CHEMISTRY COMMUNITY

Vincent Leong 2B

The enthalpy is given by several equations such as deltaG = DeltaH - TDeltaS or even just adding and subtracting enthalpy values.

Vincent Leong 2B

that corresponds to the amount of electrons involved in the overall redox rxn

Vincent Leong 2B

the maximum work is equal to the gibbs free energy. Gibbs basically indicates how much "free" energy there is to do work.

Vincent Leong 2B

most likely by the end of the week or the start of week 1 next quarter

Vincent Leong 2B

The pH meter tracks the concentration of H+ ions and I'm not too sure how it works specifically

Vincent Leong 2B

I think it means it can only attract certain charged ions (+/-)

Vincent Leong 2B

It should make sense when you write out the half reactions and get the overall chemical reaction for the cell. You'll see that the lower concentration ion comes from the oxidation of a certain metal (Product) and the ion with the higher concentration will result from the reduction of a certain metal...

Vincent Leong 2B

intensive = doesnt depend on amount of matter; extensive = depends on amount of matter

ex. intensive --> specific heat capacity; extensive --> heat capacity

Vincent Leong 2B

it doesnt matter which order you do, as long as you can isolate an exponent x or y to get the respective rxn order. The numbers of one of the reactants in both trials should cancel out.

Vincent Leong 2B

I think it's less of Le Chateliers and more of a molecular understanding of the rxn. If you have a high concentration, low volume of acid and base reacting, imagine the H+ and OH- reacting. If you have a higher concentration of either acid or base, you have more moles of H+ and OH- reacting. If you ...

Vincent Leong 2B

Yes. If you increase T = more collisions = increased rxn rate (knowing rate = k[A]) = increased k

Vincent Leong 2B

I think it should be the other way around: rate law defines molecularity.

Vincent Leong 2B

I havent looked at the textbook for this problem but its most likely related to how the coefficient for TNT in the equation is 4 so if you want the enthalpy in terms of per mol of TNT, you have to divide by 4. When they just give you deltaH or enthalpy for an entire rxn, its the enthalpy for one MOL...

Vincent Leong 2B

The catalyst does have an effect on the whole reaction (the forward and reverse rate equally). If you think of it in terms of the energy profile diagrams, the Ea is lowered for both the forward and reverse reactions (essentially if you lower the Ea for the forward rxn, you are also lowering the Ea f...

Vincent Leong 2B

you use nonelementary steps to determine whether there's a catalyst or intermediate present in the elementary step

Vincent Leong 2B

I'm guessing there will be no work to show, just multiple choice answers

Vincent Leong 2B

that involves more with biology but essentially they bind to the active site of reaction and does something at the molecular level to ease the interaction between reactants.

Vincent Leong 2B

find the orders with respect to the reactants and then add the exponents all up to get the overall order of the rxn. You can determine the respective rxn orders by using experimental data or the proposed coefficients of the rxn if it says the proposed rxn is a elementary rxn.

Vincent Leong 2B

they do have an effect. catalysts lowerr the Ea for both the forward and reverse rxns and thus the rxn would happen faster in both the forward and reverse rxns. Therefore the rate of the entire reaction would increase. Look at rxn profile to help you visualize what happens --> it would make a lot mo...

Vincent Leong 2B

The rate constant and elementary rxns can determine rate order.

Vincent Leong 2B

If you genuinely understand how to do all the homework problems along with the small detials he mentions in lecture, you should be fine for the final.

Vincent Leong 2B

I'm not too sure about A but I THINK the concept sort of goes in hand with Temperature. If you increase temp, you're increasing the movement of molecules and if you increase the movement of molecules, your reactants are colliding more or have a higher change of colliding WITH THE CORRECT POSITIONAL/...

Vincent Leong 2B

Ea = activation energy (how much energy is necessary for the R-->P ). R = constant (used to cancel out units) T= temperature (temp at which the reaction happens). A (a pre exponential factor that is used to calculate k, also corresponds to the angular dependence that reactant molecules need to have ...

