CHEMISTRY COMMUNITY

DHavo_1E

Kinetic control v thermodynamic control refers to what process is dominating the reaction process, kinetics or thermodynamics. Kinetic control infers that the fastest reaction will be taking place, whereas thermodynamic control means the reaction that is the most energy conserving, I think. Hi, Wer...

DHavo_1E

I could be wrong but Kw=[H+][OH-] and when you have K=[H+]/[OH-] it could just be talking about a reaction where you're doing normal products over reactants and they aren't asking for Kw Hi, Could you explain how they determined that the product over reactants fraction? Why is it [H+]/[OH-], and no...

DHavo_1E

By definition, disassociation means that the chemical equation will take the form AB --> A + B. So in this case, we want the net equation for the disassociation of the acid HF to be HF --> H+ + F-, and we need to adjust our anode and cathode half-reactions accordingly. In order for HF to be on the ...

DHavo_1E

Hello, How do you figure out how to write the half-reaction for this problem? The question writes: The "aluminum-air fuel cell" is used as a reserve battery in remote locations. In this cell, aluminum reacts with the oxygen in air in basic solution. a) Write the oxidation and reduction hal...

DHavo_1E

Because pH ranges from 1 to 14, you need to use the E knot cell = RTlnK/nF equation, which is derived from when delta G knot is equal to each other for both equations of -nFE knot cell and -RTknK. Then, since it begins at pH = 7, you should know that E knot cell is 0. You should be able to calculat...

DHavo_1E

Use the data in Appendix 2B and the fact that, for the half- reaction F2 + 2H+ + 2e- --> 2HF with E = +3.03 V, to calculate the value of Ka for HF. Why do you have to take the square root of K at the end to find Ka? I thought the acid disassociation constant was basically an equilibrium constant, j...

DHavo_1E

The reduction half reaction you should be considering for water is 2H2O + 2e —> H2 + 2OH- with Ecell= -0.83 V The half reaction you are thinking of is for the reduction of O2 into water Hello, I am also a bit confused on this question because the book chooses the cathode as Ni2+(aq) +2e- ->Ni(s) in...

DHavo_1E

I remember being confused on this problem too. What I ended up doing was looking at the reduction half reactions table at Appendix 2A and going from there. I looked at the equations and found which one had O2 and H+ in one equation and O2 and OH- in another equation and manipulated to to result in ...

DHavo_1E

Hello, Can anyone explain why for Q in in the equation E=E-(RTln10/nF)(lnQ), we multiply the concentrations by the pressure? This is for the question with the cell diagram Pt(s)/H2(g,1.0bar)/HCl(aq,0.075M)//HCl(aq, 1.0M)/H2(g,1.0 bar)/Pt(s). The solutions manual solved it like this: E= 0.00V-(0.0256...

DHavo_1E

Adding on, once you determine the half reactions, you are able to write the cell diagram for cathodes on the right and anodes on the left. Make sure to include the H+ in the cell diagram. Upon calculating the cell potential, you are able to determine that the reaction is spontaneous since it become...

DHavo_1E

Hello, I was confused on why when you wrote the half-reaction for oxidation, you wrote MnO4- -> Mn^2+ and not MnCl2. Could you explain why you did this? Additionally, how did you get Fe^2+ -> Fe^3+ + e- and why is oxygen not included? I thought that iron had an oxidation state of +2 in FeO as well. ...

DHavo_1E

Hello,

could you explain why there is a Ni (s) at the end of the cathode? Thank you!

DHavo_1E

Hello,

For the equation H+(aq) +OH-(aq) -> H2O(l), why is water not included in the cell diagram? Thank you!

DHavo_1E

Usually for anodes, the solid is the reactant of the half reaction. In this case, the iodine solid is the product. I don't have an explanation why this needs the platinum because I don't know as well, but I think it does have something to do with the solid being a product. Hello, How did you determ...

DHavo_1E

Hello,

When looking at the reduction half-reactions in the appendix, how do we know which one is the cathode or anode when devising the cell diagram? Thank you for the above information as well; it helped a lot!

DHavo_1E

Abigail_Hagen2G wrote:If you're given a diagram, the left is the anode and the right it the cathode. If you aren't given a diagram, the greater electric potential is at the cathode.

Hi,

What do you mean by ‘greater’ electric potential? Thank you :)

DHavo_1E

Hello,

Could you explain the purpose of inert metals like platinum in the reaction? Thank you!

DHavo_1E

I don't think they would make us draw out the galvanic cell diagrams but they might test us on the understanding of writing out the cell diagram, ie: Al(s) | Al3+(aq) || Sn2+(aq) | Sn(s) Hi, Could you explain what this cell diagram is telling us conceptually? I’m a bit confused about that. Thank you!

