CHEMISTRY COMMUNITY

Megan Chan 3A

I felt that the final was fair. There were a few questions that I found difficult, but it was balanced and the allotted time allowed for me to review all my answers multiple times. Looking forward to hopefully returning to in-person classes soon!

Megan Chan 3A

Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions: b) H+(aq) + OH−(aq) → H2O(l),the Brønsted neutralization reaction The solution manual shows that the half reactions for this are O2(g) + 4H+(aq) + 4e- → 2H2O(l) O2(g) +2H2O(l) + 4e- →...

Megan Chan 3A

An intermediate is a species that is produced, then consumed, so it would not be present in the overall rate law.

Megan Chan 3A

The second transition state is not necessarily always larger than the first one. The relative height is dependent on whether the elementary step it represents is slow or fast. For example, if the second step of a reaction is slow, then its transition state will be higher since it has a larger activa...

Megan Chan 3A

When you type out the answer, it is not necessary to include the exponent for a term raised to the first power nor the [C] term since it will equal 1 given that it is raised to the zero power, so you rate law should be Rate = k[A][B]^2

Megan Chan 3A

Hi! If you look in the solution manuals errors file, there is a typo in the question. The equation you are trying to balance should be Cl2(g) → ClOH(aq) + Cl-(aq). Hope this helps! https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14B/Solution_Manual_Errors_7Ed.pdf Ohh that makes a lot ...

Megan Chan 3A

I'm not quite sure how to balance the reaction in part d. The equation is Cl2(g) -> HClO(aq) + Cl2(g). I've tried adding water and hydrogen ions to each side but I can't seem to balance it. Is there any kind of method for balancing redox reactions with only one reactant? Thank you!

Megan Chan 3A

The final is from 9:30-11am. You can find this on the exam schedule link on Dr. Lavelle's website!

Megan Chan 3A

You would take the same approach as you did for [A], since the concentration is tripled, you would put [3B].

Megan Chan 3A

Yes! I think that's right, molecularity describes the number of molecules that participate in an elementary step, so the molecularity is equal to the sum of the stoichiometric coefficients of the reactants.

Megan Chan 3A

You need to look up the reduction potentials in Appendix 2B for H+ and Sn 2+. You can use this to solve for E naught and then plug this value and the concentrations given in the question into the Nernst equation to solve for -log[H+] which is equivalent to pH. Hopefully this helps you get started!

Megan Chan 3A

The unit for rate is molarity per second (M/s) so depending on the order of the reaction, you will determine the units for k that will cancel the other terms to give you the units for rate. For example, if you have a rate law: Rate = k[A]^2[B] the overall order of the reaction is 3, so you have M^3 ...

Megan Chan 3A

Iron rusts in the presence of water and oxygen, so your unbalanced equation should be:

Fe + H2O + O2 -> Fe2O3∙3H2O

Now you can just add coefficients to balance the equation!

Megan Chan 3A

You haven't balanced the hydrogens! They are present on the left side of your equation, but not on the right.

Megan Chan 3A

I agree with the previous response that they are used interchangeably! I'd like to add that I believe when there is no longer a T variable, that the assumed temperature is 298K, so we would need to use E = E* - RT/nF(lnQ) if the reaction occurs at a different temperature.

Megan Chan 3A

When using shorthand notation, solid species should be written on the outside and aqueous species should be towards the center closest to the salt bridge!

Megan Chan 3A

Aqueous substances should be the closest to the salt bridge and the solids should be on the outside!

Megan Chan 3A

Have you possibly typed out the equation wrong? NO-2 to NO-3 should be reduction since there is a gain of an electron. But here are some general steps for balancing redox reactions in basic solution (this method is different that the one given on Dr. Lavelle's website): 1) Balance the half reactions...

Megan Chan 3A

One of the most recognizable corrosion reactions is the rusting of iron. Rust is caused by iron reacting with oxygen gas in the presence of water to create an oxide layer. Iron can form several different oxides, each having its own unique color. Red rust is caused by the formation of iron(III) oxide...

Megan Chan 3A

Adding on to the previous reply, the oxidation number of the elements should add up to the overall charge of the molecule, so this will help you determine the oxidation number of an element after following the rules for cations, oxygen, etc.

Megan Chan 3A

You just forgot to set the side of the equation with the 50°C water negative since it is losing heat. Specific heat capacity can be used!

Megan Chan 3A

I think the best way to determine which R value to use is to look at the units and find the one that will cancel to give you the units you need for your final answer. In the case for the ideal gas law, PV = nRT, you would choose 0.08206 L.atm.K^-1.mol^-1 since these are the units for pressure, volum...

