CHEMISTRY COMMUNITY

John Liang 2I

looking at it graphically, the curve downwards will make the line tangent to the curve less and less steep. since the instantaneous rate is dependent on the slope of the tangent line, it will decrease over time. hope this helps

John Liang 2I

it looks like we will take the final in our regular location because announcements monday referred to our exam venues and review sessions. hope this helps

John Liang 2I

when we separated it out into fast and slow steps, we ignore the fast step reactants and products because these do not control the rate. an example used in class is when friends bake brownies with each person on a specific task, the rate of the production of brownies is dependent on the one slow fri...

John Liang 2I

Zero-order reactions are solely k based. the rate=k[A]^0 which makes [A]=1 and therefore rate=k. you can tell if this happens when a graph showing concentration over time is a flat line since this usually happens when an enzyme/catalyst is saturated and the rate is constant. hope this helps

John Liang 2I

yes, because homogenous catalysts would absorb being in the same state of matter. the example used in class was that the catalytic converter was solid while gas sits on top of it rather than being absorbed. it's adsorbed. hope this helps

John Liang 2I

since dG=dH-TdS, you want to find the smallest dS value. we see this in the first choice because the solid states remain most solid. hope this helps

John Liang 2I

test 2 will only cover the 2nd half of thermodynamics (gibbs) and all of electrochemistry. hope this helps

John Liang 2I

Catalysts indeed affect the rate of chemical rxns. By lowering Ea, the Arrhenius equation k=Ae^(-Ea/RT) will increase k and increase the rate. hope this helps

John Liang 2I

With acidic conditions, use h2o to balance oxygens, and then H+ to balance the H of the h2o you just used.
With basic conditions, use h2o to balance oxygens like last time, then use another h2o to balance the hydrogens of the h2o, and finally OH- to balance the "another" h2o. hope this helps

John Liang 2I

Sometimes a hw problem might show up on a final (like one did for the midterm) but the past midterm and test questions give a good picture of what the final will look like. hope this helps

John Liang 2I

N is the number of moles of e- transferred. So yes, N equals the coefficient of the e- in the equation.

John Liang 2I

You flip the sign when you want to flip the rxn around. Usually you do this for the oxidized half rxn.

John Liang 2I

The first way is taking the standard reduction potentials of each (when both are going in same direction). The second way also works bc you switch the reduced half equation and flip the E sign so it still holds to be true.

John Liang 2I

How does a rxn occurring in an acidic/basic solution assist/affect the redox reaction? In this case, how are H3O+ and OH- ions involved in oxidizing or reducing an element?

John Liang 2I

On the outline, we are expected to be able to "Use a Chemistry Van't Hoff analysis to calculate H, S, and G." Can I get clarification on what this exactly means? Using the EQ to find the other variables?

John Liang 2I

the salt bridge releases + and - ions into each side of the battery and maintains a neutral charge. for example, when Zn turns into Zn2+, it will bond with the Cl- ions of NaCl to prevent an overly positive cell. hope this helps!

John Liang 2I

Why is max cell potential directly related to the free energy difference between reactants and products in cell/redox reaction? max cell potential is the max amt of energy able to be released by the battery. since free energy difference is the energy released by a chemical equation (in this case th...

John Liang 2I

The main principle is focus on charge first when balancing the redox rxn. When the number of electrons are balanced on both sides, the elements are probably going to be balanced. this is faster than looking at the elements first. hope this helps!

John Liang 2I

What did Dr. Lavelle say was his "recommended" way of solving for the cell potential? He mentioned it during lecture earlier this week but I did not write it down :/ he recommends: 1) identify the half rxns and look up E for each: Fe3+ (aq) + e- = Fe2+ (aq) E=.77V Cu2+ (aq) +2e- = Cu (s) ...

John Liang 2I

half-reactions are taking the redox reaction and analyzing one metal at a time. for example, the Zn(s) + Cu2+ (aq) = Zn2+ (aq) + Cu(s) reaction will now become:
1) Zn(s)=Zn2+ (aq) + 2e-
2) Cu2+ (aq) + 2e- = Cu(s)

hope this helps!

