CHEMISTRY COMMUNITY

AGaeta_2C

I think its still due on Sunday night :)

AGaeta_2C

There were some weeks that I posted more than five questions for each week, would these "extras" count towards my total 50 for the quarter? Or would they just be not counted?

AGaeta_2C

Chemistry is good
Lavelle came through with that exam
Hope you all do well :)

AGaeta_2C

Dear Dr. Lavelle,

Thank you for your dedication to supporting your students and providing so many resources, even after this interesting situation came down at the university.

AGaeta_2C

I don't think we'll be specifically asked about this on the final.

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I always try to not study on an empty stomach since that lack of energy won’t do me any good. I’m just trying to establish a study routine for these next few months, but I find that completely shutting off social media and other distractions helps!

AGaeta_2C

Instantaneous rate decreases as time increases

AGaeta_2C

The order of a reaction tells you how the concentration of reactant affects the rate.

AGaeta_2C

For me, the step-up sessions and the drop-in sessions are the most beneficial.

AGaeta_2C

The units are moles per liter per time in seconds

AGaeta_2C

Molecularity is the number of species in an elementary step of a reaction

AGaeta_2C

First order reactions have linear rates.

AGaeta_2C

Everything listed on the second page of Outline 4 and all of Outline 5.

AGaeta_2C

The favorable case would be if the battery was doing work and dying. Therefore the unfavorable action would be recharging it.

AGaeta_2C

I think especially for the purpose for this class, there is no difference in efficiency between the two.

AGaeta_2C

On Friday, Professor Lavelle went over the strategy for finding the overall reaction order by comparing experimental data provided in the question. On the slide it said: (RATE 2/RATE 1) = 2 = 2.0 m therefore m=1 I understand why m would equal 1 but I'm struggling to see where the expression 2.0 m ca...

AGaeta_2C

In Friday's lecture, our first example was with 2Fe 3+ (aq) + Cu (s) --> Cu 2+ (aq) + 2Fe 2+ (aq). In this galvanic cell diagram, we used a platinum electrode in the cathode, because Fe is not a conductor, and thus platinum needed to be added. Are we supposed to know which elements are good conducto...

AGaeta_2C

Can somebody briefly explain once more why we don't use the units "J/mol" of electrons to represent work, rather we use "J/C"?

AGaeta_2C

I had a quick question for clarification on the "switch" we usually include in galvanic cell diagrams. When the switch is off, that's when the difference in electrochemical potential is greatest? And once we flip the switch on the battery, the charges are allowed to diffuse, producing work...

AGaeta_2C

Wait, nevermind... I think I see where I went wrong. I messed up on balancing with H₂O and H⁺.

Reduction: 8e- + 4Cl₂ --> 8Cl^-
Oxidation: 5H₂O + S₂O₃^2- --> 2SO₄^2- + 10H⁺ + 8e-
Combined redox reaction:
4Cl₂ + 5H₂O + S₂O₃^2- --> 8Cl^-+ 2SO₄^2- + 10H⁺

AGaeta_2C

For some reason I keep getting this question incorrect. I'd appreciate if someone can tell me where I'm going wrong? 6K.3 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 an...

AGaeta_2C

Thank you for posting this!

This additional video was pretty helpful in understanding how to balance redox reactions:
https://www.youtube.com/watch?v=N6ivvu6xlog

AGaeta_2C

The common ones I would definitely have locked down are O which is 2-, H which is 1+, F which is 1-. Elements by themselves (ex. diatomic molecules like Cl₂) are 0.

AGaeta_2C

Walking out of my midterm location, I couldn't help but overhear all the critiques/concerns students expressed after the midterm. Some of these comments surrounded the actual content the midterm tested, but also the overall structure of the exam. I wanted to create a space for students to post their...

AGaeta_2C

Why is DeltaU for constant pressure being calculated using C_v? In order words, why is the change in internal energy for a system under constant pressure (DeltaU) using the C_v value in its calculations? Shouldn't DeltaU = q + w?

AGaeta_2C

The equation used is deltaS_surroundings= (-deltaH)/T
Since temperature is in the denominator, if that value is increased, then the overall value of the expression (deltaS_surroundings) would be smaller.

AGaeta_2C

Under what conditions is q, heat flow, equal to change in enthalpy of the reaction?

AGaeta_2C

When would you use the equation dS= C * ln (T₂/T₁) and when would you include the number of moles into that equation? I notice we've used both dS= C * ln (T₂/T₁) and dS= n* C * ln (T₂/T₁) to solve problems but I'm not sure when to apply which equation.

