CHEMISTRY COMMUNITY

Andrew Pfeiffer 2E

For a reversible process (with regards to expansion), an infinitesimally small change in external pressure will cuss a change in volume, while in an irreversible process, an infinitesimally small change will not. For an irreversible process, use w=-P(deltaV). For a reversible process, use w=nRT ln V...

Andrew Pfeiffer 2E

An integrated rate law will give you the concentration of reactants or products at any time after the start of the reaction, whereas the differential rate law is a general rate law built from the chemical equation like (r=k[A][B]).

Andrew Pfeiffer 2E

You should balance the oxygens first with H2O, and then the hydrogens with H+.

Andrew Pfeiffer 2E

No, because adding OH is going to do weird things with the pH of the solution depending on whether or not the reduction reaction happens in an acidic or basic solution.

Andrew Pfeiffer 2E

From a kinetics standpoint, temperature changes K because temperature affects the forward and reverse rates of the reaction. This is because K=k/k’ where lowercase k is a rate constant. When the temperature is changed, the rate constant is affected. Therefore, K changes with temperature.

Andrew Pfeiffer 2E

Do you think we will be tested on our knowledge of catalysts on the final? If so, what are potential questions that could show up?

Andrew Pfeiffer 2E

A is the correct answer because Al and Fe undergo a simple redox reaction, with relatively low values of deltaH nought and deltaG nought. All of the other options have multiple phases or have unbalanced molar distributions, creating variety in the deltaG nought and deltaH nought.

Andrew Pfeiffer 2E

If electroplating occurs on the cathode in a Galvanic Cell, why is it introduced in 6O.3, the applications of electrolysis?

Andrew Pfeiffer 2E

Will we need to memorize anything about the Downs Process for the final exam?

Andrew Pfeiffer 2E

We'll likely encounter problems of this sort in which we'll need to calculate the molar Gibbs free energy of different alternative fuels and evaluate which fuel will be most useful. We might have to consider the molar mass of the fuel, as the molecular size of the fuel will have implications on how ...

Andrew Pfeiffer 2E

Because intermediates are by definition produced in one step and consumed in another, they are not actually present in the overall equation or rate law of the reaction.

Andrew Pfeiffer 2E

Thermal disorder has to do with how rapidly the molecules in a substance move. The more the substance is heated, the more quickly the molecules move and bounce off of each other, and the higher the thermal disorder. Increased thermal disorder leads to increased entropy. Positional disorder has to do...

Andrew Pfeiffer 2E

The order of the reactant indicates how much the rate of the reaction is affected if the concentration of (one of) the reactant(s) is changed.

Andrew Pfeiffer 2E

We can calculate it if we have all the other components of this equation (the standard change in Gibbs free energy, the energy, and Faraday's constant). Otherwise, we must use additional equations and knowledge about the conditions to indirectly solve for n and plug it back in.

Andrew Pfeiffer 2E

The terminology here appears flipped because of the meaning of the word agent. In general, an agent is a thing that brings about an action. In the case of redox reactions, a reducing agent is the atom or molecule that brings about the oxidization of another, while an oxidizing agent is the atom or m...

Andrew Pfeiffer 2E

One implication of the heat death of the universe that I've always wondered about is what it implies about the universe's time frame and origins. If the universe has always existed, then maximum entropy would have already been achieved and no more work could be achieved by the universe. But we have ...

Andrew Pfeiffer 2E

I think that you'll be okay memorizing it and applying it in problems. That being said, it's always better to understand equations in terms of their derivation, so knowing how to derive the equation certainly would help you. You'll be able to understand conceptual equations better. Hope this helped.

Andrew Pfeiffer 2E

G in non standard conditions can be calculated by using delta G = delta G^o + RT ln Q.

With standard states, it can be calculated delta G = delta H - T delta S.

Andrew Pfeiffer 2E

You'll have to use your knowledge of valence electrons and charge to determine the charges on individual atoms and ions. From there, use OIL RIG to figure out what's being reduced or oxidized.

Andrew Pfeiffer 2E

Considering most standard values (like enthalpy) are given for standard conditions/STP, maybe when nothing is specified, assume STP.

Andrew Pfeiffer 2E

Is there any meaningful difference between the term isochoric and isometric? When should we use one or the other?

