CHEMISTRY COMMUNITY

Joonsoo Kim 4L

When a catalyst is present and the reaction can follow a different mechanism with a lower activated complex, do all of the reactants follow this mechanism, or do some of them still follow the old mechanism (without the catalyst and with a higher activation energy)? In other words, when there are mul...

Joonsoo Kim 4L

If there are multiple possible mechanisms for a given reaction, does the reaction follow just one of them, or a combination of several?

Joonsoo Kim 4L

My TA mentioned that reactions don't necessarily have to be positive whole numbers; you can have negative numbers or fractions. When is this the case? Why are reaction orders commonly whole numbers like 0, 1, and 2?

Joonsoo Kim 4L

Changing the concentration in experiments can show you which order the reaction is, but it doesn't change the order of the reaction. In other words, in rate=k[A]^n, changing [A] does not change n.

Joonsoo Kim 4L

I think you use big K to convert the concentration of an intermediate in terms of concentrations of species found in the experimentally determined rate law. You can then convert big K into little k's using K=(k/k') to get your rate law all in terms of concentrations and rate constants.

Joonsoo Kim 4L

Summing the elementary steps to get the overall reaction goes for any mechanism of any number of steps. If the sum matches with the overall chemical equation, the mechanism has satisfied one of the requirements of being a plausible mechanism for that reaction. You use the pre-equilibrium approach wh...

Joonsoo Kim 4L

I think I remember my TA saying that we'll focus mainly on unimolecular reactions, since the integrated rate law becomes complicated when you introduce more than one reactant.

Joonsoo Kim 4L

Dr. Lavelle mentioned that if a reaction mechanism fulfills the criteria of A) sum of elementary steps = overall reaction and B) the rate-determining step must agree with the experimentally determined rate law, then the mechanism is possible, but not necessarily correct. Is there any way to confirm ...

Joonsoo Kim 4L

ES (where the intermediates are formed) are usually higher energy than the reactants and products because they are often the least stable. Sometimes, the intermediates are more stable than the reactants, but never more stable than the products.

Joonsoo Kim 4L

In a redox reaction, when one species is reduced, it has the power to oxidize another species (i.e. is an oxidizing agent) so a higher reduction potential would mean greater oxidizing power.

Joonsoo Kim 4L

I believe that catalysts can be used up in one step of the reaction mechanism, but then produced again in a subsequent step so it cancels out. You can tell if something is an intermediate or catalyst by looking at the order in which it shows up; if it's a reactant first then product later, then it's...

Joonsoo Kim 4L

It wouldn't work differently; the chemical equation doesn't include the catalyst since it cancel out anyways.

Joonsoo Kim 4L

If the experimentally observed rate law and the rate law found by the proposed mechanism don't match, then that means that the proposed mechanism is wrong, and the reaction occurs based on a different mechanism. Also, even if they match, it doesn't necessarily mean that the proposed mechanism is cor...

Joonsoo Kim 4L

To my understanding, A depends on the steric factor (fraction of collisions with the correct orientation) and the collision frequency, but I am pretty sure that we won't have to calculate A and it will be given to us.

Joonsoo Kim 4L

In my personal opinion, I think units are more important for organizing your thought process through a question, as opposed to the few points it'll be worth on the exam.

Joonsoo Kim 4L

When temperature increases, the Kw increases as well, which causes the neutral [H+] to increase and neutral pH to decrease. On the flip side, when temperature decreases, the Kw and neutral [H+] decrease, so the neutral pH increases. Neutral pH of 7 is only true at 298K.

Joonsoo Kim 4L

If you know the percentage of how much reactant was lost in an equilibrium reaction, you know the change in molarity ("C" in ICE) which can then help you find the C values for all other reactants and products, based on the stoichiometric coefficients. You use this to find the equilibrium c...

Joonsoo Kim 4L

When you add an inert gas, the total pressure of the system increases, but the partial pressures of the reactants and products remain the same, so there won't be a shift in the equilibrium. This is different from changing the total pressure by changing the volume of the container, which does change ...

Joonsoo Kim 4L

Deprotonation is just the dissociation of an acid when it reacts with H2O to produce H3O+. Polyprotic acids like H3PO4 have multiple hydrogens which it can give off, hence the possibility for a Ka2 value.

Joonsoo Kim 4L

In Adsorption, Dr. Lavelle mentioned that you'd want to maximize the surface area of the catalyst so that more of it is making contact with the reactants. In absorption, that wouldn't really be a problem. I agree with Destiny that it's not something that we need to worry about too much.

Joonsoo Kim 4L

What is the significance of determining the overall order of a reaction if it doesn't help you understand the individual orders or rate?

