CHEMISTRY COMMUNITY

Alyssa_Hsu_2K

Will there still be UA office hours tomorrow?

Alyssa_Hsu_2K

For example, if given the 2-d representation of ethane, how would we know whether it's staggered or eclipsed ?

Alyssa_Hsu_2K

Yes, I think Dr. Lavelle only covered torsional and steric strains thus far(up to Friday's lecture). Torsional strain, like you said, is the repulsion that occurs when the electron densities of 2 molecules are close to each other. This usually occurs under eclipsed conformations. Steric strain, on t...

Alyssa_Hsu_2K

How do we figure out the structure of ethyl methyl ether in line drawing form?

Alyssa_Hsu_2K

I know that tert is when the first carbon of the substituent is a tertiary carbon, and neo is when all but two carbons deviate from continuous chain.

If this is the case, doesn't it mean that tert and neo can be used interchangeably?

Alyssa_Hsu_2K

Is it ok if we name everything using only IUPAC rules?

Alyssa_Hsu_2K

Does Free energy of activation = activation of energy?

Hence, if we know the activation energy of something, could we then figure out delta G and whether the reaction is spontaneous or not?

Alyssa_Hsu_2K

I was wondering about that as well.
Is it because K=Ae^(Ea/RT)
Hence, the larger the Ea, the the larger the increase in K when T is raised?

Alyssa_Hsu_2K

Is it because when Ea is low, Ea/RT will essentially be 0.
Hence, in comparison, when Ea is high, the k will depend more heavily on T?

Alyssa_Hsu_2K

Hence, if we have an equation with coefficients larger than 1, we would have to multiply the rate law by that coefficient?

Alyssa_Hsu_2K

Does the unique average rate imply that all for a reaction A+B--> C+D,
that the rate of decomposition for 1 mole of A is the same as the mole of decomposition for 1 mol of B?
So the fact that A and B are different species doesn't play a role in their rates of reaction?

Alyssa_Hsu_2K

If there is a slow step before a fast step, the intermediate will not appear in the rate law at all. For example, A+B--> C+D slow C+E---> F+G fast In this case, A+B---> C+D is the rate determining step, and when we write the rate law, it will be K(A)(B). Hence, even though C is the intermediate, it ...

Alyssa_Hsu_2K

You can only find the order of reactions from coefficients if it's an elementary reaction.
If you were given the overall reaction, you cannot find its order from its coefficient.

Alyssa_Hsu_2K

Because second order reactions include initial concentration as part of the calculation for half life, each individual half life steps are going to be different. As a result, you can not treat it as a constant the way you can when it comes to first order reactions.

Alyssa_Hsu_2K

Question 9.43 says, Suppose that 50g of H2O(l) at 20.0 celsius is mixed with 65 g of H2O(l) at 50.0 celsius at constant atmospheric pressure in a thermally insulated vessel. Calculate Delta S and Delta S total for the process. I know we can find delta s by qcold=qhot. However, I don't understand why...

Alyssa_Hsu_2K

And under what circumstances are standard Gibbs free energy of formation and standard Gibbs free energy of reaction equal to each other?

Alyssa_Hsu_2K

For example, consider this question: Pt(s)/H2(g)/H+(g)//Cl-(aq)/Hg2Cl2(s)/Hg(l)
When writing the half equation for the cathode, why can't I ignore Cl- and write Hg2---> Hg? Isn't Cl not affected at all during this reaction?

Alyssa_Hsu_2K

Arrange the following metals in order of increasing strength as reducing agents for species in aqueous solution: 1) Cu, Zn, Cr, Fe In this case, I understand that the reducing agent is the molecule undergoing oxidation. Hence, the stronger the reducing agent, the smaller the reduction potential. How...

Alyssa_Hsu_2K

Suppose a reaction started out with T=290k. Then, it was heated to T=295K. Then, it was cooled back down to T=290K.
Would we still say this reaction overall is isothermal and thus delta U=0?

Alyssa_Hsu_2K

Hi, I was wondering why the sig figs for number 10 is 3 instead of 2? Doesn't the 20 degrees celsius mentioned in the problem only have 2 sig figs?

Alyssa_Hsu_2K

Hello, thank you for your answer! I think I understand your answer, but I am still a little confused about this one point: I understand that under constant pressure, we can rewrite q as H. However, I still don't get why q=mCdeltaT won't work? In other words, while it is true that delta H =mCdeltaT, ...

