CHEMISTRY COMMUNITY

Kaihan_Danesh_2J

The more likely a species is to be reduced, the better it is as an oxidizing agent. The higher the reduction potential, the more likely something is to be reduced. Therefore, the higher the reduction potential, the better the oxidizing agent.

Kaihan_Danesh_2J

I believe you need to know the E naught value for hydrogen reduction under standard conditions. I think this is the only one we need to know is 0 besides E naught in the concentration cells.

Kaihan_Danesh_2J

You do not half the concentrations, you simply are able to solve the question by using half coefficients. So essentially the only thing that changes is the Q equation because of the different coefficients which change the power to which the concentrations are put. edit: I read your question wrong, t...

Kaihan_Danesh_2J

As E decreases, G decreases as can be seen in the relation change in G = -nFE.

Kaihan_Danesh_2J

You just set up the half reactions and balance them so that they have the same number of electrons on their opposing sides and the charges are balanced in each. Once you have done that, the coefficient for the electrons you get is the value for n.

Kaihan_Danesh_2J

Entropy's relationship to gibbs free energy is summarized by the equation change in g equals change in enthalpy minus temperature times change in entropy. If entropy is positive, then the reaction is spontaneous if the change in enhalpy is smaller than temperature times entropy. If entropy is negati...

Kaihan_Danesh_2J

There are six possible outcomes of the dice, so there are 6 microstates, and you put 6 to the power of whatever number of dice you have to calculate the degeneracy. Then, plug that value into the residual entropy equation, S=Kb * lnW

Kaihan_Danesh_2J

Yes, if volume is constant, then no work has been done and internal energy just equals q.

Kaihan_Danesh_2J

The pathway should not matter for entropy, change in S is always the same for both pathways, it is the change in S of the surroundings that is different and which allows for some of these processes to occur. Entropy is a state function, so whether the pathway is irreversible or reversible, the chang...

Kaihan_Danesh_2J

The irreversible pathway allows for the expansion of gas against a constant external pressure whereas a reversible process requires that the external pressure always match the internal pressure. For example, during a reversible expansion, the pressure external constantly is the same as pressure inte...

Kaihan_Danesh_2J

You do the same thing to calculate delta S and delta G as you did to calculate delta H. Since these are all state functions, they can be calculated by summing up the values of the products and reactants and subtracting them: products - reactants. If you look at the Chempendix chart they link the sta...

Kaihan_Danesh_2J

The way I think about it is this: if adding heat is required to cause the phase change, delta H must be positive and if cooling is required, then delta H must be negative. And delta S follows the same pattern, only the reason it does is because as substances like water change phases while heating up...

Kaihan_Danesh_2J

On our formula sheets, Cp values are the specific heat capacity. Heat capacity is the ratio of energy transferred to an object and its subsequent increase in temperature. Specific heat capacity is the amount of energy required to raise the temperature of one gram of a substance by 1 degree kelvin. S...

Kaihan_Danesh_2J

We study isolated systems to learn the basic principles of thermodynamics that we can later apply to open systems.

Kaihan_Danesh_2J

Both isochoric and isometric refer to a process in which the volume is held constant.

Kaihan_Danesh_2J

Basically, there are two different values for c for the same compound and you use one or the other depending on if volume or pressure is held constant in the system. If pressure is held constant, the system can still produce some sort of work since the volume can change, so the value for c when pres...

Kaihan_Danesh_2J

No, water does not affect that because it is a solvent, it is what makes those compounds aqueous. As in, the compounds are aqueous because they are in the water. Being a solvent, any change in H2O concentration is negligible and not the focus of these exercises.

Kaihan_Danesh_2J

The phase of the compounds do matter because often times heat is absorbed or released to break or form bonds when changing the compounds to their different phases in order to allow them to react. The heat from these processes must be considered when applying Hess's Law.

Kaihan_Danesh_2J

Path B is a 2 step process. First, the system is cooled at constant volume to 1.02 atm. Volume is held constant here, so this step does not produce any work. The second step, the heating to allow the volume to expand to 7.73L, does produce work as the volume is not held constant. To calculate the wo...

