CHEMISTRY COMMUNITY

Sohini Halder 1G

I think it would never hurt to know the steps to derive them, but I feel like any test questions will focus on application, not derivation.

Sohini Halder 1G

Intermediates are FORMED and then CONSUMED within the steps of elementary reactions.
Catalysts are present from the beginning and are not consumed in the reaction; this is why only a little bit of catalyst is needed to increase the rate of a specific reaction.

Sohini Halder 1G

At lower temperatures, there is usually not enough energy to overcome an activation energy barrier, so the pathway that has the smaller activation energy will be the faster reaction and the one that is observed. This is kinetically determined. This is also different than reactions that are spontaneo...

Sohini Halder 1G

You dont need to. You know how much is changing in how many seconds; this gives you the rate.

Sohini Halder 1G

Dr. Lavelle gave an example in class that really helped me understand this. He said that if a person has a million dogs, and gives a few away, that person will still have about a million dogs. The number doesn't realllllyyy change. Similarly, for a pseudo-first order reaction, one of the other react...

Sohini Halder 1G

If I remember what Dr. Lavelle said in lecture correctly, he mentioned that they would give us experimental data or there would be a situation where only one concentration would be feasibly changing over time because the other reactants would be at a much higher concentration.

Sohini Halder 1G

Say A is your reactant. As the reaction progresses, A decreases (if first order, it decreases exponentially). Therefore, the rate is the negative change of the concentration of A over time, given by -d[A]/dt. This is equivalent to saying that the rate of the reaction is proportional (with a proporti...

Sohini Halder 1G

I am confused about 15.19. When I find k, I get 2.85 x 10-^12 mol/Ls. The answer key gives the same value but as mmol/Ls. This is wrong right? We have to convert the mmol to mol to get a power of 10^-12, but why is the units for the answer key still mmol/Ls?

Sohini Halder 1G

mathematically it makes sense because as you do anything to the concentration, nothing changes the rate of the rxn, or 0 change.

Sohini Halder 1G

Are you sharing your electrons with me because I feel a real deep bond with you.

Sohini Halder 1G

As of right now, we only know about energy barriers causing this disparity between thermodynamically and kinetically favorable reactions. I'm assuming that once we know more about the actual mechanisms that go into the reactions, we can find out this.

Sohini Halder 1G

"A voltaic cell functions only when the electrical circuit is complete. In the external circuit the current is carried by the fl ow of electrons through a metal wire. Explain how the current is carried through the cell itself." Can someone please explain this to me? The solution manual say...

Sohini Halder 1G

^^, and if you would like to have H3O+ in your final balanced reaction, do the process using only H+ first, then change them to H3O+ and account for a change in O accordingly.

Sohini Halder 1G

Maybe it's because the voltage created by concentration cells can light a bulb, hence the electrical work?

Sohini Halder 1G

No, Sn2+ is reduced to Sn4+. You need to flip whatever reduction reaction you get for the anode side to make it an oxidation reaction. You may have gotten confused because Sn4+ is listed before Sn2+ in the cell diagram, but order does not matter there, as long as it is on the left side of the double...

Sohini Halder 1G

I still don't understand this setup. Do we have to assume that Au(s) is present in both the reduction and oxidation reactions because it is the conductive metal electrode in the solution? And then knowing this, is this why Au(s) goes to both Au 1+ and Au 3+?

Sohini Halder 1G

If Delta G for formation of a product A is negative, we know that the forward process is spontaneous and will "end up" as product A, meaning that product A is stable. In the same vein, lets assume the Delta G for formation of product B is positive. The forward process that forms product B ...

Sohini Halder 1G

I'm not totally certain, but I think that if it isn't explicitly given, we may just have to derive it from DeltaG = -RTlnK

Sohini Halder 1G

When will the link be posted? Also, will the solutions that we work through during the session or an answer key be posted? Thank you!

Sohini Halder 1G

The system starts at equilibrium (think Dr. Lavelle's example where both pressures are 1 atm) but an infinitesimally small change happens which starts the reversible process.

Sohini Halder 1G

I am also very confused about this. Will we be told on the exam or do we have to draw and figure it out?

