CHEMISTRY COMMUNITY

Chloe Alviz 1E

Dr. Lavelle,

Thank you for all of your hard work these past two quarters! I'll definitely be leaving with a lot of valuable lessons as a student.
Also, thank you for all of the epic poetry and DJ'ing –– a true Renaissance man right there.

Take care and stay healthy!

Chloe Alviz 1E

"The following mechanism has been proposed for the reaction between nitric oxide and bromine: Step 1 NO + Br 2 --> NOBr 2 (slow) Step 2 NOBr 2 + NO --> NOBr + NOBr (fast) Write the rate law for the formation of NOBr implied by this mechanism." Just to clarify, if we are given a problem lik...

Chloe Alviz 1E

Can someone explain to me what the transition state theory is, and how it differs from the collision theory? Thank you!

Chloe Alviz 1E

How do we know when the entropy of the surroundings is positive or negative in a volume expansion?

Chloe Alviz 1E

I think most of the problems that we do involve expansion work (so a change in volume is given), but since you can relate work with U = q + w, I assume you can find it through this equation given the internal energy and enthalpy.

Chloe Alviz 1E

Even though Lavelle said that we won't be using steady state, would we still need to know the basic idea behind it?

Chloe Alviz 1E

For each reaction that is spontaneous under standard conditions, (K>1), write a cell diagram, determine the standard cell potential, and calculate ΔG°for the reaction: 2NO 3 - (aq) + 8H + (aq) + 6Hg(l) --> 3Hg 2 2+ (aq) + 2NO(g) + 4H2O (l) I was able to do most of this problem except for the cell di...

Chloe Alviz 1E

In the book, it says that when E°cell > 0, then K >1. What is the reasoning behind this?

Chloe Alviz 1E

"Suppose that each of the following pairs of redox couples is combined to form a galvanic cell that generates a current under standard conditions. Identify the oxidizing agent and the reducing agent, write a cell diagram, and calculate the standard cell potential from the standard potentials of...

Chloe Alviz 1E

Predict the potential of each of the following cells:

(a) Pt(s)|H₂(g,1.0 bar) |HCl(aq, 0.075M) ||HCl(aq, 1.0 M) | H₂(g, 1.0 bar)|Pt(s)

Can someone walk me through this problem? I'm having trouble with the half-reactions and setting up Q.

Chloe Alviz 1E

Is there a certain order/arrangement that phases (ex. solid, liquid, gases) are placed in the cell diagram? I do know that the cathode is on the right and anode is on the left.

Chloe Alviz 1E

What does C(gr) mean?

Chloe Alviz 1E

Calculate the standard reaction Gibbs free energy for the following cell reactions:

b) 6Fe³⁺(aq) + 2 Cr³⁺(aq) + 7 H2O(l) --> 6Fe²⁺(aq) + Cr₂O₇^2-(aq) + 14H⁺(aq)
where Ecell°= -1.29 V

Can someone walk me through how they found "n" for this problem?

Chloe Alviz 1E

"Balance each of the following skeletal equations by using oxidation and reduction half-reactions. All the reactions take place in basic solution. Identify the oxidizing agent and reducing agent in each reaction." b) Br 2 (l) --> BrO 3 - (aq) + Br - (aq) Can someone walk me through the bal...

Chloe Alviz 1E

"Balance each of the following skeletal equations by using oxidation and reduction half-reactions. All the reactions take place in acidic solution. Identify the oxidizing agent and reducing agent in each reaction." d) Cl 2 (g) ––> HClO(aq) + Cl 2 (g) For this reaction, why would Cl 2 be bo...

Chloe Alviz 1E

If the cell potential of a battery is positive, does it mean that that Gibbs Free Energy would be negative too?

Chloe Alviz 1E

What did Dr. Lavelle say was his "recommended" way of solving for the cell potential? He mentioned it during lecture earlier this week but I did not write it down :/

Chloe Alviz 1E

Half reactions are oxidation and reduction processes that occur in a redox reaction.

