CHEMISTRY COMMUNITY

Jessica Li 4F

Yes, it definitely depends on the reaction, so pay attention to the descriptors!

Jessica Li 4F

It should be given, or the rate law will be given and whichever step matches that rate law will be the slow/rate-determining step.

Jessica Li 4F

That method works! What helps me is knowing that OH- gives you one H and one O atom per molecule, while H2O gives you 2 H and one O atom per molecule. Therefore, the side with already existing O molecules will likely have H2O added to it (most of the times, it is the amount of O atoms already there)...

Jessica Li 4F

I think kinetics and electrochemistry were my favorite!

Jessica Li 4F

Thank you so much!

Jessica Li 4F

I will attend - thank you for organizing this!

Jessica Li 4F

In class today, Lavelle brought up how in reactions, the reactants need to collide with enough energy (activation energy) for the reaction to proceed. How does this apply to unimolecular reactions, where there is only one reactant and it doesn't need to collide?

Jessica Li 4F

To second off that, you can never determine a rate law like you do equilibrium constants or Q, as there are usually a bunch of intermediate steps in reactions that you have to take into account. Therefore, unless you know the whole reaction mechanism, rate laws can only really be determined experime...

Jessica Li 4F

For a zero order reaction, if you just plot [A] over time, it should be a straight line, and -k will be the slope. For other order reactions, the graphs will not be straight lines, so you cannot determine k (the slope) unless you take either the ln[A] of the reaction for a first order reaction, in w...

Jessica Li 4F

I find it helpful to write out the cathode and anode reactions, even though they are roughly the same. For instance, the half reactions would be: Anode: Metal -> Metal2+(aq, anode conc.) + 2e- Cathode: Metal2+(aq, cathode conc.) + 2e- -> Metal If you add them up, you can easily get the overall equat...

Jessica Li 4F

In addition, if you want Ecell (not Enaught cell) to be positive, you would want Q to be less than 1 so that lnQ will be negative.

Jessica Li 4F

The units cancel out because volts, coulombs, and joules are all interrelated -- one joule is equal to one volt times one coulomb.

Jessica Li 4F

How do you determine what's the most strongly reducing agent, as in 6.45? How is Al the most strongly reducing metal, even though it is very low in the standard electrochemical series?

Jessica Li 4F

Could someone explain how they did 6N.3, specifically how they got that Ecell(not standard) = 0.33 V, as it says in the solutions manual?

Thanks!

Jessica Li 4F

Potential difference is the same as voltage. Ecell is the measure of the potential difference between two half cell reactions in a redox reaction.

Jessica Li 4F

I believe KOH is just a neutral form of the OH- ion involved in one of the half reactions - I'm not sure if you actually need to include the KOH or if just OH- is fine.

Jessica Li 4F

You add Pt when there is no metal at the electrode. The only exception to this is for Hg(l) -- you can just put Hg(l) instead.

Jessica Li 4F

I'd write two half-cell equations, both involving P4 as a reactant, but one with it being oxidized and one with it being reduced.

Jessica Li 4F

I think we should be given these half-cell reactions on tests, because it would be very hard to know them right off the bat. The reason for why O3 -> O2 still transfers electrons is because although O3 is neutral overall, it still has partial charges on its individual O atoms.

Jessica Li 4F

I believe in your first half-cell equation, there should be 10 electrons transferred because you have 2 atoms of Br instead of 5. Hope that helps!

Jessica Li 4F

Perhaps he was referring to the electric potential, E. If delta E is positive, delta G will be negative. If delta E is negative, delta G will be positive.

Jessica Li 4F

I'm not super sure if this is an error, but if it is, it might be listed on Lavelle's website under his solution manual errors document.

Jessica Li 4F

You add both H+ ions and water to balance out the number of total H atoms on each side.

Jessica Li 4F

Sometimes the question will tell you if the solution is acidic or basic. Other times, if there are more H+ ions on one side, it will be acidic, whereas if there are OH- ions, it will be basic.

Jessica Li 4F

Does anyone have good tips as to how they balance redox reactions, especially the ones where you have to add H+/OH- and H2O because they're in acidic/basic solutions? Thanks!

Jessica Li 4F

Elements that are not transition-group elements usually have constant charges, especially nonmetals. In those cases, you would just use the charge of the cation or anion. For instance, potassium would be K+ (1+ charge) while chloride would be Cl- and have a -1 charge. To determine the charge of tran...

