CHEMISTRY COMMUNITY

Chinyere Okeke 2J

You want energy units over moles so that would be either J/mol or kJ/mol. And the units of activation energy has J in it our R constant would have to be 8.314 J/mol*K to make sure everything cancels out correctly.

Chinyere Okeke 2J

I don't think we'll see termolecular in this class since it's not very common but it is when the slow step (rate deciding step) of a reaction involves 3 molecules simultaneously interacting. You can see why this doesn't really occur often as the chances of three molecules colliding is very low so th...

Chinyere Okeke 2J

I think one of the previous comments explained this problem really well but to reiterate the higher the activation energy, the more temperature-dependent the rate constant is. So, since the forward reaction has higher activation energy an increase in temperature would lead to a larger increase in th...

Chinyere Okeke 2J

Since there can be heterogeneous and homogeneous catalysts, I assume there can be heterogeneous and homogeneous enzymes too since they act as catalysts. I know it is easier to assume that an enzyme is homogenous to a reactant because of the biological model that we've seen in biology classes but som...

Chinyere Okeke 2J

The graph of a zero-order reaction would be linear with a negative slope, this is because the rate law is: rate = -k[A] The graph of a first-order reaction would be linear with a negative slope, this is because the rate law is: rate = -kln[A] - for a natural log, the smaller the concentration the fa...

Chinyere Okeke 2J

To calculate the half-life of a zero-order reaction we use the equation: t1/2=[A]0/2k. So in order to calculate it, we need to know the initial concentration of the reactant and the rate constant of the reaction. And I assume we might not know if a reaction is zero-order so we would need to be able ...

Chinyere Okeke 2J

This is a good question. From what I know about reactions a change in temperature can change the rate constant which makes sense because when there is a rise in temperature, thermal energy is being added into the reaction. And I think a change in activation energy can also change the rate constant. ...

Chinyere Okeke 2J

The slowest step is important since it determines the rate of the overall reaction because the rxn can't be completed until the slowest step is finished. You can think of like when you are doing laundry, you can't use the dryer (30 min to dry) until the washer(50 min wash) is done.

Chinyere Okeke 2J

Since we are given the time the reaction took and the concentration for two different experiments, we can see that the reaction took longer when the concentration decreased. So you can use the half-life equations to find the order of the reaction that matches this.

Chinyere Okeke 2J

That's correct standard are at STP which means room temperature and 1 atm pressure! And when we try to solve for E and G we can use the standard E and G as reference points in our calculations.

Chinyere Okeke 2J

The different forms of the nernst equation are used interchangeably but it is obvious when one form is preferable over the other. Like when the temperature is standard 25-degrees or if the temperature is different. When we are using the nerst equation with either log or ln function then I don't thin...

Chinyere Okeke 2J

I think an ion-selective electrode allows us to monitor concentrations of ions in solutions, like seeing the Cl- concentration in cells. One type of ion-selective electrode is a glass electrode.

Chinyere Okeke 2J

Rechargeable batteries use reversible chemical reactions. This makes sense because the current of energy can be reversed if the battery is able to be charged (energy put into it). Chemical reactions used in non-rechargeable batteries are irreversible when a current is passed in the reverse direction...

Chinyere Okeke 2J

A pH meter is an instrument used to measure hydrogen ion activity in solutions and this means that it measures acidity/alkalinity of a solution. This tells us that the degree of hydrogen ion activity can be expressed as pH level.

Chinyere Okeke 2J

I have a hard time with this too and have an explanation for max potential in my mind that may help (it's not super scientific so hopefully this isn't wrong). When the switch is open (and current is 0) there aren't any actual electrons flowing but they are built up and have the potential to flow if...

Chinyere Okeke 2J

Can someone explain why the maximum potential occurs when the current is or close to zero? I watched the lecture but I'm having a hard time understanding this concept.

Chinyere Okeke 2J

We don't use the coefficient of a molecule to calculate the oxidation number because the oxidation number of an element is independent of the stoichiometric coefficient of the reaction. However, the coefficient IS used to balance the overall charge of the reaction so that both sides have the same ch...

