CHEMISTRY COMMUNITY

Sharon Kim 2A

I think the chart Lavelle showed in the lecture was really helpful because it showed how much energy it took to get to the gas phase. Even though they are at the same temperature, it took different amounts of energy to get to different phases. Also, since the H-bonds are strong, the high amount of e...

Sharon Kim 2A

This entire problem you're working backwards especially on the stoichiometric level. You first need to get the initial concentration of OH- by taking the pOH (0.75) and do the inverse log of 0.75 (10^-0.75), which gives you 0.178M. This is the concentration at the volume of 500ml when it was diluted...

Sharon Kim 2A

Yes, water will not always be the solvent. In one of the example problems, water was included in the expression since it was in aqueous form. Denote that all liquids are the solvent even if it's not water.

Sharon Kim 2A

When the problem says the system is at constant pressure, do we assume that the pressure is the standard 1 atm?

Sharon Kim 2A

I'm pretty sure the Le Chatelier principle will still apply in this case. If the partial pressure of the reactants increase, then the system will shift forward and vice versa.

Sharon Kim 2A

I like to think of it by referring back to the pH scale which was out of 14. Since half the pH scale is either acid or base, [H30+] and [OH-] would be 1x10^-7. Multiplying both the K values gives 1x10^-14. Kw comes into play especially in problems where it gives you the Kb value and need to convert ...

Sharon Kim 2A

If water is not part of the reaction then look for molecules that are in liquid phase. Just know that in K expressions, only molecules that are either in gas phase or aqueous can be included. Not solids or liquids. In some problems, water was actually included in the K expression and ICE table becau...

Sharon Kim 2A

The change in temperature still refers to the Le Chatelier's principle because adding heat still shifts the system. Think of heat being on the reactant side when the reaction is endothermic and heat is on the product side when the reaction is exothermic. Increasing heat in an exothermic reaction wou...

Sharon Kim 2A

This is sort of unrelated to the topic (I don't know how to make a topic on here), but in relation to Friday's lecture if the concentration of the hydronium ion is really small then do you just assume that the pH is 7? What that was Lavelle was saying? Because he gave an example of [H30+] being arou...

Sharon Kim 2A

Usually in order to know where to put the -x/+x would be what the question is giving you. If the question says it is adding more reactants then you know that in the reaction the reactants are going to lose some value of x and the products will gain some value of x, vice versa. It comes does to the L...

Sharon Kim 2A

I'm not sure about it having more moles on the left. Decreasing the volume should increase the concentration but not the amount of moles. Since the reactants have a higher concentration, the reaction will shift to the right (forward) to make Q=K again. Hope this helps!

Sharon Kim 2A

Adding more reactant shouldn't have any effect on the overall reaction itself. When adding more reactant, the reaction will temporarily be out of equilibrium and the excess reactant will be used to form more products to reach the attained ratio. During that process there would be a reaction and then...

Sharon Kim 2A

In the lecture, Lavelle mentioned that even with the pressure changing, if the volume itself isn't changing then it will have no effect on the reaction. Change in volume causes the change in concentration which changes the EQ to adjust back to the attained ratio.I am assuming if the question mention...

Sharon Kim 2A

For ICE tables in this class, I think it would be best to stick with molar concentration because sometimes you can forget to convert it back to M since most problems want it in those units.

Sharon Kim 2A

Does anyone know what Lavelle meant when heating exothermic reactions it favors the endothermic reactions?

Sharon Kim 2A

Elements starting from the 3p orbital on and elements that are able to have d-orbitals can have expanded octets. Elements that have less than octets are Group 13 elements common ones are (Al, B), free radicals (CH3), any sort of Lewis acid/base molecules. Hope this helps!

Sharon Kim 2A

We know if the ions have a larger radius is they have a less positive charge. If they have a high positive charge then that means the ion lost electrons and electrons is a key part that makes the size of ions and atoms because of repulsion. Which is the electron cloud.

Sharon Kim 2A

To determine is a salt is acidic or basic, what I do is look at the compound and see if either the cation or anion came from either a strong acid or strong base. For example NH4ClO4, CLO4 is an anion that came from a strong acid. It would be best to memorize the list of strong acids and bases and re...

Sharon Kim 2A

Usually for determining whether something is acidic or basic look for charges and protons that it accept or donate. So for acids, you should look for any hydrogens it can give away and look for positive charges. For bases, usually the it is Group 1 and 2 elements and seeing any OH- is a good indicat...

Sharon Kim 2A

I think what you are saying is correct! What I usually like to think for determining the acidity or how basic a salt is is seeing the the formula which element is usually from a strong acid or base because they will dominate over the element that is "weaker". They are usually the determine...

