CHEMISTRY COMMUNITY

William Francis 2E

Only one element acts as an oxidizing or reducing agent in any given half reaction. For instance, in the reduction half reaction MnO 2 +4H + +2e - -->Mn 2+ +2H2O, Mn acts as the oxidizing agent as it's oxidation number decreases from positive four to positive 2. However, the entire molecule Mn is in...

William Francis 2E

You may find Toolbox 6K.1 on page 538 of the textbook helpful for this. First balance the species being either oxidized or reduced in the half reaction. Then, balance oxygen by adding liquid water molecules. If the solution is acidic, add aqueous H + ions to balance hydrogen. If the solution is basi...

William Francis 2E

In an oxidation half reaction, electrons are found on the product side of the equation and the oxidation number of the species being oxidized (called the reducing agent) increases. In a reduction reaction, electrons are found on the reactant side of the equation and the oxidation number of the speci...

William Francis 2E

Unique average reaction rate is described on page 590 of the textbook. Coefficients in a balanced chemical equation do not indicate the order of the reaction. The rate at which oxygen reacts is equal to the change in the concentration of oxygen divided by time regardless of coefficients. However, th...

William Francis 2E

Does anyone know if homework is still due at the time of our discussion if we are emailing it in?

William Francis 2E

I'm a bit confused about what you mean. I don't see H 2 O in the skeletal equation given, but it does need to be added to balance the equation. The first skeletal half reaction is Br - (aq)-->BrO 3 - (aq). The oxidation number of Br increases from -1 to +5 in this, so we know it is the oxidation hal...

William Francis 2E

The textbook almost always uses Pt(s), but I'm quite sure that you could use either with equal success.

William Francis 2E

Yes, every problem I've encountered where I've had to balance a redox reaction have specified whether or not the reaction occurs in a basic or acidic solution (to know if H₊ or OH^- are available when it comes time to balance hydrogen in the half reactions as described in Toolbox 6K.1 on page 538).

William Francis 2E

I find it very helpful to write out the half reactions to really gain an understanding of the redox reaction. To do this, you will have to determine the oxidation numbers of the elements being reduced and oxidized. Toolbox K.1 on page F80 is very useful to refresh on oxidation numbers.

William Francis 2E

Potassium and chlorine respectively dissociate from permanganate (MnO 4 - ) and iron (Fe 2+ ) when the compounds are dissolved in water. Therefore, these are spectator ions that have no role in the oxidation or reduction reactions we are concerned with. This is the case when dealing with ionic compo...

William Francis 2E

The Cl- comes from the reduction of Chlorine gas (Cl 2 +2e - -->2Cl - ). After you find the oxidation reaction of chlorine, you can determine that it must also be reduced since it is the sole reactant listed. Although Cl - is not in the skeletal equation given, it must be present in the final balanc...

William Francis 2E

In other words, the half reaction with the larger reduction potential occurs at the cathode as a reduction reaction while the other half reaction is flipped to become an oxidation half reaction at the anode.

William Francis 2E

When the reduction potentials of both half reactions are positive, the more positive one will be reduced at the cathode. When they're both negative, the less negative one is reduced at the cathode.

William Francis 2E

Water's reduction potential is -0.83 V. When one reduction potential is negative and the other is positive, the positive one will be reduced at the cathode.

William Francis 2E

Toolbox K.1 found on pages F80-F81 of the textbook describes a detailed explanation of how to determine oxidation numbers. In general, oxidation numbers can be determined by assuming that a hydrogen atom in a compound has an oxidation number of +1, that an oxygen atom has an oxidation number of -2, ...

William Francis 2E

Toolbox 6K.1 entitled "How to Balance Complicated Redox Equations" on page 538 of the textbook provides all the information needed to work through 6K.1. If you need to review oxidation numbers, I suggest looking at page F80 of the textbook. Hope that's helpful!

