CHEMISTRY COMMUNITY

Michelle Wang 4I

No, it is possible that we could be asked to calculate the half life of zero or second order reactions. You just have to be careful because unlike with first order reactions, the inital/starting concentration changes the half life. So for each starting concentration there is a different half life.

Michelle Wang 4I

Typically, reactions that are termolecular are slow since it is less likely for three molecules to collide with the right amount of energy for the reaction to occur.

Michelle Wang 4I

Also, Lyndon said today that if you can add the half reactions and the electrons completely cancel out, then you don't need to convert it to delta G since they are half reactions.

Michelle Wang 4I

Catalysts work by giving an entirely new pathway for the reaction to occur with a lower activation energy. Therefore, the curve would be lower.

Michelle Wang 4I

You would use the Arrhenius equation to relate things like the rate constant with temperature, activation energy and the pre-exponential factor A. There can be questions asking you to solve for one of them given the others.

Michelle Wang 4I

How can we use the Arrhenius equation to see how the activation energy and temperature affect k?

Michelle Wang 4I

The order also goes from left to right on the anode: solid, gas, and aqueous is next to the salt bridge. For the cathode, it would be aqueous, gas, solid.

Michelle Wang 4I

There will be 8 questions total, but there will probably be like part A, B, C, etc for several of the questions.

Michelle Wang 4I

What exactly does it mean if a reaction is zero, first, or second order?

Michelle Wang 4I

For cell diagrams, does there always have to be an aqueous next to the salt bridge?

Michelle Wang 4I

“A reaction with a negative standard reduction potential favors oxidation of the reactant” is true because that means it has a -E value, and therefore it is not likely to be reduced (gain election), it is more likely to be oxidized (lose electrons). “A reactant in a reaction with a very positive sta...

Michelle Wang 4I

For question 15 part C, in the galvanic cell diagram the answer key has
Cd(s) | Cd(OH)2 (s) | KOH (aq) || Ni(OH)3 (s) |Ni(OH)2 | Ni(s)
I don't understand what the purpose of adding KOH to the anode side of the cell diagram is, and also why do we not add it to the cathode side as well?

Michelle Wang 4I

What is the point of a salt bridge?

Michelle Wang 4I

A spontaneous reaction releases free energy, so delta G would be negative.

Michelle Wang 4I

Can someone explain what oxidation and reduction mean?

Michelle Wang 4I

Can someone explain why Gibbs Free Energy is a state function?

Michelle Wang 4I

How come when a gas expands against a vacuum work = 0? What does delta S, delta U, and q equal for this situation as well?

Michelle Wang 4I

What are the effects that constant volume or temperature have on delta U and why?

Michelle Wang 4I

Oops yeah @Michelle I typed it in wrong I meant isolated system; I edited it now! Thank you!

Michelle Wang 4I

When do you use the ideal gas specific heat capacities as well as internal energy (U = 3/2nRT)? Do we have to use that internal energy formula or can we just calculate work and q and add it together like we do for other gases?

Michelle Wang 4I

The second law of thermodynamics says that when energy changes from one form to another, entropy (or disorder) increases. I think this can help us understand how the systems are changing through reactions.

Michelle Wang 4I

An example of a closed system is a beaker that can allow energy to exchange with the surroundings but matter doesn't exchange with the surroundings. An example of an isolated system would be a really well-insulated coffee mug that doesn't allow energy or matter to exchange with surroundings.

Michelle Wang 4I

Yes they are the same and I think you can use both interchangeably.

Michelle Wang 4I

Why are phase changes from solid-->liquid-->vapor endothermic reactions?

Michelle Wang 4I

Yes I believe that is correct.

Michelle Wang 4I

In the section of Prof Lavelle's notes Additional Comments, he said that the bond enthalpies of diatomic molecules are accurate, but the others are averages. Can someone explain why the enthalpies for diatomic molecules are accurate but not for other molecules?

Michelle Wang 4I

I think we don't include water in the equilibrium constant because we ignore pure solids or liquids when we write it out since it doesn't really affect the equilibrium constant.

Michelle Wang 4I

I'm kinda confused by the different between pKa and pH/pKb and pOH. Are they basically the same thing?

Michelle Wang 4I

If you go on Professor Lavelle's website, there is a link that has the constants and equations that will be given to us during the exam, https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Constants_Equations.pdf. You should know equations like K=concentration of products over reaction...

Michelle Wang 4I

The equilibrium constant isn't affected because it is a ratio of the products to reactants. So when you add more products, for example, then more reactants are formed and the ratio remains the same.

Michelle Wang 4I

You ignore pure liquids and solids in the ICE table and K equations.

Michelle Wang 4I

Hi Eruchi, In this situation we don't know the exact change in molarity that occurs in the reaction. We know that when the Cu(NH3)42+ dissociates, it forms 4 mol of NH3 and 1 mol of Cu2+. We use the stoichiometric coefficients of the balanced chemical equations to write the x that is added or taken ...

Michelle Wang 4I

Yes, it is.

Michelle Wang 4I

Please help, I don't understand salts at all. What are they and how do they lower/raise pH of solutions?

Michelle Wang 4I

If a molecule is polar, then one of its intermolecular forces would be dipole-dipole. To see if a molecule is polar, check if the dipole moments cancel through molecular geometry. If not, (and the atoms are not the same, like C-C) then the molecule is polar.

Michelle Wang 4I

I'm not sure, but I don't think we need to be able to identify which one is which just from that. If we know that the complex is flat, however, then we would know it's square pyramidal. Overall though I think just putting either square planar or tetrahedral would be accepted.

