CHEMISTRY COMMUNITY

Meachelle_Lum_1I

Why does reduction take place spontaneously at the electrode with higher concentration?
(14.93)

Meachelle_Lum_1I

Hi, does anyone know why delta H was 300 and not -300 when calculating the reverse activation energy? Didn't they give it as -300, and it's an exothermic reaction so deltaH should be negative?

Meachelle_Lum_1I

If water is a reactant, do we put this as a reactant in the rate law? Or do we not put liquids when writing the rate law?

Meachelle_Lum_1I

Is charging our phone batteries electrolysis or a galvanic example? Can someone provide an example of electrolysis and an example of a galvanic reaction?

Meachelle_Lum_1I

There will probably be an emphasis on kinetics since it was the most recent unit and we spent the longest amount of time on it. Just study everything though.

Meachelle_Lum_1I

Can rate constants be negative?

Meachelle_Lum_1I

Why does E decrease over time but E at standard conditions does not? How exactly does E decrease over time?

Meachelle_Lum_1I

For 15.3 part a when finding the reaction rate of NO2, how come we don't divide by 2, the coefficient of NO2?

Meachelle_Lum_1I

For this problem in the 6th edition, why aren't we using Au2+ for the anode rxn (instead of Au+)? Doesn't its paired anode half rxn involving (CN)2 have a 2- charge?

Meachelle_Lum_1I

When we dilute a solution, we decrease the number of ions, so I believe this has to do with Le Chatelier's Principle, in that reducing the number of ions in the anode side will cause the reaction to favor the products and the E cell potential to increase.

Meachelle_Lum_1I

I believe it's because H2 is an element in its most stable form already? Maybe it has something to do with the formation reaction being 0?

Meachelle_Lum_1I

What's the difference between unique rate law and the rate of formation/degradation?

Meachelle_Lum_1I

I don’t think it will be given but some helpful tips is that the Halogens all have a -1 charge. Column 1 all has a +1 charge. And the elements in the same row as oxygen all have -2 charge.

Meachelle_Lum_1I

I believe it’s up to 35, ending in cell diagrams.

Meachelle_Lum_1I

It’s because that equation is not the redox Reaction, but just another half reaction. So the reactions aren’t occurring at the same time as would the two half reactions and the overall redox reactions. We use G because G is a state function.

Meachelle_Lum_1I

E is not a state function, but how come we can add to get the E of the redox reactions?

Meachelle_Lum_1I

Equilibrium means the forward rate equals the reverse rate. Free energy refers to the amount of energy available to do work. So when the energy of the reactants/products aren't equal, the system is not at equilibrium.

Meachelle_Lum_1I

Oxygen is still used in all three formulas. For enthalpy, the formation reaction is the deltaH of the entire reaction. In the textbook, Hrxn is already given of H20, with Oxygen taken in to account. And for Gibbs Free Energy, since the formula is G=H-TS, if entropy and enthalpy use O2, so will Gibbs.

Meachelle_Lum_1I

Does entropy increase only when the number of moles of gas increase? Or do we look at solids and liquids too?

Meachelle_Lum_1I

deltaU=q+w
At constant pressure, we use w=-PdeltaV.
This becomes deltaU= q -PdeltaV
Since we assume constant pressure, q at constant pressure = deltaH
Thus, deltaU= deltaH- PdeltaV

Meachelle_Lum_1I

Your formula is correct, we are adding q of copper and q of water together. Try using mcdeltaT for both, where mass and heat capacity is in grams. Also double check that your units are in Joules.

Meachelle_Lum_1I

q=0 if the problem is adiabatic. w=0 if there is no volume change. q=-w if deltaU=0, which occurs when the system is in isothermal conditions. This is because isothermal occurs at a constant temperature, and for ideal gases, U=3/2nRT. For solids and liquids, q=-w because U=q+w. q and w will both be ...

Meachelle_Lum_1I

When calculating Degeneracy(W):
If the question says we have 1 molecule of a compound, does this mean the number of atoms is 1 or 6.023*10^23?

