CHEMISTRY COMMUNITY

stephanieyang_3F

You would keep going down the chain until one of them has a bigger atomic number. For CH2CH3 and CH3, CH2CH3 would have the higher priority since C is bigger than H.

stephanieyang_3F

For priority it's usually: 1. Functional groups 2. Alkenes/alkynes 3. Halides and Alkyl substituents If there are two substituents that have the same "priority" like for example, "bromo" and "chloro", you want to name it to whatever will give you the lowest number. If t...

stephanieyang_3F

Why is it for ketones only you don't have to start numbering until there are five or more carbons? How would we differentiate a "butane" which has the ketone on the second carbon compared to the first carbon?

stephanieyang_3F

For some functional groups like for example amines, in what scenario would we have to specify the functional group attached as a primary, secondary, tertiary etc functional group? Is it only when we're asked to identify what kind of functional group is attached?

stephanieyang_3F

This is something Dr. Lavelle posted on another thread. Thought it was pretty helpful:
http://web.chem.ucla.edu/~harding/IGOC/ ... _name.html

stephanieyang_3F

For every substituent, treat it like it's the chain alone first, and then at the end remove the ending and add -yl. To put this in perspective, if you were to have a substituent that had a 2 carbon chain (ethane) and a methyl group attatched to its end, you'd have "1-methyl-ethane", right?...

stephanieyang_3F

for #1, why do we need "iso" in front of propyl? wouldn't the 4- signify the branching? 4 signifies that there's a substituent on the 4th carbon of the chain and the reason you put isopropyl instead of propyl is because it's not a straight alkyl substituent CH2CH2CH3, but rather CH(CH3)2....

stephanieyang_3F

Oyindamola, yes, iso is specifcally for (CH3)2CH-- while neo is specifically for (CH3)3C--.

Christian, you have to consider them into alphabetizing. For all other prefixes (like number prefixes) you don't take them into account. That's why for 1.16 isopropyl comes before dimethyl.

stephanieyang_3F

This is something I thought about while doing 4.29, but I'm just wondering, are electrophilic addition reactions and SN2 reactions usually exothermic/exergonic? If they weren't, when would be a time where they would be endothermic/endergonic?

stephanieyang_3F

I would assume that since the slowest step determines the overall rate of the reaction it would speed up the overall rate compared to if the catalyst were to act on the fastest step. If the catalyst were to act on the fastest step, the activation energy barrier for that step would be lower but the o...

stephanieyang_3F

For the half life of a first order, the formula is t_{1/2}=\frac{.693}{k} . However for the half life of second order, the formula is t_{1/2}=\frac{1}{k[R]_{o}} . Notice that for first order reactions, the half time is constant while for second order reactions the time it takes for the substance to ...

stephanieyang_3F

You generally want to compare the experiments where the concentration of only one reactant is changing at a time. In 15.17, you'll find that the order of [C] will be zero, which makes things much easier when you're trying to find the order of [A] and [B]. Like he mentioned in class, if you're having...

stephanieyang_3F

Just to clarify, depending on the order, like zero, first, second, etc. order, the reaction rate constant units would differ right? For example, for a third order reaction, would the units for the reaction rate constant would be $\frac{L^2}{mol^2*s}$ ? Thanks!

stephanieyang_3F

Yes. The question doesn't give you any information about when dry ice sublimates so you'd assume it wouldn't change states during the process.

stephanieyang_3F

If you look closely at the question... it says "acetonitrile BEFORE the acetonitrile begins to freeze", so you wouldn't need to include the heat required to change the acetonitrile from liquid to solid. Hope I helped!

stephanieyang_3F

Mizuno, you wouldn't use Cp because you have to keep volume constant. When you're changing two variables at a time like volume and temperature, you have to keep two of the three variables constant in order to calculate for entropy or else it'd be too complicated. In the Harry Potter question, step 1...

stephanieyang_3F

You're right; the third law of thermodynamics states that the entropy of a perfect crystal at absolute zero is exactly equal to zero. However, a "perfect crystal" is achieved when there is only one microstate possible. If that's the case, W = 1, and then the entropy would be zero. The thir...

stephanieyang_3F

For this question, because we're trying to find out the value for Ka (K constant for acidic reactions), by definition, Ka is \frac{[H+][F-]}{HF} . The K value is always products/ reactants, so ideally this Ka value would represent a reaction that looks like this: HF HF\rightleftharpoons H^+(aq&#...

stephanieyang_3F

E cell isn't a state function. This can be explained by the solution of 14.27 in the homework. When you try to add the half reactions, you can't simply add the E cell, but you can find out the total E cell by adding their Gibbs free energies because the change in Gibbs free energy is a state functio...

