CHEMISTRY COMMUNITY

kateminden

On the equation sheet on the website, the half-life formulas are given. However, the equation sheet does not specify which order reaction each equation is for, so make sure that you know the difference between the equations.

kateminden

The first law of thermodynamics states that the internal energy (delta U) of an isolated system is constant. This means that delta U = 0 in an isolated system, because no energy can be transferred to/from the system and no work can be done on or is being done by the system.

kateminden

A zero order reaction is a reaction in which the rate is independent of the concentration of reactants. This means that the rate law of a zero order reaction is rate = k. I believe the reaction 2NH3 (g) --> N2 (g) + 3H2 (g) is a zero order reaction because the rate does not depend on the concentrati...

kateminden

I was wondering about this too! I got this wrong on the test, and I have still been confused about it. Thank you for all of the explanations, they were all really helpful!

kateminden

I don't think there is an exact percentage, but the newer material will most likely be more heavily focused on. There probably won't be that many questions on specifics of the older material. I think the test will probably consist of broader, simpler problems from the older material, and more specif...

kateminden

Yeah, it really just depends on what information the problem gives you.

kateminden

I was wondering this too! Also can someone please explain why a diamond is kinetically stable with respect to graphite, but thermodynamically unstable? I just can't really wrap my head around the two concepts. Thanks!

kateminden

I don't believe Professor Lavelle's are curved. I know that he does, however, curve our ending scores after our final. So even if this test isn't curved, our overall grade will be.

kateminden

In first-order reactions, the reaction rate is directly proportional to the concentration of one of the reactants. These reactions usually take the form of A → products. In contrast, a second-order reaction is one whose rate is proportional to the square of the concentration of one reactant. Second-...

kateminden

Besides platinum and graphite (carbon), other precious metals such as gold and mercury are often used as inert conductors because they effectively serve as a "source" or "sink" for electrons (they are good conductors), but they do not take part in the chemical reaction.

kateminden

If the change in enthalpy is positive the reaction is endothermic (it requires heat to form products) and so raising the temperature will cause the equilibrium to shift toward the right (i.e., more product will be formed since more heat is available). If the change in enthalpy is negative the react...

kateminden

As said above, graphite is an inert electrode. This means that it does not participate in the redox reaction and is present so that current can flow through the cell.

kateminden

Typically, you can go to your TA to ask about any points you missed but believe you should have received and they will look over your work and potentially give you points back. I think they are usually pretty fair about giving you points if you deserve them.

kateminden

Gibbs free energy is an extensive property. More matter in the system means the amount of maximum possible work available is also more.

kateminden

I think the equation sheet posted on the website is the one that will be used for the rest of the quarter, but I'm not positive.

kateminden

Thank you for asking this! I was confused about this too. I understood the Ideal Gas Law and how to apply it, but I did not quite understand the concept of "ideal behavior" of gases.

kateminden

Think about them in terms of the "area under the curve" in a graph of Pressure x Volume. Because the reversible reaction depends logarithmically on volume, while the irreversible reaction depends linearly on volume. Therefore, the area under the curve (work) is larger for reversible, as t...

kateminden

Thank you so much! This is really helpful!!

kateminden

Thank you so much!

kateminden

Yeah, I believe you can turn in any problems that have been previously assigned. We have not really learned that much new material (material that won't be on the midterm), so I do not think we would even be able to do some of the new problems.

kateminden

Yeah, I believe generally we would use atm unless a different unit was given in a specific problem.

kateminden

Like what was said above, the 2nd law states that entropy is always increasing. Over time, or as a reaction progresses, usable energy is irretrievably lost as unusable energy (heat lost to the environment, etc.) increases.

kateminden

Generally, exothermic reactions are more likely to be spontaneous than endothermic, but there are some instances when endothermic reactions are spontaneous. If the change in entropy at a given temperature is sufficient to overcome the unfavorable change in enthalpy, then the reaction will spontaneou...

kateminden

Hello! No, you do not need to memorize most of the unit conversions, constants, and basic equations because most of them will be given to you on the exam. Professor Lavelle is mainly concerned about whether or not you can apply equations, units, etc. and conceptualize, not simply memorize a list of ...

kateminden

I'm not positive, but I don't believe Professor Lavelle went over a way to solve for partial pressure without the concentration. Is there a specific problem that only gives you T and R?

kateminden

Adding on to what was stated above, the more electronegative a certain atom in a molecule is, the more it draws on the electron density of the overall molecule. This pulling of electron density makes certain molecules more polar than others. Acids become stronger as the bond between the H atom and s...

kateminden

Hello! I believe the top is x^4 because there are 4 mols of CO in the product, not 1 mol. This means that the concentration in the Kc ratio would be [CO]^4, so therefore X^4.

kateminden

Yeah, because there are only a few strong acids and bases, I believe the only way to know them is to memorize the list. I do not think there is a way to determine if something is an acid or a base (if it completely dissociates) just by looking at the given formula.

kateminden

Thank you, I was confused about this too! I know we have not learned thermodynamics in detail, but I still wanted to understand the basic concepts before the test.

kateminden

Because the equilibrium constant is fixed, and is defined by the concentration of products over the concentration of reactants, the ratio of product concentrations to reactant concentrations is constant. This means that increasing/decreasing the concentration of a certain reactant/product would simp...

kateminden

I am confused about this too! I still do not understand why the change in pressure created when an inert gas is added to a closed system at equilibrium does not affect the equilibrium.

kateminden

Pure liquids and pure solids are not included in equilibrium constant expression because their densities (and, therefore, concentrations) are constant, so they do not affect the reactant/product amount at equilibrium.