Meachelle_Lum_1I

When finding deltaH of the rxn, when are our units kJ and when are the units kJ/mol? I thought Hf is kJ/mol, but in some cases Hrxn overall is kJ/mol as well?

Meachelle_Lum_1I

Degeneracy(W) is defined as the number of possible ways a system can exist in a specific energy state. It is determined by the number of possible states, raised to the number of particles involved. For example, two particles A and B have two possible states. Thus 2 states and 2 particles is 2^2, and...

Meachelle_Lum_1I

Both, but usually specific/molar heat capacity is given in a chart. The only time we really have to calculate heat capacity is if we were given deltaH for an entire reaction, and the temperature change, and thus heat capacity = deltaH/deltaT (kJ/1 Celsius).

Meachelle_Lum_1I

I don't believe so, because if deltaU= q+w, those two variables are the only factors that affect internal energy. This is why, for an isolated system, if no work or heat can be exchanged, deltaU=0.

Meachelle_Lum_1I

What's the difference between
1) deltaU= q+w
and
2) deltaU= deltaH -PV

Is the second equation only for constant pressure? Are there certain circumstances to use each equation or are they interchangeable?

Meachelle_Lum_1I

What's the difference between deltaQ and deltaH if they both measure change in heat of a reaction?

Meachelle_Lum_1I

Q is when the reaction is not at equilibrium, K is when the reaction is at equilibrium. If Q<K, the reaction shifts right to favor the products. If Q>K the reaction shifts left to favor the reactants. Most of the time it is Q that changes, not K. K is changed by temperature only.

Meachelle_Lum_1I

Extensive Properties depend on the amount of substance given, such as heat capacity. Intensive Properties do not depend on the amount of substance given, since it's already in the calculation itself. (Specific heat capacity and molar heat capacity)

Meachelle_Lum_1I

Extensive Properties depend on the amount of substance given, such as heat capacity. Intensive Properties do not depend on the amount of substance given, since it's already in the calculation itself. (Specific heat capacity and molar heat capacity)

Meachelle_Lum_1I

Heat capacity is an extensive property and thus depends on the amount of product formed. To calculate it, we find the heat required (Joules or kilo Joules) to raise the temperature of an object by 1 degree Celsius. Heat Capacity branches off into Specific heat capacity and molar heat capacity, which...

Meachelle_Lum_1I

When adding an acid and the [H30+] ends up being less than 10^-7 due to diluteness of the acid wee added, does anyone remember what Lavelle said to do? Do we just write that the pH is 7 or do we have to add 10^-7 to [H30+] first?

Meachelle_Lum_1I

One example Lavelle gave in class was adding Helium to a reaction. Since Helium is a noble gas, it is basically unreactive and will not affect the reaction/shift equilibrium. I don't think we have to understand exactly how Helium will just "sit" in the reaction, but the main idea is that n...

Meachelle_Lum_1I

Hi! A state property is defined as not dependent on a path taken to obtain a certain state. Think about the mountain example given in class. A person who takes a longer route will have done more work but will have climbed the same altitude at the end. Thus, work is not a state property because it ch...

Meachelle_Lum_1I

For 11.87 in the 6th edition, the textbook states that X2 (g) —->2X (g) is an endothermic reaction since it takes heat to break the molecules of gas apart.

However, I thought catabolic (breaking down) reactions were exergonic?

Meachelle_Lum_1I

How can you tell if an aqueous compound acts as an acid or a base?
For example, if given NH2NH2, can’t this compound donate a H+ and also receive an H+?

Meachelle_Lum_1I

When deciding how many sigfigs to use for pH/pOH, does the rule that only numbers after the decimal count as a sig dig apply for all values in the problem (Molarity, Liters, etc.) The pH values had 2 sig figs (like 3.24) but the problem only had one sig fig so I was wondering how the rule would work.

Meachelle_Lum_1I

For water at temperatures higher than 25 degrees Celsius, we will have a different K value which changes the pH/pOH values. Does this mean that the pH for neutral water isn’t always 7? (So a pH of 7 doesn’t exactly mean neutral?)

Meachelle_Lum_1I

Adding a gas only increases the pressure of a system so the equilibrium constant will not change. However, what happens if we add a liquid like water to the system? This will not change the pressure but will it increase the volume thereby changing K?
Or does adding a liquid also have no effect on K?