Sohini Halder 1G

yep! So for instance, if heat is being released from System into Surroundings, System q will be negative and Surroundings q will be positive because the surroundings are absorbing heat. Further, System T should be higher than Surroundings T. These differing values will give you a different change in...

Sohini Halder 1G

As you increase volume or mix one substance with another, the molecules of substance X are spread out more, increasing their positional disorder.
As you increase temperature, the thermal motion of the molecules increases.

Sohini Halder 1G

Gibbs free energy also shows how much energy is "Free" to do nonexpansion work, whereas entropy relates to the possible number of states a system could be in.

Sohini Halder 1G

I think we should think about entropy as corresponding to the possible number of states that could exist, which comes from the equation S = k ln W. As volume increases, the number of possible states a gas could be in increases, so does entropy of the system.

Sohini Halder 1G

To calculate the total entropy change, you need to first find the entropy change for each reservoir: one is giving off heat, and one is absorbing heat. 400kj should therefore have two signs because q=(-) when the hotter reservoir is giving off heat and q=(+) when the cooler reservoir is absorbing he...

Sohini Halder 1G

I think that the lower DeltaH given when finding benzene with resonance shows a lowering of energy, so formation of benzene with resonance is more stable. 228kj/mol vs -1kj/mol

Sohini Halder 1G

If you are finding the standard enthalpy reaction of 1/2 N2(g) + 1/2 02(g) ==> NO(g), that will be the same as finding the enthalpy of formation of NO(g). Only when your reactants are in gas form and and pure substances that form 1 mole of a product are they the same.

Sohini Halder 1G

Dr. Lavelle mentioned that the change in internal energy of a isolated system is 0, because neither mass or energy can leave the system and therefore Delta U is = 0.

Sohini Halder 1G

C) Benzene without resonance means that there are 3 single bonds and 3 double bonds in the Lewis structure you will use for reference. It helps if you draw out the structure so you can visually see which bonds are being formed! 6 C(s) + 3 H2(g) --> C6H6(l) Breaking: 3 H-H bonds, using energy Forming...

Sohini Halder 1G

It is exchanging both matter (ex. CO2, O2) AND energy (ex. heat), so it is an open system.

Sohini Halder 1G

I got the same answer as you. For those who need help solving the problem, here is my work: Volume of water as liquid: 100 g / (1g/cm^3) = 100 cm^3 Volume of water as solid: 100 g / (0.92g/cm^3) = 108.696 cm^3 Change in volume: 8.696 cm^3 Converted to Liters: 0.008696 L External pressure: 1070 atm w...

Sohini Halder 1G

I think the extra kinetic energy allows the liquid molecules to flow past one another more easily than molecules of solids.

Sohini Halder 1G

Essentially, -q(copper)=q(water), which is saying that the heat given off by the copper is the heat absorbed by water. You have to use the q=m(delta)T equation to solve given the information you have. I set the equations to each other and solve.

Sohini Halder 1G

In this case, you can ignore the moles of reactant and product because those do not matter in the equation for finding q.

Sohini Halder 1G

Oxygen is electrognegative so the more there are, the stronger they pull the electrons away from the O-H bond, weakening it and making the overall acid stronger.

Sohini Halder 1G

I also think it is important to differentiate between Lewis, Bronsted, and Arrhenius definitions of acids and bases. Arrhenius definition states that a compound that dissociates in water to form hydronium ions is an acid, so a compound like HCL is an Arrhenius acid as well as a Bronsted acid and a L...

Sohini Halder 1G

Balance the equation. The coefficients change how you raise the numbers to a power. You are likely messing up there.

Sohini Halder 1G

I believe it is just a matter of memorization, unfortunately :( This confused me too, but I think generally, most follow the trend of the metal name followed by "-ate", such as "cobaltate". Iron (ferrate) is an exception we will just need to know.

Sohini Halder 1G

The question will always tell you. However, if the enthalpy value given is negative, it is exothermic, meaning the rxn gives off heat, and if the enthalpy value given is positive, the rxn requires heat.

