CHEMISTRY COMMUNITY

Abigail Urbina 1K

We know that equilibrium constant K is equal to k/k'. When we are deriving an expression for the rate of formation of product using the pre-equilbrium approach, can we leave the final answer with K (equilibrium constant), or do we have to simplify K to be k/k'? Will we be marked off points if we lea...

Abigail Urbina 1K

The first question of the 2015 Final asks you to solve for the specific heat of a substance. The mass is 15.9 g, the initial temperature is 25 degrees C, and the final temperature is 1027 degrees C. When we are rounding off our final answer, what is the correct amount of sig figs we have? I was orig...

Abigail Urbina 1K

Can the rate constant, k, ever be negative?

Abigail Urbina 1K

If we are given two half reactions with their standard cell potentials, how exactly are we supposed to know which is the cathode and which is the anode, and thus which reaction we would need to reverse? For example on question 3C of the 2014 Final: Fe3+ + e- --> Fe2+ Zn2+ + 2e- --> Zn(s) It says tha...

Abigail Urbina 1K

You are trying to find the value for the enthalpy of vaporization, so you use q=mCdeltaT+(1/2)m*deltaH_vap I understand how to do the algebra, however, I'm confused where the solutions got (5.4 * 10^-3 kJ/g C) for the value of C

Abigail Urbina 1K

We didn't really go too in depth in lecture on functional groups. What exactly would we be expected to know for the final?

Abigail Urbina 1K

We didn't really cover the Michaelis-Menten rate equation in lecture, so will we be expected to know this thoroughly for the final?

Abigail Urbina 1K

The question says: A voltaic cell functions only when the electrical circuit is complete. In the external circuit the current is carried by the flow of electrons through a metal wire. Explain how the current is carried through the cell itself. The solution manual says: A negatively charged electroly...

Abigail Urbina 1K

How do we predict if a metal will be dissolved in a solution?

Abigail Urbina 1K

Can someone explain to me what an electrolyte is and when exactly we would need it in our cell? What is the significance?

Abigail Urbina 1K

How do we know what the correct orientation of the molecules should be when drawing the proposed structures for activated complexes?

Abigail Urbina 1K

The book says "In some cases the original reaction with a slow rate-determining step may continue in parallel with the catalyzed reaction. However, the rate is determined by the faster path, which governs the overall rate of formation of products." I thought that the rate was determined by...

Abigail Urbina 1K

We know that the equilibrium constant K (capital K) is equal to k/k'. However is rate constant k (lowercase k) equal to k'/k? I was looking in the solution manual for question 15.61, and I wasn't entirely sure where ln(k'/k) came from or how exactly you get to that step. Can someone please clarify t...

Abigail Urbina 1K

Swetha Sundaram 1E wrote:k would be the rate constant of the forward reaction. k' would be the rate constant of the reverse reaction.

But they don't specify which is the rate constant for the forward reaction and which is the rate constant for the reverse reaction

Abigail Urbina 1K

This question gives us two different values for the rate constant of a first-order reaction. One is 0.76 s^-1 at 1000 K and the other is 0.87 s^-1 at 1030 K. How do we know which value is k and which value is k'? They don't explicitly specify it in the question.

Abigail Urbina 1K

When there are more than 1 elementary steps in a reaction mechanism, how do we determine which one is the slowest step?

Abigail Urbina 1K

If we are working with a reaction that does not have an overall order of 1, what should we look for to know that we should turn it into a pseudo-first-order reaction?

Abigail Urbina 1K

When we are deriving the rate laws, does it matter whether we use definite or indefinite integrals? Will we be able to come to the same rate law equations if we use either definite or indefinite integrals? When do we know what is appropriate to use/what we should use?

Abigail Urbina 1K

When we're solving for the order of a reactant in an equation, we usually go about by finding the experiments in the table in which that reactant's concentration changes and where the other reactants are held constant. However, if one of the reactants is independent of the rate, how does this affect...

Abigail Urbina 1K

Can someone explain to me how they found the order of reactant B? I know to look at experiments 2 and 3 to solve this. I'm just confused because the solutions manual says that increasing the concentration of B by the ratio 3.02/1.25 increases the rate by (3.02/1.25)^2, so the reaction is then second...

Abigail Urbina 1K

When we're trying to find the order of each reactant in a reaction, how exactly do we know which experiments we should work with and which ones we shouldn't? For example, when we're trying to solve for the order with respect to B, why can't we use experiments 1 and 3? We clearly see that the concent...