Meachelle_Lum_1I

I’m Lavelle’s lecture today, he said that generally when volume decreases, the equilibrium will shift to the side with less moles of gas. So if volume increases, does equilibrium shift to the side with more moles of gas?

Meachelle_Lum_1I

When asked to find the partial pressure of a gas, is this the same thing as finding the equilibrium concentration?

If so, then why is Partial Pressure = conc (RT) ?

Isn't that equation saying that partial pressure = concentration * R * T not that partial pressure = concentration?

Meachelle_Lum_1I

For 11.41 in the 6th edition, I was wondering if we would use an ICE table to solve this problem? Also, the book gave the initial amount of moles for NH4(NH2CO2) but I only used the final amount of moles of CO2 to calculate the final equilibrium concentration of NH3 so was this information necessary?

Meachelle_Lum_1I

When writing the equilibrium constant equation for gases, is it ok to use the [] notation as we do for aqueous solutions or do we have to use P (partial pressure) notation?

Meachelle_Lum_1I

When doing ICE tables and the initial concentration of a reactant/product is not given, do we just assume that it is 0?

Meachelle_Lum_1I

For 33c, since we have a weak base, we will have less OH since it doesn’t completely dissociate. However, since less OH means more H3O, this means less pOH should mean more pH. Why did Lyndon say lower pH instead?

Meachelle_Lum_1I

Why is it that Cl- does nothing when dissolved in water but F- has the potential to gain a proton and become HF?

Meachelle_Lum_1I

Does anyone have a complete list of the strong and weak acids/bases we should memorize?

Meachelle_Lum_1I

Lavelle has a worksheet on his website of all the ligand names you should know! :)

Meachelle_Lum_1I

HI is a stronger acid because H and I have a longer/weaker bond and the H+ ions will thus dissociate more, resulting in a stronger acid. However, Cl is more electronegative (another stronger acid property).

So in determining the strength of an acid, does bond length trump electronegativity?

Meachelle_Lum_1I

Why is KBr neutral when dissolved in water, but KF is not?
For 6th edition 12.65, how can we tell by looking at the salt, if it will release H+ or OH- in water?
salts:
AlCl3
Cu(NO3)2

Meachelle_Lum_1I

Hi what would be the conjugate base for B(OH)3?
Do we just add an H to it? So it would be B(OH)3H+?

Meachelle_Lum_1I

Can someone explain why As2O3 and Bi2O3 are amphoteric?

Also, in Lavelle's slides, As2O3 was labeled as an acidic oxide, so I'm not sure if that was a mistake?

Meachelle_Lum_1I

Can someone explain the exact structure of a hemoglobin/myoglobin molecule? From what I remember there are four myoglobins that make up a hemoglobin. (Each myoglobin binds 1 O2) A myoglobin = heme complex + protein. Heme complex = Fe and 4 other things/2 proteins? (Fe is octahedral but I'm not sure ...

Meachelle_Lum_1I

Hi, for the compound sodium bisoxalato(diaqua)ferrate(III), I was wondering what it meant when diaqua was in parenthesis? Also, the book wrote the water part as OH2 instead of H2O. Would it be wrong if I wrote it as H2O? Lastly, does order of the ligands matter when we're given the name and have to ...

Meachelle_Lum_1I

I was looking over the ligands naming sheet Lavelle posted and saw that there was a "nitro" and a "nitrito" but am not sure what the difference is/how to distinguish which name to use.

Thanks!

Meachelle_Lum_1I

Coordination Number is based off of the number of bonds to the central metal atom. The Complex is made up of the central metal atom and its ligands (which is connected by bonds). The bonds(Coordination #) don't exactly "affect" the Complex, it's more of a component thing.

Meachelle_Lum_1I

My TA said these would be the topics:
-VSEPR, Molecular Geometry, Lewis Structures
-Intermolecular Forces
-Hybridization
-Polarizability

Meachelle_Lum_1I

Lavelle did mention this formula in class, but we will never actually have to calculate the dipole of anything, so this equation probably won't be tested.

