CHEMISTRY COMMUNITY

Alan Chien 1J

How would you describe the geometry on pg 51 of the orgo book which gives a picture of 3-Chloro-5-iodocyclohexene? Why doesn't it have a cis or a trans in its name? Shouldn't the double bond cause some geometry? Likewise, shouldn't the two substituents also have some geometry? Would this be a correc...

Alan Chien 1J

Will we be responsible for naming functional groups located on substituents?

Alan Chien 1J

When we are talking about the planar form of cyclopentane, does it have "5 pairs of eclipsed C-H bonds"? Also, when we say pairs does that refer to all the C-H bonds that are eclipsed between two carbon atoms? Lastly, in a puckered cyclopentane (envelope conformation), I understand that th...

Alan Chien 1J

Why is the propyl group attached on the very bottom chair carbon? Could we have put ethyl and methyl there instead, and put the propyl on an adjacent carbon?

Alan Chien 1J

Why does the molecule cyclooct-4-enone have a cis- geometry?

Alan Chien 1J

Is the entropy of the surrounding 0 in this situation? If so, why is that?

Alan Chien 1J

During a irreversible expansion and an irreversible free expansion, do the two different methods affect the entropy? I realize entropy is a state function so the two would be the same but would the free aspect of the second expansion affect the entropy of the system or surroundings in any way differ...

Alan Chien 1J

Why is the entropy of the surroundings 0 during an irreversible expansion? Also when the system is adiabatic and q=0, does that affect Entropy of the system surr or the total?

Alan Chien 1J

Do all cyclical alkanes and cyclical alkenes have some sort of geometry when they have a subsitituent on them? How is it possible to tell if they are cis and trans? For example on quiz 3, we were given a 1-bromomethyl-2,3-dimethylcyclopentane. Since there are single carbon carbon bonds that can rota...

Alan Chien 1J

Are we responsible for memorizing the added tension with extra subsitutents like a methyl or how much tension an eclipsed C-H C-H produces?

Alan Chien 1J

When I look at cyclopentane, I know it has an envelope conformation when it is puckered but I does it have a cis and trans like cyclohexane. If you fill in one hydrogen, the other carbons can't alternate axial up and down because there are only 5 carbons and that would not make sense. Does it have s...

Alan Chien 1J

Will we ever be asked to draw the most stable conformation for a cyclopentane with multiple substituents attached to the carbons? If so how would we do that? For example 1, 4-dimethylpentane?

Alan Chien 1J

I believe it is purely alphabetical. The only time we take into consideration atomic mass and the size of the molecule is determing cis and trans.

Alan Chien 1J

Is it possible to determine the geometry of an alkene given just the condensed molecular formula? For example on pg 43 we have to name the compound CH3CH=C(CH3)CH(CH3)2. The answer key does not give the name with any geometric isomer but I believe it has some kind of geometry. Can anyone verify?

Alan Chien 1J

How does temperature affect the equilibrium of a reaction?

Alan Chien 1J

On page 148 of the organic chemistry textbook, why does the first step illustrate that the electrons from the single bond attack the AlCl3. I did not think that single bonds were a source of electron abundance. I would think that the lone pairs on the Cl would attack AlCl3 and the polarity between R...

Alan Chien 1J

Are these two terms interchangeable?

Alan Chien 1J

Is this path of thinking correct? Neo and Iso are meant to describe simple parent chains while tert and sec are common names that can describe fairly basic substituents? Thanks!

Alan Chien 1J

Dr. Lavelle mentioned something about this during lecture today. When deciding the numbering between two names for an alkane, does our priority for the lowest number become solely focused on the substituent that appears first (alphabetically) in our name? Thats seems to contradict a bit with what th...

Alan Chien 1J

What is the difference between the 2? They both look the same to me!

Alan Chien 1J

This is an example on the bottom of page 62 on the organic chemistry texttbook. I was not sure how the substituent prop-1-en-2-yl was named. I understand that the 1 designates which carbon the double bond starts at but what does the 2 represent?

Alan Chien 1J

When we are given a formula of (CH3)2CHCCCH2CH3 vs. CH3Ch2CH(CH3)C(CH3)3, how do we know which carbon the stuff in the parenthesis is attached to?

Alan Chien 1J

In the organic chemistry textbook, the textbook calls C5H6 cyclopentdiene. Is this correct? I thought the correct prefix for double carbon bonds should be -adiene. So shouldn't it be called cyclopentadiene? Or no?

Alan Chien 1J

Yes. A double headed arrow represents 2 electrons moving. Generally, one headed arrows will appear only for the transfer of electrons form radicals.

Alan Chien 1J

How do the two diagrams ( y axis with Pe or Free energy v. course of reaction)? Does potential energy diagrams track the change in enthalpy while the free energy tracks the gibbs free energy?

