CHEMISTRY COMMUNITY

JChen_2I

If you add water, the volume goes up causing the concentration to go down. Concentration does affect cell potential as shown in the Nernst equation.

JChen_2I

How do you tell if something is a catalyst? For example, in endgame 12, why is D not a catalyst?

JChen_2I

How comes in Endgame question 5, you were able to add the standard cell potentials to get -0.16V but you can't add the standard cell potentials in #7? Shouldn't you not be able to add them in 5 because cell potential isn't a state potential?

JChen_2I

For part D you should use the Nernst equation. E=Eo-(RT/nF) *lnQ
You have found Eo in part b. E is given as 2.20V. And Q=[Fe2+][Ce3+]^2/[Ce4+]^2 (products over reactants)
I believe the answer should be [Fe2+]=8.4*10^-6 M

JChen_2I

Thank you for all your help these past two quarters, Lyndon! Thank you for putting in so much time and effort to get us through these courses. Congratulations on graduating and best of luck in the future!

JChen_2I

Sometimes it helps to keep everything as variables and don't plug in numbers until you've isolated the variable you're trying to solve for. And of course, if you have extra time on a test, always be sure to double or triple check!

JChen_2I

Changing the mass of the electrode doesn't affect cell potential. Changing the concentration DOES change cell potential.

JChen_2I

Intermediates should not be a part of the rate law expression. Use the elementary reactions to find an equation you can set equal to the intermediate and plug it into the rate law equation.

JChen_2I

You find K2 by using the Vant Hoff equation you wrote down: ln K2/K1=-ΔH°/R [1/T2-1/T1] After finding K2, square root it to find the H3O+ concentration. Use the concentration to find the pH. This pH is the neutral pH at 10 degrees C. Compare that number with 7 to see if a pH of 7 is acidic or basic...

JChen_2I

You find K2 by using the Vant Hoff equation you wrote down: ln K2/K1=-ΔH°/R [1/T2-1/T1] After finding K2, square root it to find the H3O+ concentration. Use the concentration to find the pH. This pH is the neutral pH at 10 degrees C. Compare that number with 7 to see if a pH of 7 is acidic or basic ...

JChen_2I

Gibbs free energy depends on the equilibrium constant/reaction quotient as shown by these equations:
deltaGo=-RTlnK
deltaG=deltaGo+RTlnQ

JChen_2I

A 1.0 m NiSO4(aq) solution was electrolyzed by using inert electrodes. Write (a) the cathode reaction; (b) the anode reaction. (c) With no overpotential at the electrodes, what is the minimum potential that must be supplied to the cell for the onset of electrolysis? How do you find the minimum poten...

JChen_2I

Write balanced half reactions for the redox reaction of an acidified solution of potassium permanganate and iron(II) chloride.
Could someone help walk through how to get the half reactions? And why are the potassium and chlorines not included in the reactions?
Thanks in advance!

JChen_2I

When given a skeletal equation of a cell reaction, how do you know whether or not to include Pt? And which side would you include it on?
For example, in 6L5 why does a) Ni2+ +Zn -> Ni + Zn2+ not include platinum when b) Ce4+ + I- -> I2 +Ce3+ does

JChen_2I

The way i start this problem is by balancing the solution in acidic solution and then, at the end, adding OH- to both sides so the H+'s cancel out. The OH- + H+ turns into neutral H2O molecules (which may or may not cancel out. Therefore, Oxidation: 8OH- +P4 --> H2PO2-+4e- Reduction: 12e-+ 12H2O+ P...

JChen_2I

I think grades are adjusted a bit at the end of the quarter based on the class averages. When I had him last quarter, my final grade ended up being a bit higher than my point total would've been.

JChen_2I

One coulomb is the amount of electrical charge in 6.24*10^18 electrons. Coulomb is just a convenient unit that allows for the volt unit to be equal to 1J/C.

JChen_2I

Yes, the Focus 5 topics listed under Outline 4 will be on test 2.

JChen_2I

Test 2 will cover the second page of outline 4 (thermodynamics, focus 5) and outline 5 (electrochemistry)

JChen_2I

I think both workshops and step up sessions go over the concepts but step ups might break down the concepts more and tend to go over more homework problems. Workshops sometimes have worksheets which are helpful.

JChen_2I

Hi! I was wondering if taking 14BL and 14C together is doable or is it too heavy of a workload/not recommended?
Thanks!

JChen_2I

Step one is raising the temperature of the reactants to 200C. Step 2 is the actual reaction at 200C (deltaH is given). Step 3 is cooling the products back down to 37C. Add all these values together to get the total change in enthalpy of the reaction.

