CHEMISTRY COMMUNITY

Jar-Yee 3L

The question states tht we are using heat from the combustion of methane gas to boil 50.0 mL of water, and asks how many grams of methane are required. For the heat, do we include the heat required to boil the water completely to gaseous form? Or simply to reach the boiling point? Why? The wording s...

Jar-Yee 3L

Dr. Lavelle mentioned in class that double and triple bonds count as functional groups, right? So when we have both a double/triple bond and another functional group (alcohol, carboxyl, amine, etc) in an organic molecule, which gets priority in numbering?

Jar-Yee 3L

What is the difference between an activated complex and a transition state? Number 15.85 in the textbook asks us to draw the activated complex, and I don't know what that's supposed to mean besides drawing the TS. What's the difference? Is there a difference in drawing them?

Jar-Yee 3L

Not at all. The parent chain consists of 5 carbons (hence pent ane), and the methyl group is attached to the third carbon. No matter from which side you count from, the methyl group will be attached to Carbon number 3. So 3 is the lowest number possible for the methyl substituent group of 3-methylpe...

Jar-Yee 3L

I'm confused as to what can be an electrophile/nucleophile. I understand that an electrophile (lacking electrons) could be a cation of some sort or something with a positive charge, but a nucleophile's concept confuses me. Are all nucleophiles atoms? How come bonds can be nucleophiles? And is it pos...

Jar-Yee 3L

I have a question about the Hogwarts Problem for the midterm. Basically, Ron screws up a spell and blows up his balloon and increases its temperature as well. How come when calculating the change in entropy produced by a temperature change, the molar heat capacity C v is used? The volume changed for...

Jar-Yee 3L

For the Nernst Equation and the free energy equation of voltaic cells, does n have units of moles? It makes sense for the free energy ΔG=-nFE, because (moles)(Coulombs/moles)(J/Coulombs)= J, but if n didn't have units we would end up with J/moles, which still makes sense. It doesn't make sense when ...

Jar-Yee 3L

I'm wondering how we deal with Q, or [P]^p/[R]^r for the Nernst Equation.

Occasionally, we have a mix of both partial pressures AND molarities in the reaction quotient.

How do we take into account the discrepancy between units? For example, if one is in Torr and the other is in mol/L.

Jar-Yee 3L

What if the solid isn't a conductor? For example, I₂ (s)?

Jar-Yee 3L

I realize that reduction occurs at cathodes, and oxidation occurs at the anodes. However, given an entire, unbalanced redox reaction OR a combination of ions in solution, I find it difficult to figure out what equations to use. For example, HW 14.15b: The equation given is H + (aq) + OH - (aq) --> H...

Jar-Yee 3L

Is there a specific way to order the elements, according to their phases of matter? I know that anything to the left of the salt bridge is considered the anode (oxidation) reaction, and everything to the right of it is the cathode reaction (reduction). But within those reactions, sometimes they have...

Jar-Yee 3L

When we're asked to calculate "Gibbs free energy" of something, are we looking for G or ΔG?

I'm getting mixed messages with the book, so I'm rather confused.

Jar-Yee 3L

The question asks us to calculate ΔG for the vaporization of water. ΔG = ΔH - TΔS, so I took the steps to first look up or calculate ΔH and ΔS. I got the ΔS value correct, but in this case I used ΔH vap , while the solutions manual found the ΔH using difference of sums (ΔH f of products - ΔH f of re...

Jar-Yee 3L

Wait, but what's the difference between irreversible expansion and free expansion? The descriptions sound the same to me.

Jar-Yee 3L

Is there a difference to these two?

ΔS_rxn^o vs ΔS_f^o.

Or is ΔS_f^o the same thing as ΔS_m^o?

I'm confused.

Jar-Yee 3L

I was doing question 9.47, which asks to find change in entropy for an isothermal reversible expansion, and then an isothermal, irreversible free expansion. I used ΔS = nRln(V f /V i ) for both, which gave me the correct answer. However, when trying to calculate ΔS surr for the irreversible expansio...

Jar-Yee 3L

We're asked to calculate molar kinetic energy of Kr (g) in problem 101, given a certain temperature. I understand that taking the molar heat capacity (J mol -1 K -1 ) and multiplying it by the temperature to get molar KE (J/mol). The solutions guide says that the molar heat capacity is 3R/2. This is...

Jar-Yee 3L

For an ideal gas, ΔU=0 when ΔT=0, evidently because an ideal gas only has kinetic energy, so without heat there would be no change in internal energy. ΔU = 0 = q+w, q = -w. With ΔT = 0, then q = 0. As a result, w = -q = 0 as well. Eventually, a proof with this concept would give us ΔS=nRln(V f /V i ...

Jar-Yee 3L

Problem 27b states: "Calculate the work for the process beginning with a gas sample in a piston assembly with T = 305 K, P = 1.79 atm, and V = 4.29 L: ... (b) isothermal, reversible expansion to a final volume of 6.52 L." The equation for isothermal reversible processes with work is w = -n...

