CHEMISTRY COMMUNITY

Andie Jian 1D

The Nernst equation helps to relate cell potential to the concentrations of the species within it. It therefore also helps relate standard cell potential to the equilibrium constant.

Andie Jian 1D

Because third order reactions are so uncommon, I think Lavelle mentioned that we would not be focusing on them at all for calculations. Especially because the equation is not on the formula sheet, I think it would be really unlikely we would have to calculate anything for them. If anything, I would ...

Andie Jian 1D

Whenever the half reactions have different numbers of electrons that do not cancel out with each other, you have to convert the cell potentials into free energy instead of adding them. In the 7th edition textbook, this is explained on page 558 of topic 6M

Andie Jian 1D

You would have the largest Ecell if both E values are really positive. So, for the cathode, or the half reaction being reduced, you want to choose the reduction reaction with the highest E value. For the anode, or the half reaction being oxidized, you want to choose the half reaction with the lowest...

Andie Jian 1D

The question states: The body functions as a kind of fuel cell that uses oxygen from the air to oxidize glucose: C6H12O6(aq) + 6O6(g) --> CO2(g) + 6H2O(l) During normal activity, a person uses the equivalent of about 10MJ of energy a day. Assume that this value represents \Delta G and estimate the a...

Andie Jian 1D

The question asks what range (in volts) does a voltometer need to have to measure pH from the range of 1 to 14 at 25C if voltage is zero when pH=7. The solutions solve for the standard cell potentials, but I'm confused as to why - wouldn't you have to be solving for the cell potentials? Why are we a...

Andie Jian 1D

This question states that a pH meter is calibrated with a boric acid-borate buffer with a pH of 9.40, and the cell potential is +.060V. Then, the buffer is replaced with a solution of unknown [H+], but the new cell potential is +.22V. You need to calculate the pH of the new solution. The solution ma...

Andie Jian 1D

In a bunch of the questions in the homework like 6N.3 of the 7th edition textbook, the solutions don't convert partial pressures into concentrations when they are put into Q, even if there are non-gaseous species included as well. Furthermore, all of the partial pressures have to be converted into a...

Andie Jian 1D

Also, when drawing the cell diagram, a porous disk would just be a single line rather than a double line between the two half-cells.

Andie Jian 1D

When using the equation $\Delta G=-nFEcell$ , does the number of moles of electrons we plug in depend on the moles of ions we start out with? Or do we only look at the moles of electrons transferred within the overall reaction?

Andie Jian 1D

In class, we went over how we need a salt bridge in order to maintain neutral charges in both half-cells. I understand how the oxidation reaction will lead to a greater amount of positive ions in that cell, so negative ions will balance that charge out. However, why would the cathode side attract po...

Andie Jian 1D

When comparing whether Fe3O4 or Fe2O3 is more stable based on their free energy, why do we have to find the standard gibb's free energy of the reaction of Fe3O4 and O2 into Fe2O3 in order to determine this, rather than just comparing each of their individual free energies?

Andie Jian 1D

I was looking over the constants and equations sheet, and I didn't see the Van't Hoff equation on it. Does this mean we will need to memorize it and understand how to derive it?

Andie Jian 1D

This question asks what is common about molar entropies with a negative value. The answer is that the ones with negative entropies are all aqueous ions, because when the ions are dissolved surrounded by water molecules are more ordered that if it was the ion and the water molecules separately. Howev...

Andie Jian 1D

A reaction is spontaneous if the entropy of the universe is increasing (Stot=Ssys+Ssurr). We went over in lecture how this equation can be related to the equation for Gibb's free energy. Therefore, it makes sense that Gibb's free energy should be negative if a reaction is spontaneous, because if the...

Andie Jian 1D

I'm not entirely positive about this, but I think internal energy is more of a measure of the total energy of a system (which is why it can be changed by both work and heat). However, Gibbs free energy is a measure of only the energy that is available to be used. So, for example, during a reaction, ...

Andie Jian 1D

In 4.1 7th edition, you need to calculate the heat required to convert a block of ice into water vapor. You solve this by using changes in enthalpy, but this means that pressure must be constant in order for change in enthalpy to equal q. However, it never specified it was at constant pressure. When...