kateminden

Intermolecular forces, such as H-bonds and London dispersion forces, are between molecules, whereas intramolecular forces, such as covalent and ionic bonds, are between atoms within a molecule.

kateminden

When calculating using quantum levels, the reference or starting point En=0 occurs when n = infinity. This means that all other energy levels (n=1,2,3,...) are below the starting point, and therefore negative.

kateminden

Yes, period three elements (and those in higher periods) such as phosphorous, sulfur, and chlorine can accommodate more than 8 electrons because they have expanded valence shells (their d-orbitals can hold more electrons).

kateminden

A tetrahedral molecule has 4 regions of electron density, all of which are bonded, giving it the VSEPR formula AX4. All of the bond angles are 109.5 degrees for tetrahedral molecules. On the other hand, a square planar molecule has 6 regions of electron density, giving it an octahedral electron-pair...

kateminden

Calculating the pH of a strong acid is the same as calculating the pH of any aqueous solution. Basically, you need to know the molarity of the hydronium ion (concentration in moles per liter), and then you just use the equation pH = - log [H3O+].

kateminden

H+ in water (an aqueous solution) is the same thing as H3O+, the hydronium ion.

kateminden

Hi! Whether a molecule is bent or linear depends on if it has a lone pair. Lone pairs push the other bonds away, creating a bent or angular shape. For example, CO2 is a linear molecule because it does not have a lone pair, but H2O is a bent molecule because of the lone pairs on the oxygen atom.

kateminden

Actually, based on the equation λ = h/p, wavelength λ is inversely proportional to momentum p. This means that as the uncertainty of momentum decreases, the uncertainty of wavelength increases and vice versa.

kateminden

Ethan Yi 4E wrote:so is the test only on the things we learned after the last midterm?

Hi! Dr. Lavelle posted this announcement on his website: "Test 3 covers: End of Bonding from 3.12 (6 Ed.) and from 2D (7 Ed.); and all of Molecular Shape and Structure (see Syllabus and Outlines for details)."

kateminden

Yes, they both refer to the AX2E model. For example, H2O's molecular shape could be described as either angular or bent.

kateminden

Yeah, I believe we are expected to draw the 2-D diagram and then explain the 3-D model conceptually.

kateminden

They are the same thing!

kateminden

In CH2Cl2, the 2 C-Cl bonds create a dipole towards the Cl since Cl is highly electronegative. This makes the molecule more negative towards the Cl and away from the 2 H. It is a similar idea to how water is polar with higher negativity towards the oxygen.

kateminden

Hi! Electron distortion is when the electrons of the anion are attracted to the cation, which causes the shape of the electron density to change slightly. The distortion of the electron density shape leads to an ionic bond having covalent bond characteristics because the electrons are not completely...

kateminden

Hello,
How does molecular shape contribute to the strength of interaction? Why is the interaction stronger between two rod-shaped molecules stronger than the interaction between two spherical molecules?
Thanks!

kateminden

Thanks, I was wondering the same thing!

kateminden

The correct units are just the units you would use in any other equation with the same variables, except you use kg instead of g for mass, because momentum is kgm/s. So velocity is still m/s, and position is still m.

kateminden

Yes, because in the problem, the electron's position could be anywhere within the alveoli sac (therefore, within the diameter), and could not be outside of it, therefore that is the range or uncertainty of the position (delta x).

kateminden

This case is actually an exception to the rule. In the review session, we went over how because nitrogen has 1/2 shell stability (with 3 electrons in the 2p subshell), it actually has a higher ionization energy than oxygen, despite the periodic table trend. Similarly, carbon has a higher electron af...

kateminden

Cations are smaller than anions (and the neutral atoms) because when they lose electrons, electron-electron repulsion decreases and, therefore, the nucleus' positive charge pulls the remaining electrons inwards, towards the nucleus, decreasing the radius of the atom. Vice versa happens with anions, ...

kateminden

Yes, they are always between metals and non-metals!

kateminden

AustinGrove3B wrote:Having full or half full subshells is less potential energy than having semi full subshells. For that reason, sometimes electrons will be taken from the s subshell to fill the d subshell.

Thanks, I was confused about this too, and this example was so helpful!

kateminden

No, Professor Lavelle explained in class that applying Schrodinger's wave function equation involves pretty advanced, complicated math. I think we just need to understand the wave function conceptually.

kateminden

Yeah, I think the Test 2 Outline probably covers what we're going to be tested on.

kateminden

Thank you! I'm definitely going to use that tip from now on :)

kateminden

Technically, there is not a difference between a number in standard form versus the number in scientific notation, but scientific notation is typically used in chemistry. Scientific notation makes it easier to understand how many sig figs a number has and makes comparing amounts and values much easi...

kateminden

Yes, in class I believe Professor Lavelle mentioned that periodic tables, along with most of the equations, will be given to us for all of the tests. He said that we will be given periodic tables even if we don't actually need them on the tests, to give us "comfort."

kateminden

I am not going to pretend like I completely understand the concept of light (because I'm still pretty confused by it), but in class Professor Lavelle described photons as being discrete packets (or quanta) of electromagnetic energy that simultaneously act like waves and particles. Though photons are...

kateminden

I'm not sure exactly what the best way to study is, but I've also just been doing the practice problems. I'm making sure that I know how to do each different type of problem. I might also use the audio-visual focus-topics to review material covered in class.

kateminden

I think the solutions manual is really helpful, especially when you don't understand how to work out certain types of problems. Often just being given the right answer isn't very helpful. The manual gives the answers as well as the steps taken to get the answers. I think purchasing it is worth it.

kateminden

Thank you, I was wondering this too! To be safe, I just answered one problem from each section of the problem set, but I don't think I needed to do that. I'm pretty sure that we can choose any seven questions, even if they are all from the same section.

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