CHEMISTRY COMMUNITY

Rehan Chinoy 1K

Just think about it conceptually -- the melting of ice is spontaneous in most temperatures, so deltaG should be negative.

Rehan Chinoy 1K

Delta H equal q under conditions of constant pressure.

Rehan Chinoy 1K

(number of orientations) ^ (number of molecules). Side note: I've seen some problems where one has to use Avogadro's number to find the number of molecules.

Rehan Chinoy 1K

There was a question like this on the midterm -- if an isolated system has an internal energy of +5 J, then after 100 years it will have an internal energy of +5 J.

Rehan Chinoy 1K

It is essentially of the same function as a salt bridge. However, in the cell diagram it is indicated as 1 vertical line instead of how a salt bridge is indicated as two vertical lines.

Rehan Chinoy 1K

Yes, if you want a reaction to be the oxidation reaction you flip the sign of the associated cell potential found in the standard reduction potentials table. Otherwise, you never mess with the given cell potential.

Rehan Chinoy 1K

Eruchi Okpara 2E wrote:For the final exam, will the test specify between whether we need to use bond enthalpies or standard enthalpy reaction?

Yes, the test should specify or it should be obvious by the given values.

Rehan Chinoy 1K

Since the conversion between bar and atm is 1 atm = 1.01325 bar, do we treat them each unit as virtually the same thing?

Rehan Chinoy 1K

Yes, the standard reduction table will be provided.

Rehan Chinoy 1K

Don't study ten minutes before the final because psychologically you're telling yourself that you don't know the material well enough. Be prepared and be confident in your preparation. Some deep breaths before you enter the exam room helps too.

Rehan Chinoy 1K

Yes, like stated above, use H+ (protons) in all redox reactions unless they are basic, in which case you should use OH- (hydroxides).

Rehan Chinoy 1K

Remember that in a battery electrical energy --> chemical energy.

Rehan Chinoy 1K

Perfect, thank you both.

Rehan Chinoy 1K

Unfortunately the problems on Gibbs Free Energy are all in a designated section in the sixth edition and it's hard to comb through them and see immediately if it requires the Nernst Equation. I would just focus on the more general conceptual problems on spontaneity and the basic calculations of Delt...

Rehan Chinoy 1K

To be clear n is the number of moles of electrons in the balanced redox reaction, not the number of electrons.

Rehan Chinoy 1K

Charging a reaction is endergonic and thus not spontaneous.

Rehan Chinoy 1K

No, electrolysis will not be on the test. Only material covered during Weeks 6 and 7 will be on Test 2.

Rehan Chinoy 1K

Do we need to know/is it beneficial to know the derivation of the Van't Hoff Equation? If so, would someone like to derive it please?

Rehan Chinoy 1K

Thank you, this was very helpful. These kinds of discussion threads really help cement my understanding of the concepts.

Rehan Chinoy 1K

Did Dr. Lavelle go over specific biological examples that we should be aware of for the final?

Rehan Chinoy 1K

Remember to take the inverse of the log (exponentiate) to isolate and solve for K or Q.

Rehan Chinoy 1K

As mentioned above, n is the number of moles of electrons. To find n, one must balance the complete redox reaction (not just each half rxn separately).

Rehan Chinoy 1K

N is the number of moles of electrons in the balanced redox reaction.

Rehan Chinoy 1K

It would be safe to assume that the cell potential is positive in a galvanic cell when doing problems in the textbook or on a test because there would not be much to prove or calculate if the cell potential was negative.

Rehan Chinoy 1K

I think there is some general confusion about what spontaneity is. "Spontaneous" describes a process or reaction, so values of delta H, G, or S themselves cannot be spontaneous, but they can be used to prove spontaneity through the equation Delta G = Delta H - Temp(Delta S). The process is...

Rehan Chinoy 1K

Also KhanAcademy and YouTube in general has some pretty great content if you're struggling with the general concepts. But textbook practice problems are definitely the key to mastery.

Rehan Chinoy 1K

Homework problems on Gibbs Free Energy and Electrochemistry (up to, not including, the Nernst Eqn) would be a great place to start.

Rehan Chinoy 1K

Yes, Week 6 and 7 content will be covered on Test 2.

Rehan Chinoy 1K

As stated above, on the midterm we will be provided with the formula sheet with the R gas constant in different forms due to different units. You just have to select the correct one based on the units of the given values.

Rehan Chinoy 1K

Since work (w) is equal to (pressure) * (change in volume), if there is no change in volume then work must be equal to zero.

Rehan Chinoy 1K

Pure substances (solids, liquids) such as water do not affect the equilibrium expression.

Rehan Chinoy 1K

Le Chatelier's Principle states that if a constraint (such as a change in pressure, temperature, or concentration of a reactant) is applied to a system in equilibrium, the equilibrium will shift so as to tend to counteract the effect of the constraint.

Rehan Chinoy 1K

The 5% rule is useful because it allows us to simplify the equilibrium expression and avoid using the quadratic formula.

Rehan Chinoy 1K

Yes, you essentially answered your own question -- just pay attention to the K value and the phases of the reactants/products and the answer should be clear, just as the previous answer stated.

Rehan Chinoy 1K

The value Q is useful because in comparison to a K value for a reaction it tells us whether products or reactants are favored (which direction the reaction will go towards)

Rehan Chinoy 1K

ICE tables can be used for questions of both reactant/product equilibrium partial pressure and concentration, as long as the correct K values are used.

Rehan Chinoy 1K

Since NH3 exhibits hydrogen bonding while PH3 does not, NH3 has the higher boiling point.

