CHEMISTRY COMMUNITY

Ian_Lee_1E

I'm confused on this as well

Ian_Lee_1E

Why do Catalyst increase the rate of reverse reaction as well as forward reaction?

How does it work in terms of energy graph?

Ian_Lee_1E

When writing the reaction rate equation for bottle neck effect

I understand that you have to change the intermediate into its reverse since it is in equilibrium.

However, I don't understand to how you can get the (k1 * k2) / k'1 value

Will the k'1 be given?

Ian_Lee_1E

for this one, Professor Lavelle mentioned the reason in the "Namaste" post if you search Namaste on Chem community, youll see the answer But the basic reason is because 2H+ --> 2H+ is being cancelled out to 1H+ But if you use 2H+ you still get the same answer since you have to square the Q

Ian_Lee_1E

For this question part B What does it mean when the answer states "At equilibrium, the "rates" of the forward and reverse reactions are equal. NOT THE RATE CONSTANTS?" I get why they should not be equal since that would mean K would always equal to 0 for every equilibrium reactio...

Ian_Lee_1E

Hi! The rate limiting step is one of the elementary steps within a reaction mechanism. The rate limiting step is always the slowest step of the mechanism which is used to determine the rate of the overall reaction and limits the speed at which the reaction is occurring. The other elementary steps o...

Ian_Lee_1E

How are molecularities and mechanisms related?

Molecularities are number of species in elementary steps

and

Mechanisms are sequences of elementary steps by which a chemical reaction occurs.

Do we use molecularities to find mechanisms? Are mechanisms numbers?

Ian_Lee_1E

What are rate limiting steps? In today's lecture when professor was talking about ways to approach proposed reaction mechanism, he mentions about identifying rate limiting step, using rate law of rate-limiting step as overall rate and compare model with experiment I didn't really get what rate-limit...

Ian_Lee_1E

For this question, I used half life equation in order to find the time it would take for the concentration to decrease to one-sixteenth of its original, which would be (1/2)^4, making the time 4 x 50.5 seconds. However, the answer used the Second order integrated equation, making the answer equal to...

Ian_Lee_1E

Correct, H 2 cancels and one has 2H + ----> 2H + . Which can also be written as: H + ----> H + and E stays the same (intensive property) Remember E stays the same if one multiplies a redox reaction by some multiple (e.g., by 2). Intensive property. You can also see this mathematically. Use the Nern...

Ian_Lee_1E

What exactly is a reaction mechanism in definition?

Also, why is overall rate law the rate law of the slowest step?

Ian_Lee_1E

For this question,

Why is concentration of A close to HA in buffer system?

Ian_Lee_1E

How is the wording of the exercise suggesting that K+ ions participate in an electrolyte concentration cell reaction?

I am having trouble understanding how we know if a reaction is electrolyte concentration cell reaction and if we need electrodes for the electrolyte concentration cell reaction

Ian_Lee_1E

For this question 1. Is the given half reaction reversed to oxidation even thought it has a greater reduction potential BECAUSE the question is asking for the value of Ka for HF? 2. What is the relationship between Ka and K? because in the answer, in order to find Ka, the calculated K was square roo...

Ian_Lee_1E

For this question the answer states "A negatively charged electrolyte flows from the cathode to the anode." First of all, I was confused to how electrolytes move. I realized that salt bridge is used to balance out the oxidated ions but I still didn't really get how the electrolytes flow Se...

Ian_Lee_1E

I see what you are saying about higher reduction potential should be reduction and therefore the cathode, but for this question, I think that it wants you to use the given rxn equation. Therefore, it is asking you to look at which one actually get oxidized and reduced in this rxn and then say which...

Ian_Lee_1E

For this question, I thought the moles of electrons, or the n value, should be 2, not 1 since there are 2 Hydrogens in 1 H2

But the question puts 1 as the n value, making the E naught cell = 6 x 10^-2

Why do you not divide it by 2?

Ian_Lee_1E

For 6M. 13 part c, the even though Pb^4+ / Pb^2+ has a greater standard reduction potential then Pb^2+/Pb, the anode, which is the one that is being oxidized, is Pb^4+ / Pb^2+

Why is it the anode?

