CHEMISTRY COMMUNITY

Isabelle Hales 1J

Hi. Everything looks good except for your number of H2O molecules. For this question, the entire product (Fe2O3 3H2O) is being multiplied by 2. Hope this helps!

Isabelle Hales 1J

Hi. I am struggling quite a bit with this problem. My question is to calculate the standard potential of the cell with 1.0 M iron (II) ion solution and 1.0 M silver ion solution. I want to make sure I am using the correct standard potential values. For silver, I am using +1.498 and for iron, I am us...

Isabelle Hales 1J

Hi. I was going through the homework and had a question about standard reduction potentials. Do you flip the sign for the value if the reverse reaction is happening (as you would for deltaG values)?

Isabelle Hales 1J

I am a bit confused as to how to balance redox reactions with H2O and OH-. Can someone help walk me through how I should approach these problems? An example of a problem I am struggling with is from Sapling, Question #3. The unbalanced reaction is BrO- + Ag+ > BrO3- + Ag. Thanks!

Isabelle Hales 1J

Hi! The salt bridge allows ions, such as Cl-, to transfer between the reactions (usually from the reduction solution to the oxidation solution). As the electrons leave the oxidation side, a negative charge begins to build up on the reduction side. To continue the reaction, the cathode side must rele...

Isabelle Hales 1J

Hi! I believe you can plug in the two temperatures into T1 and T2. Then, if you know what one of the K values is, you can use it to find the other one. Hope this helps, and correct me if I'm wrong.

Isabelle Hales 1J

Hi! How exactly does a salt bridge allow ions to move from one solution to another?

Isabelle Hales 1J

Hi! Going off of this, is the melting point also found at equilibrium (deltaG = 0)?

Isabelle Hales 1J

In last Friday’s lecture, Lavelle mentioned that when K > 1, deltaG under standard conditions is negative. He said that there are more products that reactants which will cause product formation to be favored. I am a bit confused by this. I thought that if there are more products than reactants, the ...

Isabelle Hales 1J

Based off my experience in 14A, yes. However, I did have multiple/different TAs for the final but that could have been random.

Isabelle Hales 1J

Hi!! You have the right answer, you simply need to input it in scientific notation! Hope this helps!

Isabelle Hales 1J

Hi! You are on the right track with that equation! I would make sure all of your units are correct. Make sure you have the correct exponents when calculating Q and that you use deltaG in joules, rather than kJ. Hope this helps!

Isabelle Hales 1J

Qrev is the heat generated by a reversible process. The change in enthalpy is a measure of the change in heat of a system over a reaction. Thus, these two terms are describing the same value. Hope this helps!

Isabelle Hales 1J

Hi! There is also the equation deltaG = deltaG(under standard conditions) + RTlnQ which I believe would work if the system is not at equilibrium because you are using Q, not K. Hope this helps (and correct me if I'm wrong).

Isabelle Hales 1J

Hi! Enthalpy describes the amount of heat released or absorbed at a constant pressure. Entropy, on the other hand, is a measure of the likelihood that the system will be in a particular state. If there are more possible states for the system to be in, the entropy increases. (In high school, I rememb...

Isabelle Hales 1J

The second midterm is Friday, February 19th (end of week 7).

Isabelle Hales 1J

The questions mentions that the reaction in the calorimeter will release heat, which is why we get a negative q value. Hope this helps!

Isabelle Hales 1J

Hi! Remember that deltaU includes both the energy from heat (q) and the energy from compression (or work). At a constant volume, no compression/expansion is occurring and thus no work is being done. This means that in the equation deltaU = q + w, w is zero. As a result, deltaU would just be equal to...

Isabelle Hales 1J

Hi! While doing the weekly sapling hw, I noticed that there are two values for the R ideal gas constant. I do not remember going over this in class but could someone please explain when I should use each different value? I know this is very basic material but it will really help!

Isabelle Hales 1J

Hi! Going off of this in regards to friday's lecture, do reversible expansions only occur in theory? Dr. Lavelle mentioned that when we calculate the work of a reversible reaction, it will be more than the actual reaction because reactions in real life are not perfect. Does this mean we only use rev...

Isabelle Hales 1J

Hi! I am curious to know about other examples of isolated systems besides a bomb calorimeter. Also, should I not focus too much on these and instead focus mainly on open/closed systems?

