CHEMISTRY COMMUNITY

Rachel Wang

Kristin Ishaya 1A wrote:Was question 4A from the midterm a reversible expansion problem? I didn't know which formula to use so I did it both ways.

I was confused too, actually, but because pressure was constant I think it's w=-PdeltaV

Rachel Wang

So far the instances I've seen are: 1. If given a free energy vs time graph, the step with higher activation energy (higher camelback) is the rate-limiting step. 2. If one of the steps involves breaking bonds and the other doesn't, breaking bonds requires more energy. 3. If given the rate law and as...

Rachel Wang

0 order: M/s or mol/L.s

1st order: 1/s

2nd order: 1/M.s or L/mol.s

3rd order: 1/M^2.s or L^2/mol^2.s

etc.

Rachel Wang

It's very rare for 3rd order rxns to occur unless it's the overall rxn's order. The units are 1/M^2.s

Rachel Wang

Intermediates are not present at the beginning of the rxn and are instead produced during the rxn. They are always consumed by the end of the rxn. Catalysts are present at the beginning AND the end as they are not consumed by the rxn. They speed up the rxn but are not present in the overall rxn equa...

Rachel Wang

Is it possible to get a negative activation energy cuz I got one for Lyndon Bui's Practice Final 2018 Q.8D?

Rachel Wang

K is the equilibrium constant. Q is the constant when the rxn isn't at equilibrium and when compared to K can predict which way the rxn will proceed at that particular state. They are calculated the same way.

Rachel Wang

A concentration cell has the same electrode/element for both half cells BUT with different concentrations. All concentration cells have E(nought) = 0. Use the Nernst equation E = Eo - 0.05916 V/n x log Q or E = 0 - 0.05916 V/n x log Q. You get Q from the overall redox rxn, which usually has the smal...

Rachel Wang

Given the reaction: C (s) + O 2 (g) --> CO 2 (g) Why is the oxidation state of C in CO 2 4+ and why is O 2 the species that is reduced, when their oxidation states as reactants and products are essentially the same? C has an oxidation state of 0 and is oxidized to an oxidation state of 4+ because O...

Rachel Wang

Kinda stumped too but I thinkkkkk it's because Fe has an oxidation state of Fe0 -> Fe+2 + 2e-. And because there are 2 of Fe and 2 Fe(OH)2, 2x2e- = 4e- = n?

Rachel Wang

Balancing in basic is same as acidic, just add OH- to both sides to get rid of the H+, then cancel out the resulting H2O's.

Rachel Wang

Does A stay the same even if activation energy or k changes, as long as temperature is the same?

Rachel Wang

I'm a little confused under what conditions do we use

W = - P x ∆V
or
w = -nRT ln V2

Rachel Wang

I assumed I use w=-PdeltaV when it's isothermal and isobaric.

Rachel Wang

Copper pots would be most energy efficient b/c it has the lowest heat capacity constant and thus would heat up faster for an overall lower value of q.

Rachel Wang

I've found that I generally use 8.314J/mol.K because it cancels out n (mol) and T(K).

Rachel Wang

What is an example of a state function?

Is enthalpy or entropy a state function?

Rachel Wang

104922499 1F wrote:why is O3's oxidation state 0?

It's a triatomic gas with the same element -> no oxidation state

Rachel Wang

-log10H = pH.
14-pH = pOH.
-logpOH = pOH

Rachel Wang

I've seen 0.0257 as the numerator before and I think it's when they calculate RT/F at T=298K. However the eq is most likely lnQ instead of logQ.

Rachel Wang

Proceed about problem as normal; find E(nought) and plug into Nernst. If deltaG is positive the rxn is nonspont and thus the metal won't dissolve. If deltaG is negative the rxn is spont and thus the metal will dissolve.

Rachel Wang

On a practice final exam the cell diagram is

2Ag(s) + Br2(g) -> 2AgBr(s)

The cell diagram is Ag(s)|AgBr(s)||Br-(aq)|Br2(g)|Pt(s)

I'm a little confused where the Br-(aq) came from?

Rachel Wang

You cannot assume in this case. You have to find the Ecell that would be positive first, flip whichever one, and then identify the element that is reduced as the oxidizing agent.

Rachel Wang

Not sure if this helps but there's

Red Cat - Reduction occurs at Cathodes
An Ox - Oxidation occurs at Anodes

Rachel Wang

For the equation Ecell = Ecathode - Eanode, it assumes you have not already flipped the sign of the anode. Therefore Ecell = -1.18-(-1.79).
However, you made the equation E=+1.79. In this case you have already flipped the sign and hence the eq is -1.18+1.79.

Rachel Wang

The electrode is a solid metal that can conduct electricity. I2 is a solid halogen and therefore unable to conduct electricity -> hence the platinum.