William Francis 2E

First, you should balance the species being oxidized or reduced in the half reaction. Then, add water to balance oxygen. Then, add H + to balance hydrogen in acidic solutions. If the reaction is in a basic solution, you add OH - to one side of the half reaction and H 2 O to the other side to balance...

William Francis 2E

Half reactions are the two halves of a redox reaction. One half reaction is an oxidation reaction, and one is a reduction reaction. Half reactions include electrons in them. However, the textbook notes that electrons are never actually free in redox reactions. Half reactions are a convenient and sup...

William Francis 2E

Page 574 of the textbook defines Faraday's constant as "the magnitude of the charge per mole of electrons."

William Francis 2E

Since Chlorine is the only non-oxygen/hydrogen element involved in this reaction, it can be assumed that it is both the oxidizing and reducing agent. The oxidation half reaction is relatively simple to determine based upon the rules described in Toolbox 6K.1. Since you know that Cl 2 is also the oxi...

William Francis 2E

I used to get confused because it's sorta counter-intuitive that something being reduced is gaining particles (electrons). It helps me to think of reduction as the charge being reduced though. Since electrons have a negative charge, they lower the charge of whatever they are added to. That thought p...

William Francis 2E

Yes. The universe is pretty large compared to the piston, so the volume change is pretty negligible.

William Francis 2E

When pressure is constant, ΔH=q. This is because ΔH=ΔU+PΔV. Since ΔU=q+w, the equation can be rewritten as, ΔH=q+w+PΔV. Since w=-P_exΔV, that equation can be rewritten as ΔH=q-P_exΔV+PΔV. P=P_ex in isobaric conditions so ΔH=q. Therefore, C_p=ΔH/ΔT under these conditions.

William Francis 2E

ΔS° is defined as entropy change in a set of standard conditions. ΔS° won't change for a given reaction unless those standard conditions do.

William Francis 2E

Conceptually, I think it may be most important to think about Gibb's Free Energy as a convenient state function used to determine whether a reaction will proceed spontaneously. As previously stated, it is a measure of energy available to do work in a system.

William Francis 2E

Also, I believe that ΔU is equal to q+w, not ΔH, so when no work is being done on or by a system, it doesn't necessarily mean that ΔH=q. Rather, ΔH=q under constant pressure conditions. When no work is being done by a system, ΔH=q+PΔV as derived from ΔH=ΔU+PΔV with (q+w) substituted for ΔU and w=0.

William Francis 2E

I think it should be noted that enthalpy can never really be determined. When pressure is constant, ΔH=q, so the change in enthalpy is equal to heat, but enthalpy itself is not equal to heat.

William Francis 2E

Raising temperature (adding heat to system) will favor the endothermic reaction over the exothermic reaction in order to reach equilibrium. This changes the ration of products to reactants therefore changing the K value. Decreasing temperature favors the exothermic reaction.

William Francis 2E

C_v is used for systems with constant volume. C_p is used for systems with constant pressure.

William Francis 2E

ΔH is equal to q when pressure is constant. This can be derived using the equation ΔH=ΔU+PΔV. (q+w) can be substituted for ΔU. (-P ex ΔV) can then be substituted for w. This results in ΔH=q since -P ex ΔV cancels out with PΔV as long as P=P ex . I think this condition (P=P ex ) may be what was being...

William Francis 2E

Water molecules in steam have more energy at 100 degrees Celsius than the molecules in liquid water at the same temperature for the reason you stated. Even though the temperatures are the same, the steam contains more energy capable of doing work to mess up your skin upon contact.

William Francis 2E

Bond enthalpies listed state how much energy is required to break a bond between two specific atoms on average (it’s important to understand that this is just an average and not specific for whatever molecule you are working with). It takes energy to break bonds so these values are positive. Convers...

William Francis 2E

You need to take phase changes into account when calculating change of enthalpy in a reaction. The calculations we use assume that all the reactants and products are gases.