Chinyere Okeke 2J

This was a good conceptual question! The importance of the salt bridge is to ensure that the solutions stay neutral. Since there is an e- flow occurring, from the oxidation beaker to the reduction beaker, without the salt bridge the oxidation side would eventually become negative and the flow of e- ...

Chinyere Okeke 2J

I was wondering if E (cell potential) was a state function or a path function?

Chinyere Okeke 2J

When it comes to balancing redox equations I typically separate the half reactions of what is being reduced and what is being oxidized. Next I balance all the elements besides O and H. Then I add H_{2}O molecules to balance the oxygens in each equation. Then I add H^{+} to balance the hydrogen. Und...

Chinyere Okeke 2J

For reversible expansions, there is more work done as these are more efficient systems since the expansion takes longer as it occurs over small steps. This is because as the system does work, heat comes into the system to replace that work. On the other hand, for irreversible expansions, there is le...

Chinyere Okeke 2J

I think this is because when a system is at equilibrium the delta-G is 0. And when a system does work it releases energy therefore the delta-G is negative. So at equilibrium, no work is being done (by the system or the surroundings) since delta-G is not positive or negative.

Chinyere Okeke 2J

If a reaction is at equilibrium then the delta-G would be 0, therefore the reaction would not be spontaneous in either direction. We need a negative delta-G to determine a reaction to being spontaneous.

Chinyere Okeke 2J

Basically, standard means that the system (reaction) is a standard condition so temp is 298 k (room temp 25 C) and pressure is at 1 atm (~1 bar). When it is not standard the system can be at any temperature and pressure.

Chinyere Okeke 2J

It's not common because systems aren't usually at equilibrium, i.e. concentration of products and reactants are equal. So since the change in free energy is dependent on the equilibrium constant it can be said that if K=1 (eq) is uncommon so is delta-G=0 since it has the variable ln(K) in the equati...

Chinyere Okeke 2J

Entropy is the measure of disorder in a system or how Lavelle explains it in class the number of possible states a gas can be in. On the other hand, enthalpy is the measure of heat in a system. These are different concepts but they can be related to each other by the free energy.

Chinyere Okeke 2J

These are constants that we can use when looking at ideal gases. I think we can use them when we don't know the Cv and Cp but we do know that the gas is ideal (monoatomic/diatomic).

Chinyere Okeke 2J

Like the person above said you are doing the correct process by finding the moles using the ideal gas law and then using that to find the work done by using the isothermal expansion equation. However, you aren't supposed to use delta V when solving for the moles. When you do that you aren't calculat...

Chinyere Okeke 2J

An isothermal reaction is a reaction where the temperature of the system remains constant. This can be possible because despite work being done and energy being transferred these happen at the same rate so there is no change in internal energy.

Chinyere Okeke 2J

I think that if there is no change in temperature then that equation will give you a q of 0 since that equation tells us the heat due to temperature. If you want to get a q value not equal to 0 when there is no temp change you might need to use a different equation.

Chinyere Okeke 2J

Isolated doesn't allow the transfer of energy or matter so I don't think we will be doing problems with them. We could have questions where we need to identify which type of system it is: open, closed, or isolated; so know the basic characteristics of each but other than that probably not many quest...

Chinyere Okeke 2J

In this class and most Thermodynamic classes in the US, we are doing our calculations in Kelvin, this is because a lot of values/constants have Kelvin in their units not Celsius. The only time you wouldn't need to change the degrees is when you are doing change in temperature since change in 1 K = c...

Chinyere Okeke 2J

We have Cv and Cp because having gases under constant pressure is different from having gases under a constant volume. When gases are heated under a constant volume there is no way for them to expand so the pressure inside the vessel increases. When they are heated under constant pressure the gases ...

Chinyere Okeke 2J

I think what happens conceptually is that since the volume is expanding then the system is doing work on the piston. And the internal energy of the system would decrease since the system is doing work, therefore the work would be negative as energy is lost. SO the problem gave you the rate the volum...