JChen_2I

This is a problem from Hannah's acid and base review session: What is the initial pH of a buffer solution consisting of .400M acetic acid (CH3COOH) and .250M KCH3CO2? The Ka for acetic acid is 1.78 x 10^-5 The answer I got was a pH of 4.545 and I was wondering if someone could confirm that this is c...

JChen_2I

An extensive property depends on the amount you have. Heat capacity is extensive because the amount of energy required to raise the temperature by 1 degree is dependent on how much of the object you have. Molar and specific heat capacity are intensive because they have already defined the amount you...

JChen_2I

Why is deltaH=0 for isothermal reactions?

JChen_2I

Since heat of fusion for water is 6.01kJ/mol, is it correct that the change in enthalpy when going from liquid to solid be -6.01kJ/mol? And similarly going from gas to liquid would be -40.7kJ/mol?

JChen_2I

does an isothermal, irreversible free expansion mean there's no change in entropy of the surroundings?

JChen_2I

Because N2 and O2 are already in their most stable form so their standard reaction of enthalpy by definition is 0.

JChen_2I

No because it doesn't make sense for it to take negative energy to raise the temperature of a substance.

JChen_2I

Drawing the lewis structure helps a lot to visualize which bonds need to be broken and formed. In worst case scenario, you could calculate if all bonds were broken and all new bonds reformed.

JChen_2I

Entropy is always in J/K. The units for the numbers you are finding the natural log of (volume, temperature, pressure) will always end up cancelling out.

JChen_2I

Can someone explain how irreversible and reversible processes affect entropy of the surroundings and universe? For example in 4I9, why does an isothermal, reversible expansion mean there's no change in total entropy? And why does an isothermal, irreversible free expansion mean there's no change in e...

JChen_2I

How do we know when to use Hess's Law as opposed to standard enthalpies of formation where we do the enthalpies of product minus enthalpies of reactants?

JChen_2I

Calculate the reaction enthalpy for the synthesis of hydrogen bromide gas, H2(g)+Br2(l)->2HBr(g), NH3(g)+HBr(g)->NH4Br(s) dH=-188.32kJ N2(g)+3H2(g)->2NH3(g) dH=-92.22kJ N2(g)+4H2(g)+Br2(l) dH=-541.66kJ I was wondering why you don't have to divide 92.22 by 3 in order to get the reaction enthalpy for ...

JChen_2I

Could someone explain the difference between q and deltaH please? Thank you!

JChen_2I

Carbon disulfide can be prepared from coke and elemental sulfur: 4C(s)+S8(s) --> 4CS2(l) deltaH=+358.8kJ (b) Calculate the heat absorbed in the reaction of 197g of carbon with an excess of sulfur. Can someone explain why we have to divide by 4 moles of carbon to get the answer? And should we normall...

JChen_2I

The answer is 0kJ/mol for the reason you stated

JChen_2I

Does anyone know if the 14A final solutions are posted anywhere?

JChen_2I

The answer is 10.25

JChen_2I

I believe you have to search up the Ka value. I found it to be 3.0*10^-8. Use the hydronium concentration you found and plug it into the ice table. Then take the values from the ice table and set up an equation equal to Ka to solve for the initial concentration

JChen_2I

I think that catalysts only speed up reactions. They don't cause more product to form.

JChen_2I

5.61 The overall photosynthesis reaction is 6 CO2(g) + 6 H2O(l) -- C6H12O6(aq) + 6 O2(g), and H = 12802 kJ. Suppose that the reaction is at equilibrium. State the effect that each of the following changes will have on the equilibrium composition: tends to shift toward the formation of reactants, te...

JChen_2I

I think if the second Ka value is less than 10^-3 then the second protonation can be ignored

JChen_2I

A salt is produced by the neutralization of a base by an acid. Oftentimes a cation will act as the acid and an anion as the base.

JChen_2I

I believe that increasing the partial pressure of O2 is achieved by means of adding more O2 gas particles. And remember that P (partial pressure) = concentration * R * T (temperature). Doesn't increasing the partial pressure have the same effect as increasing the concentration? Why doesn't the equi...

JChen_2I

Is it right to say that the second deprotonations of all polyprotic acids (but sulfuric acid) can be ignored when calculating pH?

JChen_2I

Do we have to know how to calculate delta H? If yes, how?

JChen_2I

For this problem, you can just set the given K value equal to [products]/[reactants] and solve for the [Br2]

JChen_2I

EllieSchmidtke_4I wrote:For problem 5I.3, why would you use Kc = 160 from Table 5G.2 instead of Kc = 794 or Kc = 54?

