CHEMISTRY COMMUNITY

Linette Choi 3L

The slow step is the one that determines the rate of the overall mechanism. It should state on the question which reactions are slow or fast.

Linette Choi 3L

Yes, I would consider it an isolated system since no heat is leaving or entering the mug.

Linette Choi 3L

n is equal to the moles of electrons transferred in the reaction. For for instance if you have Cu2+ --> Cu4+ + 2e-, the number of electrons transferred is 2.

Linette Choi 3L

Definitely review the practice problems on the syllabus!!

Linette Choi 3L

Could someone please walk me through how to solve this question? Refer to the illustration below for reaction A → D. (a) How many steps does this reaction have? (b) Which is the rate-determining step in this reaction? (c) Which step is the fastest? (d) How many intermediates must form in the reactio...

Linette Choi 3L

Could someone explain why when writing cell notations, we tend to put the more aqueous compounds closer to the salt bridge and the more solid compounds farther away from it?

Linette Choi 3L

The anode is where oxidation occurs and the cathode is where the reduction occurs. Electrons should flow from the anode to the cathode.

Linette Choi 3L

Yeah that's right! With the conditions at 298K and 1 atm.

Linette Choi 3L

I used Khan Academy's chapter on Electrochemistry and I thought it was super helpful! https://www.khanacademy.org/test-prep/m ... ochemistry

Linette Choi 3L

The oxidizing agent is what is reduced (gained electrons).

Linette Choi 3L

Try clearing all of your cookies!! My sapling was glitchy last week but I didn't have any problems after I cleared all my cookies and restarted my browser.

Linette Choi 3L

The cathode receives electrons and converts the reactants into solids, thereby gaining mass.

Linette Choi 3L

N should stand for mols of electrons in the reaction.

Linette Choi 3L

Potential is the ability of the system to gain electrons (reduction).

Linette Choi 3L

I find OIL RIG super helpful!
O-Oxidation I-is L-lost
R-Reduction I-is G-gained

Linette Choi 3L

Heat capacity looks for the amount of heat needed to raise the temperature by 1 degree, so to solve this we'd use the equal q=C*dT and get C= 1/dT. Then you'd plug in your q from the equation and find the change in temp to solve the problem.

Linette Choi 3L

Heat capacity looks for the amount of heat needed to raise the temperature by 1 degree, so to solve this we'd use the equal q=C*dT and get C= 1/dT. Then you'd plug in your q from the equation and find the change in temp to solve the problem.

Linette Choi 3L

Can someone break down what we can assume about a calorimeter in terms of values of volume, pressure, temperature, etc. when we're given a problem that involves one?

Linette Choi 3L

Hello! I know that work is equal to 0 when volume is constant but can someone explain why we can assume that?

Linette Choi 3L

If it asks for Cm then it's asking for molar heat capacity which is the amount of heat needed to raise the temp by 1 degree per mol, while Cs, specific heat capacity is asking for the amount of heat to raise the temp by 1 degree per gram.

Linette Choi 3L

State property means that the value only relies on its final and initial values and not the path that it requires to get there. Some common ones are "SHUT" - S (entropy), H(enthalpy), U (internal energy), and T (Temperature).

Linette Choi 3L

You can tell if a system is endothermic (deltaH >0) or exothermic (deltaH<0) by heat but not by work.

Linette Choi 3L

It's considered a closed system.

Linette Choi 3L

I'd say it will depend on what the question entails. Just remember that 1000J = 1kJ

Linette Choi 3L

We use the equation delta U = q + w. When a system does only work, the change in internal energy (total energy of the sys): deltaU=w
When a system exchanges energy only as heat, q: deltaU= q.

Linette Choi 3L

Melissa Kulon 2D wrote:Thank you! Just to clarify, are the only terms that are NOT state functions, work and heat?

Yup! Terms such as entropy, pressure, enthalpy, or color are state functions as they only depend on the current state they are in.

Linette Choi 3L

Heat capcacity is the amount of heat needed to raise the temperature by 1 degree so it would need to be positive.

Linette Choi 3L

Hello! I believe we use deltaU = q + w. First, I used the formula q = nC deltaT, given that C is the constant‑pressure molar specific heat, to find q. q = 0.305 mol * 29.1 J/(mol*K) * 19.9K = 176.6 J. Then, I find w using w = - Pex*deltaV = -(1 atm)(deltaV). To calculate delta V (Vf - Vi), I use V ...

Linette Choi 3L

Can someone explain why deltaU = q at constant volume?

