CHEMISTRY COMMUNITY

Michelle Lee 2E

If you see the same molecule, first as a product and then used up in as a reactant, that's an intermediate.
If you see the same molecule first as a reactant and then rejuvenated as a product later on, that's a catalyst.

Michelle Lee 2E

Pseudo rate laws are where we make everything but 1 factor constant by making the number very large to calculate for the specific order.

Michelle Lee 2E

Pre equilibrium indicates a fast step before a slow step and the need to substitute the intermediate in the slow step with the equilibrium constant equation we find from the fast step.

Michelle Lee 2E

I tell bad chemistry jokes because the good ones Argon.

Michelle Lee 2E

All these chemistry jokes are so funny.
Helium, Helium, Helium

Michelle Lee 2E

Adding on, when looking at which step's rate law matches the experimentally found rate law, if something doesn't look exactly the same, there is a probability that there's a transition step. In that case, look for the step that matches the rate law the most (has the same number of components maybe) ...

Michelle Lee 2E

What if the n value we calculate using the method of initial rates was not a whole number but some weird decimal?

Michelle Lee 2E

Usually if the numbers aren't so "strange", we would be looking at the exponent to define what order the reactant is. Say we are talking about doubling the concentration of one reactant while keeping the other reactants constant. If we double the reactant and the rate for that experiment a...

Michelle Lee 2E

Is it possible at all for the slopes of zero and first order to be positive and for the slope of second order to be negative? I think I heard a ta say it could be switched? If so, then in what circumstances?

Michelle Lee 2E

Can someone please explain how we compare doubling initial concentration to how the initial rate changes, when finding the order of each reactant for multiple reactant reactions that is? For example: 1) when we double the concentration and the initial rates are doubled 2) when we double the concentr...

Michelle Lee 2E

The equilibrium constant is different for zero order, first order, and second order but the reason why they're different is because of the order (the exponent) so just keep in mind what each component of the rate law's unit is? (for example, concentrations are usually mol/L but the units can change ...

Michelle Lee 2E

In Friday's lecture, we discussed experimentally making concentrations high so we would only have to deal with the reactant with the lowest concentration. Doing so helps us simplify finding rates and rate constants and the orders of the reactants. Would this experimental process be valid with half l...

Michelle Lee 2E

Half Lives exist for all the orders of reactions we have covered. Our book doesn't necessarily use second and zero order half life equations. In lecture, we discussed these half lives depend on the concentration. Do they depend on [A] or [A]0?

Michelle Lee 2E

Always try to refer to equations like the ones we used in class. You can solve for [A]0 but don't assume [A] is [A]0.
For plots specifically for first, second and zero order, the axes are always time by [A], not [A]0.

Michelle Lee 2E

n refers to the order of the reactants. But there could also be more orders of reactants which can be denoted by other variables. Since in the recent equations we've been using, we've been dealing with concentrations which is moles per liter (like moles are already incorporated into the equation thr...

Michelle Lee 2E

So if this question is geared towards after finding n and any other exponent, we would already know the initial rate and you just found the exponents. You would just use the initial concentration from the experiment that gave you the initial rate you would use and solve for k. In the example in lect...

Michelle Lee 2E

So far in lecture we've discussed the most general form R --> P. What if there are more than just one reactant. Would we add like (1/a)(dA/dt) + (1/b)(dB/dt) OR f Is there another way to look at this? Multiplication? For example, in the lecture on Friday, to find rate we multiplied the concentration...

Michelle Lee 2E

Keep in mind that this is only for the general rate law so far. On the equation sheet, it does seem like the coefficient is mostly the denominator. I could be thinking about the concentration instead though so definitely check to be sure.

Michelle Lee 2E

Since we're only focusing on the initial rate when we're thinking of kinetics, this is where little to no product has been formed. With such little product, finding the maximum rate possible is difficult (even though we're given the rate equation through product concentration.

Michelle Lee 2E

You can also reason out that since the product has a lower concentration than the reactant, more product will want to form which makes the reaction spontaneous. A spontaneous reaction has a positive E value.

Michelle Lee 2E

If the reverse reaction is non-spontaneous for a redox reaction, does electrolysis have to occur to drive that process? Is it right to think that in order to drive a non-spontaneous reaction, that probably has a negative E(naught), electrolysis is required? Also, do all non-spontaneous reactions hav...

Michelle Lee 2E

Think of it as anode|what's happening in the reaction|cathode.
If you don't have a diagram, it's really best to look at the reduction and oxidation half reactions to see what to write.

Michelle Lee 2E

How would the moles of electrons (n) relate to Avogadro's constant?

Michelle Lee 2E

I think that the purpose of the salt bridge and the porous disk is just to transfer charged ions to keep both the solutions in the anode and cathode neutral. I don't think that the material matters because in class, we the ions transferred never affected anything but neutralizing the charge of the s...

Michelle Lee 2E

In the anode, electrons are taken away to be put in the cathode. This makes the anode solution more positive and the cathode solution more negative. Since electrons are also negative, if the negativity of the cathode builds up, then it would start rejecting the electrons (negatives repulse negatives...

