CHEMISTRY COMMUNITY

Emmaraf 1K

The questions asks which of Mn2+, Al3+, Ni2+, or Au3+ will be reduced in the presence of water when electrolyzed with water at pH 7. Reduction potential of water is -.42V at pH 7. Why is water reduced with the first 2 and why is the metal reduced for the last 2. I thought in electrolysis reactions, ...

Emmaraf 1K

The textbook acts about the process of rusting 7th edition question 6N.21 and I wanted to know what the official steps are and why the formula for rust is Fe2O3 H2O and how both oxygen gas and water act as the oxidizing agents

Emmaraf 1K

What are the units for the frequency factor, A?

Emmaraf 1K

I wanted to clarify since it's not entirely clear to me about question 11:
We wouldn't know whether to use the Cv or Cp value unless we were given the V-T or P-T diagram and saw whether volume or pressure was held constant, right?

Emmaraf 1K

It doesn't really make sense to talk about the molecularity of the overall reaction. It only makes sense to talk about the molecularity of the elementary steps. Molecularity refers to how many and which molecules must collide with each other for the reaction to be carried out at each step.

Emmaraf 1K

This statement is true for all equilibrium reactions.

Emmaraf 1K

If there is no solid metal in the anode or the cathode to carry a current, then you need to put Pt(s) in the cell diagram. Put it on the side lacking the solid metal (could be the cathode, anode, or both)

Emmaraf 1K

It's not necessarily the number of reactants but the number of molecules involved in the collision during each step of the reaction mechanism.

Emmaraf 1K

I know that last quarter it didn't matter how many posts you did per week but you just had to do all 30 posts by the end of the quarter for full points. Is it the same for this quarter or did it change to only being counted by the week? Are the posts counted by the TA or by the computer system?

Emmaraf 1K

Generally spontaneity is a measure of whether Delta G is positive or negative and is dependent on both S and H. Therefore, you need both factors to determine spontaneity. However, a negative H tends to lend itself to a negative G aka a .spontaneous reaction

Emmaraf 1K

Generally spontaneity is a measure of whether Delta G is positive or negative and is dependent on both S and H. Therefore, you need both factors to determine spontaneity. However, a negative H tends to lend itself to a negative G aka a .spontaneous reaction

Emmaraf 1K

For a reaction to occur, the correct reactants must collide with each other with the correct orientation. If they collide with the incorrect orientation, the reaction won't occur. Additionally, the collision must collide with enough energy for the bonds that need to break to break for the reaction t...

Emmaraf 1K

Basically during electrolysis, one species is always reduced and another is always oxidized but in contrast to a galvanic cell, Ecell is negative, meaning it requires energy to power the reactions to happen. Therefore, when looking at the reduction potential of water and the reduction potential of a...

Emmaraf 1K

For part B, you must find Ecell for a concentration cell where at the anode and cathode is H2--> H+ and H+ -->H2 so when you balance them there should be the transfer of 2 electrons. However, when the question is solved, n=1. Why is this?

Emmaraf 1K

I personally prefer to go to the workshop sessions that are 2 hours long since they tend to go over all the material of the past week very in depth and you get to do plenty of practice problems which are similar to the tests. However, if you go to Lyndon's Workshop 7-9 on Mondays you should get ther...

Emmaraf 1K

If there is no solid metal at either the cathode or the anode to pass an electric current, Pt must be added. If there is no solid on the cathode side, Pt is added on the right and if there is no solid on the anode then Pt is added to the left. If there is no solid on either side then Pt is added to ...

Emmaraf 1K

We learned this equation in class but to calculate K when not at 25 degrees C yet it's not on our formula sheet. Will it be on the test and if so how would it show up/would we have to memorize it?

Emmaraf 1K

The oxidizing agent is the element which really wants to be reduced. Therefore, it is the one with the highest positive standard potential.

Emmaraf 1K

There is no solid metal to generate the electric current on that side of the voltaic cell so platinum metal must be present for the half reaction to occur.