Ian_Lee_1E

Why is there a difference between Pseudo K and just K

I thought if you find the graph of ln[A] vs t for the reaction (if [A] is first order)

the k would be the slope.

Is the large excess of [B] and [C] altering the slope? if so, how?

Ian_Lee_1E

Hello For 6L 7 the equation is AgBr(s) --> Ag+ + Br- but the half reactions have Ag(s) in them. I get that Ag gets cancelled out from the reactant and product since there is 1 mole on each side, how do we know there will be Ag(s)? Also, why is cathode Ag+ (aq) + e- --> Ag(s)? I thought since Ag+ is ...

Ian_Lee_1E

Hello I have a question for the balanced redox reaction of these two. For 6L3 D, the balanced reaction was 4H2O -> 4H+ + 4OH- and the reaction was kept this way without the 4H+ and OH- becoming 4H2O But for 6L5 C Cl2 + H2 --> 2Cl- + 2H+ becomes Cl2 + H2 --> 2HCl why does the reaction in 6L5 C have p...

Ian_Lee_1E

Will Textbook Questions come out in the final as it did in the midterms?

Ian_Lee_1E

For the cell diagrams with commas

ex. Ce^4+ , Ce^3+

Do their orders matter in the diagram?

Also, Do Platinum (Pt)s get used for electrodes all the time whenever the cell requires electrode and the anode is an aqueous solution?

Ian_Lee_1E

Benjamin Chen 1H wrote:n represents the number of moles of electrons transferred in a balanced redox reaction. The 2 is specific to one half reaction and the 6 is specific to the other reaction.

Thank you for the clarification Benjamin!

Ian_Lee_1E

Andersen Chu 2H wrote:Try writing out half reactions, n is equal to the number of electrons transferred. You should see 2 and 6 electrons being transferred in the half reactions

Does n always represent the number of moles of electrons in the equation Delta G naught = -nFE naught?

Ian_Lee_1E

How does Concentration affect Cell Potential?

Ian_Lee_1E

What does it mean when professor says that the enzyme or the catalyst are saturated?

in terms of zeroth order

Ian_Lee_1E

Will the K value always be positive?

I believe in yesterday's lecture, professor said that K value is always going to be positive, keeping the half life equation for second order from becoming a negative value.

But I thought that k value can be negative whenever a reactant decomposes etc.

Ian_Lee_1E

For finding the standard reaction Gibbs Free Energy with the given cell potential, why is n "2" and "6" for a and b which are the number of moles of Ce and Fe?

Didn't the question ask for the Gibbs Free Energy of the cell reaction?

Ian_Lee_1E

What are the differences between these two cells and

once they come out in questions

how are they different in terms of solving the questions?

Ian_Lee_1E

How can a compound or a molecule be oxidyzing and reducing at the same time?

Ian_Lee_1E

Hello

I was wondering when you use E (not) = RT/nFlnK

and

E = E(not) - RT/nFlnQ

I thought E not was supposed to equal zero when it is at equilibrium so I don't really get teh equation using K

Ian_Lee_1E

Professor mentions that "we want to start of with a redox reaction with a positive E not value"

I wasn't really sure to what he meant here because some of the redox reactions have negative E not values.

Ian_Lee_1E

Hello, I was watching Dr. Lavelle's lecture and I had a quick question on standard electromotive force. So professor mentions that electromotive forces are measures of voltage with respect to standard hydrogen electrode. In this standard hydrogen electrode and transfer of e- with a different cell, t...

Ian_Lee_1E

How does Voltage change over period of time?

Also, is the E cell (V) the Voltage calculated in the beginning when everything is standardized?

Ian_Lee_1E

Could someone explain this to me? I didn't really get it from Professor

Ian_Lee_1E

For this question, I balanced the chemical equation to 2H2 + O2 --> 2H2O and I plugged in the standard entropy values in order to find the change standard change in entropy However, I was not able to use that equation but I had to use H2 + 1/2O2 ---> H20 I thought I was able to use standard enthalpy...

Ian_Lee_1E

what does it mean by

"HBr, because Br is more massive and contains more elementary particles than F in HF" ?

I get that it is more massive but I was having trouble understanding the "elementary particles" part.