Sohini Halder 1G

Myoglobin has a heme complex + protein with Fe as the metal. Hemoglobin has 4 myoglobin molecules.

Sohini Halder 1G

I believe that the octahedral shape is the maximum number of ligands that can be added. If there are more it may be unstable.

Sohini Halder 1G

I believe that when there is a compound and a dot(multiplication) sign next to it, we call it a hydrate. Otherwise water is a ligand.

Sohini Halder 1G

Add up the subscripts on AXE and you will know how many regions of electron density there are. The subscript below X tells you how many atoms are attached to the central atom. When naming the molecule's shape, you need to know both in order to discern how the lone pairs, if there are any, affect the...

Sohini Halder 1G

We write it the first way when asked for the ground state configuration of just the carbon atom itself. However, you are right, in most cases carbon forms 4 bonds and when it is in these molecules, one electron is promoted to the 2p orbital, leaving 4 unpaired electrons to form 4 bonds. Since the bo...

Sohini Halder 1G

Look for the more electronegative element in the molecule. This element will have the electrons around them most of the time. For example: H2O Because oxygen is more electronegative than hydrogen, the dipole moment will go towards oxygen. That is why oxygen-side of H2O has a slightly negative charge...

Sohini Halder 1G

Hi! This confused me at first too, as noble gases do not usually form compounds. However, Dr. Lavelle states that because Xenon is an element with multiple shells and valence electrons that are quite shielded from the nucleus's pulling power, they can be shared. You might notice that we have only se...

Sohini Halder 1G

Ionization energy and electron affinity are definitely related! Ionization energy is the energy to remove an electron, or ionize it, from an atom. Ionization energy increases left to right across a period, which makes sense because as the valence shell fills in the atom moving left to right, the ele...

Sohini Halder 1G

Hi! I've been doing relatively well in the class so far and here are the study tips I use: 1) read the chapter AND take notes before you attend the lecture on the same material -this allows me to have at least a shallow understanding of the material that Lavelle will cover in class -taking notes fro...

Sohini Halder 1G

It is because of the shielding effect. The electrons with the highest energy, and therefore least bound to the nucleus, will be removed first. That's why we write the d block elements first, to represent that the last electrons are the ones that will be removed.

Sohini Halder 1G

Fluorine is the most electronegative element, therefore its electron affinity is high because it "desperately needs" one more electron to fill its valence shell. Similarly, because it "desperately needs" an electron, it will be very difficult to remove an electron from it, causin...

Sohini Halder 1G

It needs a discrete amount. That's why only specific wavelengths are absorbed/emitted, because those exact frequencies give energies that move the electron from one level to the next. Each atom has a unique absorption and emission spectrum.

Sohini Halder 1G

It helps to think about what kind of wavelength each activity has. popcorn - microwave Reading - visible light Suntan - UV dental - x ray You can see the energy increase as you move down the list, the frequency increase, and the wavelength decrease. Using this, you can reason if your calculated answ...

Sohini Halder 1G

Set up the problem using J because that is the standard unit for energy, but you can either leave your answer in J or convert to kJ if youu want.

Sohini Halder 1G

As Dr. Lavelle said in lecture, it always helps to write out the equation in words first. For example: Energy of incoming photon = Workfunction of metal (or energy needed to remove electron) + Kinetic energy of the electron emitted After this, you can rearrange whatever way you need to for the probl...

Sohini Halder 1G

Can someone explain why all of the oxygen in the products seems to come from the original compound and not the surrounding excess oxygen?

Sohini Halder 1G

Remember that when looking at sig figs for numbers that have decimals, the placement of the ZEROES is important! For example: 0.05 has 1 sig fig. 0.0500 has 3 sig figs. In decimals, zeroes placed after a digit in the decimal places are significant, while those placed before are not. An alternate way...

Sohini Halder 1G

Remember that when looking at sig figs for numbers that have decimals, the placement of the ZEROES is important! For example: 0.05 has 1 sig fig. 0.0500 has 3 sig figs. In decimals, zeroes placed after a digit in the decimal places are significant, while those placed before are not. An alternate way...

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