Andrew Pfeiffer 2E

Ag + (aq) + e - --> Ag (s) E cathode = +0.80 V AgBr(s) + e- --> Ag(s) + Br- (aq) E anode = +0.07 80 V - 0.07 V = +0.73 V As far as devising a Galvanic cell goes, think about drawing out the diagram that Dr. Lavelle drew in class. Consider the factors that were present in the example cell (e.g. anode...

Andrew Pfeiffer 2E

To what extent do we need to know how to use the Kirchhoff Equation for thermodynamics?

Andrew Pfeiffer 2E

One of the equations relates delta G to the reaction quotient or equilibrium constant (which we use when equilibrium information is given), and the other relates the Gibbs free energy to change in enthalpy and change in entropy, which we use when we have the delta H and delta S of a reaction.

Andrew Pfeiffer 2E

We do this last step because we want to calculate the entropy of vaporization at that specific temperature. If we didn't do the last step, we would only be calculating the normal entropy of vaporization after heating up the liquid.

Andrew Pfeiffer 2E

As with equilibrium calculations, K is for when the reaction is at equilibrium, and Q (reaction quotient) is for non-equilibrium reactions.

Andrew Pfeiffer 2E

For an irreversible expansion, use w=-P(deltaX).

For a reversible expansion (isothermal), use w=-nRT(ln deltaV2/deltaV1).

Andrew Pfeiffer 2E

Try drawing out the Lewis structures for the molecules that are the reactants. From there, cross out bonds that need to be broken and draw in bonds needed to get your Lewis structures to look like the Lewis structures of the products. Hope this helped!

Andrew Pfeiffer 2E

Use 8.314 when your units are J/K*mol and use 8.206 x 10^-2 when your units are are L*atm/K*mol.

Andrew Pfeiffer 2E

It is given on our constants and equations sheet!

Andrew Pfeiffer 2E

When you are working with questions where you have to calculate the enthalpy change associated with heating a gas, and the question prompts you to "treat the gas as an ideal", you should refer to the chart.

Andrew Pfeiffer 2E

I was working on the following homework problem 4A.9: A piece of metal of mass 18.0 g at 100.0 C is placed in a vessel of negligible heat capacity but containing 50.2 g of water at 22.0 C. Calculate the final temperature of the water. Assume that no energy is lost to the surroundings. I ran into tro...

Andrew Pfeiffer 2E

If there is no non-expansion work done on the system and the pressure is still constant, then the heat consumed or released by the system will be equal to q.

Andrew Pfeiffer 2E

Q is negative because it's about the work that is done by the fuel, not the work done on the cooling chamber. The fuel is combusted and releases energy; therefore it is exothermic.

Andrew Pfeiffer 2E

Partial pressure is calculated by examining a single gas (in a mixture of gases) and identifying what its pressure would be if it occupied its volume on its own. Partial pressure is different from regular pressure, which is simply the pressure generated by all of the gases present in the container.

Andrew Pfeiffer 2E

For the general equation: ax² + bx + c = 0

x = -b +/- sqrt(b²-4ac) / 2a

You use the quadratic equation to solve for x when x is squared.

Andrew Pfeiffer 2E

These equations are both related to the first law of thermodynamics. The first equation is relevant when work done in or to the system is provided in the problem. The second equation is better suited for problems with enthalpy like heat transfers at constant pressure.

Andrew Pfeiffer 2E

Can someone please explain why burns from steam are far worse than burns from liquid water? I don't understand how the energy of from the phase change would affect the severity of the burn.

Andrew Pfeiffer 2E

Roughly 3.34 x 10^-9 m

Andrew Pfeiffer 2E

The reaction is endothermic, as it has taken in energy in the process of the reaction.

Andrew Pfeiffer 2E

Heat capacity is an extensive property because it is directly proportional to the amount of molecules you have. The more molecules, the more heat energy and the higher the heat capacity.

Andrew Pfeiffer 2E

kJ/mol I think!

Andrew Pfeiffer 2E

Does anyone know how to do 5I.33? Thanks y'all.

Andrew Pfeiffer 2E

Does anyone know how to do 5I.29? Thanks!

Andrew Pfeiffer 2E

Can anyone explain the logic behind H.1 and H.3? Thanks y'all.

Andrew Pfeiffer 2E

As far as acids go for 14B, think of acids as proton donors (H+) and bases as proton acceptors. Strong acids and bases fully dissociate, and memorize the strong acids and bases (they're listed in the book). pH is the negative log of the concentration of the H3O+, and pOH is the negative log of the c...