Chinyere Okeke 2J

Basically since you're subtracting one temperature from another it doesn't matter which units you have for change in temp since the 273 is going to cancel out either way. Delta T = (x1 + 273) - (x2 + 273), which is equivalent to (x2 - x1) + (273-273) = x2 - x1.

Chinyere Okeke 2J

I think we will know when to use a certain R value by looking at the units we are given or expected to give in our answer. Like our answer should have pressure in atm then we would probably use 0.082 atm•L/mol•K since it's easier!

Chinyere Okeke 2J

Steam burns are more severe than water burns because steam releases more energy when it makes contact with the skin. The steam changes from vapor into a liquid when it touches the skin, which releases a lot of heat (energy), and then it releases more energy in order to become the same temperature as...

Chinyere Okeke 2J

Hey! They did this because there are 4 C-H bonds in the CH4 molecule that needs to be broken and 4 C-Cl bonds in CCl4 that need to form. I think drawing out the reaction would be useful to better conceptualize this!

Chinyere Okeke 2J

Shreya Patel- 2D wrote:If we're learning it this week, do you know if it will be on the midterm?
Thanks!

I think Lavelle said in his latest email that the midterm only covers material we learned up to week 3, so last Friday! So I don't think we will be tested on that this midterm!

Chinyere Okeke 2J

A reaction with only one arrow indicate the direction in which the reaction favors. So if the arrow is pointing to the right that means that the production of the products is favored. I think that we use a single arrow for reactions that we know aren't in equilibrium (so when we are finding the Q).

Chinyere Okeke 2J

The way I look at the 5% guideline is if the percent ionization is larger than 5% then we can't approximate the X value as it is not a small enough change. This means we would have to use the quadratic formula to calculate X. The guideline has less to do with different percent ionization of differen...

Chinyere Okeke 2J

That is correct! We also use the ICE table when calculating partial pressures in order to find the equilibrium pressures. It is essentially the same process as using concentrations just make sure you remember your units!

Chinyere Okeke 2J

I agree with the replies above; one thing to note is that Kw is only 10^-14 when the temperature is at 25 degrees Celsius. Is there a constant value for Kw at other temperatures that we will be using in chem? Or is Kw at 25 degrees Celsius the only constant we know so if the example given isn't at ...

Chinyere Okeke 2J

I think for part f you have to look at the context of the problem. The reaction takes place in a plant where water is abundant so adding more water doesn't affect the eq constant as it is already in excess.

Chinyere Okeke 2J

I was also having trouble with this problem so I looked online and the equation in the (Sapling) book is incorrect. It should be:
H₂ + D₂ --> 2HD

And when you use this equation, you should get the right answer!

Chinyere Okeke 2J

I believe that as far as this class is concerned K can only be affected when the temperature of the reaction is changed. Other factors do not affect since the K constant will still be the same due to the ratio. I think changing other factors (ie. pressure and concentration) would only change when eq...

Chinyere Okeke 2J

I think you are correct as when a reaction is at equilibrium there won't be a net increase in product concentration so a Q value greater than the equilibrium constant, K, therefore there would have to be external factors or the reaction has not yet reached equilibrium.

Chinyere Okeke 2J

Just to clarify, if you remove enough reactants the system would likely be Q>K (where [products]>[reactants]) and if you add enough reactants the system could tip to be Q<K ([products]<[reactants])? Hi! I believe that you are correct as when you remove reactant, the reaction will shift so that it c...

Chinyere Okeke 2J

Since in that case, we wouldn't know for sure whether the products or reactants are favored in the reactions, the most likely best bet would be to say: If values of K are in between 10^-3 and 10^3, neither direction of the reaction is strongly favored. Emphasis on the strongly because I believe if k...

Chinyere Okeke 2J

I wouldn't assume that P/R is proportional to K since the ideal gas law is just used to convert from concentrations to pressures when trying to find the K value for pressure and to find concentrations from pressures.

Chinyere Okeke 2J

The difference is that the heme complex is a part of a myoglobin --> heme complex + protein = myoglobin.

Chinyere Okeke 2J

You look at the number of bonds to determine the shape of the complex!