Alan Chien 1J

5E(g)→4K(g)+G(g) This reaction follows a second order rate law with regards to E. What is the initial rate of decomposition of E given that the k, rate constant, is 4x10^-2 and initial concentration of E is 0.050 M. In this situation Rate (initial)=(−1/5)*d[E]/dt = k[E]^2 = 4x10 ^-2 (0.050)^2 = 1x 1...

Alan Chien 1J

The half life of a reaction is the time it takes for the concentration to halve. So yes. However, in first order reactions, the time it takes for the reaction to half is constant while in second order reactions, the time it takes for the reactant to half is not constant.

Alan Chien 1J

I am confused how the cell potential shifts given an increase or decrease in products or reactants. If I increase reactants, then the redox reaction will shift to the left. Knowing this can I say anything about the cell potential? And is what I am concluding about the cell potential is the standard ...

Alan Chien 1J

What are electrolytes?

Alan Chien 1J

Are cell diagrams solely based off of the two half reactions? As in aqueous things that might cancel out like H+ in the final balanced could show up on both sides of the cell diagram?

Alan Chien 1J

Given something like this 11.21 Calculate the standard Gibbs free energy of each of the following reactions: (a) I2(g) -> 2 I(g), K = 6.8 at 1200. K I understand the rules within calculations about mantissa and such, but would the value of K = 6.8 be considered 2SF and most probably dictate the fina...

Alan Chien 1J

The enthalpy given for glucose is the enthalpy of combusion. It is essentially the entire enthalpy of the reaction of glucose reacting with 02 to make C02 and H20

Alan Chien 1J

Kj per mol describes the enthalpy of formation while kj describes the enthalpy of the reaction.

Alan Chien 1J

Every sample of a pure element, regardless of its physical state, is assigned zero Gibbs free energy of formation.
Why is this statement false?

Alan Chien 1J

It would be considered a Galvanic/voltaic cell because the electron current, generated by the concentration gradient, generated is a spontaneous redox reaction. Electrolytic cells imply that some external supply of energy is needed to cause the non spontaneous reaction to occur.

Alan Chien 1J

Is there technically no designated anode or cathode for a concentration cell? When we calculate Ecell (=-RT/nf * ln K), we were told to always make the products the lower concentration.

Alan Chien 1J

9.82 (a) Calculate the work that must be done at 298.15 K and 1.00 bar against the atmosphere for the production of CO2(g) and H2O(g) in the combustion of 10.50 mol C6H6(l). (b) Calculate the change in the entropy of the system due to expansion of the product gases. Using w = deltanRT, I was able to...

Alan Chien 1J

Why is the total entropy in the universe 0, (Entropy system = Entropy surroundings) for reversible processes?

Alan Chien 1J

Clarification: the expansion adiabatically occurs irreversibly. Does this make the temperature of the adiabatic system higher or lower?

Alan Chien 1J

We are asked for the relative temperature difference between a system which expands reversibly and isothermally versus an adiabatic system which expands the same amount. According to PV=nRT, if the volume increases then the temperature of the system should increase right? So would the adiabatic syst...

Alan Chien 1J

On the Winter 2015 Midterm, there is a question asking for K given a redox reaction. The solutions solves for the equilibrium constant with deltag = -RTlnK. I used the Enot= 0.0592/n * log K to solve for K and got different answers. Which equation should have been used?

Alan Chien 1J

On the Winter 2015 Midterm, there is a redox reaction but the solutions solves for the equilibrium constant with deltag = -RTlnK. I used the Enot= 0.0592/n * log K to solve for K and got different answers. Which equation should have been used?

Alan Chien 1J

Should I use Enot= 0.0592/n * log K or Delta G = -RTlnK to find the equilibrium constant? Can I use either? I believe they both give different equilibrium constant values?

Alan Chien 1J

Why is it that when K =1, we assume that the reaction is at equilibrium. Doesn't K, the equilibrium constant, vary and depend on the coefficients of the reactants and products? So that when a reaction is at equilibrium, K does not have to be equal to 1?

Alan Chien 1J

Is it possible that your cathode/anode is a liquid instead of a solid like platinum or graphite? I have seen mercury show up in quite a few problems.

Alan Chien 1J

AgBr(s) -> Ag+ (aq) + Br- (aq)
How do I tell which thing is being oxidized/reduced in this reaction?

Alan Chien 1J

Pt(s)|02(g)|H+(aq)||OH-(aq)|02(g)|Pt(s)
Why do we use the reaction of
02 (g) + 4H+ (aq) +4e- -> H20(l)
for the oxidation reaction? If I were looking for an oxidation reaction with 02 and H+, wouldn't they be on opposite sides of the reaction?

Alan Chien 1J

Zn (s) | Zn2+ (aq) || Sn4+ (aq), Sn2+ (aq) | Pt(s) We are asked to find the standard potential of Sn4+/Sn2+ given the standard potential of the cell is +0.91 and the standard potential of the zinc electrode is -0.76. Knowing that Ecell = Ecathode - Eanode 0.91 = Ecathode - Eanode What confuses me is...