John Liang 2I

according to the equation w=-Pex*delta V, if external pressure is 0 (as in a vaccuum), then w equals 0. hope this helps!

John Liang 2I

You would only regard the coefficients because they represent the amt of whole molecules. the subscripts only show how many atoms there are in each molecule. you can think of the subscripts as too small to shift while the coefficients are.

John Liang 2I

How is K temperature dependent using the Van't Hoff Equations? K will change based on the Van't Hoff Equation because as we see in ln(k)=-H/RT+S/R, the K value is related to the T value. In the past, I don't think we had any equation that would explicitly relate the two variables. we can deduce tha...

John Liang 2I

How can we assume that the standard delta H and standard delta S are constant when deriving the Van't Hoff equation? Do these values not differ with temperature? Lavelle mentioned in class that although having a higher temperature will indeed increase the initial and final H and S values, the chang...

John Liang 2I

To correct what I'm saying, 1) deltaG*=deltaH-TdeltaS and 2) deltaG=deltaG*+RTlnQ are not equal to each other and the 1 will replace the deltaG* in the 2 i believe.

John Liang 2I

Can someone explain conceptually what the overall Gibbs Free Energy of a system equation means? deltaG = deltaG* + RTlnQ How does this relate to deltaG = deltaH - TdeltaS? Are these equal to each other or is the second equation supposed to be deltaG* in the first equation? Yes, deltaG* = deltaH - T...

John Liang 2I

Yes. The reason being is that liquids and solids are always in excess/change is negligible in the reversible rxns, so their molarities would cancel out in the K equation anyways. hope this helps!

John Liang 2I

Cm stands for the molar specific heat capacity of the compound. hope this helps!

John Liang 2I

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? W, or degeneracy, is a measure of how many different positions an atom can be in. For example, the C-O molecule can be in 2 positions (C-O or O-C)...

John Liang 2I

Reversible processes can be "turned" in any direction at the very slight perturbation of the system. The common one we went over in class was the slow isothermic expansion of gas in a piston chamber due to the addition of heat in gas which causes it to expand. Isothermic systems have temp...

John Liang 2I

In the question from review guide it ask what the Kc will be when the constants are halved. I think the new Kc will just be square root(Kc). Is this true, and if so is it still true if the temperature isn't at 400 K? Thanks! (Refer to pizza rolls question 1d) Yes, when coefficients are changed the ...

John Liang 2I

The definition of the first law of thermodynamics is that energy cannot be created nor destroyed. this allows us to calculate the enthalpy of a reaction, predict it using bond energies, etc. hope this helps!

John Liang 2I

indeed, you must cancel out only if the same compound is on both sides of the equation.

John Liang 2I

You can use each method based upon the information given by the problem. If given bond enthapy values, you can use bond energies, if given standard enthalpies of formation, use that, etc. hope it helps!

John Liang 2I

Because the phase change requires energy, there is a + delta H value. Hope this helps

John Liang 2I

You can think of kinetics as studying how fast a rxn will happen while thermodynamics is studying whether a rxn is favorable or not

John Liang 2I

state functions must be additive because they depend on the endpoints and not the "in-between" values. taking the mountain example used in class, to measure the altitude of the hike, you subtract the final altitude-initial altitude, disregarding any other altitudes in the middle.

John Liang 2I

Did Professor Lavelle say if there was a method preferred over another? And how would we know which one to use? He did mention that using bond energies is not as accurate as using Hess' method because bond enthalpy values are averaged across all different molecules (which gives a less precise value).

John Liang 2I

since exothermic rxns release heat, you can imagine the opposite happen if you add heat. it would be harder for the rxn to go, therefore shifting rxn left and also lowering the K value (P/R)

John Liang 2I

When you are given Ka or Kb it is a good sign that you will end up using an ice table. This is because it gives the ratio of products over reactants at equilibrium, so you can use it to find missing concentrations.