Andie Jian 1D

In this problem we need to calculate the final temperature and change in enthalpy when 765J of heat are transferred to .820 mol Kr(g) at 298K and 1.00atm. In part, b, you are supposed to do both of these things at a constant volume. I understand that you can find qv and relate it to change in intern...

Andie Jian 1D

I understand that q=-qCal, implying that C(deltaT) = -Ccal(deltaT). However, for the side for the calorimeter, would we have to switch T1 and T2 because the heat released by the system is opposite the heat gained by the calorimeter? Or is it different because a change in temperature is different tha...

Andie Jian 1D

I understand that a reversible reaction means that volume and pressure are constantly changing infinitesimally in order to maintain the same pressure inside as outside. However, is temperature also still constant during this reaction, because the change in volume balances out the change in pressure?

Andie Jian 1D

In class, we did the example: "an exothermic rxn (change in enthalpy = -50kJ) occurs in an open beaker producing net 8 moles of gast at 25C. What is the change in internal energy?" We assumed that there was no temperature change, because the temperature is taken before and after the reacti...

Andie Jian 1D

In questions like 4E.5 (7th edition), we calculate the reaction enthalpy using bond enthalpies given the reaction. However, the reaction enthalpy always ends up being set to kJ/mol. However, I'm confused why this is, because wouldn't multiplying the bond enthalpies given in kJ/mol by the number of m...

Andie Jian 1D

I don't believe we need to know specifically how it works. I think the most important part is just we should assume that whenever a reaction is described as occurring in a bomb calorimeter, it is considered a completely isolated system. Therefore, no energy or matter can leave or enter, and the inte...

Andie Jian 1D

I think you are correct. Because enthalpy is additive, if you reverse the reaction so that the product becomes the reactant, then the enthalpy should just be the negative of the forward reaction enthalpy.

Andie Jian 1D

I know that standard enthalpy of formation is always kJ/mol. However, are the units for reaction enthalpy also kJ/mol, or just kJ? I feel like I see mixed answers within the textbook.

Andie Jian 1D

The question asks whether NO or NO2 has a higher heat capacity. The answer says NO2, because capacity increases with molecular complexity. Is this a rule that we have already learned? will molecules with more bonds always absorb more energy?

Andie Jian 1D

Are there specific units we should be using for our equilibrium constants for gases? I know the partial pressures themselves within the equation technically don't have units, but should we be converting values to anything specific, like bars or atm? If so, will we be given the conversion equations b...

Andie Jian 1D

In this question, it gives you a plot of partial pressures of reactants and products. From the graph (attached below), you obtain the balanced equation 2A(g) --> B(g) +2C(g). My first question is why this balanced equation doesn't have an equilibrium symbol, and if it would be considered wrong if we...

Andie Jian 1D

In some problems in the homework (like 5I.25 7th edition), the initial amounts of the mixture include both products and reactants. Therefore, the solution manual did a check for Q first to see if the reaction will move forward or backward. Do we have to do this too each time there are both products ...

Andie Jian 1D

In this question, we are supposed to calculate the equilibrium composition. I got the quadratic equation 0= 4x 2 +1.2*10 -4 x-1.2*10 -7 , which led to x being 1.6*10 -4 . [F2]=1.0*10 -3 -x, so I got the answer 8.4*10 -4 . However, the answer key gives the answer with just two sig figs, making it 8*1...

Andie Jian 1D

Here's a helpful table from the book (pg. 410 7th edition) summarizing these rules. It also explains these concepts with more examples on that page if you need more clarification.

Andie Jian 1D

I'm not entirely sure either, but I think aqueous reactants/products may stay in because they are still reacting significantly to reactants/products (aqueous just means in water). This is contrasting to pure water, which you don't include, that the solute is mixed in. There is so much solvent (water...

Andie Jian 1D

Does anyone know if we will have to know how to convert all of the different gas units to bar? Like if a question gave something in atm and if we would need to know how to convert it to bar for equilibrium constants?

Andie Jian 1D

In the textbook, it says that the number of sig figs AFTER the decimal in the pH value should equal the sig figs of the molarity. However, many of the solutions don't seem to follow this rule. Should we just base sig figs in our pH normally, or follow this decimal rule?