Megan Chan 3A

So for part A we know that ΔS(sys) = ΔHvap / (boiling point). Using the table, you can find these values for CH4 and plug them into the equation. ΔS(surr) will just be the negative value of ΔS(sys). Part B and C will follow the same process but with ΔHfus and freezing point. Keep in mind for part B ...

Megan Chan 3A

ΔSvap = (ΔHvap) / (T @ BP) so you can just plug in the values given and solve for T!

Megan Chan 3A

First you need to convert the watts to kJ/min which can be done through dimensional analysis. Then determine the moles of each sample that were vaporized by doing (initial mass) - (final mass) and dividing by the molar mass of the sample. Now that you know how long the sample was heated (3.12 minute...

Megan Chan 3A

The enthalpy of formation of trinitrotoluene (TNT) is −67kJ/mol, and the density of TNT is 1.65g/cm^3. In principle, it could be used as a rocket fuel, with the gases resulting from its decomposition streaming out of the rocket to give the required thrust. In practice, of course, it would be extreme...

Megan Chan 3A

Yes! I think since they don't specify we can just assume 298K because that is the standard temperature.

Megan Chan 3A

The correct equation is ΔU = q + w but there are different variations you can derive if there is constant pressure or constant volume. Possibly those are the equations you see online?

Megan Chan 3A

There is heat transfer between the two volumes of water so the heat lost by the warmer water is equal to the heat gained by the cooler water: q(420.00mL water) = -q(130.00mL water) Use the formula, q = mc(Tf - Ti) and plug in the values given to solve for the final temperature! Hopefully this helps!

Megan Chan 3A

Your first need to find the specific heat of the calorimeter which is done by dividing q cal by ΔT. The heat gained by the subsequent reaction is equal to the heat lost by the calorimeter so you can solve for q rxn by setting it equal to -q cal. Because this reaction occurs in constant volume ΔU = q...

Megan Chan 3A

There is energy transfer between the ice and the water, so q(melt ice) + q(raise temp of melted ice) = -q(water). q(melt ice) = n(ΔHf water) Use q = mc(Tf - Ti) to find the energy required to change the temperature of the melted ice and water. Then just plug the values given in the problem to solve ...

Megan Chan 3A

ΔE = w + q

w = 425 kcal x (4.184 kJ / kcal)
q = -5.00 x 10^2 kJ (this is negative because heat is being released)

ΔE = (425 kcal x (4.184 kJ / kcal)) - (5.00 x 10^2 kJ)

Megan Chan 3A

Midterms were not curved in 14A because the test averages were fairly high, so I don't think they will be curved this quarter either.

Megan Chan 3A

This question is asking you to find the heat capacity of the calorimeter, not the specific heat capacity of a substance. Heat capacity is the heat required to raise the temperature of an object by 1 degree. This stays constant for the calorimeter which is why you don't need to divide by grams.

Megan Chan 3A

You can solve this by setting up an equation to represent the heat transfer between the ice cube and water: heat required to melt ice + heat required to change temp. of ice = heat released during temp. change in water. Use the heat of fusion of ice to determine the energy used to melt the ice. Use q...

Megan Chan 3A

If the strength of the bonds on the reactants is larger, then the reaction is endothermic. If the strength of the bonds on the products is larger, then the reaction is exothermic. So, you can "add" the relative bond strengths on each side of the reaction to determine whether the reactants ...

Megan Chan 3A

On Dr. Lavelle's website, he says that the correct equation is 2HD(g) ⇌ H2(g) + D2(g) so there is no change because there are equal moles of gas on each side.

Megan Chan 3A

I think the ones we need to memorize are the diatomic molecules (H2, N2, O2, F2, Cl2, Br2, and I2), all of which are gases except for Br2 which is a liquid and I2 which is a solid. It would also be good to know that mercury is a liquid in its standard state and carbon is solid. If anyone knows any o...

Megan Chan 3A

You seem to be solving the problem correctly, I think your numbers are just off because you rounded the third term in the quadratic equation too soon. Try using more decimal places for that number and your answer should change.

Megan Chan 3A

Bond enthalpies are only exact for bonds with the same atom (for example, a C-C bond), but for bonds between different atoms the bond enthalpy is an average of different bonds for different molecules. An example would be that a C-H bond would have slightly different bond enthalpies depending on the ...

Megan Chan 3A

The solvent for aqueous solutions is most likely water, so it would not be included in the equilibrium constant. I think the best thing to do is just remember that liquids and solids are excluded from the expression.

