CHEMISTRY COMMUNITY

Kyle_Phong_2J

Michelle Jin 3L wrote:Hi! First of all, e and h actually don't result in a linear plot, but f and g definitely do.

Hi,

Can you explain how you figured that out?

Kyle_Phong_2J

For this textbook question, can someone explain how to tell if parts e, f, g, and h will result in linear plots?

Kyle_Phong_2J

When temperature increases, molecules have more energy and collide more frequently, so I think A would increase as well. Ea is independent of temperature because it would not increase or decrease based on temperature and rather temperature may provide energy to overcome the activation energy barrier.

Kyle_Phong_2J

Hi,

Dr. Lavelle has a good doc that talks about different rate laws: https://lavelle.chem.ucla.edu/wp-conten ... amples.pdf

Kyle_Phong_2J

I read an old chem community post about a similar concept and other students assumed catalysts would be more efficient at high temperatures because molecules would have more energy and collide more frequently. However, there would likely be a limit to the temperature as proteins can denature.

Kyle_Phong_2J

We don't write H2O in the cell diagram because it is assumed with the OH- and H+ being aqueous.

Kyle_Phong_2J

For this textbook question, I got up to the point where lnQ=15, but when I take e^lnQ = e^15, I get Q=3269017. How does Q = 10^6?

Kyle_Phong_2J

How would you determine Q in this case?

Kyle_Phong_2J

Can someone explain why strong reducing agents have more negative standard reduction potentials in a conceptual sense? Also, is there a periodic table trend for reducing and oxidizing agents?

Kyle_Phong_2J

Assuming we add Pt(s) as a conductor because the reactants are not good electrical conductors, does the cell diagram always have the single line between Pt(s) and the reactants? I thought the single line was only for if there is a phase change, but what if the reactants were also solid?

Kyle_Phong_2J

For the anode reaction, they have Au(s) -> Au3+(aq) + 3 e-. With the half reactions that you have, it adds up to 3 Au+(aq) -> Au3+(aq) + 2 Au(s) which isn't the correct reaction.

Kyle_Phong_2J

For this textbook question, I'm confused why KOH appears in the cell diagram.

Kyle_Phong_2J

In acidic solutions, you balance O using H2O and H using H+. In basic solutions, you balance O using H2O, H using H2O, and add OH- to the opposite side of where the electrons are.

Kyle_Phong_2J

First and second order reaction plots have different y-axes and slopes. First order reactions have the y-axis as ln[A] as well as a negative linear slope while second order reactions have the y-axis as 1/[A] and a positive linear slope.

Kyle_Phong_2J

What is an example of a zero order reaction when the reaction rate is independent of the concentration of reactants?

Kyle_Phong_2J

We determine the order of a reaction when we want to see the relationship between the rate of the reaction and the concentration of reactants. First and second order reactions tell us different rates depending on how many reactants we're looking at.

Kyle_Phong_2J

Can someone clarify conceptually what's the difference between first and second order reactions?

Kyle_Phong_2J

Hi Katie,

I think you need to put the - sign of e- in superscript notation. Let me know if that works!

Kyle_Phong_2J

When Dr. Lavelle was discussing ion-selective electrodes, he mentioned something about E cell being measured and converting to H3O+. Can someone explain what he meant by this?

Kyle_Phong_2J

Cations have already lose e- and are positively charged, so they're less likely to accept more e- (low standard reduction potential). The reverse is the same for anions which have gained e- and are negatively charged, so they're more likely to accept more e- (high standard reduction potential).

Kyle_Phong_2J

I'm not sure if we mentioned other electrodes, but platinum is a good conductor that doesn't interfere with the reaction by reacting with other substances or undergoing chemical changes.

Kyle_Phong_2J

Hi,

I believe it is the other way around where if E cell is negative, K is less than 1 and if E cell is positive, K is greater than 1.

