CHEMISTRY COMMUNITY

Tobie Jessup 2E

Based on the textbook, we are supposed to use H20 and H+ for acidic and H20 and OH- for basic solutions.

Tobie Jessup 2E

Hello! We use oxidation numbers to see where the reduction and oxidation is occurring. For the 8e-, we get this in the process of balancing the equation. When we add 10H+ to the product side of the equation as we balance the hydrogens, we need to make sure that the overall charge of both sides of t...

Tobie Jessup 2E

I think it is mean to be .03 mol because when I tried that I got the correct answer, but I was also very confused on this question.

Tobie Jessup 2E

If if understand correctly, the half-if for first order reactions is not affected by concentration.

Tobie Jessup 2E

The halving of A will cause the rate to half, and then the tripling of B will cause the rate to change by a factor of 9, so overall the rate will increase by 4.5.

Tobie Jessup 2E

K can only be changed if there is a temperature change, it is measure experimentally for a specific reaction.

Tobie Jessup 2E

I think that this was to show that C is a zeroth order, so the question was seeing if we could recognize that even though the concentration of C doesn't end up affecting the rate of the reaction.

Tobie Jessup 2E

Negative orders mean that there is a tendency to form reactants, so the reverse reaction occurs.

Tobie Jessup 2E

If I remember correctly the pseudo rate constant is to make one of the reactants in a second order reaction extremely higher than the other reactant to it almost acts as a first-order reaction.

Tobie Jessup 2E

The analogy my TA gave us was that a chain is only as strong as the weakest link; so a reaction can only go as the slowest part of the reaction.

Tobie Jessup 2E

I used the previous initial rate and multiplied it by 4.5.

Tobie Jessup 2E

In order to balance a redox reaction, we can use the following steps: - separate half-reactions of the overall reaction - balance each half-reaction: - Begin with elements other than O and H. Balance oxygen atoms by adding H2O to the side that requires them. Do the same for hydrogen using hydrogen ...

Tobie Jessup 2E

I honestly had a lot of trouble with just typos and typing the problems in sapling, so make sure all the charges and correct signs/parentheses/brackets are there.

Tobie Jessup 2E

Yes! the oxidizing agent is the one that is being reduced.

Tobie Jessup 2E

Like '227 already said, they often give you those state values in the original equation corresponding to the electrodes compared to the electrolytes/ions. We already know H+ ions and OH- are typically in water, especially at room temp, and I have found that ions are usually aqueous -- which makes s...

Tobie Jessup 2E

I think that having some common ones memorized is important, but I don't think that we are expected to have them all memorized.

Tobie Jessup 2E

delta U is the change in the internal energy of a system while delta H is the change in enthalpy. For reactions with no change in the amount of gas produced, these values are negligibly different.

Tobie Jessup 2E

the second part of the equation is derived from PV=nRT an is needed because it takes into account the amount of moles.

Tobie Jessup 2E

They did this because we need to account for the melting of the water, and then the heating of the water. So there is two terms in the equation, one with a delta H for the fusion of water and one mCdeltaT for the heating of water.

Tobie Jessup 2E

When they say absorbs they are talking about the surroundings, which is why it is negative for the system.

Tobie Jessup 2E

Cp is used under conditions of constant pressure and Cv is used under conditions of constant volume.

Tobie Jessup 2E

yes! make sure to keep track of signs and put negative signs where it is necessary.

Tobie Jessup 2E

The R value must correspond with the units that the problem is using.

Tobie Jessup 2E

I found this really good answer on Quora. Please take look. So in thermodynamics, why do we only care about the external pressure when calculating the work done on a system? The key word here is the system ... it includes the piston itself as well. Work done by the system implies a transfer of ener...

Tobie Jessup 2E

Based on an example I saw in the textbook here there was a position in a vacuum, where there was no wrk done, I think that examples moving forward where objects are in a vacuum means that there is no work done. That wasn't very helpful sorry.

Tobie Jessup 2E

yes you are correct!

Tobie Jessup 2E

I think Dr. Lavelle said that the universe is an isolated system.

Tobie Jessup 2E

If you would recall the first law of thermodynamics being the conservation of energy, energy cannot be created nor can it be destroyed, only transferred between the system and its surroundings. The equation deltaU = q + w represents this fact. It pretty much summarizes the idea that the system can ...

Tobie Jessup 2E

I use the equation c=q/delta T to find the C, which is constant for the calorimeter. Then I used c to solve for q of the second reaction with the same equation since they occurred in the same calorimeter and they give you delta T for that reaction too. in this can q is equal to delta U so thats the ...