Linette Choi 3L

Can someone please show a step by step of how to solve for deltaU? I found that q=176.6 was correct and I've looked and a ton of other chemistry community posts as well but I still can't figure out how to solve for deltaU. A 0.305 mol sample of N2(g), initially at 298 K and 1.00 atm, is held at cons...

Linette Choi 3L

Can someone explain how to calculate path b for this question? A sample of an ideal gas in a cylinder of volume 3.75 L at 298 K and 2.87 atm expands to 7.39 L by two different pathways. Path A is an isothermal, reversible expansion. Path B has two steps. In the first step, the gas is cooled at const...

Linette Choi 3L

In question #12, why do we only use q= c * deltaT instead of q= n*c*deltaT?

Linette Choi 3L

In this question, why is part d. 2HD(g) + H2(g) --> D2(g) considered a no change when the pressure increases? Since pressure favors the side with less moles of gas, wouldn't it favor the products since the reactant side has 3 moles of gas?

Linette Choi 3L

Also, in the solutions manual it showed that deltaHr= deltaHf (CO, g) - deltaHf (H2O, g), why were C(s) and H2(g) omitted from this equation?

Linette Choi 3L

Can someone explain how you'd solve 4.13? Water gas is an inexpensive, low-grade fuel that can be made from coal. (a) Is the production of water gas exothermic or endothermic? The reaction is C(s) + H2O(g) --> CO(g) + H2(g) (b) Calculate the enthalpy change for the production of 200. L of hydrogen a...

Linette Choi 3L

On question 4E.5 in the textbook, why do we use the C⋯––C bond value of 518 for all 6 C-C bonds in C6H6 instead of 3 C=C bonds and 3 C-C bonds? Also why does the solutions manual not account for the 6 C-H bonds in 3C2H2 and the 6 C-H bonds in C6H6?

Linette Choi 3L

Delta Hc stands for the standard enthalpy of combustion so you would have to add O2 to each reaction before using hess's law to solve for the value of deltaHc for the given equation.

Linette Choi 3L

In this question why is the denominator of the equation Ka (1-0.024)*(0.110) instead of (0.110-x) as it is set up in the ICE table?

Linette Choi 3L

In this problem how would we know to omit the x in (0.22-2x)^2? Or when would we know that K is small enough to assume that x << 0.22?

Linette Choi 3L

(c) Use your results from parts (a) and (b) to determine which is thermodynamically more stable relative to its atoms at 1000. K, Cl2 or F2.

Can someone explain how we would find C in a different way than the solutions manual described? I'm a bit confused on why Cl2 would be more stable.

Linette Choi 3L

For problems such as the ones in 5G, 5H, or 5I that used the chart from Table 5G.2 in the textbook, how would we know when to use the value of K or Kc? Can someone explain the difference from the two and how we would know when to use one over the other?

Linette Choi 3L

Why was question 5I.1 solved without an ICE table based on the solutions manual? If we used an ICE table would we still get the same answer?

Linette Choi 3L

Can someone explain why the two values of K in this problem were multiplied instead of added?

Linette Choi 3L

We would be able to tell if the reaction prefers reactants or products based on if the reaction is exothermic (delta H is negative) or endothermic (delta H is positive).

Linette Choi 3L

If q and K are equal then the reaction is at equilibrium.

Linette Choi 3L

Could someone explain why Kc would not change in the second part of sapling week #1's when the concentration of NO2 increased?

Linette Choi 3L

I believe they are generally calculated the same way but ionization is when a hydrogen is donated and protonation is when a hydrogen is accepted.

Linette Choi 3L

I believe (OH-) is used more often but I think they mean the same thing.

Linette Choi 3L

Endothermic reactions require heat in order to occur so adding heat would lower the activation energy and cause the reaction to occur faster.

Linette Choi 3L

Only include H2O in the ice table if it is aqueous and gaseous, never a liquid or a solid. Ensure that you know which phase it is in always. Is aqueous not the same as a liquid or am I getting this wrong? Aqueous is slightly different than liquid as liquid is a pure form while aqueous is when somet...

Linette Choi 3L

I believe you only emit compounds in pure states (solid and liquid) so you would keep H20 if it was in a gaseous state.

Linette Choi 3L

You can determine which value of R to use based on the units of the other values that you need to cancel out.

Linette Choi 3L

Along with everyone else I believe it should be in atm, however, it could vary on what units the question asks you to convert to.

Linette Choi 3L

I believe Professor Lavelle used it as an example for acids and their properties, but just keep in mind that it is formed when water and carbon dioxide react with one another to form carbonic acid. This could also go with sulfur dioxide reacting with water to form sulfuric acid and nitrogen dioxide ...