Andrew Pfeiffer 2E

Yes and yes! You got it.

Andrew Pfeiffer 2E

We don't have to be able to calculate delta H yet, but eventually, when we go over enthalpy in more depth, we will learn how to calculate it.

Andrew Pfeiffer 2E

Recall that volume is integrally related to both concentration and pressure. Decreasing the volume with the same amount of reactants and products is going to increase the pressure and concentration, while increasing the volume with the same amount of reactants and products is going to decrease the p...

Andrew Pfeiffer 2E

It becomes a little clearer why K w is so small when you consider the autoprotolysis reaction of water. 2 H 2 O (l) -->/<-- H 3 O + (aq) + OH - (aq) Consider several water molecules floating about. Very (and very) occasionally, the attraction of a water molecule's H to the lone pair on another water...

Andrew Pfeiffer 2E

Hi! While I can't find any of his worksheets on Chem Community, it seems that he'll often use worksheets as the backbone of his workshops. Lyndon has workshops in Covel 210 on Mondays from 6-8. If you attend, I'm sure you will receive some of his practice problems.

Andrew Pfeiffer 2E

I think so! The oxygens have available lone pairs that could attract the positive dipole of the hydrogens in water. Hydrogen bonding occurs between H and N, O, or F, so I think it's possible.

Andrew Pfeiffer 2E

Another example would be two diethylenetriamine ligands (which are both tridentate). The prefix used would be bis-, resulting in bisdiethylenetriamine.

Andrew Pfeiffer 2E

For students continuing on to Chem 14B next quarter, does anyone know if the Sapling Access code needs to be renewed in the following quarter, or do we have access to it permanently? Thanks.

Andrew Pfeiffer 2E

Diffraction patterns when light passes through a small opening (complete with constructive and destructive interference) are evidence for light acting as a wave.

Andrew Pfeiffer 2E

When metal oxides mix with H2O, they react to form bases. When nonmetal oxides mix with H2O, acids form.

Andrew Pfeiffer 2E

Strong acids fully dissociate into its ions in water while weak acids partially dissociate into its ions in water.

Andrew Pfeiffer 2E

HCl is a strong acid, so it dissociates completely. We know that the change in pH is -log(Mfinal/Minitial), so -log(.12/1) equals .92. Therefore, the pH will change .92. Hope this helped!

Andrew Pfeiffer 2E

The molecule in question is a polydentate when

a.) the ligand has at least 2 molecules that have lone pairs and

b.) the atoms have a geometry that enables the transition metal cation to be bound in more than one place at the same time.

Andrew Pfeiffer 2E

I would say that any concept and/or equation from the homework would be fair game for the final, including those that didn't show up on the midterm. The midterm is likely a good indicator of difficulty for the quantum world problems. Hope this helped!

Andrew Pfeiffer 2E

They hybridize because it lowers the energy of the molecule.

Andrew Pfeiffer 2E

Drawing complete molecules like the one seen on the test will probably come in 14C and 14D, but you will need to know how to analyze central atoms and bond angles.

Andrew Pfeiffer 2E

Ion-ion: -250 kJ/mol
Ion-dipole: -15 kJ/mol
dipole-dipole: -2 kJ/mol
dipole-induced dipole: -2 kJ/mol
induced dipole-induced dipole: -2 kJ/mol

Andrew Pfeiffer 2E

Yes, London Dispersion Forces are stronger in larger atoms/molecules.

Andrew Pfeiffer 2E

Hybridization doesn't apply to ionic compounds.

Andrew Pfeiffer 2E

In the textbook, the molecular shape pentagonal bipyramidal (7 regions of high electron concentration) shows up a couple of times. We haven't really discussed this shape in depth in class, so will we be held accountable for it on test 2? Also, what molecules would have this shape?

Andrew Pfeiffer 2E

Sigma and pi bonds are chemical covalent bonds that come from overlapping atomic orbitals. Sigma bonds are associated with the "s" orbital, and pi bonds with the "p" orbital. In general, sigma bonds are stronger than pi bonds. They are both important for predicting molecule behav...

Andrew Pfeiffer 2E

The size of an atom can affect the molecule by throwing off the distribution of electrons. For example, a bigger molecule with more electrons will repel the other bonding groups, creating a larger bond angle. Electronegativity will attract electrons to certain atoms. With really electronegative elem...