Tobie Jessup 2E

I think that pressure can be applied, not im not sure. I know that energy can e transferred through heat, and that matter cannot be transferred in or out of the system.

Tobie Jessup 2E

this problem was tricky for me too. Remember to include all steps of the reaction, which is why they give you the delta H to solve for q. I was also able to find a helpful post if you just look it up by copying and pasting the question.

Tobie Jessup 2E

the concentrations of OH and BH are equal because for each atom/molecule of B, there is one water molecule that donates an H atom to the B molecule and turns into a molecule of OH. Therefore, OH and BH concentrations must be equal because they are connected through the reaction between a base and a ...

Tobie Jessup 2E

this reaction would be endothermic because it requires energy to break the bond between the x atoms.

Tobie Jessup 2E

you cn say x is insignificant when solving the quadratic if the k value is 10^-4 or less.

Tobie Jessup 2E

the stronger the bond, the more energy it takes to break and also the more energy released when that bond is formed. Based on relative bond strengths of the products and reactants, then you can determine if energy will be released or absorbed (exothermic or endothermic).

Tobie Jessup 2E

They are the same thing.

Tobie Jessup 2E

Salts that will increase pH (base) in water; the anion(the negatively charged component) in the salt is the conjugate base of a weak acid. This means that when the salt dissociates in the water, the anion will steal an H from water, producing OH-. Salts that will decrease pH (acid) in water will of...

Tobie Jessup 2E

strong acids and bases completely dissociate in solution, but weak acids and bases do not which is why you need to use ICE tables.

Tobie Jessup 2E

When you solve, make sure to first calculate Kb using the equation Kw=Ka x Kb because that provide Ka, but to solve the problem correctly you need to use Kb.

Tobie Jessup 2E

I think that you had to add the two values because we were finding the concentration of amine after the reaction had already occurred (the products of [BH+] and [OH-] had formed already). This means that to find the original concentration of B, we had to add back the amount that was already protona...

Tobie Jessup 2E

I also had issues with this problem, and I think it is most likely due to a mathematical error with the quadratic formula because your explanation sounds correct!

Tobie Jessup 2E

Hi! To add on, you would not want to pick a value for x that is greater than your initial concentrations because when you factor this in to find the concentrations at equilibrium, you would get a negative value. A negative concentration value would not make sense for the problem, so always choose t...

Tobie Jessup 2E

sometimes I forget to look at the original equation for the necessary stoichiometric coefficients, not sure if that is super helpful.

Tobie Jessup 2E

When I did this problem it helped to look to write each of the individual equations in their K ratios and then compare that to the K ratio of the given equation. For one of the equations I needed to use the reverse, and for another I needed to cube it to get the correct final K once you multiply the...

Tobie Jessup 2E

Hi! So first things first I would set up the ICE table to put that the initial pressure is just PCl5 (meaning the reaction would probably shift to the right since no products were there initially). From there, the change would be -x for PCl5 and +x for both PCl3 and Cl2. From there the equilibrium ...

Tobie Jessup 2E

remember to account for the stoichiometric coefficients! Thats why I had problems with this problem.

Tobie Jessup 2E

You can solve this problem using ICE tables and quadratic formula. 1bar~= 1atm, so no conversion needed, and 0.0220 would be your initial pressure for PCL5 in the ICE table, while PCL2 nand Cl2 would be 0. Solve as you would for any ICE table and then plug in the values into your Kp expression, equ...

Tobie Jessup 2E

my understanding is that homogenous and heterogenous differ in the state of which the products and reactants are in. A certain reaction is heterogenous if there is more than one state of a substance in the reaction.

Tobie Jessup 2E

Thanks everyone! ^

Tobie Jessup 2E

n/V is just moles/volume. It also confused me at first but paying close attention to the actual units it helpful!

Tobie Jessup 2E

You use Kp when all the products and reactants are gases and the question includes units like bar/atm/pa. You use Kc when the products and reactants are given in moles or molarity, even if the products and reactants are gases. If the question asks for Kp and the question only gives you information ...

Tobie Jessup 2E

I was also confused, but I was thinking that this was derived from a trigonal bipyramidal molecule, with 3 lone pairs and 2 atoms attached to the center atom. I'm not completely sure though.

Tobie Jessup 2E

trigonal pyramidal is roughly the same bond angles as a tetrahedral, 109.5, but there is not a 4th peripheral atom in one spot. Seesaw is derived from he trigonal bipyramidal, where there is on lone pair in the plane with 2 other atoms.