Ian_Lee_1E

Hello, I am getting confused with the comparison of same gas in different pressure in their entropy values. I guess I'm thinking of entropy as a more chaotic substance but I think I am wrong because I've been thinking that greater pressure leads to greater entropy since gas molecules will move at hi...

Ian_Lee_1E

Why are more Massive compounds or elements greater in molar entropy than those that have less mass?

Also, does this only apply to gas or does it apply to any state of matter?

Ian_Lee_1E

Hello, for this problem

when it states to calculate the standard entropy of the vaporization of water, can I conclude that they are trying to find the entropy change from liquid water at 85 degrees Celsius to water vapor at 85 degree Celsius?

Ian_Lee_1E

If K is a large value, ln(K) will be a positive value. Since there is a negative in the front, -RT(+number) will be negative. If K was a value less than 1, the lnK value will be a negative number giving us -RT(-number) making it positive. Delta G (standard) can only be 0 when K=1 (because ln(1)=0) ...

Ian_Lee_1E

In today's lecture, professor talked about the equation Delta G (standard) = -RT ln K, where if K value is large, then it leads to Delta G (standard) being a negatively significant value. However, I thought that delta G of a reaction at equilibrium is supposed to be 0. I don't really understand why ...

Ian_Lee_1E

Hello, For Ideal gas do we have to memorize the change in constant value in front of R (gas constant) based on whether they are monoatomic or diatomic? I think professor said that if ideal gas is monoatomic its w = -n (3/2) R ln (t2/t1) in constant volume or one for constant temperature and 2 = -n (...

Ian_Lee_1E

i dont even see the midterm grade. I have to go into myucla to view them even though before, I was able to see all of my grades on the CCLE grade section :/

Ian_Lee_1E

Also, is the irreversible process always constant volume? or can the piston of the irreversible process go up, changing the volume?

I'm generally confused on understanding the reversible and irreversible processes.

Ian_Lee_1E

Pressure would only increase with increase in moles of gas if the volume is constant ( more gas particles in one space). However, if volume is not constant ( expanding with gas), pressure can remain constant. This is why we can apply the equation w= -P\Delta V to the system to calculate work. This ...

Ian_Lee_1E

why does Irreversible process do less work than reversible reaction? I get it just with the equations as the external pressure for the irreversible pressure is significantly lower than the internal pressure. but I still don't get why reversible reaction does the maximum amount of work compared to th...

Ian_Lee_1E

Hello, Professor said in lecture 11 that "If there is no change in pressure (delta P = 0) then delta U = Qp + w = delta H - P delta V however, without the increase in moles of gas, there would not be expansion, which would make delta V = 0. He stated that if number of moles of gas changed, P de...

Ian_Lee_1E

Could someone list all the equations we went over in class so far? I believe we went over w = -P delta V w = - integral(vi - vf) PdV ----> COULD SOMEONE EXPLAIN WHEN YOU USE EITHER OR? I BELIEVE THEY ARE NOT INTERCHANGABLE q = mc delta T q = C delta T s = Kb ln W (W=degenerecy) delta U = q + w --> i...

Ian_Lee_1E

Hello,

could someone explain why kinetics is time dependent while thermodynamics is time independent?

Ian_Lee_1E

Hello, I am a little confused on why combustion is exothermic.

I thought combustion uses heat to break bonds so it was going to be endothermic.

What is the explanation to why they are exothermic reactions?

Ian_Lee_1E

Hello,

I don't understand why reversible systems do maximum amount of work compared to irreversible systems.

I don't really understand what professor is saying when he says infinite steps either.

what are the differences between the reversible and irreversible systems that lead to this outcome?

Ian_Lee_1E

Hello, so in case of work if the volume expands for the system, which means the internal pressure was greater than the external pressure, does that mean the system did work to the surrounding, decreasing the internal energy of the system? I am hearing from professor that if the work is being done to...

Ian_Lee_1E

Hello, Professor said in the lecture that work = external pressure x change in volume. does pressure have to always be external? In the example that he gave, the pressure came from inside the beaker, pushing the piston outwards. Wouldnt that be called internal pressure or am I just getting confused ...

Ian_Lee_1E

what does this mean? f) Because water is a liquid, it is by definition present at unit concentration, so changing the amount of water will not affect the reaction. As long as the glucose solution is dilute, its concentration can be considered unchanged. I understand that water is a solvent. But sinc...