Michelle Lee 2E

When the question or text has the word "ideal", does that mean the system is under constant pressure and temperature?
Also, in terms of "ideal", can the system be open, closed, or isolated?

Michelle Lee 2E

The text has to say if it is under constant pressure or volume. I don't think assuming is good enough.
I also have a question.
When the text says "ideal", does that mean constant pressure and volume because ideal would be 1 atm and 1M?

Michelle Lee 2E

In friday's lecture (and other prior unrelated lectures), Dr. Lavelle set two gibbs free energy equations equal to each other. Can we assume that we can set any of these equations for gibbs free energy equal to each other? (not standard gibbs free energy because that's not the same as just gibbs fre...

Michelle Lee 2E

Making (delta)G=0 was an assumption we had made to find out the temperature for phase changes. When we set (delta)G=0, it knocks out a potential variable and lets us see the temperature for where the in between is. Past this temperature (greater), the forward reaction is favored; below this temperat...

Michelle Lee 2E

I think the r stands for reaction, like the enthalpy, entropy, or gibbs free energy change for the entire reaction.
I assumed it was this because I think the product side would be denoted with a p or something.

Michelle Lee 2E

The graph for Gibbs Free Energy that we learned in lecture was the graph with the minimum. So I'm not too sure what you mean by when it is at a maximum it's at equilibrium. But I think the graph we learned is a good explanation. When the ball is anywhere on the graph other than the lowest point, it ...

Michelle Lee 2E

In friday's lecture, Dr. Lavelle's powerpoint said "if a system can be prepared in only one way, then S=KblnW=0".
What did he mean by prepared in only one way?
I'm not completely sure what he meant by this line and how it relates to the equation.

Michelle Lee 2E

When entropy increases, the reaction is spontaneous. That means (delta)S is positive. When (delta)S is negative, would that mean the reverse reaction is spontaneous? If so, how should we think of the difference between a non spontaneous reaction and just a reverse reaction that is spontaneous? When ...

Michelle Lee 2E

A spontaneous reaction is a reaction that occurs without help. The example he used in class really helps; the one with the closed round flask. If that example doesn't help too much you can also think of it like this. If some sort of gas was held in a closed flask by some sort of cork and that cork w...

Michelle Lee 2E

Enthalpy itself, as we learned in our first lecture, is the amount of heat released or absorbed at a constant pressure. That in variable form is: qP = change in H The new equation we learned is change in U (internal energy) is: change in U = qP + w If there is no work done like when the reaction is ...

Michelle Lee 2E

I think determining which it is: -q(sys) = q(surr) or q(sys) = -q(surr) is based on what the question is asking. If the reaction is exothermic, then it would probably be the former because the system is releasing heat and that heat is positively transferred into the surroundings. If the reaction is ...

Michelle Lee 2E

I'm not completely sure but bond enthalpies refer to breaking and forming bonds that are in the gas phase so for liquids and solids, we need to turn those liquids and solids into gases first by adding the enthalpy of the phase change, then add the bond enthalpies.

Michelle Lee 2E

It all depends on the information you're given. If you're given enthalpy changes and need to find a total enthalpy change, use Hess's Law. If you're given bond enthalpies, use the second method. If you're given standard reaction enthalpies, use the third method. This is not the only way to look at i...

Michelle Lee 2E

For example: First, think of what element(s) make up the molecule. In O2, oxygen is the one element that makes up the molecule. Next, think of the stability of the elements that make up the molecule. For O2, oxygen's most stable form is O2. Making O2 from O2 doesn't require and enthalpy change becau...

Michelle Lee 2E

Think of exothermic as something that is cooler as a result of a reaction so gas to liquid (condensation: an example is water droplets on a window from fog) and liquid to solid (freezing: ice).
Endothermic, then, would be something retaining heat like ice retaining heat and melting.

Michelle Lee 2E

You would be able to tell by looking at the written out reaction. Take H20 and H30+, you can tell H2O is a bronsted acid because it donated a H+ to H3O+, H3O+ would therefore be the conjugate bronsted base. Also if you want to relate it to Lewis acids and bases, it's flipped. Bronsted bases are in t...

Michelle Lee 2E

We know that Ka(Kb) = 10^-14 Kw=10^-14 is a constant so it doesn't change. Therefore, if Ka is small, Kb must be larger to compensate. Vice Versa, if Kb is small, Ka must be larger to compensate. This is like when we learned c=v(lambda). The speed of light is constant. If frequency is higher, the wa...

Michelle Lee 2E

Does anyone know if Dr. Lavelle is in the lecture hall right now? I don't really know if class is cancelled because Chem_mod hasn't updated since around 10 am.

Michelle Lee 2E

Even though the equilibrium constant and equilibrium quotient have no units, all quantities used in the calculations have to have the same units in order to cancel the units out. If all the quantities were in bars (units), the calculation can still be done. In this case though, not all units are the...

Michelle Lee 2E

If you can also define which is a Lewis Acid and Lewis Base, that also helps you figure out Bronsted Acids and Bases.
In most cases, a Lewis Acid is a Bronsted Base.
Also, usually, a Lewis Base is a Bronsted Acid.

