CHEMISTRY COMMUNITY

Emily_Stenzler_2H

Is delta G naught zero if something is at its boiling point? Exercise 4I.3 in the textbook assumes delta g naught is zero even thought the system is never stated to be at equilibrium. Does anyone know why this is?

Emily_Stenzler_2H

I am a bit confused as to how to determine whether products are favored or not based on Enaught values. I am also confused when we need to flip the half reactions when finding the Enaught. For example, question 6M.13 in the textbook asks us to identify if MnO4−(aq) + 8 H+(aq) + 5 Ce3+(aq)→5 Ce4+(aq)...

Emily_Stenzler_2H

Do we need to find the volts for each half reaction when finding the reaction from a cell diagram? A TA told me that we need to find these values for each half reaction and make sure Enaught of the cell is positive. I don't remember lavelle talking about this- he told us the anode is on the left and...

Emily_Stenzler_2H

Hi Emily! So basically for the cell diagrams, the order of the species in the notation doesn't really have to do with the order of the species in the half reaction. Life you said, for the cathode in this problem would be Cl2 + 2e- —> 2Cl- But in the cell notion is it presented as Cl-|Cl2 The reason...

Emily_Stenzler_2H

Hi Emily! So basically for the cell diagrams, the order of the species in the notation doesn't really have to do with the order of the species in the half reaction. Life you said, for the cathode in this problem would be Cl2 + 2e- —> 2Cl- But in the cell notion is it presented as Cl-|Cl2 The reason...

Emily_Stenzler_2H

6L.3 Write the half-reactions and the balanced equation for the cell reaction for each of the following galvanic cells: (b) C(gr)|H2(g)|H+(aq)∥Cl−(aq)|Cl2(g)|Pt(s) I am struggling to understand when we simply write out the half reactions in the order shown in the diagram (for ex H2 -> H+) and when w...

Emily_Stenzler_2H

For this one, you would have to write out the half reactions. oxidation: 3I- -> I3- + 2e- reduction: Ce4+ + 1e- -> Ce3+ You need a balanced number of electrons, so you would multiply the reduction half reaction by 2. So the moles of electrons transferred is 2. Hope this helps! Why do you keep the 3...

Emily_Stenzler_2H

For this one, you would have to write out the half reactions. oxidation: 3I- -> I3- + 2e- reduction: Ce4+ + 1e- -> Ce3+ You need a balanced number of electrons, so you would multiply the reduction half reaction by 2. So the moles of electrons transferred is 2. Hope this helps! Why do you keep the 3...

Emily_Stenzler_2H

6L.1 Calculate the standard reaction Gibbs free energy for the following cell reactions: (a) 2 Ce4+(aq) + 3 I−(aq) →2 Ce3+(aq) + I3−(aq), Ecell°=+1.08V For the above problem, how would you determine how many moles of electrons are transferred? I am getting 1 but the book says 2. Can someone explain ...

Emily_Stenzler_2H

Can someone explain how you determine the phases everything is in after balancing a redox reaction in an acidic or basic solution?

Emily_Stenzler_2H

Identify the oxidizing and reducing agents in the skeletal (unbalanced) reaction. Then, balance the reaction, including the phase (solid, liquid, etc.) of each species. The reaction takes place in basic aqueous solution. N2H4(g)+ClO−3(aq)⟶NO(g)+Cl−(aq) I've found the oxidizing and reducing agent, bu...

Emily_Stenzler_2H

These jokes are so good, I'm diene of laughter

Emily_Stenzler_2H

When calculating equilibrium constants from electrochemical data, why is it always necessary to flip one of the half reactions?

Emily_Stenzler_2H

I apologize if this is a bad question- but why are galvanic and voltaic cells called "cells"? Do they relate to cells (defined in the way we have learned in previous bio classes) in any way?

Emily_Stenzler_2H

Yes! Especially in this class, if we are finding K this is most always referring to the equilibrium constant. If we are meant to find temperature, we would likely be told to calculate temperature in K, rather than to calculate K.

Emily_Stenzler_2H

I believe a 50% or higher is needed to pass the class.

Emily_Stenzler_2H

The first relationship you need to know for this problem is that q reaction = -q calorimeter , which means that if the reaction releases heat, then the calorimeter will gain that heat (and vice versa). Because q reaction = -3.5kJ, q cal will be +3.5kJ, and that is the value we use when calculating ...

Emily_Stenzler_2H

How do you determine the number of orientations a molecule will have when using the Boltzmann equation?