Sharon Kim 2A

Make sure that bond angles will be smaller because of lone pair repulsion for the more complicated structures. They won't be the standard 120, 109.5 angles.

Sharon Kim 2A

Since there are two chlorines in the compound, it would have to be "dichloro". So the negative charge would add up to be -3 instead of -2. Since the overall charge of the coordination compound is 0, we know that the oxidation number of cobalt would have to be +3.

Sharon Kim 2A

Yes, going off of the previous post. Coordination number corresponds to the number of BONDED atoms to the transition metals.

Sharon Kim 2A

An easy way for me to find the oxidation number is to find the anion charge first and determine what the overall charge of the coordinate compound is. Whether it is 0 or -2, -1, etc. Using that to find the oxidation number so that the charges in the compound matches the overall charge.

Sharon Kim 2A

Usually with HF, it is considered to be a weak acid since the bond length is shorter. Meaning it will not dissociate as easily. I think Dr.Lavelle used HCl since it was a better example to use. I would just think with HF, it may not be as easy but I wouldn't say it is impossible.

Sharon Kim 2A

To find the coordination number is the number of bonded atoms to the transition metal. In other words, look for the number of bonds the coordination compound has. Hope this helps!

Sharon Kim 2A

We know that there are octet exceptions starting from the 3rd p-block. An element that is able to attain a d-orbital usually starting from P and on are expanded octets. Elements that have less than an octet would be Group 13 elements, radicals, H, He, Li, and Be.

Sharon Kim 2A

So far we have seen many ring structures with multiple carbons. So if there is a molecule that has a lot of carbons and you know that there is resonance, maybe that is a clue that there might be a ring structure.

Sharon Kim 2A

The number of ligands would correspond to the number of atoms (Lewis base) that are attached to the cation (central atom). So the number of bonds that molecule has.

Sharon Kim 2A

I was thinking of when Professor Lavelle mentioned the cis and trans molecule. How one cancels out and one causes a dipole moment. So I am assuming yes? Considering if the molecule only had sigma bonds. If there are pi bonds I am assuming no because structure will remain how it is because it cannot ...

Sharon Kim 2A

In today's lecture 11/23, does anyone know why in the example of ethene (C2H4) after explaining the unhybridized p-orbital he wrote sp^3 around carbon. Was it no longer sp^2 even though carbon still only has 3 electron densities?

Sharon Kim 2A

Usually you should look for symmetry and atoms that have different electronegativities within the structure. If dipole moments do not cancel it is safe to assume there are polar bonds.

Sharon Kim 2A

Can I3- can be bent as well? I was thinking that since lone pairs have a stronger force in repulsion that it will cause the structure to have a bent shape. Can someone explain more as to why it is linear?

Sharon Kim 2A

The higher the boiling/melting point indicates that it takes more energy to break the bonds (stronger intermolecular forces).

Sharon Kim 2A

Usually when referring to the strength of the bonds, it relates to the length of the bonds. However, the larger the difference in electronegativity the stronger the attraction. I am assuming that it will resemble more of an ionic character.

Sharon Kim 2A

I didn't see one electron from the s orbital go to the p orbital. In the lecture he included all the 4 valence electrons of carbon. He combined the one s orbital and the 3 p orbitals to make 4 atomic orbitals. In the 4 atomic orbitals, he fit all the 4 valence electrons in each orbital. All of this ...

Sharon Kim 2A

Yes! It is more favorable for the charges to be 0 or at least very close to 0 like a charge that it is -1. The formal charges should add up to the overall.

Sharon Kim 2A

As far as I know, I am assuming no. Usually they would give the formula already with the charges so that we will know how to draw the lewis structures and relate them to formal charges.

Sharon Kim 2A

Technically, London Dispersion forces are always present. It is a matter of what forces are dominating in those interactions. Such as London Dispersion forces being present in ionic bonds but the ionic forces are more dominant.

Sharon Kim 2A

I would put the atoms with the least ionization energies in the center and go based on the formula given in order. Carbon should be in the center as a row and be attached the H and O.

Sharon Kim 2A

When writing the electron configurations, I understood it to be written as 3d orbitals before the 4s. Is that the same thing as saying 3d orbitals are filled before 4s?

Sharon Kim 2A

The difference induced dipole-dipole and dipole-dipole is that with induced dipole-dipole interactions have non polar and polar molecules interacting such as HCL and N2. Induced dipole- dipole have a temporary dipole moment where there is a distortion with the electron cloud. While dipole-dipole are...

Sharon Kim 2A

Since he is giving an introduction to acids and bases, he didn't go as far to talk about the amount of hydrogen ions. For now he mainly focused on the concept of Lewis acids accepting electrons and Lewis base donating electrons.