The problem tells you the equilibrium mixture is at 500K which is why you use Kc=160

JChen_2I

The problem also tells you that K=7.8*10^-5 at 700K which is less than K at 600K. Therefore more ammonia (the product) would be formed at 600K since a greater K value means a greater ratio of products to reactants.

JChen_2I

6CO2(g) + 6H2O(l)->C6H12O6(aq) + 6O2(g) is at equilibrium
How comes when you compress this system, there will be little to no effect on the system? Would the reaction not shift to the formation of reactants because there's less moles?

JChen_2I

Predict whether each of the following equilibria will shift toward products or reactants with a temperature increase:
X2(g) -> 2X(g), where X is a halogen

How do halogens affect equilibrium?

JChen_2I

HClO4 is a stronger acid than HPO4. The only differing atom between them is the chlorine and phosphorus. Chlorine is more electronegative so it will help to stabilize the negative charge that will be created after the molecule gives up a proton. Chlorine is able to pull electron density towards it a...

JChen_2I

Chelates form a ring around the central atom, whereas polydentate ligands allow more than one of its binding sites to be occupied. I think that a polydentate ligand can be a chelate, but they are not necessarily a chelate. In the review session i'm pretty sure Matthew said all polydentate ligands c...

JChen_2I

How do you define chelate and polydentate and how are they different?

JChen_2I

Could someone explain why the conjugate acid of a strong base such as NaOH doesn't affect pH? Thanks!

JChen_2I

Hi,
Could someone explain the placement of lone pairs necessary on ligands for the ligand to be polydentate?

JChen_2I

Amphiprotic means that a substance can both accept and donate a proton or H+. Amphoteric means that a substance can act as both an acid and a base. All amphoteric substances are amphiprotic because if a molecule can either donate or accept a proton, that means it can act as both an acid or a base. ...

JChen_2I

ALegala_3I wrote:What does the term polyprotic mean? Can a molecule be both polyprotic and amphiprotic?

Polyprotic means an acid that can give more than one proton

JChen_2I

Hi,
Is it correct to say that all amphiprotic substances are amphoteric? If so, what are some compounds that are amphoteric but not amphiprotic?

JChen_2I

Is there a way to determine if an acid/base is strong or weak when only given their chemical formula?

JChen_2I

Which of the following can be classified as reactions between Bronsted acids and bases? For those that can be so classified, identify the acid and the base. a) NH4I + H2O -> NH3 + H3O+ + I- b) NH4I -> NH3 + HI c) CH3COOH + NH3 -> CH3CONH2 + H2O d) NH4I + KNH2 -> KI + 2NH3 Could someone explain how t...

JChen_2I

Can coordination compounds only be made with transition metals? Or can other metals act as the central metal atom?

JChen_2I

Would we get marked down if we were to write H2O instead of OH2 when the central metal is binding to the O?

JChen_2I

How do we know when a coordination complex is tetrahedral as opposed to square planar?

JChen_2I

if a central atom has two different pairs of elements attached to it, those dipoles would not cancel out because of the molecule's tetrahedral shape. The atoms are not all on one plane

JChen_2I

yup, starting from funadamentals

JChen_2I

could someone explain the different hybrid orbitals? for example, would there be a hybrid orbital with p4? what would be the orbital with 6 electron densities? 7?

JChen_2I

What section should we complete for week 9's homework?

JChen_2I

Are the angles of a seesaw molecule 90 and 120 degrees? Or should they be slightly less than 90 and 120.
The textbook answer key says it should be 90 and 120 exactly, but shouldn't the lone pair repulsion cause the angles to be slightly smaller?

JChen_2I

AX3 because there are 3 atoms (3 oxygens) attached to the central atom (nitrogen)

JChen_2I

yes, those are correct :)

JChen_2I

For 3F15, why does AsF3 have a higher boiling point than AsF5?
I guess my question is also: how is AsF5 polar while AsF3 not, as the answer key says?

JChen_2I

When carbon and hydrogen are bonded, is there a dipole moment?

JChen_2I

Are we just supposed to memorize that a tetrahedral angle is 109.5 degrees and that trigonal planar angles are 107 degrees, etc? Or is there a way that these angles were calculated?

JChen_2I

Resonance structures are alternate Lewis structures for a single compound. They're all correct and often just have double/triple bonds at different locations.

JChen_2I

Count all the total number of valence electrons and arrange them around the atoms. Then as you calculate formal charge, you can rearrange the lone pairs and see where there are double bonds.

