CHEMISTRY COMMUNITY

Ellen Amico 2L

You never include solids or liquids in the ICE table or equilibrium constants. If it's gaseous or aqueous, then it's included. In the case of H2O, the only time you would include it is if it's H2O(g).

Ellen Amico 2L

Nope! you can use any of the equations for calculating deltaG. I think it's just a way to label it relating to the reaction.

Ellen Amico 2L

The initial rate is the instantaneous rate at the beginning of a reaction (this is a derivative), the unique rate uses the average rates and accounts for the stoichiometric coefficients of the reactants and products (this is not a derivative).

Ellen Amico 2L

An increase in temperature doesn't affect the activation energy. In the case of an endothermic reaction, increasing the temperature causes the forward rate constant to increase because it'll be easier to overcome the energy barrier bc there will be more energy (however, the value of the barrier does...

Ellen Amico 2L

Anyone have any idea when we'll get our final grades since it was all online?

Ellen Amico 2L

When T = 0 K that's the absolute zero for temperature, so the molecules won't have any kinetic energy and therefore no movement, so their entropy is all based on the different positions/microstates the molecules can occupy.

Ellen Amico 2L

Yes! They're more useful than average rates since they give us the concentration at any particular point in time (this is expressed as the derivative). From the instantaneous rates, we can derive differential rate laws, integrated rate laws, etc. since they're all based on the derivatives.

Ellen Amico 2L

If you search up "endgame" on chemistry community there's a review packed created by one of the UAs.

Ellen Amico 2L

Since discussions are cancelled I think our TAs will likely email them to us, but it depends on the TA i believe.

Ellen Amico 2L

You can also treat basic solutions like acidic ones by adding H+ and H2O as you would, and then add the same amount of OH- as the H+ (and add that amount of OH- to the other side as well). On the side where there's H+ and OH-, you can combine them to get H2O and then make sure the H2O is only on one...

Ellen Amico 2L

I don't think we will actually need to use calculus to solve the problems mathematically, so it's more of a conceptual thing like when it comes to understanding what the instantaneous rates mean and also how to derive the integrated rate laws from the differential rate laws. Dr. Lavelle sent an emai...

Ellen Amico 2L

By using the initial rates and initial concentrations of the reactants, it's easier to determine the rate law. This is because when the products start to form they can sometimes affect the rate law and change it. If we only focus on the reactants at the start of the reaction we don't have to worry a...

Ellen Amico 2L

You add an inert metal when the species that is being reduced isn't a metal itself, or it's a nonconducting metal in which both the reactant and product are in aqueous form, instead of one in the solid phase and one in the aqueous.

Ellen Amico 2L

How do you find the concentration of A in part c of this question? It says: Determine the rate constant for each of the following first- order reactions, in each case expressed for the rate of loss of A: c) 2 A --> B + C [A] initial = 0.153 M and that after 115 s the concentration of B rises to 0.03...

Ellen Amico 2L

How do you determine the order of a species in a reaction from experimental data when there's no experiment where only the concentration of that species is changing?

Ellen Amico 2L

By separating it into two half reactions, it's easier to balance everything and also add in species such as water or H+ if the reaction is occurring in acidic solution. Also, when you write the net ionic equation, it's easier to cancel things out in each reaction to produce the net ionic equation.

Ellen Amico 2L

If you mean for a concentration cell, the solution of lower concentration will be the product because it will be increasing in concentration. The higher concentration solution will decrease in concentration, so it's the reactant. A voltage will be created as electrons flow from anode to cathode sinc...

Ellen Amico 2L

For n (moles) in the nernst equation, is it the moles of electrons or the moles of the substances being oxidized/reduced?

Ellen Amico 2L

For reactions that occur in acidic or basic solutions, why do we have to add water or H+/OH- ions, and how do we know how much to add?

Ellen Amico 2L

What is the purpose of the salt bridge in a galvanic cell, and what would happen if the salt bridge wasn't a neutral salt?

