CHEMISTRY COMMUNITY

Kate Chow 4H

Adiabatic means that the system is isolated and no heat is transferred. From there, you know that q=0 so using delta u = q+w, you know that delta u = w. After that, you use the information given to you in the problem to find an equation that solves for final temperature.

Kate Chow 4H

How do we draw structures for activated complexes? I came across them in the kinetics homework, but I don't remember learning them in class. Do we need to know how to do this?

Kate Chow 4H

I think that the number of posts per week vs. per quarter depends on what your TA wants. On that note, I think that the TA counts the number of posts, but I'm not positive.

Kate Chow 4H

A guess, but I would assume tetramolecular.

Kate Chow 4H

No, concentration does not affect the half life of a first order reaction. The formula for the half life of a first order reaction is 0.693/k = t. The concentration does not matter because you know that you need half of the original concentration so you always have ln 2/1 and that is always equal to...

Kate Chow 4H

I think reaction order is combined order of reactants.

Kate Chow 4H

You use different equations for delta g depending on what information you are given in the problem. There are many equations for delta g that can be further substituted, so I don't think there is one "best" equation.

Kate Chow 4H

You can actually do this problem by simply comparing the reduction potentials (E values in the table). The most negative E values will be the strongest reducing agents.

Kate Chow 4H

In cell potential diagrams, the format is always written as reactants then products, so the O2 comes first and is therefore the reactant.

Kate Chow 4H

C₂H₅OH ---> C₂H₄O
For C₂H₅OH, you know that there are 6H(+1/H), 1O(-2), 2C(-2/C). Total charge = 0
For C₂H₄O, you know that there are 4H(+1/H), 1O(-2), which leaves -2 left to be filled by the 2C. Therefore, each C will have a charge of -1 to make the total charge = 0.

Kate Chow 4H

Since delta G = -n*F*Ecell, when delta G is negative, Ecell is positive. The negatives will not cancel out in this situation. We know that -delta G means that the reaction is spontaneous.

Kate Chow 4H

When you balance a redox reaction, you have to also balance the charges as well, which means that you have to keep in mind the number of electrons.

Kate Chow 4H

When changing volume and temperature, you have to use both delta s equations (one for change in volume, one for change in temperature) and then you add the delta s values together to get delta s of the reaction. You use the values that are provided in the problem for both equations.

Kate Chow 4H

A reaction that is spontaneous usually releases energy, so it wants to form products. Forming products favors the forward reaction.

Kate Chow 4H

You use Cv when the volume is constant and Cp when the pressure is constant.

Kate Chow 4H

You divide the heat by the mols to get the right units in the end, which is just J or kJ.

Kate Chow 4H

Energy is always constant in an isolated system at equilibrium based on the principle of conservation of energy and the fact that in an isolated system, energy cannot be exchanged.

Kate Chow 4H

Kb is the Boltzmann constant. It's on the equation sheet that we're given at the beginning of all of our exams. You also always have avogadro's number because that's also on the equation sheet.

Kate Chow 4H

In a workshop today, we were using the formula $\Delta$ u = 3/2 nR $\Delta$ T. For R we used 8.314 J/K*mol but for $\Delta$ T, we used degrees celsius. I'm wondering how the constant can be in K, but the $\Delta$ T can be in celsius and how the units work out in this regard.

Thanks!

Kate Chow 4H

you can also think of delta u as the change in internal energy and delta h as the change in enthalpy in the system. Delta h has the option of including more than just the change in internal energy because it has to take into account pressure and volume as well (delta h = delta u + p(delta v)). Only ...

Kate Chow 4H

Reversible: w = -p(delta v)
Irreversible: w = -nRT*ln(v2/v1)

Kate Chow 4H

If you know Ka or Kb, you can determine the strength of an acid or base. But if you don't, there's a short list that you can memorize or know pretty well. For example, most all of the strong bases use the elements in the first 2 columns of the periodic table.

Kate Chow 4H

Yes. Molarity is equal to mol/L, so mol.L^-1 is also the same.

