CHEMISTRY COMMUNITY

HilaGelfer_2H

Thank you so much much Dr Lavelle for a great two quarters in 14A/B! It was a great year learning "good chemistry" and I could not have asked for a better professor my first year at UCLA! Thank you for working so hard to ensure that we all understood the content and by making us feel like ...

HilaGelfer_2H

Hi, You are exactly on the right track! In order to find the rate constant from the pseudo rate constants we first have to determine all the orders for the respective reactants and then use the pseudo rate constant for one of the reactants and divide it by the reactants to power of their orders. We ...

HilaGelfer_2H

Hi, You can tell if a reaction is controlled by kinetics or thermodynamics by seeing if it occurs spontaneously and if it does what is the rate at which it occurs. For instance, in his lectures Dr Lavelle used the conversion of diamond to graphite as an example. Even though this reaction occurs spon...

HilaGelfer_2H

Hi

I hope all is well.

When doing 6L.3 part d, I was confused on how to balance the equations since we are not given whether the reactions occur at basic/acidic conditions. Are we supposed to assume one condition over the other?

Thank you so much for all your help and good luck studying :)

HilaGelfer_2H

Hi,

In general, I think it is important to know that metal solids are conducting where as non metal solids do not tend to conduct electricity.

I hope this helps :)

HilaGelfer_2H

Hi, In general when balancing redox reactions you must first divide the reaction into the reduction and oxidation half reactions. Afterwards, you must balance each half reaction based on the acidic/basic conditions mentioned. In each half reaction make sure to include the number of electrons gained ...

HilaGelfer_2H

Hi, For cell diagrams, you would include what is the final combined equation of what is occurring in the galvanic cell. Therefore, I am pretty sure this would depend on a case by case scenario depending on what the final balanced equation is(but in most cases I've seen on homework it was not necessa...

HilaGelfer_2H

Hi,

The cell potential is the difference in charge between the reducing and oxidizing agents. Therefore, I am pretty confident that if water is added it should not affect the cell potential unless it is directly involved as a reducing or oxidizing agent, which isn't very likely.

Hope this helps :)

HilaGelfer_2H

Hi,

You do not multiply the cell potential, since E tells you the voltage/charge different between the reducing and oxidizing agent and that difference remains constant no matter how many molecules/electrons are involved in the reaction.

I hope this helps :)

HilaGelfer_2H

Hi,

The half life equations for each order are different because you derive the equations from each order's individual integral rate law, which is different depending on the rate of the reaction.

I hope this helps :)

HilaGelfer_2H

Hi!

If you flip the equation then you should also flip the sign of E since now the substances are flipped where the reducing agent is now the oxidizing agent and vice versa.

I hope this helps :)

HilaGelfer_2H

Hi,

The overall order of the reaction can be found by adding the individual orders of each reactant.

I hope this helps :)

HilaGelfer_2H

Hi, When balancing equations you have to know if it is under acidic or basic solution conditions. When balancing the redox reactions first split the equations into their half reactions and determine how many electrons are being lost or gained and add that to your half reactions. If the reaction occu...

HilaGelfer_2H

Hi,

Can someone help me balance this equation? Each time I do it my charges do not balance out. I've attached the problem below.

Thank you so much for all your help :)

HilaGelfer_2H

Hi, These two equations will equal one another if they both have the same stoichiometric coefficients in the balanced chemical equations or if you divide each product or reactant by their respective stoichiometric coefficient. This equation tells us that the concentration of reactants is decreasing ...

HilaGelfer_2H

Hi, Step up sessions are used to provide individuals with a step by step process on how to approach and problem solve the questions we learned that specific week. They are helpful if you need more clarification on how to approach problems in the given section we are learning. Workshops are similar t...

HilaGelfer_2H

Hi! Although both dividing and multiplying end up with the same answer, when dividing each individual reaction rate by the stoichiometric coefficient we can find a law and method that applies to all chemical reaction rates despite their stoichiometric coefficients. Consequently, this provides us wit...

