what I'm doing wrong since it seems to be a pretty straightforward calculatio...

Tiffany Chao 2H

Can someone help explain how to do 4.15 to me? I don't understand why we have to find the limiting reagent and the tabulated enthalpies of formation.

Thanks!

Tiffany Chao 2H

I'm doing some heat problems from 4A. I've noticed that to calculate q, sometimes you do q = mass x specific heat capacity x delta T. Other times, it's just q = specific heat capacity x delta T. When do you know to multiply mass or not?

Thanks

Tiffany Chao 2H

This might be a dumb question, but is a negative sign in front of the P*delta V always there? And then if the volume is decreasing (so -deltaV), then work is positive?

Thanks

Tiffany Chao 2H

How do I know when to put a negative or positive sign when calculating work on a system? For example, pushing a piston into a system?

Tiffany Chao 2H

Am I supposed to use this equation to find internal energy of a system that's at constant pressure?

Thanks

Tiffany Chao 2H

If delta H = qp under constant pressure, what's the difference between them? Like when should I use delta H and qp if the system is under constant pressure or does it not matter?

Thanks

Tiffany Chao 2H

Pressure should be the pressure of the atmosphere

Tiffany Chao 2H

Yes. Strong base = weak conj. acid. Weak base = strong conj. acid. And the same for strong/weak acids.

Tiffany Chao 2H

The reaction is a base + water. Therefore, it's important that you use Kb to figure out this question. From then, you can find pOH and then pH.

Hope this helped!

Tiffany Chao 2H

Nope just be careful that you use Ka for acids and Kb for bases.

Tiffany Chao 2H

In our notes it says if [H3O+] < 10^-7, then the solution is considered neutral. But if it's less than 10^-7, wouldn't it be considered basic? I don't understand what Dr. Lavelle meant by "neutral".

Thanks

Tiffany Chao 2H

Here: download/file.php?id=5559

Tiffany Chao 2H

Doesn't change in concentration not affect K as long as temperature is constant? And if every concentration in the reaction is changed by the same amount, it shouldn't make a difference? I understand it from a mathematical perspective but I don't understand it conceptually.

Thanks

Tiffany Chao 2H

I heard it'll be available during Week 3 as well!

Tiffany Chao 2H

Rohit Ghosh 4F wrote:You can then use the partial pressures given by the graph along with the reaction A (g) --> B (g) + C (g) to calculate the equilibrium constant.

The answer key says the chemical equation is 2A (g) --> B (g) + 2C (g)

Tiffany Chao 2H

Selena Yu 1H wrote:It says in the problem that compound A decomposes into compounds B and C. So the chemical equation would be A (g) --> B (g) + C (g)

The answer key says the chemical equation is 2A (g) --> B (g) + 2C (g)

Tiffany Chao 2H

For 6B.9 i and ii, I keep getting different answers from the answer key. For i), I do the negative log of 1.50 and get the pH to be -0.176, [OH-] to be 6.67x10^-15, and pOH to be 14.17. This doesn't match the textbook that says pH = 0.176, [OH-] = 1.50x10^-14, and pOH = 13.824. I just don't know wha...

Tiffany Chao 2H

I'm not understanding how to find the chemical equation on 5.35. Can someone help me explain?

Thanks

Tiffany Chao 2H

After I do the ICE table and try to calculate the equilibrium constant, I'm having trouble. For Kc, I have written Kc = (2x)^2/(0.114-x)^2 = 1.00 x 10^-5. I originally did this by multiplying it out and then doing quadratic equation. I got an answer, x = 7.52 x 10^-4 but the answer key did a differe...

Tiffany Chao 2H

Kc can also be used for gas problems. It just depends on the units the equilibrium concentrations are in. If its in atm or bars, you would use Kp because it's based on partial pressure. But sometimes gas concentrations can be in molarity too, which would be given in mol/L where you can use Kc to cal...

Tiffany Chao 2H

No because K is a constant. We use brackets around equilibrium concentration. The brackets indicate molar concentration so we omit actual units.

Tiffany Chao 2H

Kc is typically used for molar concentrations for aqueous and gasses. Kp is used for gas equilibrium when we use partial pressure (which will typically be indicated by equilibrium concentration in bars or atm). Q stands for the reaction quotient. On its own, it doesn't mean much. To use it, we compa...