Isabelle Hales 1J

Hi! Going off of this, is there such thing as an imperfect system? Would that be a system that does not fit the rule stated above?

Isabelle Hales 1J

Hi! I believe energy transfers between the system and surroundings as the particles interact with each other. This is why it is important to note whether you are dealing with an open/closed/isolated system. The energy lost by the system is equal to the energy gained by the surroundings. However, I b...

Isabelle Hales 1J

Hi. I am a bit confused on this as well. Does the specific heat capacity differ for water between each state or do we just need to factor in the heat required to change phases. Also, does the specific heat capacity of other substances differ between each state?

Isabelle Hales 1J

Hi. I am a bit confused on this as well but I believe the difference between the two is in terms of which substance you are describing. If you are describing a proton being added to a base to form the conjugate acid, then you would use the term protonation. However, if you are describing an acid los...

Isabelle Hales 1J

Hi! Hess's method essentially allows you to add enthalpy changes (delta H) together in order to find the overall change in enthalpy. In Lavelle's example, you are given the enthalpy changes (delta H) of two reactions. The first reaction forms nitrogen oxide and the second reaction combines nitrogen ...

Isabelle Hales 1J

Is there a general method to determining the standard state of a compound or is it something that we need to memorize?

Isabelle Hales 1J

In lecture 6, Dr. Lavelle mentioned that (as a general rule) if the K value is smaller than 10^-4, it is okay to approximate. More specifically, if X is less than 5% of the initial concentration value, we can use an approximation.

Isabelle Hales 1J

Hi! This is a general question but I am a bit confused as to how the ideal gas law PV=nRT relates to the topics of chemical equilibrium? Or are they two separate topics?

Isabelle Hales 1J

Hi! For this question, it is helpful to write out the equation. With this problem, you have to work backwards from the pH in order to find the concentrations. You also need to remember to turn the pH into pOH because you are dealing with a base reaction. Once you have found the pOH, you can use it t...

Isabelle Hales 1J

Hi! He means that you would divide X from the initial concentration. Once you turn that value into a percent and it is less that 5, then it is okay to approximate. Because it is such a small value in comparison to the initial value, an approximation will not interfere too much with the end values. A...

Isabelle Hales 1J

Hi! I am struggling a bit with question #6. Does anyone have general advice for classifying salt compounds as acidic, basic, or neutral?

Isabelle Hales 1J

Hi! Based on Chem 14A, the formulas are not usually given on exams. However, on Dr. Lavelle's 14B website, there is a sheet of equations and formulas (including those concerning acid/base reactions) which we are allowed to have with us for exams. Hope this helps!

Isabelle Hales 1J

Hi! You would need to use an ICE table (Initial, Change, Equilibrium). First, you find the initial concentration of SO3 with the moles and volume given. The initial value for the products will be 0. For the change in concentration, SO3 will change by -2X because there are 2 moles, SO2 will change by...

Isabelle Hales 1J

Hi! In todays lecture, Dr. Lavelle mentioned in an example that the reaction with a very small equilibrium constant lies very far to the left. Is it it true to say that reactions with very small equilibrium constants lie to the left and reactions with very large equilibrium constants lie to the righ...

Isabelle Hales 1J

I understand that a change in temperature of a reaction causes a change in K. However, I am a bit confused to how the K changes and in which direction with the temperature increasing/decreasing.

Isabelle Hales 1J

Going off of this, are there any chemical reactions that only proceed in one direction (and thus wouldn't have an equilibrium)?

Isabelle Hales 1J

As the volume decreases, there are more moles of gas on the left (or the reactants side) because the concentration is increasing. The reaction shifts right, or in the forward direction, because the more moles of reactant will allow for more product to form. In other words, the reaction of forming pr...

Isabelle Hales 1J

Hi! In Lavelle's Friday lecture, he mentioned that a change in pressure due to adding an inert gas would not cause a change in the reaction because the volume does not change. Thus, I believe that a change in the reaction only occurs when the pressure and volume are changing.

Isabelle Hales 1J

Hi! In today's lecture, Dr. Lavelle mentioned that as the volume decreases and there are more moles of gas on the left, the reaction will shift right. What exactly does he mean in that the reaction shifts left or right?

