CHEMISTRY COMMUNITY

Leah Savage 2F

What is the pseudo equilibrium constant? How is it found and how is it different from the normal equilibrium constant?

Leah Savage 2F

How could the cell potential/voltage ever be larger than the standard cell potential? I thought that the whole idea was that as a galvanic cell runs and approaches equilibrium, cell potential decreases from the standard cell potential at the beginning.

Leah Savage 2F

Are catalysts drawn in the activated complex? If the first step is the slow step and there is a catalyst as a reactant, is the catalyst included in the rate law? Or would this not happen? Also, are catalysts included in the molecularity of a step?

Leah Savage 2F

I think only the concentration should be included.

Leah Savage 2F

Does resonance automatically imply a lot of entropy? In actuality resonance structures aren't found multiple forms but just with bonds in between double and single bonds, so how would this increase entropy?

Leah Savage 2F

Are ketones only made up of C bound to O and two C groups? Could it be bound to a C group and something other than H?

Leah Savage 2F

Would it tell us in the problem if water is the solvent? In one problem you had to assume water was the solvent, and then later they were like "now what if there was an organic solvent instead."

Leah Savage 2F

If water is in one of the steps, should it be included in the equilibrium equation (K), or left out bc it is a liquid? If it is left out, does that mean it is also left out of the rate law?

Leah Savage 2F

In the example Lavelle did in class, he used the equilibrium equation for the step 1 (K1) and subbed this in for the intermediate in the second step. How do you know to use the first equilibrium concentration for the first step? Is it because this is the one with reactants from the problem? Is this ...

Leah Savage 2F

Once you get the equation to be ln([A]0/[A]t), you want to think of the [A]0 concentration always as 1. So in this case the [A]t would be .5, so it would be ln(1/.5). This would be equal to kt, so then divide by k.

Leah Savage 2F

I thought that if there is a negative order, the overall rate law is indefinite?

Leah Savage 2F

What does it mean that diamond is kinetically stable with respect to graphite but thermodynamically unstable?

Leah Savage 2F

So if a reaction happens really slowly, is this the case where kinetics rather than thermodynamics controls the rxn?

Leah Savage 2F

Why has the activation energy been lowered?

Leah Savage 2F

It's sort of like a limiting reactant, just like a reaction can only go until a reactant is used up; it can only go as fast as its slowest reaction.

Leah Savage 2F

The concentration of products can affect the rate and direction of reactions so it is much easier to evaluate considering only the forward direction.

Leah Savage 2F

Do galvanic cells always have a positive potential difference? If we calculate a negative potential difference, do we need to switch the cathode and anode to get a positive potential difference, even if it means changing the cell diagram?

Leah Savage 2F

Can someone explain how you know which reaction is at the anode and which is at the cathode? Do you assume the one with +.40 V is the anode, so that it can be reversed and still give a positive potential difference?

Leah Savage 2F

In 14.15A, do we have to find what would create the spontaneous rxn (Ecell positive)? Is this why we switch the oxidation and reduction? If so, do we always have to do this?

Leah Savage 2F

Can someone explain why the solutions manual puts Cl- (aq)|Cl2 (g). I thought the notation was reactant|product and Cl2 is the reactant and Cl- is the product since it is a reduction and Cl is reduced from 0 to -2.

Leah Savage 2F

When do you add Pt to a cell diagram?

Leah Savage 2F

Is Gibb's free energy only for nonexpansion work?
What is the relationship between gibb's free energy and work? what kind of system is w=-P(deltaV) for if it's not for reversible/irreversible processes? Irreversible/reversible processes do expansion work, right?

Leah Savage 2F

a) Calculate the standard Gibbs free energies of formation of the halogen atoms X(g) at 1000. K from data available in Table 11.2. (b) Show how these data correlate with the XOX bond strength by plotting the standard Gibbs free energy of formation of the atoms against the bond dissociation energy an...

Leah Savage 2F

But isn't that delta s (universe)=0 for reversible? so the system doesn't have a +delta s necessarily?

Leah Savage 2F

A tank of liquefied propane is punctured, emitting gaseous propane vapors.

Can someone explain why heat is transferred in this process? I'm kind of confused about how volume/pressure/temp would change.

Leah Savage 2F

Are irreversible processes always spontaneous? What about irreversible?