DHavo_1E

Hello,

How does the voltage difference relate to pH? Thank you :)

DHavo_1E

Hello,

Could you explain why it’s important for the ions to diffuse through a porous barrier? Im a bit confused, do the electrons move through the external wire and the ions through the porous barrier/salt bridge? Thank you!

DHavo_1E

Hello,

Will we have to know about entropy and enthalpy? Thank you!

DHavo_1E

I believe it will only cover topics from the midterm up until the end of kinetics. Would it be up until the end of kinetics or the end of electrochemistry? Based on what Prof. Lavelle said it will only be from the 2nd part of Thermodynamics dealing with Gibbs and then all of the Electrochemistry (2...

DHavo_1E

Spontaneous processes don't necessarily happen quickly, it just means that over time there is a tendency for that reaction to occur. The equation that relates spontaneity to the cell potential difference is deltaGknot = -nFEknot. This makes sense, because the more positive the E value of cell poten...

DHavo_1E

Le Chateliers principle can be used to make some predictions. 1. When (reactant) > 1, we assume Ecell > Ecell (standard conditions) 2. When (product)>1, we assume E cell < Ecell (standard conditions) Le-Chateliers principle is not useful because we cannot calculate cell potential from that method. ...

DHavo_1E

Hello,

Is there a difference between max potential and cell potential? Is the max potential describing the cell as well? Thank you!

DHavo_1E

505306205 wrote:Maximum voltage occurs right when the switch is closed, which is when there is least current flowing.

Hello,

Could you explain the relationship between maximum voltage and how there is the least amount of current flowing? What is the implication and meaning of little current flowing? Thank you!

DHavo_1E

Hello,

Could you explain the purpose of a galvanic cell and what it is? Thank you!

DHavo_1E

A salt bridge is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell(a type of electrochemical cell). It maintains electrical neutrality within the internal circuit, preventing the cell from rapidly running its reaction to equilibrium. Hello, Could you expl...

DHavo_1E

Hello, Could anyone explain how they solved this problem? I am a bit confused on the solution manual's approach regarding their usage of enthalpy of reaction and why the specific heat capacity is negative in the heat equation q=mcdeltaT as well as where they got 800g. Thank you so much! The problem ...

DHavo_1E

Hello, Could anyone explain how to determine the signs in the enthalpy equation: deltaH= deltaU + PdeltaV? In this question, work is found to be positive and when rearranging for deltaU, I would expect a negative internal energy. However, in the solution book, they wrote deltaU= deltaH + PdeltaV ins...

DHavo_1E

Hello,

Could someone explain how they got the answer for the forming bonds for part a? Thank you!!

DHavo_1E

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. Hello, Can I ask that when you found qv=deltaV whether you used the equation q=U-w? Is the enthalpy equation: DeltaH=DeltaU+pDel...

DHavo_1E

Hi,

Could anyone explain how we know the net production of moles is 1.00 when it is stated as the reactant? Thank you!

DHavo_1E

Hello,

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

DHavo_1E

On this same problem, why don't we use the molarity or volume of either of the compounds that are given to us in the problem? Why do we just multiply C(cal) by delta T. Hello, I believe we do this because the relationship q= -qcal is already established. It is simpler to find qcal with the informat...

DHavo_1E

Hello,

Could anyone explain what it means to scale the heat released to the 2mol in the equation 2C6H6(l) + 15O2(g) -> 12CO2(G) + 6H2O(l)? Why when solving for change in enthalpy, they write 2/(#of moles of benzene burned in the equation)? Thank you!

DHavo_1E

Hello, For the equation: (delta enthalpy) = (delta internal energy) + (delta n)(RT), What does it mean when the textbook says that delta enthalpy is more positive than delta internal energy for reactions that generate more gases? I don’t understand the link between this conclusion and their explanat...

DHavo_1E

Hello, For the equation: (delta enthalpy) = (delta internal energy) + (delta n)(RT), What does it mean when the textbook says that delta enthalpy is more positive than delta internal energy for reactions that generate more gases? I don’t understand the link between this conclusion and their explanat...

DHavo_1E

A constant-volume calorimeter was calibrated by carrying out a reaction known to release 3.50 kJ of heat in 0.200 L of solu- tion in the calorimeter (q 5 23.50 kJ), resulting in a temperature rise of 7.32 8C. In a subsequent experiment, 100.0 mL of 0.200 m HBr(aq) and 100.0 mL of 0.200 m KOH(aq) we...

