CHEMISTRY COMMUNITY

Deanna Necula 3B

Thank you!

What if you solve for the overall rate law instead of a rate of formation? Would the coefficients still appear then?

Deanna Necula 3B

How do the coefficients of the overall reaction come into play when deriving the rate law using the pre-equilibrium approach? For example, I don't understand where the 2 comes from in the pre-equilibrium example in the course reader.

Thanks!

Deanna Necula 3B

No. You wanted the smallest number at the first point of difference.

Deanna Necula 3B

You have to check in the redox table which has a higher reduction potential. The one with the higher reduction potential will be reduced.

Deanna Necula 3B

To find the rate constant for a first-order pseudo reaction, how do we find k'? It says in the course reader "slope" but I'm not sure which slope it's referring to.

Deanna Necula 3B

So then how do we determine cis/trans? What if one pair is E and the other is Z?

Deanna Necula 3B

So k' is -k?

Deanna Necula 3B

The ethyl will come first, but you'll still give numbering priority to the functional group.

Deanna Necula 3B

To find the rate constant for a first-order pseudo reaction, how do we find k'? It says in the course reader "slope" but I'm not sure which slope it's referring to.

Deanna Necula 3B

Well the pH tells you what your concentration of H+ is (10^-pH=[H+]). I haven't looked at the exact problem you're referring to, but if you say you're given the other concentrations, set up an equation for Q (the reaction quotient in the Nernst equation) and plug in the relevant concentrations, keep...

Deanna Necula 3B

Hi,

Which trumps which when estimating whether or not a reaction will experience an increase or decrease in entropy? For example, if a reaction produces fewer moles of gas than it uses up, but it will produce more moles overall, can we say the total entropy decreased or increased?

Thanks!

Deanna Necula 3B

Yeah, technically NO can also be ON. BF3 has all identical "peripheral atoms" (F), so it can only adopt one orientation.

Deanna Necula 3B

In the solutions manual answer for the homework question, they use R instead of C when calculating the change in entropy with respect to temperature. In other words, they replace C with R in the equation for change in entropy. Why is that?

Deanna Necula 3B

No, you only have to know the information in the sections in the "lecture outline" on the Chem 14B website.

Deanna Necula 3B

Hi, I'm a little confused about how to use allyl and vinyl. Both have double bonds in the structure, so my gut instinct when seeing a compound containing either would be to include the double bond in the parent chain rather than making it a substituent. When should I know not to do that? Or is it up...

Deanna Necula 3B

Hi,

Why is the compound 1,6-dimethylcyclohexa-1,4-diene not 2,3 dimethyl etc.?

Deanna Necula 3B

I think you can determine it from the line structure. It just depends on the location of the highest-priority substituents attached to the sp2 carbon. If they're opposite each other, then the molecule is trans. If they're on the same side, the molecule is cis.

Deanna Necula 3B

I think sometimes you can tell if there will be multiple steps, depending on how stable the product is. If, for example, substitution occurs and you end up with a positively-charged carbon, it's likely there will be another step, because the carbon does not want to be positively charged.

Deanna Necula 3B

In Self Test 4.4A (c) in Introduction to Organic Chemistry, they asked which step would have a greater activation energy if the y-axis were written in terms of potential energy instead of Gibbs Free energy. I don't quite understand why the first step, with the smaller hump, would have greater activa...

Deanna Necula 3B

No, you just describe the concentration of the intermediate in terms of the equilibrium constant K for that reaction, then you plug it into the slow step rate law. Essentially, you just want to rewrite the intermediate concentration in different terms, because it won't be present in the overall rate...

Deanna Necula 3B

In the prep quiz questions, some numbers are written in fraction format (i.e. 1/16). How do I deal with sig figs with fractions? Should I convert it to a decimal and assume it has the same number of sig figs as another value in the problem?

Thank you!

Deanna Necula 3B

Should we be concerned about the activation energy problems assigned from the textbook, since they're past the part of the textbook we're supposed to have read?

Thanks!

Deanna Necula 3B

Hi, My name is Deanna Necula and I'm a Academic News reporter for the Daily Bruin. We're looking for information about events/research occurring in the Chemistry (or related) department that would be of interest to a larger audience. If there is any research or events occurring in an academic depart...

