CHEMISTRY COMMUNITY

Taylor Newville 1C

Does anyone know of any good GEs that I could possibly take during spring quarter that'll get a requirement out of the way for a chem major. I am taking DESMA 9, it sounds really interesting and fills the visual and performance arts GE. It's also only one, two-hour lecture a week so I think it'll b...

Taylor Newville 1C

The concentration of a reactant that is zero order has no effect on the rate of its reaction. The concentration of this species can triple or halve but the rate of the reaction will stay constant. It is therefore not included in the rate law of that reaction.

Taylor Newville 1C

An intermediate is a species found only in multi-step reactions and is fundamental to a reaction's mechanism. It is formed by the reactants of one step and is consumed as a reactant in a following step. They are not included in the overall rate law of the reaction.

Taylor Newville 1C

In today's lecture, Lavelle said that k can change with temperature, activation energy, and the frequency factor. The rate constant determines the speed of the reaction, which increases with increasing temperature, decreases with increasing activation energy, and increases with increasing frequency ...

Taylor Newville 1C

Catalysts speed up a reaction (increase k) by lowering the activation energy. An enzyme is a biological catalyst because it speeds up a chemical reaction without changing the reaction or its components.

Taylor Newville 1C

I'm looking forward to the libraries opening, it's been too long since I've been to one and they're my favorite places to study.

Taylor Newville 1C

They will most likely be given to us but if not, they can be quickly derived from the rate law equations, which should be given.

Taylor Newville 1C

A zero order reaction will have a linear plot (straight line) when you plot [A] versus time. First order will have a linear graph when you plot ln[A] versus time. Second order will have a linear graph when you plot 1/[A] versus time (this will have a positive slope unlike the rest). [A] is the conce...

Taylor Newville 1C

The half life for a zero order reaction is calculated by dividing the initial concentration of the reactant by 2 times the rate constant.
t1/2=[A]0/2k

Taylor Newville 1C

It is possible to have more than three reactants but we will not need to know that for this class. Lavelle said that reactions with just three reactants are extremely rare so we will just be focusing on zero, first, and second order reactions.

Taylor Newville 1C

ln1/2[A]0 = -kt1/2 +ln[A]0
ln(0.5) = -kt1/2
t1/2 = 0.693/k

ln(0.5) = -0.693

Taylor Newville 1C

I think charge is for ions and oxidation numbers are for covalent compounds.

Taylor Newville 1C

The problem will tell you what is meant by K and its value will make it obvious too. Usually it is either the equilibrium constant or temperature in kelvins.

Taylor Newville 1C

Standard reduction potential is the likelihood that something will be reduced (gain electrons).

Taylor Newville 1C

There are 6.022*10^23 electrons in one mole of electrons.

Taylor Newville 1C

Does he offer extra credit?

Taylor Newville 1C

I'm taking Chem14C and Chem14BL next quarter. I'm not too sure what my last class will be because everything else I want/need to take is completely full. I might take my second writing two class in order to fulfill the year of English that med school requires. Not ideal at all. I've realized that ba...

Taylor Newville 1C

Work done by the system is negative (decreases internal energy) and work done on the system is positive (increases internal energy).

Taylor Newville 1C

Go to you TA's office hours and they will put you in a breakout room and then screen share your results. I would join early though because they often run out of time.

Taylor Newville 1C

For the thermochem/thermodynamics stuff I would recommend writing down all of the equations we have been using and make sure you understand all of the variables. A lot of the questions are just variations on using the same equations but solving for different variables in the equation. If you write ...

Taylor Newville 1C

I am nervous as well, but I think once we all study the material a lot more, we'll finally be able to see the whole picture of this unit and understand it a lot better. Good luck!

Taylor Newville 1C

The naught symbol (°) is used to represent a thermodynamic quantity in its standard state, such as change in enthalpy (ΔH°), change in entropy (ΔS°), or change in Gibbs free energy (ΔG°).

Taylor Newville 1C

Thank you, these are really good tips. I also read and takes notes on the textbook in addition to the lecture. I find that getting multiple explanations of a concept helps me understand it better. A lot of this material is review from AP Chemistry, so I also review my old AP Chem practice books to h...

Taylor Newville 1C

The equation lnK=−ΔHo/RT + −ΔSo/R can be used to find the temperature dependency of K. How K increases or decreases with temperature is linked to whether the reaction enthalpy is positive or negative. This is because the modified version of the equation ln(K2/K1)=−ΔHo/R(1/T2−1/T1) can be used to fin...

Taylor Newville 1C

Degeneracy is the number of possible microstates of a system raised to the number of particles in that system.

Taylor Newville 1C

There is a decrease in the disorder of the gas when it turns into a liquid. The entropy of the gas (system) decreases, but the entropy of the surroundings increases.

Taylor Newville 1C

Exercise! I find that going for a walk/quick workout (or even just doing a few jumping jacks) an hour or so before a test helps me get into "testing mode". It gives you an adrenaline boost and pumps blood to your brain. Especially because we are indoors for so long, getting a breath of fre...

Taylor Newville 1C

While doing textbook problems is definitely helpful, I don't think it is enough to do well on the exams. I find that taking notes during lecture and reading and taking notes on the textbook really helps me understand the material, instead of just memorizing how to solve the problems.