Joonsoo Kim 4L

since the equation for rate law is rate=k[A]^n, and the units for rate is mol/(L*s) and the units for [A]^n is (mol/L)^n, you can derive the units for k just by solving for units depending on the value of n.

Joonsoo Kim 4L

I believe it depends on the order in which the steps occur. For instance, a catalyst would be a reactant in a prior step and a product in a subsequent step, whereas an intermediate would be a product in a prior step and a reactant in a subsequent step.

Joonsoo Kim 4L

What does Dr. Lavelle mean by porous wall when talking about concentration cells, and how do they contribute to their function?

Joonsoo Kim 4L

Do you include everything in the overall cell reaction in the cell diagram, including H2O?

Joonsoo Kim 4L

I believe that if E(red) is strongly positive, the reaction favors reduction, so it would be a good oxidizing agent. Contrarily, if E(red) is strongly negative, the reaction favors oxidation, so it would be a good reducing agent.

Joonsoo Kim 4L

Another (less exact, but a way for confirmation) approach for me was to think about whether or not the reaction would happen by itself or not. Both of these scenarios would probably occur spontaneously given the conditions, so it would make sense that the delta G is negative.

Joonsoo Kim 4L

pH refers to the concentration of [H+] in solution, not necessarily the acidity of the solution. When the temperature is lowered, the Kw is also lowered (due to the van't hoff equation) so the [H+] of a neutral solution of water would also be lowered, which raises the neutral pH. If the neutral pH i...

Joonsoo Kim 4L

I believe you use the formula using the value you find in the table, and not the absolute value. I don't believe you'll find any negative values for bond energy, since it's the energy required to break a bond, and if that value was negative, then the bond probably wouldn't exist.

Joonsoo Kim 4L

Can someone explain why deltaH is equal to qp?

Joonsoo Kim 4L

ICE tables should be used anytime you have an equilibrium reaction, and therefore has a K value. I think Dr. Lavelle said in class that technically, most chemical reactions are equilibrium reactions, but the ones that we accept to run to completion just have a giant K value. You don't necessarily *n...

Joonsoo Kim 4L

Not sure how much this will help, but since a lot of ligands correspond to polyatomic ions, I just associate names and charges to those.

Joonsoo Kim 4L

The bond angles would be 90 degrees between the equitorial and axial domains, and 180 between the two axial domains. If you imagine a trigonal bipyramidal structure, and then remove two of the domains in the equitorial positions, it may be easier to visualize the bond angles that way.

Joonsoo Kim 4L

The way that I identify resonance is when I draw the lewis structure, a molecule with resonance will have more than 1 bond consisting of 2 or more atoms of the same element that are attached to the same central atom, except not all of those bonds are single bonds. For example, if the lewis structure...

Joonsoo Kim 4L

You may have caught an error in the textbook, or perhaps the answer key was just trying to show the initial proton transfer, and not account for the equilibrium. In any case, I don't believe that the answer key provides the full, correct equations for this problem.

Joonsoo Kim 4L

Increasing the H+ concentration makes the solution more acidic, and since stronger acids rank lower on the pH scale, increasing [H+] decreases pH.

Joonsoo Kim 4L

For writing the formula, you don't need to know which atom is attached. But it will better represent the structure of the coordination compound when you're drawing the lewis structure, though I don't know that it will necessarily be expected.

Joonsoo Kim 4L

I think all you have to know about them is that they come from losing 2 additional electrons from what is already an ion, since the S orbital in this particular atom has relatively little energy and can easily be removed. This results in unexpected ion formations- for example, Sn2+ to Sn4+

Joonsoo Kim 4L

Yes, that's correct. The units for wavelength is m (or, more commonly nm)

Joonsoo Kim 4L

What's the point of calculating pKa and pKb? I understand that they can determine the relative strength of an acid/base, but isn't that possible by just using Ka and Kb?

Joonsoo Kim 4L

Is it possible for a polydentate ligand to bond to two different transition metal cations and not form a chelate?

Joonsoo Kim 4L

What's the difference between writing an acid dissociation reaction as HA(aq) <-> A-(aq) + H+(aq) vs. HA(aq) + H2O(l) <-> A-(aq) + H3O+(aq)? For bases, we seem to always include H2O(l) in the reactants, but I've seen it written with and without for acid decomposition. Aren't H+ and H3O+ different io...

Joonsoo Kim 4L

I believe that the two Cl- in cisplatin can bond with the N atoms in DNA, since the two Cl' in cisplatin are adjacent to one another (as opposed to transplatin, where they are opposite one another) and since the N atoms in DNA have lone pairs which cause it to act as a polydentate ligand. This preve...