Emily_Stenzler_2H

A human body generates heat at the rate of about 100.W. (a) At what rate does your body heat generate entropy in your surroundings, taken to be at 20.°C? The answer key writes deltaSsurr/time = -qrev/time, resulting in -100JS^-2/293K, but then the answer is positive. Why did they just lose the negat...

Emily_Stenzler_2H

How much heat is needed to convert 80.0 g of ice at 0.0°C into liquid water at 20.0 °C (see Tables 4A.2 and 4C.1)?

The answer key shows that the equations for deltaH were used to solve this. Since the question is asking for heat, why do we not use the equation for q, and convert g to mol?

Emily_Stenzler_2H

Does q=CdeltaT whenever the reaction is closed while q=nCdeltaT when it is not closed? I am confused as to when we include n and when we don't based on the text book problems (for example, problem 4A13 doesn't consider n and I'm not sure why this is). How do we know which equation to use?

Emily_Stenzler_2H

Can someone explain the difference between finding specific heat and heat capacity? Is specific heat kj/c^-1xmol^-1? How do we solve for heat capacity?

Emily_Stenzler_2H

A calorimeter was calibrated with an electric heater, which supplied 22.5 kJ of energy as heat to the calorimeter and increased the temperature of the calorimeter and its water bath from 22.45 °C to 23.97°C. What is the heat capacity of the calorimeter? I am a bit confused on how the textbook solved...

Emily_Stenzler_2H

I believe this is because most reactions take place in 25 degrees celsius so we assume this condition if the temp is not given (learned in 14A). So I believe that's what it's doing - assuming 25 degrees celsius which would be 298K.

Emily_Stenzler_2H

How much heat is needed to convert 80.0 g of ice at 0.0 °C into liquid water at 20.°C (see Tables 4A.2 and 4C.1)? For the above question, the textbook assumes the change in enthalpy is equal to q. Don't we need to know there is constant pressure to be able to assume this? What allows us to make this...

Emily_Stenzler_2H

A constant-volume calorimeter was calibrated by carrying out a reaction known to release 3.50 kJ of heat in 0.200 L of solution in the calorimeter (q=−3.50kJ), resulting in a temperature rise of 7.32C. In a subsequent experiment, 100.0 mL of 0.200 M HBr(aq) and 100.0 mL of 0.200M KOH(aq) were mixed ...

Emily_Stenzler_2H

Can someone explain when exactly we would know that q=w?

Emily_Stenzler_2H

A particular container holds 2.85 mol of neon gas. The volume of this container can be altered by sliding a piston in or out. The volume is changed from 7.80 L to 13.70 L while at the same time the temperature is changed from 255 K to 3.20×102 K. The molar heat capacity, CV,m, for neon is 12.47 J/(m...

Emily_Stenzler_2H

Happy Valentine's day everyone! I hope you all have a great weekend + week!

Emily_Stenzler_2H

The textbook says the following:

the gas is heated from 20.°C to 100. °C (so ΔT=+80. K)

Why are 80 units in C equal to 80 units in K? Why don't we have to convert from C to K?

Emily_Stenzler_2H

I apologize if this has already been answered somewhere, but does anyone know if Chem 14C lectures will be asynchronous?

Emily_Stenzler_2H

I also look at the problem and choose the units that match the pressure units in the problem!

Emily_Stenzler_2H

I (like many others) am debating whether or not I should take 14C with 14Bl! Does anyone have tips/ insight?

Emily_Stenzler_2H

How are you guys balancing studying with relax + self care time? I find it hard to do anything but study when a midterm is coming up, but there have been multiple times when I have burnt myself out right before an exam because of this. Does anyone have tips on how to allocate time to both studying a...

Emily_Stenzler_2H

How is everyone doing in regard to working through the outlines? I know everyone has different schedules/ works at different paces, but I am still taking notes on Outline 3 (I take really thorough notes that take me a long time) and I'm worried about getting/being behind.

Emily_Stenzler_2H

I can relate with a lot of the study methods in this post. I like to rewatch the lectures before midterms/ exams and it helps me catch the little things he mentions that I may have missed the first time. It's slow and I also take thorough notes, but I believe it works and it one of the main practice...

Emily_Stenzler_2H

I also feel a bit worried. I have come to the conclusion that the exams for this class are rather hard to predict. Some midterms have been very easy, while I found others to be unfairly hard. I'm hoping that this midterm will not have any questions that are worded different than the textbook. Even t...