Vincent Leong 2B

Its not so much as it's increasing or decreasing. It's relative to the other steps in the entire mechanism/reaction. Your Ea is larger for one reaction over the other because either the problem specifies or you discover that the step is slower compared to the other fast rxns. If the rxn is fast, the...

Vincent Leong 2B

yes, it's the same thing. I would use the method of adding the reduction potential of the cathode and flipping the other rxn and adding the flipped E naught value. It makes more sense logically and it is for sure right.

Vincent Leong 2B

It can be switched. it's just general format that the book uses to keep solids or more solid like molecules on the outside of the cell diagram.

Vincent Leong 2B

Their reduction potentials give it away. Larger E (naught) = higher reduction potential = increased likelihood of reducing ; therefore, higher E (naught) species gets reduced while the lower E naught gets flipped to get oxidized. If you think about it logically, you can't have 2 reducing species in ...

Vincent Leong 2B

Look at how gibbs free energy and Eo are related than look at the nernst equation. The nernst equation has the T variable in it so it should be self explanatory how the numbers fluctuate with changing temps. Knowing Delta Go = -nFEo and delta Go = -RTlnK ; you mash the 2 formulas together and you ge...

Vincent Leong 2B

If you assign oxidation states to all reactants and all products, you can see that Au+ changes to both a more positive and less positive oxidation state. Therefore, you know that Au+ is getting oxidized and reduced. With that info, you also know Au(s) will be involved as it will be the helping hand ...

Vincent Leong 2B

it all depends on the given redox rxns you use from appendix 2. The reason why H+ is there is bc its in one of the redox rxns (either reduction or oxidation) and the platinum electrode is there to act as an inert electrode or inert metal that will conduct a current through the wire.

Vincent Leong 2B

Pt is an inert metal, not an inert electron. you need it to conduct the current caused by the loss and gain of electrons in the respective anode or cathode compartment. But you can't use any metal when you have a galvanic cell. Pt is used because it's a common inert metal meaning it doesnt react wit...

Vincent Leong 2B

Corrosion is just like the breakdown of a metal's surface when in contact with, most of the time, water. It's relevant because the breakdown is mainly because of the electrochemical rxns that happen with the metal and another chemical or molecule that causes the metal to lose e-s and thus the surfac...

Vincent Leong 2B

write out each respective half rxns and work in the same logic that you would use to write the cell diagram. We know the left side of the diagram is oxidation and the right side is reduction. Knowing this helps us know where to put the electrons in each half rxn (either as a product or reactant).

Vincent Leong 2B

Know what values of Delta G define spotaneity and at which Temperatures and type of rxn (exothermic or endothermic) at which the rxn is spontaneous.

Vincent Leong 2B

the anode is the left side of the given cell diagram and it holds the oxidation rxn. Knowing this, we know there is a loss of rxns (so electrons in this half rxn are on the product side) and the half rxn involves O2 and H+. Look at appendix 2 in the back to help you with those half rxns.

Vincent Leong 2B

I think the key to this question is knowing that there has to be a metal electrode to conduct the electrons current through the metal wire of the cell. I think it's also a huge indicator that we're considering the Au(s) on both sides because the overall reaction is missing one reactant which is not ...

Vincent Leong 2B

The charge of the cell shouldnt be affected by the stoichiometric coefficients. They are only used to balance half rxns.

Vincent Leong 2B

From what lavelle said and what I understand, the maximum work is equivalent to the maximum cell potential.

Vincent Leong 2B

the electrons being lost by say a metal such as Fe and the aqueous forms of the metal (Fe2+) make pockets on the metal. That's how the electrons and the metal ions are given off in a reaction. The redox reaction of the half reaction, Fe --> Fe2+ + 2e- represents this rusting process.

Vincent Leong 2B

delta S of the universe is 0 for reversible rxns bc the reversilble rxns can go back to their initial states. If you think of delta, its a measure of change. If your final and initial state are identical, there is no measure of change bc technically, you ended where you started. Therefore there is n...