HilaGelfer_2H

Hi,

Voltage is the electrical potential energy difference between the cathode and the anode. Consequently, it is also a measure of the chemical potential for a redox reaction to occur (must be positive for the redox reaction to occur spontaneously).

I hope this helps :)

HilaGelfer_2H

Hi, The salt bridge allows the reaction to continue occurring. As the redox reaction begins and the cathode gains more electrons, it loses its pulling ability, which could potentially stop the reaction from occurring. The salt bridge transfers negative ions to the anode and positive ions to cathode ...

HilaGelfer_2H

Hi, Can someone explain how to do this problem because my answer is not matching the one in the textbook. 4A.3 Air in a bicycle pump is compressed by pushing in the handle. The inner diameter of the pump is 3.0 cm and the pump is depressed 20. cm with a pressure of 2.00 atm. (a) How much work is don...

HilaGelfer_2H

Hi,

I saw the equipartition theorem described in our readings and was wondering if this equation and translational/rotational/vibrational energies will appear on the midterm.

Thanks beforehand for all your help :)

HilaGelfer_2H

Hi,

You can find all the homework problems that could appear on midterm 2 by looking at Dr Lavelle's syllabus and completing the problems stated under thermochemistry and thermodynamics.

I hope this helps :)

HilaGelfer_2H

Hi, Since all systems are not fully effective and there is no way for the system to reach 0K, everything in reality has some amount of residual entropy. I don't think there is exact cutoff for what is considered small since it depends on the data presented, but it is important to remember that the m...

HilaGelfer_2H

Hi, Since in an isothermal reversible pathway the temperature remains constant, the system and the surroundings are considered to be at thermal equilibrium. As a result, DeltaS of systems equals 0. Furthermore, since an isothermal reversible pathway consists of infinitesimally small steps, the small...

HilaGelfer_2H

Hi, you use the whole number coefficients to balance the equation; however, for calculating entropy/enthalpy you need to see how many moles of the substance are involved in the reaction since you will have to divide or multiply accordingly depending on how you balance your chemical equation. I hope ...

HilaGelfer_2H

Hi, Work done on an irreversible pathway tends to occur quickly and take less work since the system is pushing against a lower external pressure. On the other hand, the work done by a reversible pathway maximizes the amount of work performed by the system since the external pressure is almost exactl...

HilaGelfer_2H

Hi, During his office hours, Dr Lavelle confirmed that the midterm will include all of outline 3 remaining post midterm 1 and outline 4. As a result, I would assume we will finish outline 4 by tomorrow's lecture, since I am pretty certain that is the last lecture for the midterm. I hope this helps :)

HilaGelfer_2H

Hi, As many others have said I would definitely recommend doing the problems listed on the outline and syllabus since those also appear on the exam. UA sessions and TA office hours can also be helpful if you have specific questions or topics you’d like to review. Lastly, the most helpful thing for m...

HilaGelfer_2H

Hi, Enthalpy means the amount of heat released or absorbed in a reaction at a constant pressure. Since Enthalpy, which equals DeltaH, requires a constant pressure, you can only solve for qp/ethanlpy when the system has a constant pressure. if the system has a constant volume rather than a constant p...

HilaGelfer_2H

Hi, So for heat capacity of calorimeters, it is a constant for that piece of equipment, therefore we are not interested in the mass of a substance in the calorimeter. We know that q=mcdeltaT, but since for the calorimeter the heat capacity is the same no matter the mass we can disregard the m and yo...

HilaGelfer_2H

Hi, The first thing you have to do is find the calorimetry heat capacity by using the equation q=mcdeltaT. In order to do that you put in the information found in the first sentence of the question and find the missing value c, which you can apply to the second half of the question since c is a cons...

HilaGelfer_2H

Hi, I am confused on how to approach this question. I know we need to use the specific heat capacity of water but I am not sure exactly how. If you combine 420.0 mL of water at 25.00 ∘C and 130.0 mL of water at 95.00 ∘C, what is the final temperature of the mixture? Use 1.00 g/mL as the density of w...

