CHEMISTRY COMMUNITY

Steph Du 1H

It is constant pressure. It couldn't be constant temp because you have a deltaT term in your equation meaning temperature is changing.

Steph Du 1H

If [OH-] increases, pH should increase. Remember that pH less than 7 is acidic and pH greater than 7 is basic. More OH- means more basic so higher pH. If you increase OH- and therefore increase pH, the rate decreases because OH- is in the denominator.

Steph Du 1H

Common to weak acids is the carboxyl group, COOH. So whenever you see a COOH, it is going to be a weak acid. C6H5NH2 is an amine which is a compound derived from ammonia by the replacement of one hydrogen atom in NH3. Amines are commonly weak bases. We also know it is a base because the N in NH2 has...

Steph Du 1H

I believe you can either put KOH or just OH-. It's just a convention to add a cation to the OH- to stabilize it. You add OH- because the half reaction is done in a basic solution with OH- anions.

Steph Du 1H

In some reactions, two or more different products can be formed by different pathways. If the product formed by the faster reaction predominates, the reaction is considered to be under kinetic control. If the predominant product is the more thermodynamically stable, the reaction is considered to be ...

Steph Du 1H

I think it is because since our pseudo rate law = k'[A]^n, [A] is actually changing. So we aren't looking at the initial concentration of [A]. [B] and [C], on the other hand, we consider to be constant since they are in large excess.

Steph Du 1H

I think it is because at equal concentrations/at equilibrium, there is no longer an electrochemical/concentration gradient. For example, take an Ag+ concentration cell. The left (anode) is 0.1 M Ag+ and the right (cathode) is 1 M Ag+. The electrons are attracted to the greater positive charge on the...

Steph Du 1H

It tells you how the concentration of B increases in 3 mins. Convert that to the rate at which the concentration of A decreases in 3 mins, subtract that from the initial [A} and you have your final [A] (it will be 0.009). Then use the integrated rate law for a 1st order reaction to get k. The same a...

Steph Du 1H

No, for cell diagrams we don't include the coefficients! Just the species.

Steph Du 1H

Also, you may sometimes see C(gr) used instead of Pt. This is graphite which is another an inert metal that can be used as an electrode.

Steph Du 1H

Look at Appendix 2B for the half reactions and then balance.

Steph Du 1H

For these questions, you are supposed to use Appendix 2B and find the half reactions that result in the reaction given.

Steph Du 1H

6L.7 Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions: (b) H+ (aq) + OH- (aq) → H2O (l) I am not understanding how to write the cell diagrams. I know what the half reactions are but how do I know which to put into the cell diagram? W...

Steph Du 1H

Write the half-reactions and the balanced equation for the cell reaction for each of the following galvanic cells:
(d) Pt(s)|O2 (g)|H+(aq) || OH-(aq)|O2 (g)|Pt(s)

I don't understand how to figure out the reactant and product for the half reaction. Do I use O2 or H+? And OH- or O2?

Steph Du 1H

I know that for the right answer, they assign K as +1 but why did they do this over assigning Cl as -1? Aren't they both monoatomic ions which have an oxidation state that is their ionic charge?

Steph Du 1H

I think this is referring to whether something is thermodynamically stable or unstable. It is stable if ΔG of formation is negative, because it means that the product spontaneously forms. It is unstable if ΔG of formation is positive, because that means it takes energy to form that product.

Steph Du 1H

Can you be more specific as to what you are trying to balance? Balancing a chemical equation with stoichiometric coefficients is the normal way in which you ensure that both sides have an equal number of each element.

Steph Du 1H

First I wrote out the balanced equation: 2 HCl (aq) + Zn (s) --> H2 (g) + ZnCl2(aq) ZnCl2 (aq) is the same thing as Zn2+ and 2Cl- because they dissociate into their ions Then we have to find the limiting reagent: HCl: (0.500 mol/L)(0.8000 L) = 0.4 mol HCl Zn: (8.5 g) / (65.39 g/mol) = 0.13 mol Zn 0...

Steph Du 1H

4.7 (a) Calculate the work that must be done against the atmosphere for the expansion of the gaseous products in the combustion of 1.00 mol C6H6(l) at 25 C and 1.00 bar. (b) Using data inAppendix 2A, calculate the standard enthalpy of the reaction. (c) Calculate the change in internal energy, deltaU...