Vincent Leong 2B

entropy would be negative if youre decreasing entropy or youre forming bonds (typically) because the molecules are becoming more compact or have less room for residual/positional orientations.

Vincent Leong 2B

I think the cell diagram only gives the knowledge necessary to tell what species are being reduced and oxidized. From knowing this, we can deduce what species are the reactants and products. To get to the overall reaciton, I recommend writing out the full half reactions, balancing each half reaction...

Vincent Leong 2B

6L.9 (a) Write balanced half-reactions for the redox reaction of an acidified solution of potassium permanganate and iron(II) chloride. (b) Write the balanced equation for the cell reaction and devise a galvanic cell to study the reaction (write its cell diagram). How do we know what to use for the ...

Vincent Leong 2B

the conducting solid of the electrode allows for a current to flow. The Voltage numbers that are given by appendix 2 are achieved by the redox reactions that occur in the galvanic cell; They are recorded when electrons are moved from the solution, attach to the cathode or anode, and flow through the...

Vincent Leong 2B

I think the answer key refers to appendix 2 a lot so that's probably where they got that value.

Vincent Leong 2B

I dont think the type of salt in the salt bridge matters because they separate into its respective cations and anions to neutralize the ions in the cathode/anode containers. In terms of problems, i dont think they ever ask for the specific salt in the salt bridge.

Vincent Leong 2B

The salt bridge is to maintain charge balance because electrons move from 1 half cell to another.

Vincent Leong 2B

If there are no solids in one of the half reactions, an inert metal electrode is necessary in order to conduct electrons on that side of the reaction. Basically, if there is aq, liquid, or g in your half reactions, make sure to add Pt(s) (we choose Pt bc its a common metal electrode) to the cell dia...

Vincent Leong 2B

The key takeaway is that you dont change the E standard cell potential value whenever you balance equations. We're used to multiplying by a factor or dividing by a factor when balancing more than 1 equation (ex. enthalpy change) but for Ecell, leave the value as is and just add or subtract values.

Vincent Leong 2B

I think the key concept to understand is that theres delta S done by temp and volume change. So detaS(t) + deltaS(v) = DeltaS(sys). Delta S total, universe, and surroundings all depend on how you calculate deltaS(sys). Just make sure you use the 2 formulas we learned in class to calculate deltaS of ...

Vincent Leong 2B

I was thinking that as well. I think H2 is just a special case for diatomic gases and that not all diatomic gases in their stable form have a Delta G of zero because its rare for molecules to have no entropy or enthalpy change

Vincent Leong 2B

the delta U for an isolated system is 0

Vincent Leong 2B

Delta G always needs to be calculated. I can't think of the case where Delta G is 0 for a diatomic or any molecule because Delta G consists of enthalpy, temperature, and entropy. Essentially, if you have a delta G of 0 youre saying the molecule has no entropy change and no enthalpy change which is u...

Vincent Leong 2B

You cant find delta H because enthalpy is heat at constant pressure. To get Delta H you need a constant pressure or else you're solving for q or heat which would be mCdeltaT

Vincent Leong 2B

delta s total refers to the universe and the delta s system + surr are self explanatory (your system and whatever you define as your surroundings). An example is a cup and a bench. The cup is my system and its on my surroundings (the bench). The key concept is that for a reversible system delta S to...

Vincent Leong 2B

what is the simplest way to think about the first law of thermodynamics?

Vincent Leong 2B

What is the difference between the 2nd and 3rd law? What are their respective equations supposed to say about the system or the general word statement?

Vincent Leong 2B

i think the only key takeaway from an adiabatic system is that q = 0.

Vincent Leong 2B

I think of reversible as the example lavelle showed us in class. If you have a gas in a chamber and the gas pushes against a piston, it willpush it until a new volume is set so the piston stops moving and the gas stops pushing against that piston. In this process, the external pressure is constant a...