Chinyere Okeke 2J

This was a nice review for me of molecular shapes! Tetrahedral is when there are 4 bonding e- pairs bonded to the central atom, while square planar is when there are 6 regions of e- density around the central atom but 4 are bonding e- pairs and 2 are lone e- pairs!

Chinyere Okeke 2J

I'm a little late but this question was a good review for me moving on to other chemistry classes so thanks! Lavelle said in a lecture that when the coordination sphere is overall negative you add the -ate to the end of the TM name!

Chinyere Okeke 2J

I think the conjugate acid would be water because the (OH) group of the Ba(OH)2 will be attracted to the H and form water!

Chinyere Okeke 2J

I think you also have to include sodium at the beginning. It would be sodium triamminetrichlorocobaltate (II) (Co has a +2 charge because you get a -1 overall charge outside the brackets due to only a +1 charge from Na to balance it out, and the 3 Cl- atoms give a -3 charge, meaning that to get an ...

Chinyere Okeke 2J

Yes, terminal atoms can be hybridized, but typically, at least in Lavelle's examples, we are only looking at the hybridization of central atoms. However, I do think so problems from the textbook outline want you to give the hybridization of all the atoms in a compound when you are describing the sig...

Chinyere Okeke 2J

Anything that isn't covered/mentioned in the lecture will not be on the final. So the topics on chem community with an asterisk '*' will most likely not be tested on. They are just additional resources and/or for people who are interested in learning more about a certain topic, like black bodies.

Chinyere Okeke 2J

Great question, as this has important biological consequences! Water is amphoteric, meaning it can act as either an acid or a base. Whether it is an acid or a base depends on its surrounding solution so in a solution with basic compounds it will act as an acid, and vice versa.

Chinyere Okeke 2J

Strong acids are classified by the degree of dissociation in solution. HF not only has a short bond (which makes it harder to dissociate) it also has an F- (which has high electronegativity). Therefore, HF is unable to completely dissociate into H+ and F- making it a weak acid.

Chinyere Okeke 2J

You get delocalized pi bonds when there is resonance in a structure. This means that the double or triple bonds can be in different positions so the electrons (pi bond) are delocalized around the molecule.

Chinyere Okeke 2J

Yes, an h-bond is a type of dipole-dipole interaction as both molecules must have a dipole moment to form an h-bond. But I don't know if when asked a question about the bond types if we should say it has both h-bonding AND dipole-dipole OR if we should just say it has h-bonding?

Chinyere Okeke 2J

I think dipole-dipole interactions it occurs between two molecules that have dipole moments (polar molecules) And dipole-induced dipole is when a molecule with a dipole moment interacts with a molecule with no dipole moment. And the dipole will induce a temporary dipole moment on the nonpolar molecu...

Chinyere Okeke 2J

lwong Dis1L wrote:What are delocalized pi bonds?

I think a delocalized pi bond is when a structure has resonance, therefore a double bond (which includes a pi bond) can be in multiple positions so its electrons are delocalized.

Chinyere Okeke 2J

You know every type of bond can only have one sigma bond, and the rest are pi bonds. So a single bond has one sigma, double bond has one sigma and one pi, and a triple bond has one sigma two pi.

Chinyere Okeke 2J

Wow, this is awesome. Y'all are so resourceful. To add to this my personal trick/(the way I do it) is to use the atomic radius trend as a basis for the other trends (electroneg., electron affinity, ionization) in the periodic table. It helps me know other trends and also understand why the trends ar...

Chinyere Okeke 2J

Electronegativity is the ability of an atom to pull electrons towards it. So an atom that smaller (small atomic radius) with a higher charge will have higher electronegativity as it holds onto its electrons tighter and has a stronger effective nuclear charge since there are fewer electrons shielding...

Chinyere Okeke 2J

A triple bond has 1 sigma bond and two pi bonds. This is because the sigma bond occupies the internuclear axis so there can't be more than one sigma bond. Therefore in a triple bond, there are 2 more pi bonds above and below the internuclear axis.

Chinyere Okeke 2J

I believe that in the case of cis-dichloroethene and trans-dichloroethene what makes them non-polar and polar is the dipole moments and how they either cancel out or don't cancel out. They can't rotate because of the double bond between the two C's which fixes them in place, but I would look at the ...

