CHEMISTRY COMMUNITY

Adam Vuilleumier 2K

Since you know concentrations, unique rate (from table), m and n (from prior calculations), you only have one unknown variable (k). Therefore, by plugging in all these values, you'll be able to get k.

Adam Vuilleumier 2K

You can derive the half life equation for a second order rate law equation by saying that 1/[0.5A]initial = -kt + [A]initial

Adam Vuilleumier 2K

Unique rate takes into account the stoichiometric coefficients of each product/reactant. This means that the unique rate of any product or any reactant will be the same.

Adam Vuilleumier 2K

I think what you're referencing if the idea that we talked about in class today that states that higher order reactions are less likely to occur because they involve more specific types of collisions (ie. four molecules colliding at the same time). I think if you know the general idea of why higher ...

Adam Vuilleumier 2K

So far, the only calculus-type material that we've covered is that integrating the rate law gives you an equation in terms of time rather than concentration. That and the simple fact that the derivative of 1/x is lnx. If you know these things along with the idea that a derivative is the instantaneou...

Adam Vuilleumier 2K

To find n and m, use the "division method" we discussed in class. What this means is you use the equation rate = k[R]^n and plug in two of the experimental values so you have two equations with all the constants filled in except m and n. Then you divide them so that either m or n cancels, ...

Adam Vuilleumier 2K

At one of the review sessions for the midterm, one of the UAs said that derivations are fair game on the midterm. I assume this means they might be present on a test too.

Adam Vuilleumier 2K

The term nF is used to describe the quantity "charge". Since Faraday is the number of coulombs per mole of electrons, you need to n to cancel moles in the equation. This yields a coulomb value.

Adam Vuilleumier 2K

You use Pt(s) in a diagram to allow electron current to flow. Without platinum or some other solid conductor, electrons would not be able to flow from the anode to cathode and there would be no voltage.

Adam Vuilleumier 2K

Yes, since delta G = delta G standard + RTlnQ, and Q is concentration of products over reactants, Gibbs free energy does depend on concentration.

Adam Vuilleumier 2K

Gibbs free energy is not measured directly (like enthalpy is by a calorimeter). Rather, it is a calculated value that we get when we know enthalpy, entropy, and temperature.

Adam Vuilleumier 2K

Since G = H - TS, entropy and enthalpy are correlated since they must subtract to equal Gibbs' free energy.

Adam Vuilleumier 2K

Degeneracy is calculated by finding (number of equivalent energy states)^(number of particles)

Adam Vuilleumier 2K

I was in a review session the other day and the UA who was leading it said that we should match our answer's units with the units given in the problem. Therefore, if energy is given in kJ, we should give our answer in kJ, not L. atm or any other unit.

Adam Vuilleumier 2K

Enthalpy is the amount of heat energy in a system. It is equivalent to q when pressure is constant. So when a problem gives you the enthalpy instead of the q value, chances are that pressure is constant.

Adam Vuilleumier 2K

At the end of class today, Dr. Lavelle said that the system's temperature is constant despite work being done because heat from the surroundings is being added. I didn't understand how the system could be isothermal then if there is heat exchange between the system and its surroundings.

Adam Vuilleumier 2K

Degeneracy and entropy are directly related concepts by the Boltzmann equation S=kBlnW. As degeneracy (W) increase, so will entropy. Degeneracy is just a more solid and concrete value, because it is the number of possible states that some particle can be in. Entropy is just a manipulation of this nu...

Adam Vuilleumier 2K

Residual entropy has to do with the placement of certain atoms within a molecule. For example, a molecule like CH3Cl would have residual entropy because there are four different places (degeneracy) where you can put the Cl in the tetrahedral shape. A molecule like O2 on the other hand only has one c...

Adam Vuilleumier 2K

Catabolic reactions, or reactions that break bonds, release energy and are therefore exothermic. (Think about ATP - when the phosphate group is broken off, energy is released). Because of this, I'm pretty sure neutralization is exothermic because of the acid and base bonds it breaks, not because of ...

Adam Vuilleumier 2K

Temperature and heat are different things in chemistry. Temperature is measured in Kelvin or degrees Celsius, but heat is an energetic concept measured in joules.

Adam Vuilleumier 2K

Are state properties like enthalpy intensive or extensive properties? Or are they neither?

Adam Vuilleumier 2K

Solids and liquids do not have concentrations because their atoms are already as condensed as possible.