Kyle_Phong_2J

Can someone explain why we're only using E naught instead of E? How does this affect our answer if we use E?

Kyle_Phong_2J

Hi,

Will there be a key posted for this week's worksheet? Hope you are feeling better!

Kyle_Phong_2J

I reached the point where -530.4 kJ/mol produces 2 mol of sulfur, but I'm confused how they got to the final answer of -4.96 x 10^5 kJ. I multiplied -530.4 kJ/mol by the 1870 mol (calculated at the beginning) but that doesn't give the same answer.

Kyle_Phong_2J

4.19 in the textbook asks about average kinetic energy and I'm confused how they get the equation average kinetic energy = 3/2*RT. Can someone explain this more conceptually and did we derive this in class?

Kyle_Phong_2J

Hi, your approach is right, so make sure you are using the correct enthalpy values. Also, remember to multiply the standard enthalpy for H2O by 2 since there are 2 moles of this if you forgot.

Kyle_Phong_2J

For 4.15 in the textbook, I'm not sure why there is a negative sign when they use -4.184 J/degrees C*g as the specific heat of water.

Kyle_Phong_2J

Most of the time, I think we use Kelvin, but there are sometimes we use Celsius depending on the formulas we're working with. For instance, we might be doing q=m*c*deltaT where the c's units can be J/degrees C*g. I'm not sure what you mean by when they are equal to each other since Kelvin = Celsius ...

Kyle_Phong_2J

For this question, I solved it using q=mcdeltaT where the specific heat is the unknown variable. However, I assumed m to be 1 and I'm not sure if we are supposed to do that or if so, how do we know to do this?

Kyle_Phong_2J

At equilibrium, delta G = 0. We can visualize this with that curve Dr. Lavelle drew where the x-axis is the process and the y-axis is G (free energy). At the minimum of the graph, delta G = 0 which means there is no free energy for work to be done.

Kyle_Phong_2J

Hi,

I think you are correct that it would apply to other substances since we are using a general formula.

Kyle_Phong_2J

The first thing to do is to convert W to J/s. Then, multiply this value by the time heated to get the energy supplied in J.

Kyle_Phong_2J

We discussed in lecture today how if delta S total > 0, the process is spontaneous (favorable). When Dr. Lavelle says favorable, does that mean the products or favored? What does he mean by this?

Kyle_Phong_2J

Professor Lavelle mentioned how when molecules are more complicated (have more atoms), they have more vibrational and rotational states, and a higher standard reaction entropy. I understand how rotational states work, but can someone explain what vibrational states are conceptually?

Kyle_Phong_2J

How do you tell the difference between a reversible and irreversible expansion?

Kyle_Phong_2J

Endothermic reactions have stronger reactant bonds (which requires more energy to break) and weaker product bonds (require less energy to break). Exothermic reactions have weaker reactant bonds and stronger product bonds. We can use the table of relative bond strengths to see whether the reactants o...

Kyle_Phong_2J

When pressure is increased, the system favors the side with fewer moles and vice versa. In heterogenous reactions, we would ignore reactants and products that are in the solid and liquid states.

Kyle_Phong_2J

I'm watching the lecture for today and I'm not understanding why Dr. Lavelle put the 'Small' error in S for Thermodynamic Property and 'Large' error in S for Statistical. Can someone elaborate on this?

Kyle_Phong_2J

When pressure increases, the gas molecules move faster and are more likely to collide with one another. Having more moles of gas molecules means that there would be more collisions, so the equilibrium shifts toward the side with less moles so there are fewer collisions.

Kyle_Phong_2J

pH + pOH = 14
pKa + pKb = pKw = 14
pKa = -log[Ka] and pKb = -log[Kb]
When we are given one value, we can often solve for the other value such as being given pH = 3 so pOH would be 11.

Kyle_Phong_2J

For this textbook question, there is initially 200 mL of solution and 5 mL of that is transferred elsewhere and diluted to 500 mL. How do you know what values to use for this dilution?