HilaGelfer_2H

Hi! Dr Lavelle said we do not have to worry too much about sig figs for this course so I would recommend doing it with the most accurate values and then comparing it to answers in the question. My TA in discussion said that If your value varies slightly from one of the answer choices then to choose ...

HilaGelfer_2H

It was in the readings he assigned and it was included in some homework problems so I would say we do have to know how. However, during his office hours Dr Lavelle said that he would give us the Ka1 values for the problem and the Ka2 value if it was significant. The textbook says that only H2SO4 has...

HilaGelfer_2H

Hi,

For part B, you need to use the ideal gas law to find out how many moles of hydrogen you have and then from there you can multiply the amount of moles by the enthalpy change of the reaction (which is in per mole) to find your final answer.

Hope this helps :)

HilaGelfer_2H

The midterm will be structured like last quarter and will all be multiple choice. There might be partial credit but I am not sure.

Hope this helps :)

HilaGelfer_2H

Hi,

I know how to approach 4D.15 in the textbook, but I keep getting the right answer with a positive sign where as the book labels it as -312KJ.mol^-1. Does anyone know what I might be doing wrong?

I attached the problem below.

thanks beforehand :)

HilaGelfer_2H

Hi,

Yes, all the lectures from week 1 to the end of week 3 are covered on the midterm. Week 4 lectures are not covered on the midterm.

Hope this helps :)

HilaGelfer_2H

Hi In order to identify if a salt is acidic basic or neutral you have to break it apart into the anion and the cation. Then the key rule to remember is that the conjugate acids of weak bases make a salt acidic and the conjugate bases of weak acids make the sale basic. Based on these rules, you can t...

HilaGelfer_2H

Hi,

When I study for Chem midterms I have found it helpful creating a study guide based on the learning outcomes/outline points on Dr Lavelle's website. This way you can connect his lectures to our outlines which include the topics we will be tested on.

I hope this helps :)

HilaGelfer_2H

Hi,

Strong bases include oxides and hydroxides of groups 1 and 2 metals and some of the strong acids include all the hydrogens bonded to halogens except hydrogen fluoride which is a weak acid.

Hope this helps :)

HilaGelfer_2H

Hi,

For the enthalpy of formation, you can account for phase changes by adding the enthalpy required/released in that phase change to your enthalpy of formation calculations.

Hope this helps :)

HilaGelfer_2H

Hi!

During Dr Lavelle's lecture he said we could account for compounds/elements not in the standard state by adding the enthalpy for the phase change required to put them in their standard state.

Hope this helps :)

HilaGelfer_2H

Hi!

I think the cut off is like any other weak acid where Ka < 10^-4.

I hope this helps :)

HilaGelfer_2H

Hi,

I think you have to log onto sapling by first accessing CCLE and clicking on week 1 to open the website through the class website. When I did that the first day of the quarter, it let me apply me credit from last quarter and immediately enter the homework assignments.

I hope this helps :)

HilaGelfer_2H

Hi,

To solve part be you would use your answer for part i and then the equation M1V1 = M2V2 to determine the original hydroxide concentration before dilution.

I hope this helps :)

HilaGelfer_2H

Hi, Can anyone help me with how to approach this problem? At a certain temperature, the given reaction has an equilibrium constant of Kp=393 PCl3(g)+Cl2(g) --> PCl5(g) PCl5 is placed in a sealed container at an initial pressure of 0.0250 bar . What is the total pressure at equilibrium? Thank you in ...

HilaGelfer_2H

Hi,

I think in this scenario the lactic acid can also react with the HBO2 resulting in a lowering on the compounds concentration.

I hope this helps :)

HilaGelfer_2H

Hi! For this problem you would want to first write a balanced chemical equation followed by making an ICE table. Afterwards, using the values in your ice table you can fill in the concentrations and set them equal to your Ka value. After you find what x equals, you can use it to identify your hydron...

HilaGelfer_2H

Hi, yes your reasoning is exactly correct. A decrease in SO3 through condensation will result in more product being formed to minimize the effect of the change resulting in both of the reactants reacting to form BOTH SO3 and NO resulting in an increase of NO compared to its initial value. I hope thi...