Leah Savage 2F

Can someone explain why ΔG is 0 at equilibrium, and why it is 0 at the temperature of a phase change? For example, in problem 9.91, it says: Using values in Appendix 2A, calculate the standard Gibbs free energy for the vaporization of water at 25.0 C, 100.0 C, and 150.0 C. (b) What should the value ...

Leah Savage 2F

Considering positional disorder, would you expect a crystal of octahedral cis-MX2Y4 to have the same, higher, or lower residual entropy than the corresponding trans isomer? Explain your conclusion.

What are the microstates for the trans isomer?

Leah Savage 2F

Why can you assume 1 mol in problem 9.13? It says: 9.13 During the test of an internal combustion engine, 3.00 L of nitrogen gas at 18.5 C was compressed suddenly (and irreversibly) to 0.500 L by driving in a piston. In the process, the temperature of the gas increased to 28.1 C. Assume ideal behavi...

Leah Savage 2F

Nanotechnologists have found ways to create and manipulate structures containing only a few molecules. However, orienting the molecules in speci c ways to assemble such structures can be difficult. Calculate the entropy of a solid nanostructure made of 64 molecules in which the molecules (a) are all...

Leah Savage 2F

9.19 says: Calculate the standard entropy of vaporization of water at 85 C, given that its standard entropy of vaporization at 100. C is 109.0 J K 1 mol 1 and the molar heat capacities at constant pressure of liquid water and water vapor are 75.3 J K 1 mol 1 and 33.6 J K 1 mol 1, respectively, in th...

Leah Savage 2F

I believe that the entropy of an isolated system would reach a maximum at equilibrium, because there wouldn't be any more huge net changes. However, if the system is not isolated (for example if heat can be transferred from the system to the surroundings), the entropy of the system at equilibrium do...

Leah Savage 2F

If you think of the idea of a spontaneous process, it would make sense that it would often correspond to a positive entropy. The universe always wants to be more disordered and be going to less energy, so this would make a positive delta S and a negative delta H.

Leah Savage 2F

Is it true that the entropy of the universe is always increasing because some form of energy is always released by a reaction, even if it's endothermic?

Leah Savage 2F

8.113 says: Water gas is an inexpensive, low-grade fuel that can be made from coal. (a) Is the production of water gas exothermic or endothermic? The reaction is C (s) + H2O (g) --> CO (g) + H2 (b) Calculate the enthalpy change for the production of 200. L of hydrogen at 500. Torr and 65 C by this r...

Leah Savage 2F

Where does the solutions manual get 800 g to use in q=mc(delta T)?

Leah Savage 2F

Which enthalpies of formation do you use?

Leah Savage 2F

8.3 says: Air in a bicycle pump is compressed by pushing in the handle. If the inner diameter of the pump is 3.0 cm and the pump is depressed 20. cm with a pressure of 2.00 atm, (a) how much work is done in the compression? (b) Is the work positive or negative with respect to the air in the pump? (c...

Leah Savage 2F

8.99 says: Hydrochloric acid oxidizes zinc metal in a reaction that produces hydrogen gas and chloride ions. A piece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800.0 mL of 0.500 m HCl(aq). If the initial temperature of the hydrochloric acid solution is 25 C, what is the fina...

Leah Savage 2F

I believe you use R=8.314 J/(K)(mol). I don't think it really matters which you use; but you must pick the one that cancels with the units you already have in the problem to leave you with joules.

Leah Savage 2F

I'm pretty sure it is because the problem says "temperature rise of 7.32 C" rather than giving a specific temperature in celsius. This temperature rise would have been the same whether the degrees were celsius or kelvin, so either units can be used in the problem.

Leah Savage 2F

So when there are enthalpies of combustion, do you always have to use Hess's Law? What other cases would you use Hess's Law in?

Leah Savage 2F

For problem 57, which says: Determine the reaction enthalpy for the hydrogenation of ethyne to ethane, C2H2(g) 2 H2(g) S C2H6(g), from the following data: Hc (C2H2, g) 1300. kJ·mol 1, Hc (C2H6, g) 1560. kJ·mol 1, Hc (H2, g) 286 kJ·mol 1. Why is this problem solved in the solution manual by using the...

Leah Savage 2F

To add on, in liquid water, hydrogen bonds connect each water molecule to approximately 3.4 other water molecules. When water freezes into ice, it crystallizes into a rigid lattice that increases the space between molecules. Ice is less dense than water because of this.