Steph Du 1H

Suppose that 50.0 g of water at 20.0°C is mixed with 65.0 g of water at 50.0°C at constant atmospheric pressure in a thermally insulated vessel. Calculate ΔS and ΔStot for the process. What is wrong with my calculation for Tf? (50 g)(4.184 J/C*g)(Tf-20°C)=(65 g)(4.184 J/C*g)(Tf-50°C) and then solvin...

Steph Du 1H

You would just do 4.76 kJ/0.579 mol and that gives you your desired unit of kJ/mol.

Steph Du 1H

Also remember that there are more ∆S equations than just these 2. There is also ∆S=nR lnP2/P1 and ∆S=C lnT2/T1.

Steph Du 1H

Yes, it is Boltzmann's constant.

Steph Du 1H

I believe it is used when the temperature is constant.

Steph Du 1H

A particular container holds 4.89 mol of neon gas. The volume of this container can be altered by sliding a piston in or out. The volume is changed from 7.60 L to 11.90 L while at the same time the temperature is changed from 279 K to 362 K. The molar heat capacity, CV,m, for neon is 12.47 J/(mol · ...

Steph Du 1H

For an isothermal, reversible expansion, you use the equation w=-nRTln(v2/v1).

Steph Du 1H

That looks right to me but can you give the question number so that I can look at my work?

Steph Du 1H

I'm not too sure how that equation is derived but molar heat capacity of an ideal gas at constant pressure is greater than at constant volume. This is because at constant pressure, not all heat supplied is used to raise the temperature. Some returns to the surroundings as expansion work and C=q*delt...

Steph Du 1H

They can be interchanged under certain conditions. Delta H and q are equal to each other at constant pressure. See 4C in the textbook for more information.

Steph Du 1H

Yes, that equation is correct but remember it has a negative sign in front. R and T are constant but delta N will change. You look for the reactions where there is a positive delta N (meaning an increase in the number of moles of GAS) which will result in a negative work value meaning the system doe...

Steph Du 1H

I was under the impression that we do assume it's a perfect system because otherwise, there's a lot of other things we need to take into account to get the right answer.

Steph Du 1H

Just to add onto the above, the reason that the C⋯––C bond value is between the single bond value and double bond value is because resonance is a blending of the two.

Steph Du 1H

I think the point of doing it infinitesimally small increments is so that it is reversible and it does more work/is more efficient. The point of it is when this is used in real life such as in an engine, where you would want to be more efficient. In reality though, it is not possible for it to be tr...

Steph Du 1H

I am taking 14BL right now. You do a lab activity with your group every week and it is usually watching videos, answering questions, or doing virtual simulations/experiments like Phet. You also do weekly individual lab reports where you watch a TA do an experiment and are given their data, then you ...

Steph Du 1H

I am kind of confused about what we went over in lecture today. What does it mean that a system at equilibrium is a reversible process whereas systems not at equilibrium are irreversible? Why do systems at equilibrium do more work? What is the point that it does more work? How is the external pressu...

Steph Du 1H

5.35 The following plot shows how the partial pressures of reactant and products vary with time for the decomposition of com- pound A into compounds B and C. All three compounds are gases. Use this plot to do the following: (a) Write a balanced chemical equation for the reaction. (b) Calculate the e...

Steph Du 1H

Dr. Lavelle commented on this in his lecture: When you're using bond enthalpies and have drawn the appropriate lewis structures, you can either: 1) break all of the bonds in the reactants and then form the products, or 2) break only the bonds that change from reactants to products. If you choose op...

Steph Du 1H

An amine is an example of a weak base. Amines are derived from ammonia by replacement of one hydrogen atom in NH3, so an example would be CH3NH2. In this case, it is telling you the pH of the solution which you can use to calculate pOH. This gives you the concentration of OH- at equilibrium. So in y...

Steph Du 1H

I don't think we usually see salts where both the cation and anion are weak. Here's what I go by: A salt solution is neutral if the cation and anion are the conjugate base/acid of a strong acid/base. A salt solution is acidic if the cation is the conjugate acid of a weak base and the anion is the co...

Steph Du 1H

It should be ( 2(H2O enthalpy of formation) + (K2S enthalpy of formation)) - ( (H2S enthalpy of formation) + 2(KOH enthalpy of formation) ). Make sure you are also finding the correct enthalpy of formation in regards to the state of the substance. The (aq) enthalpy of formation will be different fro...