Megan Chan 3A

Seraphina Joseph 1C wrote:What outlines are being tested on Midterm 1?

I believe everything that we have gone over up until this Friday's lecture will be tested on the midterm, so chemical equilibrium, acids/bases, and the beginning of thermochemistry. Lavelle will probably be sending out an email to confirm the topics too!

Megan Chan 3A

Adding on to the previous reply, if you approximate by omitting x, you should also check that it is less than 5% of the initial concentration to make sure it is okay to approximate. Otherwise, the quadratic formula should be used to solve the equation.

Megan Chan 3A

% ionization = [H3O+]eq / [HA]initial x 100%

Megan Chan 3A

When I solved this problem, I was also getting 1.5%. What I did was use the pKa to find Kb which I found to be 1.622x10^-6. Then I set this equal to (x^2)/(0.0073-x) and solved for x. I got x = 1.08x10^-4 = [OH-] (1.08x10^-4) / 0.0073 x 100% = 1.5% I'm not sure if I am doing the problem wrong or if ...

Megan Chan 3A

This only applies to reactions with gases, so I don't believe there would be any problems involving a change in pressure with aqueous solutions.

Megan Chan 3A

I also think that the textbook problems from the outline are very helpful and is great practice for exam questions. Another thing I would recommend is the UA sessions. They provide worksheets that have past exam problems and also are great at explaining the topics from lecture and answering any ques...

Megan Chan 3A

They are different for every UA!

Megan Chan 3A

You need to make an ice table and use the initial pressure and K value to find all the partial pressures at equilibrium. PCl3 Cl2 PCl5 I 0 0 0.0230 C +x +x -x E x x 0.0230-x Once you have the partial pressures of each gas, add them together to find the total pressure.

Megan Chan 3A

Yes, that's right, find the molarity of H2O by doing 2.5mol / 50L = 0.05M. You can disregard C because it is a solid and won't affect the equilibrium constant. Then you can put this value into the icebox and solve for the concentrations of CO and H2O at equilibrium and plug these values along with [...

Megan Chan 3A

I believe so! The UA said that everything should be the same as 14A with the exception of the change in timing for exams which would be taken in lecture rather than discussion.

Megan Chan 3A

1. Balance the equation: 2AsH3 (g) <--> 2As (s) + 3H2 (g) 2. Solve for the concentration of AsH3 and H2 by dividing the given molar amounts by 3L (you can disregard As in this problem because solids are not included when solving for Q) 3. Plug in the values of the concentrations into the reaction qu...

Megan Chan 3A

For this problem, you would use the ideal gas law, PV=nRT. Since you are given the concentrations of each gas, you can rearrange the equation by dividing both sides by V to get P=(n/V)RT. n/V is equal to concentration (mol/L) so plugging in the concentrations of the gases given into (n/V) of the equ...

Megan Chan 3A

Yes! This is definitely something you should be familiar with.

Megan Chan 3A

Question 6D.11 from the textbook asks: Decide whether an aqueous solution of each of the following salts has a pH equal to, greater than, or less than 7.

The answer key says that Cu(NO3)2 has a pH less than 7. How do we determine that?

Megan Chan 3A

I think if you are given a base, like in these questions, the best approach is to find the pOH first and use that to find the pH. You would first need to find the concentration of OH- from the given base and calculate the pOH using -log[OH-]. We know that pH + pOH = 14 so that's how you can solve fo...

Megan Chan 3A

They both represent water, so it doesn't really matter which way you decide to write it. The reason it is sometimes written as OH2 rather than the conventional way (H2O) is to show that it is the oxygen atom that attaches to the TM cation.

Megan Chan 3A

This molecule has trigonal bipyramidal electron geometry, so the three lone pairs would be trigonal planar while the I atoms would be axial forming a 180 degree bond angle.

Megan Chan 3A

Yes! The geometry of a coordination compound is determined by the number of bonds on the TM cation, or the coordination number. I think there are many different geometries but we will mostly be dealing with either 4 or 6 bonds on the TM cation. If the coordination number is 6, the shape will be octa...

Megan Chan 3A

Adding on, while we won't need to distinguish between square planar and tetrahedral for a coordination number of 4, I think there are a few specific coordination compounds we should know the shape of. I think it'll be helpful to memorize that cisplatin and the heme complex are square planar, but oth...

Megan Chan 3A

I actually don't think the order for writing the chemical formulas matters according to what the UAs have said, but the convention seems to be writing the TM cation first and then the ligands next.