John Liang 2I

Hi all, I heard earlier that the Fall quarter final would be available in Young 3034 during week 3. However, I assumed that Dr. Lavelle would send out an email with the times we could come to pick up the exam. I never received instructions about when and how to pick it up. Does anyone know if the e...

John Liang 2I

K will not change based on changing the volume. What might change is that a Lechatlier shift to the right or left may occur, but the K value remains constant.

John Liang 2I

decreasing the volume increases the amount of pressure because of the equation PV=nRT. You can see that if the V value is lowered, the P value must increase in order to keep being equal to the other side.

John Liang 2I

we know they are the same concentration by referring to the original chemical equation of water protolysis:

H2O(l) = H3O+(aq) + OH-(aq)

because of the one-to-one stoichiometric coefficients, the moles of hydronium ion produced will equal the moles of hydroxide ion produced. Hope this helps!

John Liang 2I

(a) When the pH of 0.10 M HClO2(aq) was measured, it was found to be 1.2. What are the values of Ka and pKa of chlorous acid? To find [H3O+], you take the antilog of the pH, and [H3O+] also equals [ClO2-]. For the Ka equation, Ka = [H3O+][ClO2-]/[HClO2]. How do you know that [HClO2] = (.10 - .06) w...

John Liang 2I

The autoprotolysis of water gives rise to Kw because of its equilibrium expression. K=products/reactants, so Kw=[H30+][OH-] while the H2O(l) is left out of the equation since it's a liquid. Hope this helps!

John Liang 2I

Le chatlier’a principle is that a chemical rxn at equilibrium will do its best to minize effects to the system. Meaning, if product is added to a rxn at equilibrium, the reverse rxn would increase to “balance” out the inequality. It’s used to predict how a rxn at equilibrium would change if we distu...

John Liang 2I

Yes, we are still going over equilibrium in class so it’s acceptable to turn in.

John Liang 2I

It depends on your TA, so i would ask him or her about it during discussion.

John Liang 2I

I'm a little confused on Le Chatelier's Principle, can someone define exactly what it is and explain why the in class example Lavelle gave with it? N2+3H2=2NH3 He said that if N2 were increased, NH3 was increased, and H2 was decreased, the only way to increase the yield of NH3 without adding more r...

John Liang 2I

What happens if N2 is increased, NH3 is increased, or H2 decreased? What does it mean when they go to the left or right? Yep! you are correct on both. Both NH3 is increased and H2 decreased. Going left or right simply means where the rxn shifts in response to adding products or reactants. In this c...

John Liang 2I

In today's lecture, for the last past exam question that we went over, can anyone please explain again why the H was bonded to the O atom instead of the Cl atom? (molecule is HClO3) Thank you! That is simply the common structure of all oxoacids. (H3PO4, H2SO4, HNO3, etc.) If the H was bonded to the...

John Liang 2I

can someone please explain why as the s character o bonds increase, the bond angle increases? Thanks.

John Liang 2I

Could anyone explain conjugate acids and bases in words? I understand how to identify them in an equation, but is there any way to describe them in words? the conjugate base to an acid is the same compound except it now has lost its H+. For example, HCl is an acid. When HCl donates a proton, Cl- is...

John Liang 2I

hemoglobin, myoglobin, cisplatin are the main compounds of bio significance. hope this helps!

John Liang 2I

Q2A: "Hydrogen bonding is important for nucleic acids. Draw how two separate nucleic acids below (guanine (G) and cytosine (C)) will interact via H-bonding to form a GC pair." - can someone explain where the H-bonds are and how they're formed? Q2.B: "There are two different base pair...

John Liang 2I

From what I read on 3F.5, hydrogen bonding can happen between H and N, F, or O molecules, but only if the H is already attached to a N, F, or O? So hydrogen bonding is technically not possible if that does not happen(as in, it would just be a regular interaction if one free H attached to an O, and ...

John Liang 2I

Aiden Metzner 4E wrote:Does hybridization of orbitals change depending on if the bonds are sigma or pi?

no, just look at regions of e- density. for example, NO3- has three sigma bonds and one pi bond but still has hybridization of sp2 because there are 3 regions of e- density around N. hope this helps!