Ellen Amico 2L

I don't think this is something we need to know for this class in particular since he never mentioned it in lecture and it doesn't appear in any homework/previous test problems.

Ellen Amico 2L

The max cell potential is E = -work/charge. It can also be calculated by finding the the maximum work, which is Wmax = ΔG = -nFE. They're equivalent because the max potential is related to the free energy difference between the reactants and products in a cell. This also why we can still calculate t...

Ellen Amico 2L

Why is the net charge -1 for acetic acid? Could someone explain the reasoning behind the answer?

Ellen Amico 2L

Could someone explain why the answer is C? I'm confused on how adding NH4Cl would lower the pH considering its ions are conjugate acids and bases.

Ellen Amico 2L

I'm confused on why question 6D isn't answer choice C because going from solid to liquid would lead to an increase in entropy, and this seems to be largest relative to the other answer choices. Could someone explain this?

Ellen Amico 2L

We calculate E cell using the equation Ecell = Ecathode - Eanode. Ecathode and Eanode are taken from the table of reduction potentials that will be given to us during the test.

Ellen Amico 2L

The purpose of having the Pt is that it's an inert conductor that can transfer electrons. It's only needed when there's no conducting solids so it can do the work of moving the electrons from the anode to the cathode. Copper is a conducting solid, so the Pt isn't needed on that side. However, iron i...

Ellen Amico 2L

How do we know when we need to reverse a reduction equation (to make it oxidation) and therefore multiply the reduction potential by -1?

Ellen Amico 2L

I don't think we ever need to know how to derive any of the equations Lavelle shows us in lecture, it's more to help us with a conceptual understanding. You just need to know the equation itself and when it's used.

Ellen Amico 2L

Can someone explain pizza rolls question 6? I'm confused on why ΔS = 0 and ΔU = 0. Also, I added up 2 calculations for the heat, but I can't seem to get the right answer. If anyone has tips on how to approach the problem please let me know!

Ellen Amico 2L

My TA mentioned this in discussion and said that at constant pressure for an ideal gas, specific heat (Cp) = 5/2R. At constant volume for an ideal gas, Cv=3/2R. So for part a you would plug in 5/2R into q = nCp∆T and for part b you would plug in 3/2R into q = nCv∆T.

Ellen Amico 2L

Can someone explain the concept behind the ∆S = q/T equation? I'm confused on what it really means and when to use it.

Ellen Amico 2L

I was looking at my lecture notes and in class we discussed that the Boltzmann equation has a small error for S but a large error for w. What does this mean exactly?

Ellen Amico 2L

Do we actually need to calculate the integral for the work equation when the system is at equilibrium or do we just calculate using -P∆V?

Ellen Amico 2L

When finding calculating bond enthalpies, do we account if there's multiple of the same bond? For example, for CH3 do we multiply the C-H bond enthalpy by 3? Also, will calculating bond enthalpies be a skill we really need to know for the test since it's the least accurate method of calculating enth...

Ellen Amico 2L

Anything that allows for energy to be exchanged with its surroundings but not matter is a closed system. One example is a pressure cooker.

Ellen Amico 2L

If there's an increase an entropy due to the reaction, the universe will respond by decreasing the entropy somewhere else in order to keep entropy constant overall.

Ellen Amico 2L

The question says: "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 ...

Ellen Amico 2L

If the % ionization is less than 5%, then we can use assumption and get rid of the x in the denominator since the initial and equilibrium concentrations will be very similar.

Ellen Amico 2L

Yes! Because enthalpy is a state property, we can add or subtract the enthalpy changes in order to get to an overall reaction since the path we take to get there doesn't matter.

Ellen Amico 2L

During a phase change, all the added heat energy is going towards breaking the bonds rather than increasing the temperature. Therefore, the temperature won't change.

Ellen Amico 2L

Any of the problems from the Thermochemistry section of the syllabus!

Ellen Amico 2L

We have to pick them up ourselves in Young Hall 3034 between 9-5pm.