Vincent Leong 2B

I dont know of any biological examples that Lavelle specifically went over in class. I'm guessing you could apply delta H or delta S within a rxn in your body? I'm not sure but as far as I know, there hasn't been any mentioned biological examples.

Vincent Leong 2B

Only when we consider degeneracy and the question mentions something about possible energy states.

Vincent Leong 2B

Thats sort of hard to generalize because it ultimately comes down to the context of the question. I think understanding what is actually is happening and really paying attention to what's given should hint at which equation you should use. For example, whenever i see "a metal being placed in wa...

Vincent Leong 2B

The easiest way to conceptualize this is to look at the equation: delta S = q/T; if T is low, the fraction is overall larger assuming q stays the same. That would mean Delta S is larger at low T or more significant.

Vincent Leong 2B

the most common units that we'll always be working with is J, g, K, L, and atm. It's good practice to convert to these units because a lot of the given units of, for examples, constants will be given in J/mol or kJ/mol. the 8.314 constant is also in K or kelvin units so leaving things in celsius can...

Vincent Leong 2B

the negative sign doesnt play much of a role until you get the final Delta S value. If the final calculated delta S is larger than 0, you have a spontaneous rxn while a delta S thats less than 0 is a non spontaneouos process.

Vincent Leong 2B

Reversible and irreversible mainly build on the concept of whether the external pressure is constant or not. In a reversible process, the work done by volume expansion goes against a external pressure that fluctuates or changes in very small increments. In a irreversible process, the work done by vo...

Vincent Leong 2B

if work is negative, the change in volume is positive and thus the system is doing work on the surroundings. if work is positive, the change in volume is negative and thus the surroundings is doing work on the system. Think of it in terms of the piston example: if the volume is increasing, the gas i...

Vincent Leong 2B

Yea. I personally find diagrams or the lewis structures to be really useful in determining which bonds are formed and which are broken which will help you find delta h using bond enthalpies.

Vincent Leong 2B

Yea. I personally find diagrams or the lewis structures to be really useful in determining which bonds are formed and which are broken which will help you find delta h using bond enthalpies.

Vincent Leong 2B

4D/8
Ethanol is a renewable and clean-burning component of gasoline:
2 C2H5OH(l) + 6 O2(g) --> 4 CO2(g) + 6 H2O(l) delta H = -1368 kJ
What is the change in internal energy for the reaction of 1.00 mol C2H5OH(l)?

how do you solve this? What was the final answer?

Vincent Leong 2B

There on his main website and they should be math assistance. The key concept is integration and derivatives.

Vincent Leong 2B

In an closed system, it is more obvious if there's a change in volume or not as there will ussually be a lid that is pushed on by a piston. If in a closed system, they dont mention a initial volume or a volume change, then we can definitively say that work is 0. In an open system, it's sort of simil...

Vincent Leong 2B

use PV = nRT to solve for n which are moles of the gas. Then use the molar mass and the moles you solved for to solve for grams of gas.

Vincent Leong 2B

I'm unsure what you specifically want to know but just know as entropy increases, it tells us that theres an increasing amount of disorder as we break and form bonds. This just implies that every process that occurs shifts the order of the universe and thus causes chaos, as microscopic as they may be.

Vincent Leong 2B

Degeneracy is directly related to entropy and helps find positional entropy. With entropy solved, we can use delta H and entropy in an equation we'll learn later on to determine whether an equation is spontaneous or not.

Vincent Leong 2B

I believe there's no extra credit. There wasnt any extra credit in his 14A class so i wouldnt be surprised if there was none this quarter as well.

Vincent Leong 2B

Theres a section in 4C reading that gives a nice table for atoms, linear molecules, and nonlinear molecules. The C constants vary depending on their molecular structures and whether Volume or Pressure is constant. It should be figure 4C.3

Vincent Leong 2B

4C/3 : Calculate the final temperature and the change in enthalpy when 765 J of energy is transferred as heat to 0.820 mol Kr(g) at 298 K and 1.00 atm (a) at constant pressure; (b) at constant volume. Treat the gas as ideal. I don't understand this question at all. If anyone could it explain it thro...

