CHEMISTRY COMMUNITY

paytonm1H

thank you so much Lyndon!

paytonm1H

Kc is the equilibrium constant with respect to the equilibrium concentrations. Kp is with respect to equilibrium partial pressures (for gases). K is one of these two.

paytonm1H

A is the frequency factor for the reaction. It is specific to each reaction.

paytonm1H

adiabatic means no heat exchange (but there can he heat already stored in the system, there is just no heat loss or gain). in an adiabatic system, q=0

paytonm1H

Madison Hurst wrote:So is it technically a second order reaction, but because one of the concentrations is so big its basically constant?

it is pseudo first order with respect to the reactant you are focusing on, I think

paytonm1H

when reactant molecules collide to form products, the reactants must collide with enough energy and in the correct orientation to overcome the activation potential. We didn't go into much detail about orientation. Just know that the frequency factor (A) in the Arrhenius equation represents the perce...

paytonm1H

deltaG for a spontaneous process represents the energy that is free to do useful work

paytonm1H

the definition of enthalpy is the change in heat of a system at constant pressure. so q = deltaH at constant pressure.

paytonm1H

rate laws can be written directly from the molecularity of each elementary step. this is because elementary steps directly show how molecules collide in each step of a proposed mechanism.

paytonm1H

why, in free expansion, are work and heat transfer both equal to zero?

paytonm1H

is the change in internal energy always equal to zero for isothermal, reversible expansion?

paytonm1H

can we still pick up our 14A final? and if so, where?

paytonm1H

This equation is used to find the concentration of A when 0.018 M of B has formed. Essentially, it is just balancing using the stoichiometric coefficients, and it is not an equation we explicitly learned. It makes sense because 3 moles of B forms from 1 mole of A, so it only took 1/3 of 0.018 M of [...

paytonm1H

6.79 asks for the average current flowing through your body in the course of a day, given the Gibbs free energy and that all the energy we use comes from the reduction of O2 in the glucose oxidation reaction. Can someone explain how to do this problem?

paytonm1H

6.57 asks for the Ka value for HF given the two half reaction reduction potentials. After calculating K using the Nernst equation, the solutions manual takes the square root of K to get the Ka value. Can someone explain why they do this?
Thanks!

paytonm1H

In 6O.3, it asks to compare various aqueous ion solution to water at pH=7 to see which species will be reduced at the cathode. The answer key uses E=-0.42V for the reduction potential of water. However, in the appendix, the standard reduction potential of water is -0.83V. Can someone explain how the...

paytonm1H

for part (b) of 6N.1, the answer key uses 2 for n, but both the anode and cathode half reactions only have one mol of electron transfer. Is this a solutions error or am I missing something?

paytonm1H

do you ever include H2O (l) in galvanic cell diagrams? and if so, when?

paytonm1H

how do you know what liquids are conducting??

paytonm1H

thank you!!

paytonm1H

when writing the cell diagram, how do you know when to write Pt(s) on each electrode side??

paytonm1H

I'm working on part d of 6K3. It asks to balance the reaction of chlorine in water Cl2(g) --> HClO(aq) + Cl2(g) I got the oxidation half rxn as Cl2 + 2H20 --> HClO + 2H+ the final answer from the key is H20(l) + Cl2(g) --> HOCl(aq) + H+(aq) + Cl1(aq) I don't understand why the Cl2 in the reactants c...

paytonm1H

can someone explain the reasoning behind the three steps to solve for enthalpy of #12B

paytonm1H

I just experienced the same thing. I don't understand why deltaS=nRln(T2/T1) is valid in this problem.

paytonm1H

In this question, it asks you to calculate net change in entropy for a change in volume and temperature, but you aren't given the moles of gas. It tells you to assume ideal behavior (P=1.00atm), and the answer key also says to assume 1 mol of gas. Is this always true for gas at ideal behavior?

paytonm1H

4.19 asks for the molar kinetic energy of Kr(g) at two temps 1.0C apart, in which the difference is the molar heat capacity of Kr(g).