Kaihan_Danesh_2J

Because the volume is held constant, you do not need to factor in work to calculate the total internal energy, or delta U. In short, since volume is held constant, q = delta U

Kaihan_Danesh_2J

Weak Bases are bases that are not listed in the strong bases list and weak acids are acids not listed in the strong acids list. If you are referring to whether something is an acid or base specifically, then you can usually tell based on their 1.) lewis structure or 2.) their charge. In the case of ...

Kaihan_Danesh_2J

You can just memorize the strong acids and bases based on what Professor Lavelle told us (group1/2 hydroxides and oxides, etc.). But, if you are referring not to just strong acids and bases but relative acidity and basicity, then you have to review the topics from the previous quarter which include ...

Kaihan_Danesh_2J

Making x negligible is alright anytime K is 10^-4 or less. It does not matter the situation, if the K value is that small, you can make x negligible. But, it can only be negligible for the denominator because you must still solve for the x in the numerator of the K equation.

Kaihan_Danesh_2J

pKa is the acid dissociation constant and it determines the strength of an acid—lower pKa, stronger acid, higher pKa, weaker acid. pH, however, is only a measure of the amount of hydronium in a solution. pKa = [A-][H3O+]/[HA]. pH = -log([H3O+]. So, to convert between the two, you'll have to convert ...

Kaihan_Danesh_2J

First, you would divide the Kb value by 1*10^-14 to convert it to a Ka value because, now, you are dealing with the conjugate acid of nh3, nh4+. Then, you would set up the Ka equation for the reaction between nh4 and water to hydronium and nh3. Finally, if you use an ice table, you'll see that hydro...

Kaihan_Danesh_2J

I'm not sure if that equation is correct, but the way I did it was I got the equation (0.154-x)/x^2 = 0.0032. Then, I rearranged the equation so that it becomes a quadratic. It should look like this: 0.032x^2+x-0.154. Then, you just use the quadratic to get the x value. And you must use the most rea...

Kaihan_Danesh_2J

The way I think about it, if the pKa is smaller than the pH, then the acid in question can contribute to making the solution more acidic, so the species in which the H+ is lost is the predominant species. if the pKa is larger than pH, then the acid likely will not contribute to making the solution m...

Kaihan_Danesh_2J

Chemical equilibrium is the general term for equilibrium in a chemical reaction whereas acid equilibrium is chemical equilibrium only the reaction in question involves an acid in water.

Kaihan_Danesh_2J

To find the initial B concentration, you have to add the concentration of BH+ to B.

Kaihan_Danesh_2J

The way I see this relationship is as follows. The lower the pKa, the stronger the acid, the higher the pKa, the weaker the acid. If pH < pKa, the solution is "more acidic" than the acid, so the non charged species is the predominant species. If pH > pKa, the solution is "less acidic&...

Kaihan_Danesh_2J

It would not shift either way because a change in pressure not due to compression does not shift the equilibrium one way or another. Professor mentioned that, for example, increasing pressure by inserting an inert gas into a container would not affect the equilibrium of a reaction.

Kaihan_Danesh_2J

yes, the autoprotolysis of water occurs constantly, before and after any neutralization reaction and at the same rate, so you do not have to consider it in your calculations.

Kaihan_Danesh_2J

The sign of delta H only changes the effect of changes in temperature. When delta H is positive, the reaction is endothermic, then heating up the reaction will shift the equilibrium to the right and increase the value of K. When delta H is negative, the reaction is exothermic, then heating up the re...

Kaihan_Danesh_2J

the value of x is, I think, 0.478/10 and this is because CH4, in the ice table, increases by x and since, at equilibrium, there is .478/10 M CH4, x is .478/10. The question is the same as all the other ones we do only they give the number of moles for both reactants AND the concentration of the prod...

Kaihan_Danesh_2J

I don't think the professor would test us on equations that cannot be solved with the quadratic formula unless the equilibrium constant is low enough to the point where you can assume the x-value you subtract from the reactants in the denominator is 0.

Kaihan_Danesh_2J

If the forward reaction is endothermic and heat is added, then more products will form and K will go up. If that reaction is cooled, then the reactants will be favored and K will go down. If the forward reaction is exothermic and heat is removed(cooling), then more products will form and K will go u...

