CHEMISTRY COMMUNITY

Emily Vainberg 1D

thanks Dr. Lavelle! I had a great two quarters learning from you! :)

Emily Vainberg 1D

Did any of the TA's tell you what the questions are going to be focused on for the final? Will it be multiple choice? Mostly thermodynamics or kinetics?

Emily Vainberg 1D

I think all discussion are also cancelled, so you need to email your TA to get your test 2 back. I know some TA have been sending pictures to students.

Emily Vainberg 1D

There are different units depending on which order of the reaction you have.

zero order= molL−1s−1
first order= s−1
second order= L mol−1s−1

Emily Vainberg 1D

You figure out a reaction is zeroth order the same way you figure out reactions are first or second order. Compare the concentrations given with the changes in initial rate. Rate=k[A]^0 which makes [A]=1 and therefore rate=k.

Emily Vainberg 1D

If you look at the graoh of 1/[A] v Time for a second-order reaction, the slope is equal to K. The y intercept =1/[A]i. This means the graph for a second order reaction would be linear.

Emily Vainberg 1D

For part A of this question, did you get that the rate of O2 was 1.32 moles.L-1.s-1 ?

Emily Vainberg 1D

I didn't even use the formula for this question, I just looked at the equation. when you compare the reactants, one mole of NH3 is used for every 3 moles of H2. This means that for part a) The rate of consumption of N2 is (1/3) times the rate of consumption of H2.

Emily Vainberg 1D

When the questions asks you to write a balanced half reaction, would we include Pt (s) if we needed to add it as a conductor into the equation? I know we include it when we write out the cell diagram, but what about the half reactions?

Emily Vainberg 1D

When you are writing a cell diagram based on a given redox reaction, do you use write the reactants and products after you balance them, or the way they are given in the equation. I am referring to problem 6L.5 c) When I balance the equation I get Cl2 + 2e- --------> 2Cl- but the answer key writes t...

Emily Vainberg 1D

the problem is Cl2(g) + H2(g) -----> HCl (aq)

I am confused on how you are suppose to create half reactions when there is only one product. Can someone explain how they wrote the half reactions for this problem.

Emily Vainberg 1D

You need the moles of electrons transferred in a cell in order to make sure the units cancel out and you get your final answer in the correct units.To find n write out the half reactions to determine the amount of electrons are being transferred. Make sure not to add the electrons from both sides. I...

Emily Vainberg 1D

What would help with this problem is just drawing out the cell diagram. Since the Fe are both aq, you have nothing to label the rod above the water solution for Fe. This means you need to use Platinum (most common conductor). Always have one solid and one aq for each half redox reaction.

Emily Vainberg 1D

Standard Condition is always going to be 1mol/L and all gases at 1 bar with a temperature of 273.15K.

Emily Vainberg 1D

The purpose of a salt bridge is to maintain charge balance because the electrons are moving from one-half cell to the other. The electrons flow from the anode to the cathode.

Emily Vainberg 1D

the textbook shows how balancing an acidic solution vs a basic solution is a bit different. Will the problem always tell u whether the problem is acidic or basic?

Emily Vainberg 1D

If the problem says there is an external force does that always mean the reaction is irreversible? Can there ever be an external force in a reversible reaction?

Emily Vainberg 1D

Oh okay thank you! Does it not matter that the volume was changing initially in the problem?

Emily Vainberg 1D

I am looking over my lecture notes and found this formula

S= n ln 2R
- can someone explain when you use this formula rather than the typical formula for residual entropy ?

typical formula:
S= Kb * ln W

Emily Vainberg 1D

So I am having trouble doing problem 5 for the pizza roll review. So far this is what I have in trying to find the total entropy: 1) Entropy for Volume change in container 1 = (2.25moles He) (R) (ln 4/1) entropy = 25.93 2) Entropy for volume change in container 2 = (1.49 moles Kr) (R) (ln 4/3) entro...

Emily Vainberg 1D

When does the Internal Energy (delta U) = delta H?

