CHEMISTRY COMMUNITY

Abigail Sanders 1E

I was also wondering this as well since the final was multiple choice and I thought we would get our score right away. I think Lavelle probably has to look over grades and chemistry community to assign grades before he makes them public for everyone.

Abigail Sanders 1E

Since the final exam is on ccle, will the system mark students off based on how you round? For example I know when you are solving with natural logs keeping the entire decimal vs keeping the significant figures before you calculate gives a very different answer eventhough both are correct. Is there ...

Abigail Sanders 1E

The graph of [A] vs time would not be linear for a second order reaction. It is only linear for zero order. For second order the graph of 1/[A] vs t is a linear function with a slope equivalen to +k.

Abigail Sanders 1E

I don't think we have to know anything else about heterogenous catalysts and adsorption as Dr. Lavelle said in lecture that we will mainly be looking at homogenous catalysts. We only have to know that heterogenous catalysts act by adsorption and thus by the reactants sitting on the catalyst's surface.

Abigail Sanders 1E

I know that Dr. Lavelle posted a powerpoint of what he would have gone over in wednesday's lecture if that is helpful. It is on his class website under week 10 review.

Abigail Sanders 1E

Yes so because k is the reaction constant for the forward reaction k' is the reaction constant for the reverse reaction. Thus the equilibrium constant K for the forward direction is equivalent to k/k'. This can be determined because at equilibrium of a+b --> c + d, the rate of the forward reaction (...

Abigail Sanders 1E

Does anyone know when you can infer reaction order based on the number of molecules in a reaction? For example the reaction A+B --> C+D could be rate=k[A]^2[B] or rate=k[A][B] depending on experimental data provided. But, in reaction mechanisms we assume that the rate law for A+B--> C+D is rate=k[A]...

Abigail Sanders 1E

Hello everyone, thank you for your kind words. I was looking forward to this review session, but unfortunately it will have to be cancelled. Judging from Dr. Lavelle's email, I am sure you all will do well on the final, and I wish you the best with all of your future endeavors. Good luck, and have ...

Abigail Sanders 1E

Will this review session still be held now that all classes are cancelled? Lyndon's sessions are normally the most helpful thing before the finals/midterms.

Abigail Sanders 1E

For question 6.65 can someone please explain why K is equivalent to [H+]/[OH-], i thought that Kw was equivalent to the product of the two concentrations.

Abigail Sanders 1E

Normally you can tell the phases of the reactants and products from the equation given. For example if there was the reduction of Co2+(aq) to Co(s) then Co would be a solid and Co2+ would be aqueous. Then, in the cell diagram, you separate phases in contact by a single line (|) and sides of the galv...

Abigail Sanders 1E

I believe in the textbook it states that K>1 when E°cell >0. This makes sense because at equilibrium E°cell= RT/nF * ln K so, when K>1, ln k is a positive number and E°cell will be greater than 0. Furthermore, the sign of K, or K>1 can also be determined from delta G as Delta G = -RTlnK at equilibri...

Abigail Sanders 1E

I believe you would look at the table of reduction equations in Appendix 2B and find the one that has the correct other reactants so that when you add the oxidation and reduction reactions, the overall redox equation is the one the question is looking for. On the quiz I believe we will be given the ...

Abigail Sanders 1E

The way I understand it is that ΔG is equivalent to the free energy of the reaction, or how much energy the reaction will give off. Standard Delta G or ΔG° is the standard free energy of the reaction at STP at equilibrium. So, when the reaction is not at equilibrium you can use ΔG=ΔG°+RTlnQ to calcu...

Abigail Sanders 1E

When you reverse a reaction, the cell potential should switch to the negative value of its current value because you are essentially switching the anode and cathode standard potentials. So, if cathode = 0.9 and anode is 0.5, the cell potential for #1 would be 0.4 but, for #2 if you reverse the react...

Abigail Sanders 1E

Yes, what I do is I write out the half reaction for each oxidation or reduction reaction and then make sure I flip it if necessary so that the electrons cancel and it is in the right oxidation/reduction form. Then, I combine the two balanced half-reactions for the overall reaction for the cell and d...

