CHEMISTRY COMMUNITY

Matthew Chan 1B

For the most part, it is just plugging in your values and solving. Sometimes you will need to do some intermediate calculations in order to obtain all the necessary values to calculate the desired answer.

Matthew Chan 1B

Since k is the rate constant, if everything else is constant, if you increase the value of k, the reaction rate will increase.

Matthew Chan 1B

Zero order: [A] vs time (linear)
First order: ln[A] vs time (linear)
Second order: 1/[A] vs time (linear)

Matthew Chan 1B

If you know the K for the forward reaction and if you also have the overall K, you can then determine the reverse K and therefore the rate for the reverse reaction.

Matthew Chan 1B

t_1/2=[A]₀/2k

Matthew Chan 1B

I think we will usually be using:
G=-nFE
G=-RTlnQ

Matthew Chan 1B

If they are in the same state, then use commas.
Ex: Fe³⁺(aq),Fe²⁺(aq)
If they are different states, then use the vertical lines to indicate that.

Matthew Chan 1B

If you have an ion-gas reaction or if you do not have a solid state metal/conductor on that part of the galvanic cell, then you need to include some kind of conductor like Pt(s)! Platinum is only one of them, but it's probably the most common one to use.

Matthew Chan 1B

You just need to multiply the reaction(s) to balance out the number of electrons, and then you should be able to cancel them out since they will then be equal.

Matthew Chan 1B

Hi there! I think for this problem, we just have to look in the Appendix for half reactions that, when we combine them, will give us the resulting equation. Hope this helps!

Matthew Chan 1B

We know that Cl₂ both oxidizes and reduces, so $Cl_{2}\rightarrow 2Cl+2e^{-}$

Matthew Chan 1B

My guess is, same as Nicholas, that we know that electrochem reactions proceed forward to give a positive E_cell value.

Matthew Chan 1B

In a basic solution, make sure whatever is being oxidized or reduced is balanced first, then begin to balance out the oxygens and later hydrogens with water. Then balance out the inequality that the H2O creates with water on the other side and then the same number of OH- ions on the side you origina...

Matthew Chan 1B

This is to ensure that both sides of the equation are balanced, based on whether the solution is acidic or basic. In this case, it appears to be acidic.

Matthew Chan 1B

The 14H+ becomes 8H+ because it cancels out with the 6H+ on the other side when you are combining the two half reactions. In addition, you get the 3 as the coefficient to the molecules with carbon because you have to multiply the oxidation half reaction, which has 2 electrons by 3 in order to cancel...

Matthew Chan 1B

Yes, since at 0K atoms can't move, so there is no disorder since there is no other possible state/position that the atoms could be in, which means zero entropy.

Matthew Chan 1B

Endothermic reactions require heat, so if there is not enough heat provided to the system, then the reaction cannot continue. In this sense, heat can be thought of as the "limiting reactant".

Matthew Chan 1B

If you're asked for the enthalpy change of some reaction or whatever, you'll typically be provided with values that correspond to the standard enthalpies of formation for certain components of the reaction that you can then use in your calculations to determine enthalpy change of a reaction, etc.

Matthew Chan 1B

Oxidation state is the charge of the atom with ionic chemical bonds. Oxidation number is the Roman numerals that we use to represent the oxidation state of the atom.

Matthew Chan 1B

I think it's good practice to know a few of them, like SO₄^2-, or CO₃^2-... stuff like that.

Matthew Chan 1B

Whether you use mass or the number of moles for these calculations just depends on what units the values you're given use, or if you're expected to have an answer with certain units.

Matthew Chan 1B

Enthalpy is the heat transfer at a constant pressure... so keeping that in mind, that is why we use C_p.

Matthew Chan 1B

Matthew Chan 1B wrote:When there is work done on a system, the work is negative. However, when the system itself does work, then work is positive.

Sorry, I meant it the other way around haha! Work is negative when the system does work. When the system has work done on it, then work is positive. Sorry about the typo!

Matthew Chan 1B

Work is negative when the system does work itself, which makes sense since the system is "losing" work/energy, which makes sense why it would be negative.

Matthew Chan 1B

Heat is the form of energy transfer from a one temperature to another, like from hot to cold. Enthalpy is the heat transfer at a constant pressure. So the heat added or lost by the system is the enthalpy change.

Matthew Chan 1B

When there is work done on a system, the work is negative. However, when the system itself does work, then work is positive.

Matthew Chan 1B

DHavo_1E wrote:Hello,

Can I ask why for constant volume we do not get an answer for the change in enthalpy? Thank you

This is because at a constant volume, q_v = $\Delta U$ of the system.