Isabelle Hales 1J

Hi! I have a question relating to this topic. Can hydrogen bonds only occur between atoms that have dipole moments? Or can a H that is partially positively charged bond to a fully negatively charged ion? It would be great if someone could clear this up for me, thanks!

Isabelle Hales 1J

I had the same question. I keep counting 7 for both. Here is an image: Screen Shot 2020-12-12 at 5.38.34 PM.png For A, there are 5 nitrogens each with one lone pair, and 2 carbons that can accept an H. For G, there are 2 nitrogens, 2 oxygens each with 2 lone pairs, and one carbon. What am I missing...

Isabelle Hales 1J

Hi! In Lec #27, Lavelle discussed the properties that make an acid strong or weak. Strong acids lose protons easily, thus they have weaker/longer A-H bonds because the H+ is easier to remove. Additionally, a stronger acid will have a more stable resulting anion as the acid is more likely to lose the...

Isabelle Hales 1J

Hi! You are given the concentration of [OH-] ions in the solution. With this information, you can find the pOH value by calculating the -log([OH-]), just as you would for [H+] ions. Once you have found the pOH value, you can subtract that number from 14 to get the pH value. Lastly, with the pH value...

Isabelle Hales 1J

I am bit confused why HBrO4 is a stronger acid than HBrO3, HBrO2, etc. Can someone help me understand this?

Isabelle Hales 1J

Hi. Thanks everyone for all of their tips. I find it really helpful to go over the sapling hw questions and check for concepts that I do not understand as well. The textbook problems are also quite helpful with checking my understanding. I have also been talking through some concepts with other stud...

Isabelle Hales 1J

Hi! To add on, we usually use pH more often when discussing solutions. However, you may only be given the concentration of OH- ions and can only figure out the pOH. In this case, you can actually use the pOH to find the pH because of their close relationship by subtracting the pOH value from 14.

Isabelle Hales 1J

Yes! Remember that pH is a measure of the concentration of [H3O+] that ranges from 1.0 M to 1.0 x 10^-14 M. While the pH value changes by just 1 from 6 to 7, the actual concentration is changing from (1.0 x 10^-6 M) to (1.0 x 10^-7 M), thus decreasing by 10x. Hope this helps!

Isabelle Hales 1J

Hi! Are there many other amphoteric compounds outside of the metalloid band (such as water)?

Isabelle Hales 1J

Hi. Weak acids/bases are not fully ionized in solution, which means that they do not fully separate into the charged molecules. Strong acids/bases are completely ionized in solution. Hope this helps!

Isabelle Hales 1J

Hi! You are correct! Dr. Lavelle mentioned that the coordination number is the number of bonds.

Isabelle Hales 1J

How do we determine how many bonds a transition metal can make with other ligands/molecules? In Dr. Lavelle's example, Ni could form 6 bonds with other ligands. Is this the case for all transition metals? Do we need to memorize how many bonds each TM can make or can it be easily determined?

Isabelle Hales 1J

Hi! From my understanding, as you mix other substances into the solution with a transition metal, some of the new e- rich molecules will mix into the compound and replace H2O as ligands. While the TM is being dissolved, it can still only bind to a certain amount of molecules. In Dr. Lavelle's exampl...

Isabelle Hales 1J

A delocalized pi bond occurs in molecules that have resonance structures. When the double bond can occur in more than one place, the atoms bind in such a way that they share electrons, causing the bonds to be in between single and double. When this occurs, the pi bond is delocalized because it is no...

Isabelle Hales 1J

Hi! A polar bond involves an unequal sharing of electrons. In a polar bond, the electrons are pulled more to one atom than another because of a difference in electronegativity. However, in a nonpolar bond, the electrons are shared equally. Similarly, in a polar molecule, electrons are pulled in such...

Isabelle Hales 1J

Do the electrons in hybrid orbitals have different properties/characteristics compared to electrons in their normal orbitals? Will hybridized electrons behave differently? Also, does hybridization and the mixing of orbitals affect the shape of an atom or molecule?

Isabelle Hales 1J

Hi! I also find it helpful to visualize the molecule as a sphere/globe. The atoms that are placed equidistant around the middle of the molecule are the equatorial atoms (on a globe, they would be around the equator). The atoms that are above or below the ring of atoms in the middle are the axial ato...