Abigail Sanders 1E

The 4i and 4j sections were included on the midterm I believe so they are not a part of the homework problems included for test 2. It will just be the 5G and 5J problems as well as all the electrochemistry problems.

Abigail Sanders 1E

Yes the answer above is correct. I remember Dr. Lavelle mentioning in class that the midterm covered all the chapter 4 problems on the thermodynamics outline so, test 2 should cover the second group of problems from sections 5G.3, 5G.4, 5J.3. Then, of course, all the homework problems from electroch...

Abigail Sanders 1E

For this homework problem, why does the answer key calculate the reaction quotient Q in the Nernst Equation with both partial pressures and concentrations, rather than converting all to a common parameter so that the units cancel? For reference the cell diagram is Pt(s)|Cl2(250 Torr)|HCL(1.0 M)||HCL...

Abigail Sanders 1E

Can anyone explain which cell diagrams/reactions include Pt(s) as an inert electrode vs when it is incorrect to include it in the cell diagram? Also, what are the other types of electrodes that can be used and when are these used?

Abigail Sanders 1E

I am pretty sure you can use either for calculations. The two equations are inherently the same, you are just using a different log base (10 for log and e for ln) so the constants are different because there is a conversion factor between the bases. I also don't think you need to memorize them becau...

Abigail Sanders 1E

In the solutions manual it says that for 6N #1 part b the answer should be K=1X10^4 however, it comes to this answer by saying there are 2 moles of electrons involved in the reaction when, in reality there is only one? Is this a solution manual error or am I missing something? E17C5C0C-A093-44E8-A38...

Abigail Sanders 1E

Can anyone fully explain why question 6B on the midterm has answer A? I guessed that it is A because it is this reaction has the smallest change in entropy but I am not sure if that is correct.

Abigail Sanders 1E

This equation also allows us to compare two K values K1 and K2 and observe how the change in temperature either increases or decreases K. The equation for comparing is: ln (k2/k1) = -(delta*H)/R * (1/T1 - 1/T2) . However, as stated above, the Van't Hoff Equation's purpose is to show the temperature ...

Abigail Sanders 1E

When the question asks for net charge of the molecule it is basically asking if the molecule will dissociate in a given PH. Thus, if the molecule does give off a hydrogen ion it will be a negatively charged molecule and if it does not it will be neutral. In this question, the pka of the acid is less...

Abigail Sanders 1E

Yes, because entropy is a state function, it does not matter for the change in entropy for the system if it underwent reversible or irreversible expansion. This is why for question 8 you know that for both a and b the change in entropy for the system is 0.910 J/K. The change in total entropy or chan...

Abigail Sanders 1E

Can anyone explain the rationale behind the answer for midterm question 3B? On the test I used the rationale that the reaction with the fewest moles would produce the largest change in temperature for the same heat but this was marked incorrect? For reference the answer Dr. Lavelle posted is B.

Abigail Sanders 1E

This is because in the question, they are asking for the change in internal energy which is equivalent to the equation ∆U=∆H-P∆V. W=-P∆V so, this equation can be written as ∆U=∆H+w. They are simply setting -22kJ equivalent to P∆V to substitute it into the equation ∆U=∆H-P∆V. You are correct that the...

Abigail Sanders 1E

∆S is related to ∆H by the equation ∆S=∆H/T. This is because ∆H is equivalent to qp and ∆S=qrev/T so, we can then say that ∆S=∆H/T.

Abigail Sanders 1E

To clarify the last two responses, Gibbs Free energy is 0 both at the boiling point and when the reaction is at equilibrium.

Abigail Sanders 1E

The monatomic gas has the greatest entroy because 1. it is a gas so the particles experience a lot of disorder and 2. because monatomic gas means that none of these particles are diatomic so, there is more of them contributing to greater disorder in that gas. Diatomic molecules do not has redidual e...

Abigail Sanders 1E

Q is positive or negative depending on if heat is added or removed from the reaction. If heat is added to the system that you are finding the internal energy for then it will be positive, if it is removed, Q will be negative.

Abigail Sanders 1E

Yes, in a reversible process, the pressure wants to remain at equilibrium so it tries to maintain the pressure external to equal the internal pressure. This is why the process is reversible because the pressure of the surroundings and the system change at the same time so reversing the process would...