Kyle_Phong_2J

Can someone explain how the deprotonation of a weak acid is affected by its concentration? This was something I came across in the textbook (Section 6D).

Kyle_Phong_2J

The autoprotolysis constant of water (Kw) is 10^-14 at 25 degrees C and we know that KA multiplied by KB is Kw which we use in acid base reactions.

Kyle_Phong_2J

In class today, Dr. Lavelle said that there was 0.15 M NH4Cl. When he created the ICE table for NH4+ + H2O -><- NH3 + H3O+, he put the initial concentration of NH4+ as 0.15 M. Why is it the same concentration as NH4Cl and what happened to the Cl?

Kyle_Phong_2J

I think Dr. Lavelle was just reinforcing that the product of KA and KB is KW or 10^-14 (a constant). If KA increases, KB decreases and vice versa. We may need to find KA and KB to determine the strength of acids and bases. For instance, we know that a compound that has a low pKA and high KA is a str...

Kyle_Phong_2J

Hi,

Can someone explain how we can tell it's 2A in the graph? I understand that it's 2C because there are about double the amount of C than B. However, how does A relate to this?

Kyle_Phong_2J

Autoprotolysis is when a molecule like water ionizes itself. 2 H2O -><- H3O+ and OH-

Kyle_Phong_2J

I'm not sure if I am understanding your question, but the concentrations of H3O+ and OH- are equal when there is neutral water. Unless it is neutral water at 25 degrees C, the two concentrations are most likely not equal.

Kyle_Phong_2J

This part asks about whether Cl2 or F2 was more thermodynamically stable relative to its atoms at 1000 K. What does this mean and how can you tell when something is thermodynamically stable?

Kyle_Phong_2J

Since you know it doesn't make sense to have a negative x-value, you can disregard the -0.013. Then, you use 0.011 as x and add all the pressures together for the total pressure at equilibrium. It would look like x + x + 0.071-x since these are the pressures at equilibrium.

Kyle_Phong_2J

For this textbook question, it states that the mixture is at 500 K, but later it is given that K=25. What is the difference between these two K values?

Kyle_Phong_2J

Atm represents atmospheric pressure at sea level. 1 atm = 1,013.25 millibars (a conversion I found online) and I'm unsure if there is a threshold for what's considered high in terms of atm.

Kyle_Phong_2J

I think of it as the equilibrium readjusting itself based on the change of temperature. Since heat is lost, the reactants are favored because they produce heat that makes the overall reaction exothermic.

Kyle_Phong_2J

Can someone give an example of solving for X and one of the values being larger than the initial concentration? I recall the example we did in class had one positive and one negative value for X.

Kyle_Phong_2J

Can someone explain why H2O is omitted in the concentration problems we discussed in lecture?

Kyle_Phong_2J

The conjugate base loses the H+ ion, so it would be (CO3)2- for your example.

Kyle_Phong_2J

Some others that were mentioned are:
- Cr (insulin control)
- Fe (myoglobin, hemoglobin, e- transfer in oxidation of carbohydrates, proteins, fats in respiration chain)
- Mn, Ni, Cu, Zn (enzyme functions)

Kyle_Phong_2J

We can look at the periodic table trend for atomic radius size and see that I > Cl. Therefore, the bond length between H and I is longer and weaker compared to H and Cl. This means it is easier for the H to be donated, so HI is a stronger acid.

Kyle_Phong_2J

(en) is NH2CH2CH2NH2 or ethylenediamine and is a neutral ligand. The ligands that we need to know can be found on Dr. Lavelle's coordination compound sheet: https://lavelle.chem.ucla.edu/wp-conten ... pounds.pdf

Kyle_Phong_2J

Hi,

I don't think we are expected to know the ones higher than hexa.

Kyle_Phong_2J

Hi,

When the complex is at the end, we start the name with the cation (K). The name in this case would be potassium hexacyanoferrate(II).