HilaGelfer_2H

Hi, Your reasoning for part II is completely correct. Since the reaction is exothermic, by decreasing the temperature the reaction favors the side with a lower enthalpy which in this case is the products. Regarding part I, when the volume is decreased (pressure is applied/increases) the reaction wil...

HilaGelfer_2H

Hi, The answer to this would be D since even though the reaction has reached equilibrium it is impossible for the reaction itself to stop occurring naturally if possible. However, since at equilibrium the forward and reverse reactions occur at the same rate the concentrations of the reactants and pr...

HilaGelfer_2H

Yes exactly! Solids/liquids which are pure substances are not included in the calculation of k.

hope this helps :)

HilaGelfer_2H

Hi!

Yes that is exactly correct! Furthermore, since water, the solvent, is a pure substance (liquid) it is not included in the calculation of K.

I hope this helps :)

HilaGelfer_2H

Hi,

NH2 is a monodentate ligand. However the ligand in the photo, is not only NH2 it consists of two NH2 molecules which are part of the whole ligand and provide the two lone pairs. Therefore, not NH2 but the whole ligand containing two NH2 compounds is bidentate.

I hope this helps :)

HilaGelfer_2H

Hi, Just like you wrote, you can find the work function if the minimum frequency needed to remove an electron is given in the problem.Likewise, if the problem gives you the threshold energy (work function) needed to remove an electron you can calculate the threshold frequency by using the equation E...

HilaGelfer_2H

Hi, For AX3E2 the central atom has five regions of electron density. Therefore, its electron arrangement must be trigonal bipyramidal. When choosing the lone pairs you have to choose lone pairs that minimize electron pair repulsions. By placing our lone pairs in the equatorial position, it minimizes...

HilaGelfer_2H

Hi,

Ligands are lewis bases which donate an electron pair to the central metal atom. Each ligand can consist of one or electron density regions of the central atom depending on if it is polydentate or not.

I hope this helps! :)

Good luck studying for the final!

HilaGelfer_2H

Hi, I don't think we need to know the exact ranges of each type of EM radiation, other than visible light which has wavelengths between 400nm to 700nm. However, I think we do have to know how the order of the EM spectrum so we can compare which type of light has a shorter wavelength/high frequency. ...

HilaGelfer_2H

Hi!

She had her review session on acids and bases today and she posted her slides. I've attached them here too just in case you still can't find them

I found her slides really helpful!

Acids and Bases Review.pdf: (1.55 MiB) Downloaded 199 times

HilaGelfer_2H

Hi! The relative strength of an acid is determined by how easily the acid can lose an H atom as well as how stable the resulting anion becomes (depending on if resonance exists and the electronegativity/electron pulling power of the anion). Using these two criteria you can determine which acid is st...

HilaGelfer_2H

Hi
I found this sapling link with oxidation number rules really helpful!
https://sites.google.com/site/chempendix/oxidation

HilaGelfer_2H

Hi!

The coordination number of the complex compound reflects how many ligands the TM is bonded to.

I hope this helps :)

HilaGelfer_2H

Hi,

Transition metals are located in the active site of the enzyme and help the molecules bond to the enzyme.

I hope this helps :)

HilaGelfer_2H

Hi,

An organometallic compounds refers to ligands bound to carbon/carbon being a central part of the compound since any organic molecule must include carbons.

I hope this helps :)

HilaGelfer_2H

Hi, Although in the lectures Dr Lavelle did not give us the exact electron affinities for noble gases, he said that all noble gases have negative electron affinities since they do not release energy when an electron is added but rather they require energy for an electron to be added. I hope this hel...

HilaGelfer_2H

Hi!

Nickel 2+ can have enough room for the bonds since it also has an empty p-shell it can utilize if necessary.

I hope this helps :)

HilaGelfer_2H

Hi!

Hybridization occurs as molecules come closer together and begin to form bonds. As atoms come closer together, the electron density of the atoms shift/change in order to allow the optimal amount of bonds necessary for the compound to form.