John Liang 2I

Alexis Robles 3A wrote:For the final, do we need to write the name for coordination compounds in the New IUPAC Name Convention or is the regular/"old" one fine?

Lavelle said during lecture that either one is acceptable.

John Liang 2I

In today's lecture review, why do we consider the size of the nucleus (the 1.7 x 10^-15 or something) as the delta x? I don't conceptually understand why we plug that value in for the uncertainty in position. Are we saying that within that value or less is where the electron could be located? And s...

John Liang 2I

How does CH2 2+ work if the C has no octet?

John Liang 2I

Annie Ye wrote:For each pair, determine which compound has bonds with greater ionic character: (a) HCl or HI; (b) CH4 or CF4; (c) CO2 or CS2.

a) HI
b) CF4
c) CS2

John Liang 2I

coordinate bonds form when a single molecule donates 2 electrons to form a bond (this is instead of covalent bonds when each atom "donates" one each). A ligand (like NH3) is any molecule that donates 2 electrons to form a coordinate covalent bond. In class today, NH3 donates N's lone pair ...

John Liang 2I

nonpolar molecules would have induced dipole-induced dipole attractions, aka london dispersion forces.

John Liang 2I

dipole-dipole are between polar molecules such as CHCl3. Induced dipole-induced dipole are between nonpolar molecules such as CH4. Hope this helps :)

John Liang 2I

How does one determine which dichlorobenzene isomer has the largest dipole moment between 1 and 2? Is it 1 because the angles of the dipoles are more "skewed" in one direction than 2?

John Liang 2I

For the general bond angles, you do just need to memorize what they are based upon the shape. Then, with lone pairs, you can reason qualitatively if the bond angles would distort to be larger or smaller than their original angle. For trigonal pyramidal atom, for example, the regular bond angle is 10...

John Liang 2I

All molecules that don't have dipole moments do have London dispersion forces. You can treat this force as the "default" force between molecules if there are no dipole-dipole or ionic intermolecular forces. This is because naturally, molecules create temporary dipoles with each other for a...

John Liang 2I

Pentene being linear-shaped allows the pentene molecules to fit closer together and have a shorter distance between each other, while neopentene being spherical-shaped increases the distance between the atoms. A shorter radius between atoms leads to stronger attraction and higher boiling point. Hope...

John Liang 2I

You can think of vapor pressure as the opposite of boiling point. Vapor pressure is how much gas exists alongside a liquid amount of a substance. Substances with a stronger imf lead to a higher boiling point and lower vapor pressure (molecules are more attracted and less likely to evaporate).

John Liang 2I

Are the bond angles of POCl3 distorted from due to P being different from Cl? How would each bond angle be affected?

John Liang 2I

In the notes, C5H12 has a lower intermolecular force than C18H38. Can someone please explain how the increase in mass per molecule makes the induced dipole-induced dipole attraction greater? Is this a matter of atomic radius, electron cloud, and polarizability? How can this be if the organic compoun...

John Liang 2I

Nope! The only notable "exception" in periodic trends that is of interest in our class is that oxygen's ionization energy>nitrogen's ionization energy.

John Liang 2I

Yes! Once you get to period 3 p block atoms (P,S,Cl, etc) and below, the presence of the d orbital allows for the "breaking" of the octet rule.

John Liang 2I

yup! just remember that there are exceptions to electron affinity and ionization energy (N>O) rules.

John Liang 2I

Icl2+ has two lone pairs and two single bonds to cl.
Icl4- has two lone pairs and four single bonds to cl.
Icl3 has two lone pairs and three single bonds to cl.
Icl5 has one lone pair and five single bonds to cl.

hope this helps!

John Liang 2I

Electron density shape is purely looking at the electron domains. For example in water, there are two bonds and two lone pairs: four electron domains (regions) in total, tetrahedral. Now for molecular shape, lone pairs play a role because they affect the bond angles. 2 bonds and 2 lone pairs (2,2) m...