Ellen Amico 2L

Bond enthalpies are for breaking bonds in gases, so you need to add in the enthalpy of phases changes for reactants/products that are liquid or solid. Additionally, they're averages from many different molecules, which makes them less accurate. Using Hess's Law or the standard enthalpies would be a ...

Ellen Amico 2L

5.35 says: The following plot shows how the partial pressures of reac- tant 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) Calcula...

Ellen Amico 2L

I don't think so, considering it's not listed on Outline 2. If he doesn't mention it in class either I don't think it's something we need to know.

Ellen Amico 2L

The only time K is changed is when the temperature is changed (endo/exothermic reactions). For changes in concentration or pressure, the reaction will adjust so it can eventually go back to equilibrium and therefore remain at the same K value.

Ellen Amico 2L

It just depends on what units the data is given in. If it's concentrations, then you would denote that with Kc, but if you're given it in pressure, then denote it with Kp. If for some reason you need to convert between concentration and pressure, you can use PV=nRT.

Ellen Amico 2L

There's no practice tests but if you do all the homework problems in the textbook and review your lecture notes you should be well prepared!

Ellen Amico 2L

You can approximate when K is less than 10^-3 because the change in the initial concentration will be so small that x will be a negligible value.

Ellen Amico 2L

Because of Le Chatelier's Principle. the reaction will adjust in order to keep the ratio of products to reactants the same. For example, if there's a higher concentration of reactants, the reaction will adjust to have a higher concentration of products. The actual concentrations will change, but the...

Ellen Amico 2L

The rule for changes in pressure applies to only moles of gas, so you don't consider the solid and therefore there are more moles of gas on the right, so the reaction shifts to the left and favors the reactants.

Ellen Amico 2L

It just depends on what unit the the products and reactants are given in. If it's in molar concentration, then the equilibrium constant is denoted as Kc. If it's given in partial pressures, then it's denoted as Kp. They both mean the same thing, but it's just to differentiate between both.

Ellen Amico 2L

The reasoning really comes from comparing Q (reaction quotient) to K. In the example he used, the concentration doubled due to the change in pressure, so Q < K and the reaction shifts right. There was more moles on the left, so this is consistent with the short reasoning. I wouldn't worry too much a...

Ellen Amico 2L

Changes in pressure or volume cause changes in concentration (due to PV=nRT), and changes in concentration don't affect K due to Le Chatelier's Principle, so that's why pressure and volume don't affect K, only temperature.

Ellen Amico 2L

For 6A on the midterm, I thought the question was asking about the quantum number l because the question mentions subshells, but what are the correct values for this question?

Ellen Amico 2L

I'm going over the midterm and I was wondering what the correct graph would look like for question 2B, and why?

Ellen Amico 2L

I don't think this is something we need to know a lot about for the final. Ka and Kb were mentioned in a review session yesterday, so I would just know the formula for calculating them and know that they relate to weak acids/bases. However, I think that most of our calculations will have to do with ...

Ellen Amico 2L

Here's what my TA told me:
metals form basic oxides
nonmetals form acidic oxides
elements diagonally from Be to Po on periodic table and some d-block elements form amphoteric molecules.

so for example, SO2 would be acidic since S is a nonmetal.

Ellen Amico 2L

Yes!
Strong acids: HCl, HBr, HNO3, H2SO4, HClO4 and HI
Strong bases: oxides and hydroxides of group 1 and 2 metal cations (e.g. NaOH, CaO)

Ellen Amico 2L

The methyl group on acetic acid is electron donating and stabilizes the structure of the compound more, and this also destabilizes the negative charge on the conjugate base. If the conjugate base is less stable, then the corresponding acid is less acidic. Formic acid has a hydrogen attached rather t...

Ellen Amico 2L

You can also look at the ions and see if they're conjugate acids/bases of strong or weak acids/bases. For example, if the ions are the conjugate acid and base of a strong acid and a strong base, then you will know the salt is neutral.