Alyssa_Hsu_2K

I understand that during isothermal reactions, delta U=0. I also understand that if work is being done, then w cannot be zero hence q cannot be 0. However, I don't understand the conceptual basis behind q being 0. I thought the equation for q is q=mCdeltaT. If there is no temperature change, shouldn...

Alyssa_Hsu_2K

it will essentially cover all of chapter 8, 9, and bits of 11.

Alyssa_Hsu_2K

Hello,
I was wondering why we don't need to take into account the temperature change that may arise when reactants break their bonds.
Thanks!

Alyssa_Hsu_2K

Hi, thank you for your response! However, I'm still a little confused. On course reader pg. 18, when we have the reaction CH4+2O2--> CO2+2H2O, we take into account the standard enthalpies of both the products and the reactants. However, this particular equation is not at chemical equilibrium, which ...

Alyssa_Hsu_2K

On pg. 20 of the course reader, there is the following example: In a constant P calorimeter at 25 Celcius mix 50.0ml each of 1.0 M HCL and 1.0M NaOH. After Mixing, the temperature= 31.9 degrees Celcius. (H+)+(OH-)--> H2O. How much heat is released? In this case, do we assume that all of H and OH- ha...

Alyssa_Hsu_2K

You can also think of it like this:
Endothermic reaction: Hfinal-Hinitial= positive number. This means that the system has gained energy.
Exothermic reaction: Hfinal-Hinitial= negative number. This means that the system has lost energy.

Alyssa_Hsu_2K

When given standard reaction enthalpies of each individual reactants, you must add all the product enthalpies together and decrease that by the total reactant enthalpies.

Alyssa_Hsu_2K

Determine which acid in each of the following pairs is stronger and explain why:
1. HF or HCl
2. HClO or HClO2
3. HBrO2 or HClO2
4. HClO4 or H3PO4
5. HNO3 or HNO2
6. H2CO2 or H2GeO3

Thanks!

Alyssa_Hsu_2K

Hi, can you elaborate on your answer a bit?
Where did your 14.00 come from?

Thanks!

Alyssa_Hsu_2K

The problem says: Calculate the pH at each stage in the titration for the addition of 0.15M HCl to 25.0mL of 0.110M NaOH. In Part b,we were told to calculate the pH after the addition of 5.0ML of acid. I was wondering why the significant figures for this question isn't 2? I thought since 5.0 only ha...

Alyssa_Hsu_2K

During an analysis of an unknown acid HA, a 0.010 M solution of the sodium salt of the acid was found to have a pH of 10.35. Find the formula for the acid. I don't really understand why my way is wrong. Below is my line of thought: Since they tell us that sodium salt was produced, we can infer that ...

Alyssa_Hsu_2K

Say we were only given NH3(aq), and asked what is its conjugate acids and conjugate base. I know acids are the proton donors and bases the proton acceptors. However, in this case, how would we know whether NH3 or H2O is the proton donor or acceptor? Seeing that Oxygen is more electronegative than Ni...

Alyssa_Hsu_2K

For example, I know that when K is large, there is a larger ratio of products to reactants, hence the reaction favors the product. However, I don't really know what " favors the product" means. After all, I thought to be in equilibrium is have the same rate of reverse and forward reactions...

Alyssa_Hsu_2K

I believe you can convert between pressure and concentration with the Pv=NRT equation.

Alyssa_Hsu_2K

I think this question require information we haven't learned yet.

Alyssa_Hsu_2K

It is said that when we look at heteronuclear MO, the more electronegative atom has lower energy. As a result, it contributes more to the bonding energies. However, can someone explain why this is?

Alyssa_Hsu_2K

Percent yield is actual yield/theoretical yield. Actual yield is determined experimentally and will be given to you in the question. You can calculate theoretical yield by first determining the limiting reactant, then use its molar ratio to calculate the amount in grams for a final product. Then, yo...

Alyssa_Hsu_2K

Can you elaborate on how we can figure out that Sulfur is more electronegative than Carbon ?

I know that Oxygen is more electronegative than Sulfur, and that Oxygen is more electronegative than Carbon.

Thanks !

Alyssa_Hsu_2K

How would you know whether CO2 or CS2 has greater ionic character? I understand that O is more electronegative than S, but how would I know how that compares to Carbon's electronegativity? In other words, just because O is more electronegative than S doesn't mean it's further away from C. Moreover, ...