stephanieyang_3F

For galvanic cells, which have to be spontaneous in order for it to run, E cell has to be positive. You can use the relationship deltaG = -nFEcell to explain this phenomenon. In order for delta G to be negative, which indicates that the reaction is a spontaneous one, E cell must be positive. For ele...

stephanieyang_3F

If you use the half reactions given in the Appendix instead of trying to balance it out on your own you'll find that those are the only two half reactions regarding Au. Also the second half reaction is flipped to become the anode so you can maximize the total standard reduction potential of the cell.

stephanieyang_3F

You would need to add on an electrode if there isn't any solid or if there isn't any solid that is a conducting, solid metal.

stephanieyang_3F

How do we know which Fe2+ half reaction to use from the list of half reactions?

stephanieyang_3F

For 14.15 b, could I have done it by using the half reactions: 2H+ + 2e- -> H2 and 2H2O + 2e- -> H2+2OH- ? I got the same E cell potential but my cell diagram doesn't include O2 (g) like it would have if I had used: O2 + 4H+ +4e- -> 2H2O O2 + 2H2O + 4e- -> 4OH- Can anyone explain if it makes any dif...

stephanieyang_3F

Hi! My understanding is that you derive your half-reactions that are listed in the Appendix. The Appendix lists Au+ +1e- ---> Au and Au^3+ + 3e- -----> Au. Naturally you'd want to set up the balanced redox half reactions giving you Au+ -> Au3+ + 2e- for your oxidation reaction and Au+ + 1e- -> Au fo...

stephanieyang_3F

Hi Maya! Yes it makes sense but I'm just having a hard time trying to understand it.. Do you mind explaining it with an example? Thank you so much!

stephanieyang_3F

How does the value of a standard cell potential relate to its reducing/oxidizing strength?

stephanieyang_3F

You use w = -PdeltaV when pressure is constant. Prof. Lavelle used delta n RT to show that reactions at constant pressure can also do work when there is a change in the number of moles of gas.

stephanieyang_3F

Yes you can think of it like that at constant pressure. Well this makes sense because for phase changes (at constant temperature) the enthalpy of the reaction is basically delta H fusion, the amount of heat needed to convert 1 mol of solid to liquid. In this case delta H is negative since its the fr...

stephanieyang_3F

Keep in mind it's CHANGE in entropy = q(rev)/T, so for entropy changes at higher temperatures, it's less significant than for entropy changes at lower temperatures. The same amount of heat could be transferred into the system, but for a lower temperature, that amount of heat is going to cause more e...

stephanieyang_3F

A higher pressure would mean that a substance is more constricted to move around. The entropy would therefore be less when the pressure increases.

stephanieyang_3F

Usually entropy increases from solid -> liquid -> gas because there's more disorder associated with a gas (molecules are free to move around and are not bounded) compared to that of a solid (molecules are in rigid structures).

stephanieyang_3F

This question is similar to a question that was discussed in some discussions regarding 9.19 in the problem set. You need to calculate the entropy change to raise that liquid water to 100 C, then calculate the entropy change for the phase change, and finally calculate the entropy change for the cool...

stephanieyang_3F

Not sure if we learned it since thermodynamics/thermochemistry doesn't relate to time at all. Maybe you mean temperature? or something like that?

stephanieyang_3F

If you use the equation ΔU = 3/2nRΔT (internal energy for ideal gases) you'll find that a constant temperature will make ΔU = 0.

stephanieyang_3F

Elle, do you mean deltaS(surroundings)=negative deltaH(rxn)/T?

stephanieyang_3F

Part B is basically asking how much heat is absorbed when that much amount of carbon reacts with S8. Since sulfur is in excess, the amount of carbon in the reaction goes to completion, which means it will determine the reaction's yield of heat. Since the original reaction said 358 kJ is associated w...

stephanieyang_3F

Elementary particles refer to protons and electrons.

stephanieyang_3F

Since entropy is a state function, it doesn't really matter whether the process is irreversible or reversible, as long as the change for the path is the same. Example 9.5 in the book shows a similar question that has a good graph that visually shows why we can use these two equations to find the tot...

stephanieyang_3F

Kathy can you explain what you mean by that? What kind of example should we refer to understand that concept?

stephanieyang_3F

When we supply heat into a system, typically that heat is used to raise the temperature of that substance. (ex: we heat up water from 3.0 C -> 45.0 C). When a phase change occurs, the heat supplied into the reaction is used to break the intermolecular bonds of a substance instead of raising the temp...