A great way to remember them is OIL RIG –– "oxidation is loss (of electrons), reduction is gain."

Hope this helps!

Chloe Alviz 1E

When would we use a comma versus a single line between the reactants and products?

Chloe Alviz 1E

How would we know when to use platinum as a cathode/anode in a battery?

Chloe Alviz 1E

I know when ΔU = 0, but in these cases, how do I interpret q=-w?

Chloe Alviz 1E

What are the units for enthalpy, exactly? Can they be written in the form of KJ/mol?

Chloe Alviz 1E

Given two different molecular formulas, how would we determine which one has a higher residual entropy?

Chloe Alviz 1E

Use data in Table 4C.1 or Appendix 2A to calculate the entropy change for (a) the freezing of 1.00 mol H₂O(l) at 0.00 8C; (b) the vaporization of 50.0 g of ethanol, C₂H₅OH, at 351.5 K.

Can someone walk me through this problem? Thanks!

Chloe Alviz 1E

What is the minimum value that K has to be for it to be considered "small" (so the 5% rule can be used?)

Chloe Alviz 1E

Carbon disulfide can be prepared from coke (an impure form of carbon) and elemental sulfur: 4C(s) + S 8 (s) --> 4 CS2(l) ΔH°= +358.8 kJ (b) Calculate the heat absorbed in the reaction of 197 g of carbon with an excess of sulfur. (c) If the heat absorbed in the reaction was 415 kJ, how much CS2 was p...

Chloe Alviz 1E

What's the difference between nonlinear and linear molecules for heat capacity?

Chloe Alviz 1E

Why does Lavelle avoid using the term "disorder" when he explains entropy? If we were asked to describe it on an exam, how should I define it?

Chloe Alviz 1E

Why is ∆U = 0 for isothermal expansion (or compression) of an ideal gas?

Chloe Alviz 1E

When exactly does enthalpy equal "q"? Are there certain conditions we need to follow?

Chloe Alviz 1E

Can someone explain to me the difference between non-expansion and expansion work? Also, how does this concept of expansion pertain to enthalpy?

Chloe Alviz 1E

Air in a bicycle pump is compressed by pushing in the handle. The inner diameter of the pump is 3.0 cm and the pump is depressed 20. cm with a pressure of 2.00 atm. (a) How much work is done in the compression? (b) Is the work positive or negative with respect to the air in the pump? (c) What is the...

Chloe Alviz 1E

In the book, it states that the heat lost in an reaction is heat gained by the calorimeter (-q=q_cal). Can someone explain why this is?

Chloe Alviz 1E

Can someone explain the "reversible process" that they talk about in section 4A.3 of the reading? I was thrown off when they mentioned how a "reversible reaction can be reversed by an infinitely small change in a variable".

Chloe Alviz 1E

Do we need to know how to calculate equilibrium concentrations for polyprotic acids?

Chloe Alviz 1E

For temperature increases in a system that is in equilibrium, why would the K value change?

Chloe Alviz 1E

We don't really need to know this concept; as long as you know what values you plug into for the formula you should be fine.

Chloe Alviz 1E

For this question, I'm stuck on part b:

Calculate the pH of ... (b) 0.055 M AlCl₃(aq).

Although I know the process of finding the pH, I'm not sure how to write the equilibrium reaction for this salt. Can someone walk me through it?

Chloe Alviz 1E

For reactions that are in homogeneous equilibrium (all reactants and products are gases, in this case) how would we know whether we need to use partial pressure units or molar concentration?

Chloe Alviz 1E

How can we tell if a reaction is endothermic or exothermic besides looking at the sign of ΔH?

Chloe Alviz 1E

I have a question on one part of this question: Consider the equilibrium CO(g) + H 2 O(g) <--> CO 2 (g) + H 2 (g). b) If the partial pressure of CO is decreased, what happens to the partial pressure of CO 2 ? Why would the partial pressure of CO 2 decrease if the moles of gas on each side are equal?

