CHEMISTRY COMMUNITY

SarahCoufal_1k

I think it's just an online test on CCLE now and you submit at the end

SarahCoufal_1k

Also, will he be posting the link to his chem 14B website?

SarahCoufal_1k

Does LaVelle's last email mean it's M/C now?

SarahCoufal_1k

Is it still in the 3 hours, or do we have more time now?

SarahCoufal_1k

On the midterm one of the questions was:
Which combination of solutions of HCl and NaOH would produce the largest ∆T?
the answer was: 50 mL of 2 M HCl with 50 mL of 2 M NaOH
can someone explain reasoning behind this please?

SarahCoufal_1k

I wouldn't be surprised if we get our tests back on Wednesday as the Ta's might have to work on the final so they get the tests over with first. Do the TAs make our final or Dr. Lavelle? This is a good question. I'm not too sure but would like to know TAs make the discussion tests. Lavelle makes mi...

SarahCoufal_1k

n is the number of electrons transferred in the redox equation. When you do your reduction half reaction there is a certain amount of electrons transferred and same case for the oxidation half reaction. You then have to multiply the equations to balance the electrons. when they match and cancel that...

SarahCoufal_1k

n is the number of electrons transferred in the redox equation. When you do your reduction half reaction there is a certain amount of electrons transferred and same case for the oxidation half reaction. You then have to multiply the equations to balance the electrons. when they match and cancel that...

SarahCoufal_1k

Because all cell potentials are given in the reduction form and since the anode is being oxidized you switch the equation around so it represents that. Then when. calculated Ecell you can either do Ecathode-Eanode. or Ereduction+Eoxidation but you have to reverse the sign of the oxidation one

SarahCoufal_1k

I don't think so but I would maybe ask him or a TA. Because we go over derivations a lot but we don't seem to do them on any homework problems or tests.

SarahCoufal_1k

why does reaction free energy depend on pressure and the equilibrium constant?

SarahCoufal_1k

The energy barrier (the hump) is really large so it would take a lot of energy to do the reaction so it's most likely to stay as the reactant.

SarahCoufal_1k

No. There are the peer learning sessions like step up and office hours though

SarahCoufal_1k

Gibbs free energy is the best determinant. I think it's best since it includes both entropy and enthalpy in its formula. If it's negative it is spontaneous

SarahCoufal_1k

Use an inert electrode. The one they used in class a lot is platinum

SarahCoufal_1k

use commas if they are in the same phase, if different phases use lines

SarahCoufal_1k

First, you should balance the species being oxidized or reduced in the half reaction. Then, add water to balance oxygen. Then, add H + to balance hydrogen in acidic solutions. If the reaction is in a basic solution, you add OH - to one side of the half reaction and H 2 O to the other side to balanc...

SarahCoufal_1k

I think it is always from anode to cathode. Oxidation is loss of electrons at the anodes so then they flow to the cathode where there is the gain of electrons which is reduction

SarahCoufal_1k

N is the number of electrons being transferred.

SarahCoufal_1k

In the examples in discussion for balancing redox equations it seems like you first add H2Os to balance Oxygens. Can you then add either OH-s or H+s depending on the EQ, or is one preferred over the other?

SarahCoufal_1k

The reduction is spontaneous when standard reduction potential is positive

SarahCoufal_1k

W refers to the number of possible states of a molecule with equal energy. W= #of states^# atoms.

SarahCoufal_1k

Why exactly does delta G have to be negative to be considered spontaneous? If delta G is negative that means it is going from higher energy(reactants) to lower energy (products). If it is going from high to low, you need very little (if any input of energy) so it will happen spontaneously. On the o...

SarahCoufal_1k

Gibbs free energy has to be negative. B/c delta G= Delta H - T*DeltaS the others contribute to delta G

SarahCoufal_1k

We will be getting midterms back Wednesday after lecture. The TAs will be outside

SarahCoufal_1k

Why is entropy a maximum at equilibrium. ( looking at the equation deltaS= q/T)

SarahCoufal_1k

4G.5 is asking about positional disorder. It gives a random octahedral in the sis structure and trans structure. It says the Cis compound has 12 different orientation while the trans compound only has three. Why are there so many more orientations for the cis one?

SarahCoufal_1k

If expanding, volume is increasing and pressure is decreasing. It will favor the side of the reaction with more moles Lavelle taught a shortcut in class that If volume is decreasing, shift towards the side of the reaction with less moles. So your question is just that in reverse. I think it is impor...