Alan Chien 1J

I believe Pt Electrode would work in most situations because it is an inert metal and wouldn't take part in the reaction.

Alan Chien 1J

Will Dr. Lavelle's review session be bruincasted?

Alan Chien 1J

When we are expected to write the equation for the combustion of a substance, for example of glucose, are the products H20 always in liquid and C02 always gas?

Alan Chien 1J

Change in entropy is defined as 3/2*n*R*change in temperature. If the reaction is isothermal, then the temperature will not change and entropy will be 0. Change in entropy is q + w, and if entropy is 0, q = -w.

Alan Chien 1J

How is it possible in this situation that the Total entropy of the universe is positive but the entropy of the surroundings is 0? Shouldn't entropy of the surroundings be the -entropy of the system.

Alan Chien 1J

So could the entropy of any of the phase changes be calculated at any temperature?

Alan Chien 1J

Can someone explain what it means to calculate for the entropy of a phase change such as a vaporization for a substance below its boiling point? How is it possible to have something vaporize if it has not reached the boiling point?

Alan Chien 1J

Are reversible reactions by definition isothermal?

Alan Chien 1J

I think what the chem mod is saying is that delta u = q + w. But in an adiabatic expansion q is 0. Therefore U = W. By using a separate equation for U ( change U = 3/2 nRchangeT), you can indirectly solve for W.

Alan Chien 1J

When a reaction occurs under standard conditions, is it assumed that the pressure is constant? (qp = enthalpy)

Alan Chien 1J

qcal = ccal * change in temperature is the equation to find the heat or change in heat of a calorimeter. However, the equation can be used in different situations given different information. For example, if the question only provided mass or number of moles of a substance and a change in temperatur...

Alan Chien 1J

Gases in general have the ability to expand. In a bomb calorimeter, the gases are held at constant volume by definition so that the change in enthalpy is the same as the change in internal energy. To my understanding, since the question did not mention a bomb calorimeter, and the number of moles on ...

Alan Chien 1J

I believe heat (q) itself is path function and not a state function. Rather, enthalpy is a state function and is equal to the heat (q) when pressure is held constant.

Alan Chien 1J

For pH values, only the numbers after the decimal are considered when determining significant figures. For example, 7.1 is just 1 SF. 7.89 is 3 sig figs.

Alan Chien 1J

Decreasing the partial pressure of a reactant is synonymous with decreasing the amount/concentration of that substance in the reaction. According to Le Chatelier's principle that states the the system will try to reduce the effects of any changes, more reactant would be produced to make up for remov...

Alan Chien 1J

We are required to know about Lewis Acids and Bases for the midterm as it was covered in lecture. I asked my TA and the equations should given to us, at least that was the case for the quizzes up to now, so I expect the midterm to be the same.

Alan Chien 1J

I would give the radical to the lesser electronegative atom, because the more electronegative atom (more inclined to gain an electron) would more likely have its octet filled first.

Alan Chien 1J

Do all atoms in a molecule undergo hybridization? For example, in CH20, the Carbon and Oxygen both have a hybridization of sp2. Would it be wrong to say that Hydrogen has a hybridization of s?

Alan Chien 1J

On question 25 (a) in the Chapter 4 HW, we are given the molecule CH2CL2. I drew out the molecule as a Carbon with 4 covalent bonds, which would lead me to believe that it is non polar (AX4). However, the answer key says that CH2CL2 is polar because of its dipole moments, which I understand. So, is ...

Alan Chien 1J

Polarity can be identified in two different ways. There can be polarity in the bond between two atoms and overall polarity in a molecule. Polarity of bonds can be determined by the differences in electronegativity. To determine the polarity in a molecule, first determine the shape. Then examine the ...

Alan Chien 1J

Knowing how many regions of electron density are around the central atom, or are around the atom you want to describe the hybridization of tells you what the hybridization is. For example, the carbon in Ethene has 3 regions of electron density (2 single bonds, 1 double bond). Then we associate 3 reg...

Alan Chien 1J

The wavelength is raised to the negative one power because that is just another form of writing the equation. Similar to how sec^-1 is the same as /sec, lambda ^-1 is just dividing by lambda.

Alan Chien 1J

Although not the most efficient way, I personally calculated the energy levels of n=1 through 5 which surprisingly does not take that much time and compared the differences until I found one that matched the energy emitted given the wavelength was 102.6 nm.

Alan Chien 1J

On the Chem 14A website, Professor Lavelle shared a useful guide titled Everything You Want to Know about Sig Fig : https://lavelle.chem.ucla.edu/class-websites/chem14a/"onclick="window.open(this.href);return false; To answer your question, if the last digit ends in 5, then we would round to the nea...

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