CHEMISTRY COMMUNITY

Amy Xiao 1I

In addition to being able to see our grades, does anyone also know if we will be able to review our answers? I would like to see the questions I got wrong. How will we be able to see these? Is he releasing the final exams online? We'll be able to see specifically what we missed because we took the ...

Amy Xiao 1I

Dr. Lavelle, thank you for taking care of us throughout the whole quarter. I admire how quickly you acted to make sure we would be assessed as fairly as possible during these chaotic times, and I really appreciate how selfless you are when you take so much time out of your own day to organize study ...

Amy Xiao 1I

For Number 1 on Test 2, why would you not multiple S or H by 2, because you need to balance the equation first and that doubles the moles of product?

Amy Xiao 1I

Do we need a printer? I can't imagine doing Chem through my laptop.

Amy Xiao 1I

So from the cell diagram, we know that the anode on the left is the oxidation half-reaction and the cathode on the right is the reduction half-reaction. For the oxidation half-reaction we have two species they give you, H 2 (g) and HCl (aq). In this case, since HCl is aqueous, you can think of it a...

Amy Xiao 1I

Yes, I would probably do homework on kinetics for this week.

Amy Xiao 1I

Calculate the time required for each of the following second-order reactions to take place: (b)A->2B + C,when [A]0 = 0.15 mol/L, for the concentration of B to increase to 0.19 mol/L, given that kr = 0.0035 L/mol.min in the rate law for the loss of A. Why does the solutions manual include the part wh...

Amy Xiao 1I

I think one should cram (if you must cram) the full day/night two days before the final and then take it easy the day before; sleep and have a good meal and just review lightly in the 24 hours before the final!

Amy Xiao 1I

Substance A decomposes in a first-order reaction and its half life is 355 s. How much time must elapse for the concentration of A to decrease to ... (c) 15% of its initial concentration; (d) one-ninth of its initial concentration? Why does the solutions manual do the math differently for each step. ...

Amy Xiao 1I

Dinitrogen pentoxide, N2O5, decomposes by first-order kinetics with a rate constant of 3.7 x 10^-5 /s at 298 K. (a) What is the half-life (in hours) of N2O5 at 298 K?

I am somewhat confused by how the units cancel. Why are the seconds applied to ln2 (numerator) and not the 3.7x10^-5 (denominator)?

Amy Xiao 1I

Determine the rate constant for each of the following first- order reactions, in each case expressed for the rate of loss of A: (a) A -> B, given that the concentration of A decreases to one-half its initial value in 1000. s Answer: k=0.693/1000s Where did the solutions manual get the 0.693? There w...

Amy Xiao 1I

I agree, after asking a UA, we both agreed that there was an error in the solutions because the number of electrons only changes by one in the reduction.

Amy Xiao 1I

What other situations makes use of the Nernst Equation (for our purposes in this class)? Would you only use it when given a concentration cell?

Amy Xiao 1I

When you have multiple of one state of matter, does it matter the order in which you put them with the commas?

Amy Xiao 1I

In the cell diagram for part (a) in the solutions manual, they do not include Pt(s) on the left side, but it is present on the right side. I thought Pt(s) had to be present on both sides, can someone explain why this happens?

Amy Xiao 1I

2Hg^(2+) + 2e- --> . Hg2 ^(2+) (see image for clarification)

Can someone explain how this reaction works? I cannot figure out where the 2 e- come in since it seems to me like Hg keeps the same charge.

Amy Xiao 1I

I was reading 5J3 and there was a box on page 432 that explained the changes on equilibrium constant of temperature. They explained through kinetics and mentioned the Arrhenius equation. Can someone explain what this all means?

Amy Xiao 1I

You would start balancing the Oxygen atoms by adding H2O on one side, but then you need to balance the Hydrogens. For basic solutions, there is a surplus of OH- compared to H+ in acidic solutions, so you would then add OH- to balance the Hydrogens.