Kate Chow 4H

I don't think so. I think Dr. Lavelle will provide the constants that we need.

Kate Chow 4H

Usually when you have all gasses in the reaction, you use Kp. There's not necessarily an advantage to either Kc or Kp. It often comes down to the information that is provided in the problem.

Kate Chow 4H

You're given that k = 41 when N2+3H2 --> 2NH3. In part c of the question, the reaction is now 2N2 + 6H2 --> 4NH3. Notice that all of the coefficients were simply multiplied by 2 in part c. If you calculate K using products/reactants with the coefficients as the powers, you realize that the coefficie...

Kate Chow 4H

Ksp has to do with solubility, but we haven't gone over it in class yet. I wouldn't worry about it until it's discussed in class.

Kate Chow 4H

I'm also a little confused about this, but I'm pretty sure that you use Kp for gasses unless the problem specifically tells you to use Kc. Also, in some problems, you can determine whether to use Kp or Kc depending on the units given. I know there are instances of this in the assigned problems.

Kate Chow 4H

Q is just a calculation of equilibrium at a certain point within the reaction. Q changes as the reaction shifts towards equilibrium. However, Q = K at equilibrium.

Kate Chow 4H

Since you know that NH4HS is a solid, you can put 0 in for the whole column. You can add "x" to both NH3 and H2S in the change row (C) because you eventually want to find their equilibrium concentrations. You're given Kc so you can use the equilibrium equation: Kc=[NH3][H2S]. Kc=[0.2+x][x]...

Kate Chow 4H

I think that Dr. Lavelle said that calculating K is a 14b topic, so we won't need to know how for the final.

Kate Chow 4H

In acetic acid, the CH3 is actually electron donating, which means that it adds electron density and makes it harder to remove the H. The easier it is to remove an H, the stronger the acid. So, because it's hard to remove the H in acetic acid, acetic acid is weaker than formic acid.

Kate Chow 4H

No, a double or triple bond doesn't change the hybridization. You can treat them the same way as you do a single bond. They count as "1".

Kate Chow 4H

I'm not exactly sure what you mean by atoms, but when there are 3 bonds and 1 lone pair, the VSEPR formula is AX3E.

Kate Chow 4H

I think that you're correct in that it's just telling you to know the order of magnitude of different types of forces.
So, ion-ion > ion-dipole > H bonding > dipole-dipole interactions > dipole induced dipole > London dispersion forces.

Kate Chow 4H

The 2 preceding the sp(x) just tells you which period (row in the periodic table) he's talking about. It relates to electron configuration. For example, Oxygen's electron configuration is 1s2 2s2 2p4, so when you have to write the hybridization of oxygen, it would be 2sp(x)

Kate Chow 4H

Increasing molar mass means that the molecule has more electrons. When there are more electrons present, there are stronger London Dispersion Forces, and thus stronger attraction. Also, in larger/heavier molecules, the valence electrons are further away from the nucleus, which allows for easier temp...

Kate Chow 4H

I agree. I think Test 3 picks up right after what the midterm tested on and definitely includes hybridization.

Kate Chow 4H

I didn't know what s character was either, but Chem_Mod did a really nice job explaining this previously: viewtopic.php?t=1255

Kate Chow 4H

Since sigma bonds are present in every bond, but pi bonds are only present in double and triple bonds, anything with both a sigma and pi bond (1 for double, 2 for triple) will be shorter and stronger than something with only a sigma bond. Essentially, if a pi bond is present, that will be shorter an...

Kate Chow 4H

I agree. When drawing the VSEPR models, you only show the wedges to show 3 dimensions. The solid wedge comes towards you and the dotted wedge away from you. The bond type isn't really relevant when drawing 3D models.

Kate Chow 4H

Adding onto the previous reply, a bent shape occurs when there are 2 bonded atoms and a lone pair on the central atom. The lone pair pushes down, making the bond angle closer to 120 degrees instead of the 180 degrees seen in a linear shape. Regarding molecular interactions, you can look at symmetry ...