Ian_Lee_1E

In the answer key, the Kc value that it uses is

1.6 x 10^-2, which is 1 / 6.1 x 10^-3

In the chart given by the textbook *Table 5G.2*, it says the K c value is 1 / 6.1 x 10^23.

I'm confused to which one is the correct one.

Ian_Lee_1E

hello I remember professor stating that if k is smaller than 10^-3 more reactants are at equilibrium than the product, which makes total sense. However, I heard that if K is greater than 1 it favors right and if its less than 1 it favors left. what is the point of division between favoring right or ...

Ian_Lee_1E

I'm still confused on the Constant pressure and Constant volume calorimetry and capacities If its constant pressure capacity, does that mean that it is in open space, so its not sealed and the pressure will be the same as outer pressure, usually 1atm? Also if its constant volume, the pressure inside...

Ian_Lee_1E

When the question asks

How much heat was released from the system

Do we hvae to put negative to the energy value since it is exothermic? or since the question is already asking for a negative value, do we just put the absolute value of the energy?

Ian_Lee_1E

Professor mentioned about perfect system where the energy absorbed or released from the system will be opposite of the energy of surrounding.

do we assume that every reaction we encounter in the problems are in perfect system?

Ian_Lee_1E

Professor mentioned in his lecture that the reaction of HCl and NaOH is neutralization reaction

What is neutralization and what makes that reaction one of it?

Ian_Lee_1E

when writing the Molar Heat capacity professor wrote J * k^-1 * mol^-1

I understand that the unit of energy had to change from kj to j since moles are much smaller than grams. However, why did it have to change from celsius to K? Is it the same reason?

Ian_Lee_1E

Hello

I'm still confused to what Standard Enthalpy of Formation is

Professor said that by definition, it is standard reaction enthalpy for the one mole of substance from its elements in their most stable form.

How do we know for each compounds what their standard enthalpies are?

Ian_Lee_1E

How can Heat not be a state property? in what case would that be?

Ian_Lee_1E

Do water vapors change phase siilar to photon where enough energy is required in order to release an electron?

And I was wondering if we would have to memorize any particular "minimum" energy for such famous compounds like water molecules.

Ian_Lee_1E

It basically goes like this. when vapor meets our skin, which is around 36 degrees Celsius, the vapor, due to thermodynamic equilibrium, cools down to a liquid state with temperature of 36 degrees celsius. In order for this to happen, the vapor first needs to condense to a liquid. If we see the char...

Ian_Lee_1E

Professor mentioned that the way we have constant temperature for a reaction is that we just measure the heat that was given off during the reaction and wait for the system to come back to the equilibrium, to the initial temperature. How do we know when to measure the temperature given off? do we me...

Ian_Lee_1E

Hello,
professor mentioned about carbohydrate converting into co2 and h2o when it gets broken down as energy.

Is that the cause of sweating? The increase of H2o gets released outside of our body to cool down the body as energy gets released?

Ian_Lee_1E

you guys are saying I can approximate, but I'm not sure what you guys mean when yall say i can approximate.

Does that mean x = approximately 0?

Ian_Lee_1E

does that mean if the percent ionization is less than 5 percent we approximate x as 0?

Ian_Lee_1E

why do we have to check whether K is less than 10^-4 in order to approximate? what does that mean?

and if that condition is not met, what happens?

Ian_Lee_1E

I have a question from the recent lecture. Professor said that if X is less than 5 percent for the weak acid percent ionization, the approximation is valid. However, couldnt different weak acids have different percent ionization, leading to an x value, which is the H3O+ composed from ionization, big...

Ian_Lee_1E

Why is Kw always considered 14?

Weren't there questions where Kw equals to 2.1x10^-14 or other values?

why would Kw always be considered 1 x 10^-14 in any aqueous solutions?

Ian_Lee_1E

H2 should go onto the right side of the equation. this should be a typo I think there were already a lot of students who asked about this, finding the typo. Also, maybe the reason why neither the products or reactants are favored is because as pressure increases that means more collision occurs, lea...

Ian_Lee_1E

Favoring left means that the equation is producing reactants more efficiently than products.

Towards the left means the same thing!