Linette Choi 3L

Can someone help explain how to draw arrows when determining dipole moments and how we can assume polarity of the molecule with this method?

Linette Choi 3L

I first count up all of the electrons, so for instance, if the molecule was AlCl3, I would have 24 electrons (3 + 3*7). Then, I draw out the structure, keeping the oxidation states in mind. So since Cl has an oxidation state of 1- and Al has one of 3-, I would deduce that Al would have 3 single bond...

Linette Choi 3L

The full name is the electric field dipole moment and it is the electric field created by the asymmetry in charge. Dipole moment = q x d where d is the distance between the charges (partial charges) The larger the asymmetry in charge, the bigger the dipole moment. The larger the distance between th...

Linette Choi 3L

How do you determine if a bond is slightly less than 109.5 or the general bond angle? For instance, in the textbook, on problem 2E.21, for the compound N2H4, the shape is trigonal pyramidal so I assumed the bond angle would be 109.5 but the textbook is saying its 107. How would we determine this?

Linette Choi 3L

https://lavelle.chem.ucla.edu/wp-conten ... pounds.pdf
This is a link on Dr. Lavelle's website. Hope this helps!

Linette Choi 3L

The longer (weaker) the bond, the more easily the atoms will disassociate protons, making the acid stronger. Thus you could identify which acid has a longer bond length in order to determine which one is stronger.

Linette Choi 3L

The oxidation state is the charge of the atom. Ex: the oxidation state of Fe (II) would be 2+.

Linette Choi 3L

Similar to what Scot said, I usually find the charges on all the ligands first and then compare it to the overall charge. For instance, in a compound like FeCl2, we know that Cl typically has a charge of 1-, so thus, we can assume that the oxidation number of Fe will be 2+ in order to balance out th...

Linette Choi 3L

HBr would be stronger than HCl as it has a shorter bond length, meaning the bond would be more difficult to break. However, like Khoa stated, the strength of an acid is defined by how easily it dissociates into a proton. Thus, with HBr having a longer bond length, it would more readily give away its...

Linette Choi 3L

How do we know if a ligand forms a chelate? For instance, I'm a bit confused on how EDTA is a chelate as it doesn't appear to have a visible ring of atoms that includes the central metal atom.

Linette Choi 3L

Could someone explain the significance of ligands? Also how would we be able to identify if a molecule was an octahedral complex or tetrahedral complex based off of them?

Linette Choi 3L

Axial bonds are on the vertical plane on the central atom and will typically be 180 degrees while the equatorial atoms will be horizontally distorted, or on the "equator" of the atom. A good example can be shown on the sapling HW #5. The axial atoms are directly vertical, coming out from t...

Linette Choi 3L

Hi can someone help explain how we would find the hybridization for a chelating ligand such as the one Professor Lavelle used in lecture #24? (Co2+ bonded to NH2-CH2-CH2-NH2)

Linette Choi 3L

All bonds have 1 sigma bond, however, double and triple bonds have additional pi bonds along with the sigma bonds.

Linette Choi 3L

Yup, if the resonance structure still had the double bond then it would still have the sigma and pi bonds. The bonds would just be delocalized along with the electrons since resonance structures move them around.

Linette Choi 3L

Yes all covalent bonds would have sigma and pi bonds. However, single bonds only have sigma bonds while double and triple bonds would have both sigma and pi, with the pi bonds making the double and triple bonds more difficult to break.

Linette Choi 3L

Since the S has 3 oxygen bonded to it, we know it would be trigonal and along with the lone pair on S we can deduce that the shape is trigonal pyramidal.

Linette Choi 3L

Can someone explain to draw the lewis structures for #11 on topic 2C and how to tell if Xe can have more than 4 bonds?

Linette Choi 3L

Can someone explain how iodine is able to have up to 7 bonds on #3 on topic 2C? How do we know if the element can have an expanded Octet?

Linette Choi 3L

It should be the sum of all the charges. Hope this helps!

Linette Choi 3L

I know how formal charge is used to help identify the best resonance structure, but how are oxidation numbers and bond lengths used? Also how do we know which method is the most plausible?

Linette Choi 3L

Single bonds have 1 sigma bond, double bonds have 1 sigma bond and 1 pi bond, and triple bonds have 1 sigma bond and 2 pi bonds. Pi bonds are significantly harder to break as they do not allow rotation and have a higher energy state than sigma bonds. Thus, bonds with more pi bonds are more difficult...

Linette Choi 3L

Covalent radius is half the distance between the centers of atoms in nonmetals or metalloids while atomic radius is half the distance between the centers of neighboring atoms.