William Francis 2E

A bomb calorimeter is completely sealed and insulated, making it an isolated system. It maintains constant volume, so it must withstand very high pressures or else it will blow up like a bomb).

William Francis 2E

The biggest difference to me seems to be that isolated systems include insulation. For instance, a completely sealed water bottle is a closed system (since it can still exchange energy with the surroundings through the medium of the bottle’s material), but a well-insulated bottle is an isolated syst...

William Francis 2E

100 degree Celsius steam contains substantially more energy (and therefore more potential to harm a person's skin) than liquid water at 100 degrees Celsius since the steam has all of the heat the liquid water has PLUS the energy that was required to vaporize the liquid water.

William Francis 2E

I think acids can still be considered weak with a Ka higher than 10^-3. Strong acids almost completely dissociate in water, so Ka values are never even really given for them since they would be so large. 10^-3 has (somewhat arbitrarily) been deemed the cutoff point to distinguish small K values from...

William Francis 2E

pKa's are used for acids and pKb's are used for bases. The K value for a reaction between an acid and liquid water is deemed a Ka value. A Kb value is the K value for a reaction between a base and liquid water. pKa's are calculated by taking the negative log of the Ka. pKb's are calculated by taking...

William Francis 2E

You can check that your approximation is valid after solving by showing that your calculated x value is less than 5 percent of the initial concentration of the weak acid or base being used.

William Francis 2E

Kp should be used with gases since you're dealing with partial pressures rather than concentrations.

William Francis 2E

Yes, I believe you can determine the equilibrium concentrations of all molecules present in a reaction at equilibrium given the concentration of one reactant or product at equilibrium and a balanced chemical equation. I doubt this situation is very common though.

William Francis 2E

We don't need to know how the K equation, [products]^x/[reactants]^y is derived. I do not believe that making the coefficients exponents has any obvious conceptual implication.

William Francis 2E

Endothermic reactions are hungry for heat. You can think of heat as a reactant in the equation necessary for the reaction to run: AB + heat <--> A + B. Therefore, when the reaction gets heat, it favors the forward reaction, resulting in a raised concentration of the products. The opposite is true fo...

William Francis 2E

I don't think it really matters when the question does not specify which to use. The calculations are the same.

William Francis 2E

The moles of a substance used in a reaction may differ from the number of moles of the substance actually found in the solution. We cannot assume the concentration of a reactant based upon the concentration of another reactant because we have no idea how much of it was actually added to the solution...

William Francis 2E

I'm pretty darn sure that at least 5 posts per week are still required for full credit. The grading breakdown from the syllabus lists "Weekly Online Discussion (5 pts per week)" as a section of the grade. I believe that each point corresponds to one post, meaning that five posts per week w...

William Francis 2E

<-> means a reaction is at equilibrium while -> means the reaction is overall travelling in one direction. It doesn't matter which is used in an equation when computing Q since Q values can be calculated for reactions at equilibrium as well as for reactions not at equilibrium.

William Francis 2E

It would be the inverse of the one reactant concentration since K_c is always calculated with product concentrations over reactant concentrations. If no reactants' or products' can be input then you just input 1.

William Francis 2E

Temperature affects K c because the forward reaction is either exothermic or endothermic. If the forward reaction is endothermic, then the K c value will be larger when the temperature is higher since the forward reaction will absorb the added heat and produce a greater proportion of products to rea...

William Francis 2E

Nope. You do not include solids and liquids when calculating Q. It's just like calculating K except the reaction may or may not be at equilibrium.

William Francis 2E

Yes, I think that you can usually assume this. Hydrogen bonds only occur with Nitrogen, Oxygen, and Fluorine since these are the most electronegative atoms, so they can result in relatively strong dipoles.

William Francis 2E

I find it helpful to draw lewis structures when trying to determine the strength of an acid relative to another as it allows me to assess how strongly hydrogen atoms are bonded to the molecule. The easier those bonds break, the more acidic it is.

