CHEMISTRY COMMUNITY

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Joslyn_Santana_2B wrote:W 2011 Q2

Why is 3H2 ignored in the 1st rxn? Why is H2 ignored in the 2nd rxn?
I thought it was because of a phase difference in the 1st one, but in the 2nd rxn there is no phase difference and it's still ignored.

Elements in their most stable form have a dG of 0

ntruong2H

I was wondering why it is correct, when time is supposed to be in seconds? I got to completely different answers when I converted to seconds as opposed to leaving it in minutes. Time does not have to be in seconds. Time is the unit that is given to you in the problem. If an explicit measurement of ...

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Sue_Park_2D wrote:Why do we use "hydroxy" to indicate the alcohol groups in naming #9b instead using the -ol ending?

Hope this helps!
viewtopic.php?f=91&t=21092&sid=6a2f50fad7a86019f743fde47e37fd73#p59963

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Jasmine Cha 1A wrote:Hi, so I know that n is defined as the number of electrons transferred in a reaction, but do you have to write out each half rxn to figure this out? Thanks!

Yes. As you will not know how many electrons you will need for the balanced redox reaction

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When is an OH group on an organic molecule named as a -hydroxy- and when is it named as an -ol? A specific example of this can be seen on Winter 2015 Final Question 9B where in 1,3,4,5,6-pentahydroxy-2-hexanone hydroxy is also an -OH group. Here, the -OH group was a hydroxy because the ketone got t...

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This is not possible. They would always be opposite of each other. You cannot specify anything at this point if they are on the same carbon. Okay thank you. Also, can we start with either chair conformation and then draw in the substituents? As long as the substituent is placed equatorially the ans...

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Question 5A on the winter 2014 final in the back of our course reader solves for the rate constant at the end but doesn't show how. I understand how to find which order the reaction is and the rate law but not how to find the rate constant with the given information. Please help/explain. Thanks! Ba...

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If there are two different substituents on the same carbon, do we still have to indicate cis/trans? For instance, in the molecule 1-ethyl-1-methylcyclohexane, do we have to say trans (or E) - 1-ethyl-1-methylcyclohexane? Since I know the ethyl will be placed equatorial down and the methyl will be pl...

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KelseyKobayashi_2M wrote:Also for this question on the other molecule, is the cycloheptane a substituent because of the alcohol?

Yes. The main chain must always include the functional group since functional groups have the highest priority in naming.

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Why do we assume that the expansion and temperature change aren't simultaneous? My intuition is to assume that since volume expands from 3.0 to 3.5 liters, we wouldn't use the equation with constant volume but with constant pressure. Are these types of problems usually this way (where volume expand...

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Dianna Grigorian 1C wrote:What is the difference between geometric isomers and constitutional isomers?

I believe geometric isomers only occur when there are double bonds. So cis and trans forms of the same molecule are geometric isomers.

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Guillermo Vega 1H wrote:I don't really understand when we should compute with the formula w=-nRTln(V2/V1). Is this for only isothermal expansions or are there other situations in which we can use the formula?

Use this formula when pressure is not constant.

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For question 1B, aren't you supposed to use Cp instead of Cv? In the answer, Cv is used but volume is not constant while temperature is. The variable in the subscript in these is supposed to be the variable held constant, right? No. This is supposed to be constant volume. This is basically a two st...

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For question 9A, why is the correct answer trans (E) when the 4 Carbons and respective hydrogens have a higher atomic number than the Chlorine on the opposite side? you have to do it atom by atom. So compare the the atomic number of Cl to the atomic number of C in the carbon chain. Cl has a higher ...

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Hi everyone! Can anyone explain why the methyls of the cyclohexane in Question 6A of the 2013 Final are numbered 1,5,5,6 and not 1,2,3,3? It's because you have a double bond. When you have a double or a triple bond, you must count going across across the bond. You started at the double bond, but th...

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Theresa Dinh 3F wrote:O is a catalyst so it's not actually involved in the reaction

O is not a catalyst. Catalysts must first appear as a reactant in the first step and finish as a product in the last step. Intermediates are produced during the first step and used up as a reactant during the subsequent step.

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How are we able to determine what the rate limiting step is when we're only given the mechanisms the students create? Thanks. Well, if you think about this, you are trying to find the rate law that matches with the one you found from part A. For any of the students' proposed mechanisms, if you set ...

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Isn't O an intermediate? You are correct. O is an intermediate. This is essentially a pre equilibrium problem. You are trying to see if setting step 2 as the slow step will give you the rate law you wanted and calculated in part A of this question. First, find the rate law for the slow step. This w...

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Tara_Shooshani_3N wrote:Since U=0, w=-q

So q and w don't equal 0? I thought that since it's isolated, it can't expand/contract and heat can't be exchanged so they'd be = 0

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In an isolated system, I know dU=0. Does this also mean that q and w=0?

What are the values for q and w in a closed system?

Thanks.

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The problem is: draw the staggered conformation for rotation about the C1-C2 bond in 2-methylbutane. Can someone please explain how we would go about drawing this? Thank you.

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When looking at a newman projection or structure, how can we tell which type of strain there is? I understand torsional strain occurs in eclipsed conformations and that steric strain occurs when two atoms are trying to be in the same volume. However, when just simply looking at a projection, how can...

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For the first part: E & Z are not necessarily the same as cis and trans, but they are similar. Cis/trans is used to designate orientation around a double bond when there are only two substituents that are important. E/Z is used for more complicated situations i.e when there are multiple substit...

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IMG_1371.JPG For 1.28, why do we have to specify E or Z? I understand that E=trans and Z=cis; however, I do not get why or when we have to use these? For instance in our course reader, on page 102, there is a similar example to problem 1.28 and we did not have to specify E or Z? Thank you. The ques...