John Liang 2I

Yup! all subatomic/super duper small particles all exhibit wave-like properties. Because of the de Broglie equation wavelength=h/mv, a very small mass will give a larger wavelength and vice versa. Electrons are emphasized in class and in our textbook since their particle/wave duality affects how ato...

John Liang 2I

The "cutoff" number to see if a particle has wavelike properties is wavelengths of 10^-15. Any smaller than that and it's too hard to measure any wavelike properties (a baseball is about 10^-34)

John Liang 2I

Your formal charges should "make sense" when drawing resonance structures. Take your first structure and see if you can move any double bonds. Then, take the formal charge of the atoms with your resonance structure and evaluate them. Usually, they should be 0 or negative 1 and positive 1.

John Liang 2I

If Sn has 4 valence electrons and loses 4 electrons, does it have 0 valence electrons or 8 new valence electrons? are the two the same?

John Liang 2I

How does one determine how many electrons Mn has?

John Liang 2I

Remember that when calculating formal charge for lds of ions, the sum of the formal charges must add up to the overall ion's charge.

John Liang 2I

The formal charge is important to calculate in order to find out which lewis dot structure is most prevalent in nature, and therefore the most accurate.

John Liang 2I

Formal charge refers to singular atoms in the lewis dot structure of covalent bonds, not ions. The ions themselves have an overall charge, but not a formal charge. For example, the lewis dot structure of F- is F with seven dots around it, no bonds formed. On the other hand, the F atoms in SF6 do hav...

John Liang 2I

"Why are s-block metals typically more reactive than
p-block metals? "

Is this because s block metals have an easier time giving up an electron while p block metals have a harder time giving up an electron? Due to orbitals?

John Liang 2I

11d. " How many orbitals are in subshells with l equal to (a) 0;
(b) 2; (c) 1; (d) 3?

How does one differentiate between the definitions of shell, subshell, and orbital?

John Liang 2I

how does one find the minimum position uncertainty of the bowling ball?

John Liang 2I

How does one find the needed uncertainty in velocity of the electron?

John Liang 2I

how does one find the wavelength needed to eject the electron at the given velocity?

John Liang 2I

Since this question describes the photoelectric effect, can one use Ek=1/2mv^2 to calculate the energy of the electron, then apply it to E=hc/wavelength to get 34 nm? answer solutions uses debroglie

John Liang 2I

The Photoelectric Effect shows that the Energy of the photon must be equal to or more than the Energy required by the work function. My question is, if the E of photon = E of work function, doesn't that mean there is 0 E kinetic energy of the ejected electron? How would an electron be ejected if it'...

John Liang 2I

"When neutrons pass through a crystal is a diffraction pattern observed? What does this experiment tell us about neutrons?" I do not remember this in class. do neutrons exhibit wave properties because they are so small? or do only electrons do so because of their electromagnetic properties?

John Liang 2I

"True or False? An electron in the n = 4 state in the hydrogen atom can go to the n = 2 state by emitting electromagnetic radiation at the appropriate frequency." Is there a distinction between an atom absorbing electromagnetic radiation and emitting it? Can't only light emit radiation? Ar...

John Liang 2I

" Can molecular spectroscopy be used to identify molecules?" Is the answer no, since the bonds between electrons within the molecule make it complicated to excite electrons?

John Liang 2I

"If the energy of a photon does not match the energy difference between two energy levels in a hydrogen atom is the photon's energy absorbed?" Is absorbed and emitted interchangeable when describing an electron being excited up an energy level?

John Liang 2I

"In photoelectric experiments, typically what part of the electromagnetic spectrum is the incoming light?" Is this visible light, since scientists are able to control the color as opposed to higher frequency/more dangerous lights? (UV)

John Liang 2I

in question 7a, how does one find the stoichiometric coefficients for the chemical equation?

John Liang 2I

in question 19, how does one find the subscripts for the empirical formula for caffeine?

John Liang 2I

in question 25, how does one find the number of times it takes to dilute the solution to no effect for the medicine?

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