Jessica Li 4F

You can memorize it with the acronym OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Jessica Li 4F

Work is negative when a system does work, which means that it expands. If you do work, you lose energy, so it is negative. On the other hand, work is being done on the system when it is compressed. Think of it as the system can expand by itself, but you have to do something to the system to compress...

Jessica Li 4F

N_a stands for the number of atoms, or if you’re dealing with molecules, molecules, hence the N and a. You use Avogadro’s number to find the number of atoms or molecules if given moles, because the equation to degeneracy W is the number of possible states to the power of the number of atoms.

Jessica Li 4F

Can you have an irreversible reaction that involves no change in temperature or delta U, or could you have a reaction at constant temperature but a nonzero value for delta U?

For instance, 4.17 is at constant temperature but delta U does not equal to 0.

Jessica Li 4F

It wouldn’t necessarily be an isolated system because you can still do work on the system.

Jessica Li 4F

I believe so as well, as it cannot exchange energy with its surroundings at all.

For an adiabatic container, however, you should always have q=0 since no heat is exchanged, but work can still be done, so w does not necessarily equal to 0 unless at constant volume.

Jessica Li 4F

I think you can assume that reversible equations are isothermal. Think of a phase change, which is at equilibrium and isothermal.

Jessica Li 4F

Delta U is 0 for an isothermal reaction because it is a state function and overall everything cancels out. This means that q + w = 0.

Jessica Li 4F

The link is working for me - try it again, maybe it wasn't fully uploaded earlier!

Jessica Li 4F

When pressure is constant, there is expansion work done, which can be calculated using the equation w = -P(deltaV). However, at constant volume, delta V is 0, so w will be 0 and no work will be done.

Jessica Li 4F

Where do you find the values needed to find the standard heat of reaction in this problem? How do you calculate it as well?

Thanks!

Jessica Li 4F

How do you know when water is a gas or liquid in a combustion reaction?

Jessica Li 4F

How can you tell when work is positive or negative when calculating the internal energy, such as in 4B.5?

Jessica Li 4F

Okay that's what I thought but wasn't sure. Thanks!

Jessica Li 4F

Does anyone know the answer for part a. of this question? Do you have to take into account the raised temperature of the water or the copper kettle, or both?

Thanks!

Jessica Li 4F

That makes a lot more sense now - thank you so much!

Jessica Li 4F

Can someone explain 4D.3b please and how to calculate the change in internal energy please? Thanks!

Jessica Li 4F

Those values should be given to you (I.e. the heat of vaporization of water is 40.7 kJ/mol), but it is up to you to multiply those values accordingly depending on how much of something you have (I.e. moles of water).

Jessica Li 4F

Agreed. Combustion is a unique type of reaction in which an organic/carbon-based molecule reacts with oxygen to produce CO2 and water.

Jessica Li 4F

If a reaction is spontaneous, it means that no additional energy is needed for the reaction to occur. It will just happen naturally (although this doesn’t indicate how fast a reaction will occur). You can determine spontaneity using Gibbs free energy or delta G — if the value is negative in a reacti...

Jessica Li 4F

A phase change occurs any time you convert a substance, between the three states of matter/phases: solid, liquid, and gas. There is also a fourth stage but it should be irrelevant to the class. You can determine which changes are exothermic and endothermic by comparing the relative energies of the d...

Jessica Li 4F

I actually got 8.6 - I think you forgot to convert the given pKa value to pKb, as the molecule you are dealing with is actually the base.

Jessica Li 4F

Yes, you could just calculate the mmol to moles (1 mol = 1000 mmol) and then divide that by the volume (0.500 L) to get the concentration.

Jessica Li 4F

When determining how many sig figs to include in our end values, do we factor in the sig figs of given K values too? For instance, if you want to find the concentration of H2 and have the concentrations of I2 and HI up to 3 sig figs, but your given K value only has 2, would you round the value of [H...

Jessica Li 4F

For 5G.1 part c, would the explanation that it is false because the concentrations of the products and reactants don't affect the value of K itself be correct?

Jessica Li 4F

I think it is a typo and it should actually refer to Table 5G.2, which other problems refer to. However, I think there is also an error between the given Kc value in the table and the one in the solutions manual, so I'd just be careful of that.

Jessica Li 4F

Could someone explain how to do 5.39? I'm having trouble with the given Kc -- it should be 6.1 * 10^23 according to the table in the textbook but the solutions manual seems to say it's 6.1 * 10^-3, so I'm a bit confused.

Thanks!

Jessica Li 4F

How much of ideal gases will we have to know for Test 1? Thank you!