Kaihan_Danesh_2J

Yes, this is due to the fact that compression, decreasing the volume to increase pressure, causes a change in the concentration of reactants and products since concentration is equal to number of moles per liter(VOLUME), but if the volume does not change and you, say, insert an inert gas to change t...

Kaihan_Danesh_2J

Q is used when the reaction is no longer in equilibrium. For example, a change in pressure due to compression will result in the reaction shifting a certain way. For that instant where one way is favored, we use Q to determine which way is favored. And we do this by comparing Q to K because inevitab...

Kaihan_Danesh_2J

q is simply the constant "k" when the system is not in equilibrium, so q can be calculated at any time that is not equilibrium, meaning it represents an instant and not the system after time has passed. So, when pressure increases due to volume changes, the "q" is now necessary t...

Kaihan_Danesh_2J

Yes, you are right. Solids and liquids are not included in any of the equilibrium concentration or constant calculations.

Kaihan_Danesh_2J

On question 18 for sapling week 7/8 it says that the C-H bonds in both a molecule with two carbon chain and a molecule with a 4 carbon chain each are in the same plane while in a molecule with a 3 carbon chain, they are perpendicular to the axis on which the molecule lies. Why is this the case? The ...

Kaihan_Danesh_2J

Im sure you can derive them in certain ways, but I think the best way is to memorize the strong acids and bases because those are the only acids and bases we are studying in this class in terms of calculations.

Kaihan_Danesh_2J

Titration is a method of determining the concentration of OH- in a base if the basic solution's concentration is unknown or H30+ if the acidic solution's concentration is unknown. Essentially, during titration, one continually drops small amounts of an acid or base of known concentration into a solu...

Kaihan_Danesh_2J

Ionization is simply the acquisition of a charge and since acids tend to lose, for example, a hydrogen, the hydrogen becomes positively charged and the attached molecule becomes negatively charged. Since both these components of the previous compound acquired charges, they are ionized. This typicall...

Kaihan_Danesh_2J

On the sapling, they were considered bases so I think it is safe to say that we would consider group one and 2 hydroxides bases. Also, since the TAs are our grad student learning mentors, I would think that their insight is correct, especially because they go by Professor Lavelle's curriculum.

Kaihan_Danesh_2J

Ka stands for the acid ionization constant, which tells you the extent to which a weak acid will dissociate in water. Because strong acids usually completely dissociate or almost completely dissociate, a measure of the extent of their dissociation is not necessary and since we are only doing calcula...

Kaihan_Danesh_2J

Yes, in some sense it does. Strong acids are those that lose protons easily, so the lower the electronegativity difference between two atoms in a compound, the more likely a proton can be released from that compound. Also, the stability of an acid after losing a proton depends on the electronegativi...

Kaihan_Danesh_2J

Can an amphoteric compound be both a strong acid and strong base or is it just a compound with little acid/base character?

Kaihan_Danesh_2J

ClOH is a stronger acid than BrOH and IOH because ClO- is much more stable than BrO- and IO- because it has a strong negative dipole toward the Cl compared to the dipole toward the Br or I as the electronegativity difference between Cl and o is much greater than that between Br and O and I and O. Be...

Kaihan_Danesh_2J

I think the book just skipped the reasoning for why it has a lower pKb. Your reasoning I believe is correct and it is why BrO- has a lower pKb.

Kaihan_Danesh_2J

The dipole moment always points towards the more electronegative element. So, you can either just identify the more electronegative element or use the arrow to identify it.

Kaihan_Danesh_2J

https://images.app.goo.gl/7t5wCcM9Z5523UdbA

This chart was extremely helpful for visualizing molecular shapes and the reasons why they are the way they are in relation to their ‘base’ forms, which are those without any lone pairs.

Kaihan_Danesh_2J

All expanded octets must have hybridized orbitals because there are usually quite a number of bonds when an atom has an expanded octet meaning there must be enough lone electrons to create those bonds. Thus, the hybridized orbitals are necessary and often include the d orbital in the hybridization.

Kaihan_Danesh_2J

Resonance always implies electron delocalization because if the bonds of a molecule have an equal chance of being in different places in the molecule, which is the case with many resonance structures, then the electrons will just be delocalized as it is equally favorable to be in each of those places.