William Francis 2E

Linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, and square planar shapes are symmetrical which means that molecules of these shapes are nonpolar if all atoms bonded to the central atom are the same.

William Francis 2E

If a ligand binds to a central atom at more than one sites, then it is a polydentate ligand.

William Francis 2E

Yes, if one of the attached atoms is different, it is polar. So if all the atoms are the same, then it is nonpolar? I believe that if the three atoms attached to the central atom are the same, the molecule will be nonpolar because the three potential dipoles would cancel out. I look at it as vector...

William Francis 2E

My TA did confirm that we need to memorize the common ligand names as provided on page 724 of our textbook for the final exam. Fun stuff!

William Francis 2E

I'm not exactly sure how anionic ligands affect naming, but I do know that if the complex in a coordination compound is an anion, you use the latin name of the central atom in naming the compound (for example: "ferrate" instead of "iron").

William Francis 2E

This has to do with the bond length as the longer the bond, the more easily it is broken. This means that if the bond is longer, then it would be considered a stronger acid in comparisons such as HF and HI as in this case HI bond is longer making it easier to lose the H+. How do you tell if it has ...

William Francis 2E

Strong acids contain longer bonds that are more easily broken, causing H+ ions to be more readily emitted. For instance, the bond in HI is longer than that of HF since Fluorine is more electronegative than Iodine. As a result, HI is more acidic than HF.

William Francis 2E

The oxidation number of a metal can be determined by looking at the ligands bonded to the central metal atom in a coordination sphere and the overall charge of the complex. We're supposed to memorize the charges of common ligands as given on page 724 of the text book. The charges of the ligands plus...

William Francis 2E

Ionic bonds are classified as bonds similarly to covalent bonds meaning that they are intramolecular forces. When two oppositely charged ions interact, they form a bond and become one molecule so the attractive force is within that molecule rather than between two (hence, it is intramolecular). Conv...

William Francis 2E

I believe that this molecule would be nonpolar since carbon, hydrogen, and iodine all have very similar electronegativities. Carbon and iodine have electronegativites of 2.5 and hydrogen has an electronegativity of 2.1. There isn't substantial enough electronegativity difference between atoms to res...

William Francis 2E

When a central atom is bonded to two atoms with no lone pairs, it is linear. If one lone pair is present on the central atom, the molecule is bent since the lone pair repels the bonded atoms. I find it useful to count how many areas of electron density there are for the central atom before I determi...

William Francis 2E

Permanent dipoles exist in polar molecules. When oppositely charged dipoles attract between molecules, it is called a dipole-dipole force. Induced dipoles are temporary and present in nonpolar molecules when electrons unevenly distribute charge. These are Van Der Waal's forces.

William Francis 2E

If a substance is in a gas phase, it means that the forces holding the molecules together have been overcome by the energy added. For instance, the hydrogen bonds in liquid water are broken when the water is heated, causing the water to enter a gaseous phase. So the dipole-dipole forces have the sam...

William Francis 2E

A double bond contains one sigma bond and one pi bond. A triple bond is one sigma bond and two pi bonds. You can remember this because sigma bonds correspond to orbitals in the s subshell of valence electrons. Since there is only one orbital in the s subshell, only one sigma bond can be made; the re...

William Francis 2E

The De Broglie equation comes in handy when you need to find the wavelength of something with mass or if you are given the wavelength and required to find the values of other variables (mass or velocity).

William Francis 2E

This question does not ask for an exact bond angle to be given for ClO2+. I think it's asking more generally for the shape of the molecule. Since the central chlorine atom has one lone pair and is bonded to two oxygen atoms, we know that the molecule will have a bent, trigonal planar shape with an O...

William Francis 2E

After you've worked to ensure that the atoms in the molecule you're drawing satisfy the octet rule (unless they're exceptions), you should calculate the formal charge of each atom in the molecule by subtracting the number of bonds it's formed and the number of unpaired electrons around it from the n...