Adam Vuilleumier 2K

Percent protonated is used to describe weak bases and percent deprotonated is used to describe weak acids. Percent ionization is for both

Adam Vuilleumier 2K

A good way to tell if something is an acid or base (if you're not familiar with the molecule already) is to draw a Lewis structure. Then, you can see if there is a free hydrogen that can be donated, or a free lone pair that can take up a hydrogen.

Adam Vuilleumier 2K

LeChatelier's principle states that all reactions will adjust to change in a way that stabilizes the equilibrium. When temperature is changed, K also changes (unlike in problems with pressure changes). Since the K is lower as the temperature rises, this means that the hotter reaction favors the reac...

Adam Vuilleumier 2K

A better rule for when we can assume x is insignificant is when x is less than 5% of the initial value. (I found this rule in the worksheet that we got in discussion)

Adam Vuilleumier 2K

Since Kc uses concentrations which are approximations, I know that we do not include mol/L units in our calculations. For Kp and partial pressures, do we leave out the units as well?

Adam Vuilleumier 2K

Like we said in class, the more stable side of the equation (R or P) will be the side that has a greater concentration at equilibrium. Therefore, if the equilibrium constant ([P]/[R]) is a large number, that means that there is a high concentration of product and the product is more stable. If K is ...

Adam Vuilleumier 2K

Since K is the equilibrium constant, you should use the concentrations present when the reaction reaches equilibrium.

Adam Vuilleumier 2K

To clear up what was said above, there is no way to determine the concentration of a solid, however, liquids are not included in the Kc for a different reason. Dr. Lavelle said that the amount of liquids, or solvents, on both sides of the reaction are almost the same. Thus, the concentrations cancel...

Adam Vuilleumier 2K

When we are talking about the electron configuration and not the actual shape, tetrahedral shape is under the tetrahedral class of electron configuration, meaning it has 4 areas of electron density. Square planar is under the octahedral class of electron configuration, meaning it has 6 areas of elec...

Adam Vuilleumier 2K

Just know that pOH is the -log[OH-] and a low pOH means that the solution is more basic. While we're on this topic, does anyone know why a low pKA means that it is a stronger acid?

Adam Vuilleumier 2K

I think what you're looking for is the "Naming Coordination Compounds" link midway down on Professor Lavelle's ucla website.

Adam Vuilleumier 2K

The problem becomes a simple dimensional analysis problem when you remember that pH is just -log[H3O+] in mols of H3O+/Liter. Make sure to remember that since it's a log function, the greater the H3O+ concentration, the LOWER the pH will be.

Adam Vuilleumier 2K

Interesting addition that we went over in my discussion section: sigma bonds are stronger than pi bonds. Even though pi bonds have two areas where the electron densities overlap, sigma bonds have a larger area of overlap, which makes the bond stronger.

Adam Vuilleumier 2K

They relate to resonance structures because sigma bonds are typically single bonds and double bonds are typically formed from one sigma and one pi bod. Depending on which area of electron density you place the pi bond, there will be different resonance structures.

Adam Vuilleumier 2K

Another trick I use is the fact that "sigma" sounds like "single". Therefore, I can remember that single bonds are usually sigma bonds.

Adam Vuilleumier 2K

Since you can only infer the number of areas of electron density from hybridization, you cannot be sure about bond angle. To know the bond angle, you need to know the types of atoms involved and the amount of bound atoms/free electrons.

Adam Vuilleumier 2K

In case you needed the answers for the problem, they are a) sp b)sp2 c)sp3 d)sp3

Adam Vuilleumier 2K

Polarizability is the term we use to refer to a low electron affinity/electronegativity element's ability to have its electrons pulled away from it. The higher the polarizability, the more likely it is to form a bond with covalent character.

Adam Vuilleumier 2K

Dipole-induced dipole is probably a little stronger because it includes a dipole in the bond. A London or Van der Waals bond does not have a dipole. It has two induced dipoles which are extremely weak.

Adam Vuilleumier 2K

For this problem, you should only be comparing the O to the S since C is present in both molecules. We know that the S has a larger atomic radius because it is lower in the group. Elements with bigger radii have a lower electron affinity and therefore a lower electronegativity. Because of this fact,...

Adam Vuilleumier 2K

I was just wondering why we put the 8 electrons around the anion, but we don't for the cation. Don't they both have filled valence shells?