Emily_Stenzler_2H

According to my TA, you can go to any TAs office hours to review your exam! Sometimes they make you wait until they've finished answering questions first since you have to go into a private breakout room.

Emily_Stenzler_2H

The proton is not speaking to the other proton, he’s mad atom.

Emily_Stenzler_2H

I think it would help if you could more closely identify the trends of mistakes you are making. Are they typically careless errors? Conceptual misunderstandings? Were you on the right track on most problems or were the solutions nothing you would have even guessed? I think identifying these factors ...

Emily_Stenzler_2H

I do feel the midterms last quarter were not as challenging. Hopefully, since the average was down this time compared to last quarter, the next midterm will be a bit easier!

Emily_Stenzler_2H

In regard to non-fiction books, I think the Sociology of Health and Illness by Peter Conrad is super interesting!

Emily_Stenzler_2H

You can't cancel if the phases of the same compound are different. They have to be the exact same phase and compound in order to be able to cancel if they are on opposite sides of different equations. Do you know why this is? Is this because certain values change based on the phase? If so, what val...

Emily_Stenzler_2H

If you have extra time after doing ALL textbook problems, I recommend rewatching some of the harder lectures. I've picked up on small things I missed the first time that were conceptually important on the midterm.

Emily_Stenzler_2H

I don't have a full time or part time job but I just want to say how much I admire everyone who is balancing a job and schoolwork! I already feel so overwhelmed with classes and a couple extracurriculars, I can't imagine also having a job on top of everything :/ You guys are so amazing!

Emily_Stenzler_2H

What value do we use for p when finding the number of moles for part a of question 14 on sapling? Do we use the difference for this part?

Emily_Stenzler_2H

I personally found the test to be a little more difficult than I had anticipated. I was able to do all the problems from workshops and the homework; however, the midterm's problems seemed a little harder. Maybe I was just nervous. Can anyone else relate? I really felt the "Maybe I was just ner...

Emily_Stenzler_2H

I love classic rock :-) CSNY, The Beatles, Steely Dan, all great stuff

Emily_Stenzler_2H

I love this post! It's so important for us to all stay positive! We're all so capable of achieving our future goals and one rough midterm certainly isn't going to stop us from achieving our dreams :-) I spend the weekend doing some thrift shopping and baking with my roomie!

Emily_Stenzler_2H

Week 1 - 3!! I'm scared! Is anyone else scared? I definitely feel nervous for the midterm but I would recommend going to UA sessions especially if you’ve finished all the textbook practice questions! I feel that the worksheets and practice problems they have are really helpful in prepping for exams...

Emily_Stenzler_2H

Additionally- I've started to write little reminders for things I typically make careless errors on. For example, I know I have written something like "5.6" instead of "5.6x10^-5" down from my calculator numerous times (not looking all the way to the end of the number) so I am no...

Emily_Stenzler_2H

I can totally relate to this! Sometimes it helps me to 'talk' through every step of the problem in my head so I am double checking that everything I'm writing down is making sense and in a way double checking that all my values are being written down correctly.

Emily_Stenzler_2H

I believe K unspecified is referring to Kc, so we would use units of molarity.

Emily_Stenzler_2H

Yasmina Zaarour 1G wrote:The Kw for water is 1.0x10^(-14) only when temperature is at 25C, but the question gives you a new value of Kw for water because it's asking about at 37C. Take the square root of the given value (2.1x10^(-14)).

ohh I missed that! Thank you!!

Emily_Stenzler_2H

When solving this problem, why do we not have to do the same procedure as 15, where we needed to find the combustion reactions for each compound? Additionally, why is N2 not taken into account when finding the standard reaction enthalpy?

Emily_Stenzler_2H

How would we come to the conclusion that the molar concentration of OH- in neutral water is 1.4 x 10^-7? What calculation would be done to come to this conclusion? I tried taking the square root of Kw=1*10^-14, since [OH-] would equal [H3O+], but I didn't end up with 1.4x10^-7

Emily_Stenzler_2H

Vivian Chang 3L wrote:I think the only ones we need to know for this course are the strong acids H2SO4, HI, HBr, HNO3, HCl, HClO3, HClO4 and the strong bases are Group 1 & Group 2 metal oxides/hydroxides. Hope that helps!

Thank you!!

Emily_Stenzler_2H

When H2SO4 dissociates the first time, it produces H3O+ and HSO4. Wouldn't this make HSO4- it's conjugate base? In the second deprotonation, HSO4- is acting as a weak acid. Why is this?