Isabelle Hales 1J

To add on, I find it best to work backwards when determining the 3D molecule that involves lone pairs. With the square pyramidal shape, we know that one region was removed from an octahedral shape. Thus, we had a total of 6 regions of electron density but removed 1 to create the square pyramidal sha...

Isabelle Hales 1J

Hi, going off of this, I understand how to determine the polarity of a molecule but I am a little confused how to determine if an individual bond is polar or non-polar. Is it simply a large difference in electronegativity between the two atoms?

Isabelle Hales 1J

Hi! I have heard Dr. Lavelle use both molecular shape and geometry to refer to the 3D shape of a molecule. After looking online, I believe there is a distinction between electron geometry and molecular geometry. Molecular geometry specifically refers to molecules that involve lone pairs, while elect...

Isabelle Hales 1J

Hi! The formal charge on the carbon atom would be -2, not -1. When we find the formal charge, we evaluate 4 - ((4/2)+4), which is equal to 4 - (2+4), or 4 - 6, which gives us a formal charge of -2. Additionally, the overall molecule does not have a charge so the formal charges of each individual ato...

Isabelle Hales 1J

Hi. I approached this problem by looking at how I could change the bonds around the carbon. For the first resonance structure, I looked at the bonding of the oxygen and saw that the double and single bonds could be switched between the two oxygens. For the second resonance structure, I considered ho...

Isabelle Hales 1J

Hi! I am a bit confused on this too. How can you tell if the dipoles in a molecule will cancel each other out? Also, what is the difference between a dipole and an induced-dipole?

Isabelle Hales 1J

Electrons are constantly moving around within an atom. They do not stay in one place, which also explains why an orbital is a wave function of the electron's position (because it is always changing). As the electrons move around, they tend to congregate in certain spots which is where we get electro...

Isabelle Hales 1J

Hi! Intramolecular forces are forces that exist within a single molecule, such as the force that bonds the hydrogen and oxygen atoms together in a water molecule. On the other hand, intermolecular forces are forces that involve more than one molecule, such as van der Waals interactions. Also, lookin...

Isabelle Hales 1J

Are there any cases when a group 13 element forms a complete octet? Or is this too unrealistic to happen in the molecules we are studying? Should we expect group 13 elements to not form a complete octet and instead become Lewis acids?

Isabelle Hales 1J

Electron affinity is the energy that is released when an electron is added to an atom (the opposite of ionization energy, the energy required to remove an electron). Electron affinity increases towards the top right. The atoms in group 16 & 17 want electrons to become stable, while the atoms in ...

Isabelle Hales 1J

A Lewis acid is a molecule that can accept electrons. In Dr. Lavelle's lecture, the Boron in BF3 is deficient in electrons, as it only has 6 rather than 8, which makes it want to accept more electrons. A Lewis base, on the other hand, is a molecule/atom that gives away electrons. Dr. Lavelle describ...

Isabelle Hales 1J

Hi! I have another question with this problem. Can someone explain what the difference is between the endings "-ite" and "-ate"? What do they tell us about the ion/chemical formula?

Isabelle Hales 1J

To start, consider how many bonds the carbon needs to have. Because it needs to create 4 bonds, we can deduce that there is going to be a double bond to the oxygen, and then two single bonds to each of the hydrogen. Once you have bonded the structure together, calculate how many lone pairs each elem...

Isabelle Hales 1J

Hi! I believe Dr. Lavelle said that once you begin filling the 3d orbitals, the 4s orbitals become higher in energy. In other words, after you pass atomic number 20 (calcium), then 3d becomes lower in energy than 4s. This makes sense for writing the electron configurations because they are written i...

Isabelle Hales 1J

Hi! I have another question about the ml values. How do we know which sub-orbital the ml value is referring to? For example, if l = 2, we could have ml = 1 or possibly ml = -2, but which specific d-orbitals do those values correspond to?

Isabelle Hales 1J

Hi! To start, for the last question, only 2 electrons can have the same three quantum numbers. No two electrons can have all 4 of the same quantum numbers but two can have 3 of the same quantum numbers with different spin quantum numbers. For the second question, l = 1. This means that we are in the...

Isabelle Hales 1J

I am not entirely sure why electron configurations are written from lowest to highest. However, one of the benefits is that for atoms with larger atomic numbers, the beginning of the electron configuration can be written as another element for shorthand. Also, using the shorthand makes it very clear...