Sharon Kim 2A

We usually use them to see which lewis structure is best to use because the formal charges tells you which ones are the most stable. Finding the structure that have zero charge or close to zero is preferable.

Sharon Kim 2A

Usually, by looking at one structure you can recognize where certain bonds can be placed or replacing a lone pair to make a double bond. I think by practicing with many Lewis structures, overtime you'll just remember which structures have resonance. However, I wouldn't say there's a definite shortcut.

Sharon Kim 2A

Yes, even in polar covalent in the sharing region there is an unequal sharing of electrons making one side partial negative and partial positive. As Professor Lavelle said, it is very unlikely to ever find a pure covalent bond. I think it goes for both concepts too like distortion. Such as ionic bon...

Sharon Kim 2A

Usually, I think of it as electronegativity, ionization, affinity gets stronger an arrow goes diagonally up right of the periodic table. Then for atomic radius (size) increasing, I think of an arrow going diagonally down to the left of the table. That has helped me through the years I took chem!

Sharon Kim 2A

Yes, I was having a lot of trouble with that problem as well because I didn't understand exactly what the problem was asking. But you have to count up the number of wavelength cycles that are complete/whole. The ones that cut short are not compatible.

Sharon Kim 2A

For this question, it gave the energy value of the ejected electron in eV. To get the unit into eV/atoms do we just multiply by 6.022E23? I don't know if I would do that though because it is over moles. Is doing that technically making the units over atoms?

Sharon Kim 2A

I think my TA said that for this class, there are no competition between students so there won't be a curve for exams.

Sharon Kim 2A

Are nodal planes basically the "axis" lines in the model?

Sharon Kim 2A

I was thinking that if the light has a higher frequency, the energy emitted would be the same as long as the the energy (frequency) is reaching the energy threshold. The same energy would be emitted that was initially absorbed.

Sharon Kim 2A

Does anyone know the concept of 4s being a higher energy level? I was thinking that ground states have higher energy since it is the closest to the nucleus, but why does 4s or any level higher than the ground state have the title of "higher energy level"? Is there a difference between ener...

Sharon Kim 2A

Was the midterm review session schedule posted anywhere on the website? The link was there before and the date still had to be posted, but now it's not there.

Sharon Kim 2A

I still do not understand how the energy level at 5, n = 5 and l = 0. I am still going off go how l is based off of n values like "n-1". I can't seem to find the connection.

Sharon Kim 2A

Usually the final answer should in nm? It's just in the problem the wavelength comes out be in m initially.

Sharon Kim 2A

The speed of light equation (c = lambda * frequency) was used to derive the De Broglie equation. With that equation, it was derived by the combination of Energy = pc and E = hv. The speed of light equation was rearranged to be v = c/wavelength. From E=hv (substituting v), we get E=hc/wavelength. Mak...

Sharon Kim 2A

Usually the SI unit for the wavelength that comes from the speed of light formula is in m. However, for most of the questions, I've found they want the answer to be in nm. When going to from m to nm, multiply by 1E9 and from nm to m multiply 1E-9.

Sharon Kim 2A

For these series, basically do we need to know that for UV it has longer lengths for Lyman series going from n=1 to n=2 and for Balmer it is shorter and starts from n=3 to n=2 to n=1. Not completely sure about this concept.

Sharon Kim 2A

Not completely related to this topic but will the question tell you if light is acting like a particle or wave? Or do we have to figure it out somehow?

Sharon Kim 2A

Not sure either but I'm pretty sure for visible light we should know that it ranges from about 400nm to 700nm.

Sharon Kim 2A

I was confused as to why there were two separate energies in this problem. For the formula E=hc/l is E the total energy from the light source or the energy per photon? I was confused since there was the 32 j/sec and then the formula was used to find another energy value.

Sharon Kim 2A

To add on, even though it may seem obvious, I think it is so important to take advantage of the office hours and peer sessions opportunities! But not just show up. Do the textbook problems, Sapling hw, and review over the lectures. Come prepared with questions that target weak areas of understanding.

Sharon Kim 2A

To change it to integers, normally wouldn't you multiply the entire equation by the denominator of the coefficient that's not a whole number

Sharon Kim 2A

I usually write the magnesium sulfate formula as you would and write out how many H20 there are by the prefixes. Same goes for all other formulas that have hydrate next to it.

Sharon Kim 2A

When doing sig figs I always look for leading zeros and when finalizing an answer I've learned it is always best to go for the smallest sig fig given in the problem. But if for some reason it isn't clear, three sig figs are a safer option.

Sharon Kim 2A

In some of the more difficult types of problems, would the mass of the products change because some of it will go off as gas?

Sharon Kim 2A

So there is no zoom link or a video under the media sources tab?

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