Matthew Chan 1B

Hey! The 101.325 J value is just a conversion that we should know. I think it's just good to memorize it as it's definitely important, since we measure work in joules and not L.atm!

Matthew Chan 1B

If this offers any reassurance, I also got 490 Joules for my answer. Either we're somehow all wrong or the solutions manual is wrong. Chem_Mod?

Matthew Chan 1B

Remember that \Delta H is always q, but q is not always \Delta H . We also have to keep in mind that \Delta H is only equal to q at a constant pressure. If we have constant volume, then q is equal to \Delta U . That being said, when we have a system that is being held at a constant pressure, we gene...

Matthew Chan 1B

I believe that extensive properties depend on how much of a sample you have, while intensive properties don't really depend on quantity of the sample or the size of the sample, etc.

Matthew Chan 1B

When we have an irreversible reaction, there is a sudden change, since everything happens right away. It's like pulling the pin out of a piston and so the piston will immediately extend if there is some pressure within it after the pin is removed. When we have a reversible reaction, we have work whi...

Matthew Chan 1B

Transitioning from a liquid to a gas requires the breaking of bonds, which is why so much more energy is needed to make this change in comparison to the transition from solid to liquid.

Matthew Chan 1B

If you look at the heating curve of water, there is a much bigger "flat line" going from liquid to vapor than from solid to liquid, which basically tells us that there is more energy required to vaporize. With this higher amount of energy required to vaporize compared to transition from so...

Matthew Chan 1B

Only at constant pressure and volume! :)

Matthew Chan 1B

If there was a system where no energy was lost or gained/with all perfect conditions, q (of the system) = -q (of the surroundings). This is so that if there is any change in one, the other will inversely adapt/change, since if there is heat lost by the system, the surroundings will gain that heat.

Matthew Chan 1B

I think that the negative sign indicates that the reaction is exothermic, so the system itself is losing heat, while the surroundings are gaining heat.

Matthew Chan 1B

Wait, they'll be returned in this week's discussions?

Matthew Chan 1B

A state property is a quantity that is independent of how the substance was prepared

Matthew Chan 1B

You will still be able to pick them up thius week

Matthew Chan 1B

Hess's Law states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes

Matthew Chan 1B

Could someone explain the example that Lavelle went over with steam and why it has much more severe burns than liquid water?

Matthew Chan 1B

Knowing how to draw the lewis structures won't specifically be tested (I think), but it will just help with being able to understand what happens during a reaction when doing problems with acids/bases, salts, etc. so that you can see the proton/electron transfers.

Matthew Chan 1B

It's usually because H2O is in excess, which means that we cancel it out, since there's no real change to it. I think Lavelle gave the analogy: If someone has one million dollars, and they give ten dollars to someone else, we don't say that they have $999,990. We just say that they have about a mill...

Matthew Chan 1B

As mentioned above, we only account for the Ka values for the weak acids, since we know that the strong acids basically dissociate 100%. Also, when x is 5% of the initial concentration, we can get rid of the x since it would be such a small negligible amount anyway.

Matthew Chan 1B

I was just wondering if we have to know how to convert between K and K c . It's done in Example 5H.1 Suppose you are a scientist studying the reactions of SO2 and O2. If you wish to use gas molar concentrations, you must first convert the equilibrium constant K to Kc. At 400 Celsius, the equilibrium...

Matthew Chan 1B

I was wondering if someone could help explain the example that Dr. Lavelle gave in class when discussing the reaction from N₂ to 2N. I understand that N₂ is more stable and therefore it is favored, but then how does that make the reaction endothermic? Thanks!

Matthew Chan 1B

I think that we will be given these values on a constants sheet or something like that. Just make sure that when dealing with units of pressure, pick one unit and be consistent with it throughout the problem. Be sure to not interchange them.

Matthew Chan 1B

Both are units for measuring pressure. They are both okay to use, just make sure that you stay consistent with whichever unit you choose or with whatever is given in the problem.

Matthew Chan 1B

I think if we just look at the homework problems that he has assigned and look at the difficulty level of those, that would give us a pretty good idea of what the test would look like. Also I assume whatever has been mentioned in lecture up to Test 1 would be fair game as well.

Matthew Chan 1B

Yeah, as Tanmay said above, I don't think that we have to know how to calculate it yet.

Matthew Chan 1B

R just represents the gas constant, which is 8.314 J.mol^-1.K^-1

Matthew Chan 1B

Oxalate follows the polydentate "template" that Dr. Lavelle mentioned: it has atom--spacer atom--spacer atom--atom. We can see that as O--C--C--O and the O's are the atoms that bind to the metal. Therefore, it's bidentate.