Kyle_Phong_2J

Hey Brandon,

I believe there is no E at all. Another example we went over in class was CH4 being written as AX4 without an E since there are no lone pairs.

Kyle_Phong_2J

Hi,

My question has chlorine outside of the brackets, so it would not be considered a ligand. However, both chloro and chlorido are the same and chlorido is just the newer name.

Kyle_Phong_2J

The general formula for neutralization reactions is acid + metal hydroxide -> salt + water. For J9 in the textbook, the solutions show a different form. Can someone explain how to approach these questions?

Kyle_Phong_2J

For the textbook question 2.45 Part B, how do we identify the composition of the bonds and hybridization of the lone pairs?

Kyle_Phong_2J

When we went over the equation HCl + H2O ⇌ H3O + Cl in class, Dr. Lavelle mentioned that the arrow is technically more right and a only a little left because HCl is a strong acid. Can someone explain this further?

Kyle_Phong_2J

Ligands are the molecules that attach to the central transition metal atom and they are e- pair donors. You can identify them as the "other things" inside the bracket aside from the transition metal. For instance, [Fe(CN)6]^4- has the ligand (CN)6. Dr. Lavelle has a list of common and impo...

Kyle_Phong_2J

When there are 4 ligands, the coordination complex can be tetrahedral or square-planar. How can we tell which one to use?

Kyle_Phong_2J

I remember Dr. Lavelle mentioning that transition metals often bind to cage-like molecules. Can you explain why this happens?

Kyle_Phong_2J

Hi,

I think we say ammine when NH3 is a ligand.

Kyle_Phong_2J

Can someone explain why the name of the coordination compound for #1 of this week's Achieve has just chloride instead of trichloride since there is Cl3?

Kyle_Phong_2J

Hi,

I think radicals also follow the VSPER model. If there are 2 bonds and 1 unpaired electron, it would result in a bent shape, but would the bond angles be closer to 120 degrees since there is less repulsion than a lone pair?

Kyle_Phong_2J

Can someone explain how transition metal cations can form complexes with H2O?

Kyle_Phong_2J

In the NH3 example, 3 unfilled sp3 orbitals form σ-bond with H 1s orbitals. Why are the hybridized orbitals unfilled? When they form σ-bonds, does this fill the orbitals?

Kyle_Phong_2J

I remember Dr. Lavelle mentioning in yesterday's lecture that it is easy to form coordinate compounds. Can someone explain why this is?

Kyle_Phong_2J

So does this expression only model an attractive force since we want to go to lower energies?

Kyle_Phong_2J

There are a total of 34 e- in SF4. If we draw the lewis structure, S is the central atom as it has the lowest ionization energy. Then, we place the 4 F's symmetrically around S with a single bond. We can place 3 lone pairs on each F, but this would only add up to 32 e- and we still have 2 e- left. T...

Kyle_Phong_2J

In the interaction potential energy expression -a1*a2/r^6, why is there a negative sign?

Kyle_Phong_2J

Hi Meghan,

Diethyl ether (and ethers in general) do not have hydrogen bonds. If you try drawing the lewis structure, you'll see that H forms covalent bonds with C and O is only bonded with C.

Kyle_Phong_2J

Hi Dominic,

Yes, polarizing power is proportional to the charge of the cation and inversely proportional to the size of the cation. A small cation would have a higher polarizing power since there is a smaller radius and the positive center would have a stronger effect on the anion electrons.

Kyle_Phong_2J

Hi,

When calculating formal charge, we count each e- individually. For example, 2 lone pairs would be considered 4 lone pair electrons, so we use 4 in the formula.

Hope this helps!

Kyle_Phong_2J

Hi Noah,

I can't say what it'll look like on the test, but since hydrogen bonds are a specific type of dipole-dipole interaction, it'll likely have only have hydrogen bonding or dipole-dipole as an answer choice.