SarahCoufal_1k

What exactly does it mean for a process to be reversible? In the review session today, they said something is reversible when ΔS total=0 and since ΔS total= ΔS system + ΔS surroundings then ΔS system = -Δ S surroundings The textbook also defines a reversible process as a process that can be reverse...

SarahCoufal_1k

In the review session today they said enthalpy is always heat, but heat is not always enthalpy. The definition of enthalpy is heat at constant pressure. So if it isn't at constant pressure, the heat can't be described as enthalpy

SarahCoufal_1k

Another giveaway is the phases of the reactants and products. For example, if the reactant is a solid and both products are liquids, it's most likely endothermic

SarahCoufal_1k

ELu 1J wrote:Are you only going over the answers at the Sunday review session or the Monday one as well? Thank you so much :)))

they usually go over it in the three hour session on Sunday only

SarahCoufal_1k

Mathematically if delta V is negative (volume is decreased/compressed) then work is positive. Logically because the system is compressed, like with a piston, work is being done on the system so it's positive. If delta V is positive (volume is increased/expanded) then work is negative. The system is ...

SarahCoufal_1k

open system allows the transfer between matter and energy
closed system allows the transfer of energy but not matter
isolated system doesn't allow the transfer of energy or matter

SarahCoufal_1k

yes, the negative sign is always there. When volume is decreased/compressed, delta V is negative so work is positive overall. when volume is increased/expanded, delta V is positive and so w is negative overall. This is because when it is compressed, work goes into the system, and when it is expanded...

SarahCoufal_1k

There is no extra credit. Even though you missed 5, still do them. Because it sometimes just counts if you have 50 at the end of the quarter and you might still get full points.

SarahCoufal_1k

You use an ICE box to find an unknown concentration. That can be either initial concentration or equilibrium concentration. When you are given Ka or Kb that will be used to find one/more than one of the equilibrium concentrations

SarahCoufal_1k

Can someone explain again why a burn from hot steam would be way worse than a burn from hot liquid?

SarahCoufal_1k

you need to look at the units of the other variables provided. Because you use r in the equation pv=nrt --> p=(n/v)*rt, you have to look to see if the concentration was given in molarity or in partial pressure which would be atm or bar. Choose the r with matching units because then they should all c...

SarahCoufal_1k

It depends on the units of the other. variables provided. Choose the r with the matching units and all but the variable you are looking for should cancel out.

SarahCoufal_1k

I believe there are 6 phase changes.
3 are endothermic: Melting (solid to liquid) vaporization (liquid to gas) sublimation (solid to gas)
3 are exothermic: freezing(liquid to solid), condensation(gas to liquid) and deposition (gas to solid)

SarahCoufal_1k

I'm also confused by 6D.15. In part B, the solutions manual replacer AlCl3 with Al(H2O)6 ^3+. and have a Ka=1.4X10^-5. That seems like a given constant but I can't find it in any of the text.

SarahCoufal_1k

it's used to convert between pressure and concentration. When you are finding K for gasses that could. be in either pressure or concentration. You manipulate the equation to get a. simpler conversion. It starts as PV=nRT. then divide by V for P=(n/V)RT. And n/v is moles/Liter which is the definition...

SarahCoufal_1k

Exothermic forward reactions release heat and so when the temperature increases it favors the endothermic reverse reaction which also favors the reactants. In the same way, endothermic forward reactions favor the temp increase and forward reaction which also favors the products. This will permanentl...

SarahCoufal_1k

you find pOH by taking the -log[OH- concentration]. you find pH by taking the -log[H30+ concentration].When it is base you use the OH- concentration and when it is an acid you use the H30+ ion concentration. Also when the equation/problem only gives you one, you can find the other because pH + pOH=14

SarahCoufal_1k

when the K is less than 10^-3, the x subtracted. is going to make such a small difference it's acceptable and easier to just ignore it. Only ignore when it is addition or subtraction though, you should include it in multiplication or division I believe. If concentration. found is less than 5% of the...

SarahCoufal_1k

yes! pOH + pH = 14 so you can. just subtract 14 from whichever. concentration you have and you have the other one as well

SarahCoufal_1k

5 posts a week, supposedly they check at the end of each week

SarahCoufal_1k

You omit them because there is usually so much excess that the actual amount then used is insignificant in terms of molar concentration.