JChen_2I

Electronegativity increases to the right and going up. Electronegativity is the electron-pulling power of an atom. So as you go to the right on the table, the outer electron shell becomes increasingly full so the atom would want an electron more because it's closer to becoming stable with a full ele...

JChen_2I

Does CN- have a triple bond? If yes, why a triple bond and not a double bond with 2 lone pairs on nitrogen?

JChen_2I

Are resonance structures only the lowest energy structures with the best formal charge? Or can they be structures with non-optimal formal charges?

JChen_2I

Hi,
Does anyone have the answers to the worksheet from Lyndon's workshop during week 3 10/14? They'd be much appreciated, thank you!!

JChen_2I

i believe this is how you do it: a) 103 because ml can be any value between [-l, l] so there are 51*2+1 possible orbitals bc) 1 because they give you a ml value so there's only one possible orbital d) if n=57 then there are 57 possible l values and for each of them they'd have l*2+1 orbitals (or ju...

JChen_2I

We should always use formal charge but remember that the sum of the formal charges of all the atoms should equal the charge of the compound. Check that the octet rule is satisfied and keep in mind that certain elements can have expanded or incomplete octets too.

JChen_2I

the equation we would use is mass*indeterminacy of velocity*indeterminacy of position should be greater than or equal to h/4pi Plug in 0.0028kg for mass and 0.68 for indeterminacy of velocity because the velocity could be 0.34m/s above or below the given value. Isolate and solve for the position and...

JChen_2I

htatshwe_3L wrote:So it's everything including material from test 1, not everything after test, right?

Yup, from week 0 to the end of focus 2D

JChen_2I

How long does an object's wavelength have to be to be considered to have wavelike properties?

JChen_2I

Carbon has a lower ionization energy.
Carbon is 153.9 kJ mol‑1 while chlorine is 349.0 kJ mol‑1. Chlorine is also closer to the top right corner of the periodic table which means it has higher ionization energy

JChen_2I

Because ONF is a neutral compound, you would want the formal charges to equal 0

JChen_2I

Magnesium has 2 valence electrons so it will generally give them up to atoms with almost full electrons. Arsenic has 5 valence electrons so it needs 3 more to complete its shell. Therefore if you have 3 magnesium atoms, there will be 6 electrons to give. And with 2 arsenic atoms, it needs 6 electron...

JChen_2I

Cobalt is in group 9 so it has 9 valence electrons in its neutral state. Then because the charge the 3+, it means there are 3 less electrons than it does during its neutral state so there are 6 valence electrons.

JChen_2I

How do we know that bromine gets 10 valence electrons?

JChen_2I

Give the notation for the valence-shell configuration (including the outermost d-electrons) of Group 5 transition metals

Could someone please explain how the answer to this is (n-1)d^5ns^2?
Thank you!

JChen_2I

i believe this is how you do it: a) 103 because ml can be any value between [-l, l] so there are 51*2+1 possible orbitals bc) 1 because they give you a ml value so there's only one possible orbital d) if n=57 then there are 57 possible l values and for each of them they'd have l*2+1 orbitals (or jus...

JChen_2I

could some please explain how electrons are paired and parallel? in what situations are they paired and when are they parallel?

JChen_2I

I believe the photoelectric effect works the same way but different metals have different threshold energies/energy required to remove an electron.

JChen_2I

The Balmer series includes all spectral line emissions that have a final n value of 2 while the Lyman series includes all spectral line emissions with a final n value of 1. Because they are all emission lines, the initial n value must be greater than 2 and 1 respectively.

JChen_2I

could someone explain the difference between orbitals, shells, and subshells please? thank you!

JChen_2I

a) lambda = c/v 589*10^-9 m = 3.0*10^8 m/s (v) v = 5.09 * 10^14 Hz E=hv E=6.626*10^34m^2kg/s * 5.09 * 10^14 Hz E=3.37 * 10^-19 J (per atom) b) 5.0 mg * (1g/1000mg)*(1 mol/22.99g) * (6.022*23 atoms/1mol) = 1.3096 atoms 1.3096 atoms * (3.37*10^-19J) = 44.2 J c) 1 mol * (6.022*23 atoms/1mol) * (3.37*10...

JChen_2I

We know that the E(photon)=threshold energy + E(kinetic) Threshold energy is the amount of energy required to remove an electron from a metal which is what we found in (b) to be 1.66*10^-17 J E(kinetic)=1/2mv^2 where m=9.11*10^-31 kg (mass of electron) and v=3.6 * 10^3 km/s or 3.6*10^6m/s so E(kinet...

JChen_2I

A UA told me we have to memorize the order of the spectrum. And for visible light know that violet has the shortest wavelength while red has the longest.

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