Ellen Amico 2L

My TA told me these rules:
metals form basic oxides
nonmetals form acidic oxides
elements diagonally from Be to Po and some d-block elements form amphoteric molecules

so for example, SO2 is acidic because S is a nonmetal.

Ellen Amico 2L

My TA told me our main focus should be strong acids and bases, but doing the homework problems and reading the textbook sections is what I'm doing to study.

Ellen Amico 2L

Yeah it depends on the compound you're working with, so you have to think about how lone pairs or protons are being donated. For an oxide like CaO, you end up with 2 OH- because the oxygen takes a hydrogen from water so you form 2 OH-.

Ellen Amico 2L

I would remember all of them just to be safe. My TA sent a chart he made with the shapes and there's more variations on there then we went over in class.

Ellen Amico 2L

You need to draw the arrow with the plus on one end, which the arrow pointing towards the more electronegative atom and the plus side towards the less electronegative atom. You can also draw the partial negatives and positives using a lowercase sigma. Do this for every bond on your structure and the...

Ellen Amico 2L

I don't think this is something we need to know for Test 2, it's just going to cover anything after the midterm which includes Topic 3F (intermolecular forces) and VSEPR.

Ellen Amico 2L

Maya Pakulski 3D wrote:So we basically just have to memorize which bond angles go with which shapes?

Yes that's honestly the easiest way, but conceptually you should know that the reason the bond angles occur is due to repulsion between lone pairs and bonding pairs.

Ellen Amico 2L

You should know where the bond angles are in the structures if there's multiple. For example, in trigonal bipyramidal, you should know the 120 bond angles are between the 3 atoms in the same plane, the 90 degree angles are between the trigonal plane and the atoms above and below, and there's a 180 a...

Ellen Amico 2L

The dispersion depends on the polarizability because the the more polarizable an atom is, the more it'll affect the electron density and the more the electrons will be attracted to each other. Therefore, there will be a larger fluctuation in the dipoles and the potential energy of the interaction wi...

Ellen Amico 2L

They should just sum up to the overall charge! For example if your structure is an ion that has a charge of -2, the formal charges should add up to that. But the formal charges on the atom for each resonance structure will vary because the bonds vary.

Ellen Amico 2L

Lavelle said in lecture today that they'll be given back next Wednesday during lecture!

Ellen Amico 2L

Interionic forces are between ions, which are charged molecules, and intermolecular forces are between molecules, which are covalently bonded. The interionic forces are ion-ion and ion-dipole, and the IMFs are van der waals forces and hydrogen bonding.

Ellen Amico 2L

The question is: Draw the Lewis structures that contribute to the resonance hybrid of nitryl chloride, ClNO2 (N is the central atom). I'm confused as to why there even needs to be resonance structures if both oxygens can have double bonds since nitrogen has enough electrons for it... is there a reas...

Ellen Amico 2L

There's going to be 8 questions (with parts i'm assuming) and it covers up to Outline 3 topic 2D. I think the best way to get an idea of knowing what the questions will be like is looking at your discussion section questions and also going to Lavelle's review session of past exam questions.

Ellen Amico 2L

I would always choose the structure that has the lowest formal charge, since that's what we were taught in lecture/discussion. Also, we wouldn't know what the experimental evidence is for a structure on the test so just go by the FC!

Ellen Amico 2L

1 eV = 1.60e-19 J, so convert from keV to eV to joules (dimensional analysis is easiest).

Ellen Amico 2L

I'm planning on doing all the textbook questions in the syllabus for the review, quantum, and chemical bonds. This should cover everything!

Ellen Amico 2L

I'm planning on doing all the textbook questions in the syllabus for the review, quantum, and chemical bonds. This should cover everything!

Ellen Amico 2L

Not sure why I'm having trouble with this but I need some help on 7 from the textbook:
You need to prepare 510. g of an aqueous solution containing 5.45% KNO3 by mass. Describe how you would prepare the solution and what mass of each component you would use.