Chinyere Okeke 2J

I believe delocalized pi bonds are in molecules that have resonance since e- in resonance structures are delocalized (the double/triple bond is equally distributed in the molecule). So I think a delocalized pi bond would occur when the e- involved are able to move around the nuclei of the atom.

Chinyere Okeke 2J

To draw the Lewis Structure with the lowest charge on each atom (so basically the most stable structure) an LA mentioned a tip: when the molecule has a net charge that is negative, you give the most electronegative element the charge. So for NCO- you would assign the O the -1 formal charge. And that...

Chinyere Okeke 2J

We are not expected to know how to get chemical formulas from compound names YET. Lavelle mentioned that he will give us the formula but I'm assuming later we will be expected to figure it out on our own, once he's taught us how to do so.

Chinyere Okeke 2J

We aren't expected to calculate the exact electronegativity for atoms nor are we expected to know all the electronegativities of elements. HOWEVER, we should know the general trend of electronegativity on the periodic table. The trend is as we go across a period the electronegativity of elements inc...

Chinyere Okeke 2J

London Dispersion Forces (LDF) are intermolecular forces that occur between ALL molecules when they interact. Be it ions with ions, dipole-dipole, nonpolar-nonpolar, polar-polar interactions. This is because LDF occurs when electron clouds temporarily fluctuate as they get close and gain temporary c...

Chinyere Okeke 2J

The conversion for picometer (pm) is

1 picometer = 1.0 x 10^-12 meter

So for example to convert 145 pm would be 1.45 x 10^-10 m

Chinyere Okeke 2J

Thank you for confirming my theory, I did it and it worked! I'm still a little confused about how to determine oxidation states of compounds where it isn't just one element and a bunch of O's, but I get the concept. Did he go over this in the lecture or are we expected to read the book to understand...

Chinyere Okeke 2J

I also started on the homework and was confused about oxidation states because he didn't really go into depth about them. I think they are the charges of the atoms (how many electrons they lose or gain) but different from formal charge. I know that O usually gains 2 e- so it usually has an oxidation...

Chinyere Okeke 2J

To add on to this question, what are some more theoretical approaches to determining whether a bond is ionic or covalent? Audra gave the example of observing its solubility in water, but if we were in a situation where we are unable to do that how else could we determine the type of bond? I know we...

Chinyere Okeke 2J

All salts are ionic bonds, but not all ionic bonds are salts I believe. We can tell that a bond is ionic because the atoms involved usually have a charge as one atom loses electrons and another atom gains electrons.

Chinyere Okeke 2J

#12. FInd the oxidation number of Cl in ClO₄^-

I tried -1 but it says it is incorrect. Do we need to add the oxidation numbers of all O's so (4 x -2) -8 and then say that since the net formal charge of the molecule is -1, the oxidation number of Cl has to be 7?

Chinyere Okeke 2J

I think it depends on the surroundings of the molecules. I know in aqueous solutions covalent bonds are usually stronger than ionic and in biology most of the molecules looked at are in aqueous solution. However, in chemistry I was also taught ionic bonds are generally stronger because the coulombic...

Chinyere Okeke 2J

I think to tell the difference between nonpolar and polar (there is a charge) you look at the relative electronegativity between the atoms, for example since C and H are close together on the periodic table they have similar electronegativities so a CH4 molecule would be nonpolar because there is no...

Chinyere Okeke 2J

I was confused about this too haha. The ground state is usually when the electrons fill all the shells from lowest energy to highest energy so as you can see in this example of the excited state an electron is in the 3s orbital when the 2p orbital isn't filled.

Hope that helped!

Chinyere Okeke 2J

It goes that way because usually 3d orbital is lower energy than 4s orbital with the exception of certain elements like Ca and K. This is because an e- in the 3d orbital will have lower energy than 4s, only after the 4s orbital is occupied.