Emmaraf 1K

Correct

Emmaraf 1K

If you have more oxygen atoms on one side, add enough H2O to the other side so that both sides have the same number of oxygen atoms. Then, you will have a disproportionate number of H atoms on the side you added H2O to so add enough H+ to the other side of the reaction so that there are the same num...

Emmaraf 1K

An extensive property is a value that changes depending on the amount of something (grams, moles, etc) whereas an intensive property doesn't change depending on amount. Heat capacity is extensive since it's units are only J/K whereas specific heat capacity is specific to a single gram of something (...

Emmaraf 1K

Degeneracy never equals 0 since having a degeneracy equal to 0 is like saying something doesn't have a single state which it can occupy aka it doesn't exist. The question on the midterm I believe you are thinking about had a degeneracy=1 so entropy=0 because S=klnW and lnW = 0. The molecule could on...

Emmaraf 1K

Can you post the explanation to 14. I feel like you need the enthalpy of CH4? Would you calculate it by finding the change in entropy associated with the loss of CH4 and O2 by calculating their entropies/mol (step 1), then finding the change in entropy as the reaction proceeds by doing delta H of th...

Emmaraf 1K

Jim Brown 14B Lec1 wrote:
Jim Brown 14B Lec1 wrote:I think you need the bond enthalpy for ethane, then use it to heat water to 100º C and to reach its ∆Hvap

Enthalpy of combustion*

Okay that makes 10x more sense . Thanks!

Emmaraf 1K

I am completely lost on #12. How would you even start?

Emmaraf 1K

For part b, you are asked to find S of surroundings, system, and total for an irreversible isothermal expansion. In the solution, it says w=0 since it's a free expansion (what does this mean?) and that delta U=0 (but why if it is irreversible?). It later says delta S surroundings = 0 and total entro...

Emmaraf 1K

A higher heat capacity means a substance can better withstand and input of heat and will not increase its temperature as easily. Hence, a higher heat capacity means a less steep slope.

Emmaraf 1K

Any element in its most stable state has an enthalpy of formation of 0. If you look at a periodic table which has states of matter labelled, the element in that state of matter has an enthalpy of formation of 0 except for diatomics which must have 2 atoms present at their most stable state of matter...

Emmaraf 1K

I believe you are talking about calculating entropy of say the vaporization of water at 298K. We don't know any equation for the entropy of the vaporization of water at 298 degrees, we only know it at water's boiling point 373K. However, enthalpy is a state function so if we can undergo a bunch of p...

Emmaraf 1K

Tbh I think some of the textbook questions forgot to include number of moles. That phrase is used to denote to use the 3/2 R and 5/2 R values for C in the entropy equations.

Emmaraf 1K

In irreversible expansion, the outer pressure will suddenly drop from say 2 to 1 ATM and so the piston will constantly be pushing against 1 ATM of pressure. In reversible expansion, the pressure outside slowly drops bit by bit from 2 to 1.9 to 1.8 to 1.7 (not actually by .1 but by an infitesimally s...

Emmaraf 1K

For q=mcdeltaT, use Cv under constant volume and Cp under constant pressure.

Emmaraf 1K

Be aware of the heat/temperature curve for water. A lot of the problems we have done have had ice at zero degrees so it will just start to melt when heat is added since zero is the melting point. If we start at less than zero degrees, say -3, then to get to the melting point we need to raise the tem...

Emmaraf 1K

If volume does not change then work is 0

Emmaraf 1K

Yes entropy changes. I believe the equation you gave is for isothermal expansion so temperature is constant over the course of the process. However in the equation S=q/T it is evident that temperature does matter.

Emmaraf 1K

Temperature is supposed to be given for #6 I believe.
EDIT: Okay the review system I went to did end up giving us the temperature but you can calculate it n your own using PV=nRT where pressure and volume is the pressure and volume of the system after the isochoric step so V=10l and P=10atm

Emmaraf 1K

It would be nonspontaneous since entropy is negative and Gibbs free energy is also positive since deltaG=deltaH-TdeltaS

Emmaraf 1K

Enthalpy is heat at a constant pressure. When pressure changes, the amount of final heat within a system is dependent on how the pressure has changed so pathway does matter in this scenario.