DHavo_1E

Hello,

I am also confused about this! Additionally, what makes a reaction irreversible and what are its implications in relations to work and internal energy? Thank you!

DHavo_1E

Hello,

What would be an example of an intensive property? Thank you!

DHavo_1E

Hello,

I am a bit confused about the concepts in this theorem. Would you be able to explain it? Thank you!

DHavo_1E

Hello,

Could I ask whether anyone could give an example of a closed system? Thank you!

DHavo_1E

Do we need to be able to differentiate between translational energy, rotational energy, and vibrational energy? If so, what is the difference. Hello, I believe that knowing what they are would not hurt! Translational energy is simply kinetic energy (or energy of an atom or molecule due to its motio...

DHavo_1E

During reversible reactions, products and reactants are always being made (going back and forth), while in irreversible reactions products can't revert back to what it was as a reactant. I also think that during reversible reactions, the system stays at thermodynamic equilibrium, and during irrever...

DHavo_1E

Megan Kirschner wrote:Would anyone be able to describe the diagram you all are referencing? I was sick and unfortunately missed that lecture :/

Hello,
The diagram looks like this:

https://commons.wikimedia.org/wiki/File:Water_Phase_Change_Diagram.png

DHavo_1E

Hello,

In response to this question, could anyone explain why the bond enthalpies in Dr. Lavelle's example were positive AND negative?

DHavo_1E

Hi,

I would like to add Dr. Lavelle's example with the diagram of a mountain, about how altitude was a state function because it was a set amount and only depended on initial and final values while two hikers (A and B) had different values depending on the pathway they took (similar to work).

DHavo_1E

Since I don't know which exercises you're referring to (when you mention that some exercises do 5/500 instead of 500/5), I'm not sure if there's a particular reason; but it's just a matter of finding a ratio of solvent between the original and the diluted, and as long as you know the magnitude of d...

DHavo_1E

Well let's look at the changes for A B and C. A: -10 B: +5 C: +10 The ratio of A/B/C is -10/+5/+10 or simplified -2/1/2. Negative change indicates the reactant while positive indicates products. The ratio above represents the coefficients for the equation. For every 2 A molecules used, 1 B and 2 C ...

DHavo_1E

Hello,

Could anyone explain how 2A in the answer 2A(g)-> B(g) + 2C (g)? Thank you!

DHavo_1E

So because it is a very weak acid/base it only dissociates by a small concentration. Therefore, since your KA or B is [products] / [reactants] and only a small concentration will dissociate into the product, Ka or B is a very small number divided by a much bigger number which = a very small number ...

DHavo_1E

What is the R constant in PV=nRT, I keep hearing there are different R constant and I am confused about that? Hello, For the Ideal Gas Law (PV=nRT), R= 8.314J/molK, which is the universal gas constant. I believe there is also differing R constants depending on units used. I found the section "...

DHavo_1E

Hello,

I am also confused about this question. Could you also explain what it means to be thermodynamically stable? Thank you!

DHavo_1E

Hello, I agree with the statements above and would like to add looking at the sections targeted in the textbook (6A.4, 6B, 6C.1-6C.2, 6D, 6E, 6E.1) in the syllabus as well as do all the homework problems assigned to get an idea of what is expected for acids and bases. Additionally, I would also look...

DHavo_1E

Hello,

Could someone explain what the textbook means when it says that a solution of a weak acid in water would have a very, very low concentration of OH- ions to maintain autoprotolysis equilibrium? Thank you!

DHavo_1E

Hello, I also was confused on this derivation. In addition, do you understand the statement at the end of pg.170 where it says "the times required for the same amount of two substances to effuse through a small hole are inversely proportional to the rates at which they effuse" and "th...

DHavo_1E

ShastaB4C wrote:So would it be incorrect to put brackets around a partial pressure?

Hi,

I would write them using parentheses because brackets are a notation used for concentration.

DHavo_1E

I agree with the statement above that K would be 1/concentration of the gas or liquid, and would like to add that this is because the activity of pure substances and liquids is 1, which accounts for the right side of the equation of solids and liquids. K = [(activities of products)^n/(activities of ...

DHavo_1E

Hi, I agree with the statements above that Q is a reaction quotient at a time that may or may not be at equilibrium and is solved the same way as if solving for K. It helps determine where the reaction is at. For example, if Q < K, then the equation will lie to the right (because a smaller Q means t...

DHavo_1E

It probably depends on what you are given in the problem (the partial pressures or the concentrations). You could do either Kp or Kc, as long as you keep track and do not confuse the two because the values will not be the same. And in Wednesday's lecture Lavelle showed us how to convert between Kp ...