Abigail Sanders 1E

For this question we will use the equations (delta S)=(delta q)/T and q=nC(delta T). First, we will find the delta q. This is found by the equation (delta q)=nC(delta T). Then, we will divide this equation by T to find the entropy. This is equivalent to nC(delta T)/T. Then, to find the overall chang...

Abigail Sanders 1E

In this question it is actually power, not work that is given. The power is given in Watts which are equivalent to Joules per second. Thus to solve this question you will find the energy/ heat in 100W which is 100J (over the time span of 1s). Then we have an equation relating entropy to heat which i...

Abigail Sanders 1E

Generally, the basis for when moles/mass is used depends on the units of the constant C you are using. If the C given is J/g then this is the specific heat capacity and you will use grams of substance. However if the units for the constant C are J or KJ/mol then you will use moles in the equation an...

Abigail Sanders 1E

Does anyone understand the thought process behind how this problem is solved? I understand why, to find the entropy of vaporization at 85 degrees, you would have to add the change in entropy from 85 degrees to 100 degrees and from 100 degrees to vaporization but, I do not understand why there is a t...

Abigail Sanders 1E

I believe the common unit for deltaH in Dr. Lavelle's material is KJ*mol-1. The only times you would use just KJ is when you are given a specific amount of moles that are reacting and you are finding the change in enthalpy for that quantity.

Abigail Sanders 1E

I think the enthalpies will always be given to us if we need them on the test or I suppose Dr. Lavelle would give us a variety of tables for us to locate these values. In the past he has given us a constant sheet with other constants so I hope that he would do the same. Otherwise, they would have to...

Abigail Sanders 1E

Like stated previously, it is helpful to convert to whichever constant is given in the question. A helpful strategy for this is to look at the units in the constant given to know what units to include in your equation. So, to answer your question it is better to convert to moles if the molar heat ca...

Abigail Sanders 1E

There is a negative sign in the worl equation because we are looking for work on the system, rather than on the surroundings. Work on the system is positive when volume decreases for a gas and work on the system is negative when volume increases. This can be represented by the equation W=-P*(delta)V...

Abigail Sanders 1E

How are delta H and q different in terms of heat? In the homework questions these variables are used interchangeably, is there a way to distinguish when you use one or the other and what does each mean?

Abigail Sanders 1E

Steam causes very severe burns because the change in enthalpy from boiling water to vaporized gas is significantly larger than that of boiling water reducing in temperature when it comes in contact with your skin. It requires a lot of energy to vaporize a liquid so, when this gas comes in contact wi...

Abigail Sanders 1E

Yes, the conjugate see-saw applies both ways. This means that if an acid is very strong then its conjugate base will be very weak. Additionally, if a base is very strong then its conjugate acid will be very weak. This can be shown through the equation Ka x Kb =Kw. If an acid has a very high Ka then ...

Abigail Sanders 1E

I believe in the 6E.1 reading it says that if Ka2 is more than 10^3 times smaller then the change in pH for polyprotic acids can be considered insignificant. This can be found by dividing ka1 by ka2 and seeing if the quotient is greater than 1000.

Abigail Sanders 1E

To find the pH of 0.055 M AlCl3 first, you show how Al 3+ and Cl- dissociate in solution. Because Cl- is the conjugate base of a strong acid it has no effect on the pH. Al3+ however can act as a weak acid by binding OH- to reduce its overall + charge. Then, it is a simple ICE table from here because...

Abigail Sanders 1E

The temperature remains constant during boiling even though heat is supplied because the heat supplied is being used as energy to break the bonds. It takes energy to break a bond so all of the heat applied will be used to break these bonds.

Abigail Sanders 1E

I believe if you are given a pKa value you will have to use the concentration of H3O+ from that pKa equation to find the pH of the solution. Like if pKa=[H3O+][CH3COO-]/[CH3COOH]=2X10^-5 then you would have to know the other concentrations in the reaction to be able to find the concentration of H3O+...

Abigail Sanders 1E

This problem relies on the formula that M(initial)*V(initial)=M(final)*V(final). In order to solve this problem it is necessary to work backwards. To begin with you will find the molarity of OH- of the solution at the end of all of the dilutions as the pH is given. Thus you find the pOH and plug the...