stephanieyang_3F

Someone also posted a version of their work on this thread: viewtopic.php?f=76&t=17620 which I found really helpful.

stephanieyang_3F

These are standard enthalpies of combustion. For combustions reactions, it's always in the same form: some hydrogen/hydrocarbon burned in excess O2 gas yields to CO2 gas and H2O liquid. Keep in mind that these enthalpy values are based on one mole of C2H2, C2H6, and H2. So when you do manipulate the...

stephanieyang_3F

All this means is that the heat is being transferred. -q would mean that in respect to that point (whether it is sys/surrounding or wherever it's coming from) it would have lost heat and whatever is gaining that heat would have a positive q. Since energy is always conserved, the amount of heat lost ...

stephanieyang_3F

It doesn't really matter. The reason why we sometimes use moles and sometimes use grams is to correspond the units with the type of heat capacity you choose to use. Specific heat capacity is specifically with the amount of heat to raise 1 gram (for liquid water 4.184 J/gC) and molar heat capacity is...

stephanieyang_3F

Basically for this question, you're going to be dealing with a system approaching equilibrium... first thing you wanna know is that: the heat gained by the water in the ice cube will be equal to the heat lost by the initial sample of hot water. In other words, both these values will be equal to each...

stephanieyang_3F

We use w = -nRTln (V2/V1). This is the equation we use for work at variable pressure.

stephanieyang_3F

I stumbled upon this concept in 8.29. Which would have a higher molar heat capacity: NO or NO2? I've read from other posts that the more atoms present in the molecule, the higher the molecular complexity and thus the more possible bond vibrations that can absorb added energy. What if two molecules h...

stephanieyang_3F

I'm interested! :)

stephanieyang_3F

Also when you're writing out the reaction, and in this question they're asking for the standard enthalpy of formation of these compounds, do you assume that the compound is in the gaseous state? If so, why do we do that?

stephanieyang_3F

Why does carbon need to be in its gas phase? How do you know when you need to "atomize" a compound?

stephanieyang_3F

Yeah that's exactly right! I like to draw them out cause I tend to mess up when I just visualize it in my head... Eventually you won't even have to go through each step cause you'll be so used to it. For even harder structures it's really useful to know how to approach the problem especially if you ...

stephanieyang_3F

For these bond enthalpy questions, it's really important to be able to visualize the lewis structure so you know which bonds are formed and broken. When it comes to drawing lewis structures, you should focus on satisfying the octet rule above all things. Making sure each atom in the structure has 8 ...

stephanieyang_3F

q(v) is heat at constant volume and q(p) is heat at constant pressure. Think about the formula ΔU = q+w. ΔU is the change in internal energy of system. If you think back to the piston model mentioned in lecture, energy used as work of expansion (the gas inside piston is pushing against the piston, c...

stephanieyang_3F

In problem 8.57, they asked for the reaction enthalpy for that particular reaction. If the given enthalpy of combustion values are in kJ/mol already, do we assume that the reaction enthalpy in the answer also be kJ/mol? Cause usually reaction enthalpy is in kJ and that's what I would've put if the g...

stephanieyang_3F

Not sure how often we'll actually have to use these equations but we should probably understand the concept behind them. So work is one of the ways energy can be transferred from its system/surroundings. They're actually the same equation (you work it around the bit and one equals the other). If you...

stephanieyang_3F

During lecture, Prof. Lavelle gave us three methods to calculate the ΔH(rxn). One of these methods is by using the bond enthalpies. So keep in mind in a chemical reactions, when bonds break, energy is absorbed or required and when bonds form, energy is released. It's important to know the structure ...

stephanieyang_3F

You won't always be given the ΔH of the reaction. It depends on the question and what kind of information the question provides. To provide an example, when we use the second method he described during lecture, you're given the bond enthalpies for what bonds are formed and broken. From that given in...

stephanieyang_3F

Professor Lavelle said that each week ends midnight Sunday.

stephanieyang_3F

You can think back to the idea that as new bonds form, energy is released and when bonds are broken, energy is required. The standard enthalpy of formation is the net amount of energy that is either released or absorbed per mole of a product in a particular reaction. The majority of the standard ent...

stephanieyang_3F

I think "state" depends on the type of state function you're dealing with. An example would be something like a change in volume and the initial volume or "state" would be something like 30 mL to its final volume or "state" as 100 mL. I don't think "state" is ...

stephanieyang_3F

Enthalpy is the amount of heat released or absorbed at a constant pressure whereas heat is one way that energy can be transferred between a system and its surroundings as a result of a temperature difference. Like Angela mentioned, heat is a path dependent function while enthalpy is a state function...

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