Michelle Wang 4I

Since HCl is a strong acid, we can assume that there is 100% dissociation. Since there is .025 M HCl, then there is .025 moles of HCl. When HCl dissociates, there would be .025 moles of H3O+. You would then divide that by the volumes to get the molarity, and use that to solve for the two pH levels.

Michelle Wang 4I

If you have the oxidation state of the transition metal, then you can look at the charges of the ligands and add them together. If the oxidation state of the TM is +3, but the total charge of the ligands is -2, then the total charge of the coordination compound is +1.

Michelle Wang 4I

N2O only has 16 valence electrons, but for there to be a lone pair on central nitrogen with a double bond on oxygen and a single bond with nitrogen, there would be 18 electrons. I think that the stablest structure would actually be a triple bond between the nitrogens, and a single bond on the oxygen...

Michelle Wang 4I

Yes, fluorine is more electronegative so it would have a stronger intramolecular bonds (bonds within the molecule between C and F) and it would require more energy to break this bond over CHI3. However, melting point questions are referring to the strength of the intermolecular forces. In this case,...

Michelle Wang 4I

Can someone explain again how the shape of a molecule can determine whether it is polar or non polar?

Michelle Wang 4I

I’m not positive, but I think that electronegativity increases down a row and decreases down a group with no exceptions. Of course, the last group (noble gases) do not have electronegativity values since they have a stable configuration.

Michelle Wang 4I

H2O has hydrogen bonding due to the large electronegativity difference between H and O, which causes a dipole in the molecule. With multiple molecules, the partial negative charge on the O interacts with the positive H charge, forming a hydrogen bond. This increases the boiling point of H2O since th...

Michelle Wang 4I

Yes, as Swetha said electronegativity is a periodic trend. Electronegativity is how much an atom wants to gain another electron to try to get a full shell. Therefore elements on the top right (fluorine) are the most electronegative since they want a full shell, while elements on the bottom left are ...

Michelle Wang 4I

I'm not positive, but I think I remember Professor Lavelle saying that those are what we have to know for exams. However, I think it is good to know how to find the configuration of all the elements.

Michelle Wang 4I

If the question asks you to draw the Lewis structure of a molecule that has resonance, do you just draw one of the structures or do we need to draw all of them?

Michelle Wang 4I

Do we need to know how to recognize molecules just from the name? I keep getting conflicting information where some TAs tell me I need to be able to recognize the polyatomic ions from the name (like hydrogen sulfate) while others say that the formula will be written in parenthesis so we don't need t...

Michelle Wang 4I

It might have been acceptable in your high school class, but since in Lavelle's lecture he said that the lines were only for bonds then representing the electrons with lines is incorrect. Drawing each dot is somewhat tedious but do it anyways so that you won't be marked off on the midterm/tests!

Michelle Wang 4I

Effective nuclear charge is the attraction between the protons and valence electrons. As the effective nuclear charge increases, the radius of the atom decreases since the valence electrons are being pulled closer to the nucleus. That is why atomic radius decreases from left to right down a row.

Michelle Wang 4I

Electronegativity is the ability of the atom to attract electrons into its orbital. For example, fluorine is the most electronegative element because it wants to reach the stable configuration of neon. Ionization energy is the amount of energy necessary to remove an electron from an atom. The two ar...

Michelle Wang 4I

1 eV is equal to 1.602e-19 J. It's on the constants and equations page, https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

Michelle Wang 4I

If you are talking about the energy of the photon, it is always positive because a photon is basically the form that energy takes when it is released by an electron moving to a lower energy level. A photon cannot be negative energy, as if the electron was absorbing energy to move to a higher energy ...

Michelle Wang 4I

The class is graded on a point basis, out of 500 points. You can find more info on it on the syllabus.

Michelle Wang 4I

The sunday posts you made would count for week 2. The cutoff for the three posts of the week are sunday nights.

Michelle Wang 4I

I didn't know where to post this question, but what date is the midterm? I couldn't find it on the syllabus.

Michelle Wang 4I

I think that the covalent radius is the measure of the size of an atom that forms part of a covalent bond. The atomic radius is just the distance from the center of the nucleus to the electron cloud.

Michelle Wang 4I

To find the moles of O, you first convert the masses given of CO2, H2O, and N2 to find the mass of C, H, and N respectively. Then you subtract the masses of those three elements from the given weight of .376 g to get the mass of O. Use molar mass to get moles of O.

Michelle Wang 4I

Yeah, mass percent composition is the percent of how much one element makes up a molecule by weight. For example in H2O, hydrogen makes up about 11% of the molecule.

Michelle Wang 4I

I think I remember Professor Lavelle saying that for now we don't need to know the phases, but that in the future we will need to know (and write the phases on chemical equations) without having them given to us.

Michelle Wang 4I

I understand how to use the formulas to convert from frequency, wavelength, and the energy of the photon. But I don't get how to identify which event/activity the radiation corresponds with. Can someone explain how you know what event it is?

Michelle Wang 4I

For some reason when I first saw this problem I thought it would be one where we use the formula Minitial x Vinitial = Mfinal x Vfinal since it had to do with preparing a solution. Can someone explain why we don't solve it that way?

Michelle Wang 4I

I found the molecular and empirical formula, but I wrote the answer with Os last instead of first (which is the correct way to write it). Would I be marked off for not writing the proper order of elements, even if I have everything else correct? Is there a way to know what order the elements should ...

Michelle Wang 4I

Is there a particular number of sig figs that we should be using when looking at the periodic table? For example, should we use 16.00 or 15.999 as the atomic mass of oxygen? I know that the difference is slight but I'm just wondering.

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