Abigail Urbina 1K

Why is it that for a first-order reaction, the half-life of a substance is independent of its initial concentration? Why can the concentration of A be used as its "initial" concentration at any stage of the reaction? Can someone please clarify this?

Abigail Urbina 1K

Rate = k[A]^n, where n is the exponent that represents order of the reaction. Concentration is always going to have units of M/L, which is the equivalent to mol.L -1 .s -1 . Be sure to apply the exponent to the units of the concentration as appropriate. You want to isolate k to be alone on one side ...

Abigail Urbina 1K

To find the overall order of a reaction, you can simply add all the individual orders of each reactant in a reaction.

Abigail Urbina 1K

The unique rate of a reaction takes into account the stoichiometric coefficients of a reaction. It is the rate of appearance/disappearance of any species in a reaction divided by its stoichiometric coefficient.

Abigail Urbina 1K

When do we have to add an inert electrode to a cell diagram?

Abigail Urbina 1K

How do we know that the standard cell potential is 0 in this particular problem?

Abigail Urbina 1K

In the cell diagram for part c of question 13 on the homework, why is the right side of the salt bridge written as Cl-(g)|Cl2(g)? Why is it not Cl2(g)|Cl-(g)?

Abigail Urbina 1K

In the full reaction, reactant O₃ is in an aqueous state. Why is it that when we are writing the half reaction O, O₃ is in a gaseous state?

Abigail Urbina 1K

I was looking over some lecture notes on the Nernst Equation and was a little bit confused about something. We learned that E=E°-(2.303RT/nF)logQ. Can someone clarify where the 2.303 value comes from?

Abigail Urbina 1K

When writing a cell diagram, does the charge of an ion have to be taken into consideration to determine the order in which components are written? Is Fe3+(aq) always going to be written before Fe2+(aq)? Or does it not matter?

Abigail Urbina 1K

In the textbook, it says that a hydrogen electrode is denoted H+(aq)|H2(g)|Pt(s) when it is the right hand electrode in a cell diagram and Pt(s)|H2(g)|H+(aq) when it is the left-hand electrode. I don't quite understand why they are listed in that order because with respect to which side it is as an ...

Abigail Urbina 1K

Is there anything significant that changes in our calculations if we use the molar convention for finding the reaction Gibbs free energy?

Abigail Urbina 1K

In determining the oxidation numbers of transitional metals, does it all just depend on the charges of the other atoms it forms a molecule with? Like for example, in the compound MnO 4 - , does the 7+ charge depend solely on the oxygen charges? Do we rely on the non-transitional metals in an ion as ...

Abigail Urbina 1K

When it is not explicitly stated in the question whether pressure or volume are constant, when do we know which to use?

Abigail Urbina 1K

When we're calculating questions involving solving for q (heat transfers), I'm a little bit confused on when we would use the specific heat of ice and when we would use the specific heat of water - for questions involving phase changes where ice melts.

Abigail Urbina 1K

Under what circumstances would delta H be equal to delta U? Can someone explain this? In the notes we wrote that the volume of the reactants would be equal to the volume of the products if pressure was constant. However, I still don't get how delta H then becomes equal to delta U

Abigail Urbina 1K

I was wondering the same question. Saw this exact same concept applied to a question on the 2015 Winter Midterm

Abigail Urbina 1K

Can someone give some important to remember examples of the following?

When:
a) Delta S=0
b) Delta G=0
c) Delta U=0

Abigail Urbina 1K

When there is a temperature change and we are being asked to solve for delta S, how do we know whether to use Cp or Cv?

Abigail Urbina 1K

Can pressure not be constant when reversible expansion is taking place?

Abigail Urbina 1K

Can someone clarify to me when we use the units of kJ/mol for delta H versus when we use just kJ for delta H?

Abigail Urbina 1K

Why do we have to set delta G equal to zero when we are trying to find the temperature at which a reaction is spontaneous? I just find this a little confusing because I typically associate spontaneous reactions with negative values for delta G

Abigail Urbina 1K

Can someone explain/clarify what exactly degeneracy is and what its overall importance is?

Abigail Urbina 1K

Usually, the units for entropy are J/K. However, it shouldn't really matter. One thing to be mindful of, though, is keeping your units consistent throughout your calculations. Let's say you are adding up different values for entropies. You want to make sure you are adding entropies that all have uni...

Abigail Urbina 1K

Using a + or - sign in your calculations allows you to keep track of something that is either being gained or being lost, especially for problems that ask you to solve for any delta values.