Kaihan_Danesh_2J

The polarity of the bond depends on the electronegativity difference between the two bound atoms. The polarity of the molecule depends on whether the dipole moments cancel out. Keep in mind that ‘cancel out’ means the dipole moments are exactly opposite of each other on the 3 dimensional representat...

Kaihan_Danesh_2J

I think that you would want the negative charge on the more electronegative element and you would also want like charges to be as far away from each other as possible.

Kaihan_Danesh_2J

I don't think they are meant to be seen that way. A better way to understand it, I believe, is that the behavior between electrons and light are similar because that is all the two models describe for each.

Kaihan_Danesh_2J

I think you just need to know the trends. The trends are especially useful for questions that ask, for example, whether NH3 has molar polar bonds or HF. HF would have molar polar bonds because fluorine is more electronegative than N and since H is the constant in each, the difference in electronegat...

Kaihan_Danesh_2J

It depends on the dipole moment of the molecules. If there is a molecule with a lone pair and a slightly negative charge on the atom with the lone pair, that atom will likely make a coordinate covalent bond with another molecule in which the central molecule has a slightly positive charge. So, it is...

Kaihan_Danesh_2J

The stronger the bond, the higher the melting point. Since ionic bonds are stronger than covalent bonds due to it being a bond between to atoms with opposite charges, the bond will be much stronger than a covalent bond, resulting in ionic compounds having higher melting points.

Kaihan_Danesh_2J

I believe the same question mentioned that the most optimal method was not using the oxidation numbers to predict the resonance structures.

Kaihan_Danesh_2J

How are we to know which resonance structure is most plausible using oxidation number?

Kaihan_Danesh_2J

I understand that there are atoms that can have an expanded octet, but why exactly? I know we were given an explanation but I still don't really understand why.

Kaihan_Danesh_2J

I just understood the question as asking whether the bonds had more double or triple bond character. If you look closely, you'll see that one says overwhelmingly, for example, C-single bond-N character and the complement to that says ample C-double bond character-N. You just take the bond lengths gi...

Kaihan_Danesh_2J

My bad, I did it in increasing order, just reverse the order from my previous reply.

Kaihan_Danesh_2J

N2, C4H8, C3H7OH, LiCl. Hydrogen bonds are not as strong as ionic bonds meaning the boiling point of a molecule with ionic bonding would have a higher boiling point.

Kaihan_Danesh_2J

I think that the test will include everything we need to be able to answer the question with just a formula sheet. By that I mean if the electronegativity difference between two atoms is not already clear by the distance between them on the periodic table, they will give us the electronegativity val...

Kaihan_Danesh_2J

So the formal charge formula we use is F.C. = V - (s/2+L). For carbon, there are 4 valence electrons, 4 shared electrons, and 4 lone electrons; for nitrogen, 5 valence, 8 shared, 0 lone; and for oxygen there are 6 valence, 6 shared, 2 lone. Just plugging those values in will give you the formal char...

Kaihan_Danesh_2J

For the sapling question about the resonance structures of perchlorate ions, how can we find out which lewis structure is most plausible using given bond lengths?

Kaihan_Danesh_2J

Some covalent character means that the electrons bonding a molecule together are partially shared, not fully. The amount of ‘covalent character’ is determined by the difference in electro negativities of the atoms. The lower the difference, the more covalent the character of the bond even if it’s io...

Kaihan_Danesh_2J

Helium is an exception to the rule and this is the case for many noble gases when it comes to trends. Because noble gases will not form molecules, He will never be in a situation where it would attract a pair of electrons.

Kaihan_Danesh_2J

The relieving of oxygen upon ionization is because the opposite spins in the 2px subshell results in more electron electron repulsion because the other two subshells, 2py and 2pz, have only one spin direction. Thus, the atom would be more stable if they all spun in the same direction with one electr...

Kaihan_Danesh_2J

Conceptually, this is like other questions we’ve done. The incident photon energy is the energy of the light that we would calculate in the photoelectric questions using E=hv and which you can find with wavelength. The energy of the electrons is the kinetic energy, but is given. So, what you’re calc...

Kaihan_Danesh_2J

I believe all the ones besides the basics (milli, kilo, cents, etc.) that we would need to know are on our formula sheet.