Isabelle Hales 1J

Hi. I am a little confused on this topic. I thought Dr. Lavelle used the term "degenerate orbitals" to also refer to Hund's Rule, in which electrons occupy each separate orbital until they have to pair up within the same orbital. Is the term exclusively used to refer to one-electron systems?

Isabelle Hales 1J

Hi. I am still a bit confused about this. Is it correct to say that the Hamiltonian value represents the probability of finding the electron while the energy wave function represents the orbital itself? It would be great if someone could explain what exactly the equation is representing with the two...

Isabelle Hales 1J

Hi! I wanted to add something that really helped me understand the right sign to use. For the Rydberg equation, we get a negative value because we are calculating the decrease in energy as the electron moves from a higher shell to a lower shell. However, when we use that value to calculate the frequ...

Isabelle Hales 1J

Hi! I had a question related to the Rydberg constant. I kept seeing "Rydberg constant for hydrogen" when I was looking up the value of the constant. Does the constant change depending on which element is involved?

Isabelle Hales 1J

Hi! I am also a little confused with Schrodinger's Equation. In what context would we use the equation? In other words, what kind of question/problem would it help us solve? I understand it might not be on the midterm but I am curious as to how the equation is applied to the real world.

Isabelle Hales 1J

For this question, I believe the context of n1 and n2 is based on what values were placed in the Rydberg equation. With this equation, n1 is always the final energy level that the electron ended up at, while n2 is the energy level where the electron began. It is a little confusing for me at times as...

Isabelle Hales 1J

Delta E is negative when electrons emit light because energy is being released as electromagnetic radiation. As the electron goes down in energy levels, it is losing energy as it emits light, which is why we see a negative sign. However, when calculating the energy of the photon that was emitted, we...

Isabelle Hales 1J

To add on, Lavelle said that it is easier to consider a free electron to have zero energy. The bound electrons have less energy because as the electron goes down each energy level towards the nucleus, it is technically losing energy. This is where we get the negative sign. Additionally, the reason t...

Isabelle Hales 1J

Hi! I am still a little confused on this question. It mentions that the burst of photons (in part 2) is coming from a light with a different frequency. Wouldn't that impact the work function because we used the frequency in part one to calculate it? Or is the work function specific to the kind of me...

Isabelle Hales 1J

Hi! To add on, I believe quantum is used to describe the principle of the photon model. The energy of light is not continuous, as in the wave model, but rather quantized, meaning it jumps from one value to the next. I think "quantum" is a more general term and can be used to describe any s...

Isabelle Hales 1J

From my understanding, the Lyman series refers to when an electron drops from energy level n = 2+ to energy level n = 1, emitting light in the UV region. The Balmer series corresponds with the electron dropping from energy level n = 3+ to n = 2, which emits light in the visible region.

Isabelle Hales 1J

The theoretical yield is the maximum amount of product that can be produced if everything goes perfectly (no residue left behind, no side reactions, etc.). In order to find the theoretical yield, you use the moles of the limiting reactant. This may be causing some confusion because even if you have ...

Isabelle Hales 1J

Hi! I used the same method to find the mass of C (using the moles in CO2 and multiplying by the molar mass). One thing I would be cautious of is finding the mass of O. I tried to find the mass of O through the same method but got ~1.65 g of O in the CO2 alone (which is more than the 1 g sample that ...

Isabelle Hales 1J

Thanks for all the helpful solutions! I am still a bit confused on how the 37.5% concentration is applied in the problem. How does the concentration of the solute (37.5%) impact the mass that you would calculate from the number of moles(~271 g)? Are you trying to find 37.5% of the 271 g? Or is the p...

Isabelle Hales 1J

I understand the basics of identifying a limiting reactant. However, I am a bit confused on how it works in certain cases when you have a balanced equation with different stoichiometric coefficients. For example: 1 A + 2 B = 2 C + 1 D Let's say you are going through this problem and find that reacta...

Isabelle Hales 1J

To begin, I would figure out how much oxygen is used in the reaction. We know that the reaction occurs in a 26.45 g crucible. Once the reaction has taken place, the crucible with the product weights 28.35 g. Because of the law of conservation of mass, the mass of the product will equal the mass of t...

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