Amy Xiao 1I

Can someone explain why we use half-reactions? Dr. Lavelle said it makes it easier to understand the redox reaction as a whole but I am just more confused.

Amy Xiao 1I

What section of the textbook should we do for this week? The syllabus points us to 5G but I can't find the Van't Hoff Equation there.

Amy Xiao 1I

What I did was I found Ka and used the pH given to find the concentration of H+. Since the equation for Ka is [H+][A-]/[HA], you can find the ratio of conjugate base to acid by dividing Ka by the concentration of H+. But how would you know if it stays as a salt in the stomach or becomes something e...

Amy Xiao 1I

I did it the way you advised and still got 22.85C as my answer for Tf. For your Step 3, shouldn't the right side of the equation be negative (e.g. -265 g*specific heat of water...)? Why do you not use the specific heat of ice on the right side? (Using the specific heat of ice gives me 20.93C for my ...

Amy Xiao 1I

10: Matt asks me for water with no ice at a dining hall. Just to spite him, I measure 25.0 g of ice at 0.00 oC and drop it into 265 mL of water at 25.0 oC. What is the final temperature of the water?

Can someone provide a step-by-step guide on how to solve this?

Amy Xiao 1I

I also can't get the answer for q and w; it seems like he subtracted 23.3 and 9.12 to get 14.2, but I don't understand why you wouldn't add the work values you get for each part of the problem together instead. Edit: it seems that work will be positive if the system is being compressed, like in the ...

Amy Xiao 1I

Can someone walk me through how to solve 4A of Pizza Rolls? I am confused about how to use Hess's Law.

Amy Xiao 1I

3 E and 3 F Can someone explain why these two statements are false? I don't really understand the wording of 3E, and is it ever possible to have a negative heat capacity? 3E: Removing heat from a system is exothermic, so temperature must decrease. 3F: Heat Capacities can be negative for simple molec...

Amy Xiao 1I

In terms of homework or test problems, they will most likely tell you explicitly that the system is being changed reversibly.

Amy Xiao 1I

What is the significance of having error in either the W or the S?

Amy Xiao 1I

Hi Amy Xiao 1I! To answer your question, the question is asking for the rate at which entropy is generated. Rate is typically some value per unit of time. For example, we have MPH, the rate of how far you're traveling (miles) per unit of time (hour). Because we want the rate of entropy generated, w...

Amy Xiao 1I

Calculate the standard entropy of vaporization of water at 85C, given that its standard entropy of vaporization at 100C is 109.0 J/Kmol and the molar heat capacities at constant pressure of liquid water and water vapor are 75.3 J/Kmol and 33.6 J/Kmol, respectively, in this range. Why is it necessary...

Amy Xiao 1I

In the solutions manual for 4F.1a, why does it say that the rate of entropy generation is Entropy(of surroundings)/time? What does time have to do with this problem?

Amy Xiao 1I

Areena H 2K wrote:From lecture, we talked about a bomb calorimeter which is an isolated system and insulation, and also an open calorimeter, and adiabatic calorimeters as well.

I don't remember Dr. Lavelle mentioning adiabatic calorimeters, can you explain what those are and their importance?

Amy Xiao 1I

Remember that the definition of enthalpy is the change in heat at a constant pressure.

Amy Xiao 1I

I agree with Justin, I think that since you are dividing by a volume/density, which always has to be positive, you have to get a positive answer.

Amy Xiao 1I

The way the solutions manual solves it is that it first converts the 197 g of Carbon to mols -> 16.40 mol C. It also divides the energy absorbed by 4 moles of Carbon by 4 so that you will know how much 1 mol Carbon absorbs. After that, you can just multiply that energy by however many moles of carbo...

Amy Xiao 1I

Because the change in thermal energy is equal to q + w and since there isn't any work being done on the system, I believe that the answer is simply delta U = q and q = delta H so the answer is -320 because that is the value of delta H in the reaction for 1 mol of OF2. Where did you get the 320? The...

Amy Xiao 1I

Who is Gibbs, and why is he giving out free energy?