Adam Vuilleumier 2K

Dr. Lavelle uses FC=valence electrons - (free electrons + bound electrons/2) because it helps us to understand that each line, or bond, in the Lewis structure represents 2 electrons. You can also use FC=valence electrons - (dots+lines) though because counting each line is the same thing as multiplyi...

Adam Vuilleumier 2K

Covalent bonds are defined as any type of bond between atoms where electrons are shared. Therefore, almost all bonds have covalent character because certain elements will attract electrons from other elements depending on their electron affinities. For example, an atom with a high electron affinity,...

Adam Vuilleumier 2K

As Dr. Lavelle said in class, the octet rule is better described as the "octet guideline". But for the most part, the elements found in many organic compunds (like carbon, nitrogen, and oxygen) are most stable with a complete octet (8 valence electrons).

Adam Vuilleumier 2K

As Brian stated, delocalized electrons are spread out between atoms in equal proportions. Another way of saying this is that each atom has an equal electron density around it. It is important not to confuse "delocalized" with "electron movement". This is because electrons often b...

Adam Vuilleumier 2K

An atom with a high electron affinity (low ionization energy) will usually be the central atom in a molecule. This is because it can easily form bonds with other atoms without using a lot of energy. Central atoms typically have the most bonds, so it makes sense that an atom that likes to form bonds ...

Adam Vuilleumier 2K

The solutions manual does not include the Aufbau diagram for any of the answers, just the written form. And the use of noble gases in the shorthand is accepted by most chemists, so I'd assume that this is ok as well.

Adam Vuilleumier 2K

I know that we covered this a little bit a the end of class today, but how do we know that there are other exceptions in the lower periods of the d-block? I know we don't need to know this for class, but I just want to see if it connects to other concepts. Thanks!

Adam Vuilleumier 2K

For all the 6th Edition people out here, our question was to calculate the difference as a percentage of the neutron wavelength. This calculation can be done as follows: (wavelength of proton/wavelength of neutron) x 100%.

Adam Vuilleumier 2K

Hi,
I think you may be confusing velocity with frequency. In the equation c=lambda*v, the v is referring to the frequency, not the velocity of the neutron. The De Broglie equation is needed because only this equation includes v for velocity. Hope this helps

Adam Vuilleumier 2K

The kinetic energy that the posts above have described are referring to the physical velocity that the electron has left over after it is ejected from the metal sheet. If there was only just enough photon energy to eject the electron, there would be no energy left over to move the electron.

Adam Vuilleumier 2K

Hi, Not sure if this is 100% correct, but I believe that the lower bound for indeterminacy in velocity should be 0. My reasoning is that if delta v is 0, then that means delta p also has to be 0 based on v=p/m. Since all of this is based on photons and the De Broglie equation, momentum can never be ...

Adam Vuilleumier 2K

Hey guys, My TA told us that we would have all the formulas and constants we need provided for us on the front page of each test (including Avagadro's number!). However, I find it helpful to memorize certain equations so I can be more familiar with how to use them and so I don't have to keep flippin...

Adam Vuilleumier 2K

Mass percent composition problems will always add up to 100% if all the elements in the molecule are listed in the problem. I'm guessing that the reason you aren't getting 100% is round-off error.

Adam Vuilleumier 2K

The term "work" is a physics term. Work refers to the amount of kinetic energy it takes to complete a task. For example, it might take 1 joule of work to lift an apple one meter off the ground. In this context though, I'm pretty sure the work equation is the "phi" symbol that Pro...

Adam Vuilleumier 2K

I've heard of other states of matter like plasma and Bose-Einstein condensates. Will we be covering how to handle those in this class? Is there any difference in balancing chemical equations with plasmas, etc. in them? Are there even any chemical reactions that involve plasmas?

Adam Vuilleumier 2K

One way I use to remember the formula for percent yield is that I know percent yield will always be under 100%. Therefore, the smaller value (actual yield) will always be divided by the larger value (theoretical yield). By using this, we will always know that the equation for percent yield = actual/...

Adam Vuilleumier 2K

Another interesting definition of accuracy - Accuracy can be defined as when the average experimental value is close to the true value. For example, if the three following measurements were taken of a 30 mL sample of water: 20 mL, 35 mL, and 35 mL, the average value of this measurement is 30 mL. By ...

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