Tiffany Chao 2H

Can someone help me explain how 5H.3 works?

Thanks

Tiffany Chao 2H

Starts on Tuesday, 1/21, and will be in your discussion.

Tiffany Chao 2H

I understand that when we calculate Kc, we omit the units and put brackets instead to indicate molar concentration. However, is it the same rule for when we calculate Kp? Do parentheses indicate gas concentration so therefore we don't need to put units like bars or atms?

Thanks

Tiffany Chao 2H

Can someone explain to me a section of the notes? It was when he asked "Q2: A biological acid, HA, has pKa = 4.22. What's the Ka value of the acid?" I don't understand the part after that says "Is the acid neutral or negatively charged at pH 6?" and the little section under it. T...

Tiffany Chao 2H

I tried looking for a list on Dr. Lavelle's website but I can't find it. Can someone tell me what are the strong and weak acids and bases that we need to know for the class. Thanks!

Tiffany Chao 2H

I don't completely understand why EDTA 4- has 6 bonding sites. I understand that there is 2 from the 2 nitrogen atoms which each has a lone pair. However, there are 4 double bonded oxygen atoms with 2 lone pairs each and 2 single bonded oxygens with 3 lone pairs each. Why is it that the single bonde...

Tiffany Chao 2H

Our notes mention cisplantin for chemotherapy, myoglobin/hemoglobin for oxygen transportion in blood cells, chromium to assist insulin, iron for myoglobin/hemoglobin, cobalt for vitamin B12, and Mn, Ni, Cu, Zn for enzyme function.

Tiffany Chao 2H

Oxides and hydroxides with group 1 and 2 elements. Also, metal oxides react with water to form strong bases.

Tiffany Chao 2H

You would typically use it for weak acids and bases because the ions wouldn't completely dissociate and thus, an equilibrium is formed. A strong acid or base in water would completely dissociate, which is when you would use an arrow.

Tiffany Chao 2H

No. That will be 14B.

Tiffany Chao 2H

I typically start off by writing all of the variables the problem gives me. For example, if it gives me wavelength = 900 nm. Then, I write the variable the problem is asking for, so like energy = ? for example. Lastly, I write out all the equations that contains the variables (like wavelength and en...

Tiffany Chao 2H

J. 9. says: Identify the salt that is produced from the acid–base neutralization reaction between (a) potassium hydroxide and acetic acid, CH 3 COOH; (b) ammonia and phosphoric acid; (c) calcium hydroxide and bromous acid; (d) sodium hydroxide and hydrosulfuric acid, H2S (both H atoms react). Write ...

Tiffany Chao 2H

Can someone help me explain how to do J.17.:
In each of the following salts, either the cation or the anion
is a weak acid or a weak base in water. Write the chemical equation
for the proton transfer reaction of this cation or anion with water:
(a) NaC6H5O; (b) KClO; (c) C5H5NHCl; (d) NH4Br.

Thanks!

Tiffany Chao 2H

Did Dr. Lavelle ask us to use the new naming rules or it doesn't matter? For example, cyano (old) vs cyanido (new). Thanks

Tiffany Chao 2H

I've been hearing mixed answers. What's the correct number of H-bonding sites and what's the explanation? Thanks

Tiffany Chao 2H

What's the difference between a Bronsted acid/base and a Lewis acid/base? It seems that they're opposite of each other but that's making me confused. Thanks

Tiffany Chao 2H

To add onto Mingdah, pH and pOH have an inverse relationship.

Tiffany Chao 2H

It could but I think all that we should know will stay in between 1 and 14.

Tiffany Chao 2H

I am extremely confused while reading the notes for naming coordination compounds. Is anyone able to give me a brief summary to help me better understand the rules?

Thanks

Tiffany Chao 2H

If you're talking about a molecule like CHCl3, it's polar. However, the polarity of it doesn't affect the fact that its tetrahedral and all bond angles are 109.5 degrees. The only time bond angles/shape is affected would be due to lone pairs.

Tiffany Chao 2H

I think the most significant thing we need to know about ion-ion forces is that its the strongest IMF. But I believe the test will mainly focus around dipole-dipole, LDF, and H-bonds.

Tiffany Chao 2H

I believe hydrogen can form H-bonds with any N, O, F that have lone pairs regardless of whether it's already attached to a hydrogen or not.