The solutions manual gives the average kinetic energy = (3/2)RT. Did we cover this in class? Where does this equation come from?

paytonm1H

In the final step pf 4.15 in the solutions manual is said
-2000J=(-4.184 J/Cg)(800g)(deltaT)

I understand they are using the q(water) = -q(zinc oxidation) = mCp(deltaT)

Where are they getting the 800g water from?

paytonm1H

Hi all. I am currently working on 4E.5 in the 7th edition textbook. The question asks to estimate the reaction enthalpy using bond enthalpies for the formation of benzene: 3C2H2(g) --> C6H6(g) 3 C-C triple bonds are broken and 6 C-C 1.5 bonds are formed. Why isn't the formation of 6 C-H bonds includ...

paytonm1H

Hi all. I was wondering up to what part of the syllabus we will be tested on for the midterm. I have the seventh edition and I know that Lavelle said the midterm will cover thermodynamics to the end of entropy. Is that through section 4I in the textbook?
Thanks.

paytonm1H

responding to the question on heat lost by ice = - heat gained by water... The hear gained by the water is not negative. This relationship is simply implying that the heat lost by the ice and the heat gained by the water are equal and opposite one another. So you can use that relationship to find th...

paytonm1H

The ideal gas law should be your go to:
PV=nRT

paytonm1H

I would also caution to make sure your stoichiometric coefficients are correct when writing shorthand version of weak reactions.

paytonm1H

I believe both will give you the correct answer, only as long as all of your partial pressures are in the same unit (either all bar or all atm).

paytonm1H

In problem 5.61 the photosynthesis reaction is given: 6CO2(g) + 6H2O(l) --> C6H12O6(aq) + 6O2(g) Part (b) asks what the effect would be if the system was compressed. Since there are more moles on the product side (disregarding moles of water on reactant side), I would think that compression would sh...

paytonm1H

Hi all,
I am currently working through 5I.27 and keep getting 8.7119 or 0.0738 for x, but the answer key has 9.2 or 0.071 for x. I have the quadratic equation set up the same as the answer key but can't find any mistakes in my calculations. Has anyone else run into this problem?
Thanks!

paytonm1H

K=1 means that the concentration of the products is equal to the concentration of the reactants at equilibrium. It's not impossible, its just not common in everyday chemical reactions.

paytonm1H

My understanding is that because pure liquids remain extremely dilute and solids barely dissociate, their effects on equilibrium are disregarded. Therefore they are not included in the equilibrium constant.

paytonm1H

My bad, I mean self test 5G.3A

paytonm1H

In example 5G.3 in the 7th edition (page 401), it asks to find the equilibrium constant using the net ionic equation. Can someone help me understand how to do this? Or is this even relevant to what we have gone over in lecture?
Thanks!

paytonm1H

When given the momentum of a gamma ray photon, how do you find the velocity of a 200g snail? How can photons have momentum when they are massless?

paytonm1H

Yes, ion-ion is stronger because an ionic bonds is stronger than an ion-dipole intermolecular bond

paytonm1H

Once again, thank you so much Lyndon! You are a life saver!!

paytonm1H

polarizability increases down a group and decreases across a period, because more electronegative atoms are harder to remove an electron from

polarizing power decreases down a group and increases across a period, because smaller, more highly charged cations have greater power

paytonm1H

Weak acids and bases will be discussed more thoroughly in 14B. For the 14A final, a conceptual understand that weak acids and bases do not completely ionize will probably suffice. Only calculations on strong acids and bases where they completely ionize will be on the final.

paytonm1H

HOCl is stronger because Cl is more electronegative, pulling the electrons close towards it. This weakens the O-H bond, making it easier to lose a proton(H+).

paytonm1H

Transition metals can have expanded valence shells, so the ligands act as Lewis bases (donate electrons) to form coordinate covalent bonds. To find the coordination number, add up all the ligands attached to the central ion. Bidentate would attach at two places, so a bidentate ligand would count as ...

paytonm1H

Radicals have an unpaired electron, making them very reactive.

paytonm1H

This is because the greater the oxidation number of the central atom, the stronger the acid. When the oxidation number of the central atom is high, it has a greater electron withdrawing power (pulling electrons towards it), hence weakening the O-H bond (making it easier to lose the H/proton).

paytonm1H

Since salts as acids and bases are weak, there probably won't be much on the final asking about them (Lavelle said that we will only be tested on strong acids and bases, weak will be covered more in 14B). Conceptually, I believe we should know that salts containing acids will lower the solution pH a...

paytonm1H

Hi all. In the 7th edition, question 6D.11 asks whether each salt has a pH greater than, less than, or equal to 7. Part (e) gives AlCl3. Why so small, highly charged cations react with water to form acidic solutions? The answer key says that the pH is less than 7 and gives the equation: AlH2O)6 3+(a...