Abigail Sanders 1E

Yes, as stated previously, temperature is the only factor that can alter K in a reaction at equilibrium. For future reference I believe we also have to know how pressure/concentration alter the direction of the reaction but these changes to not affect K.

Abigail Sanders 1E

I believe that to find the Ph you find the PKa. Then, you would find the concentration of the H30+ in the pKa formula. However, are you sure you are refering to problem 6D.15 because that problem asks only for the pH and gives you a molarity of NH4Cl?

Abigail Sanders 1E

I believe he said in class that for weak acid dissociation, if the concentration of H+ is less that 1X10^-7 you can assume that the solution itself is essentially neutral. I do not know if this is essentially what your question is regarding. If your question is about why liquids are excluded if thei...

Abigail Sanders 1E

Basically the reasoning is that the equilibrium expression 2HCL--H2 +Cl2 is the same as HCL+HCL--H2+CL2. thus, in the first equilibrium constant k=[H2][Cl2]/[HCL]^2 while the second one is k=[H2][Cl2]/[HCL][HCL]. These two expressions are equivalent and show why the stoichiometric constant is raised...

Abigail Sanders 1E

I think we should be able to use either if the problem does not specify Kc or Kp. I know that if a concentration is given we should use Kc and if a pressure is given we should use Kp but, I believe in gas phase, as illustrated in class, you can give equilibrium constants in either concentration or p...

Abigail Sanders 1E

From my experience with Dr. Lavelle last quarter we typically only need to know the information presented in lectures for the exams. So, if the Ksp appears in lecture I think it would be fair game for an exam or midterm. Additionally, if Dr. Lavelle doesn't assign any homework problems regarding the...

Abigail Sanders 1E

Yes, changing the volume of something changes the equilibrium becuase it changes the concentration of either reactant or product. Because concentration in Molarity is equal to moles/L, increasing the volume of reactants would decrease the concentration of reactants and shift the reaction to the left...

Abigail Sanders 1E

I still don't really understand why solids aren't included, is it just because they don't have a concentration? yes, simply stated solids don't really have a concentration so they are not included. In more complex terms the amount of the solid does not affect the amount of reactant at equilibrium s...

Abigail Sanders 1E

Would thiocyanto and isothiocyanto have different lewis structures depending on which atom is bonded to the central transition atom?

Abigail Sanders 1E

I think when the N's attached to the central metal have three bonds, they are neutral. One of the nitrogens attached to cobalt only has two bonds, so it probably has a negative charge. The cobalt oxidation state is probably +1 I looked up the answer and it said that the oxidation state was 3+ for c...

Abigail Sanders 1E

: b12.gif (4.09 KiB) Viewed 334 times

How would you find the oxidation state of the Cobalt TM at the center of the ring structure for B12 in this attatched image? I tried to look at the ligands but I can't tell which ones contribute to the oxidation state.

Abigail Sanders 1E

Once the molecule loses one H+ ion, it normally now has a negative charge. Because negatively charged atoms attract positively charged ones it will be increasingly harder to lose another positive charge as the molecule would like to remain as stable and as neutral as possible. Thus, the additional H...

Abigail Sanders 1E

Yes, water is both amphoteric and amphiprotic as it can react with both acids and bases, making it amphoteric, AND it can either accept or donate an H+ ion making it amphiprotic.

Abigail Sanders 1E

I believe that polydentate means that a ligand can bind to multiple molecules at different locations. Simply stated, polydentate describes the number of available bonding sites on a ligand. If a ligans is chelating, this means that it forms a type of ring structure than can bond to the same central ...

Abigail Sanders 1E

As was stated previously, amphoteric describes a molecules ability to react with an acid or a base while amphiprotic is a molecules ability to accept or donate hydrogen ions. Additionally, amphiprotic molecules can also be amphoteric but amphoteric molecules are not always amphiprotic. For example, ...

Abigail Sanders 1E

HI is the stronger acid as there is a weaker bond between HI than there is between HCl. This is due to both the atomic radius and the size of I vs Cl. Because I has a greater number of electrons, there is less nuclear charge felt at its valence electrons so, the HI bond is weaker and easier to break...