Deanna Necula 3B

Mass. Greater mass means more fundamental units (and therefore more ways for the units to be scrambled about and generate disorder/greater entropy).

Deanna Necula 3B

It's an aqueous species, so it means it's dissolved in water. The water is the anode because it's lower down in the electrochemical table, so it's more likely to be oxidized than the sulfate.

Deanna Necula 3B

I haven't looked at the exact problem you're referring to, but generally the farther apart on the electrochemistry table they are, the larger the difference between their standard reduction potentials (therefore the larger their Ecell)

Deanna Necula 3B

Equilibrium (no likelihood of proceeding either direction)

Deanna Necula 3B

The reaction with the more positive (less negative) reduction potential will be the one undergoing reduction (it has a higher potential for reduction). The other one therefore must be the anode, and must be reversed.

Deanna Necula 3B

Hi, Does it matter in which order I put same-phase species in my cell diagram, if they are separated by commas? Also, how do I know where to put "miscellaneous" ions such as "H+" in my cell diagram? Also, in hw 14.15 c), they separated two species at the cathode that were of the ...

Deanna Necula 3B

Will we be told if a reaction is basic or acidic?

Deanna Necula 3B

I think that when a reaction is isothermal, that just means that the internal energy change is 0. The temperature as a whole does not change from the start to the end of the reaction, but it doesn't mean that no work or no heat transfer has occurred; it just means that any factors that can affect th...

Deanna Necula 3B

You're solving for ∆G=0 because at that point the reaction shifts from spontaneous to non-spontaneous and vice versa. Therefore, if you calculate the temperature of reaction that would allow you to have ∆G=0, below (or above, depending on the problem) that temperature you will have negative ∆G, mean...

Deanna Necula 3B

The external pressure of 3 atm. The "P" in that equation always represents the external pressure.

Deanna Necula 3B

I think for this instance, because the volume is changing, you can assume it already takes into account the change in pressure (because by changing volume you change pressure). So, you just have to use the volume change for your work equation.

Deanna Necula 3B

Hi,

For Winter 2014 workbook problem #11, in Step 2 ("getting back to the original internal energy"), ∆U suddenly becomes positive 158 J when just previously we had calculated -158 J for the change in internal energy. Why does it become a positive value?

Thank you!

Deanna Necula 3B

Because it's an isothermal reaction, you know that ∆U=0. Therefore, q must be -w, which you've already calculated. And I think it asked for change in internal energy, not enthalpy.

Deanna Necula 3B

The textbook example 9.5 states: "In one experiment, 1.00 mol O2(g) was compressed suddenly (and irreversibly) from 5.00 L to 1.00 L by driving in a piston, and in the process its temperature was increased from 20.0 C to 25.2 C. What is the change in entropy of the gas?" I know that to cal...

Deanna Necula 3B

I think you're right that entropy as a singular, absolute value is highest at high temperature, but the change in entropy is maximized the lower the initial temperature. You can see this effect from the equation ∆S=qrev/T; the lower the temperature of reaction, the higher the change in entropy. Conc...

Deanna Necula 3B

I remember there was a different way to do sig figs last unit, when calculating pH using log. For this unit, when we are using ln, must we apply the same sig figs rule? Also, is it applied right after using ln or log, and that truncated number is used in further calculations, or is it applied at the...

Deanna Necula 3B

I'm unsure as to why entropy would decrease when there is an increase in pressure. Is this statement only applicable if the pressure increase is done by decreasing the volume? That would make more sense to me. But if, for a given volume, the pressure increases (say by pumping in more molecules of ga...

Deanna Necula 3B

In problem 8.49, we needed to know temperature in order to use the equation PV=nRT. However, there was no given temperature in the problem. The solutions manual assumed 25C. When can we assume that a reaction is occurring at standard state (this particular problem gave no mention that the problem wa...

Deanna Necula 3B

8.21 asks to calculate the final temperature of water after adding a piece of copper. However, the solutions manual calculations inputs the same final temperature variable for the copper and the water in the equation "heat lost by metal=-heat gained by water", and solves for it that way. H...

Deanna Necula 3B

In example 8.3 in the textbook, where we were asked to calculate the heat necessary to increase the temperature of (a) 100 g (b) 2 mol of water from 20 to 100 degrees Celsius, the textbook used Kelvin in (b) because molar heat capacity is expressed in Kelvin rather than Celsius. However, it used 80 ...