AGaeta_2C

Since both parts have to do with phase change, you can use dS = q/T = dH/T. Thus, the calculation is 1.00 mol(-6.01 kJ/mol)/273.15 K. And, the answer is -0.0220 kJ/K. For part b, you can use the same equation. First, calculate the moles of ethanol and multiply it by the molar enthalpy of vaporizati...

AGaeta_2C

On the constant and equation sheet, I saw the equations C p = (5/2) R, C v = (3/2) R, and U= (3/2)nRT, and it brought up some questions. I know the equation to calculate heat capacity under a constant pressure is C p = (5/2) R, but for which kinds of molecules does this apply to? monoatomic or diato...

AGaeta_2C

Will the review session on that day be covering past exam problems?

AGaeta_2C

From outline four, topics 4F through 4J will be covered.

AGaeta_2C

Lavelle explained this in lecture once that increasing the pressure will shift the reaction to the side with less moles of gas (this is the simplified way of thinking about it)

AGaeta_2C

It is crucial that you choose the version of the gas constant that has the same units as the problem you're working on, just so they cancel out and you don't introduce new units.

AGaeta_2C

Thankfully it is always the same sheet used so it's pretty essential to understand it and how each equation is derived. :)

AGaeta_2C

For 14B, we really only have to worry about work of expansion and work of compression. ΔU = q + w where ΔU is the total change of (internal) energy of a system , q is the energy in the form of heat gained/lost by the system , and w is the energy in the form of work done on/by the system. A negative ...

AGaeta_2C

Actually, is this line of thinking correct?:
The change in internal energy (ΔU) is equal to the change in enthalpy (ΔH) because there is no energy being done through work (w).
ΔU= q_p + w
ΔU = ΔH + (-PΔV)
ΔU = ΔH + 0
ΔU = ΔH

AGaeta_2C

Why is ΔU equal to ΔH only when pressure is constant and volume doesn't change?
Is this relationship shown through any of the equations we have been working with?

AGaeta_2C

Any energy lost/gained by the system must equal the energy gained/lost by the surroundings.

AGaeta_2C

I'm having trouble mentally keeping track of all the variables we use in thermochemistry. U= internal energy P= pressure V= volume H= enthalpy ; ΔH= change in enthalpy, which sometimes is equal to q(??) q= heat ; q p = heat under constant pressure, q v = heat under constant volume w= work C= heat ca...

AGaeta_2C

Have the tests been returned yet for some? Does anyone know when is the latest we'd get our test back?

AGaeta_2C

I hope this helps :)

AGaeta_2C

Hello, In response to this question, could anyone explain why the bond enthalpies in Dr. Lavelle's example were positive AND negative? Bond energy can be described as the energy it takes to break a bond, in this case, the values would be positive. Therefore, the energy it takes to form bonds would ...

AGaeta_2C

It states that because enthalpy is a state property, changes in enthalpy are additive. in other words, enthalpy change in each step of a multistep rxn can be added to give TOTAL enthalpy change.

AGaeta_2C

I think we would just get the bond enthalpy values from a chart.

AGaeta_2C

Hess's Law describes that because enthalpy is a state function, it must be additive (enthalpy change at each step of a multistep reaction can be added to give TOTAL enthalpy change).

AGaeta_2C

How do you know the standard state of a molecule? Its easy for molecules like nitrogen and oxygen, but how would we know for other molecules?

AGaeta_2C

How do we validate the approximation of X? I know as long as X is less than 5% of the initial concentration, then the approximation is valid. However, how do you actually calculate if X is less than 5% of the initial conc.?

AGaeta_2C

Which types of molecules don't affect the pH of a solution? For example, as discussed in 1/17 lecture, when calculating the pH, why do we not include Cl - in the reaction between HCl and NH 4 + to form the salt NH 4 Cl? Professor Lavelle said Cl - was not included because it is not involved in the r...

AGaeta_2C

Unfortunately I was unable to attend lecture and was wondering if anyone would be kind enough to post their notes from Wednesday, 1/15. I would be greatly appreciative!

AGaeta_2C

Apologies for the cramped paper and sloppiness!

AGaeta_2C

I'm still having trouble with being able to determine which components of a reaction are (weak/strong) acids or bases, or salts. I remember that we just had to memorize strong acids/bases for 14A. But if anyone can share tips on how to identify the others in a reaction please help! Also, the effects...