Megan Chan 3A

I believe there has to 6 bonds for the shape to be octahedral. If there are only 4 bonds the shape would either be square planar or tetrahedral.

Megan Chan 3A

I'm not exactly sure what you mean, you may have heard incorrectly during lecture? During the lectures about coordination compounds, we learned that the number of bonds on a transition metal cation will determine its shape. The shape of the coordination compound will be octahedral if there are 6 bon...

Megan Chan 3A

When drawing Lewis structures, we want to find the most stable molecule, and if the central atom can have an expanded octet, then that is a valid structure. Since As is in the third period, it can have and expanded octet, and by changing one of the As-O single bonds to a double bond, the molecule wi...

Megan Chan 3A

Hybridization is determined by the number of regions of electron density, so lone pairs would be taken into account. Whether the bond between atoms is a single, double, or triple bond doesn't really matter because that bond will still be counted as one region of electron density. So, the number of e...

Megan Chan 3A

Yes! The shape is cyclical when the carbon atoms form a ring. Then you would just attach the hydrogen atoms to the carbon atoms and choose the type of bonds accordingly to use the right number of electrons.

Megan Chan 3A

For the molecular shape trigonal bipyramidal, the atoms in the equatorial position will be the four atoms that are arranged horizontally and form trigonal planar geometry. The atoms in the axial position are the ones arranged vertically and form linear geometry with the center atom.

Megan Chan 3A

Single bonds can't be pi bonds because pi bonds are formed when two lobes of an orbital on one atom overlap with two lobes of an orbital of another atom. This forms a double bond where the atoms cannot rotate freely without breaking the bond.

Megan Chan 3A

I don't believe that is the case. A double bond consists of a pi bond but it is not called one. Can anyone confirm?

Megan Chan 3A

I think you're right that this has to do with the fact that I has a much larger radius than F so it is more polarizable. Since it is more polarizable, the temporary dipoles will be larger, and CHI3 will have much stronger LDFs, causing the normal melting point to be higher.

Megan Chan 3A

Elements located in the p-block of period 3 or later are capable of having expanded octets. Boron and aluminum are capable of having an incomplete octet (they are stable with 6 electrons).

Megan Chan 3A

Effective nuclear charge increases across a period because the number of protons is increasing but the the number of shielding electrons stays the same. I'm not sure if there is a trend going down a group since the number of protons is increasing, but so is the number of shielding electrons.

Megan Chan 3A

[Ar]3d6 shows the configuration of a neutral Cr atom, and to find the metal with a 2+ charge, you count forward two elements on the periodic table from Cr since a 2+ charge is obtained when the metal loses electrons. Fe is the element that would give you an identical electron configuration to Cr whe...

Megan Chan 3A

This is a list of oxidation rules copied directly from Sapling, I found that it explained it all very clearly so hopefully this helps :) A neutral element that is not part of a compound has an oxidation state of zero. Monoatomic ions have oxidation states equal to their ionic charges. The sum of the...

Megan Chan 3A

The electron will be removed from the outermost shell, so using the periodic table, you can see which orbital the valence electrons are located in and that's where the e- will be removed to form the +1 ions. Hopefully this helps!

Megan Chan 3A

Hydrogen bonding isn't actually a bond, it is a special case of dipole-dipole attraction between two molecules so the hydrogen atom will still only have one bond.

Megan Chan 3A

The sum of oxidation numbers for each element must add up to the overall charge of the molecule (so in this case, it should equal -1). Oxygen has an oxidation of -2, and since there are four oxygen atoms, the sum of oxidation states for oxygen is -8. Now, you can pick an oxidation state for the chlo...

Megan Chan 3A

I was doing the Sapling homework and on question 9, it asks which Lewis Structure is most plausible based on the oxidation number. I found the oxidation number but I'm not sure how to apply this to determine which resonance structure is the most likely. If anyone could help, thanks!

Megan Chan 3A

Adding on to the previous reply, ionic bonds are between a metal and nonmetal (a transfer of electrons) while covalent bonds are between two nonmetals who share electrons.

Megan Chan 3A

What does it mean for a bond to have overwhelming or ample bond character when given the Lewis Structure and bond length data and how do we know which description fits the chemical bond given?

Megan Chan 3A

Electron affinity refers to the amount of energy released when an electron is added to a neutral atom in the gas phase, while electronegativity is how well an atom can attract electrons, so they are not the same thing.