I hope this helps :)

HilaGelfer_2H

Hi,

Dr. Lavelle said in today's lecture that we will go over naming, but since we haven't yet I don't think you need to know it until he covers it during lecture.

I hope this helps :)

HilaGelfer_2H

Hi,

in these cases I would look at the size of each individual atom since the larger the atom the longer the bond length. You can use the trends of ionic/atomic radii to determine which atom is larger.

I hope this helps :)

HilaGelfer_2H

Hi,

When solving this problem, I would first look at the size/charge of the cation and anion respectively. A smaller sized cation with a greater positive charge will lead to stronger ionic character, thus making the answer to this question CAO rather than KCL.

HilaGelfer_2H

Hi,

When approaching this problem, I would first draw out the lewis structures and from there you can see if any atom in each molecule has an unpaired electron, which would make it a radical.

I hope this helps :)

HilaGelfer_2H

Hi! The central atom is always the least electronegative and in this example it is carbon. Since Carbon in CH3CL is bonded to each atom, the molecule consequently has 4 bonding regions with no extra electrons/lone pairs. Therefore, its VSEPR notation must be AX4 since the structure describes a centr...

HilaGelfer_2H

Hi,

All molecules can have instantaneous dipole moments that only last temporarily since electrons continuously move in the molecule. Therefore, while CO2, like all other nonpolar molecules, may not have a permanent dipole and it has instantaneous dipoles that are constantly changing.

HilaGelfer_2H

Hi, By stating that the molecule is arranged tetrahedrally, he references how the shape of the molecule shows a central atom(in his case sulfur) surrounded by 4 regions of electron density. However, although the shape consists of those four regions, when we name the shape of the molecule we only ref...

HilaGelfer_2H

Hi,

I was reading the textbook and saw that there was an angular momentum equation. Do we need to know that for the midterm or will we not be tested on it?

Thank you so much beforehand :)

HilaGelfer_2H

Hi! You can predict the number of valance electrons based on the atom's placement in the periodic table as well as based on whether it is more likely to accept or lose its electrons. For instance, Oxygen, which is in group sixteen of the periodic naturally has six valence electrons, and becomes an O...

HilaGelfer_2H

Hi!

I attached the link with the rules to finding oxidation numbers that sapling created. I hope this helps :)
https://sites.google.com/site/chempendix/oxidation

HilaGelfer_2H

Hi! You are right that the four hydrogens connected to the nitrogens could create a hydrogen bond with water. However additionally, the two nitrogens each have one set of lone pairs and the oxygen molecule has two lone pairs open for hydrogen bonding. Thus, the maximum number of water molecules that...

HilaGelfer_2H

Hi, In his lecture Dr. Lavelle explained that when the difference is between 1.5-2 we would have to look at how the compound actually behaves in experiments such as its ability to dissolve in water in order to identify whether the compound acts more as an ionic or covalent bond. I hope this helps :)

HilaGelfer_2H

Hi!

A lewis acid is electron deficient meaning it needs a lone pair of electrons in order to complete its octet. On the other hand, a lewis base has enough electrons to complete its octet and can share one of its lone pairs with the lewis acid.

I hope this helps :)

HilaGelfer_2H

Marley Magee 2H wrote:Does anyone know where they'll be posted? Just under grades on CCLE, right?

Hi!

Yes Dr. Lavelle said they will be posted on CCLE under grades. I hope this helps :)

HilaGelfer_2H

Hi, One of the limitations in lewis structures is that they do not reflect bond length or resonance (when each bond length is between a single and double bond). When discussing the bond length, it depends on which atoms are bonding to each other as well as if there is resonance in the compound. Thus...

HilaGelfer_2H

Hi! The schrodinger equation is meant to explain the movement of an electron around an orbital, which can be described through many different functions since each electron behaves differently. When you take the double derivative of the wave function which is the hamiltonian (H) you get the wave func...

HilaGelfer_2H

When will our final grades from the midterm be posted? Also, I heard someone say Lavelle may be offering partial credit? How would that be the case because they are multiple choice? Hi! I am in Dr. Lavelle's office hours and he said that all grades including chemistry community should be updated by...