Chinyere Okeke 2J

If you are given the quantum numbers for n, l, and m sub l, how would you go about finding the number of electrons in the atom? For example: n=7, l=2, m sub l=-1 For quantum numbers, I think you can use them to find the maximum number of electrons. Lets use the numbers 2, 1, 0, +1/2 First you would...

Chinyere Okeke 2J

What is the uncertainty when it is in percentage. Like for example, if a problem says an e- has a velocity of 4.5 m/s with an uncertainty of 1% is the delta-v 1 or is it 4.5(0.01)?

Chinyere Okeke 2J

The way you rounded it is correct. I also get nervous when rounding numbers with sigfigs like that. But you should remember to round only at the end when you have your answer because rounding in the intermediate steps could cause you to get the wrongs answer.

Chinyere Okeke 2J

I heard from a UA that there will be some easy calculation questions worth few points to serve as grade buffers and there will be some long, multi-step questions that will be worth more.

Hope this info helps with your studying!

Chinyere Okeke 2J

The EΨ means the energy of the wave function. This value also corresponds to the orbitals (since the second derivative of the wave function = the probability of finding an electron). In other words, the Hamiltonian (the double or second derivative) of the wave function = the energy of the wave func...

Chinyere Okeke 2J

For this problem for the second part you want to find the lowest and highest wavelength so you are looking for the lowest and highest energy changes. This means that you need to calculate the energy change from n=5 to n=4 and the energy change from n=5 to n=1. From there you can solve for the wavele...

Chinyere Okeke 2J

I think that the diameter 10^-15 problem he went over in the lecture has more to do with the size of an atom and if an electron could be located inside the nucleus of an atom (which he proved is impossible as the uncertainty in velocity was greater than the speed of light) while not being able to me...

Chinyere Okeke 2J

I believe that we answer in meters (m) because that is the SI units for distance, however, if the question asks for the answer in different units (ie. nanometers (nm) or picometers (pm)) then you will have to convert it. But in general, it's best to leave the units as meters. I do think there are so...

Chinyere Okeke 2J

So is delta v equal to the change in velocity, essentially the uncertainty they give us in the problem? Also, I was under the impression that when we were using uncertainties like ± 5.0 m/s we had to double it for it to be equal to delta v. I thought this because in this case the answer could be an...

Chinyere Okeke 2J

Electron transition basically when an electron goes from one energy level to another depending on if it is emitting energy or absorbing energy. If the electron has absorbed energy then it will go to a higher energy level, and vice versa if an electron emits energy as a photon then it will go to a lo...

Chinyere Okeke 2J

Basically, in the photoelectric effect for an electron to be emitted the energy of the light needs to be high enough that it meets the threshold energy of the electron. The scientist who found the photoelectric effect discovered that even when they increased the intensity of the light the electrons ...

Chinyere Okeke 2J

The velocity of an electron that is emitted from a metallic surface by a photon is 3.6×10^3 km.s^-1 (a) What is the wavelength of the ejected electron? I did this problem using DeBroglie's equation and got the correct answer, but I was wondering if you could use the equations we learned in the photo...

Chinyere Okeke 2J

In most cases, it will be Kelvin that we use for SI units because those are the units typically used for calculation, because it's related to pressure, moles, and volume. I believe that we could also consider and use Celcius since that is the unit used to measure temperature. I don't think Fahrenhei...

Chinyere Okeke 2J

During Friday's lecture, Prof drew the diagram of the oscillating waves and said that they were electric fields oscillating, which I understood, but then he mentioned how there were magnetic fields 90-degrees to the electric fields and I'm confused as to how they come into play? The other diagram he...

Chinyere Okeke 2J

Basically, to put it in simple terms, a vacuum makes sure that there are no extraneous particles that could interfere with the experiment. The vacuum ensures that the e- are isolated and that there are no particles (i.e. water) that the e- could potentially interact in the air. If the e- were to int...

Chinyere Okeke 2J

5. The γ-ray photons emitted by the nuclear decay of a technetium-99 atom used in radiopharmaceuticals have an energy of 140.511 keV. Calculate the wavelength of these γ-rays. Since I feel like many people might be unfamiliar with the units: keV is the SI units for volt or the electric potential and...

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