paytonm1H

This problem gives you a table of varying [H30+], [OH-], pH, and pOH values and you are asked to fill in the rest. The first line gives the [H30+] concentration as 1.50 mol/L. When plugging that into -log[H30+], I get -0.176, however, the answer key says that the pH is 0.176 (positive). Can someone ...

paytonm1H

Does the pH of water increase or decrease with increasing temperature? Why?

paytonm1H

Hi all!
I was wondering if anyone has any suggestions/easy ways of deciding if an oxide is acidic, basic, or amphoteric.
Thanks!

paytonm1H

Can someone please explain the difference between the covalent and hydrogen bonds in water? Thanks!

paytonm1H

In the book it says that in the original convention, the electric dipole is represented by an arrow that points towards the negative partial charge, but then continues to say that the book will use the modern convention, where the electric dipole is represented by an arrow that points to the positiv...

paytonm1H

In the textbook, it states that in PCl5, the P atom expands its valence shell to ten electrons by using one d-orbital. Wouldn't the lowest energy be if the P atom used two d-orbitals with the two electrons having the same spin (Hund's Rule)? Why do they only fill one orbital?
Thanks!

paytonm1H

Polarizing power is the ability of a cation to distort an anion. Small cations have the greatest polarizing power because there is a greater concentration of positive charge in a smaller area, attracting the electrons from the anion.

paytonm1H

Hi all! I'm working on question 2D.7(a), where its asking if AlCl3 or KCl bonds are more covalent. The difference between the electronegativity of AlCl3 is 1.5 and the difference for KCl is 2.2. Since covalent bonds are identified as having a EN difference of <1.5, shouldn't the AlCl3 be considered ...

paytonm1H

Thank you so much for all your help today Lyndon!! You are incredibly generous with your time! Good luck on your midterms and get well soon!!

paytonm1H

What is the difference between copper(I) ion and copper(II) ion?

paytonm1H

Hi all! In lecture this morning, Dr. Lavelle presented the example of phosphorus pentachloride for expanded valence shells. He wrote that the molecule's ten valence electrons' configuration is s2p6d2, using one d-orbital. Wouldn't the two electrons in the d-orbital occupy two separate orbitals, inst...

paytonm1H

Electronegativity increases to the right, but this trend does not include the noble gases. So Fluorine has the highest electronegativity, not Ne or He. This makes sense because electronegativity refers to an element's ability to form covalent bonds, and noble gases don't bond.

paytonm1H

In regards to the DeBroglie equation, can light have momentum even though photons are massless?
Thanks!

paytonm1H

Using the Einstein equation, how do you calculate the number of photons emitted from a light source??
Thanks!

paytonm1H

My TA told us that we will be tested on the following concepts:
wave/particle duality
photoelectric effect
De Brogile's equation
Heisenberg's indeterminacy equation
electromagnetic spectrum

He also said that his test is problem-solving heavy! I do not believe that orbitals will be on this test.

paytonm1H

The photoelectric effect is significant because it showed that light has both wave-like properties and particle-like properties. The finding was that increasing the intensity of the light would not result in electrons being emitted from the metal surface. Instead, you have to increase the frequency/...

paytonm1H

Hi all!
I'm looking for some basic clarification on the atomic spectra... what is the difference between atomic absorption and atomic emission spectroscopy and what does each line/wavelength on the spectrum represent?
Thanks!

paytonm1H

Hi all!
Has anyone completed problem 1A.15? How do find the initial and final values of n when given the line observed at 102.6 nm?
Thanks!

paytonm1H

Hi all!
Each unique metal surface has its own threshold energy needed to remove an electron... if this threshold isn't met, is the energy of the incoming photon absorbed by the surface? I'm just curious to know where this energy goes (thinking about the conservation of energy).
Thanks!

paytonm1H

Hi all!
In the post-module assessment for the limiting reactant calculations module, there were two questions that asked you to find the product formed under the criteria of 1 atm and 25 degrees Celsius. Do these change the way we calculate product or can they be disregarded?
Thank you!

paytonm1H

Thank you so much! It's always encouraging to hear that our hard work will be applicable for many years to come!

paytonm1H

Hi all!
Wondering if I should be reviewing chemical structure properties for the upcoming test... do you think there will be questions where we will need to know basic shapes, polarity, and bond type??
Thank you!

paytonm1H

m1v1=m2v2 will be your best friend! Also, here is Lavelle's video module on molarity and dilution of a solution!
https://lavelle.chem.ucla.edu/wp-conten ... n.wmv.html

paytonm1H

Does anyone have any recommendations for a review video on how to read the periodic table and what the columns/rows mean??
Thanks!!

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