Kaihan_Danesh_2J

Hey, so as I understand it, there’s portions of the receiving wall which do not make contact with the electrons and it’s due to the constructive and destructive waves caused by the electrons, which is a wavelike property. Also, the electrons bend as they go through the edge of the wall, meaning it’s...

Kaihan_Danesh_2J

Do empirical formulas have to have whole numbers or can they be numbers like 1.5, 2.5, etc

Kaihan_Danesh_2J

For combustion analysis questions, when finding the mole ratios, do you guys round off when finding the number of moles for each element: c, h, and o.

Kaihan_Danesh_2J

Does anyone know when midterm scores will be released?

Kaihan_Danesh_2J

Hi I’m still confused on how to do the question where you’re given the energy level an electron is excited to and the color it emits and then are asked to find the energy released when the electron fell energy levels. How would you find the lower energy level it is excited to?

Kaihan_Danesh_2J

I don't believe we will have to derive the molecule's structure from name alone. On one question in the sapling, we had to know the molar mass of an organic molecule given by name and our TA said we will not have to derive the chemical structure to find the formula. Besides basic and common molecule...

Kaihan_Danesh_2J

It is not in the correct order because when I did them, they did not follow the order in which we learned the concepts.

Kaihan_Danesh_2J

I believe so because many of the spectroscopy questions require that the answer for wavelength be given in nanometers or some unit that makes the answer practical. For example, writing wavelength in meters would not be proper because wavelength is so small.

Kaihan_Danesh_2J

I would say because it is such a simple process that it is good to know how to do it. Just include the ions that are involved in precipitate/product formation.

Kaihan_Danesh_2J

Hey, I think most chemical reactions will be given and if they are not it is likely because it a simple balancing equations problem. We will likely need to know chlorine gas is Cl2 because that was part of the review I believe.

Kaihan_Danesh_2J

I think the best thing to do is ensure that the number of sig figs is considered at the very last calculation. That would mean storing values in your calculator for calculated values that must go into other equations or dimensional analyses. I do not think any question will tell us how many signific...

Kaihan_Danesh_2J

I round my answers in the last calculation because I was told that that gives the most accurate answer. I do not know exactly why that is, but it was the rule of thumb in both my regular and AP Chemistry class in high school.

Kaihan_Danesh_2J

Hey, using the iPad has been a big help for me because it allows me to organize notes, lectures, and homework. I use a screen protector which makes it feel a little more like paper and I feel like it makes the light from the iPad less harsh, but I am not too sure if that is really the case. Also, if...

Kaihan_Danesh_2J

Hi! So I was just wondering whether the textbook questions or the sapling homework would be more useful in our review for the exam. Also, are the audio-visual module questions similar to what we will see on the exam?

Kaihan_Danesh_2J

What is the difference between the Lyman Series and the Balmer series. Is the wavelength of light emitted by the electrons different for each because one involves UV light and the other visible light? And, if so, why does this distinction matter?

Kaihan_Danesh_2J

My TA said during our discussion section that we could use our phones as an external webcam, but I am not too sure if this is 100% confirmed. I think the reason we need it is because a lockdown browser would shut down any running applications, especially zoom, which uses a webcam.

Kaihan_Danesh_2J

The TA in my class assured us that this is beyond the scope of what we have learned so far and is more of an organic chemistry thing. So, no, we will not have to know the molar masses of these compounds off the top of our heads or through calculations: if we do get them, the molar masses will be giv...

Kaihan_Danesh_2J

I believe it is because solids and liquids have constant concentrations and Kc shows to us the change in concentration after a chemical reaction. The only things that significantly change in concentration are aqueous substances and gases.

Kaihan_Danesh_2J

You are right. To put it simply, the stoichiometric coefficients are there to create balance in the chemical reaction, not to denote the molar mass of the compounds. So, if you were to find the molar mass of a compound, you would just add the up the masses of the elements without multiplying them by...

Kaihan_Danesh_2J

I understand the process of finding the number of significant figures in the questions we have been assigned, but I never understood why we round the way we do and why we round differently depending on whether we subtract/add or multiply/divide. Does anyone here know the answer?

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