SarahCoufal_1k

Dina 2k wrote:did the Professor mention how many discussion sections you can miss without losing points?

you turn in your homework during discussion, so I think if you don't go you would lose those 5 points of hw

SarahCoufal_1k

everything on sapling learning is voluntary and just an extra resource I believe

SarahCoufal_1k

It allows us to convert between the partial pressure and molar concentration. the ideal gas law is PV=nRT. You then modify the equation so P=nRT/V and n/v is mol/L which is concentration. When you isolate that, p/RT=n/V which mean p/RT=concentration n. This means that partial pressure/ (gas constant...

SarahCoufal_1k

I think the best way is to memorize the strong acids and bases and if you come across one that is not on that list, it is weak. Fundametals J has a table as well that lists them. Strong Acids: HCl HI HBr H2So4 HNO3 HCLO4 HCLO3 Strong Bases: these include basically all of the group one metals + hydro...

SarahCoufal_1k

Yes, it is probably most efficient to memorize those so you can write out compound names correctly

SarahCoufal_1k

He was saying that 14B will start with titrations and finding out molarity when it is a weak acid/base not strong one

SarahCoufal_1k

cumulative final

SarahCoufal_1k

When the overall complex in brackets is negative add -ate to the end of the transition metal name. However, in iron make it ferrate. (example of another would be nickel-nickelate)

SarahCoufal_1k

A lot of resources on Lavelle's website!!

SarahCoufal_1k

I think the TA review sessions help a lot, try to go to as may as you can even on the same subject for reinforcement etc.

SarahCoufal_1k

inorganic acids are. often much stronger than organic acids as well. Also there is a helpful table about strong acids and bases in the textbook. I believe it's in fundamentals J

SarahCoufal_1k

I think it's because the overall compound has a negative charge. Usually when a transition metal is used it has an overall positive charge so it's odd when it's negative. Lavelle said adding -ate to transition metals sounds weird too so use that to remember it's an odd compound with an overall negat...

SarahCoufal_1k

Will the final have emphasis on material post midterm and discussion tests. Or is it a likely split between everything we learned?

SarahCoufal_1k

The grades are up on myUCLA !

SarahCoufal_1k

Acids are proton donors. Bases are proton acceptors. Something that is acidic has a lot of protons in it, because the acids lost them (i.e donated) in the solution.

SarahCoufal_1k

Bent has 1 or 2 lone pairs- either AX2E or AX2E2. They are bent because the lone pairs repel the electrons on the bonded atoms.

SarahCoufal_1k

I do the arrow thing as well. I draw an arrow going from bottom to top on the side, and left to right. Lastly a diagonal aloo from bottom left to top right. These are all for increasing electronegativity and ionization energy.

SarahCoufal_1k

^ there is more information about the final on his syllabus

SarahCoufal_1k

Is Hybridization on the test? or is it only until sigma and pi bonds?

SarahCoufal_1k

Dr. Lavelle said he will go over this more in depth on Monday as well

SarahCoufal_1k

Yes you can, and they will. usually be the same. Looking at the AXE formula, X is number of bonded pairs to central atoms and E is number of lone pairs. With this you can understand the number of electron densities but also the number of bonded atoms. So you can use both to figure out the shape and ...

SarahCoufal_1k

As said above, the strength of repulsion would affect the angles, which will also affect the shape. a lone pair lone pair would have greater repulsion and so the bond angle would be larger because they can't get as close to each other

SarahCoufal_1k

I believe Dr. Lavelle said that molecules with the same AXE formula have the similar molecular shape so yes. For Example AX4 is usually tetrahedral and AX2E1 is usually trigonal pyramidal

SarahCoufal_1k

I think anything not on the midterm to the beginning of the new outline is ok

SarahCoufal_1k

Molecules like O2 ad Br2 and F2 etc. do not have dipoles. This is because they have the same electronegativity so each atom pulls the electrons an equal amount towards their own nucleus. Dipoles occurs when an atom pulls the electrons towards it more than another atom

SarahCoufal_1k

Basically most elements in period 3 and beyond. This is because there is now the D sub shell that can be filled. In addition Boron can be satisfied with either 8 or 6 electrons.