Steph Du 1H

4E.5 Use the bond enthalpies in Tables 4E.2 and 4E.3 to estimate the reaction enthalpy for (a) 3C2H2(g)SC6H6(g) In the answer, they add the energy put in to break 3 carbon triple bonds and subtract the energy released when 6 carbon bonds are formed, but why don't you also subtract the energy release...

Steph Du 1H

I think that's more of a generalization. In reality, you could have A LOT of a weak acid vs. a little of a strong acid and the weak acid would have a lower pH.

Steph Du 1H

For [B] did you make sure that it is $x - 10^{-4.719}$ ?

Steph Du 1H

An amine is an example of a weak base. Amines are derived from ammonia by replacement of one hydrogen atom in NH3, so an example would be CH3NH2. In this case, it is telling you the pH of the solution which you can use to calculate pOH. This gives you the concentration of OH- at equilibrium. So in y...

Steph Du 1H

K stands for equilibrium constant. So the subscript a or b denotes whether the equilibrium constant is referring to the acid or base. They are calculated in the same way.

Steph Du 1H

A salt solution is neutral if the cation and anion are the conjugate base/acid of a strong acid/base. A salt solution is acidic if the cation is the conjugate acid of a weak base and the anion is the conjugate base of a strong acid. A salt solution is basic if the cation is the conjugate acid of a s...

Steph Du 1H

I think it is because the products are aqueous and you know Ca(OH)2 dissociated into its ions which could only happen in water.

Steph Du 1H

Hi, I'm still having trouble understanding why we need to use the value that's less than the initial condition. Could someone provide an explanation using equations/a calculation if possible? Thanks so much. The solution to the quadratic equation gives you x, or the change in concentration. For exa...

Steph Du 1H

Also, the reason that the partial pressures of the reactions and products don't change is because when you're increasing pressure by adding an inert gas, you aren't changing the volume which means pressure remains the same. This can be seen in the equation PV = nRT.

Steph Du 1H

A possibility could be if you had a reaction and then you added a bunch of products? So that would lead to a greater Q value and then the reaction would proceed in a reverse reaction.

Steph Du 1H

Because the ratio that stays the same has the pressures raised to the power of their stoichiometric coefficients. This is the equilibrium constant. The actual pressures/concentrations are going to be different because the two containers have a different amount of reactant which will lead to a differ...

Steph Du 1H

If the bond being broken for the compounds you are comparing is different, you compare bond length (ex. HCl and HBr). If the bond being broken is the same, then you compare electronegativity (ex. CCl3COOH and CH3COOH).

Steph Du 1H

Also, remember to know bis- (2), tris- (3), tetrakis- (4), and pentakis- (5) for naming coordination compounds in which the ligand is polydentate or already has a prefix in its name.

Steph Du 1H

To add onto the above, the resulting anion having resonance and thus being more stable is what makes the acid more likely to give off its proton (thus a stronger acid).

Steph Du 1H

The number of regions of electron density = the number of hyrbridized orbitals. NH3 has 4 regions of electron density and sp3 is the hybridization with 4 hybrid orbitals.

Steph Du 1H

When looking at what has a higher boiling point, we look at what has the strongest intermolecular forces. They both have dipole-dipole which I think are the same strength. I'm not sure if the higher electronegativity of S would make the dipole-dipole interaction stronger. H2Se would have strong LDFs...

Steph Du 1H

So I understand that the conjugate acid of weak bases produce acidic solutions and the conjugate base of weak acids produce basic solutions whereas strong acids/bases result in a neutral solution, but why is it that weak acids/bases result in a basic/acidic solution?

Steph Du 1H

The number of hybridized orbitals is equal to the number of regions of electron density. So, if you had a central atom with 3 regions of electron density, you would need 3 hybridized orbitals: sp2. Then, you'd have one unhybridized p orbital left.

Steph Du 1H

If you put the 2 lone pairs into the axial position (one above and one below), then they would be interact with the 3 atoms in the equatorial position that are 90 degrees away. If you put the 2 lone pairs into the equatorial position, they would interact with the 2 atoms in the axial position that a...

Steph Du 1H

I don't think that we need to look at polarity for determining the strength of an acid. The strength of an acid can be determined based on bond length (longer bonds = weaker bonds = easier to remove H+ = stronger acid) and the stability of the resulting anion (resonance and electron withdrawing atom...

Steph Du 1H

I don't think that we'll be tested on actually calculating the equilibrium but you should know that a larger Ka value means that the products are favoured and it is thus a stronger acid. Looking at the size of the K value only applies to comparing the strength of weak acids. Strong acids don't have ...