Deanna Necula 3B

A greater electronegativity means that H will acquire a more positive partial charge. Therefore, it's more likely to dissociate and "attach" onto a base, because the base will supply it with the electrons it needs. Also, the more electronegative the halogen the more the electron density is...

Deanna Necula 3B

You know that the M2+ ion has lost two electrons. Therefore, for each electron configuration, just tack on two extra electrons to determine what each M is without any lost electrons. However, be careful; remember that after the d-orbitals are filled up, next come the "s" orbitals for that ...

Deanna Necula 3B

The equilibrium constant K for a certain chemical reaction is the "goal" of that particular reaction, because it describes a favorable state where reactants and products are constantly being replenished and the direction of the reaction is not skewed towards any one direction. Any changes ...

Deanna Necula 3B

The answer to the Lewis structure of N2O on the Fall 2012 midterm has a central N connected to another N and an O via double bonds. However, my drawing has a central N connected to an N via a triple bond and a single bond linking to the other O. In my Lewis structure, the oxygen (which is more elect...

Deanna Necula 3B

Why does CH3 have less electron withdrawing power than H? Is carbon (which is the atom attached to the rest of the structure) not more electronegative than H, and thus better able to withdraw electrons from the O-H bond to strengthen the acid?

Thank you!

Deanna Necula 3B

I have uploaded the video in a different (.mov) format.

Deanna Necula 3B

It's actually tridentate.

Deanna Necula 3B

Typically dissociation reactions are endothermic, because it requires energy to break bonds. Similarly, reactions involving a compound being "built" are typically exothermic, because energy is released when you form bonds.

Deanna Necula 3B

They give you the equilibrium concentrations for all the products and reactants so just set up the K expression and calculate K by plugging in the equilibrium concentrations. Then redo your ice box, starting with half the initial quantity of BrCl. Set your new equilibrium concentrations equal to the...

Deanna Necula 3B

The thing is, if you add more of a certain reactant, the final concentrations can't be the exact same as before, because now you have more matter. The only thing that is always the same at equilibrium is your K constant. This doesn't necessarily imply that your concentrations are the same; instead, ...

Deanna Necula 3B

I'm not sure cis molecules will always chelate...if there's an obstructing atom in the structure between the two atoms that could potentially chelate, then it won't likely chelate because that atom will prevent them from being close enough to do so. For example, look at (b) and (c) 17.36 in the text...

Deanna Necula 3B

Yes, even when there's a cation outside the complex you must add -ate at the end of the transition metal name along with the oxidation state in roman numerals. The cation outside the complex implies that the complex itself is anionic, and whenever the entire coordination complex is anionic you add -...

Deanna Necula 3B

Yes, there are in total 2 moles on the left side of the equation and three on the right. Therefore, there will be a greater perceived pressure on the right side than on the left, because when you decrease the volume (which is how we're assuming we increase pressure because otherwise the change in pr...

Deanna Necula 3B

There is an example in the textbook of a chemical equation involving the dissociation of ionic compounds, where an aqueous product is not included in the equilibrium expression. The textbook says that the reason it's not included is because the concentrations of the spectator ions cancel and only th...

Deanna Necula 3B

The bromine atoms have a different electronegativity than the hydrogen atoms, so the dipole moments won't be of equal size. Therefore, they won't fully cancel each other out. Tetrahedral structures are only non-polar when all the peripheral atoms are identical.

Deanna Necula 3B

The question is: calculate the moles of nickel chloride in 20 mL of a 0.01 M solution. In this problem I convert the volume to liters, where the number of liters would be 0.02L. Therefore, does my answer require the number of significant figures that I get during the conversion or as many as there a...

Deanna Necula 3B

When determining bond angles, should they be calculated around each central atom? For example, if you have two carbons linked together surrounded by hydrogen atoms, should the C-H bond angle be calculated around each carbon? So you would have two bond angles around a central atom, not just one. Than...

Deanna Necula 3B

In the textbook, it says that the molecule O2 has 2 unpaired electrons. However, when I draw the Lewis structure (and count the number of valence electrons, which happen to be even), no electrons are unpaired.

Deanna Necula 3B

If there is a greater difference in electronegativities between atoms present in an ionic bond, is that bond considered "stronger"? Does the same concept apply to covalent bonds?

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