Linette Choi 3L

Yes, as the atomic radius increases, the nuclear charge will decrease as electrons are further from the nucleus. It is the same principle with ionic radius as the lower nuclear charges indicate that the radius is larger.

Linette Choi 3L

I believe selenium can have an expanded octet due to the fact that it is in the same group as Sulfur and follows the same pattern of expanded octets and bonding pairs.

Linette Choi 3L

How do we identify delocalized electrons in resonance structures? Also, if there are multiple resonance structures how would we be able to differentiate between delocalized double bonds or triple bonds?

Linette Choi 3L

I don't believe there is a way to determine the electron spin from where we are now. For now, I would just keep in mind that electrons in the same orbitals go (spin) in different directions.

Linette Choi 3L

I believe formal charge is used to determine the most stable Lewis structure possible. The question will most likely clarify if you need to find it or not.

Linette Choi 3L

Could someone provide some more examples of an element like Sulfur being able to have an expanded octet? I am a bit confused on why this exception is possible and what other elements it could apply to.

Linette Choi 3L

According to the Aufbau principle, electrons will occupy the lowest energy orbitals first before higher energy orbitals. The 3D having 5 orbitals over 2, would be higher in energy, thus filling up the 4s orbital before 3d.

Linette Choi 3L

I was a bit confused on this as well, but according to some chemistry websites, hydrogen has some properties similar to alkali metals (group 1) such as forming cations but it also has some properties of halogens (group 17) such as the tendency to reject electrons to make itself stable. But like Sean...

Linette Choi 3L

When we're asked to write out the full electron configuration, is it okay to use the shorthand notation for the noble gas or would it be best to write out the whole thing? I believe the question will ask for the noble gas configuration if needed. However, I noticed on sapling that it would already ...

Linette Choi 3L

Hello! Atomic radius would increase as you got across a period as more electrons are added to each shell due to the increasing nuclear charge from left to right. https://www.thoughtco.com/chart-of-periodic-table-trends-608792 <-- There is a good chart on this website to help see the periodic trends.

Linette Choi 3L

Hello could someone help explain to me how to solve part B of this problem? I correctly answered A but I am not sure how to go about solving part B. A.) When a metal was exposed to photons at a frequency of 1.15×1015 s−1, electrons were emitted with a maximum kinetic energy of 3.30×10−19 J. Calculat...

Linette Choi 3L

I also don't believe you need to know how to calculate it. However, you can determine the spin by the number of electrons in the atom. For instance, using Hund's rule, for an element like Carbon which is 1s2 2s2 2p2, the two S orbitals are filled with arrows, however with the remainder of the arrows...

Linette Choi 3L

No two electrons in the same atom can have the same 4 quantum numbers as the numbers completely specify an electron's orbital and spin state in the atom.

Linette Choi 3L

This equation should be used when there is uncertainity in calculating the position or momentum of an electron as we cannot do so simultaneously. Most of the questions have asked us to solve for velocity so I would use those examples to prepare.

Linette Choi 3L

Yes! De Broglie's equation is most commonly used to describe the wave nature of electrons, but can be used for any particle with mass.

Linette Choi 3L

Can someone explain how to complete M11a in the textbook fundamentals? I looked at the solutions manual and I am not sure why they subtracted the 4.47g O2 from the 5.77g of O2 and why this same method was not used with the O2 obtained from the second reaction.

Linette Choi 3L

The Sapling Homework for weeks 2, 3, and 4 will all be due on Sunday midnight of week 4 and counts for 15 points.

Linette Choi 3L

Higher intensity would only make the light brighter and increase the number of photons, not increase the energy of each individual photon. However, increasing frequency would increase the energy of the photons, allowing them to be able to eject the electrons.

Linette Choi 3L

Yup! Increasing the light intensity would just mean the light is getting brighter and the number of photons in increasing but it would not change the frequency or energy of the individual photon where it would have enough energy to eject an electron.

Linette Choi 3L

I believe that the question should clarify if the molecule is diatomic or not. However, if the question has you write out the chemical reaction, you should be able to identify that the elements H, N, F, O, I, Cl, Br are all diatomic molecules and should be written as "H2, N2, etc..." when ...

Linette Choi 3L

Since both V1 and V2 are in the same units, it should be okay to keep them both in mL. However, I would recommend keeping your units in L since molarity is mols/liters.

Linette Choi 3L

Hi! I also found this website https://www.dartmouth.edu/~genchem/sigfigs.html which shows some examples at the end for reference as well.

Linette Choi 3L

Typically I use exactly the molar mass that is given from my periodic table (which rounds the decimals to the hundredth place). However, using 4 decimal places is a safer method to get a more accurate answer.

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