Alyssa_Hsu_2K

When trying to draw O3S -2, we can draw a structures that obeys the octet rule and 3 that doesn't(by having a s=o double bond). I was wondering if we can say that the structure that obeys the octet rule is in resonance with the 3 that don't obey the octet rule? After all, the structure that obeys th...

Alyssa_Hsu_2K

Is it true that : 1. covalent bonds can have ionic character if there is a difference in electronegativity between the two atoms. The higher the electronegativity, the stronger the ionic character. 2. ionic bonds can have covalent character because when an anion is placed next to a highly charged ca...

Alyssa_Hsu_2K

I believe the book said the specific reason for that involves quantum mechanics. Another note, I don't think being able to draw resonance structures "proves" that the molecule is more stable than expected. It's just that the combined structure is more stable than any single one of the reso...

Alyssa_Hsu_2K

Are people interested in having a session this Sunday so we can work on workbook questions for quiz 2?

Alyssa_Hsu_2K

So in other words, for example, once u get to the forth period, you can hold 32 electrons. Cause if n=4, then l can equal to 0, 1, 2, 3. S can hold 2, p can hold 6, d can hold 10, and f can hold 14??? And phosphorus and sulfur, they each can hold an additional 10 electrons in their d-orbitals, is th...

Alyssa_Hsu_2K

Adding on to what Alexandra said, The reason light emitted is negative is because we have set energy=0 as a reference point. Energy=0 happens when the electron is at such a high energy that it is detached from the atomic orbital. Hence, it is important to recognize that energy,in reality, can never ...

Alyssa_Hsu_2K

The problem says: Xylitol, a sugar substitute, has a mass composition of C 39.43%, O 52.58%, H 7.88% and a molar mass of 152.15 g x mol^-1. What is its molecular formula/? I did: (1) 39.43g C/12.011= 3.2828 mol C 52.58g O/15.999=3.2865 mol O 7.88g H/1.008= 7.8175 mol H (2) Divide by smallest mole nu...

Alyssa_Hsu_2K

I'm sorry, but I'm still a little confused. When the question says, the speed is within 3 x 10^2 kilometers times inverse seconds,doesn't that mean all of the numbers smaller than 3 x 10^2 apply? If so, then why are we only including the difference between 3 x 10^2 kilometers times inverse seconds a...

Alyssa_Hsu_2K

What if the problem instead says the speed is known to be within 3 x 10^2 kilometers times inverse seconds? Would delta V be 3 x 10^2 kilometers times inverse seconds?

Alyssa_Hsu_2K

The question says: Estimate the minimum uncertainty in the position of a marble of mass 1g given that its speed is known to within plus and minus 1.00 millimeters times inverse second. Why is the delta velocity in this case 2 x (1.0 x 10^-3) meters time inverse second ? Where did the 2 come from? Is...

Alyssa_Hsu_2K

So for instance, a Lyman series always transitions to n=1, the Balmer series n=2, and the Paschen series n=3? Is this also why each series usually produce electromagnetic radiation and different points of the spectrum? (Visible light, ultraviolet...etc)?

Alyssa_Hsu_2K

The problem says: The velocity of an electron that is emitted from a metallic surface by a photon is 3.6 x 10^3 km/s. What is the wavelength of the ejected electron? I know I can use 0.5mv^2 to find the energy of the electron. However, I can't use E=hv and c=wavelength x frequency to find wavelength...

Alyssa_Hsu_2K

On the textbook example 1.5 on pg. 14, the problem goes: Suppose you are developing a radiation detector to be used on a spacecraft and decide to use a thin layer of metallic potassium to detect certain ranges of electromagnetic radiation. You need to make some estimates of the physical properties i...

Alyssa_Hsu_2K

Hello, So basically the unit for chemical equations is always moles? Ex. H3PO4+Ca(OH)2--> Ca3(PO4)2+H2O 1. In this problem, we can say 1 mole of H3PO4 and 1 mole of Ca(OH)2 can produce 1 mole of Ca3(PO4)2 + 1 mole of H2O? 2. If I wanted to find the number of Ca(OH)2 molecules, I'd multiply 1 mole of...

Alyssa_Hsu_2K

On the book, it says when we see 2Na+2H2O--> 2NaOH+H2, we can say that 2 moles of Na, 2 moles of H2O produces 2 moles of NaOH and 1 mole of H2. However, I don't understand why. Personally, I See 2 atoms of Na, 2 molecules of H2O--> 2 molecules of NaOH and 1 molecule of H2. I don't know how they can ...

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