DHavo_1E

K or Kp is used when there are gases in the chemical equation, unless stated otherwise because gases can be discussed in terms of molar concentration. Meanwhile, Kc is for concentrations in mol/L for solutes in a condensed phase. Hi, I was wondering if you could explain why gases would be discussed...

DHavo_1E

Hello, I am confused about the Self-test 5G.3A in the textbook that asks about writing the equilibrium constant for the reaction 2AgNO3 (aq) + 2NaOH (aq) -> Ag2O(s) + 2NaNO3 (aq) + H2O(l) and to look at the net ionic equation. I don't understand why the 2Ag is included in the answer (K= 1/ [Ag+]^2[O...

DHavo_1E

Lewis Acids are electron acceptors, but what is the best way to determine that from a Lewis Structure? I'm having a little trouble visualizing the concept of Lewis Acids in terms of where an electron pair can be added on an atom. For Lewis Bases, I can see where lone pairs could be donated, but I'm...

DHavo_1E

Which would be the stronger acid HClO or HClO2? I know that you first look at bond length and then anion strength but how would you figure this one out? Hello, For future reference, I would like to add that you look at bond lengths only for binary acids (H-A), because for oxoacids such as HClO and ...

DHavo_1E

In organic molecules, how do we know if there is an "implied" Hydrogen? Hi, For the example he showed in class (NH2CH(CH3)CH2CH2COOH), you can determine whether there is an "implied" hydrogen by counting the number of bonds. For example, if a carbon was at the peak of the line s...

DHavo_1E

What is the difference between a carboxylic acid and an oxoacid? How can you tell which is which? Hi, For future reference, I would also like to add that for carboxylic acids, you can determine the strength by the electronegativity of the groups bound to it. For example, for trichloroacetic acid (C...

DHavo_1E

I don't think it would necessarily be raised, but if a molecule chelates then less of that molecule is needed to bond to the metal. So there would just be less ligands needed in the coordinating complex. The coordinate number doesn't increase but there are less ligands, so the charge on the metal s...

DHavo_1E

Hello,

For the example in the textbook:
[Fe(OH2)6']^2+(aq) + Cl-(aq) -> [FeCl(OH2)5]+(aq) + H2O(l)

Why is replacement less complete when chlorine is added to an Iron (II) solution?

Thank you in advance!

DHavo_1E

They're both red proteins. A Fe atom bound to 4N forms a heme complex, and a heme complex bound to a protein forms a myoglobin (which transports O2 in muscle cells). Four myoglobin-like molecules exist in each hemoglobin, binding four O2 molecules. That's pretty much everything Prof Lavelle mention...

DHavo_1E

Drew Myers 3A wrote:How does one differentiate between inorganic and organic bases?

Hi,
I believe that organic based contain carbon, while inorganic bases do not have carbon, hydrogen, and oxygen at the same time (I.e. h2O which is inorganic).

DHavo_1E

Hi,

I agree with the statement above, and would like to add that this differs from the Lewis definition which discusses the transfer of electrons (acids- electron pair acceptor vs. base-electron donor).

DHavo_1E

The strength of an acid depends on two things: The strength of the A-H bond, and the stability of the resulting anion. Bond strength is related to the length of the bond, and because Iodine has a much larger atomic radius than Fluorine, HI has a much longer, and therefore weaker, bond. The hydrogen...

DHavo_1E

Hello!

Could someone explain what Pka and ka are conceptually and how they are different? Thank you!

DHavo_1E

Hi,

Could you explain how its structure relates to its function, or why its important as a biological molecule? Thank you in advance!

DHavo_1E

Hi,

Could someone explain why inorganic acids are stronger than organic acids? Thank you!

DHavo_1E

Hi,

Where does cisplatin bond on the DNA to stop translation, and thus cause the cell to die? Thanks in advance!

DHavo_1E

Hello,

In class, Dr. Lavelle drew out the structure of this organic compound, and I was wondering why for the CH bonds he write CH2 instead of drawing C as a central atom bonded to two hydrogens on either side. Thanks!

DHavo_1E

Hi,

Could someone explain when or why a cation would be outside the coordination sphere? Thanks in advance!

DHavo_1E

Hi,

Could someone reiterate the significance of Fe as the central atom in the Heme complex, and how that relates to its function? Thank you!

DHavo_1E

How does an atom having preference affect the molecular shape ? Dr. Lavelle brought up several times how Fluorine will change the dynamic since it has preference. Hello, The example he gave in lecture talked about CCl4 vs. HCCl3 and their preference for attraction. If a Na+ cation was held near the...