It's great to know both Lewis and Bronsted Acids and Bases though.

Michelle Lee 2E

Solids and Liquids are not included in the Equilibrium Constant equation so in the class example Ca(OH)2 (s) ---> Ca2+ (aq) + 2OH- (aq), if we looked at the reverse reaction, would it just be the inverse? In chemical equilibrium, does this mean that this chemical equation will not change in the slig...

Michelle Lee 2E

Kp is the denotation for finding the equilibrium constant only through partial pressures. Partial pressures are only through gasses. Kc is the denotation for a general equilibrium constant that doesn't use partial pressures so if you have a partial pressure equated after experimentation, you have to...

Michelle Lee 2E

In response to Saichandra's post, I agree with the description of what dipole moments do. However, I'm unsure of how polarity determines shape of the molecule. In the example in class today, cis-dichloroethene and trans-dichloroethene, one was polar, one was not but I don't see how their shapes chan...

Michelle Lee 2E

Bond lengths and bond energies also also relate to electron affinity and differences in electronegativity. Shorter bond lengths (more bonds = doubles and triples), higher bond energy, larger difference in electronegativity.

Michelle Lee 2E

Sigma bonds can form between two s-orbitals and between an s-orbital and a p-orbital.
Pi bonds form between p-orbitals, and p-orbitals and d-orbitals; but I don't really understand how the d-orbital hybridization work.
Can someone explain d-orbital hybridization?

Michelle Lee 2E

For the cis-dichloroethene and trans-dichloroethene, did Dr. Lavelle say which one had rotational ability? Both have carbon and carbon linked by a double bond, so wouldn't both forms have pi-bonds and both can't rotate?

Michelle Lee 2E

In response to Guangyu Li's post, what would be the bond for a triple bond then? And is there any specific reason for the names, i.e. sigma and pi? We didn't learn this is lecture.

Michelle Lee 2E

We learned in Oct. 27th's lecture that when using formal charge, the best structure would be when the middle element has a 0 formal charge. Why is this? Also what if there is no 0 formal charge for the element in the middle of the lewis structure, or is it always possible to have an element in the m...

Michelle Lee 2E

I agree with the two responses given. Would the resonance structure be wrong if one of the forms of the Lewis Structure was missing and we didn't go back to check that it was missing? Also, Formal Charge allows us to figure out the best or closest model/structure to the actual structure (to my knowl...

Michelle Lee 2E

I don't know which one is most common (I've never gone through a resource that said one was more common than the other but others may say differently) but I can explain why they are the most common. In Fe^2+ : [Ar]3d^6, the two electrons that are removed are from the 4s orbital which makes true the ...

Michelle Lee 2E

Give the notation for the valence-shell configuration (including the outermost d-electrons) of ... Do we need to know these for the all the exceptions in the periodic table? (d) "coinage" metals (Cu,Ag,Au) is an example they use to explain this. But what about Tungsten, Niobium, Ruthenium,...

Michelle Lee 2E

Overall, I'm still a little confused. I would like to add as well the model that Dr. Lavelle always uses for describing the shells in hydrogen. (The one with the positive charge--nucleus--at the bottom of the model and horizontal lines above it to signify the rising shells. He always mentions that a...

Michelle Lee 2E

This is definitely a question worth Chem_Mod to answer though. However, I would say we don't have to because he said in lecture that it was fine if we write it out as just 2p^3. I also believe we won't have to because he mentioned that writing the full configuration out, 2px^1 2px^2 2px^3 just makes...

Michelle Lee 2E

How do we measure intensity? In the book on page 4, it says intensity is the square of the altitude. However, the above response defines intensity as the number of photons. Then on page 9, the Stefan-Boltzmann law is explained. How am I supposed to relate all this information?

Michelle Lee 2E

Cam Bear 3G, Your response helped me understand the general differences. However, I was wondering if you had any insight to some new questions I have on Constructive and Destructive Interference. What are we supposed to understand about constructive and deductive interference? Do we need to understa...

Michelle Lee 2E

I had problems with the wording of this question as well; to understand it, I had to look at it theoretically. Say the frequency decreases; we know that the wavelength increases. [from the equation c=(lambda)(nu)] As the wavelength increases, the oscillation itself, the wave model, extends. [you can...

Michelle Lee 2E

If you want to find how much, in grams, of excess reactant remains, you can use stoichiometry. If you're given how much, in grams, there is of the reactants originally in the question, find the limiting reactant first by converting the elements/molecules to moles and looking at the molar ratios; loo...

Michelle Lee 2E

I went back and looked into formula units. The way I understand it now is that formula units are like molecules and atoms; it is in the same category that describes what the atomic item at hand is. A formula unit is different in that it deals with ions (cations, anions). Ions don't behave like molec...

Michelle Lee 2E

This post helped me understand the term "formula units". However, in Question E.21, when it asks for "the number of molecules and formula units in ...", should I consider formula units and molecules to be the same thing? If not, how should I look at what this question is looking ...

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