Vincent Leong 2B

you can refer to hess's law to explain why delta H is positive or negative. The formula involving the summation of products minus the summation of reactants should indicate where heat lies in the reaction and whether bonds are being broken or formed.

Vincent Leong 2B

Elements in their standard state are not formed, they just are. That's the why technically don't have a delta H of formation value. They just naturally exist in that form and so they don't require energy to form.

Vincent Leong 2B

Is the concept of intensive and extensive properties similar to the concept of state function? If not, what's the difference between the two terms?

Vincent Leong 2B

It's helpful to think of decreasing/increasing V or increasing/decreasing P in terms of Q and K. When you change the volume (basically changing the pressure), you're changing the Q value and whether the new Q is larger or lesser than K, you know which way the reaction will shift.

Vincent Leong 2B

There will be a given table of bond enthalpy values that you can use to ultimately calculate delta H.

Vincent Leong 2B

Bond enthalpies are calculated averages with the exception of diatomic molecules (ex. H2, O2, N2) which are exact measurements.

Vincent Leong 2B

The approximation is only used when K is small enough and you know this approximation works if the x you solve for is < 5% of the initial concentration.

Vincent Leong 2B

There are several types of solutions that will produce buffers but the only one we've covered is a salt with a weak conjugate acid/base and a weak acid/base.

Vincent Leong 2B

No the test only includes chemical and acid/base equilibrium

Vincent Leong 2B

Their concentrations just don't differ when transitioning from the initial to equilibrium. In essence, if you were to include them into the K constant, it would essentially be 1 or negligible.

Vincent Leong 2B

If you think about it logically, it requires energy to break chemical bonds. If it takes energy to break bonds, energy must be absorbed by the reactants to break those bonds and so that energy comes in the form of heat. If the enthalpy of the reaction or delta H is positive, it means more energy is ...

Vincent Leong 2B

This question is testing the concept that Kreverse = 1/Kforward. Because the Chinese researcher flipped the initial reaction, the K constant of the chinese researchers reaction will be the inverse of the K constant calculated in the first part of the question., which should be answer D

Vincent Leong 2B

bond forming = exothermic = n2 + h2 --> nh3

bond breaking = endothermic = PCl5 --> PCl3 + Cl2

Vincent Leong 2B

high pka = low ka = weak acid
low pka = high ka = strong acid

Vincent Leong 2B

Weak acids are the only types of acids that we'll consider Ka values. In strong acids, there will be no Ka value because it will always be undefined as the acid will dissociate completely, leaving no concentration of reactant left.

Vincent Leong 2B

I believe its by the end of this week in some room in young hall. make sure to ask Dr. Lavelle about it on wednesday or check for his old email about it

Vincent Leong 2B

Why do we use the coefficients in a reaction as exponents in the K constant? What do they imply?

Vincent Leong 2B

Do solids and liquids affect equilibrium and but not the equilibrium constant or does it have no impact on both? Why?

Vincent Leong 2B

Just focus on Kb and the logic of how pkB is related to pOH or pH should follow. Kb is a measure of how strong a base truly is. The larger the Kb is, the more basic a molecule is meaning that the larger the Kb, the more products (OH- and ion) that forms or the more readily it dissociates. If you hav...

Vincent Leong 2B

Yes, a pH can be below 0. This just tells us that the molarity of hydrogen ions is greater than 1. Theoretically it is possible but in a lab, that can rarely be observed.

Vincent Leong 2B

They ionize completely because the H+ ion attached to them fully dissociates into 2 separate molecules. When looking at their ice table, their x variable is equal to the initial concentration of a reactant. This is what defines a strong acid or base: their ability to fully dissociate into its ionic ...

Vincent Leong 2B

inert gases don't react with the reactants so they ultimately don't consume the partial pressure or the concentration of the reactants. The fact that partial pressure of the reactants (which directly impacts the K constant) is not effected, the K constant is therefore not effected.

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