Taylor Newville 1C

I assume that unless the grading scale is explicitly stated otherwise, it follows the standard scale of the majority of courses at UCLA: "A range from 90-100%, B range from 80-89.9%, C range from 70-79.9%, D range from 60-69.9%, and F given to 59.9% or lower. Within each letter grade range, a m...

Taylor Newville 1C

What's the difference between a closed system and an isolated system? Can an isolated system not absorb heat or lose heat to its surroundings? An isolated system is insulated, which means it can't exchange matter or heat with the surroundings. A closed system is not insulated so the only constant i...

Taylor Newville 1C

Grades are out now!

Taylor Newville 1C

Steam causes a more severe burn than a liquid (even if they are at the same temperature) because it releases both the energy of cooling down to body temp and the energy of a phase change (from vapor to liquid) onto the skin.

Taylor Newville 1C

K is considered small when it is less than 10^-3, but Dr. Lavelle said that K should be less than 10^-4 to use successive approximation. Always check that the percent ionization is less than 5% to know that x is insignificant because there are many exceptions to the rule.

Taylor Newville 1C

I plan to take 14BL next quarter. I am taking LS23L right now, which has a similar format to 14BL. I would recommend taking a lab only if your other courses are not too much work as they require a lot of time. I would especially wait to take 14BL if you are taking a writing two class in the spring b...

Taylor Newville 1C

Lorena_Morales_1K wrote:Why is bond formation exothermic again?

Bonded atoms/molecules are more stable. Unbound atoms require energy to keep them apart which is released when they form a bond, making bond formation exothermic.

Taylor Newville 1C

The definition of delta H is the change in enthalpy of a system. The definition of exothermic is that heat is released during a reaction and the enthalpy of the system decreases (which can only ever be represented by a negative delta H). The opposite is true of endothermic. So, you are right and the...

Taylor Newville 1C

I doubt that more classes will open up, the same thing happened this past quarter where only a fraction of the number of spots LS7B had was available for LS7C. It was just first-come, first-serve. I talked to my counselor about this and she said its because these classes are made to "weed out&q...

Taylor Newville 1C

Think of the difference between the C-H bonds in CH4 versus CH3Br. Bromine is much larger and more electronegative than hydrogen so it will pull the central carbon closer to itself, weakening the other bonds in the molecule. Therefore, the C-H bonds in methane will have a slightly different bond ent...

Taylor Newville 1C

The temperature will only stay the same when heat is being applied during a phase change. Energy would be used to change the bonding between the molecules. Energy is added to break bonds and energy is subtracted to bond molecules to one another. These energy exchanges are not changes in kinetic ener...

Taylor Newville 1C

You would use Kelvin. If you are ever unsure about what units to use, look at the units of the equation's constant to see what needs to be canceled out. For the Ideal Gas Law, the gas constant R= 8.314 J/mol·K, so the temperature needs to be in Kelvins too.

Taylor Newville 1C

Yes. During a change in phase, energy is used to change the bonding between the molecules. For example, when water is melting, added energy is used to break the bonds between the water molecules and when water is freezing, energy is subtracted as the molecules bond to one another. These energy excha...

Taylor Newville 1C

Endergonic and exergonic refer to changes in free energy (delta G). Endothermic and exothermic refer to changes in internal energy of molecules, measured as heat given off or taken up (delta H).

Taylor Newville 1C

Will we ever use the reverse of Ka (1/Ka), or just always use Kb when we are going in the opposite direction of an acid/base equation?

Taylor Newville 1C

Can someone who had Lavelle last quarter for 14A explain what his midterms are like?

Taylor Newville 1C

This Youtube video gives a lot of examples of how ICE boxes are used:
"Ice Table - Equilibrium Constant Expression, Initial Concentration, Kp, Kc, Chemistry Examples"

Taylor Newville 1C

Even in K is less than 10^-4 and we are able to approximate, will we get the correct answer if we use the quadratic formula instead?

Taylor Newville 1C

Can someone who had Lavelle for 14B explain what Respondus is and how we use it for the midterms?

Taylor Newville 1C

You can use either K or 1/K for this question and get the same answer. If you use K, make sure you set it equal to [NO]^2/[N2][O2]. And if you use 1/K, make sure you set it equal to [N2][O2]/[NO]^2.

Taylor Newville 1C

In Dr. Lavelle's lecture on Le Chatelier's Principle, he talks about how adding an inert gas to a container does not effect the reaction since there is no change in the concentration of reactants, products, and volume. What is the exact definition of an inert gas and why does adding one to a contai...

Taylor Newville 1C

A good example of this concept is question 3 of the Week 1 Sapling Assignment. I'm not sure if everyone has the same questions with the same values on sapling, but for the question I was given, you solve for x using the quadratic equation and get two positive values. However, one is bigger than the ...

Taylor Newville 1C

Sapling is required! In the past, I believe it wasn't, but now it is. You buy the bundle that Lavelle has up on the site, it's around $70ish dollars I believe for multiterm access. It gives you access to the ebook and other resources. Sapling is also where you do the homework which I believe is wor...

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