Rehan Chinoy 1K

Yes, shape means molecular geometry. Professor Lavelle will be explicit about it on the final though.

Rehan Chinoy 1K

Lewis acid: substance that donates H+ (proton)
Lewis base: accepts H+
Bronsted acid: accepts electron pair
Bronsted base: donates electron pair

Rehan Chinoy 1K

Yes, water is amphoteric and amphiprotic.

Rehan Chinoy 1K

The compound [Co(NH2CH2CH2NH2)2(CN)(Cl)]Cl (pardon the lack of subscripts) would be written as chlorocyanobisethylenediaminecobalt (III) chloride because the ethylenediamine already has a "di-" in it, or in other words it is already a polydentate molecule, so if there are two in the comple...

Rehan Chinoy 1K

It's also helpful to see if the molecule has nitrogen, oxygen, or sulfur atoms with lone pairs because these are common binding sites for ligands and thus would make the molecule polydentate.

Rehan Chinoy 1K

Rehan Chinoy 1K

The oxidation number is the charge of the central transition metal in the coordination compound, which can be found if you know the charge of the ligands and the overall charge of the compound. The coordination number is the number of ligands that can be bonded to the central transition metal in the...

Rehan Chinoy 1K

It would be helpful to know the charges of the more common polyatomic ions, especially for determining the oxidation state of the transition metal in a coordination compound.

Rehan Chinoy 1K

AlexandraZuniga1L wrote:Does a polar molecule have to have polar bonds with dipoles that do not cancel?

Yes, polar molecules have to have polar bonds with dipole moments that do not cancel.

Rehan Chinoy 1K

It would be less than 120 degrees because the electron geometry is trigonal planar not tetrahedral. Thus, when one of the regions of electron density is a lone pair, the bond angles become less than 120 degrees. You would be correct if the electron geometry was tetrahedral i.e. there was one more re...

Rehan Chinoy 1K

I believe it has to do with the bond angles. The two lone pairs cause the angles between the central atom and each ligand to be less than 90 degrees, whereas the angles between the ligands and the central atom in a trigonal planar molecule are 120 degrees.

Rehan Chinoy 1K

The question should be how do determine if a molecule is polar. This, just as the above answer indicates, is determined by first finding the shape of the molecule using VSEPR and then seeing if the dipole moments cancel each other out or not.

Rehan Chinoy 1K

Also keep in mind that formal charge only applies to compounds and not individual atoms.

Rehan Chinoy 1K

You only need to indicate shape in VSEPR models, and even then it is not necessary to depict a 3D drawing of the structure. Simply naming the shape (trigonal planar, linear, etc) will suffice.

Rehan Chinoy 1K

Yes, Helium (He) has the greatest ionization energy.

Rehan Chinoy 1K

Considering the shortcut would save you approximately 2 seconds for multiplying by 2 and then dividing by 2, I would just stick to how you've been doing it if it's getting you the correct results. :)

Rehan Chinoy 1K

When drawing a Lewis Structure, the atom with the lowest ionization energy is the central atom. So, for example, hydrogen is never the central atom.

Rehan Chinoy 1K

The central atom is the one with the lowest ionization energy. So, for example, hydrogen is never the central atom in a Lewis structure.

Rehan Chinoy 1K

Bound atoms are more stable and have lower energy than separate atoms because forming a chemical bond releases energy.

Rehan Chinoy 1K

Choose the atom with the lowest ionization energy to be the central atom; hence, hydrogen is never a central atom.

Rehan Chinoy 1K

Yes, as stated above, 5p^1 represents one electron in the outermost 5p subshell of Indium, so when Indium loses an electron to become the cation In+ it will lose the outermost electron, or 5p^1. When Nitrogen gains 3 electrons to become the anion N^3-, 3 electrons are added to the outermost energy l...

Rehan Chinoy 1K

Yes exactly, in your example of the 2s^2 orbital, the superscript of 2 indicates that there can be 2 electrons in the 2s orbital.

Rehan Chinoy 1K

Atomic radius increases down and to the left on the periodic table because electrons are farther away from the nucleus. Ionization energy decreases farther down the periodic table because it is easier to remove an electron when they are farther away from the nucleus. Electron affinity increases up a...

Rehan Chinoy 1K

Is the intensity of the black body radiation related to a wave's amplitude? Or is it related to the number of electrons emitting a certain frequency?

Rehan Chinoy 1K

Yes, intensity is very different from frequency. We care about the fact that a photon needs to be of a greater energy than the electron to remove the electron, which is determined by a higher frequency as high frequency corresponds to high energy. Intensity merely means how many photons interact wit...

Rehan Chinoy 1K

Yes, I have not seen "f" used in many textbooks or academic sources; I would stick with "v" (nu).

Rehan Chinoy 1K

Yes, I believe all matter exhibits wavelike properties, but with our current techniques for measuring these wavelike properties we can only measure it for objects with relatively small mass and momentum, hence the 10^-15 (m) or 10^-18 (m) threshold.

Rehan Chinoy 1K

Yes, you can't throw in an extra O atom because of the law of conservation of mass. Instead, manipulate stoichiometric coefficients such that the chemical equation is balanced, or that each side of the equation has the same number of atoms of each element.

Rehan Chinoy 1K

You want to make sure all the stoichiometric coefficients are whole numbers. So if you have 3.5, you will most likely multiply all of the coefficients by 2 or a multiple of 2 to ensure whole numbers.

Rehan Chinoy 1K

I don't think it matters, I just prefer to keep it in liters because molarity of a solution is moles/liters. But for the M1V1 equation, since you are essentially finding a value using a ratio, whether you choose mL or L does not matter as long as you are consistent and that you specify.

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