Emily Vainberg 1D

"How much heat is required to convert a 42.30g block of ice at -5.042 degrees C into water vapor at 150.35 degrees C?" How come for this problem the answers use delta H for the heat? I thought you would be using q since they did not specify that you were calculating the change in enthalpy??

Emily Vainberg 1D

can someone explains what this term means?

Emily Vainberg 1D

Can someone tell me why the answer for this question is negative? I got positive 312KJ. When you sum the products and subtract the sum of the reactants I got: (-1560) - (-1872)= +312

Emily Vainberg 1D

I watched a video online where it showed how you calculate a reaction that goes through multiple phase changes. 0 degrees to 170 degrees. In this video they had to split it up first calculating the energy from 0-100 degrees and then from 100-170 degrees. Is this something we will need to know for th...

Emily Vainberg 1D

Are we going to finish the whole Thermodynamics outline for the midterm?

Emily Vainberg 1D

For 4A. 13 the question is asking "what is the change in the internal energy"
- I am really confused why we are not using this equation : (delta U)= w + q

The textbook is saying to use C= q/t

Emily Vainberg 1D

The trick for this problem is to know that the pump is a cylinder. You use the diameter they give you to calculate volume. the change in volume = (1.5)^2 (pi)(-20) = -.14137 L then you plug this into the equation for w= - (pressure) (change in volume) work= -(2atm) (-.14137L) work= .283 atm.L work= ...

Emily Vainberg 1D

Are any phase changes exothermic? All the ones we went over in class were endothermic.

Emily Vainberg 1D

SO in class we learned about Hess's Law, but this was not in the textbook for 4A. Which section of the Unit (Chapter 4) did we already go over in class?

Emily Vainberg 1D

How do you know which value for R to use?
For instance for problem 4A.5, my final answer should be in J but i am also using volume so I don't know whether to use the first or second value

R =
8.314 J·K-1·mol-1
8.206 x 10-2 L·atm·K-1·mol-1
8.314 x 10-2 L·bar·K-1·mol-1

Emily Vainberg 1D

so for basic calculations of work, we need to use the volume. If the word problem refers to a piston, are we assuming the shape is a cylinder? For 4A.3 a bike pump was a cylinder, is a piston also a cylinder?

Emily Vainberg 1D

Question 5 states: What is the pH of a .030 M solution of NaC6H7O2? The pKa of C6H8O2 is 4.76. C6H7O2-(aq) + H2O(l) ⇔ C6H8O2(aq) + OH-(aq)

For my answer I got the pH= 10.858
Did anyone else do this problem and get the same answer?

Emily Vainberg 1D

For the test 1 practice worksheet, I don't understand how to solve problem 6.

1 M H2 and 2 M Cl2 are mixed with the reaction H2(g) + Cl2(g) ⇔ 2HCl(g). Kc=50.
What are the equilibrium concentrations?

Emily Vainberg 1D

For problem 1a) in section 6D I understand how the ice table is set up but I don't get where the solution manual got the value for Ka. How did they get the value of Ka to be 1.8 x 10^-5?

Emily Vainberg 1D

Since there are more moles on the right, this change will cause the reaction to shift left and increase the partial pressure of 02

Emily Vainberg 1D

In Outline 1, it says we should know how chemical equilibrium relates to "Biological examples. For example: ATP hydrolysis; osmotic pressure". Can someone tell me what section I can find this information in the textbook?

Emily Vainberg 1D

For 5J.5 I got that all parts of the problem favored the reactants, except part d where there was no change. Can someone confirm this answer for me?

Emily Vainberg 1D

In endothermic reactions (h= +), when temperature increases which direction does the equilibrium shift towards?

Emily Vainberg 1D

When you increase pressure in a reaction (by compression) you count the moles to see when side of the reaction is favored. Ex. If you compress: F2 (g) <----> 2F(g) I know that since there are 2 moles on the right versus one mole on the left, then compressing a reaction would cause the reactants to b...