Abigail Urbina 1K

In part c of 33, the chemical reaction is SO 2 (g) + Br 2 (g) +2H 2 O(l) --> H 2 SO 4 (aq) + 2HBr(aq) Entropy decreases in this reaction. The solution manual says that entropy decreases because the total number of moles decreases. However, like in previous problems, is it sufficient to say that entr...

Abigail Urbina 1K

The sign of delta S depends on the direction of heat flow. Delta S is simply q_rev divided by T. q_rev is negative whenever the system is losing heat. From that information, you then know that the surroundings gain heat. If the system is gaining heat, then q_rev is positive.

Abigail Urbina 1K

Whenever a question states that we should assume ideal gas behavior, are we always supposed to plug in 1.00 mol of gas into an equation when the amount of moles is not given? Are there certain circumstances (other than when they actually give us the amount in moles of gas) where we should not automa...

Abigail Urbina 1K

If asked to calculate the changes in entropy and we are given temperature in terms of Celsius, do we always have to convert the temperature to be in terms of Kelvin? I know both can technically be considered correct, but is there a preferred method?

Abigail Urbina 1K

Do we have to be familiar with superheating for the Week 3 test?

Abigail Urbina 1K

Update: found my mistake, I had an error in one of the digits

Abigail Urbina 1K

I'm a little bit confused as to how the solution manual came up with the value of \Delta H= -242 kJ/mol in part a of 8.67. I understand that to find the enthalpy of the reaction, you need to use the equation of bonds broken-bonds formed. I still can't seem to get that same value though. Can someone ...

Abigail Urbina 1K

For problem 8.49 on the homework, I am a little bit confused on how to first approach the problem. The solution manual starts solving the question using the approach of the PV=nRT equation. However, how do we know just based on the wording of the question that there is a net production of one mole o...

Abigail Urbina 1K

For questions involving heat transfers between a system and its surroundings, I'm a little bit confused on how you determine where the negative sign goes (which q is going to take the negative sign). If q is negative, doesn't it mean that the temperature is going down, meaning it is losing heat? And...

Abigail Urbina 1K

I know that Dr. Lavelle went over this in lecture yesterday a little bit, but can someone please explain to me exactly what an irreversible expansion is? And why is it irreversible? If I understand this correctly, I know that a reversible expansion can be reversed through an infinitesimal change (sm...

Abigail Urbina 1K

CH 3 CHCH 2 (g) + H 2 O(g) --> CH 3 CH(OH)CH 3 (g) I understand which bonds are broken and why (because I drew out the Lewis structures). However, for the bonds that are formed from this reaction, I am confused as to why the solution manual says that 1 mol of a C-C SINGLE bond is formed. Also, it sa...

Abigail Urbina 1K

I'm having a little bit of trouble distinguishing the differences between intensive and extensive properties. We were told that extensive properties are dependent on the amount of a substance, so one could imply that intensive properties would be the opposite. However, how are intensive properties n...

Abigail Urbina 1K

I know that the units of standard enthalpies of formation is always kJ/mol. However, for problem 8.63 in the textbook solutions manual, why is the solution only given in kJ and not kJ/mol? Are there certain instances where the answer should only be written with units of kJ? Should we just stick to a...

Abigail Urbina 1K

When calculating q for the calorimeter, why are you multiplying it by 2.49 K and not 7.32 K? How do you know which value to use?

Abigail Urbina 1K

We know that the change in enthalpy is equal to q p , and we also know that enthalpy is a state property. However, we were taught in lecture that heat, which is q,is not a state property because it depends on the path it was taken to get to its final state. How is it not a state property when the ch...

Abigail Urbina 1K

We've seen so many different equations that can be used to calculate q such as q=nCdeltaT or q=mCdeltaT. How do we know when to not take mass or moles into account and just use the equation q=C*deltaT? I was having a hard time finding why this equation was used in 8.53 when a mass was given.

Abigail Urbina 1K

For homework problem 8.51, I understand how to come up with the calculation formatting, but I'm a bit confused with the signs of some of the values in the solutions manual. Can someone explain the reason the final answer is +23.9x10^3 kJ/L, a positive value? Energy is being released so I would origi...

Abigail Urbina 1K

How exactly are we supposed to approach part c in homework problem 8.45?
4 C(s) + S8(s) --> 4 CS2(l) deltaH=+358.8 kJ
c) If the heat absorbed in the reaction was 415 kJ, how much CS2 was produced?

Abigail Urbina 1K

For questions such as homework problem 8.41, where you are asked to calculate enthalpy changes where phase changes are involved, how do you know when to account for the molecular weight of a substance? In other words, if you are given the mass of a substance, how exactly do you know when to divide t...