Emmaraf 1K

When stuff is happening from the outside on the system, work is positive. When the working is doing stuff on its own accord, work is negative. It is similar to how when there is an input of heat from the outside, heat is positive and when heat is released by the system, q is negative.

Emmaraf 1K

The volumes are normally given to you. Also, sometimes the question will say that the system expands by x amount in which case you add the initial volume and x to get the secondary volume.

Emmaraf 1K

Water contains hydrogen bonds which require a lot of heat in order to break so its temperature doesn't rise very easily because most of the energy goes into just trying to break these hydrogen bonds. Therefore, water has a high boiling point because it is hard to generate the heat necessary to overc...

Emmaraf 1K

At what concentration of equilibrium hydronium ions must we consider the autoprotolysis of water as being an important factor in pH calculation. Is it 10^-6 or 10^-7?

Emmaraf 1K

Yes. Although if you are referring to a certain problem it seems odd to me they would give you volume with partial pressures unless there was a second part to the question but for your purposes you are right.

Emmaraf 1K

Yes, changing the volume is like compressing/decompressing the container which the reaction is taking place so pressure and volume have an inverse relationship. A decrease in volume is an increase in pressure and an increase in volume is a decrease in pressure. Therefore a volume decrease/pressure i...

Emmaraf 1K

Given the equilibrium equation 2O3 <--> 3O2, why will the Partial pressure O2/Partial pressure O3 value change if you start with a different concentration of O3?

Emmaraf 1K

You can always use either but make sure you use Kp for partial pressure and Kc for concentration.

Emmaraf 1K

Emmaraf 1K

Basically what was meant in lecture is that when you get the hydronium ion concentration is less than 10^-7, this value is really representing the change in hydronium ion concentration (think "x" in the ICE table) from the initial hydronium ion concentration, which was 10^-7. Therefore, si...

Emmaraf 1K

I emailed my TA about some of the issues with negative pH in the 7th edition textbook and she agreed that it looked odd and would ask Dr. Lavelle about it. I think the solutions manual is just wrong in a few cases.

Emmaraf 1K

Correct.

Emmaraf 1K

I'm going to email my TA about this tomorrow but I think some questions in the 7th edition are incorrect in the cases where the pH/pOH aren't in the range of 0 to 14 since there are a few problems in the textbook (I remember one of them was a table where H+ and OH- concentration were 1.5M) where the...

Emmaraf 1K

Looking at the table, you have to divide the reaction into two parts so imagine the reaction happening in two steps to get to the final equation. The first step will have a K value as will the second step. Therefore, to find the final K value, multiply the Ks from the first and second step to get th...

Emmaraf 1K

Yes, you would just put 1 (but in the numerator since the products are in the numerator and the reactants are in the denominator)

Emmaraf 1K

It is my understanding that adding in an inert gas doesn't affect equilibrium/create a shift because it will behave as an ideal gas meaning it won't have a volume. If volume isn't changing, then the concentration isn't changing so the equilibrium stays where it is. However, what happens if say a liq...

Emmaraf 1K

I don't believe adding an inert gas changes the equilibrium regardless of whether it is an open or closed system. Assuming the inert gas behaves as an ideal gas, the gas is said to take up no space aka that it has no volume. Therefore, if volume isn't changing then the concentration isn't changing i...

Emmaraf 1K

I think you might be looking back at the problem in the wrong order from when we took the notes. It was given that the dead muscle tissue has an ATP concentration at equilibrium 7.214x10^-11 but that originally it was 8.435x10^-3. We didn't solve for the 7.214x10^-11 value since it was give to us. W...

Emmaraf 1K

NH4I(s) --> NH3(g) + HI(g)
I clearly see that if the NH4I was separate as ions the NH4 gives a proton/hydrogen to the I but they don't exist as separate ions and isn't that necessary since otherwise why does the textbook give a hint to write the net ionic equation?