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Will we be mostly tested on alkane and alkene naming, since these are the ones most heavily covered in class? We have not covered how to name or draw/identify line structures for alkynes much, so I was wondering if there will be minimal coverage on alkynes? Thank you.

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In the book, it says that "Catalysts produce a less negative dS(double dagger) value by bringing the reactants together before the reaction occurs and placing them in the correct orientation." I am confused as to how this does not make dS even more negative. Since catalysts put the molecu...

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In the book, it says that "Catalysts produce a less negative dS(double dagger) value by bringing the reactants together before the reaction occurs and placing them in the correct orientation." I am confused as to how this does not make dS even more negative. Since catalysts put the molecul...

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When writing rate laws do we have to include the charges? like [CH3C+CH3] and [Br-]?

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This is problem 4.30 from the green book: "The activation energy in organic reaction is known to be 125 kJ/mol. Will this reaction occur at room temperature?" The answer is no. However, can someone please explain why this is the answer or how we were supposed to know this? Thanks.

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Page 85 of the course reader has an example of a reaction profile with intermediates. As Tyler described, intermediates are represents as a minimum between two peaks. How can you tell how many there are though? Because the example in the course reader, there is 1 minimum between the two peaks, but ...

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Jeannie_Kim_1I wrote:When alphabetizing, we would take iso- and neo- into account, but not sec- or tert-.

Okay thank you! Is there a particular reason for this or is this something to just memorize?

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When using prefixes, such as iso- , neo- , sec - , or tert- , do we take these into account when putting the names in alphabetical order? I know when dealing with numerical prefixes, such as di, tri, etc we do not. However, I was wondering if the same rule applies for these? Thank you.

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Hello, I was wondering, when looking at or drawing reaction profiles, how do we account for the formation of intermediates? Further, how do we know, based on the amount of "humps" in a reaction profile, how many intermediates are formed? Thank you.

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Are catalysts included in the rate law? Also, if they are, do they thus affect the order of the reaction?

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If a step is "very very slow," and that is the slowest specification for the rate of any step, it is the rate determining step. Because that step determines rate, a faster step prior to it would not affect the rate of the reaction, and therefore would not be included in the rate expressio...

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Is there a reason why number six's answer has three significant figures despite the given concentration 0.35M having two significant figures? Do we not bother with sig figs when ln is involved? When dealing with natural logs, only the digits after the decimal point are counted as significant figure...

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Can someone please explain to me what happens if we have a step that is said to be "very very slow"? Does this now mean that this is the rate determining step? Also, can someone please explain what happens if there is a fast, equilibrium step that comes before the slow step? Do we now incl...

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I know this week's quiz covers through page 73 in the course reader, but can someone let me know which section that corresponds to in the textbook? I believe it is through section 15.10, but nothing after that. Can someone please confirm this? Also, can someone also let me know which problems corres...

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If the conditions under which the reaction occurs are not explicitly stated, how do we know whether to use the heat capacity for constant volume or to use the one for constant pressure? Thank you.

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The question reads as follows: "Using the standard cell potentials: F_{2} (g) + 2H^{+}(aq) + 2 e^{-} \rightarrow 2HF (aq) E^{o} = +3.03v F_{2} (g) + 2e^{-} \rightarrow 2F^{-} (aq) E^{o} = +2.87v Calculate the value of Ka for HF." I was wondering, why...

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When trying to figure out how many sig figs to use, do we take into account the values given for temperature? Or do we ignore how many sig figs were reported for temperature? For instance, if a problem said it occurred at 25 degrees Celsius, and the rest of the values of the problem (say volume, mol...

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I still don't understand where the equation q(ice) = n*∆H + m*C_sp * ∆T comes from?? Also what is C_sp? The equation for ice breaks down as follows: The n*∆H part is where we first have to melt the ice. n is the amount of moles of ice times ∆H, the molar heat of fusion (fusion is the process of goi...

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Hi! The question asks to calculate K and dG0 at 298 K for the following redox reaction: Mn2+ + Br2 --> MnO4- +Br- I was able to solve for dG0 by using dG0 = -nFE0 and obtained the same answer as the answer key. When I try to solve for K using dG0 = -RTln(K) I find the K is equal to 1.35x10^-72. The...

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Hi, can someone please tell me what the units should be for \Delta H when using bond enthalpies? Because the examples in the book vary on whether or not the units for the final measurement are reported in kJ/mol or just kJ. For example, like the problem on our quiz, what were the units supposed to b...

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It would have to do with the type of reaction that is given. In the question, it is specified that this reaction is in a nickel-cadmium cell, which needs a salt bridge. The previous parts (a) was specifying a solubility equilibrium and part (b) was specifying a Bronsted neutralization reaction, bot...

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The KOH_{(aq)} or KOH (aq) acts as the salt bridge between the two solutions, which allows for ion flow. The conducting electrode refers to the solid pieces of metal in solution, acting as the cathode or the anode. The previous questions only used one metal species as both cathode and anode...

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Hi, I was wondering if someone could please help me with question 14.15, specifically part c? The question reads: Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions: c) Cd(s) + 2Ni(OH)_{}3 (s) \rightarrow Cd(...

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Can someone please explain exactly why reversible reactions require more energy and heat than irreversible reactions? Thanks!

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Hi, I'm having a little difficulty solving 8.99? The problem is as follows: Hydrochloric acid oxidizes zinc metal in a reaction that produces hydrogen gas and chloride ions. A piece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800.0 mL of 0.500 M HCl (aq). If the initial tempe...

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Why is it that although heat is being added at a sample's melting and boiling points, the temperature remains constant and the curve therefore looks flat? I thought that since heat is being added that the temperature would increase and the graph thus would have an increasing slope? Similarly, can so...

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