DHavo_1E

Lavelle mentioned that the potential energy is proportional to -q1q2/r^6, when q is the polarizability of an atom (dependent on the number of electrons and the size of the atom), and r is the distance between the atoms or molecules. I think this measures how strong the energy is that is pulling the...

DHavo_1E

Hello,

In that case, would H2O have a lone pair next to each other, or on opposite ends and why? Thank you!

DHavo_1E

This is what one of the TA's said: - Use VSEPR model to determine the shape of inorganic, organic, and small biological molecules, cations, and anions - Use bond dipole moments and shape to determine if a molecule is polar or non-polar - Use shape and polar or non-polar properties, identify intermo...

DHavo_1E

Anne Tsai 1F wrote:I think XeF2 is nonpolar because the fluorines are arranged so that the molecule has a linear shape, and thus there is no dipole moments.

Hi,

Could you explain how XeF2 has a linear shape when there are three lone pairs or why there are no dipole moments?
Thank you!

DHavo_1E

Hi, The explanation above was very clear! I would also like to add that shape is then identified by the relative location of atoms, NOT lone pairs and that if there are no lone pairs, the electron arrangement of a molecule equals the molecular shape (ex: if there are four regions of electron density...

DHavo_1E

Hello, I would like to add on the electrostatic is also used in the phrase "electrostatic attraction" which refers to an attractive force between two things oppositely charged ( one positive charge and one negative charge). This interaction is favorable and explains how ionic bonds form as...

DHavo_1E

I believe I recall that hybrdization makes the orbitals suitable for electron pairing/bond formation, but I'm sure we will go into more detail on this in lecture this week. Hello, Does hybridization also lower the energy within orbitals, helping the atoms reach a lower energy state and become more ...

DHavo_1E

Hi,

Would that mean a larger electric dipole moment implies a greater polarizability, and a smaller dipole moment less polarizability? Thank you!

DHavo_1E

For the purposes of this class they are the same, but in induced dipole moments the molecule that induces the dipole moments has a permanent dipole moment while the same does not occur in London Dispersion Forces. Hi, Could you explain what you mean by this further? What constitutes a permanent dip...

DHavo_1E

How do you know when a molecule is polar or nonpolar? What are the differences? 2E.27 Predict whether each of the following molecules is likely to be polar or nonpolar: (a) C5H5N (pyridine, a molecule like benzene except that one OCHO group is replaced by a nitrogen atom); (b) C2H6 (ethane); (c) CH...

DHavo_1E

Hi,

I would like to add that the schrodinger equation determines the energy of an electron and the probability of finding an electron at a specific location. Additionally, the square of the wave function gives the probability density in a specific region of a volume.

DHavo_1E

Hi, I would like to add that at first light was seen as a wave which led scientists to believe that increasing intensity would increase energy and thus release more electrons (such as how a big wave at the beach has more energy than a small wave as Dr. Lavelle said). However, it was found that incre...

DHavo_1E

For reference, I am doing 1B.9, which gives the wavelength of the light and watts of the light and I am stumped on how many photons are being emitted in 2 seconds. I used E=hv to find the energy of a singular photon and then I divided that by the watts which gave me the answer in the back of the bo...

DHavo_1E

Hi, For Lewis structures, I agree with the response above. The atom that is most electropositive (least electronegative) is in the center. Oxygen is further to the right than nitrogen, meaning it is more electronegative because more protons attracts more electrons, making nitrogen less electronegati...

DHavo_1E

Could someone explain the difference between probability vs probability density? Thanks! Hi, How I see it is probability density telling whether or not it is likely to be in a specific region of a volume, and probability as how much likely. For example, at a point, the probability density could be ...

DHavo_1E

For electrons to be delocalized, it means that the electrons are shared within the entire molecule. In the other examples we have seen,it has always been about electrons being shared between two atoms. For example, the electrons in CH4 are shared between each of the carbon and hydrogen bonds. Howev...

DHavo_1E

I understand that there are not truly any definite values that dictate whether a bond is covalent or ionic. However, in the book it says that a good rule of thumb is that an electronegativity difference of two means that the bond has so much ionic character, it is best regarded as ionic. Are there ...

DHavo_1E

Can someone explain why the difference between a compound of widely separated gases and tightly compacted ones is so drastically different? Lattice energy is the energy given off when oppositely charged ions in the gas phase form a solid. It measures the strength of a bond between ions so if it is ...

DHavo_1E

From what I understand, you always count the subshells when finding ground-state configurations. Even when finding an ionic configuration, you still count the subshells, just some of the electrons have moved to other energy levels so it may seem as if you "aren't counting" them. For Mn in...

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