Andie Jian 1D

The question asks whether BrO- or C17H19O3N is a stronger base. The solution uses their respective pKb values to determine their strengths as bases. However, is there a reason to logic which is stronger without the values, or would we have to be provided the values in order to know?

Andie Jian 1D

The question asks you to write the chemical equation, draw the lewis structures, and identify lewis acid and base of the reaction H2O2 reacts with sulfur trioxide to form H2SO5. I really don't understand a lot about this question. I understand that the chemical equation would be H2O2 + SO3 --> H2SO5...

Andie Jian 1D

For 7th edition 6A.15 and 6.5, the questions ask you to draw the structures of SO2 and SO3 respectively. I thought having all of the oxygens with double bonds would be the most stable structure, providing the lowest formal charges. However, the answers show SO2 with 1 double and 1 single bond, and S...

Andie Jian 1D

The order within the formula and when writing the actual name may be different. The alphabetical order of the ligands when written out for the naming is independent of the alphabetical order when writing out the formula.

Andie Jian 1D

For polydentates, even if an oxygen has multiple lone pairs, will that oxygen form multiple bonds or just one? Also, in 9C.5, why does oxalate only count as a bidentate whereas CO3^2- can be both a mono or bidentate?

Andie Jian 1D

For part c of this question, the formula for tetraamminediaquacobalt(III) is [Co(NH3) 4 (OH2) 2 ]Br3. However, in the solution manual instead of OH2, it is written as H2O. Does anybody know why this is? Also, in part d of this question, the formula for sodium bisoxalato(diaqua)ferrate(III) is writte...

Andie Jian 1D

I don't think we have to know the exact values of the dissociation energies. I would just focus on the relative trends of dissociation energy due to changes in atomic radius (greater radius means lower dissociation energy), multiplicity (greater multiplicity means higher dissociation energy), and nu...

Andie Jian 1D

Ya, a sigma bond doesn't become a pi bond, its just that more types of bonds are added on as you go from single to double to triple bonds. I found this image really helpful in visualizing how a pi bond and sigma bond interact in a double bond. https://ds055uzetaobb.cloudfront.net/image_optimizer/3ab...

Andie Jian 1D

The model for C6H4O2 given below the question does not include the extra valence electrons that should fill the oxygens' octets. Each one of those oxygens should have 2 extra lone pairs, meaning there are 40 valence electrons for C6H4O2. Therefore, the reduced product should have 42.

Andie Jian 1D

I understand that lone pairs push down the bonding pairs leading to smaller bond angles than expected given the VSEPR shape. However, for t-shaped, see-saw, and square pyramidal, I am confused if the lone pairs push on the equatorial bonds, the axial bonds, or both. Therefore, I am also confused on ...

Andie Jian 1D

Oh okay this makes things clearer! But I'm still a little confused, because wouldn't flourine's greater electronegativity lead to stronger dipole-dipole bonds too? Or is this difference less important than the difference in london forces?

Andie Jian 1D

I'm not completely sure, but I do think you would need to know the VSEPR shape in order to figure out the bond angles, because that takes into account the electron repulsions that create those angles.

Andie Jian 1D

In the 7th edition book problem 3F.5 part c, between CHI3 and CHF3, CHI3 has a higher melting point because of london forces. I understand iodine is a larger molecule with a lower electronegativity, meaning it is more polarizable and therefore has stronger london forces. However, flourine is a highl...

Andie Jian 1D

I think you would treat it like each central atom is the center of its own shape. So since each nitrogen has 3 bonding pairs and 1 lone pair, the molecule would look like 2 trigonal pyramidal shapes put together. I'm not totally sure about this though, sorry!

Andie Jian 1D

In class, I think it was mentioned that it is okay if we just draw normal lewis structures if the question doesn't specify because sometimes the shapes can get really complicated. However, we will have to be able to name the actual VSEPR shape. As for lone pairs, I think it doesn't really matter whi...

Andie Jian 1D

What is Induced dipole? An induced dipole is formed due to random electron movement. Sometimes, randomly, an electron cloud will form more on one side of a molecule than another, creating a temporary chance "induced dipole". This chance dipole then can induce a dipole in a neighboring mol...

Andie Jian 1D

I think you are right that it would, because no matter if it was connecting with a coordinate covalent bond, the electrons will be repelling each other the same as any other tetrahedral molecule, forming the same shape with 109.5 degree angles.