Ellen Amico 2L

Yes the number matters here! The l = 0 always refers to the s-orbital, no matter what shell it's in. So l is always the same for each type of orbital.

Ellen Amico 2L

The principal quantum number refers to the shell and the orbital angular momentum number refers to which orbital the electron is in.
So for this problem it would be:
a) 6p - n = 6, l = 1
b) 3d - n = 3, l = 2
c) 2p - n = 2, l = 1
d) 5f - n = 5, l = 3

Ellen Amico 2L

They normally don't have electrons in the d-orbital, but because they have a n=3, the d-orbital exists and is available to them to fill up with more than 8 electrons, which allows them to have an expanded octet.

Ellen Amico 2L

Instead of writing out the entire electron configuration starting at 1s for an element, using the brackets of the closest previous noble gas is a shortcut and the configuration for an element will continue from there.

Ellen Amico 2L

Does anyone know what topics the midterm will cover? Will the high school review content from test 1 be on it too?

Ellen Amico 2L

I started off by finding the frequency using the wavelength and then I used the equation: frequency = -R[1/(n1)^2- 1/(n2)^2] and solved for n1, which should get you a number close to 3. I figured out n2 because the question mentioned UV radiation, which equates to the Lyman series and n2 is always 1...

Ellen Amico 2L

I don't think it's something we really have to know for the test, since each electron has one of those spin numbers. Not totally sure on why it's +/- 1/2, but I think it's because of a theorem, which we definitely don't need to know. I would just remember that it exists and is a way of specifying a ...

Ellen Amico 2L

It's not that l has multiple possible values, but ml does. l represents the different subshells in an energy level (e.g. l = 2 represents the d subshell). This mean the values of ml for the d subshell can be -2, -1, 0, 1, 2. These values represent the 5 different orbitals of the d subshell.

Ellen Amico 2L

Yes! This matters because it violates Hund's rule, which states not only that each orbital is occupied by a single electron first, but also that all the unpaired electrons need to have parallel spins (the same spin direction).

Ellen Amico 2L

I was super confused about this in class at first too but this is really helpful! Thanks so much for the tip!

Ellen Amico 2L

The "v" for frequency isn't really a v but a greek letter, so I think of it more like a 'wavy' v and try to make it clear in my work. It's also helpful to know which equations use frequency and which use velocity off the top of your head.

Ellen Amico 2L

Can someone clarify or give examples of what the particle properties of an electron are and what its wavelike properties are? I feel like there are certain experiments that show each but I'm confused on what these properties are.

Ellen Amico 2L

Destructive interference refers to when two waves collide "out-of-phase" in that the peak of one wave interacts with the trough of another, resulting in a smaller, weaker wave. Basically you can think of two waves that don't line up.

Ellen Amico 2L

I find that it's not really necessary since the question usually just involves the numbers. Just make sure to keep track of which energy level is the final one and which one is the initial, so a diagram might help you in that aspect.

Ellen Amico 2L

The question is: "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?" I calculated the molarity of the solution, but how do I find the concen...

Ellen Amico 2L

I asked my TA specifically about balancing equations and he said you don't have to show it step by step if you just can see it and know how to balance it. Some questions have parts that make you go step by step, but I think you should just show as much work as you personally need. He did emphasize t...

Ellen Amico 2L

The first test has to do with the modules that review high school chem.
So the topics are:
Empirical and Molecular Formulas

Balancing Chemical Equations

Limiting Reactant Calculations

Molarity and Dilution of a Solution

Ellen Amico 2L

Basically it's asking the for the length of a chain of 1 mol of atoms strung together. Since there's 6.02 x 10^23 atoms in a mol (avogadro's number), you would multiply the radius of one Ag atom by the number of atoms in 1 mol, which gives you the total length.

Ellen Amico 2L

Hi, just wondering if anyone has tips for studying for Lavelle's tests? I know there are plenty of practice problems in the textbook, but is there anything else/other resources people recommend doing? Thanks!

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