William Francis 2E

The bond angle is linear (180 degrees) because the central atom (Be) is bonded with two atoms (both Carbon) with no extra lone pairs to potentially distort the shape of this bond angle. I would assume that the bond angle would be 120 degrees if the central atom was a Boron atom bonded to three Carbo...

William Francis 2E

Guanine and cytosine are held more tightly together than adenine and thymine in DNA because three hydrogen bonds link them as opposed to the two linking adenine and thymine.

William Francis 2E

I was confused about that too, but I think I have it figured out now. In many cases, the question will imply where the electron in the hydrogen atom begins or ends. For instance, if ultraviolet light is emitted by the change in energy level, this light can be recognized as part of the Lyman series, ...

William Francis 2E

Iodine has a larger atomic radius than bromine, fluorine, and chlorine as a result of its added shells of electrons. All of iodine's electrons are just sloshing around with potential to unevenly distribute charge throughout the atom. This sloshing results in Van Der Waal's forces or London Dispersio...

William Francis 2E

Oxygen's electron configuration is 1s2 2s2 2p4. Since a p subshell houses three orbitals that can contain up to 6 electrons, a p subshell containing 3 electrons is considered half full with one electron in each orbital (as per Hund's Rule). Half full subshells can be somewhat desirable for atoms (al...

William Francis 2E

Polar bonds occur when two atoms of substantially different electronegativities bond with each other. For the purposes of this class, I believe we can generally label hydrogen and carbon as having low electronegativity while nitrogen, oxygen, and fluorine have high electronegativity. I mention these...

William Francis 2E

Also, I'd add that it's important to remember what SI units are used for constants, so that you can be sure the units cancel out in your calculations. For instance, lengths used in calculations involving Planck's constant should be in meters, masses should be in kilograms, and times should be in sec...

William Francis 2E

Shorter bond lengths are stronger because the attractive charges are in closer proximity to each other. I like to picture James Bond as a short, stocky, strong man who will not easily be broken. Because "bond" is a chemistry thing, but it's also the last name of the fictional character, Ja...

William Francis 2E

I believe that electron affinity refers specifically to the energy required to add an electron to a neutral atom while ionization energy refers to the energy required to remove an electron from a neutral atom.

William Francis 2E

Effective nuclear charge describes the attraction between an atoms nucleus and an average electron in some shell, n. The effective nuclear charge is equal to the number of protons in the atom's nucleus minus the number of electrons in between the nucleus of the atom and a specified shell of electron...

William Francis 2E

Shorter bonds tend to be stronger bonds because the charged subatomic particles (protons and electrons) are in closer proximity to each other.

William Francis 2E

Antimony (Sb) has an electron configuration of [Kr] 4d10 5s2 5p3. Valence electrons are the electrons in an atoms outermost shell. In this case, the fifth shell of electrons is the outermost and it contains 2 electrons in the s subshell and 3 in the p subshell. So, in total, it has 5 valence electro...

William Francis 2E

Yep! We can do questions from The Quantum World or Chemical Bonds since these are the topics we've been covering this week.

William Francis 2E

Also, Germanium would have two unpaired valence electrons in its expected ground state since it has one more electron than Gallium in the 4p subshell and these electrons don't begin to pair in p subshells until the fourth electron is added.

William Francis 2E

As an example, Gallium's electron configuration can be written as [Ar] 3d10 4s2 4p1. The fourth shell of electrons is the outermost shell for Gallium, so we can determine that it has 3 valence electrons in total. However, the 2 electrons in the 4s subshell are paired, so Gallium has only one unpaire...

William Francis 2E

The lecture slides listed H, He, Li, and Be as exceptions to the octet rule. However, I think that Dr. Lavelle said that we'd discuss more exceptions in the next lecture.