Emily Vainberg 1D

Dr. lavelle said that for total partial pressures you add all the partial pressures together.
Do you ever use this value for ICE tables. I'm a bit confused when you would use this value.

Emily Vainberg 1D

Can we use ICE tables when we are referring to partial pressure (like when values are given in bars)?

Emily Vainberg 1D

For 51. 27 I got Q< k which means the reaction proceeds left, but I don't get part C. It asks "What is the equilibrium composition of the reaction system?"
Can someone explain what this means?

Emily Vainberg 1D

You actually don't need to use an ICE box for this problem. You said yourself you known that NH3 is 0.2. So when you plug that into the equation you get H2S= (1.6x10^-4)/(.2)

Emily Vainberg 1D

can someone explain when we are allowed to cancel out the x to make solving the quadratic equation easier?
I'm referring to when we set up the equation after making the ice box.

Emily Vainberg 1D

You use Q, which the the reactant quotient. You calculate it the same way as you find K (concentration of product/concentration of reactants). If Q<K the reaction proceeds to the right because the forward reaction is favored. If Q>K the reverse reaction is favored, so reaction proceeds to the left. ...

Emily Vainberg 1D

For ICE tables, when we find change in equilibrium should we also use x in relation to the stoichiometric coefficient?

Emily Vainberg 1D

If it says 4 H3PO4(aq), do we include that when we calculate K and Q? I know we can't use solids and liquids but i'm not sure about aqueous. Also if it says H20 is aqueous do we use that in the equation?

Emily Vainberg 1D

On tests, will we always be given the state of the reactants and products? Like will they tell us if it is a gas, liquid, or solid?

Emily Vainberg 1D

Q and K are both calculated the same: Concentration of products/ concentration of reactants The only difference is that K is used when the reaction is at equilibrium, which is a specific point. Q can be used throughout the reaction before equilibrium is reached. Comparing the two lets you predict wh...

Emily Vainberg 1D

Does anyone know what the test 2 average was?

Emily Vainberg 1D

I know that all Bronsted Acids are Lewis acids, but not all Lewis acids are Bronsted acids (and the same for bases). How do you know if something is a Bronsted acid then?

Emily Vainberg 1D

Does anyone know where the answers are for the textbook problems on Coordination Compounds? The section for the homework problems is 9C in the textbook but I can't find that section in my sixth edition solution manual.

Emily Vainberg 1D

How do you find pKa from pH values?

Emily Vainberg 1D

what does the little a represent in the log function?

Example: pOH= -log a (OH-)
then the a is transformed into brackets making it: [OH-]
does 'a' just mean concentration?

Emily Vainberg 1D

Is the value for Kw always 1x10^14? In the textbook that is the value they used for the example but I'm not sure if that is always the value.

Emily Vainberg 1D

Did we go over diamagnetism and paramagnetism in lecture? It is from a chapter we already finished learning but I don't remember hearing this before.

Emily Vainberg 1D

yes you can definitely do that as long as you write somewhere on the paper that it is for both weeks!

Emily Vainberg 1D

If you have discussion on Friday and won't have it this week will the grades for Test 2 be posted before the next discussion? Or will we only get our scores back 3 days before the final?

Emily Vainberg 1D

Hybridization is always happening. When you have a molecule, the atoms are moving around which create energy and cause hybridization to occur. For any given molecule you can find hybridization orbitals. Ex. NH3 is 2sp3

Emily Vainberg 1D

yes! the final is definitely cumulative, I think there will be much less questions on the fundamentals unit and will be more heavy towards things we learned after the final.

Emily Vainberg 1D

Sigma bonds are the only bonds present when there is a single bond. As the number of bonds increase, then you start using pi bonds. For example a triple bond has one sigma bond and two pi bonds. Also, the atoms can't rotate if there are pi bonds present since they are stronger and can't break apart.

Emily Vainberg 1D

You should look up the big VSEPR chart and use that for reference. The 'less than' signs refer to when you go across a row. So Trigonal pyramid (which has 1 lone pair and three bond pairs) is less than 109 degrees because the shape to the left is tetrahedral (4 bond pairs, 0 lone pairs). The bond an...

