CHEMISTRY COMMUNITY

Baoying Li 1B

For 4.17 part c, how do you know the remaining number of moles if 0.045 mol? What is the approach? Why is it not 0.15 mol as half of the O2 and all of the SO2 was being used?

Baoying Li 1B

Hey. How do you distinguish the intermediates and catalysts by looking at the reactions? What should be appeared in the overall reaction?

Baoying Li 1B

What is required for a cell to do work?

Baoying Li 1B

I think it's different because electrolysis takes place in an electrolytic cell that uses electron current to drive non-spontaneous redox reaction. A galvanic cell converts chemical energy into electrical energy and it happens spontaneously. If you want to make a galvanic cell become an electrolytic...

Baoying Li 1B

I think Dr. Lavelle has mentioned that we are only going to be tested on the pre-equilibrium approach and he also covered details of the pre-equilibrium approach during the lecture.

Baoying Li 1B

An endothermic reaction always has greater activation energy and a slower rate than the opposing exothermic reaction. When you reverse an endothermic reaction, it would become an exothermic reaction. Since the exothermic reaction is spontaneous, it requires less energy than the endothermic reaction ...

Baoying Li 1B

Are rate laws all instantaneous reaction rates or average reaction rates? When do we use the unique average rate in calculations?

Baoying Li 1B

Under what conditions do we only consider use the rate law of slow step as the overall rate laws? Is that always the case?

Baoying Li 1B

Hey, can someone describe the characteristics of the graph of rate laws? Like what’s the between the first order and the second order? What about the zero order?

Baoying Li 1B

I think the problem will indicate that which reaction is the slow step.

Baoying Li 1B

In the solution manual, the equation of first order is written as ln[A]initial/ln[A]final= kt. Is it the same equation ln[A]final/ln[A]initial = -kt that we discussed in the lecture?

Baoying Li 1B

Can someone please explain why a negative charge electrolyte flows from the cathode to the anode instead of the opposite direction?

Baoying Li 1B

How come when you flip the reaction at the anode to make it an oxidation reaction, you don't have to flip the sign of the electrode potential at the anode? For example, for Zn (s) -->Zn^2+ (aq) + 2e-, the solution manual says that the electrode potential in this for is -.76 V. Why wouldn't it be +.7...

Baoying Li 1B

Yes. But in this case, you are going to add the cell potential of both half-reactions instead of subtracting the cell potential of the anode from the cell potential of the cathode.

Baoying Li 1B

Both free energy and maximum work are negative because of the work done by the cell on the surroundings. However, when we say how much work can be done by a system, we always ignore the negative sign of the work. It makes no sense if you say the maximum work that the cell can perform is -8.4 kJ per ...

Baoying Li 1B

"You don't need to include K if there is no information about potassium in the question. You can just put OH-(aq) to indicate alkaline conditions."--Dr. Lavelle
You can find this post in the endorsed section. I believe the name of the post is something about the cell diagram.

Baoying Li 1B

Partial Pressure can be quoted in either Torr, atm, or Pa. But the most common one is atm. I think that's why it adds an extra step to convert Torr to atm(It actually makes the equation easier to calculate). But I think you would get the same answer by just using Torr.

Baoying Li 1B

Can somebody list the steps of writing a cell diagram?

Baoying Li 1B

For HW 6N #1 b), I think the n(the number of mole of electrons) is 1, but in the solution manual shows that the n=2 and In 3+ needs 2 electrons to be reduced to In 2+. I am wondering how does it happen?

Baoying Li 1B

If I am correct, there are two ways to calculate the Stand Cell Potentials. The first way is to use Le Chatelier's Law, adding the standard potentials of the reactions. The second way is to use the formula Standard Cell Potentials = Standard Potential of Cathode - Standard Potential of Anode. But wh...

Baoying Li 1B

You can find it in the table of content under Appendix2, named "2B Standard Potentials."

Baoying Li 1B

What is "n" in the Nernst Equation and Gibbs Free Energy Equation? Do they have the same meaning? How do we determine it when we are giving a chemical equation? For Hw PROBLEM 6L#1 a), how do we know that the n is 2?

Baoying Li 1B

Sorry, it should be a negative side on the right. I did the question in this way and got the right answer. Perhaps, you may want to check your calculation again:) Good luck for the midterm

Baoying Li 1B

10: Matt asks me for water with no ice at a dining hall. Just to spite him, I measure 25.0 g of ice at 0.00 oC and drop it into 265 mL of water at 25.0 oC. What is the final temperature of the water? BEFORE SOLVING THIS PROBLEM, THINK ABOUT THIS: The ice and the water are in the same container. Sinc...