Tobie Jessup 2E

Melis Kasaba 2F wrote:I'd probably say all the ones he brought up in lecture. Here's a helpful chart: [img]VSEPR%20Geometries.png[/img]

This is super helpful thank you!

Tobie Jessup 2E

I had the same question as well, glad we are all on the same page!

Tobie Jessup 2E

I was also confused on this shape, but there was a good explanation in the textbook that helped me out!

Tobie Jessup 2E

I'm also a bit confused on the differentiation, can anyone clarify?

Tobie Jessup 2E

Thanks for this new resource, looks like they'll be helpful for the final!

Tobie Jessup 2E

Coordination numbers correspond with the number of atoms bonded to the central atom, so if there are 6 atoms bound to the central atom then it can only be octahedral.

Tobie Jessup 2E

My initial thought for this question was that it wasn't seesaw because seesaw has more than 4 regions of electro density? not completely sure though.

Tobie Jessup 2E

Hi guys, so I know a ligand is anything that attached to a transition metal and gives off a pair of electrons (correct me if I'm wrong), but would that be the formal definition of a ligand? Also, what would be the main importance of ligands? At its simplest, a ligand is something that bonds to a ce...

Tobie Jessup 2E

Hybridization correlates with the number of regions of electron density. This problem is confusing since there is a double bond, but a double bond is only considered to be 1 region of electron density, so for this problem there is 4 regions and therefore the hybridization is sp^3.

Tobie Jessup 2E

Hybridization correlates to the number of regions of electron density! Dr. Lavelle outlines them in the lecture from Monday 11/23 which has been very helpful for me.

Tobie Jessup 2E

Double bonds count as one region of electron density not two! Because of this the As actually only has 4 regions of electron density and therefore hybridization of sp^3.

Tobie Jessup 2E

Joseph Hsing 3H wrote:If you can imagine the Lewis structure in your head from the molecular formula, then you can check for resonance and as a result delocalized pi bonds. I would recommend just drawing out the different structures just to be safe though and more practice.

This was helpful thank you!

Tobie Jessup 2E

Yes! Each bond and each lone pair are considered regions of electron density, where electrons are most likely to be found.

Tobie Jessup 2E

Its been easiest for me to compare the electronegatvities of the atoms to determine in the molecule is polar, but shape is also an indicator as to whether or not an atom is polar.

Tobie Jessup 2E

The strength of intermolecular interactions increase with the number of individual interactions/bonds, so the 3 hydrogen bonds in G-C bonding makes them strong than A-T bonding which only has 2 hydrogen bonds.

Tobie Jessup 2E

I also believe that the question is asking about intramolecular forces based on the language of the question. If the question was wanting to test the students knowledge on IMFs I think there would be specific language about LDFs or other intermolecular forces. The question is most likely questioning...

Tobie Jessup 2E

You should think about how a square pyramidal shape could be formed from any of the basic shapes by removing bonding pairs and replacing them with lone pairs. For instance, you can form a square pyramidal shape by taking an tetrahedral shape and removing any one of the bonding pairs and replacing i...

Tobie Jessup 2E

From one of Dr. Lavelle's lectures, I remember him ranking repulsion strength as so: lone pair-lone pair> lone pair-bonding pair> bonding pair bonding pair.

Tobie Jessup 2E

You pretty much just have to memorize the rules for oxidation numbers. Oxygen always has an oxidation number of -2, except in peroxides and F2O. If you add up the -2 charges of all 4 oxygens, you get -8. In order for the charge of the overall molecule to be -1, Cl must have an oxidation number of +...

Tobie Jessup 2E

When I took chemistry in high school, I had to memorize the oxidation numbers. Group1 is +1, Group2 is +2, Fluorine is -1, Group15 is -1, Group16 is -2, Group17 is -3, Oxygen is usually -2(not when with fluorine or peroxides), Hydrogen is +1 when paired with non-metals and -1 when paired with metal...

Tobie Jessup 2E

Use the octet rule to first determine lewis structure but the lowest formal charge is used to determine which is most stable between difference resonance structures.

Tobie Jessup 2E

This is super helpful I've been having trouble with these, thanks!

Tobie Jessup 2E

Hi! From my understanding, there's no easy way to know or calculate how many resonance structures there are for a molecule, you just have to practice knowing how to identify them when drawing the Lewis structure. Hopefully, we will be given relatively simple ones if asked on the midterm, like C4H4,...