Andrew Pfeiffer 2E

Because the VSEPR formula is geared towards one central atom surrounded by electron groups, you can only apply it to one of the carbons. Treat the other carbon as a surrounding atom with a bonding group in between. With either of the carbons, you will get the same result. Hope this helped!

Andrew Pfeiffer 2E

In most scenarios, you'll be able to give a good guess right off the bat based on the electronegativity difference between the molecules in question. However, you should always draw the Lewis structure, because dipole moments might cancel out resulting in a non-polar molecule. Hope this helped!

Andrew Pfeiffer 2E

It is determined that triiodomethane will have stronger London dispersion forces on the basis of polarizability. Triiodomethane is bigger than trifluoromethane, and its electrons are held more loosely (and can be distorted more easily). Thus, triiodomethane undergoes stronger London dispersion force...

Andrew Pfeiffer 2E

For this type of question, you're going to want to first draw out the Lewis structures of the molecules in the problem. Using the lecture content from this last week, you will determine which intermolecular force applies to the molecules in the question (London dispersion, hydrogen bonding, etc.). A...

Andrew Pfeiffer 2E

I was reading through the textbook, and the subject of dimers briefly came up in 3F.5. The only description of dimers here is "pairs of identical molecules", but this feels a little obscure, since there are lots of pairs of identical molecules. Can someone clear up what a dimer is more spe...

Andrew Pfeiffer 2E

We are not asked in our homework problems to directly use the equation, but it would be used when an exact measurement of the attractive and repulsive forces between molecules is necessary for a calculation or lab procedure.

Andrew Pfeiffer 2E

Water molecules undergo intense hydrogen bonding between the oxygen and hydrogen atoms. This makes it significantly harder for the intermolecular forces to be overcome to melt ice. Conversely, H 2 S has dipole-dipole interactions (S being negative and H positive). While these dipole-dipole interacti...

Andrew Pfeiffer 2E

During the Dino Nugget Review Session, it was stated that the Rydberg equation can be used for the calculations, but if you get the answer wrong, you will not receive partial credit for your work. Perhaps the best bet is solving the question Dr. Lavelle's way and using the Rydberg equation to check ...

Andrew Pfeiffer 2E

For this question, you will have two resonance structures. In both structures the N will have a formal charge of +1 (that is, it will have four shared pairs of electrons with no lone pairs). Cl will always have a formal charge of 0 (that is, it will have a single shared pair of electrons and three l...

Andrew Pfeiffer 2E

From the Schrodinger equation (which isn't really applied directly in textbook calculations) one can calculate the wave function for any particle, as well as its corresponding energies. Now, the application of the Schrodinger equation to quantization comes from a thought experiment involving a parti...

Andrew Pfeiffer 2E

Dr. Lavelle talked about how we will only be responsible for working with elements of the first four periods/rows on the periodic table. If by many-electron atoms you mean atoms of large elements with high energy orbitals, then you shouldn't have to worry about completing big calculations. However, ...

Andrew Pfeiffer 2E

Electron affinity is the change in energy (in kJ per mole) when an electron is added to a neutral atom (in the gas phase) to make a positive ion. Electronegativity simply is the tendency to attract shared electrons (or a distribution of electrons) toward itself. Both of them share the same trend (in...

Andrew Pfeiffer 2E

Boron has only 3 valence electrons. In order to satisfy the octet rule, it would have to gain 5 electrons from bonds. However, there are no known compounds where boron bonds with five other atoms. Boron has shown tendencies, thus, to follow the sextet rule: it gains 3 additional electrons through bo...

Andrew Pfeiffer 2E

So in order to calculate the formal charge of a molecule, you need to use the equation FC = (Valence electrons) - (lone pair electrons + (shared electrons/2)) for each atom in the molecule. For example, in the part (a) the formal charge for the oxygen atom on the left of the structure would be solve...

Andrew Pfeiffer 2E

For a further explanation of part A, Magnesium usually gives up 2 valence electrons, while Arsenic usually takes 3 valence electrons. Thus, two Arsenic atoms would take 6 valence electrons, and 3 Magnesium atoms would give 6 valence electrons.