SarahCoufal_1k

When a bond is stronger, it needs more energy to be broken. There is more energy in higher temperature. So when a bond is really strong, it needs more energy to be broken (and go to different state) i.e it has a higher boiling point/melting point. Similarly when a bond is weak, it needs little energ...

SarahCoufal_1k

Can homework this week be from the chemical bonds section? (outline 3, section 2 of the textbook)

SarahCoufal_1k

At least a week

SarahCoufal_1k

I think the question on the exam may make it clear whether it wants multiple versions. For example if it is talking about resonance or if it says "make the lowest energy structure". Some lewis structure have the same formal charge when changed and others are variable. I think when you are ...

SarahCoufal_1k

A radical is an atom, molecule, or ion with at least one unpaired electron. This is highly reactive, because this single electron is unstable and the molecule doesn't want it.

SarahCoufal_1k

It's mainly so you won't mess up and remove it from the wrong shell. This is especially important when you get to the fourth row and start using the D block. Because you would fill up an electron from low to high energy, 4S starts out lower energy than 3d, but once 3d has an electron now it's a lowe...

SarahCoufal_1k

In 1.B.27,the question says: a bowling ball of 8 .00kg is being rolled down an alley lane at 5.00 +/- 5.0 m/s. what is the minimum uncertainty in its position? For uncertainty in velocity (delta v), would it be 10 or 5? I thought it would be 10. because the range is from 0-10, however the solutions ...

SarahCoufal_1k

In light of classes being canceled, are the step-up programs/ midterm review sessions still happening today?

SarahCoufal_1k

I believe Threshold Frequency is the amount of energy of a photon required to displace an electron on a metal surface. The photon needs to at least match the threshold, which will result in a kinetic energy of 0.

SarahCoufal_1k

You should probably know the groupings, because that might help determine how they react with other elements/atoms.

SarahCoufal_1k

I think he was just talking about the different types to inform us. Also to show how the length can be shorter when attraction is greater. I think he just used the numbers to help us understand better, not have a way to calculate it.

SarahCoufal_1k

After Atomic number 20 (i.e. 20 electrons) 3d has a slightly lower energy level and you want to fill them from low to high. So do 3d and then 4s

SarahCoufal_1k

Resonance structures are different versions of lewis structures for the same molecule and there are equivalent models.

SarahCoufal_1k

If it is in a plus 3 state, shouldn't we subtract 3 electrons instead of add?

SarahCoufal_1k

Because it's a transition week you can either turn in homework on either Quantum or Chemical bonds

SarahCoufal_1k

They don't have enough electrons to begin with to then go form an octet. It would take a lot of energy to do so I think. Hydrogen can only have two in its first shell, so it has its own version of the octet rule

SarahCoufal_1k

I think a problem on a test would be to draw out the lewis structure, but it would only ask for one way. But all variations would get credit. I would ask Dr. Lavelle to be sure.

SarahCoufal_1k

I think increasing intensity of light only increases the amplitude/height of a wave but does not change wavelength or the energy emitted. So more photons would be emitted but the energy of each photon wouldn't change. To change the energy of a light wave, the frequency has to change. As we learned, ...

SarahCoufal_1k

I'm not sure how to identify them. But constructive interference occurs when two light sourced are in phase with each other, such as peak on peak. This can result in bigger peaks. Destructive interference occurs when the light waves are not in phase with each other and as a result are smaller peaks

SarahCoufal_1k

during your calculations, do not round to sig figs until the final answer. I think when you are doing a problem you can write the number as 3.456.. to show you have the rest of the number saved in you calculator and are going from that

SarahCoufal_1k

On Lavelle's main page of his website there is a document that answers a bunch of questions about sig figs. One thing I noticed that was important was in addition and subtraction go by the fewest number of decimal places. But in multiplication and division just the fewest number of sig figs

SarahCoufal_1k

I think the best way to do conversions like this is to use dimensional analysis. nm to meters to angstroms. I think also be aware of what your answer looks like. For example, if you are going. from nm to angstroms thats going. from 10^-9 to 10^-10. An angstrom is smaller scale, and there are more an...

SarahCoufal_1k

It's the units of Hz. 5 Hz and 5 s^-1 and 5 cycles per second are all the same/

SarahCoufal_1k

For the sake of our class, I think it's reagents/reactants on the left side and products on the right side.

SarahCoufal_1k

I don't think you have to show as many steps as in discussion. However, if you mess up the final answer, but show your correct process, you might receive partial credit.

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