Abigail Sanders 1E

conjugates are the molecules formed when the acid/base either donates or accepts a hydrogen ion. For example in the HCL molecule HCL is an acid and the product Cl is the conjugate base. I recommend reading the textbook on this topic as it is very helpful.

Abigail Sanders 1E

H are included in possible hydrogen bond locations if they are bound to N, O, or F. Because they are bound to these atoms, hydrogen has a slightly positive charge due to the other atoms high electronegativity. Thus a hydrogen bond can form between a slightly negative O, N or F on another molecule an...

Abigail Sanders 1E

The two formulas are no different in structure except that they are used in different orders to indicate which atom binds to the central atom in the molecule. In H2O, O always binds to the central atom so the order of the atoms in the formula depends on their placement around the central Atom. H2O i...

Abigail Sanders 1E

Like stated previously a bronsted acid is something that can donate a hydrogen and a Bronsted base is a molecule that can accept a hydrogen. Contrarily, a lewis acid is a molecule that accepts electrons while a lewis base is a molecule that donates electrons. A Bronsted acid can be classified as a l...

Abigail Sanders 1E

Is there a list of acids names and formulas we should memorize for the final? In several homework problems it asks to either name an acid or base or to draw the formula from the name in order to predict the reaction. I was wondering if on the final we will also be required to do this or if the formu...

Abigail Sanders 1E

The two different molecules that are Tshaped are AX3E2 and AX3E3. The bond angles for T shape are less than 90 degrees for both.

Abigail Sanders 1E

Atoms with higher polarizing power will create anions that are more polarized. The increased polarizability of a molecule contributes to stronger IMF such as London Forces. Stronger forces create bonds that are harder to break so they require more energy and thus more energy to reach the boiling/mel...

Abigail Sanders 1E

Melting point generally depends on the type of intermolecular forces present in molecules. For example, dipole moments could contribute to a polar structure which would create dipole-dipole interactions. The strength of the double bond and its affect would depend on the size of the atoms involved an...

Abigail Sanders 1E

Tetrahedral and trigonal pyramidal have the same electron structure: 4 electron density regions surrounding a central molecule) but they have different molecular structures. For tetrahedral there are 4 electron density regions around the central atom that consist of 4 different atoms. In trigonal by...

Abigail Sanders 1E

Lone pairs not on the central atom do not account for the shape of the central atom. However, lone pairs on an outside atom do account for the electron and molecular structure of that atom.

Abigail Sanders 1E

AsF3 has a higher boiling point than AsF5 as AsF3 is polar and thus has dipole-dipole interactions between molecules. AsF5 on the other hand, is non-polar and thus only has weak London Forces. Due to the stronger dipole-dipole interactions, it is harder to break the bonds in AsF3 and thus its boilin...

Abigail Sanders 1E

CHF has a lower boiling point than CHI as Fluorine has a smaller radius and electron cloud, and thus does not have as high of a polarizability. In CHI on the hand, Iodine has a very large radius and thus a very large electron cloud, this means that it has a very high polarizability. Atoms with a hig...

Abigail Sanders 1E

Hydrogen Bonds can only be formed on atoms or molecules with N, O, or F but, these hydrogen bonds can also form between 2 different molecules. It only works with N, O, and F as these atoms have very high electronegativities and will (most likely) have negative formal charges.

Abigail Sanders 1E

I agree with the previous responses, it is very helpful to draw ou thte structures for these molecules before determining their interactions. Overall, structures that are nonpolar will only have very weak london forces. Structures that are polar will have dipole-dipole and structures that are polar ...

Abigail Sanders 1E

I think that it is really helpful to have the lewis structure because then you can more easily determine polarity and hydrogen bonds but I do not believe it is necessary unless the question asks specifically for it,

Abigail Sanders 1E

Smaller cations have a higher polarizing power as their smaller nucleus has a high nuclear charge due to the small radius. This small radius can cause a distorted electron cloud in an anion, especially if the anion is very large like Iodine.

Abigail Sanders 1E

Higher electronegativity atoms do tend to draw more electrons towards them as electronegativity is a measure of an atoms tendancy to attract electrons. As an example, Oxygen has a very high electronegativity and often has a negative formal charge due to its attraction of electrons.