Andie Jian 1D

Ohhh okay thank you so much!

Andie Jian 1D

In 7th edition problem 2B.9, the lewis structures for Cl-, K+, and Na+ don't include brackets around them. Does anyone know why this is? Would we get points off on exams if we did draw brackets?

Andie Jian 1D

I'm not totally sure, but I don't think there is necessarily a limit. I think in the end it really just depends on trying to get the formal charges as close to 0 as possible.

Andie Jian 1D

In the 7th edition textbook, the solution for the lewis structure for ClONO3 (2C.5 part c) assigns the Nitrogen in the molecule single bonds to 2 oxygens, and a double bond to another oxygen. However, wouldn't the most stable state assign the nitrogen 2 double bonds and a single bond, because that m...

Andie Jian 1D

You're totally right in that the uncertainty of velocity should be 10m/s. The textbook and solution manual made a mistake. You can see the error addressed in the "Solution Manual Errors in the 6th Edition" link Dr. Lavelle has on his site (it is not listed on the 7th edition errors link, b...

Andie Jian 1D

Degeneracy means the same energy. For example, with hydrogen, every subshell in the same energy level will be degenerate, because there is no electron-electron repulsion countering the pull of the nucleus, as there is only one electron. However, with multi-electron atoms, the subshells within the sa...

Andie Jian 1D

Their p-orbitals of Cr and Cu only need one more electron to reach the much more stable state of all the p orbitals having 1 electron (5 in total), or 2 electrons in each orbital (10 in total). Therefore, in those cases, an electron will jump down from the higher energy s orbital to complete the orb...

Andie Jian 1D

I might have heard this wrong, but I think it was explained in class how a larger negative charge increases energy. I'm a little confused why this is, because wouldn't a higher positive value also increase energy according to [(q1)(q2)]/r ? Also, when he says, does he mean the total value of (q1)x(q...

Andie Jian 1D

I'm not completely sure myself, but I'm thinking it probably has to do with electron-electron repulsion. So, if there are multiple electrons, you have to take into account the differences in their movement due to electrons trying to avoid each other. But, I'm not totally sure how this affects their ...

Andie Jian 1D

The answer would have 2 sig figs. the x10^ part of scientific notation does not count as a sig fig.

Andie Jian 1D

Looking at the equation you cited, v=E/h or E=hv, because h is a constant, v, or frequency, is directly proportional to E, and is, in fact, the only factor that changes it. Therefore, it could be said that the energy of a photon is defined by its frequency.

Andie Jian 1D

I think he was maybe saying that when things have a large enough mass, their wavelengths become so small that they are basically negligible. For example, all things made of matter have wavelike properties, including things like cars. But, you don't see a car squiggling with waves down the road. The ...

Andie Jian 1D

I think he said that the minimum detectable wavelength was 10^(-18) m

Andie Jian 1D

The textbook gives us the Rydberg formula, which is \nu = R \left \{ \frac{1}{n_{1}^{2}}-\frac{1}{n_{2}^{2}} \right \} So I think it has to have something to do with combining the equations E=hv, and E=-hR/n^2 , so you get v=-R/n^2. Then, if you were trying to find v1-v2, you would get \nu = -R \lef...

Andie Jian 1D

I know during lecture we defined c as 3.00x10^8m/s, but in the solution manual for homework, it is defined as 2.998x10^8m/s. Is there any preference for how many sig figs are preferred for this constant?

Andie Jian 1D

I totally agree with your answer, but I think it is also really important to take into account the molar mass along with the number of atoms. For example, H2 would still account for less percent than O in water. I don't think actual calculation was necessary for this question, but maybe it had 4 sig...

Andie Jian 1D

Hi! This might be a stupid question, but I was just wondering that if when trying to find a whole number ratio to get the empirical formula, I find the wrong ratio from a non-whole number ratio? For example, what if I multiply my non-whole number ratio by 3 and in the end, every number is extremely ...

Andie Jian 1D

Hi guys! Does anybody know if we can round off our answers each step to the correct amount of sigfigs? I'm not sure if I should use rounded off steps or if I should store each step's answer on my calculator to use for the actual calculation until the end, only writing down the rounded off sigfigs fo...

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