Jar-Yee 3L

Interesting. Does this mean that I shouldn't be looking at the words "reversible" or "irreversible" in such a literal sense? If so, then how should I be looking at it?

Jar-Yee 3L

I'm just having a few problems clarifying the below equation: w = -PΔV We were told in lecture that everything in this equation, with the exception of external pressure, is in respect to the system. w sys = -P ext ΔV sys Does w sys refer to work done by the system? Or work done on the system? Or wor...

Jar-Yee 3L

In my discussion section, we were given two different equations for work. The first one is for an irreversible process: w = -P ext ΔV The second is for an isothermal reversible: w = -nRTln(v f /v i ) Both use changes in volume, but I don't know when to use one equation or the other. For example, sel...

Jar-Yee 3L

At what number do the section 8.11 problems begin?

Jar-Yee 3L

Ah, thanks. How would that affect the pH of the solution?

Jar-Yee 3L

It's given. All K_a values can be searched up in the index of the textbook.

It's dumb, I know.

Jar-Yee 3L

You stated that it would gain an H atom.... Which isn't making sense to me. If BF 3 stripped an H atom off a water molecule, it would technically leave OH - in the solution, and actually raise the pH instead. BF 3 is a Lewis acid. Why would it raise the pH? Do the Lewis definitions of acids and base...

Jar-Yee 3L

We were asked to calculate the pH of a 3.38 g sample of alanine, NaCH 3 CH(NH 2 )CO 2 , dissolved in 50.0 mL of solution. We were given K a1 and K a2 . I tried doing the ICE table twice and solving for [H 3 O + ] twice, adding them up, and calculating the pH. I got 1.30 (which, I warn you now, is co...

Jar-Yee 3L

So in HW 12.79, we're asked to find the pH of a .15 M H 2 SO 4 solution. Since sulfuric acid is a strong acid, I assumed maximum ionization/deprotonation/dissociation and just calculated pH with -log[H 3 O + ], and did -log(.15)=.82. However, upon looking at the solutions guide, they started a new I...

Jar-Yee 3L

Why is Boron Trifluoride a lewis acid? Until now I always stuck to the Bronsted-Lowry definition of acids and bases, because H + transfer was so much easier. But now I'm stuck with BF 3 , a compound which I cannot see any way to tie in with the Bronsted definitions. The only thing I can think of is ...

Jar-Yee 3L

So I know that some compounds have multiple binding sites, and are hence polydentate. For example, Carbonate (CO 3 2- ) and oxalate (C 2 O 4 2- ) both have oxygen atoms that can act as separate binding sites due to their negative formal charge (and hence attraction to the transition metal cation's p...

Jar-Yee 3L

I apologize ahead of time for the stupid questions... I understand that strong acids dissociate completely, while weak acids don't. (Same with bases) However, I don't understand how to tell a strong acid from a weak one. Is there a certain way to do that? How do I look at the chemical formula of an ...

Jar-Yee 3L

If a homogeneous equilibrium reaction consists of both reactants and products of the (aq) and (g) phases, how will we find the equilibrium constant? I'm wondering because with gases we use (P chemical ) and with aqueous solutions we use [chemical] instead. How do we incorporate both gases and aqueou...

Jar-Yee 3L

When bonding H and F, since the valence shells for each are different, how would we draw a molecular orbital for a single bond between the two?

Jar-Yee 3L

I realize that there are exceptions to the octet rule, and that period 3 and higher elements have d-orbitals in the valence shell that can take in extra electrons. But how many more electrons can you accommodate? Is there a set answer for certain elements? This goes back to Ch. 4 homework, question ...

Jar-Yee 3L

It's because the world is in 3 dimensions. On paper (since paper is obviously 2-d), it's easier to think of all the atoms lying on a 2-dimensional plane. But the world isn't like that. A third dimension offers way more space for the atoms to move away from each other. Hence, for a tetrahedral struct...

Jar-Yee 3L

Okay... I'm really confused on one thing. Is it true that orbitals constitute subshells, which constitute shells? Or is that the wrong way to think about it? Also, when writing in a certain notation: would it be that, say, 3p would be a subshell, and 3p x an orbital? And what would be represented by...

Jar-Yee 3L

Hi all! I had a quick question on writing the notation of certain d-block atoms. I remember that Professor Lavelle suggested we write the orientation of the p orbitals to give the electron configuration notation more of a conceptual meaning. So for example, for a neutral atom of Carbon (6 protons, 6...

Jar-Yee 3L

Hello everyone! In the online modules (and hopefully on Friday), Prof. Lavelle went over that the indeterminacy of a variable (ex. Δv) is equal to the size of the "spread" of the possible values for that variable. For example, if v=10±1 m/s, then the values range from 9 to 11. Hence, the i...

Jar-Yee 3L

You said it yourself! While calculating ΔE, you find that ΔE= -4.09x10^-19 J. A negative value of ΔE simply tells us that there was a drop in the energy level of the electron (from the 4th energy level to the 2nd). Hence, the negative change in energy just tells us that the energy lost 4.09x10^-19 J...

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