Tiffany Chao 2H

The one with the bigger molar mass (and more electrons). It allows for more interactions which increases the London Dispersion Force.

Tiffany Chao 2H

Sigma bonds allow for the molecule to rotate while pi bonds don't allow it to rotate because they are side by side. Like Lavelle showed in class, if you put 3 EXPO markers in between your two pointer fingers and twist, two markers would fall (pi) while one marker remains in place (sigma).

Tiffany Chao 2H

Lone pairs create a strong electron-electron repulsion which pushes all other atoms closer to together. For example, if you had a trigonal pyramidal with a lone pair at the top, it would push the atoms that are part of the structure closer together, creating a smaller bond angle between the atoms.

Tiffany Chao 2H

Even if there is, it's easy to remember that wedges mean the bond is towards you and dashes are pointing away from you.

Tiffany Chao 2H

Just treat drawing Lewis structures for molecules as drawing a normal Lewis structure. Afterwards, go onto VSEPR.

Tiffany Chao 2H

You are correct in that they both have London forces. However, CHI3 has stronger London forces because it has more electrons than CHF3, therefore stronger London forces.

Tiffany Chao 2H

I think the most important thing we need to know is that the atoms C, N, O, F must fulfill the octet rule. Any other atoms can either have an expanded octet (or don't fulfill the octet rule at all).

Tiffany Chao 2H

8 but it's likely that there'll be different parts to each questions such as a. b. c. etc.

Tiffany Chao 2H

I highly doubt it. Just know about the concepts that came to light (haha) about the photoelectric experiment (ex. particle-like properties.)

Tiffany Chao 2H

You can put it in any unit you want unless the question specifies what unit it wants your answer to be in.

Tiffany Chao 2H

The smaller radius and highly charged cations have the strongest polarizability.
For anions, larger atomic radius and less electronegativity are more highly polarizable.

Tiffany Chao 2H

I'm in the 12:00 pm lecture. Dr. Lavelle said that we won't be having any class this Wednesday. He just forgot to tell the other lectures.

Tiffany Chao 2H

Hi, I'm getting confused on the difference between ionic character in covalent molecules and covalent character in ionic molecules. Based on my notes, it seems like for both types of molecules, the ionic/covalent character is that electrons are unevenly distributed. For example of ionic character in...

Tiffany Chao 2H

Orbitals don't necessarily have to do with the ionization energy levels. Just know the periodic table trend for ionization energy: The further right you go in a row, the higher the ionization energy is. The further down you go in a column, the lower the ionization energy.

Tiffany Chao 2H

He told me that there are no more video modules for the topics after Heisenberg. He said the best way to get something similar to the video modules was to use Sapling Learning.

Tiffany Chao 2H

The 2+ charge means that 2 electrons are removed. The outermost/highest energy electrons are usually the first to go, thus the removal of the 2 4s electrons.

Tiffany Chao 2H

The atom that goes in the middle is the one with the lowest ionization energy. You can figure that out using the periodic table trends for ionization energy. The further right you go in a row, the higher the ionization energy is. The further down you go in a column, the lower the ionization energy. ...

Tiffany Chao 2H

I think you should just stick with the one Dr. Lavelle gave us. The problem probably wouldn't give you bond length in the first place so you couldn't figure out the formal charge with the equation your TA gave you.

Tiffany Chao 2H

velocity: m/s
threshold energy: J or kg. m^2. s^-2
Planck's constant: 6.626 x 10^-34 Js or kg.m^2. s^-1
frequency: Hz or s^-1

Tiffany Chao 2H

One question.

Tiffany Chao 2H

Depending on the different experiments that have been done, we can determine whether an light acts like a particle or wave depending on the situation. Particle-like property: This can refer to the Photoelectric experiment. So basically, if we shoot light at a piece of metal, it has to surpass the th...

Tiffany Chao 2H

Work function is the threshold energy. It basically means that its the minimum energy needed from the light source to shoot off an electron.

Tiffany Chao 2H

Hi,

F. 21.: 1F.21 Identify the following elements as metals, nonmetals, or
metalloids: (a) lead; (b) sulfur; (c) zinc; (d) silicon; (e) antimony;
(f) cadmium.

Will we have to know the answer for elements like antimony or cadmium?

Thanks

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