Emmaraf 1K

How do you determine which is the strong base out of BrO- and C17H19O3N? Did the solutions manual just use the list of pKa values in the textbook?

Emmaraf 1K

How do you know Bi2O3 is an amphoteric compound? Do you just assume because Bi is close to the metalloids even though it isn't one itself?

Emmaraf 1K

Why does letter b showcase a reaction between a Bronsted acid and a base? The hint says to give a net ionic equation so you can clearly identify the reactants and where the movement of the proton is but in the question, NH4I won't be separated in the net ionic equation since it's a solid. Therefore,...

Emmaraf 1K

How do you determine the oxidation numbers for Pt and Cr? What does the "en" and "edta" mean?

Emmaraf 1K

Why do the answers for b and d not include "ate" at the end of the transition metal so why isn't it "cobaltate"? The solution manual for some of them includes the "ate" like for letters a and c and then all of them include "ion" at the end. Why is this?

Emmaraf 1K

Yes, if the cation is a conjugate acid of a strong base and the anion of the salt is a conjugate acid of a strong acid then the salt will have no effect on pH

Emmaraf 1K

Without seeing reactions which show a compound as and acid and then a base, is there any way to tell an amphoteric compound just by looking at its Lewis structure? Is there a general way to tell? Also since the metalloids are generally amphoteric is there a list we should know?

Emmaraf 1K

Since conjugate bases of weak acids form basic solutions and conjugate acids of weak bases form acidic solutions does anyone have just a clean list of strong acids and bases to memorize where we just can assume if the acid/base isn't on the list then it's weak? I believe for strong bases it's those ...

Emmaraf 1K

A polyprotic acid is one which can break off a hydrogen ion more than once so if an acid contains more than one hydrogen atoms then it is polyprotic such as H2SO4 or H3PO4

Emmaraf 1K

Could someone go through the specific steps for solving 2.45 on the 7th Ed. How do you determine whether hybrid orbitals or unhybridized orbitals are used in the sigma or pi bonds and how do you determine the lone pairs?

Emmaraf 1K

You can only figure out VSEPR structure by knowing the hybridization type since hybridizations just tell you the numbers of regions of electron density. d2sp3 is octahedral, dsp3 is trigonal bipyramidal, sp3 is tetrahedral, sp2 is trigonal planar, and sp is linear. To get molecular geometry, you nee...

Emmaraf 1K

The negative sign indicates attraction and all these interactions are attractive.

Emmaraf 1K

If I recall correctly, the HCN has a triple bond between the C and N while the other molecule only has a single bond. Single bonds are longer/weaker than triple bonds.

Emmaraf 1K

Emmaraf 1K

Larger hydrocarbons have more electrons which means greater polarizability which means greater attractive forces/stronger bonds between molecules. Stronger bonds/greater attractive forces between molecules results in the molecules forming a solid because solids are more tightly held together.

Emmaraf 1K

I get that larger molecules have more electrons, but why does that cause greater polarizability? Is it because the more electrons there are, the more chances there are for electrons to become unequally distributed to create a partial negative and partial positive charge within the molecule?

Emmaraf 1K

The answer is a combination of the other two posts. For one, P has a larger atomic radius, thereby making the bond longer. Secondly, N has a higher electronegativity meaning it pull electrons in the bond closer to it thereby making the bond shorter.

Emmaraf 1K

Bond length isn't related to the number of bonds in the molecule. The bonds within SO2 and SO3 are all double bonds so the bond strength/bond length is considered equal. You can think of the bond lengths as averages of the lengths of all the bonds in the molecule and so given the length of an S=O bo...

Emmaraf 1K

For question 2A15, the anticipated ion formed by Cadmium is a +2 which I understand because it has 2 electron in its 5s orbital which it wants to get rid of but what about the exceptions to electron configurations in the transition metals like copper and chromium (since while we don't need to know t...