Emily Vainberg 1D

Are we just suppose to know what bond angles a shape has, or specifically the angle between each of the atoms in the given molecule. For example, SCl4 is a seesaw which has both 120 degree angles and 90 degree angles. Do we need to know where each of these angles are in the molecule. - also I know f...

Emily Vainberg 1D

Does anyone who has taken AP chem know how easy sigma & pi bonds are? He said he is going to go over it in class for a few minutes, but with the test so soon I wanted to know how difficult the concept is.

Emily Vainberg 1D

How do you determine if a molecular is polar or non polar?

Emily Vainberg 1D

We went over this really briefly in class one day, so I was wondering if we need to know how to identify what is an acid and what is a base for Test #2.

Emily Vainberg 1D

When you look up the full VSEPR char online, there are a total of 15 shapes. Some (like Bent and Linear) repeat. Should we know all of the ones on the chart or only the ones he went over in class for Test 2?

Emily Vainberg 1D

These two types of intermolecular forces are confusing me. How do you know if something is a permanent dipole vs. a temporary dipole that can be induced. (I know h2o is permanent but I don't understand with other molecules)

Emily Vainberg 1D

Does anyone know if he scales the class at the end of the quarter? Or do you have to get 465/500 points to get a solid A in the class?

Emily Vainberg 1D

Is hydrogen bonding just a strong type of a dipole-dipole interaction?

Emily Vainberg 1D

When they give a molecule such as CH4 and ask to identify the intermolecular forces, do they mean the forces between multiple of the molecules? Or just in the molecule itself. I am a bit confused because my TA doing practice problems and kept drawing multiple molecules.

Emily Vainberg 1D

When energy is absorbed, energy is positive
When energy is emitted, energy has a negative value

Emily Vainberg 1D

I also was confused on this question. I believe you were suppose to use the energy found in the previous problem, and relate this to the number of photons using Avogadro's number. Then once you had the photons convert to moles of Hydrogen atoms using the grams per mole of hydrogen.

Emily Vainberg 1D

Since the d shell has space for more electrons (10 in d shell compared to 6 in p shell) then certain elements can have expanded octets. The most common elements with expanded octets are Sulfur, phosphorus, silicon, and chlorine.

Emily Vainberg 1D

The electronegativity of Oxygen is 3.44
Then compare electronegativity of Oxygen with that of B and C. Carbon has an electronegativity of 2.55 while B is 2.04. Since Carbon is closer to Oxygen in terms of electronegativity, that means it has a more covalent bond that that of Oxygen and Boron.

Emily Vainberg 1D

Basically it is referring to solubility. If a certain compound has more covalent character then it is less soluble in water. Compounds with elements close together have similar electronegativity, so they are covalent. So if you want to find out which element is less soluble in water look for compoun...

Emily Vainberg 1D

The higher the electronegativity of an atom, the greater its ability to attract shared electrons. The electronegativity of atoms increases as you move from left to right across a period in the periodic table. If you look at the periodic table, Florine is one of the most electronegative atoms because...

Emily Vainberg 1D

When you look at a lewis structure, the lone pairs are the dots around the electrons, NOT the electrons that are shared through the line. Also, if there are 4 dots (example: Oxygen has one line representing two shared electrons and four dots around the electron) that means there are 4 lone pairs, NO...

Emily Vainberg 1D

If a problem says: Indium(III) sulfide
Does the (III) just give the charge for Indium or is it telling us how many atoms of Indium there will be?

I'm assuming charge since the formula is Ir2S3.

Emily Vainberg 1D

Can someone explain how to make the lewis structure for sodium hypochlorite. I don't understand how the textbook solutions just add a + sign for sodium. I get that if you add +1 then Sodium turns into a full valence, but how come they are allowed to do that?

Emily Vainberg 1D

I know that Ni2+ is: [Ar] 3d6 4s2
Since this turns into [Ar] 3d8, I don't understand why the textbook says there are two unpaired electrons.
Can someone please explain!