Ian_Lee_1E

Professor said in the lecture that we neglect the water concentration in the Kw equation because it is excess. What does that mean?

why do we have to neglect it, leading to just the calculation of [H3O+][OH-]?

Ian_Lee_1E

What are super acids and super bases?

Is that when the Kw does not equal to 10^-14? when would those cases be?

Ian_Lee_1E

When discussing about the change in pressure Professor said that when Volume Decreases and more moles of gas are on the left, then reaction will favor products if volume decreases and more moles of gas are on the right, then reaction will favor reactants How can there be more moles on the left and o...

Ian_Lee_1E

Hello, For the first example and the last example of lecture 2 that Professor Lavelle went over, which was 1.50 mol PCl5 is placed in a 500ml reaction vessel and decomposes at 250 C to form PCl3 and Cl2, Kc = 1.80, All 3 compounds are gases at 250 C why is professor using concentration instead of pa...

Ian_Lee_1E

Why is this true?

"If one starts with higher concentration of reactants, the equilibrium concentrations of the product will be larger"

Ian_Lee_1E

Why is K=1 rare?

Ian_Lee_1E

What is the meaning of partial pressure?

Ian_Lee_1E

Hello,

I realized that for examples like N2 + 3 H2 <-> 2NH3 the reaction needs to be at a higher temperature since the gas molecules have to collide in order to create a molecule.

What happens if the reaction was in a lower temperature?

Will the reaction even take place?

Ian_Lee_1E

Hello

When professor was talking about how the change in concentration of solvent was immeasurable, can someone clarify what he was talking about?

It was towards the end of the lecture where he talked about the molar concentration of a pure substance does not change in reaction.

Ian_Lee_1E

Hello,

I was wondering where I could find Professor's Review Slides shown on Wednesday

Ian_Lee_1E

How does resonance stabilize an anion?

Ian_Lee_1E

Also, what does it mean to withdraw electron density??

Ian_Lee_1E

Hello, Professor said as he was explaining relative acidity that "Acid more readily loses H+ if resulting anion is stabilized by resonance and/or electron withdrawing atoms which delocalize and stabilize the negative charge." He also stated that whenever charges delocalize it is most stabl...

Ian_Lee_1E

Hello,

what forces are considered favorable??

And how do you compare whether ones more favorable?

Ex.

NH3 has hydrogen bond, london dispersion, and dipole dipole.

which force is the most favorable?

Ian_Lee_1E

I realize from the photoelectric effect that the electrons are emitted when each photons have enough energy to meet the threshhold.

And I thought professor said that if each photon energy does not meet the quota, then they just go through the atom.

Then, how do electrons get excited?

Ian_Lee_1E

hello,

I was wondering how you know when drawing the lewis structure that the 2 Hydrogens go to the oxygen rather than making 6 single bonds, 4 oxygen and 2 hydrogens attached to Se.

Ian_Lee_1E

Hello,

Can someone clarify why Oxides are strong bases?

Ian_Lee_1E

Hello,

I was watching lecture 26 and saw that Methylamine, which derives from NH3, ammonia, is a weak acid.

I was wondering what property of NH3 makes it a weak base.

Ian_Lee_1E

What can and cannot be polydentate?

Ian_Lee_1E

Hello, when professor talked about Ethylene - Diamine - Tetra - Acetate, the Ethylene, NH2-CH2-CH2-NH2, can rotate to make a ring, said by professor in his lecture. Is this related to the ideal bonding geometry of ligand that can form more than one bond to the same TM cation (Chelate)? Where ligands...

Ian_Lee_1E

Hello,

Professor mentioned that Pi bonds can break when the central atoms rotate.

Are there any cases when they do rotate and what is the cause of their rotation?

Ian_Lee_1E

Hello, I was wondering how many bonds can T.M. complexes make in a solution.

Some say they are tetrahedral and some say they are octahedral. How do we know how many bonds can each transition metals make?

Ian_Lee_1E

Hello,

I was confused to whether there are two pi bonds forming in a single pi bond because there are areas of bonding up and down if you look at the 3D graph.

Ian_Lee_1E

Hello As professor was giving an example using Ethene, professor said "energy gap is small between hybridized and normal p orbital, so you do not spin pair.: What does that mean? I understand that since sp2 is a mix between s and p orbitals, it is going to be closer to the p orbital. But I do n...

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