Kyle_Phong_2J

Why does N2 shift between partially positive and partially negative? Are the bonded/lone pair electrons moving between the 2 N atoms?

Kyle_Phong_2J

We discussed in lecture how if the difference in electronegativity between 2 elements is greater than or equal to 2, it is an ionic bond. If the difference in electronegativity is less than 1.5, it is a covalent bond.

Hope this helps!

Kyle_Phong_2J

Hi Joseph,

In most cases, ionic bonds are stronger than covalent bonds. I think if the ionic bond has more covalent bond character it may be possible that the covalent triple bond is stronger, but that's a very specific situation.

Hope this helps!

Kyle_Phong_2J

Hi,

I'm still confused about how e- distortion and e- polarizability are related. Can someone elaborate how they are related?

Kyle_Phong_2J

Calculating electronegativity involves subtracting two values on the electronegativity chart that Dr. Lavelle showed us, but formal charge uses the FC = V - (L + S/2) equation.

Kyle_Phong_2J

Hi,

I remember Dr. Lavelle telling us in lecture today that radicals are highly reactive and only exist for a short time. Can someone explain why they only exist for a short time and what happens to them?

Kyle_Phong_2J

Hello,

To find the mass of a helium atom, you take the molar mass and multiply it by 1 mol/Avogadro's number. Remember to also convert this value from g/atom to kg/atom. For the rest of the question, you are correct to convert angstroms to m and plug those values into the uncertainty equation.

Kyle_Phong_2J

When drawing Lewis structures for covalent bonds, I'm confused when you know to put lone pairs vs when to use double and triple bonds. I was told that some elements can't have double bonds with other elements, so I was wondering how you know when to use double/triple bonds and when to draw lone pairs.

Kyle_Phong_2J

Hi Stephanie, I don't have the same Achieve #4 as you, but I will try to help based on the information that you've given. What is the unit for wavelength? Also, the equation E = hc/lambda solves for the energy per photon, but it sounds like the question is asking for the number of photons. If they g...

Kyle_Phong_2J

Hi, If an electron is excited to the next energy level how would that affect the quantum numbers and electron configuration? Let's say that the electron configuration of Be at ground state is 1s^2 2s^2 and an electron from the 2s subshell is excited, would n=3 and the electron configuration be 1s^2 ...

Kyle_Phong_2J

Hi,

If the question doesn't give any more information on the quantum numbers, any of those values for ml would be correct.

Kyle_Phong_2J

Hello,

It is correct to double the velocity if we do not know the direction. For 1B27, it is a typo and the correct answer is in the Student Solutions Manual Errors page where it uses 10 m/s as uncertainty in velocity.

Hope this clarifies things!

Kyle_Phong_2J

Hi,

For #18 on the Post-Assessment, I just ignored the part where the question states, "Assume that we know the position of an electron to an accuracy of 1% of the hydrogen radius" when using the Heisenberg equation. Was the 1% just here to throw us off or does it actually have a purpose?

Kyle_Phong_2J

Yes, this assumes that the energy is the same as in part A. The only part of the question that changes is that there's 5 mg of sodium instead of one atom.

Kyle_Phong_2J

Bohr's frequency condition is the equation E = hν where E is the energy difference when an electron goes from a higher level to a lower level of energy (like n=4 to n=1), h is Planck's constant, and v is the frequency of the photon. We use this equation when trying to find frequency or switching var...

Kyle_Phong_2J

I remember during lecture that we learned a wavelength must be greater than 10^-15 for it to have measurable wavelike properties. However, this is hard for me to understand conceptually. Can someone elaborate on this threshold?

Thank you!

Kyle_Phong_2J

I think it depends on what values are given in the context of the question and what we are trying to find. For instance, if the question is asking for the wavelength of the EM radiation when an electron goes from n=3 to n=1, we would use the equation 1/λ=R(1/n1^2-1/n2^2.

Hope this helps!

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