Jessica Li 4F

You are allowed to cancel out the x, or deem it negligible to get an approximation of a K value or concentration of a reactant/product, if the K value is very small. Usually the cutoff is 10^-4, so if it is less than that, you can solve the equation without including the x. This usually happens when...

Jessica Li 4F

Kc applies to molar concentration (M) of gases or aqueous solutions, while Kp applies only to gases and is respective to their partial pressures.

Jessica Li 4F

The concentrations of solids and liquids don't change much compared to those of aqueous solutions and gases, so they are negligible/cancel out as 1 basically.

Jessica Li 4F

For any given concentrations, the ratio will always be the same, regardless of what the concentrations are, so they don't change the value of K. However, temperature affects K because changes in temperature shift the reaction toward a certain side. Think of temp. as a reactant or product in the reac...

Jessica Li 4F

Myoglobin can bind one O2. Since four myoglobins make up a hemoglobin molecule, hemoglobin can bind 4 O2 molecules.

Jessica Li 4F

Do we have to know the names of acids and bases and their corresponding formulas? For instance, bromous acid?

Jessica Li 4F

To echo the above statements, I know there are a lot of problems in the textbook that he assigns, but I'd just do all of them, or least all the different kinds of problems, as he does usually pull one question from the textbook and puts it in the test. I also think the UA workshops are very helpful,...

Jessica Li 4F

Organic molecules usually have carbon atoms involved in their makeup, while inorganic molecules don't.

Jessica Li 4F

Ligands donate a whole electron pair to the central atom in a coordination compound. For instance, a lone pair in O2 contributes to the whole of a bond between O2 and Fe when it bends to Fe in a molecule like myoglobin.

Jessica Li 4F

Usually I'd look for the ions that stay in the same state of matter/that repeat on the reactant and products side of the reaction. For instance, if you have Na+(aq) on one side and Na+(aq) on the other, it is a spectator ion. However, if you have Na+(aq) and NaF(s) on the other, it would not be a sp...

Jessica Li 4F

We will probably have to know types of acids generally and their corresponding strengths.

Jessica Li 4F

Yeah, Lavelle hasn't really gone over amphoteric compounds in lecture yet, so I think it would be best to just know that H2O is the standard amphoteric compound because it has the same concentration of H+ and OH- ions.

Jessica Li 4F

Since the F atom in HF pulls the H so strongly, it's hard for the H to "escape" and break the bond between them two to result in their respective ions. In addition, the trend goes that HF < HCl < HBr < HI in terms of acid strength because the bond strengths (and lengths) are increasingly w...

Jessica Li 4F

It is easier for molecules to dissociate as you increase the temperature, as this increases energy. So you can think of it with any molecule - if you increase energy, it is easier to break bonds.

Jessica Li 4F

To add onto the coordination compounds, if the compound as a whole is negatively charged, you want to add an -ate to the ending of the central atom instead.

Jessica Li 4F

To add onto the coordination compounds, if the compound as a whole is negatively charged, you want to add an -ate to the ending of the central atom instead.

Jessica Li 4F

Solvents can vary in their pH, and therefore the concentration of overall H+ and OH- ions there are in a given solution. The presence of the H+ and OH- already in the solvent would affect how much of the solute can dissociate.

Jessica Li 4F

How do chelating ligands work exactly? What are more examples of chelating ligands and their uses?

Jessica Li 4F

Can someone explain how cisplatin binds to DNA again? I understand that it binds to an exposed N atom on guanine, but do the Cl atoms then get displaced for Pt to bind to nitrogen? Also, would the bonds be intramolecular or intermolecular? Thanks!

Jessica Li 4F

Does anyone have the answer for 2.57? It doesn't seem to be included in the solutions manual, and I just want to check to see if my answer is correct or not.

Thanks!

Jessica Li 4F

Does H2, since its linear, have a sp hybridization even though it doesn't have "access," per say, to p orbitals in its ground state electrons? Would it not have hybridized orbitals in general?

Jessica Li 4F

Could someone explain the answer to the following problem?

For the following nitrogen oxide compounds:
a. Draw the most stable lewis structure.
b. Determine the shape.
c. Determine whether the compound is polar or nonpolar.

Nitric Oxide (NO)

Nitrogen Dioxide (NO2)

N2O

Thanks!

Jessica Li 4F

I agree with all of the comments above. 2E has a lot of great practice, but if you want to do more, there should be a lot of other worksheets on Google. I think a good method to memorize the shapes and angles is to start with the VSEPR formula and overall shapes without taking into account lone pair...

Jessica Li 4F

Polarizability doesn't relate to an electron, but rather the whole atom. The polarizability of an atom is essentially how easily its electron cloud can be distorted.