Baoying Li 1B

Delta U = 0 for ideal gas when the process is isothermal since the change of internal energy depends on the change of temperature.

Baoying Li 1B

For this question, delta U = q + w since it does not specifically indicate the specific process. When you calculate w=-delta n RT, you would need the temperature, which is 298 K (we usually assume that the reaction is in standard temperature if it does not indicate).

Baoying Li 1B

4D.1 Carbon disulfide can be prepared from coke (an impure form of carbon) and elemental sulfur: 4 C(s) + S8(s) -> 4 CS2(l) delta H° = 358.8 kJ Calculate the heat absorbed in the reaction of 197 g of carbon with an excess of sulfur. Can someone walk me through this and break it down conceptually? 1)...

Baoying Li 1B

When the problem asks you to find the heat of phase changes and rising temperatures, you need to consider/mutiply the moles of certain substances. Thus, delta H is KJ or J in this case. When you ask for the reaction enthalpy and the heat(given the heat and the moles), the delta H would be KJ/mole. B...

Baoying Li 1B

You need to calculate two changes in enthalpy of this process. 1) the enthalpy change of the ice melting. 2) the enthalpy change of rising the ice temperature from 0 Celcius to 20 Celcius. Adding these two values would give you the total heat required.

Baoying Li 1B

5. Lex Luthor is trying to design a gaseous mixture of Krypton gas and Helium gas to make Superman weak and have a high voice at the same time! But first, he wants to quickly review entropy. He finds a sealed box that has two compartments. He puts 9.00 g of Helium gas in the first compartment and 12...

Baoying Li 1B

Can anyone explain the process in detail of how to solving #5 parts a and b? I still couldn't get the right answer.

Baoying Li 1B

How to find the number of ways atoms can be arranged in the Boltzmann’s formula? For instance, the HW problems 4G. 5, why is the W=12 to the 6.02*10^23?

Baoying Li 1B

Cm is the molar specific heat of the substance that you are calculating.

Baoying Li 1B

I think we would consider the complexity of molecules to determine their relative molar entropy. For molecules like I2, F2...diatomic molecules/monatomic molecules, we would consider their mass to determine their relative molar entropy.

Baoying Li 1B

what does it mean when it is says delta U= q+w is an adiabatic system?

An a diabetic system is the system through which no heat transfer can take place, q=0. Since delta U = q+w, delta U=w in an adiabetic system.

Baoying Li 1B

For the Homework Problem 4.1, when we raise the temperature of the ice cube from -5.042 Celcius to 0.00 Celcius, why do we multiply the mass of the ice cube by the specific heat capacity 2.03 J per Celcius per grams instead of the specific heat of water (4.18 Celcius)?

Baoying Li 1B

I think they will be provided.

Baoying Li 1B

When you're trying to find the value of work, when would pressure be negative when calculating -P*deltaV, making the work positive? Work is positive when the force and the distance moved are in the same direction, which means the work is done by the gas(Delta V is negative). I think the pressure wil...

Baoying Li 1B

When calculating the change of entropy and enthalpy, when do we consider the situation of constant pressure and constant volume? What is the difference?

Baoying Li 1B

The midterm will cover the materials on Chemical Equilibrium, Acid and Base, Thermochemistry and Thermodynamics. I believe that Lavelle will finish talking about the Thermodynamics in the following week.

Baoying Li 1B

The calculation is supposed to be -1300+1560-2*286=-312.
To obtain the final equation:
The second equation needs to be reversed.
The third equation should be double.

Baoying Li 1B

What are the types of heat capacity?

Baoying Li 1B

What is the difference between constant pressure and constant volume? How would the heat, work, and internal energy respond to these two states?

Baoying Li 1B

When you can replace the Celsius with Kelvin without doing the conversion?

Baoying Li 1B

What do these symbols represent specifically? H, delta H, Delta H naught, qp, q.

Baoying Li 1B

Can someone explain again why steam at 100 Celsius causes a worse burn than boiling water at 100 Celsius? I know Dr. Lavelle has talked about this during the lecture. I just want to make sure that if I get it right.

Baoying Li 1B

Yes. The test will be taking during the discussion. If your discussion is on Tuesday, then you will have a test on that day. It covers outline 1 and outline 2’s materials(chemical equilibrium & acid and base equilibrium). Good luck!

Baoying Li 1B

Using Ice box can help us organize our thoughts and avoid to make mistakes. I suggest to use it just in case or it may help you get some partial credit.

Baoying Li 1B

When do we use the equilibrium sign (the double arrows) in the chemical reaction? Based on my knowledge, if the reactants completely dissolve in the reaction, we can use it. However, I have seen the chemical reaction of weak bases and weak acids also use the equilibrium sign. But couldn’t they disso...