Emily_Stenzler_2H

The Cl- anion is neutral, but the Al3+ is not. From the reaction AlCl3 + 3H2O <=> Al(OH)3 + 3HCl, HCl is a strong acid, so it will dissociate in water to form H+ and Cl-. As a result of this reaction, the solution will be acidic, with a pH < 7. so does this mean AlCl3 is a weak base since its conju...

Emily_Stenzler_2H

I am a bit confused as to how to determine if CH3NH3Cl is a strong acid or not. This is how I was thinking, I would appreciate if someone can let me know if this thought process is incorrect Since Cl- is a spectator ion, we disregard it and only look at CH3NH3+ which we can determine to be a strong ...

Emily_Stenzler_2H

Which chart should we be studying to memorize the strong acids+ bases? I believe chart 6C.3 has more strong acids and bases than the chart we had studied for 14A- is this the chart we should be referring to for 14b?

Emily_Stenzler_2H

When solid NH4HS and 0.400 mol NH3(g)were placed in a vessel of volume 2.0 L at 24 °C, the equilibrium NH4HS(s)⇌NH3(g)+H2S(g), for which Kc=1.6×10−4, was reached.What are the equilibrium concentrations of NH3 and H2S? In regard to the above question, I am a bit confused why the fact that we are give...

Emily_Stenzler_2H

In this textbook question, we are asked the following: In an experiment, 2.0 mmol Cl2(g) was sealed into a reaction vessel of volume 2.0 L and heated to 1000. K to study its dissociation into Cl atoms. Use the information in Table 5G.2 to calculate the equilibrium composition of the mixture. I thoug...

Emily_Stenzler_2H

The question is the same type as the one answered her https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=49&t=72607&sid=71ad910c0d27a25e2f6004ba1d1b9342 Thank you! Were you able to make the assumption that [OH-]=[NH4] simply because the molar ratio is 1:1? Does the pH not apply here when c...

Emily_Stenzler_2H

Since we know pH is a certain value, you just have to keep in mind that pH+pOH=14. So you would have to do 14-pH=pOH Using this pOH value, you can now solve for, first, [OH-], and then, using this concentration and the Kb value, the initial concentration. The % protonation would be the [OH]/[initia...

Emily_Stenzler_2H

I believe in one of the UA sessions, we were solving an ICE table for a weak acid and we were told we couldn't assume that the concentration of H3O+ and the conjugate base were the same, even though they had the same molar ratios, because the solution had an acidic pH. Is it true that we need to tak...

Emily_Stenzler_2H

Is the content from Friday's lecture the last lecture content that will be on the upcoming midterm?

Emily_Stenzler_2H

I don't know of any inclusive study guide that has been made, but in my experience taking 14A with lavelle, the outlines and 'learning objectives' on the outlines make a pretty good study guide. If you do all the textbook problems (and can do them correctly) and understand all the topics on the outl...

Emily_Stenzler_2H

Start with the hard concepts! Don't put them off until the end. If you understand certain concepts, then you don't need to spend the same amount of time on them as topics you don't understand. This may seem obvious, but I know it's sometimes tempting to work on easier problems because it feels as th...

Emily_Stenzler_2H

The 'mountain' example lavelle used in lecture really helped me understand this, showing that two people climbing a mountain could get to the same point with various paths, but in the case of state properties we don't care about those paths, just where the climbers started and ended. I recommend rew...

Emily_Stenzler_2H

If I'm not mistaken, I thought he had classified it as an acid last quarter, but I agree with the above posts. It will probably be clearly indicated on the midterm if HCLO3 is used.

Emily_Stenzler_2H

Can someone please tell me exactly what parts of the periodic table can be used to determine if a weak compound is an acid or base? I've heard people mention looking at the first two groups and something about the halogens, but I'm still unclear exactly how to determine this and I would really appre...

Emily_Stenzler_2H

Referring to textbook question 6A.21 -

How do we know [H3O+]=[OH-]? Do we assume they are always the same if the problem just says "in water"?

Emily_Stenzler_2H

Exothermic reactions? I studied them before they were cool.

Emily_Stenzler_2H

Study stuff you are struggling with first! Prioritize harder concepts. In order to truly check mastery, do tons of textbook problems. Once you can do a handful all right on your own, you can be pretty confident that you have the concept down.

Emily_Stenzler_2H

I think I missed the discussion last quarter when Dr. Lavelle explained how we know something is a salt...can someone please explain? Is it anything with Na? Will Na always be acting as an acid when it dissociates from a compound?