Megan Chan 3A

Polarization is when there is a separation of the positive and negative charge. An ion has high polarizing power, correct me if I'm wrong, when it has a small radius and is highly charged. The nucleus will have a strong pull on the electrons, so it causes distortion.

Megan Chan 3A

Adding on to the previous reply, when you look at the orbitals in the order of the periodic table, 4s comes before 3d so it will be occupied first, but once the 3d orbital is occupied, 4s will have a higher energy, so it can be written after 3s to demonstrate increasing energy levels.

Megan Chan 3A

The UA sessions are super helpful and I also highly recommend the textbook problems! I also like to take notes on the textbook chapters in addition to the lecture just in case I missed anything and it also can be helpful to clear up anything I may be confused about.

Megan Chan 3A

Oh I see, I think you may be right. I overcomplicated the problem and solved for frequency which has the unit s^-1 and multiplied that with the energy to get J.s^-1, but 750 watts should be correct from your explanation. Thank you!

Megan Chan 3A

The question asks: In each second, a certain lamp produces 2.4×10^21 photons with a wavelength of 633 nm. How much power (in watts) is produced as radiation at this wavelength (1W = 1J⋅s^−1)? However, the solution only provides the energy of the photons (750 J) which I was able to solve for, but I w...

Megan Chan 3A

Wavelength are set properties of a pulse of light, so it will not change when the number of photons changes. Each photon has the same wavelength and frequency. Energy, however, is proportional to the number of photons, so since there are 100 photons, the energy will be multiplied by 100.

Megan Chan 3A

Yes! You have it right, M(L) ranges from L to negative L.

Megan Chan 3A

Both are valid equations to find the energy of a photon. The one you use will depend on the information given during a specific problem. They can also be used together by doing hv = work function + KE

Megan Chan 3A

This isn't a mass percent problem, it only consists of dimensional analysis. So, for part A the process of solving the question would be: 3.00g CaBr2 x (1 mole CaBr2 / 199.197g) x (1 mole Ca2+ / 1 mole CaBr2) You would repeat the same steps for part C, just make sure to convert kg to g first. Hopefu...

Megan Chan 3A

I believe this is the excited state of neon. You can tell because there are 10 electrons, but the 2p subshell has not been fill yet an electron is in the 3s orbital.

The ground state configuration is how you would normally write it for neon.

1s^2 2s^2 2p^6

Hopefully this helps!

Megan Chan 3A

Velocity gives us the speed and direction of an object while momentum is the tendency of an object to stay in motion. Momentum is dependent on mass while velocity is independent of mass.

Megan Chan 3A

I am not 100% sure, but I think we need to memorize the order of the electromagnetic spectrum (radio waves to gamma rays) and to know that the visible light region has a wavelength ranging from 400nm to 700nm.

Megan Chan 3A

Schrodinger's Wave Function tells us the probability of finding an electron in a certain position. I don't think we will be going into more detail about the equation other than the basic concepts discussed in today's lecture. For orbitals, I think we will be going more in depth in the next few lectu...

Megan Chan 3A

How would you go about solving this problem? I found the change in energy by using E = hc/lambda but I wasn't sure how to find the initial and final energy levels. In the ultraviolet spectrum of atomic hydrogen, a line is observed at 102.6 nm. Determine the values of n for the initial and final ener...

Megan Chan 3A

First, to find the energy per photon, you would use the equation: E = hc/lambda = (6.63x10^-34 x 3.00x10^8) / 1850x10^-9 which gives you 1.075x10^-19 J per photon. To find the number of photons, you divide the total energy (given in the question) by the energy per photon. 11J / (1.075x10^-19 J/photo...

Megan Chan 3A

Yes, light intensity is the number of photons. Increasing intensity can cause more electrons to be ejected if the energy per photon is greater than or equal to the energy required to remove an electron.

Megan Chan 3A

This question is asking for the energy needed to remove an electron from ONE sodium atom, so you must divide the work function by Avogadro's number.

(150.6 kJ/mol) / (6.022x10^23 atoms) x (1000 J/1 kJ)

= 2.501x10^-19 J

Megan Chan 3A

This question is asking us to find the frequency of the light so we can use the equation hv - (work function) = kinetic energy We already know the work function (2.501x10^-19 J) and the kinetic energy (1.99x10^-19 J) from the previous questions, so we can plug these values into the above equation an...

Megan Chan 3A

I've been having trouble getting the correct answer for this problem so if anyone could help thanks :) A 1.50-g sample of metallic tin was placed in a 26.45-g crucible and heated until all the tin had reacted with the oxygen in air to form an oxide. The crucible and product together were found to we...

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