AGaeta_2C

What is the difference between strong acids/bases and weak ones in terms of calculating K?

AGaeta_2C

Is K_w the equilibrium constant for water or the ionization constant?

AGaeta_2C

what would happen if the reactant(s) and/or products are different states? If the R or P are in different states, you calculate K c , since aqueous and gaseous solutions exist in concentrations. You really only calculate K p when all the R and P are gaseous, but even then, it is generally acceptabl...

AGaeta_2C

How do you convert from Kc to Kp? I believe this was briefly discussed in the second lecture. To convert between partial pressure and concentration for a gas, you use the Ideal Gas Equation, PV=nRT. Where, P= pressure, V= volume, n= number of moles, and (RT) are constants. To calculate pressure of ...

AGaeta_2C

To clarify,
solids are not included in the K expression because they don't necessarily have a "concentration," and liquids are not included because they consist of a pure substance?

Is this thinking correct?

AGaeta_2C

Where will the questions for test one be pulled from? The textbook?
Also will it cover material from Acids and Bases or just Chemical Equilibria?
Thank you:)

AGaeta_2C

I was confused on this, too. Is this thinking correct?:

Because the equilibrium constant is a ratio of product concentration (moles/volume) to reactant concentration at equilibrium, it is unaffected by increases in pressure (by adding an inert gas) as long as the volume stays the same.

Thank you:)

AGaeta_2C

Hey guys I just wanted to share these videos that I found really helpful with visualizing molecular shape! And an attached chart provided by one of the TAs in a different forum. https://www.youtube.com/watch?v=nxebQZUVvTg&t=904s https://www.youtube.com/watch?v=cdo6FtSU_k8 Also, there's this help...

AGaeta_2C

I think it's just the "old" way vs the "new" way idk. It's correct to use both but it's good to be consistent with it.

AGaeta_2C

For some reason the word titration just threw me off so it really took me a minute to put something down.

AGaeta_2C

I appreciate all the resources that are provided to help us succeed in this series! Especially chemistry community because of its accessibility. Thank you, Professor Lavelle!

AGaeta_2C

I think my number of posts is off by one, is this a problem for anyone else?

AGaeta_2C

curry 1E wrote:I get that for a conjugate acid you add an H and add + to overall charge. But, for part a), would you add the H to the H of CH3 or the H of NH2?

You would add an H to the Nitrogen atom, since the carbon atom has a complete octet already.

AGaeta_2C

Jaklin Astvatsatryan wrote:.

thank you so much i really appreciate it !

AGaeta_2C

My TA differentiated between these types of acids and bases during discussion and I figured it might help someone if I posted it here. Arrhenius Acid: gives off H+ Base: gives off OH- B-L Acid: proton donor Base: proton acceptor Lewis Acid: electron pair (usually nonbonding 2e-) acceptor Base: elect...

AGaeta_2C

I'm extremely confused when it comes to naming coordination compounds. In my notes I have "add 'o' to end of name if its an anion (ex. cyan ide --> cyan o ; chlor ide --> chlor o )" How do you know when the atom is an anion? Do we do this for ligands or for the metal aspect of the compound...

AGaeta_2C

What's the most important difference between weak acids and bases vs strong acids and bases as it pertains to this class?

AGaeta_2C

In Monday's (11/25) lecture, Professor Lavelle mentioned that we wouldn't be using the ICE table for solving the pH/pOH of weak acids and bases until 14B. But during discussion my TA was going to go through an ICE box example. Will we be using the ICE table this class or for 14B? Or will we just be ...

AGaeta_2C

I was wondering if anyone would be kind enough to post or share their notes for the lecture before Thanksgiving (11/27). Like many students I had to fly back home. You would be doing me a tremendous favor!

Thank you

AGaeta_2C

Has anyone found out when our test 2 grades will be announced?

AGaeta_2C

VSEPR Table.pdf Really? Do you know where I can find his post?? Wow! Thank you so much this helps a lot! Quick question about the chart so in domain 6 is the angle for AX5E less than 90 degrees? And is the reason why the bond angles in AX4E2 still 90 degrees because of the two lone pairs in the axi...

AGaeta_2C

I'm guessing that for the purpose of this class we have to know:
single bond: sigma bond
double bond: sigma bond + pi bond
triple bond: sigma bond + 2 pi bonds

AGaeta_2C

Why are sigma bonds more stable? Similarly, it was said in lecture that pi bonds are weaker because the electrons "are more exposed." In what way can electrons be "more exposed"? Thank you in advance!