Joonsoo Kim 4L

You also have to determine the shape of the molecule, which also has an effect on the polarity of the molecule. For instance, on paper, NH3 seems like it would be a nonpolar molecule, since all of the dipole moments cancel each other out. However, since it is a trigonal pyramid (not trigonal planar)...

Joonsoo Kim 4L

I'm confused as to why the uncertainty equation seems to only account for one-dimensional movement of a particle. I understand the derivation of the equation, but in reality, wouldn't an electron (or anything that can move) be able to move in 3-D space?

Joonsoo Kim 4L

It wouldn't really be accurate to say that dipole forces would ever occur over hydrogen bonding, since hydrogen bonding is just a type of strong dipole-dipole interaction which happens when a Hydrogen atom is bonded with a Nitrogen, Oxygen, or Fluorine atom in a molecule, which results in a highly p...

Joonsoo Kim 4L

If I heard correctly, I believe what Stephen said is correct and all material covered in the midterm ended on Wednesday's lecture. But, it's always a good idea to learn it now so you understand it better for the final!

Joonsoo Kim 4L

You would have to use information about other properties of the compound to determine whether it is a covalent bond or an ionic bond, such as solubility in water.

Joonsoo Kim 4L

You can tell if you need a double bond if the number of valence electrons needed for each atom to form an octet exceeds the number of valence electrons actually available. One way to determine which atoms are double bonded is by looking at formal charge, and the "best" case would be where ...

Joonsoo Kim 4L

It's difficult to define a trend for covalent radius, since a covalent bond often involves two different elements, and not just one.

Joonsoo Kim 4L

I'm pretty sure that the f-orbital corresponds only to when the angular momentum quantum number (l) = 3, and when the principal quantum number (n) = 4 or 5. I believe there are no 6f or 7f orbitals.

Joonsoo Kim 4L

Disclaimer: I'm in no position of authority to say this with certainty. If I'm interpreting the question correctly, I think you're asking about how we'll distinguish between electron configurations of isoelectric atoms/ions. Honestly, I would bet my money that questions will either a) tell us if the...

Joonsoo Kim 4L

the "E" in E=hv is the energy per photon , so if a question asks how many photons a light source has, as long as the question gives you (or gives you a way to find) the frequency and total energy of the light, you can use E=hv to find the energy per photon, and then divide E total /E per p...

Joonsoo Kim 4L

The Rydberg formula is just a simplified version of the formula, ΔE=[(-hR)/(N₂²)-(-hR)/(N₁²)]=hν. Cancel out and substitute some terms and you should end up with c/(Rλ)=1/n₁²-1/n₂², and it should be pretty clear what the two values of n are.

Joonsoo Kim 4L

I believe there is no "intensity" of photons- instead, the intensity of a light source relates to the number of photons (i.e. an intense light would correspond to many more photons than a dim light). In the wave model, the intensity of a light relates to the amplitude of the wave (imagine ...

Joonsoo Kim 4L

Electrons will only be emitted by a metal surface due to a light ray if the incoming light has energy equal or greater than the threshold energy. According to the wave model of light, that would mean that the amplitude of the light would have to be large in order to achieve the threshold energy (thi...

Joonsoo Kim 4L

The negative sign indicates that energy is lost from the electron in the process of jumping from a higher to lower energy level. The opposite of this value (positive number) would be the energy gained by the photon that is emitted as a result of this jump between energy levels.

Joonsoo Kim 4L

My high school teacher used lowercase "f" to represent frequency, which was pretty useful in the context of high school chemistry, and which wasn't ever confused with v or even capital F for force. I'm not sure if a new variable "f" will be introduced later on, but this has worke...

Joonsoo Kim 4L

Think about the units of what you have and what you need, and then you can then identify which conversions you can use to get from point A to point B. For instance: You have the mass (and I would assume you have the identity of the substance as well) You need no. atoms If you have the mass of a samp...

Joonsoo Kim 4L

I'm pretty sure that the combustion of a hydrocarbon follows this pattern: Hydrocarbon + (x)O2 -> (y)CO2 + (z)H2O Where (x), (y), and (z) are stoichiometric coefficients you assign to balance the equation. An analogy from class that helped me to remember this was oxygen metabolism: when you consume ...

Joonsoo Kim 4L

Technically, it isn't "impossible" to have two limiting reactants. But, the conditions needed to create this scenario would be *highly* unlikely, because you would have to have the exact multiple of one limiting reactant as another, based on the mole ratio. For example, in the balanced equ...

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