Baoying Li 1B

Since pH=-log[H3O+}, the concentration of H3O+ would be 10 to the -pH.

Baoying Li 1B

Can someone explain the process of solving this problem?

Baoying Li 1B

You can compare the equilibrium constants of this reaction under different temperatures to see whether the reaction is exothermic or endothermic. If the equilibrium constant in the lower temperature is higher than the equilibrium constant in the higher temperature, the reaction is exothermic. The op...

Baoying Li 1B

For problem#20 in Post-Assessment-Part 3, what are the steps of solving this problem? What is the answer? What does the "c" mean in the "c+2x" which represents the equilibrium concentration of SO3?

Baoying Li 1B

You are right with both Kc and Kp.
Kc is the equilibrium constant of the concentration of the aqueous solution.
Kp is the equilibrium constant of the partial pressure of the gas.

Baoying Li 1B

I think it's one over the concentration of the substance as a reactant(neither a solid or pure liquid). This shows the equilibrium constant of the forward reaction whereas the reverse reaction will only be the concentration of the substance.

Baoying Li 1B

Yes. Use partial pressure to find the equilibrium constant, Kp for the gas. Use concentration to find the equilibrium constant, Kc for aqueous solution.

Baoying Li 1B

I believe that changing the volume will not affect the equilibrium constant because the direction of the reaction will change respectively in order to minimize this effect(Le Chatelier's Principle). If volume decreases and more reactants on the left side, the reaction proceeds to the right to produc...

Baoying Li 1B

How do you where the lone pairs lie in a trigonal pyramidal shape and in a seesaw shape?

Baoying Li 1B

Why are both As2O and Bi2O3 amphoteric compounds?

Baoying Li 1B

There's a problem with Vitamin B12 in one of UA's worksheets. You can search "Karen" and click the title "Karen Sun workshop." There's a worksheet called "Worksheet F19--Coordination Compounds." I think that's very helpful. What is the structure of Vitamin B12? I know t...

Baoying Li 1B

What is the difference between Ka, Kb, and Kw?
Also, what is the difference between Ka1 and Ka2?
Do we need to calculate Ka and Kb in the final?

Baoying Li 1B

Memorize all the strong acids and weak strong. Others will be weak acids and weak bases.

Baoying Li 1B

How do you know whether a compound is amphoteric, basic, or acidic? Are there any methods?

Baoying Li 1B

Bond order= #of bonds of an atom divide by the # of atom it is connected with. single bond=1 double bond=2 It just means the average bond number.
The bond order of Cl2 is 1.

Baoying Li 1B

1. Reaction btw acid and base--Neutralization
2. Writing [H3O+] AND [OH-] as logarithms
3. Relative acidity (Binary, oxoacid, carboxylic)
Hope this helps!

Baoying Li 1B

What are the rules to distinguish which is a stronger acid or a weaker acid?

Baoying Li 1B

How can you determine the oxidation number of the transitional metal in a complex?

Baoying Li 1B

How can we distinguish conjugate acid and conjugate base? Is there any shortcut or a clear/systematic way to identify them?

Baoying Li 1B

How do you know the coordination number of a complex? Like Ba[FeBr4]2

Baoying Li 1B

I think that the molar mass only works for London dispersion forces. The dominant intermolecular force of KCl and KI is ion-ion which depends on the polarity. The higher the difference of electronegativity between elements, the higher the boiling point. Cl has a higher electronegativity than I. So K...

Baoying Li 1B

First, draw the Lewis structure. Observe the bond between atoms.
Use these rules can help you find Sigma bonds and Pi bonds:
Single bond-one sigma bond
Double bond-one sigma bond and one pi bond
Triple bond-one sigma bond and two pi bonds

Baoying Li 1B

2F.3. How many sigma bonds and how many pi bonds are there in a) H2S? b) SO2? How do you find how many sigma and pi bonds there are in a molecule? So, the first thing is to draw a Lewis structure. a) Both H-S bonds are single bonds. A single bond contains a sigma bond. So in total, there are two sig...

Baoying Li 1B

London dispersion forces depend on the mass of a molecular. The greater the mass, the greater the forces. Boiling point increases due to the increasing molar masses, increasing surface tension, increasing intermolecular forces. Also, larger polarity results in greater intermolecular attractive force...

Baoying Li 1B

Because the number of valence electron of C and O is 10, 5pairs. If you draw a double bond between C and O, you will need 12 electrons to finish the octet. Also, you can check the formal charge of each element. The lower the formal charge, the more stable the molecule is.

Baoying Li 1B

Hi guys. I have a question that why is the angle of trigonal pyramidal less than 109.5 degrees but not less than 120 degrees since the angle of trigonal planar is 120 degrees. Also, same as the angular shape, why is the angle not less than 180 degrees but less than 120 degrees?