Tobie Jessup 2E

The Lewis base donates the electrons in coordinate covalent bonds, but Lewis bases don't solely create coordinate covalent bonds.

Tobie Jessup 2E

As your move across the periodic table, more protons are added to the nucleus and therefore there is a stronger positive charge that pulls the electrons closer to the nucleus.

Tobie Jessup 2E

I am also a bit confused with formal charge. To my understanding, I know that calculating the FC of single atoms is important when checking the stability of a molecule because rearranging bonds to have atoms with FCs of zero creates a more table structure overall. Hopefully that was a bit helpful!

Tobie Jessup 2E

The three examples of violations to the octet rule we have talked about in class so far are electron-deficient molecules, radicals, and elements with an expanded octet. Typical electron-deficient elements, like Boron and Aluminum (Group 13 elements) with 3 valence electrons would need to gain an ad...

Tobie Jessup 2E

We say that the first electron spins up, but that value doesn't mean much since the positive and negative directions don't have meaning. I know that 2 electrons in the same sub shell do not spin the same direction.

Tobie Jessup 2E

l values correspond to s, p, d, and f quantum numbers so l=0 is s, l=1 is p, l=2 is d, l=3 is f.

Tobie Jessup 2E

If there is either a trough-trough or a peak-peak interaction then it is constructive, but if it is trough-peak then it is a destructive interaction.

Tobie Jessup 2E

If the electrons were solely acting as particles, they would pass through the two slits in the board and make clear lines, but since they act like waves we see the pattern in the picture where that are points of overlapping waves.

Tobie Jessup 2E

I remember that n1 is alway the lower energy shell because having a lower value in n1 ensures that the answer that comes out of the equation will be positive, whereas if the larger number was first the number would come out negative. Not a very scientific way to explain but an easy way that I've bee...

Tobie Jessup 2E

Do you guys know if the midterm is all multiple choice?

Tobie Jessup 2E

For me I usually do a lot of practice problems the days before and then make sure to also go over my notes before the test. I also think its important that we know the formulas and units within the formulas so we can check our answers/work on the test.

Tobie Jessup 2E

The units of wavelength is meters (m) and the unit for frequency is Hertz (Hz), which is s^-1. Nano meters aren't very commonly used and it is also important to keep track of these units in these calculations, like understanding that Hz is really s^-1.

Tobie Jessup 2E

From what I understand, the speed of light is always said to be constant c. c is also the fastest speed, so it is telling if an answer to a question is more than the speed of light because it must be incorrect.

Tobie Jessup 2E

Is using our phone or another device for the midterm okay instead of a webcam?

Tobie Jessup 2E

Dr. Lavelle sent an email detailing the update to the homework change. It is now due week 4 and includes weeks 2,3, and 4 but the material from week 4 will not be on the midterm.

Tobie Jessup 2E

In high school I was taught to use 3 sig figs but I think a good rule of thumb is to avoid rounding util the end and use the same amount of sig figs that is used in the problem.

Tobie Jessup 2E

Kelvin itself is the unit, unlike Celsius and Fahrenheit.

Tobie Jessup 2E

I dot think it hurts to memorize some, but from what I know formulas will be given to us!

Tobie Jessup 2E

I also have not taken chemistry in 4 years now so these responses were very helpful! It is definitely frustrating relearning concepts that I have forgotten but Dr. Lavelle has definitely given us a huge amount of resources to help us.

Tobie Jessup 2E

It definitely helps to start with the element that occurs the least and work from there! sometimes there can be a bit of tail and error to balance each side of the equation.

Tobie Jessup 2E

Since the solution is solely CaCl and water, you know that the solution is 36% CaCl by mass (given) and therefore 64% water by mass. The total mass is 855.1 g you can multiply the total mass by .36 to find the mass of CaCl in the solution and .64 to find the mass of water in the solution

Tobie Jessup 2E

I also had trouble determining how to approach this question, but this thread was very helpful. I think it was really key to understand how to use the grams of products given to find the mass and moles of C, H, and O. From there, the rest of the problem was generally straight forward!

Tobie Jessup 2E

I also had trouble with this question since I hadn't worked with determining sig figs in a super long time but something that really helped me was understanding the differences between leading and trailing zeros, like some of our peers said above^

Tobie Jessup 2E

I was also very confused on how to approach this question at first, a key issue I had was recognizing the 1:1 ratio of product to reactant to determine the mols of product and therefore the theoretical yield. So thanks to this thread I was able to solve this problem!

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