Amy Xiao 1I

Silver walks up to Gold in a bar and says, "AU, get outta here!"

Amy Xiao 1I

What homework problem sets should we be doing for this week on enthalpy?

Amy Xiao 1I

If the bond enthalpies are not available you can calculate the standard reaction enthalpy using the standard enthalpy of formation of all products and subtracting that from the standard enthalpy of formation of all reactants. To add on to that, Dr. Lavelle talked about this being the third method t...

Amy Xiao 1I

[quote="Connor Ho 1B"] For part (a), here's how I went through it: [b]2. From the table in the chapter, we know that the K b of NH 3 is 1.8 x 10 -5 . We would plug this number into the K equation for this chemical reaction (Note: this is less than 10 -3 , so you can later estimate x by rem...

Amy Xiao 1I

"In a gas-phase equilibrium mixture of H2, I2, and HI at 500. K, [HI] is 2.21 x 10^-3 mol/L and [I2] is 1.46 x 10^-3 mol/L. Given the value of the equilibrium constant in Table 5G.2, calculate the equilibrium molar concentration of H2." How do you solve this? I looked at the table and I th...

Amy Xiao 1I

Jamie Lee 1F wrote:41 is two sig figs, so your answer should also be two sig figs, so 1.7 x 10^3!

If that is so, can you explain why the answer for 5I.1 is 1.4 when the answer you get is 1.370, and every other given number has more sigfigs than 1.4?

Amy Xiao 1I

I am also confused, can someone explain which two equations would form this one?

Amy Xiao 1I

It's also important to note that for a homogeneous gas equilibrium, we can calculate Kp, but we cannot calculate Kp for a heterogeneous equilibrium that involves both gas and aqueous species. Thank you for pointing that out! We would have to convert everything into concentrations, not partial press...

Amy Xiao 1I

If you have 41^2, that becomes 1681. Why is the answer written as 1.7 x 10^3?

Amy Xiao 1I

It's also important to note that for a homogeneous gas equilibrium, we can calculate Kp, but we cannot calculate Kp for a heterogeneous equilibrium that involves both gas and aqueous species. Thank you for pointing that out! We would have to convert everything into concentrations, not partial press...

Amy Xiao 1I

I think if you are not given any values, you would simply write out the Q expression with the molecular formula placeholders in the square brackets. Q and K would be the same if no values are given, considering Q is calculated during a specific point in the reaction when there must be actual values ...

Amy Xiao 1I

Basically, it's telling you to ignore spectator ions like Li+, Na+, K+, Rb+, Sr2+, Ba2+, Cl-, Br-, I-, NO3-, ClO4-, SO4(2-), etc. because these ions occur in the same form on the left and right side of the chemical equation. So when you're writing your equilibrium constant expression they'd cancel ...

Amy Xiao 1I

Thank you for clarifying why aqueous solutions cannot have partial pressure; I was under the impression that you could convert either to partial pressure or concentration for each of them.

Amy Xiao 1I

"A final point is that, when a reaction involves fully dissociated ionic compounds in solution, the equilibrium constant should be written for the net ionic equation by using the activity for each type of ion. The concentrations of the spectator ions cancel and so do not appear in the equilibri...

Amy Xiao 1I

Drake Choi_1I wrote:Does anyone know when Test 2 will be passed back?

Your TA should have passed it back by now in discussion.

Amy Xiao 1I

Get enough sleep the night before and don't cram the day of. Pack all your materials the night before so you don't scramble for it. Eat a good breakfast and give yourself some time to relax before it starts.

Amy Xiao 1I

Why does the answer (0.08567) have only 4 sig figs when the information given to you is 102.557 (nm) and 100 kJ? Does the use of Avogadro's number (6.022 x 1023) affect this?

Amy Xiao 1I

"Light with a wavelength of 102.557 nm excites a hydrogen-atom gas sample . . . If 100 kJ of energy was absorbed by the gas sample, how many photons in total caused electronic excitations? How many moles of hydrogen were excited assuming one photon interacted with one unique hydrogen atom?"...