Chloe Alviz 1E

I still can't quite understand how equilibrium works with temperature (the "exothermic" and "endothermic" seem to be throwing me off). I've been trying to read the textbook for help but I'm quite lost. Can someone explain the process to me?

Chloe Alviz 1E

Do solids and liquids apply to Le Chatelier's principle? For example, if we removed a solid reactant, would there be any effect on the equilibrium?

Thanks!

Chloe Alviz 1E

During lecture, Dr. Lavelle mentioned how as Ka increases, Kb decreases. What is the reason for this?

Chloe Alviz 1E

A reaction mixture that consisted of 0.400 mol H 2 and 1.60 mol I 2 was introduced into a flask of volume 3.00 L and heated. At equilibrium, 60.0% of the hydrogen gas had reacted. What is the equilibrium constant K for the reaction H 2 (g) + I 2 (g) <=> 2 HI(g) at this temperature? Can someone help ...

Chloe Alviz 1E

When we write the K expression of a reverse reaction, would we flip the products or reactants around? Or would the expression remain the same?

Chloe Alviz 1E

Q, or the "reaction quotient", measures the relative amount of reactants and products at a certain time. It essentially describes a reaction when it's not in equilibrium and what direction will be taken for it for equilibrium to be restored (hence the favoring of a forward or reverse react...

Chloe Alviz 1E

I was able to understand and follow most of this example, but I noticed that in Step Four that they removed the "-2x" and "-x" from the (3.21 - 2x)² and (6.21 - x) from the K expression. What is the reasoning behind this?

Chloe Alviz 1E

For several problems, I noticed how they used either "bar" or "Pa" for the units of partial pressure. Which set of units is the "default" one? Also, how would we convert between bar and Pa (kPa as well)?

Chloe Alviz 1E

What's the difference between K and K_c?

Chloe Alviz 1E

Does the coordination number have any correlation to whether a ligand is polydentate?

Chloe Alviz 1E

In some of the naming coordination compounds on the homework set, I noticed how some complexes had the name "cyano-" or "cyanido". How do we know when to use which?

Chloe Alviz 1E

What page in the textbook can we refer to acid rain?

Chloe Alviz 1E

Which of the following ligands can be polydentate? If the ligand can be polydentate, give the maximum number of places on the ligand that can bind simultaneously to a single metal center: (a) HN(CH 2 CH 2 NH 2 ) 2 ; (b) CO 3 -2 ; (c) H 2 O; (d) oxalate How can we tell if something is mono or polyden...

Chloe Alviz 1E

How do we know if a coordination compound has a tetrahedral or square planar shape?

Chloe Alviz 1E

How do we determine the oxidation state of the metal atom / ion in a complex?

Chloe Alviz 1E

Do molecules with a shape such as bent or seesaw have hybrid orbitals?

Chloe Alviz 1E

How many sigma bonds and how many pi bonds are there in (a) H₂S and (b) SO₂?

For SO₂, the answer says that there are two sigma bonds and one pi bond. Why do they use the Lewis structure with one double bond and one single bond rather than the one with two double bonds?

Chloe Alviz 1E

Can you explain how to find the hybridization? I've always struggled with it I would recommend looking at the example listed in 2F.3 (page 122 on the pdf). You should first identify the molecular shape about the central atom - the number of atomic orbitals it has is equivalent to the hybridized orb...

Chloe Alviz 1E

In the textbook, it says that all single covalent bonds are sigma bonds. If this is the case, how would we define the bond for an ionic compound such as NaCl?

Chloe Alviz 1E

When you have a double bond, is it correct to label it as "σ + π"? Also, given a molecule with multiple single bonds, is it correct to label every single bond with a "σ"? I didn't get my test back last week (since I left campus early) and this question has been buzzing around my ...

Chloe Alviz 1E

When can we tell when a bond angle is "slightly less than" what is expected?
For example, if we look at CH₃^- from problem 2E.15, why are the bond angles considered to be "slightly less than 109.5 degrees"?