Jessica Li 4F

For tetrahedral molecules (or of any shape really) like CHCl3 or POCl3 that are polar but don't have any lone pairs, why are the bond angles all 109.5 degrees (according to the solutions manual)? Would not the more polar atoms attached to the central atom push the other atoms away slightly more?

Jessica Li 4F

I think the more accurate shape of ClO2+ is bent, but if you were to describe it generally (without taking into account the lone pair on Cl), it would have a trigonal planar shape because there are 3 regions of electron density.

Jessica Li 4F

Why does the different structure of the 2 molecules, pentane and 2,2-dimethylpropane, make their boiling points different?

Jessica Li 4F

In resonance structures, electrons are not just limited to the single/double bonds as seen in the different Lewis structures, but actually delocalized, or spread out, among all of these bonds. Therefore, no resonance structure is the actual Lewis structure of the molecule. Rather, the molecule is a ...

Jessica Li 4F

To second that, the more electronegative atom usually wants more electrons. If it is that the radical electron would give that element more electrons, that it would get the electron on the Lewis structure and vice versa. However, formal charge is also important, so I'd keep that in mind.

Jessica Li 4F

For #10 on worksheet 3, why is wavelength not related to speed per De Broglie's equation?

Here is the question: 10. Which of the following statements regarding electromagnetic radiation is true?

Jessica Li 4F

For the Rydberg equation, does the negative sign cancel out?

Jessica Li 4F

Emission means releasing energy, which means that delta E itself will be negative, as energy is lost.
The energy of a photon, however, can never be negative, so it is always the positive absolute value of delta E.

Jessica Li 4F

It would be when the compound itself is charged/a polyatomic ion.To add on to the previous answer, since SO₃^2- has a 2- charge overall already, the formal charges (if correctly calculated) should also come out to -2.

Jessica Li 4F

I think stability depends a lot on the formal charges of the atoms involved. Since atoms with expanded valence shells tend to have lesser formal charges (hence why they are expanded rather than not), I would say that they are definitely stable.

Jessica Li 4F

I think it's helpful if you have time to write down the electron configuration. Then, you can just count all the subscripts in the highest n level and add them up. This helps prevent confusion with the d orbitals. For instance, for Zinc, the config would be 1s2 2s2 2p6 3s2 3p6 3d10 4s2. Since n =4 i...

Jessica Li 4F

I believe that covalent bonds have partial charges because they only have semi-ionic characteristics, while ionic bonds have actual full charges (i.e. Na+ and Cl- (the plus and minus charges)) and their own semi-covalent characteristics.
Correct me if I'm wrong though!

Jessica Li 4F

However, since phosphorus has a technically empty 3d orbital, it can bond to other elements in a way so that it can have up to 10 valence electrons, as with PCl5.
Hope that clarifies things!

Jessica Li 4F

For elements who cannot have expanded octets, you want to draw bonds in a way so all elements (except H and He) have octets. For instance, CO2 would need double bonds, while CF4 only needs single bonds. For elements who can have expanded octets, you want to calculate the formal charges of all the at...

Jessica Li 4F

I think there's only one midterm (on Nov. 6), and that one will be cumulative all the way from Test 1.

Jessica Li 4F

P, S, and Cl are definitely not the only elements that can have expanded octets. They are just when the expanded octet for elements begins - elements with associated d orbitals should all be able to have expanded octets.

Jessica Li 4F

When drawing Lewis structures, does the angle at which we draw them have to be accurate? For example, with NO3-, making the oxygens perpendicular to N versus at a slanted angle?

Jessica Li 4F

The question is "Photoelectric experiments gave rise to a new equation relating the energy of light to its frequency. Select the right equation."

Is the answer E = hv instead of the one with work function and kinetic energy, and why?

Jessica Li 4F

I think it depends on what values you are given and what the question is asking for. The Rydberg equation is usually used when you're trying to find which energy levels an electron is jumping to/from, or when you're trying to find the wavelength of emission when given the energy levels the electron ...

Jessica Li 4F

You could also draw in possible paths the electron could take/be to show that you are uncertain of where exactly the electron is.

Jessica Li 4F

What equations relevant to the quantum unit will be given on a test? Will we have to memorize the Schroedinger Equation, etc.?

Jessica Li 4F

If an atom has an electron configuration such that some electrons are paired in an orbital, and you are trying to find an electron in a paired orbital, the fourth quantum number would help distinguish between the one with an up spin and the one with a down spin. Therefore, yes, the fourth quantum nu...

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