Baoying Li 1B

Because when you draw the Lewis Structure, there are two lone pairs on top of Iodine. These two lone pairs make the original structure(tetrahedral) bend to a square planar structure. The arrangement is octahedral. It doesn't mean that the structure is the same as well.

Baoying Li 1B

What do we need to know in order to get well-prepared for Test 2?

Baoying Li 1B

Is there a relationship between the arrangement of an atom and the hybrid orbitals used by the atom?

Baoying Li 1B

What is the difference between the arrangement and shape of a molecular compound? When do lone pairs matter?

Baoying Li 1B

Which bond is longer: (a) the CN bond in HCN or in H3CNH2? (b) The NF bond in NF3 or the PF bond in PF3? For part A, I got that the bond in H3CNH2 is longer, is this the correct answer? For part B, I got that the NF bond in NF3 and the PF bond in PF3 were the same length--is this correct? You got th...

Baoying Li 1B

There's no lecture tmr. Professor Lavelle has sent an email to confirm this. Good luck on your Midterm!

Baoying Li 1B

I think we need to know that the energy level starts from n=1 for Lyman series(UV region) and it starts from n=2 for Balmer series(visible region, 700nm-400nm).

Baoying Li 1B

Once I told Chemistry a joke, there's no reaction.

Baoying Li 1B

No, you don't have to remember the mass of an electron. It will be on the reference sheet which will be provided to you during the midterm as test 1.

Baoying Li 1B

Yes, I think your summary is pretty good. Just be aware that the formal charge is not always equal to zero, but the more close to zero, the more stable the structure is. Elements like P, As, Cl, As and etc. usually break the octet rule because of their odd valence electrons. Lastly, make sure the to...

Baoying Li 1B

Hey, do we need to know about the oxidation state for the midterm?

Baoying Li 1B

Electronegativity increases as you go down to a group and decreases as you go across a period.

Baoying Li 1B

You can count the valences of electron of the Lewis structure and check that if it's equal to the number of valence of electrons you have calculated. Since there are 42 valences of electron in XeOF2, Xe has one lone pair.

Baoying Li 1B

For this problem, I got 21 for the number of valence electrons of NHF2. But the answer shows only 20 valence electrons. I wonder if I calculated the number of valence electrons wrong?

Baoying Li 1B

What does En represent? According to the notes, if En is greater than 2, it's an ionic bond; if En is less than 1.5, it's a covalent bond. What if the En is in between 2 and 1.5? What would it be?

Baoying Li 1B

Hey guys, can someone explain the process of assigning formal charges in detail? What is it mean to count the bonding? Why is the resonance the most stable when Fc equals to 0?

Baoying Li 1B

I wonder why the answer is 3d^10, not 3d^8 4s^2? Shouldn't we remove the electrons of the outermost energy level first? Or it depends on how we write the electronic configuration?

Baoying Li 1B

There's actually no rules of putting the dots for electrons. As long as you put the right number of valence electons, you will be fine. Make sure that pair up two electrons first and the put the next eletron.

Baoying Li 1B

Hi, can anyone explain why does 4s come before 3d since the energy level n=3 is lower than n=4? I know Professor Lavelle has explained this, but I am still confused.

Baoying Li 1B

Both atomic radius and ionic radius show the same trend: increase down a group and decrease down a period. However, the atomic radius is always smaller than the ionic radius. Ex. Na+=1.02 Angstroms Na=1.54 Angstroms

Baoying Li 1B

In the unit "keV", "k" stands for kilo(10^3) and "eV" stands for electron volt, a unit of energy equal to the energy acquired by an electron in being accelerated through a potential difference of 1 volt. Therefore, "keV" is kilo-electron volt. For problem 1B5,...

Baoying Li 1B

First, you need to find the velocity of a proton by using the formula v=c/wavelength. Then you can find the amount of energy per proton by using the formula E=hv. You use the total energy in the 2s divide by the energy of one proton. You will get the total amount of protons in this energy. In order ...

Baoying Li 1B

Hey guys. What are the answers to #40, 43.44? And can you show the process of how to solve one of them?

Baoying Li 1B

When you are solving a problem, how do you know which fundamental equation you are using? What are some keywords in the problems that may help? For example, E =pv, hv, hc/wavelength.

Baoying Li 1B

A. What is the kinetic energy of the ejected electron? 1.99*10^-19J Your answer is correct. 29. B. How much energy is required to remove an electron from one sodium atom? The energy that is required to remove an electron is the work funtion, which is 150.6 kJ.mol^-1. You need to find the energy per ...

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