Chloe Alviz 1E

I read in the textbook that the hydrogen bond is the strongest intermolecular force; however, other sources I've looked at said that the ion-dipole interaction is the strongest force. Which one is stronger?

Chloe Alviz 1E

The strength of London Dispersion Forces is determined by the polarizability - or how easy the electron cloud is distorted - of an atom. Remember that larger atoms have higher polarizabilities because more electrons means more shielding from nuclear charge. Therefore, the larger the molecule, the st...

Chloe Alviz 1E

Why does CHI3 have the higher boiling point if CHF3 is more polar (higher electronegativity)?

Chloe Alviz 1E

For a) HCl vs. NaCl, why does NaCl have the higher boiling point? What factors do we use to compare these two?

Chloe Alviz 1E

Draw the Lewis structure and give the approximate bond angles of

a) C₂H₄
...
d) N₂H₄

Although drawing the Lewis structures wasn't difficult, how do I determine the shape of these molecules since they have two central atoms?

Chloe Alviz 1E

I'm a little stuck on part b of this question:

b) How many different OSCl bond angles are there in this molecule (SOCl₂)?

How do I interpret this question?

Chloe Alviz 1E

An example of a linear molecular shape would be N₃-. It's from 2E.17.

Remember that a linear molecular shape has a bond angle of 180, so it should appear as a straight line.

Chloe Alviz 1E

Are there any tricks to drawing Lewis Structures for molecules with three or more different types of atoms?
Ex. CH2F2

Chloe Alviz 1E

What is vapor pressure exactly, and how can we determine if a molecule has a "high" or "low" vapor pressure based on its intermolecular forces?

Chloe Alviz 1E

Does polarizability have any connection to a molecule being polar?

Chloe Alviz 1E

What trends can we use to determine the polarizing power of an atom?

Chloe Alviz 1E

Why is the ionization energy of oxygen lower than that of nitrogen?

Chloe Alviz 1E

For each pair, determine which compound has bonds with greater ionic character: (a) HCl or HI; (b) CH4 or CF4; (c) CO2 or CS2 For (c), I looked at the answer key and I noticed that the CO2 was considered the most ionic because "C" and "S" have nearly identical electronegativities...

Chloe Alviz 1E

Place the following elements in order of increasing electronegativity: antimony, tin, selenium, and indium.

Why does Selenium have the biggest electronegativity out of all of them if it has less protons?

Chloe Alviz 1E

What does it actually mean to have "ionic" or "covalent" character?

Chloe Alviz 1E

You are correct! They are interchangeable terms - basically you should instantly know that it's the minimum energy required to eject an electron from a metal!

Chloe Alviz 1E

When I think about the trends, I like to think about them conceptually! Going across a row (L to R), the electrons occupy the same shell, so there is less shielding from the nucleus. As a result, they have a greater pull to the nucleus and are much smaller in radius. When you go down a group, the el...

Chloe Alviz 1E

Although I know that nodal planes are where there is zero electron density (and they are in p- and d- orbitals), what's their significance? What can we understand about an orbital through the nodal plane?

Chloe Alviz 1E

On the learning outcomes for the Quantum world, one of the guidelines was "Use the Bohr frequency condition to explain the origin of the lines in the spectrum of an element". What exactly is Bohr's Frequency condition?

Chloe Alviz 1E

For Cu+, why is the configuration written as [Ar]3d¹⁰ instead of [Ar]3d⁸4s²? Could someone also explain the configuration rules regarding the d-block?

Chloe Alviz 1E

"How many unpaired electrons are predicted for the ground-state configuration of each of the following atoms: (a) Bi; (b) Si; (omit C) (d) Ni?" Can someone walk me through this problem? My logic for "unpaired" electrons were the orbitals with only a single electron, but I don't t...

Chloe Alviz 1E

I don't think that we need to calculate the effective nuclear charge - I think as long as you know the basic concept of it you will be fine.

Effective nuclear charge is the pull that the nucleus has on an electron, taking into account the shielding from other electrons.