Amy Xiao 1I

It should also be noted that if O happens to have 2 lone pairs, H-bonding can happen at each of those sites, meaning that O can H-bond twice.

Amy Xiao 1I

Would water be considered both amphiprotic and amphoteric?

Amy Xiao 1I

Which one do we use in lecture most often, and is it necessary to specify on exams?

Amy Xiao 1I

For each (a) and (b), you would get two equations, one where the compound acts as a base and one where it acts as an acid.

Amy Xiao 1I

If given the molecular formula of a ligand would you be able to figure out how many possible bonding places of the ligand there are without having to draw the Lewis structure?

Amy Xiao 1I

What is the purpose of having the "diaqua" in parentheses in sodiumbisoxalato(diaqua)ferrate(III)? What does that tell you?

Amy Xiao 1I

If you were just given a transition metal, would you be able to estimate what coordination numbers it would have? I am under the impression that coordination numbers tell you how many bonds that TM can form.

Amy Xiao 1I

Leslie Almaraz wrote:When given the name of the compound, does it matter the order in which you formulate the atomic symbols?

Could you clarify what you mean by "atomic symbol"?

Amy Xiao 1I

Can anyone clarify about the homework under Molecular Shape and Structure in the syllabus that is 2F? Has Lavelle ever gone over the topics in 2F, and should we do homework problems from that set?

Amy Xiao 1I

Hybridization is when atomic orbitals are mixed into hybrid orbitals so that chemical bonding is easier. Basically, electrons can move into new hybrid orbitals and these orbitals are used to create bonds with other hybrid orbitals in different atoms in order to for a molecule. Can you explain what ...

Amy Xiao 1I

If there is no central atom, there would simply be no A in the formula. For example, in I3-, the formula is X2E3 because there is no central atom. (This is problem 2E13A)

Amy Xiao 1I

Is bent or angular the best term to use to describe a trigonal planar/tetrahedral with one lone pair? VSEPR charts online use the term "bent," but the textbook calls them "angular".

Amy Xiao 1I

I can see where you would be confused, because I am as well. All I know is that on the tests, when there are complex compounds, the test question seems to indicate where the central atoms are and what they are bonded to, and then you could probably go from there.

Amy Xiao 1I

I think so, because mixing ions into water (which is polar) would cause the more positive dipole on the H of water to attract/be attracted to the chlorine ion. I think the term polar would apply to ions in this case; polar seems to just be if there is a net charge that is not zero.

Amy Xiao 1I

Can elements that break the octet rule only do so until their formal charge becomes zero, or is the formal charge not a major factor for the octet rule?

Amy Xiao 1I

Is it possible for a central atom to have multiple sigma bonds, each connected to a different atom? Or does the one sigma bond rule hold true for the entire atom?

Amy Xiao 1I

Can someone explain parts B and C? For B, I am confused about why the hydrogen bonding in water is stronger than the attractive forces in diethyl ether. I thought diethyl ether would also have hydrogen bonding. For C, why does a lower surface area due to the compactness of dimethylpropane cause it t...

Amy Xiao 1I

Can someone explain why CHI3 has a higher melting point that CHF3? If fluorine is more electronegative than chlorine, wouldn't it have a stronger hold on the electrons and cause there to be a higher melting point instead?

Amy Xiao 1I

Can someone give a quick and easy-to-remember summary for all the forces? (Ion-Dipole, Dipole-Dipole, London, and Hydrogen Bonding) Can you also provide examples where multiple can exist in the same molecule?

Amy Xiao 1I

To summarize, all molecules have dipole moments unless they are between the same element and/or symmetrical?

Amy Xiao 1I

Your explanation was super helpful, but can you explain why you don't know where the O is coming from? Why can't you just add the mol of Oxygen from CO2 and H2O?