AGaeta_2C

Can someone explain the difference in the symmetry of electron density between a sigma bond and a pi bond? In lecture it was said that sigma bonds have electron densities with cylindrical symmetry around the internuclear axis, whereas pi bonds have electron densities on each side of the internuclear...

AGaeta_2C

In response to your question about the test, I think the test specifies which bond angle (e.g. What can you say about the bond angle between F-C-H?). In that case you'd have to specify if the bond angle is less than or greater than 109.5°.

AGaeta_2C

A: central atom
X: atoms bonded to central atom
E: lone pairs
Molecules with the same formula have the same general shape (bond angles vary)

Examples:
NH₃: AX₃E
H₂O: AX₂E₂
CH₄ and CH₃Cl: both AX₄ (with tetrahedral shape)

AGaeta_2C

I think just to be safe you should draw it as accurately as you can. However if the question asks you to name the molecular shape then that should be the top priority. I hope this helps!

AGaeta_2C

Why do lone pairs "take up" more space than bonding pairs?

AGaeta_2C

I think lone pairs are taken into account when determining the electron distribution shape, not the molecular shape. According to my TA, the molecular shape is usually. what professor Lavelle asks for on tests.

AGaeta_2C

MKearney_4G wrote:So would you say that electrons farther from the nucleus (higher energy) are more easily distorted?

I think you can say the electron cloud of the anion in question is more easily distorted.

AGaeta_2C

Also why does the high polarizability of iodine make it a solid at room temperature?

AGaeta_2C

Can somebody explain why heavier elements have greater polarizability? Thank you :)

AGaeta_2C

for mini dino nuggets 3a and 3b can someone please explain the difference between polarizing power and most polarizable? Polarizing power is used when referring to cations , which give up an electron. Polarizability refers to anions , which gain an electron. These terms both point out the covalent ...

AGaeta_2C

In the answer key for the mini dino nuggets, the dipole moments in question 2a are going towards the negative partial charge when in the textbook it says to go towards the positive charge. What would be correct? On page 96 of the textbook it shows two. different conventions for representing the dip...

AGaeta_2C

The Pauli Exclusion Principle basically states that no more than 2 electrons can occupy a single orbital. Hund's Rule states that due to electron/electron repulsion, orbitals must be occupied by one electron before they can be occupied by a second electron.

AGaeta_2C

I think it'd be best to know the definitions in your own words and to be able to identify the trends of each across the periodic table. I doubt we need to memorize the definitions word for word tho..

AGaeta_2C

Hund's Rule states that due to electron/electron repulsion, orbitals must be occupied by one electron before they can be occupied by a second electron.
The Aufbau Principle states that electrons occupy lower energy orbitals before higher energy orbitals.

AGaeta_2C

Ionization energy is defined as the energy needed to remove an electron from an atom in the gas phase. Ionization energy increases across a period and decreases down a group. For this reason, He has the highest first ionization energy.

AGaeta_2C

I know that according to the Aufbau Principle, the 4s state is occupied before electrons enter the 3d state because the 4s state is lower in energy at first. But why does the 4s orbital become higher in energy once the 3d state is occupied? Thank you in advance :)

AGaeta_2C

I wasn't sure where to post this, but why are resonant molecules more stable? Thank you in advance!

AGaeta_2C

For example, the shorthand electron configuration for Barium is [Xe] 6s^2. I hope this helps!

AGaeta_2C

I was wondering if we are required to know the Pauli Exclusion Principle and Hund's Rule by name on tests?

AGaeta_2C

I find Khan Academy rly useful! Also YouTube videos in general go a long way for me, too. Shorthand e configuration is just the symbol of the noble gas in the previous period followed by the additional e configuration after that (got that from google).

AGaeta_2C

I think we should just to be sure, since there's problems about it in the homework and it was talked about in discussion. :)

AGaeta_2C

Yeah it's the same sheet that can be found on the website. However, my TA said there are some equations that aren't found on the sheet (?) so...

AGaeta_2C

The number of sig figs you keep always depends on the lowest amount of sig figs used in the question. For ex., if the question uses the numbers 7.0g, 3g,and 0.003004g, your answer should use one sig fig, because its dependent on 3g.

AGaeta_2C

Accuracy is the proximity of your value to the actual value. Precision is the closeness of all your values. Your value is more accurate if its closer to the actual value. Your values are precise if they're all relatively close to each other.

AGaeta_2C

It's best to apply the sig fig rules at the end of all calculations so you get the most accurate answer :)

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