William Francis 2E

I just read something that said that violations to the aufbau principle are more common for elements with higher atomic numbers than for those with lower atomic numbers. Why is this?

William Francis 2E

I like doing as many of the practice problems as I can and reviewing the modules. Practice makes perfect! As a side note, perfection is ultimately unobtainable. But practice is still always a good time.

William Francis 2E

Here's another fun fact that's relevant because it concerns how the momentum of light can affect relatively small particles: the 2018 Noble Prize in Physics was awarded to Arthur Ashkin for his work with optical tweezers. Optical tweezers are beams of light that can be used in labs to hold microscop...

William Francis 2E

I agree that it's a mass thing. Now, I'll add in some moderately fun facts. For instance, a baseball is estimated to be about 1.59x10^29 times more massive than an electron. I wanted to find something that has about the mass of 1.59x10^29 baseballs because I figure that's always a productive way to ...

William Francis 2E

Since you got that (1/n2^2)=.112, you can solve for n2 by first multiplying each side of the equation by n2^2. Then, you'll get that 1=.112(n2^2). From here, you divide each side by .112 to get that (n2^2)=8.93. If you then take the square root of each side you find that the value of n2 in the equat...

William Francis 2E

To start, you need to convert grams of C57H110O6 to moles. The molar mass of this fat is (57 times the molar mass of Carbon) + (110 times the molar mass of Hydrogen) + (6 times the molar mass of oxygen). Once converted to moles, you can multiply this number by 55 to convert it to moles of water sinc...

William Francis 2E

Is anyone familiar with Dr. Lavelle's rule for rounding to the nearest even number when there are 4 sig figs and you can only have 3? I don't think I've heard him talk about this. Is this a rule that isn't standard, but Dr. Lavelle wants us to apply? And what do you mean by the nearest even number ...

William Francis 2E

Perhaps I'm misinterpreting your question, but didn't you say that the wavelengths were already given? If not, what was given to be converted into wavelengths?

William Francis 2E

To solve (0.0398mol/0.25L)=(0.00215mol/xL) for x, I would start by multiplying each side of the equation by x. The equation then becomes (0.0398x/0.25)=(0.00215). From here, I would multiply each side of the equation by 0.25, then divide each side by 0.0398 to get x=0.0135 as the final answer. Since...

William Francis 2E

Frequency and intensity are variables manipulated for the light shining on the metal surface in the photoelectric experiment rather than for the electrons. In the equation Ek=1/2(me-)(ve-)^2, the "m" variable represents the mass of the electron (which I believe should be a constant), and t...

William Francis 2E

Update: I believe I have cracked the case. 2.27 grams of BaCl2 is equivalent to 0.0109 moles of BaCl2. There must be the same number of moles of BaCl2 as there are of BaBrx since these are the only two compounds including barium. Therefore, there are 0.0109 moles of BaBrx. We also know that there ar...

William Francis 2E

Is it worth noting that chlorine and bromine usually come as Cl2 and Br2 rather than just Cl or Br when they are by themselves? I too am having trouble knowing what the balanced equation would look like for this question.

William Francis 2E

Yes! You are correct. Furthermore, scientific notation could potentially be used to make it more clear that you answer is written with a certain number of sig figs. For instance, 400 could be written as 4.00 x 10^2. This may be preferable to writing "400." in some cases since the decimal m...

William Francis 2E

Fe2O3 + 2CO --> 2Fe + 2CO2 is balanced for iron and carbon but not oxygen. There are five oxygen units of oxygen on the left side of the equation but only four on the left. Since CO and CO2 are the only two compounds in the equation with carbon, they must have the same coefficient. Similarly, since,...

William Francis 2E

After you calculate the moles of each molecule involved in the reaction, you must compare the ratio between quantities of the different reactants to the ratio of their coefficients in the balanced equation for the chemical reaction. For instance, in the chemical equation 2NA+2H20-->2NaOH+H2, there i...

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