Andrew Pfeiffer 2E

Magnesium arsenide: As₂Mg₃

Indium III Sulfide: In₂S₃

Aluminum hydride: AlH₃

Aluminum telluride: H₂Te

Bismuth III Flouride: BiF₃

Andrew Pfeiffer 2E

Resonance structures are necessary when you can draw the same arrangement of atoms with different arrangements of e-. This occurs on account of the number of double bonds and lone pairs on each atom (you can find the best arrangement using formal charge, which we learned about last lecture).

Andrew Pfeiffer 2E

Either of the first two options should suffice. Drawing a resonance hybrid would be the quickest way to display resonance, but drawing all of the structures is helpful in the sense of seeing all the options. Doing both is probably unnecessary, but if specified by the question, would be suggested. Ho...

Andrew Pfeiffer 2E

It does not seem to have an effect on the calculation of the problem. Its inclusion in the problem is more along the lines of a fun fact than a necessity for solving the problem, as they provide the particular weight of the ball used in this problem. If you were able to go through the calculation, s...

Andrew Pfeiffer 2E

As succinctly as I can put it, electron shells are made up of orbitals that have the same principal quantum number (n). Orbitals are regions around the nucleus of an atom where one is most likely to find a corresponding electron.

Hope this brief distinction helped!

Andrew Pfeiffer 2E

You were right that 64 J will be emitted! For this question, you need the equation E = (hc)/lambda Plugging in for Planck's constant, the speed of light, and the frequency of violet light, you can find the the energy of the photon to be 4.7 x 10 -19 J/photon. Divide 64 J by this number to get the nu...

Andrew Pfeiffer 2E

We know that the third quantum number m l helps show the individual orbitals within a subshell. We know that m l = 2l + 1 Plug in 0, 2, 1, and 3 for l to get answers one orbital, five orbitals, three orbitals, and seven orbital respectively. See page 34 in the book for a helpful explanation on what ...

Andrew Pfeiffer 2E

Within the examples in the book, it looks as though the uncertainty equation will give you the range of uncertainty (in your example, 10) which can be divided by 2 in order to get the +/- uncertainty (in your example, 5). Hope this helped!

Andrew Pfeiffer 2E

De Broglie's equation is best suited for finding the wavelength for a moving object with mass. If you're working with photons, or things that don't really have mass, use Einstein's equation. Hope this helped!

Andrew Pfeiffer 2E

Start with the equation v = c/lambda. Plug in 2.998 x 10 8 for c and 102.6 x 10 -9 for lambda. This will give us 2.922 x 10 15 seconds -1 . After that, we can use the Rydberg equation to solve for n 2 . Plug all the values into the Rydberg equation (see textbook page 7), and you should end up with n...

Andrew Pfeiffer 2E

While I'm not sure it'll be required, I am sure memorizing the frequency, color, and wavelength of EM radiation will serve helpful on homework and tests as well! Particularly with UV, visible light, and infrared radiation. Even if you don't memorize the exact ranges, having a ballpark estimation can...

Andrew Pfeiffer 2E

When Dr. Lavelle was lecturing last week on the photoelectric effect, I was a little confused on the distinction between the frequency and amplitude of a wave. I know that frequency is the number of wave cycles per second, and that amplitude is the height of a wave, but which of these two contribute...

Andrew Pfeiffer 2E

You cannot use the (lambda)(v) = (c) equation or the e = (h)(v) equation for electrons, but you can use them for light!

Andrew Pfeiffer 2E

The length of a bond in a chemical compound depends on the compound at hand. Consider looking up a chart of experimental bond lengths!

Andrew Pfeiffer 2E

Whenever you're dealing with a number that can't be expressed with a simple decimal (i.e. a single digit followed by a decimal point and the consequent digits), scientific notation can really help clean up messy decimals. That being said, you can use your own judgment as to when numbers need to be c...

Andrew Pfeiffer 2E

Fractions can be used as an intermediate step when balancing equations, but you never want fraction coefficients in your final balanced equation. For example... Given the unbalanced equation C 4 H 10 + O 2 ---> CO 2 + H 2 O. First you would balance H and then balance O. After that, you should have C...

Andrew Pfeiffer 2E

I noticed in lecture that when dealing with the unit of molar mass, grams per mole, Dr. Lavelle writes the unit as g.mol -1 . I've always written the unit simply as g/mol. I know that this is minutia, but wanted to know if I should begin expressing my units the same way, or if the way that I've been...

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