Matthew Chan 1B

In water, can SO₃ (sulfur trioxide) form hydrogen bonds with the water molecules?

Matthew Chan 1B

Elizabeth Harty 3A wrote:How do you know if there are sigma bonds available for rotation?

dont sigma bonds already have the ability to rotate? so if its just a single sigma bond then it can rotate but if theres pi bonds then you cant rotate. did i answer your question?

Matthew Chan 1B

For the Mini marshmallow worksheet, for problem 2, we are asked to write the name/formula for the coordination compound, etc. in problem 2a, the coordination compound is [Ni(NH 3 ) 2 O 2 ]Br 2 . However, in the answer key, the compound is something completely different. I just wanted to check to see...

Matthew Chan 1B

the C=C bond is the strongest because atoms that share the same/similar energies that overlap will have the strongest bonds. Thus, the homonuclear bond between C=C is the strongest out of the other options. I think this is correct... but not entirely sure

Matthew Chan 1B

How would you determine whether a ligand can bind at multiple sites (or be chelating?) Is there a certain angle threshold? For example like in 9C.7 (I've attached an image of the isomers of diaminobenzene that the book uses). Note that B and C should be switched around to represent what the textbook...

Matthew Chan 1B

The OH₂ is just to draw attention to the fact that the oxygen is what is contributing the electrons to the central metal atom

Matthew Chan 1B

Do we arrange on the basis of the element's letters or the names of element/compounds?

Matthew Chan 1B

Are we still allowed to use -o like fluoro and chloro rather than -ido?

Matthew Chan 1B

Yes, ligands are considered lewis bases because they are donating an electron pair.

Matthew Chan 1B

Induced dipole is when there is a molecule that is not polar or has no dipole moment, but then it experiences a temporary dipole moment because of the dipole moment of another molecule. A dipole dipole moment is where there are two molecules that experience attraction between the partially positive...

Matthew Chan 1B

In Focus 2.61, it asks you to draw the Lewis structure for HOCO, and to identify whether or not it is a radical. Evidently, the lewis structure for HOCO results in: H-O-C=O, with the Carbon having a single electron on it, or a radical. How do we know that carbon is the atom that will have the single...

Matthew Chan 1B

Induced dipole is when there is a molecule that is not polar or has no dipole moment, but then it experiences a temporary dipole moment because of the dipole moment of another molecule. A dipole dipole moment is where there are two molecules that experience attraction between the partially positive ...

Matthew Chan 1B

Lewis acids and bases are defined as being able to accept or donate electron pairs, while Bronsted Lowry acids and bases are defined as being able to accept or donate hydrogen ions (H+).

Matthew Chan 1B

A chelating complex is a complex that contains a ligand that forms a ring of atoms that includes the central metal atom.

Matthew Chan 1B

Can someone explain what the Heme complex is and what significance/applications it has? Thanks.

Matthew Chan 1B

Orbitals hybridize whenever the resulting molecule will be lower in energy.

Matthew Chan 1B

Paramagnetic compounds have unpaired electrons while diamagnetic compounds the electrons all have paired spins.

Matthew Chan 1B

If I have discussion on Tuesday, will Hybridization be on Test 2? Is it something that's going to be discussed Monday lecture?

Matthew Chan 1B

With enough practice and being able to determine if something is bent or linear, etc. you can likely determine the correct bond angles because after lots of practice, it should be second-nature to remember what bonds correlate with what shapes.

Matthew Chan 1B

Due to electrons always moving around and never staying in one set place, they sometimes create temporary dipole moments, which is why there are london forces in every bond.

Matthew Chan 1B

Dipole moments cancel when there are moments facing in opposite directions, effectively canceling each other out.

Matthew Chan 1B

This is kind of a dumb question, but do we have to memorize the bond angles?

Matthew Chan 1B

When there is a large difference in the electronegativity between two elements, the electrons from the large electron dense regions of the large anion are pulled away from that region and (more electrons = more shielding, so electrons are held less tightly) closer to the small cation and the electro...

Matthew Chan 1B

We need to draw dipole moments for these bonds between the atoms, as there will inevitably be some pull of electrons one direction or the other between two atoms. Then, looking at the broader picture, we can eventually determine if the moments cancel each other out or overall point in a certain dire...

Matthew Chan 1B

Electrons don't actually stay in place like how they look in Lewis structures. They are not stationary. They are constantly moving around and thus, the resonance structures allow us to get a better picture of what the actual molecule looks like, since resonance depicts how the electrons may move aro...

Matthew Chan 1B

You should put the more electronegative element in the center, and then try and minimize formal charges of that atom and the atoms that surround it.