Steph Du 1H

See "Naming Coordination Compounds" on Dr. Lavelle's website which gives all the neutral and anionic ligands. en is ethylenediamine, NH2CH2CH2NH2. Its two nitrogens have lone pairs which makes en a bidentate ligand.

Steph Du 1H

You consider the electronegativity because you are trying to determine the stability of the resulting anion. If the resulting anion is stable, it makes it so that the acid is more likely to lose its H+ atom, so that is what makes it a stronger acid.

Steph Du 1H

I find them super helpful! I go to Matthew's workshops most of the time and what he does is he posts a worksheet beforehand on Chemistry Community and during the session, we do 1-2 questions at a time on our own then he goes over them with us. It's good practice in addition to Sapling and the homewo...

Steph Du 1H

You will find delocalized pi bonds in molecules that have resonance. This is because, in resonance, the electrons are delocalized over a pair of atoms, so the pi bond would be a delocalized pi bond.

Steph Du 1H

In response to your question about shape, shape can affect the function of enzymes, for example, since enzymes have a specific shape to their active site that allows molecules to bind/not bind with them.

Steph Du 1H

For molecules that are tetrahedral, it will always be polar if the 4 atoms attached to the central atom are different. A tetrahedral molecule can only be nonpolar if they are the same 4 atoms attached to the central atom.

Steph Du 1H

To visualize what the people above said, here's a helpful image.

: download.png (8.65 KiB) Viewed 159 times

If you were to rotate an atom attached by a pi bond, the orbitals would no longer overlap side by side and they break.

Steph Du 1H

Adding onto the above poster, radicals have very short lifetimes and they damage DNA since they can react with it. That is why we need things like Vitamin C and Vitamin E which react with radicals before they cause damage.

Steph Du 1H

I think just a general thing to remember is that C and H have very similar electronegativities. The position of H on the periodic table isn't representative of its electronegativity.

Steph Du 1H

H, He, Li, and Be don't need an octet. P, S, Cl and other p-block elements that are in row 3 or later can have an expanded octet. And group 13 elements like B and Al normally don't form a full octet because it is difficult for them to gain 5 e- to get a full shell. I think those are all the exceptio...

Steph Du 1H

Kind of as a follow-up question, how would you determine which element in a molecule would create a bigger difference in electronegativity? Like when trying to compare two molecules' iconic character? I think you look for the molecule where the elements are furthest apart on the periodic table. If ...

Steph Du 1H

They aren't in the same row and atomic radius increases as you go down. Br is further down than Na.

Steph Du 1H

The ns subshell is always filled before the (n-1)d subshell where n is the principal quantum number. So 4s is filled before 3d, and 5s is filled before 4d, and so on. This is because when the ns subshell (ex. 4s) is not yet filled, it is lower in energy than the (n-1)d subshell (ex 3d). Once the ns ...

Steph Du 1H

Maddie Turk Disc 2C wrote:So just to clarify, all mollecules have London Disperson forces right? regardless of what molecule it is , it will always have that?

Yes, I think all molecules have London dispersion forces. They are the weakest of the intermolecular forces.

Steph Du 1H

To determine the difference in electronegativity, you can use a periodic table that displays electronegativity values. For a C-H bond, the difference would be: 2.55 - 2.20 = .35 (electronegativity of C minus electronegativity of H). In this case, the difference is too small for the compound to be r...

Steph Du 1H

I think all elements in row 3+ of the p block can have an expanded octet, but the most common ones to have an expanded octet are P, S, Cl which is what Lavelle mentions specifically in the lecture as an example. And adding on to the person above, the other exceptions are H, He, Li, and Be which don'...

Steph Du 1H

Draw the Lewis structure, including typical contributions to the resonance structure (where appropriate, allow for the possibility of octet expansion, including double bonds in different positions), for (a) periodate ion; (b) hydrogen phosphate ion; (c) chloric acid; (d) arsenate ion. Am I supposed ...

Steph Du 1H

Why does having polar double bonds make something a Lewis acid? Like CO2 for example is a Lewis acid. Why can't it donate the lone pairs on oxygen?

Steph Du 1H

I like to use flashcards. I don't take notes on the textbook but if there's important stuff from Lavelle's notes that aren't intuitive/requires memorization then I'll make flashcards and go over them before the midterm.

Steph Du 1H

I think it describes when an anion's electrons are pulled towards the cation. It creates a shared region which gives ionic bonds some covalent character. Easily/highly distorted electrons are also describes as being highly polarizable meaning they are easily pulled into the shared region.