Emmaraf 1K

But @Sara Lakamsani 4C why do you include the 3d orbitals in the number of valence electrons? Doesn't valence electrons only come from the outermost shell which would be when n=4 for Mn? I understand the correct answer is 7 but why do you count the 2 4s electrons and the 5 3d electrons when counting...

Emmaraf 1K

I think another rule to follow is if formal charges are all else equal (in terms of magnitude), then the correct structure should have the negative formal charges on the atoms with the largest electron affinities.

Emmaraf 1K

Since the atoms which can have an expanded octet do so by accessing their d orbitals, does that mean some atoms could form 9 bonds (since 1 s orbital, 3 p orbitals, and 5 d orbitals? Then, for some atoms which can access their f orbitals, could they even form 16 bonds (1 s orbital, 3 p orbitals, 5 d...

Emmaraf 1K

Lewis dot structures are inaccurate because electrons don't actually just stay stagnant in bonds but are found throughout the molecule and can move/have momentum. Therefore, the electron density at different points in time is constantly changing but on average, there is an equal electron density at ...

Emmaraf 1K

Another easy way to calculate formal charge is to just do Valence electrons - # of bonds - number unpaired electrons = formal charge since then you don't have to divide by 2 when counting# bonded electrons. It speeds up the calculation and makes you less prone to making mistakes in my opinion.

Emmaraf 1K

The electron shielding doesn't outweigh the increase in nuclear charge when moving left to right on the periodic table. Therefore, since the electrons are pulled stronger into the nucleus, it's harder to remove and electron so ionization energy is greater. The added electron shielding going from Cs ...

Emmaraf 1K

Look at the nuclear charge of the isoelectric anions/cations/atoms. The higher the nuclear charge, the smaller the atom/cation/anion will be because the electrons will have a stronger pull inward thereby making the radius smaller.

Emmaraf 1K

You put the subshells in order of their principal quantum number and then their angular momentum quantum number. Going from the s to the d subshells, for example the 4s and 3d, the 4s subshell becomes a higher energy when the 3d starts to be filled which is why the 4s should be written afterwards wh...

Emmaraf 1K

Are you referring to a specific question? Looking at a chart online for ionization energies, it says the ionization energy for sulfur is less than oxygen.

Emmaraf 1K

Do we know the reason why the 4s subshell is a lower energy than the 3d UNTIL the 3d subshell starts to be filled. What causes the switch in energy to occur?

Emmaraf 1K

Response to Lily Smith on electron spin: In addition to the other commenter, electron spin is more of a concept to help us understand the nature of electrons and why it's possible to have two electrons in a single orbital. Technically, electrons don't exactly "spin" in the traditional sens...

Emmaraf 1K

When writing electron configurations for atoms in their ground state, why do we always start with electrons having an up spin when filling a subshell? If the ground state just denotes the lowest energy state of the atom, why does it matter whether we fill one orbital with up spin and then the next o...

Emmaraf 1K

There are certain exceptions to those trends for electron affinity and ionization energy. Carbon and nitrogen is one of those exceptions for electron affinity (as is all of group 4 vs group 5 elements). The exception arises because nitrogen has a half filled p subshell whereas carbon's p subshell on...

Emmaraf 1K

Particle in a Box questions give us the right mindset to apply the Schrodinger's Equation since we can think of electrons as being the particles inside the box which is a molecule. We use the box concept to denote that that the particle can't be found outside the box (which is why the PE inside the ...

Emmaraf 1K

It is believed that the half filled d subshell and half filled s subshell is a more stable configuration than when the s is completely filled but the d shell only has 4 electrons since having a half filled subshell increases stability.

Emmaraf 1K

We don't need to know anything past the 3d subshell for the test but after looking information about Tungsten up, it oddly doesn't follow the half-filled d subshell exception like chromium so you do the electron configuration normally, including the lanthanides which represent the 4f orbitals.Theref...

Emmaraf 1K

Various experiments displayed the wave-particle duality of electrons. The photoelectric effect showed how electrons acted as particles because when a photon with a certain threshold frequency was absorbed by an electron, the electron was shot out like a particle (kind of like billiards). In contrast...

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