Emily Vainberg 1D

h bar is just a simplified way of talking about Plank's constant. If the book uses h bar they are talking about h divided by 2π.

ℏ= reduced Planck's constant

ℏ= h/(2π)

Emily Vainberg 1D

Does anyone know how many Units are going to be on the midterm?
I'm assuming it will be the first three: Fundamentals, Quantum World, and Chemical bonds but I was wondering if anyone knew for sure.

Emily Vainberg 1D

Help with Textbook Question!
How many unpaired electrons are predicted for the ground-state configuration for Ta

For Ta I got: [Xe] 4f14 5d2 6s2
I thought this would mean that there are 2 unpaired electrons but the answer key says there are 4. Can someone please explain?
Thanks!

Emily Vainberg 1D

Is Dr. Lavelle going to upload more modules or is the Heisenberg Principle the last one he will post?

Emily Vainberg 1D

In the textbook it keeps asking about: Zeff E
I'm not sure what this notation means can someone tell me? Thanks

ex: Is Zeff e for an electron in a 1s-orbital is the same as Zeff e for an electron in a 2s-orbital?

Emily Vainberg 1D

During the lecture Dr. Lavelle said the following equations are for electromagnetic radiation. He said we should not be using them for electron calculations. speed of light= (wavelength)(frequency) Energy= (planks's constant)(frequency) If we can't use these equations for electron calculations, whic...

Emily Vainberg 1D

Does anyone known how long we will be working on the "Quantum World" Unit?
Are we going to start the next unit (Chemical Bonds) week 4?

Emily Vainberg 1D

Can someone please explain the following problem (Atomic Spectra Module Question #42) An excited hydrogen atom emits light with a frequency of 1.14 x 1014 Hz to reach the energy level n = 4. In what principle quantum level did the electron begin? I use the frequency to calculate the total change in ...

Emily Vainberg 1D

Question on the Atomic Spectra Module asks: Calculate the frequency of a photon emitted by a hydrogen atom when an electron makes a transition from the fourth to the second principal quantum level. I did the calculations and got A. -6.17 x 10^14 Hz However, it said my answer was wrong. Are frequenci...

Emily Vainberg 1D

Problem: Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 10^5 m.s-1. The work function for sodium is 150.6 kJ.mol-1. QUESTION: How much energy is required to remove an electron from one sodium atom? I would assume that the answer is just 1.5 x 10^5 since they giv...

Emily Vainberg 1D

I had this question too, since the book used it in a confusing way. I asked my TA and he said that formula units is the empirical formula but for an ionic compound. However, since we don't need to know what an ionic compound is yet, when they ask for formula units they just means molecules.

Emily Vainberg 1D

For this question you are not required to know what health benefits the solution brings. What you do need to know is that when there is no more active reagent, the solution would not have any effects. This would mean there would be no health benefits because there is no solution left to use.

Emily Vainberg 1D

No, the test is only on the Fundamental Topics that were in the unit 1 outline.

Emily Vainberg 1D

Since we don't have a scientific calculator for tests and quizzes, should we round to a few decimal points after each calculation since the basic calculator can't store the numbers?

Emily Vainberg 1D

When calculating percent yield, is the actual yield always given in the problem? We only should know how to calculate theoretical yield and percent yield right?

Emily Vainberg 1D

Here are some tips when counting significant figures - pay attention to where the decimals are placed ex: 1000. vs. 1000 - Since there is a decimal at the end for the first number it has 4 significant figures where as the second number only has 1 significant figure - think about what numbers would s...

Emily Vainberg 1D

For the hw problem L.7, part b asks, "What mass of oxygen is needed to oxidize this amount of tristearin?" Can someone explain how you would go about solving this problem?

Emily Vainberg 1D

When you are finding the molarity of a solution, do you always use the full value on the periodic table or can we round to three decimal points?
Ex. if there are 15 grams of Hydrogen would you do (15/1.00974) to find the amount of moles?

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