Matthew Chan 1B

Dipole moments arise from differences in electronegativity in a covalent bond between two atoms. The arrow of the dipole moment is generally pointed towards the element that is more electronegative. They represent the direction in which the electrons are pulled toward, given the differences in elect...

Matthew Chan 1B

Can someone explain why a bigger electron density region means that there will be a longer bond?

Matthew Chan 1B

On the periodic table that we got for test 1, we didn't get electronegativity values. However, if we're just comparing values, we have to know the trends of electronegativity across the periodic table. The trend is that electronegativity increases from left to right, and increases from bottom to top...

Matthew Chan 1B

Do we have to know about sigma and pi bonds for the upcoming midterm? I don't remember anything being said in class, but I see this topic thread, so I'm not sure now. Does anyone know? Thanks.

Matthew Chan 1B

You would draw the lewis structure of each ion and draw them next to each other.

Matthew Chan 1B

My TA told me whenever drawing Lewis structures, we should always try and calculate formal charges. We should do this to make sure we're drawing the most accurate structure that we can.

Matthew Chan 1B

Ionization Energy: the energy required to remove an electron from a gas-phase atom

Electron Affinity: the energy released when an electron is added to a gas-phase atom

Matthew Chan 1B

I don't think that it was explicitly mentioned in the outline for the Quantum World unit, and I never heard him state it in class.

I could be wrong, though.

Matthew Chan 1B

Chromium: [Ar]3d 5 4s 1 Takes the electron from the 4s subshell and adds it to the 3d subshell because 3d will be more stable with that fifth electron, since it will now be half filled. Copper: [Ar]3d 10 4s 1 Takes the electron from the 4s subshell and adds it to the 3d subshell because 3d will be m...

Matthew Chan 1B

The second ionization energy is higher than the first ionization energy because, in short, it takes more energy to remove an electron from a positively-charged atom (cation) than it does to remove an electron from a neutral atom. When an electron is removed, that means there's more protons than elec...

Matthew Chan 1B

Effective nuclear charge means the amount of force that the electrons in the valence shell experience. And so, as the effective nuclear charge increases, that means the electrons are being held on to more tightly, which means that it will require more energy to remove the electrons from the valence ...

Matthew Chan 1B

Subshells, aka the angular momentum quantum number, are represented by the letter l . We calculate l by taking our principle quantum number n , and subtracting one from it. l = n -1. Then, to calculate how many orbitals are in the subshell, we take the value of l and all numbers equal to and in betw...

Matthew Chan 1B

I think that it's something we just have to accept as fact. It's a pretty complicated mathematical equation, but basically it's said that the probability of obtaining any possible measurement outcome is equal to the square of the corresponding amplitude. Not sure, but hopefully this helps. Also, Tif...

Matthew Chan 1B

There is an error in the solution manual. Check Dr. Lavelle's solution manual errors PDF on his website; he addresses that mistake.

https://lavelle.chem.ucla.edu/wp-conten ... rs_7Ed.pdf

Matthew Chan 1B

$\psi$ : tells us what the height of the wave at a certain position (x,y,z) is. It basically tells us something about the wave of an electron given a certain position
$\psi ^{2}$ : tells us the probability of finding an electron within a certain space.

Matthew Chan 1B

Higher numbered shells (like n=2 to n=1) have higher energy because of the force that it takes for the nucleus to hold onto the electron. When the electron is closer, it takes less energy to keep the electron. However, as the electrons are further out/further away from the nucleus, it takes more ene...

Matthew Chan 1B

I was wondering if ionization energy and threshold energy are related. Are they the same? Or is there a difference? The textbook gives a definition that seems pretty similar to what threshold energy is. Thanks in advance for clarifying.

Matthew Chan 1B

Yes, different elements will hold onto their electrons with different amounts of strength. Thus, we can say that the threshold energy does indeed vary between elements.

Matthew Chan 1B

A shorter wavelength means that the energy of the radiation increases. Fluctuation in the wavelength of the radiation affects the energy. Intensity is not affected by changing the frequency/wavelength. You can think of the intensity as the amplitude of the wave. Hope this helps.

Matthew Chan 1B

Non-metals will not exhibit the photoelectric effect due to the fact that they do not have any 'spare' electrons in their outer shells. However, metals do contain these 'spare' electrons. These electrons are the ones that can be dislodged/removed by a photon of light that has sufficient energy to do...

Matthew Chan 1B

While I was working through this problem, I wasn't sure how I was supposed to already know that n 1 =1. The problem reads: In the ultraviolet spectrum of atomic hydrogen, a line is observed at 102.6 nm. Determine the values of n for the initial and final energy levels of the electron during the emis...

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