Abigail Urbina 1K

NO2 is more complex than NO considering it has one more atom in it. This makes the molecule more complex. Since there are more atoms present, there is room for more energy to be absorbed, hence a larger capacity for heat.

Abigail Urbina 1K

When asking to calculate the amount of heat released, how do we find what the heat capacities are for the Kr gas? I understand that to find the amount of heat released, the formula is mass x heat capacity x change in temperature. I have mass and the change in temperature, but how do I find the heat ...

Abigail Urbina 1K

Heat capacity refers to the amount of heat that is required to change the temperature of an object by a certain degree. Specific heat capacity, however, refers amount of heat that is required to raise the temperature of an object per unit mass. In other words, you can obtain that by dividing the hea...

Abigail Urbina 1K

This is because the energy being supplied in the form of heat disrupts the INTERmolecular forces that hold the liquid or solid together. Nothing is happening intramolecularly.

Abigail Urbina 1K

What are the ultimate differences between the two methods we were taught in lecture yesterday to calculate change in enthalpies? The first was Hess's Law and the other was using the bond enthalpies. I was trying to watch videos online to clarify this concept but was a little confused. Where does the...

Abigail Urbina 1K

Can someone please re-explain/clarify why it is that the solution is still considered neutral even when [H3O+] < 10^-7 ? How exactly does this concept tie in with the reasoning that autoprotolysis generates 10^-7 mol/L ?

Abigail Urbina 1K

For those of you who went to the review session last night to go over the questions on the practice final, could you explain #21 to me? We are being asked to find the oxidation state of iron in the heme complex. In order to find it, we must find the formal charges of the nitrogen atoms. He demonstra...

Abigail Urbina 1K

Why is NO2 not drawn with two double bonds? If it is drawn with two double bond and one lone electron on the N central atom? Wouldn't all the formal charges of the atoms be 0 if it was drawn that way? if it's drawn with two single bonds and one double bond, there would be formal charges on the N ato...

Abigail Urbina 1K

For the ground-state electron configuration of I-, is it acceptable/correct on a test to answer it as simply [Xe]? Or is it preferred to write [Kr]4d10,5s2,5p6 (without the commas, of course)? Or are both acceptable?

Abigail Urbina 1K

Since we know that strong acids and strong bases are completely ionized in water unlike weak acids and bases, when we are calculating for pH and pOH in reactions that involve strong acids or bases, is it necessary to set up an ICE box? Like we do when solving for concentrations involving weak acids ...

Abigail Urbina 1K

The CCl3 group in trichloroacetic acid that is bonded to the carboxyl group, -COOH, is more electronegative (electron-drawing) than the CH3 group in acetic acid that is bonded to the carboxyl group. With respect to the definition of a Lewis acid, an electron acceptor, this would make CCl3 more acidi...

Abigail Urbina 1K

When you are comparing HCLO4 and H3PO4 to determine which one is a stronger acid, you know right away that Cl is more electronegative than P. However, does the number of H atoms in the molecule affect its strength in acidity? There is the same number of O atoms, but there is a different number of H ...

Abigail Urbina 1K

Question 12.49 on the homework says: Which is the stronger base, the hypobromite ion, BrO- , or morphine, C17H19O3N? Justify your answer. The answer to the question is that the stronger base is the hypobromite ion. I understand that for oxoacids, the more oxygen atoms there are in a compound, the st...

Abigail Urbina 1K

I don't understand how to solve part b of #27 on the chapter 12 homework. Can someone explain how they went about to setup the calculation for part b? I didn't quite get how they did it in the solution manual A careless laboratory technician wants to prepare 200.0 mL of a 0.025 m HCl(aq) solution, b...

Abigail Urbina 1K

I understand that if an oxide is amphoteric, it can react with both acids and bases. However, how can you tell just by looking at a chemical formula whether that molecule will be amphoteric or not. For example, in question 12.17d, how are you able to tell that Bi2O3 is amphoteric and not simply basi...

Abigail Urbina 1K

As I was working through some of the problems in Fundamentals J of the textbook, I was doing some questions involving writing out the complete and net ionic equations of particular reactions. When writing out ionic equations, how do we know when exactly to split up a molecule into its particular ato...

Abigail Urbina 1K

The easier it is for the acid to dissociate, the stronger of an acid it is. Keep in mind that single arrows are used in chemical reactions for strong acids to denote that it is being COMPLETELY ionized when it is in water. Stronger acids can donate more protons in comparison to weak acids because th...