Abigail Sanders 1E

Yes, hydrogen bonds only form with N O F as those atoms often have very high electronegativity as well as often have lone pairs. The large electronegativity allows the positively charged H to interact with it in a slight bond.

Abigail Sanders 1E

I believe that an increase in frequency is directly related with an increase in the kinetic energy in electrons ejected from a surface due to the photoelectric effect. An increase in frequency does not increase the number of elecctrons emitted, however an increase in intensity will increase the numb...

Abigail Sanders 1E

I believe that when comparing two different covalently bonded molecules the bond that is more polar is the bond with the largest difference in electronegativity.

Abigail Sanders 1E

The atomic radii decrease even though the number of protons increases as the increase in protons increases the effective nuclear charge on the electrons in the atom and pulls them in closer. This decreases the atomic radius.

Abigail Sanders 1E

Hello! I was wondering if anyone knows whether or not we will have discussion sections week 9. This is the week of Thanksgiving so I am trying to plan when to go home.

Abigail Sanders 1E

I do not know if it will be provided on the midterm but I expect that memorizing the equation will save you valuable time while completing the questions. Formal charge is something that is calculated in almost every bonding question so it would be valuable to memorize.

Abigail Sanders 1E

As was previously stated, formal charge does not always have to be calculated to find the lowest energy structure, however, it often aids in deciding which structure has formal charges closest to 0 for the central and outward atoms.

Abigail Sanders 1E

Na has a smaller first ionization energy compared to Cl as ionization energy increases across a row on the periodic table. This is because as you move across a row the numbers of protons and electrons increase making the attraction between the two greater as well. This makes it increasingly hard to ...

Abigail Sanders 1E

I asked my TA the same question and he said to make sure you have it done by the end of the week (Sunday) so that would probably mena 11:59. I know he also mentioned that Dr. Lavelle has his own way of recording the participation points so I would post before then if possible so that you do not miss...

Abigail Sanders 1E

Like the previous replies have said, expanded octets can occur in elements of row 3 and beyond. When checking for resonance and finding the lowest energy structure, make sure to calculate formal charge along with the bonding as ti will help you reach the final stage where resonance can be examined.

Abigail Sanders 1E

The spin number is important because it shows that there are two electrons in each orbital that are spinning in opposite directions to counteract the magnetic field. The numbers are important when filling to keep track of which electrons are in which orbitals as electrons fill in +1/2 first and then...

Abigail Sanders 1E

I agree, since nothing can have a velocity higher than the speed of light I would assume that something in the calculation would be incorrect. Maybe retry the problem or message if you still have questions about a specific example.

Abigail Sanders 1E

I believe Dr. Lavelle said we will only need to be familiar with s,p,d blocks for quizzes and exams however, I am sure there will be a few homework questions that involve the material so it might be helpful to know how F fills.

Abigail Sanders 1E

4s will come before 3d as it has a slightly lower electrostatic energy. This allows the subshell to be filled before 3d. Once 4s is full, then 3d has the lower energy so it fills with electrons.

Abigail Sanders 1E

Atomic radii decrease as you go across a period in the periodic table as the increased amount of protons in the nucleus has a stronger pull on the electrons. This stronger pull decreases the size of the radii by a very small, but quantifiable amount.

Abigail Sanders 1E

momentum is defined as p(momentum)=m(mass)*v(velocity). Thus, momentum is not the same as velocity becuase an object's momentum is dependent on its mass and velocity. Two objects traveling at the same speed but with extremely different masses will have very different momentums.

Abigail Sanders 1E

When frequency decreases energy decreases and wavelength increases. This is because E=hv which is a direct relationship between energy and frequency where h is a constant. Additionally, c=(wavelength)(frequency) which is an indirect relationship because the multiple of these two values is equal to a...

Abigail Sanders 1E

In regards to the previous post, I believe the subshell level can be higher than 3(l=3) it is just that we will not be studying subshells higher than l=2 in this course. I believe the highest level we study is level d. In response to the primary question, subshells are calculated by subtracting 1 fr...

Search found 112 matches