Chloe Alviz 1E

You can generally memorize the number of orbitals, but if you want to be technical... You can figure out the number of orbitals per sub shell through the quantum numbers! The subshells are depicted by l = 0,1,2,3 (which is s,p,d, and f, respectively). The magnetic quantum number m represents the ind...

Chloe Alviz 1E

For this question, one of the parts stumped me: "Of the following sets of four quantum numbers { n , l , m l , m s }, identify the ones that are forbidden for an electron in an atom and explain why they are invalid: b) (5, 0, -1, +1/2)" Why is the m l invalid for this question? Wouldn't -1...

Chloe Alviz 1E

For this question - which asks if the following statements in each listed part are true - part "c" stumped me the most: c) Electrons having l = 2 are better at shielding than electrons having l = 1. According to the answer key, this statement is false, since p-electrons are "less able...

Chloe Alviz 1E

A vacuum is a container with low pressure or virtually no matter. If you can recall, the photoelectric experiment was carried under a vacuum so the photons (light) wouldn't be intercepted by particles in the air.

Chloe Alviz 1E

Are we responsible for knowing the regions (and their respective wavelengths) of the EM spectrum? Or we given them during an exam?

Chloe Alviz 1E

The DeBroglie equation doesn't apply to photons because, like what the previous poster said, they do not have a measurable mass.

It applies to any particles with a mass - not just electrons! One homework problem in 1B asked for the wavelength of a proton and neutron using the equation.

Chloe Alviz 1E

When scientists first carried out the photoelectric experiment, they first shone a longer wavelength (or smaller frequency) of high intensity light on a metal. However, this didn't eject any electrons. It wasn't until they increased the frequency of the light (or shorter wavelength) that the electro...

Chloe Alviz 1E

(Page 15 in the book, 152 on the pdf.) Here is the question: "Suppose you are developing a radiation detector to be used on a spacecraft and decide to use a thin layer of metallic potas- sium to detect certain ranges of electromagnetic radiation. You need to make some estimates of the physical ...

Chloe Alviz 1E

While I was reading the text, I noticed the following statement about Lyman and Balmer lines: "The Balmer series consists of the lines with n1 = 2 (and n2 = 3, 4, . . .). The Lyman series is a set of lines in the ultraviolet region of the spectrum with n1 = 1 (and n2 = 2, 3, . . .)." Altho...

Chloe Alviz 1E

You can use Planck's constant when you are determining the energy of a photon (E = hv) and the energies at different n levels ( En = -hR / n^2)
It is also used in De Broglie (lambda = h/p).

Chloe Alviz 1E

1A.3 asks:

"Which of the following happens when the frequency of electromagnetic radiation decreases? Explain your reasoning.

[I was able to get a, b, and d]

c) The extent of the change in the electrical field at a given point decreases."

How do I interpret this problem?

Chloe Alviz 1E

My question refers to a problem from the post-assessment module: "Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 10^5 ms-1. The work function for sodium is 150.6 kJ mol-1...." In this question, what does the "work function" refer to? Is it su...

Chloe Alviz 1E

Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 10^15 s^-1 before it can emit an electron from its surface. A. What is the minimum energy needed for this effect? B. If the metal is irradiated with 194 nm of light, what is the maximum KE of emitted electrons? I was able to s...

Chloe Alviz 1E

Question: A solution is prepared by dissolving 0.500 g of KCl, 0.500 g of K2S, and 0.500 g of K3PO4 in 500. mL of water. What is the concentration in the final solution of (a) potassium ions; (b) sulfide ions? Can someone walk me through this problem? I've walked through it (part a, specifically) by...

Chloe Alviz 1E

Question G7 asks how much of each mass component we would need to prepare 510 g of an aqueous solution (containing 5.45% KNO3 by mass). What are the mass components besides KNO3? Can someone walk me through this problem?

Chloe Alviz 1E

How do significant figures work when we are adding decimal values together (ex. 0.38 + 0.7)? Also, should I wait to round my values until the end of the problem?

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