Rachel Wang

A porous disk is also known as a salt bridge. It allows ions to balance charges in the two half-cells when electrons are being exchanged and offsetting the charges.

Rachel Wang

In this question they provided heat capacity constants in both J.K-1.mol-1 and J.C-1.g-1. You'll have to pay attention to which temperature the use based on which heat capacity constant you use. Personally I find it very helpful to write down the units as you are solving instead of only incorporatin...

Rachel Wang

The problem states :
Determine the rate constant for each of the following
first-order reactions, in each case expressed for the rate of loss
of A:

But part C) has the rxn equation : (c) 2 A -> B + C

Isn't this a second order, not a first order, rxn in respect to A?

Rachel Wang

In Hmwk question 9.7 we used the equation
∆S = n C (ln T2/T1)
but in question 9.13 we use the equation
∆S = n R (ln T2/T1).

When do we know which one to use?
Thanks!

Rachel Wang

le Chatelier's principle is the idea that if a restraint is applied, the equilibrium will shift so as to counteract the effect of the restraint. Adding products will shift the rxn left; adding reactants will shift the rxn right. Adding pressure (decreasing volume) will shift rxn to side with less ga...

Rachel Wang

The reaction quotient is the same equation as K, but for any time in the reaction - not just at equilibrium. Compare Q to K to predict which direction the reaction will proceed. If Q<K, reaction proceeds foreward and makes more products. If K<Q, reaction goes backwards and makes more reactants.

Rachel Wang

Endothermic: requires heat to carry forward the reaction and therefore deltaH is on the left. Increasing heat in this case would produce more products. Exothermic: releases heat once reaction has occurred and therefore deltaH is on the right. Increasing heat causes equilibrium to favor reactants and...

Rachel Wang

Assuming A is also a gas, increasing the pressure (decreasing volume) would shift the equilibrium to the side with less gas molecules - in this case left.
Decreasing the pressure (increasing volume) would shift the equilibrium to the side with more gas molecules - in this case right.

Rachel Wang

It's good to memorize these three:

ethylenediamine (en) - bidentate
diethylenetriamine (dien) - tridentate
ethylenediaminetetraacetato (edta) - hexdentate

Rachel Wang

Assuming formal charge is the same, do central atoms n=3 and up (ie nitrogen) prefer to have an extended octet or not? Does the extended octet expend less energy in ground state?

Rachel Wang

Rachel Wang

Usually if it's symmetrical we can assume they cancel out. :^)

Rachel Wang

The lone electron is on Carbon because oxygen is more electroegative and therefore is more likely to attract enough e- to fill its octet.

Rachel Wang

I can tell what kind of hybridization an atom has, but I don't understand the chemical reasoning behind hybridization.

Rachel Wang

Look for the number of nitrogens and oxygens with a negative charge. Usually they have lone pairs for additional bonding.

Rachel Wang

chromium - assists insulin in control of blood sugar
iron - myoglobin, hemoglobin, e- transfer in the oxidation of carbohydrates, proteins, fats (respiratory chain)
cobalt - vitamin B12
Mn, Ni, Cu, Zn - critical for enzyme functions

Rachel Wang

Delocalized e- bonding occur everytime there's resonance involved, esp. evident with the 6 carbon ring.

Rachel Wang

I think UV light is typically used because it has sufficient energy (higher frequency than visible light) to eject e-.

Rachel Wang

Here's the question: The velocity of an electron that is emitted from a metallic surface by a photon is 3.6 * 10^3 km/s. (a) What is the wavelength of the ejected electron? I tried doing this question two ways: once using the de Broglie equation, which gave me the right answer (2.0*10^-10m), and onc...

Rachel Wang

You find in part a) the kinetic energy of the ejected e-: 5.909x10^-18 J. You find in part b) the work function, aka threshold energy because it's the minimum required energy to eject the electron, not the incident light. To find the energy of the incident light, E - 1.658x10^-17 = 5.909x10^=18 E = ...

Rachel Wang

Balmer series is the visible region of the light spectrum and Lyman is the UV region. When the electron jumps from quantum level n=2 to n=1, this releases much more energy than when it jumps from n=3 to n=2. Lyman's principle quantum level is n=1 because UV light has higher energy. Balmer's principl...

Rachel Wang

I did it a little differently from the book but got similar answers (around .05 off - not sure if that's a huge problem), so hope this helps c: You have to find the limiting reactant of P4O6 first. I converted 5.77g P4 to moles: The molar mass of P4 is 123.88g 5.77g/123.88g = 0.0466 mol P4. The rati...

Rachel Wang

Here is the question: H.21 The psychoactive drug methamphetamine (“speed”), which is sold as the prescription medication Desoxyn, C10H15N, undergoes a series of reactions in the body; the net result of these reactions is the oxidation of solid methamphetamine by oxygen gas to produce carbon dioxide ...

Search found 49 matches