Steph Du 1H

Electron affinity refers to the amount of energy released when an electron is added to a neutral atom in the gas phase, while electronegativity is how well an atom can attract electrons, so they are not the same thing. Adding on to this, electronegativity is calculated form ionization energy and el...

Steph Du 1H

For atoms with an electronegativity difference that is greater than 2, it is an ionic bond but it also has some covalent characteristics. So, it is still an ionic bond but there is some sharing of electrons where the anions electrons are pulled into a shared region by the cation. Likewise, atoms wit...

Steph Du 1H

High polarizing power only describes cations. The higher the charge and the smaller the volume of the cation means that there is a higher charge density and thus higher polarizing power. They are ions that pull the electrons of an anion into a shared region, distorting it and giving it covalent char...

Steph Du 1H

The principal quantum number (n) gives the energy and size of the orbital. It is known as the shell. The angular momentum quantum number (l) gives the shape of the orbital where l=0 is s-orbital, l=1 is p-orbital, l=2 is d-orbital, and l=3 is f-orbital. This is a subshell. The magnetic quantum numbe...

Steph Du 1H

Multiplying the enthalpy of fusion with the mass gives the joules of energy required to melt the ice. Using E=hc/lambda, you can calculate the energy of one photon with the wavelength of 831nm. Then, to find the number of photons, you divide the joules of energy to melt the ice with the energy of on...

Steph Du 1H

Why is it that the d-orbital has more energy than the s-orbital because the electrons in the d-orbital experience a lesser effective nuclear charge than electrons in the s-orbital? Why would a lower charge mean a greater potential energy?

Steph Du 1H

When it says that a high energy electron drops to a lower level, it is saying that there is energy released. This energy is released in the form of a photon, which is a packet of energy, and that photon is light which has characteristics like wavelength or frequency and a lot of questions will ask y...

Steph Du 1H

If atoms/ions are isoelectronic, then would they be the same size since they have the same number of electrons and same electronic configuration? So like the examples that Lavelle gave during lecture, would Na+, Mg2+, and F- all have the same atomic radii?

Steph Du 1H

I know psi squared is the probability density but what does the wave function itself actually represent? How does the Hamiltonian of psi give the energy of psi? What does the energy of psi/wave function mean? What exactly is a "solution" to a wave function?

Steph Du 1H

I think the wave function is a mathematical model that tells us how electrons are arranged around a nucleus in orbitals and the probability of finding an electron at a certain distance away from the nucleus. The solutions to the wave function are the orbitals.

Steph Du 1H

I believe it refers to the number of electrons at a certain distance away from the nucleus. So certain areas have higher electron density probability while other areas, such as nodal planes, have 0 electron density probability meaning there are no chance of electrons being there.

Steph Du 1H

I don't think we need to know how to calculate energy for atoms other than hydrogen. The equation e=-hR/n^2 only applies to hydrogen atoms since it's based on empirical data of H.

Steph Du 1H

The Lyman Series all contain the transition n=2 to n=1 and Balmer Series all contain the transition n=3 to n=2 From my understanding, these are the "common transitions" because they are the biggest energy gaps in the series (the energy levels converge as they increase). In all honesty, I ...

Steph Du 1H

The mass of an electron is a constant and can be found on the equation and constants sheet that Dr. Lavelle has on his website.

Steph Du 1H

n₁ represents the final energy level that it drops to and n₂ represents the initial energy level that it starts at. I find the numbers confusing as well so I usually just us n_f and n_i instead.

Steph Du 1H

The question says it is in the ultraviolet spectrum. Thus, we know it is the Lyman series (Lyman series emits ultraviolet light), and the Lyman series refers to electrons that drop to the ground state, n=1. So then you would use n=1 as your final energy level.

Steph Du 1H

The only way you can determine whether the empirical formula is also the molecular formula is if the question gave the molar mass of the compound. If it doesn't give it, then you would not be able to determine if it is the molecular formula. You can only determine whether it is the empirical formula...

Steph Du 1H

Adding on to Shanna, the mass of an electron is a constant and any other constants can all be found on the equation sheet on Lavelle's website.

Steph Du 1H

Lyman is when an electron drops to the ground state n=1. Balmer is when an electron drops to n=2. Electrons in the Lyman series release more energy than Balmer: a drop from n=2 to n=1 is greater than n=5 to n=2 even though it drops only one level. This is because the energy difference becomes smalle...

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