Kate Chow 4H

When you draw the Lewis Structure, you see that iodine has 2 lone pairs. Since iodine has 2 lone pairs, it makes it t-shaped instead of trigonal planar. Molecules that are trigonal planar don't have lone pairs on the central atom.

Kate Chow 4H

2nd ionization energy is higher than 1st ionization energy because it's always harder (takes more energy) to remove an electron from an already positively charged ion.

Kate Chow 4H

Radicals are very reactive because they have an odd number of electrons and they want to do anything they can (bond, etc.) to get rid of that 1 electron.

Kate Chow 4H

You can also just count the number of electrons and see if the number is odd or even. Odd = radical. There's not really a need to draw out the Lewis Structure if you don't want to.

Kate Chow 4H

Mn electron configuration: [Ar]3d5 4s2 - Since 4s is of higher energy level thad 3d, you remove an electron from 4s before you would remove from 3d.
Similarly, Au electron configuration: [Xe]4f14 5d10 6s1 - Again 6s is of higher energy level, so you remove an electron from 6s before 5d or 4f.

Kate Chow 4H

There's actually a thread on this that I'll link here: viewtopic.php?t=16277
but the overall gist is that it's because of electron repulsion.

Kate Chow 4H

Additionally, since 5s is a higher energy level than 4d, electrons would be removed from 5s before they're removed from 4d.

Kate Chow 4H

The number of dots that are put around each element relates to the number of valence electrons that element has. For example, C has 4, N has 5, O has 6, F has 7 valence electrons. The number of valence electrons correlates with the number of the column (group).

Kate Chow 4H

Based on the number of valence electrons calculated, you can determine which types of bonds to use. For example, CH4 uses a single bond whereas C2H4 uses a double bond. In C2H4, you have the same number of hydrogens but another carbon, so you have to use a double bond to stay within the confines of ...

Kate Chow 4H

Can someone go over all the trends in the periodic table that we've learned and the reasoning behind them? I think I mixed a few up.

Thanks!

Kate Chow 4H

Correct! To convert from moles to grams, you multiply by the molar mass. Use dimensional analysis if you are unsure. L3b. 0.650mol LiH * 1 mol Li3N/2 mol LiH * 34.83 g Li3N/1 mol Li3N = mass of Li3N that will produce 0.650 mol LiH You can see that all the units cancel out except for g Li3N, which is...

Kate Chow 4H

1 Joule = 1 kg * m^2 * s^-2, which can also be written as kg*m^2/s^2

You can convert Joules to SI units when you're using dimensional analysis to help you solve a problem. This could come in handy whenever you're solving for energy, as energy is often measured in Joules.

Kate Chow 4H

It doesn't matter because the only difference between 6.626 X 10^-34 and 6.63 X 10^-34 is the number of significant figures. I would use 6.626 X 10^-34, as more significant figures will usually lead to a more accurate answer in the end.

Kate Chow 4H

An angstrom is just a unit of length (think meter as another example) that is equal to 1*10^-10 m or 0.1 nm. I know Dr. Lavelle mentioned that angstroms are used to express wavelengths, so they are probably applied in lab in that way as well.

Kate Chow 4H

First, start with the formula E=hv, where E=energy of photon, h=constant, v=frequency. Extent of change in an electrical field is basically the energy of a photon. You are given that frequency (v) decreases and since h is a constant, you know that E (energy of photon) must also decrease. Hope this h...

Kate Chow 4H

I'm pretty sure the 3 posts are due at the end of the week, no matter when your discussion is.

Kate Chow 4H

I think you would have to use scientific notation, so 100 = 1.0 * 10^2.

Kate Chow 4H

I know this sounds stupid but:
Have No Fear Of Ice Cold Beer
hydrogen, nitrogen, fluorine, oxygen, iodine, chlorine, bromine

Kate Chow 4H

Based on what Dr. Lavelle said in class, for now it's fine if we look up the formulas of compounds. Eventually, we'll have to memorize some of the more common compounds, but he'll teach us and also let us know when.

Search found 61 matches