Amy Xiao 1I

I think that we need a blue book, since it is an official midterm, as well as a scientific calculator for basic calculations. Other than that, some extra pens would be good to have on hand too. And I haven't heard anything about the midterm locations yet, so he will probably be sending out an email...

Amy Xiao 1I

For part e, why are there two unpaired electrons for Ni2+?

Amy Xiao 1I

For Gallium to give away 3 electrons requires lower energy compared to accepting 5 electrons.

Amy Xiao 1I

Going off of what Connie said, the 3d block is already filled so you don't have to include those electrons in when you count; a good rule of thumb for me is to see what column (group) the element is in and use that number. Sb is in the 5th group from the left if you disregard the d block of metals.

Amy Xiao 1I

The textbook problem 2A.1c states that Mn has 7 valence electrons, but it is very clear about "including d electrons."

Amy Xiao 1I

If that is the case, why even have the Octet rule? I understand that it is an easy rule of thumb for many elements, but if it does not really apply after the d-orbital comes into play, is it really helpful?

Amy Xiao 1I

I am also confused. I thought covalent bonds were ones in which the electron was already free to move around, so I am not understanding how an electron can be free to move around the entire molecule instead. What allows it to do so?

Amy Xiao 1I

I'm not too confident on this topic either, but I'm pretty sure you remove or add an electron to the highest energy level depending on whether you want a cation or anion. For example, to change Zinc from its ground-state to Zn+, you take the ground state configuration [Ar]3d^104s^2 and remove an el...

Amy Xiao 1I

Do you know any good jokes about Sodium Hypobromite?

NaBrO

Amy Xiao 1I

Why are noble gases good at singing?

Because their resonance is the most stable!

Amy Xiao 1I

Have you seen the new James Bond movie?

The name's Bond. Covalent Bond.

Amy Xiao 1I

Why is Helium the best audience for a comedy show?

Because it always says He He He.

Amy Xiao 1I

The more you know about the velocity, the less you know about the speed, and vice versa. The reason for this according to Dr. Lavelle is that the process of observing either one of those influences the other. If you set up more sensors to get accurate velocities, you will move the electron off cours...

Amy Xiao 1I

Dr. Lavelle uses those terms practically interchangeably when discussing the photoelectric effect.

Amy Xiao 1I

I think Dr. Lavelle went over this in class while explaining the photoelectric effect. He did not go over black body radiation much, and atomic spectra relates to electrons, not electromagnetic radiation, so that narrows it down to either electron diffraction or the photoelectric effect. I chose to ...

Amy Xiao 1I

My Chem teacher argued that Iron Man was a woman.

Fe-male

Amy Xiao 1I

I reached out to Alpha Chi Sigma and was told that tutoring would start Week 3, so make sure to contact them once Monday starts!

Amy Xiao 1I

What element best describes me?

The element of indecision: Imnotsurium

Amy Xiao 1I

I took Quiz 1 after running into the room 3 minutes late and I was panicking, but I calmed down and thought out my answers. I would say that the best thing to do is review everything that you need on the syllabus: pen, non-graphing calculator, etc. If something will be given, don't try to remember e...

Amy Xiao 1I

I think that Lily is right in saying that there is no correlation between light intensity and the kinetic energy in the equation E=hv.

Amy Xiao 1I

C would be the correct answer. Think of the "change in the electrical field" as slope-- how steep a wave goes. As frequency decreases, the waves stretch out farther and the slope would be less steep because they are now farther apart.

Amy Xiao 1I

What effect does the magnetic field have on the electrons, then?

Amy Xiao 1I

I personally feel like the general rule of thumb for round should include only integers ending in 0.5 or 0.33. Anything else, can either be rounded up or down. For example: If your result is a number that is 2.55, obviously round that to just 2.5. I understand that you would round 2.55 to 2.5, but ...

Amy Xiao 1I

What should the threshold be for decimals when solving for empirical formulas? I think that if the results are 0.1 away from the nearest whole number, then you round to that whole number. Otherwise, I think you multiply to the nearest whole number. However, I see in some example problems, like one ...

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