HilaGelfer_2H

Hi! Since quantum numbers represent a certain a state for the electron, knowing the electron configuration you'd have to know which shell/subshell the question was referring to so you could write the correct quantum numbers for that state. Once you know which state you referring to you can decipher ...

HilaGelfer_2H

Hi! Quantum numbers were a little confusing for me as well when I first watched the lecture. But afterwards, I watched a khan academy video on it which really helped me fully comprehend quantum numbers and electron configurations. I've attached the video below so you can watch it. I definitely recom...

HilaGelfer_2H

Hi! The second ionization energy is always higher than the first one since when the atom loses an electron is has a net positive charge. Consequently, when something has a net positive charge the nuclear charge increases holding the electrons more tightly towards the nucleus. As a result of this inc...

HilaGelfer_2H

Hi! When I was watching the lecture regarding trends in the periodic table, I noticed that the radii of isoelectronic particles such as O2- and F- still have a big difference in their ionic radius. I was wondering why this was so since they have the same number of electrons? Is this because of how t...

HilaGelfer_2H

Hi, I think the first sentence refers to describing the shape of the orbitals, how many subshells of the orbitals there are and any other descriptions of the orbital in general. And the second sentence refers to the relationship amongst the orbitals themselves and how they affect the electron's beha...

HilaGelfer_2H

Hi, I hope all is well. I just watched the audio-visual focus module on the heisenberg indeterminacy equation and noticed that Dr. Lavelle said that the delta v (uncertainty in velocity) must be less than the speed of light. Thus, I was also wondering if there was a limit on the uncertainty in posit...

HilaGelfer_2H

Hi! In this question, the -1 refers to how many orbitals of the subshell f there are. Since our l value is 3, we know that our ml values can be any integer from -3 to 3. Therefore, the -1 part of this problems reflects that you have three orbitals(-3,-2, and -1) of the f suborbital on shell 6. Knowi...

HilaGelfer_2H

Hi,

The electron density distribution refers to the arrangement of the electrons in atom (and the probability of finding the electron at that certain space).

I hope this helps :)

HilaGelfer_2H

Hi! We will not be tested on the particle in a box on the midterm. Dr Lavelle said in his lecture today that our midterm will only go up to the last slide he did which introduced the different atomic orbitals and did not discuss a particle in a box. In general, our midterm will only contain all the ...

HilaGelfer_2H

Hi! When comparing the different types of orbitals Dr. Lavelle mentioned that the s orbital unlike the others has no nodal plane and a symmetric electron density distribution. Does this then mean that there is no point in the s orbital for which we get an electron density distribution of 0? Furtherm...

HilaGelfer_2H

Hi! H in Schrodinger's equation is called the Hamiltonian and refers to the double derivative of the wave function(psi). This equation is thus stating that the double derivative of the wave function will show us (be equal to) the mathematical wave function of the electron's energy. I hope this helps!

HilaGelfer_2H

Hi! Here are a couple of things that help me. When watching the lecture, I make sure to take detailed notes and pause the video when needed. When we finish a certain topic in a video I also pause the video and try to write down or verbally explain the topic to myself in order to test what I actually...

HilaGelfer_2H

Hi! In the example in the lecture light was emitted by the hydrogen atom, meaning that the electron lost energy resulting not only in it decreasing its principal quantum level from 4 to 2 but also experiencing a negative change in energy. However, if the opposite was occurring and the electron was b...

HilaGelfer_2H

Hi! When explaining that waves cancel each other out, Professor Lavelle is referring to the destructive interference of waves, which occurs when waves interact with one another out of phase. When this occurs, a photo of the resulting wave will look shorter since the overlap of the two waves cancel e...

HilaGelfer_2H

Hi! When figuring out the answer I could deduce that C was the correct answer but I am not completely sure on why C is correct. Can someone explain this to me in more detail? Which of the following happens when the frequency of electromagnetic radiation decreases? Explain your reasoning. (a) The spe...

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