Meachelle_Lum_1I

Drawing the Lewis Structure usually helps! Once you draw the structure you can see if there is a net dipole moment - if there is one then it is polar. A helpful tip though is that C bonded to H is always nonpolar. Also, if the Lewis Structure is symmetric, it is usually nonpolar because the dipoles ...

Meachelle_Lum_1I

For this question, when finding bond lengths you would look at the average of the two since resonance is the blending of structures. Thus, you would take the average of 140pm and 120pm to get 130pm as the bond length.

Meachelle_Lum_1I

The bond angles depend on the shape of the molecule so you’ll already know the smaller and bigger angles included. However, if you’re asking how to tell which angle is “90” or which one is “120” within the structure, you’ll have to visualize how the molecule is put together. There are many shapes to...

Meachelle_Lum_1I

No, I believe only lone pairs on central atoms would affect the shape. This is why the trigonal pyramidal shape (3 bonded and 1 lone pair) has a bond angle that’s slightly less than a tetrahedral (4 bonded and 0 lone pair) bond angle of 109.5 degrees. Lone pairs on non central atoms do affect the re...

Meachelle_Lum_1I

Hi, in chapter 4 question 43 (6th edition), the question asks what would happen to the bond angles as the “s” character increases?
Can someone explain what this means?

Meachelle_Lum_1I

Which will have a higher melting point: CHI3 or CHF3?
I assumed it was CHF3 since Fluorine is the most electronegative and will have a strong pull on the electrons of the other atoms. Can someone explain why CHI3 is higher?
(6th edition 6.5c)

Meachelle_Lum_1I

Hi, there is a question in the homework that mentions isobal analogy. (3.101b Edition 6) Can someone explain what this is and if it's important to know?

Meachelle_Lum_1I

Hi, which molecule is more ionic/soluble in water?
MgO or BaO?
Since Mg is more electronegative than Ba, I would assume MgO is more ionic, but the textbook solutions say otherwise. (This is problem 3.79b 6th edition)

Meachelle_Lum_1I

Bigger atoms means a larger atomic radius, and as we can see from our periodic trends, the bigger the atom/molecule, the more electrons it will have. The electrons can cause distortion in nearby atoms, which ultimately affects the polarizability of the electron.

Meachelle_Lum_1I

Yes, as mentioned above ionic bonds tend to be between oppositely charged atoms. The general guideline is that if the electronegativity difference between the two is greater than 2, it is classified as an ionic bond. Also just wanted to note that while ionic bonds are stronger than covalent bonds, a...

Meachelle_Lum_1I

Hi, VSPER stands for Valence Shell Electron Pair Repulsion Model, and we use this rule to determine the shape of the molecule. As the name implies, this rule determines where bonding electrons and lone pairs will be in respect to the central atom.

Meachelle_Lum_1I

Hi, in Edition 6, Ch 3 question 39, the question states: Write the complete Lewis structure for each of the following compounds: (a) ammonium chloride; (b) potassium phosphide; (c) sodium hypochlorite. Do we have to know the chemical formula of these? Also, the Lewis structures for these molecules w...

Meachelle_Lum_1I

Yes, those are just given calculations. The only thing Lavelle wanted us to get from those numbers is that resonance is the blending of structures and that the real structure is takes the average of the bond lengths.

Meachelle_Lum_1I

Some tips I find helpful:
- The atom with the lowest ionization energy ends up in the middle for lewis structure.
- Carbon always makes four bonds
Also, just remember that every element except H, He, Li, and Be follow the octet rule.

Meachelle_Lum_1I

Hi Gabby! I also think if you search up "garlic bread" you can get more midterm practice problems.

Meachelle_Lum_1I

Hi, I was looking at the CH 2 HW problems, and after taking Test 2, just know up to question 39/41. Basically, once you reach electron configuration, you've gone too far. Just know stuff discussed in lecture (s,p,d,f and possible quantum numbers)

Meachelle_Lum_1I

I would say the only important thing to know is that 400-700nm wavelength is Visible Light, and that 50/100-400nm wavelength is UV light. Also, based off the hw, probably know that Lyman series is n=1 and Balmer series is n=2.