Abigail Urbina 1K

When we are asked to write the equilibrium expression K for a reaction and we are not told whether to write that equation with respect to concentration or partial pressures, how do we know which way to write the expression? In question 11.3 on the homework, I originally answered it in terms of molar...

Abigail Urbina 1K

I was confused at first as well when I encountered some of the questions on the homework. Then I realized that the cation always precedes the coordination sphere and the anion always proceeds the coordination sphere. To think of it in simpler terms, think of a compound as simple as NaCl - the cation...

Abigail Urbina 1K

At face-value, it is not too difficult to determine whether or not a ligand has more than one binding site for electrons. This is because we are pretty familiar with drawing Lewis structures and taking lone pairs of electrons into consideration. However, I'm a little bit confused when it comes to de...

Abigail Urbina 1K

It would be linear if there are no lone pairs of electrons on the central atom. This is because the lone pairs are responsible for any repulsion between themselves and the bonded electrons. The bent shape/structure of any molecule can be attributed to this repulsion phenomenon. A linear shape would ...

Abigail Urbina 1K

What exactly is important for us to know when it comes to differentiating between sigma and pi bonds? I understand that sigma bonds are known as every first bond between two atoms, and a pi bond is every second or third bond, etc between two atoms. In lecture we were taught that sigma bonds are when...

Abigail Urbina 1K

The definition of electronegativity itself is "the electron-pulling power of an atom." The best way to think of it conceptually without having to memorize any numbers is to think of it in terms of electron affinity. The trend for electron affinity increases as you go from left to right acr...

Abigail Urbina 1K

You should aim to draw a Lewis structure with atoms with a formal charge as close to zero as possible. When you add all the formal charges of each of the atoms together, you get the overall formal charge of the compound itself. Lower formal charges correspond to more stability within those compounds...

Abigail Urbina 1K

In the textbook, it says that the modern convention for representing a dipole (which is used throughout the book) is having an arrow point toward the positive partial charge. In the original convention, a dipole is represented with an arrow pointing toward the negative partial charge. In lecture, we...

Abigail Urbina 1K

Upon doing some of the practice problems and looking at the solutions manual, I saw that Tungsten didn't follow the same exceptions rule as copper and chromium do when it comes to electron configuration. You would originally think that you would leave make the d shell half-filled and leave the s orb...

Abigail Urbina 1K

To answer the first question, the reason the electron configuration for silver is [Kr]4d^10 5s^1 instead of [Kr]4d^9 5s^2 is because when you are one electron away from a half-full or fully d shell, it is actually lower in energy to fully fill the d shell and to leave the s orbital singly occupied w...

Abigail Urbina 1K

The above responses are great, but to elaborate on orbitals, orbitals can be thought of in terms of probability density. Instead of thinking of electrons as moving around in an orbit, meaning only round and round in a 2D flat space, think of the possible area (in terms of volume) that an electron ca...

Abigail Urbina 1K

I understand that the allowed values for the magnetic quantum number range from l,l-1,...-l, but once we solve what those values are, what exactly do those numbers tell us? I know that the magnetic quantum number labels different orbitals of a subshell in terms of their orientation, but how does the...

Abigail Urbina 1K

Is there a particular "threshold" in length that a wavelength must have in order for us to be able to detect the wavelength or observe its characteristics? What is considered "too small" for us to detect?

Abigail Urbina 1K

Can someone help me with how to solve this problem? The question reads, "An excited hydrogen atom emits light with a frequency of 1.14 x 1014 Hz to reach the energy level n = 4. In what principle quantum level did the electron begin?"

Abigail Urbina 1K

Use the equation that directly includes wavelength, since that is the quantity you are looking for. The Ek formula should be utilized when you are looking for mass, velocity, or excess energy electron

Abigail Urbina 1K

If the energy of the photon is equal to the threshold energy, then there will be no excess energy. All kinetic energy from the electron is attributed to excess energy of the incoming photon. If there's no kinetic (excess) energy, then no electron will be ejected. Therefore, when you're looking at wo...

Abigail Urbina 1K

It might be easier to calculate and see the final answer in meters. However, when asked to compare with other wavelengths, and with questions that ask for different types of radiation, converting your answer to to units of nanometers in the end is very helpful.

Abigail Urbina 1K

In the video module under "Atomic Spectra," it discusses how electrons continually become "more negative" when they go down energy levels and emit more photons. When discussing Rydberg's formula, we were given -R=[(1/n1^2)-(1/n2^2)]. In the book, the same formula is given but jus...

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