Emily_Stenzler_2H

In lecture today (1/13) what did Dr. lavelle mean when he said CH3COOH was acting as a base (22:36)? I thought anything with -COOH was a weak acid.

Emily_Stenzler_2H

On sapling, there were a couple questions asking about the relationship between Q and K when temperature is increased for a reaction. For example, if the reaction was endothermic then (to my understanding) more reactants were made and K increased. According to sapling, K is now greater than Q. Why d...

Emily_Stenzler_2H

The textbook states "at equilibrium, there is no tendency to form more reactants or more products". If this is true, then why do we say that "equilibria favors reactants/products"?
I am a bit confused and would really appreciate if anyone could clear this up!

Emily_Stenzler_2H

do we need to memorize how to convert between units of temperature?

Emily_Stenzler_2H

Hi! I've found that my biggest problem with studying is I never spend enough time on the stuff I'm shaky on. I like going through concepts I know how to do, just to confirm I do, but it ends up taking way too much time. While it can be daunting to do practice problems on concepts you don't like/fin...

Emily_Stenzler_2H

Focus studying on what really helps you, rather than just trying to get through all possible study materials! For me it helped to really focus in on doing textbook problems to study since the amount of potential study resources for this class can get overwhelming, and I found the textbook problems w...

Emily_Stenzler_2H

I believe anything we post on here gets points! I'm not sure if they check how thoughtful the posts are, so I would try and have at least 5 considerably helpful posts each week :)

Emily_Stenzler_2H

John Tucker Must Die is a really funny light chick flick as well!

Emily_Stenzler_2H

The movie Little Miss Sunshine is so heartwarming and cute (even though some aspects of it are more serious). I also recommend Frozen or Tangled as some good Disney films :-)

Emily_Stenzler_2H

If the exams are the same format as 14a, I recommend joining the zoom at least 5 minutes early! It gets a bit chaotic when everyone joins and the TA has to go around checking papers.

Emily_Stenzler_2H

I think this may vary based on your TA, so I recommend doing a little over 5 each week to be safe. We were also told last quarter that they have to be posted in the right place to count, so make sure you are doing this as well!

Emily_Stenzler_2H

Megan Sparrow 1A wrote:To clarify, these midterms are held in class this quarter. Last quarter they were held in discussion section times. They are still within the same time frame of 50 minutes for each midterm. You can also look at alternate times if you are an international student.

Thank you!

Emily_Stenzler_2H

Does anyone know the date of our midterms? I can't find the Test and Exam Schedule (mentioned in the syllabus).

Emily_Stenzler_2H

I believe he said we are mainly using atm in this class, so I would convert to this unit (using the R for atm) if you are unsure! Just make sure all units match.

Emily_Stenzler_2H

Practice problems! These have really helped me work out points of confusion and accurately test myself for mastery of the course content. Once you can do a handful of problems from a certain concept and get them all correct, you can be pretty confident that you've mastered that concept!

Emily_Stenzler_2H

Only pure solids and liquids do not effect equilibrium! The activity of these substances is 1 (if I interpreted this correctly from the textbook :))

Emily_Stenzler_2H

Additionally, he pointed out that equilibrium mixture, equilibrium concentration and equilibrium composition all mean the same thing, just in case anyone was getting confused with those terms as well!

Emily_Stenzler_2H

In the textbook, a formula is given to directly convert Kp to Kc. Do we need to be able to use this formula, or can we make these conversions by using the pV=nRT equation?

Emily_Stenzler_2H

In outline 1 when the textbook provides both thermodynamic and kinetic explanations, do we need to be focusing on the thermodynamic as well as the kinetic explanations? I know thermo will be covered later, but since we have not discussed it yet can we focus on the kinetic explanations?

Emily_Stenzler_2H

Can someone describe exactly what an ideal gas is, as well as an inert gas?

Emily_Stenzler_2H

victoria_dao_3G wrote:Besides shows, what are some movies you guys are watching right now? I'm trying to get into the holiday spirit, so any Christmas movie recs?

some really good movies (not Christmas ones tho) I recommend are Knives Out, Paris Texas or the Farewell, depending on what your movie preference is!

Emily_Stenzler_2H

Nathan for you + the office >>>

Emily_Stenzler_2H

I tell bad chemistry quotes because all the good ones Argon.

Emily_Stenzler_2H

During Kate's UA session on Thursday, students were discussing additional steps necessary when finding equilibrium for ionic equations. Can someone explain how we would approach ionic equations/ finding K for these equations?

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