Meachelle_Lum_1I

I think the only important thing to know here is that in each of these series, (Balmer/Lyman), the principle quantum number for the lower energy level is the same for each absorption line. That's why n=1 for all of Lyman series,etc.

Meachelle_Lum_1I

For the Schrodinger Wave Function, I think Lavelle just wants us to know that it's a wave function that explains where the s,p,d,f orbitals come from. You won't have to apply/use the Schrodinger equation. He may give us some s,p,d,f practice problems Friday though.

Meachelle_Lum_1I

The d-block can hold up to 10 electrons. This is because each orbital can contain up to two electrons, and the d subshell has 5 orbitals. I'm not sure what you mean about how you can get the number of electrons by looking at the periodic table, but hope this clarified the electrons a d subshell can ...

Meachelle_Lum_1I

eV= electric Volt
kEv is like "kilo" electric Volt so just multiply that number by 1,000.
The conversion for volts to joules is 1eV=1.602*10^-19 Joules.

Meachelle_Lum_1I

Kinetic energy is calculated by 1/2mv^2. Use the given velocity in the problem and the mass of an electron (9.11*10^-31 kg), and substitute these values into the equation.

Meachelle_Lum_1I

There's the principle quantum number (shell) which simply goes by n=1, 2, 3... and is the average e- distance from the nucleus. Then, there's the angular quantum number (subshell) represented by l, which ranges from 0 to (n-1). This determines the e- orbital shape. l=0 is the s subshell l=1 is the p...

Meachelle_Lum_1I

Hi! For G5, you would use Volume = number of moles/Molarity. This is derived from our typical Molarity=n/V equation.
Just make sure to find the number of Na moles in Na2Co3 (2 Na moles per Na2Co3), and use that number of moles in your actual equation.

Meachelle_Lum_1I

Hi! I guess the best way to answer your question would be to summarize the four quantum numbers. There's the principle quantum number (shell) which simply goes by n=1, 2, 3... and is the average e- distance from the nucleus. Then, there's the angular quantum number (subshell) represented by l, which...

Meachelle_Lum_1I

Hi, that formula represents the excess energy of an emitted electron. It is also known as kinetic energy of the electron. One way you could solve for kinetic energy is:
E(incoming light/photon) - E(work function/energy to emit electron) = E(kinetic energy of emitted electron)

Meachelle_Lum_1I

Hi! Here's a little legend to help: If problem asks- Amount of energy to remove electron = work function Number of photons= usually means amount of INCOMING light Excess energy=kinetic energy=1/2mv^2 Also, a general formula to help: Energy(photon) - Energy to remove e- = Kinetic energy of electron (...

Meachelle_Lum_1I

Hi! I think the difference in units just depends on the number of elements given. When figuring out whether to use atoms/molecules in your answer, it depends on whether or not there's one element given (ex: Hydrogen) or more than one element given (H2O). I believe we refer to atoms if there is just ...

Meachelle_Lum_1I

Hi Chem 14A,
So there are 6 outlines on the chem website...does outline 2 correspond with week 2 homework?
And since there are 10 weeks and only 6 outlines, are the other 4 weeks for midterms or is there another outline?
Just wanted to figure out how to find each weeks' hw. Thanks in advance!

Meachelle_Lum_1I

Hey! So when calculating the required moles, think of the "4 to 5" ratio as 'how do I get from 4 to 5'. To get from 4 to 5, we multiply by 1.25. And we use that same number (1.25) to calculate the required amount. Just make sure to keep the elements in the same sequence when multiplying (i...

Meachelle_Lum_1I

Hi! You're definitely on the right track. :) The initial volume would be 0.02L because that's the amount you're actually bringing in to the new flask. Even though we technically started with 0.15L, that was only used to calculate the number of moles of our solute, which stays constant throughout the...

Meachelle_Lum_1I

Hi Christine! :) I believe you meant to ask why is Volume(initial) .02L and not .15L? And it's because we're transferring 20 ml of the solution into the new flask. We use this new amount because it's what we're actually taking into the new container. Even though we technically started with 0.15L, th...

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