Leah Savage 2F

Problem 8.31 says calculate the heat released by 5.025 g of Kr(g) at 0.400 atm as it cools from 97.6 C to 25.0 C at (a) constant pressure and (b) constant volume. Assume that krypton behaves as an ideal gas. Can someone explain why the molar heat capacity is different at a constant pressure rather t...

Leah Savage 2F

When temperature rises, the pH of water decreases, but the concentrations of OH- and H3O+ must still be equal, right? Does this mean that pH+pOH only equals 14 at 25 degrees celsius?

Leah Savage 2F

Can someone explain what Dr. Lavelle meant Friday when he explained that a weak acid with an H3O+ equilibrium concentration smaller than 10^-7 will still have a pH of 7? I'm confused because I thought that pH would rise with a smaller amount of acid.

Leah Savage 2F

In number 8, it says, will the equilibrium constant K increase, decrease, or remain constant after the following changes to the exothermic reaction 4A(g) + 3B(g) ⇌ 2C(g)? When the stoichiometric coefficients are doubled, it says increase or decrease, but I thought that it would definitely decrease? ...

Leah Savage 2F

Oh oops! Thanks so much!

Leah Savage 2F

In the XeO2F2 lewis structure, why aren't there double bonds to both oxygens? If there were, Xe and O would both have formal charges of 0. But in the real formal charge, only one O has a double bond and the other one has a formal charge of -1 and Xe has a formal charge of +1.

Leah Savage 2F

In (ICl2)-, I thought that the lewis structure would put a double bond between I and one of the Cl's, so that the Cl would have the negative formal charge because it is electronegative. But the real lewis structure puts 3 lone pairs on I and then single bonds to each Cl. This gives I the negative fo...

Leah Savage 2F

Why does N have a lower electron affinity than C, but not a lower electronegativity?

Leah Savage 2F

Is it always better for all the atoms in a molecule to have an octet and possibly have formal charges than for them to have 0 formal charge but more than an octet? For example, in O3, or ozone, is the reason that there aren't 2 double bonds because the central O would have more than an octet? Becaus...

Leah Savage 2F

Does the addition of an inert gas ever impact the equilibrium? Can it change the volume of a system?

Leah Savage 2F

Would we be expected to know this without having the pkb or pka or kb or ka provided? There are a lot of problems where you have to order strength of acids/bases. I know there are rules for binary acids and oxoacids, but what about for a problem like this one?

Leah Savage 2F

12.53 asks: Suggest an explanation for the different strengths of (a) acetic acid and trichloroacetic acid; (b) acetic acid and formic acid. For B, I don't really understand why formic acid is stronger than acetic acid. The solutions manual says that the -CH3 in acetic acid has electron-donating pro...

Leah Savage 2F

If a molecule is polydentate, does that mean that it can chelate as well? Is something only polydentate if it's multiple bonding sites can bond to the same metal?

Leah Savage 2F

Can someone explain why this is true? "If one starts with higher concentrations of reactants, the equilibrium concentrations of the products will be larger." I don't really understand what they mean by "the equilibrium concentrations of the products will be larger." Larger than t...

Leah Savage 2F

I don't know if it's extremely important, but I think it's good to generally know that bond order refers to the number of chemical bonds between a pair of atoms.

Leah Savage 2F

I think the bond angle always refers to the angle between two bonded atoms, which can be influenced by lone pairs on the central atom.

Leah Savage 2F

It's definitely helpful to know the overall electron geometry for each molecule, though, because it can allow you to better see the overall shape that is manipulated when atoms are replaced with lone pairs!

Leah Savage 2F

The T-shape shows up twice, once for a molecule with shape AX3E2, and once for a molecule with shape AX3E3. Both times the axial plane is made up of atoms, and the equatorial plane includes one equatorial atom and one or two lone pairs. This creates a t-shape for both, since the 2 axial and 1 equato...

Leah Savage 2F

Generally you draw a lewis structure and then do some trial and error until you find a lewis structure with the lowest overall formal charge possible! That will be the most stable/lowest energy.

Leah Savage 2F

[Ar] 3d6 for an 3+ cation means that 2 4s valence electrons plus one d electron were taken away. This makes sense for Co, which normally is 3d7 4s2. Two 4s electrons and one d electron was taken away from Co3+. For Fe 3+, two 4s electrons and 1 d electron were taken away leaving 3d5, so this must be...

Leah Savage 2F

Energy increases more drastically as you go down the periodic table and increase quantum numbers, but also increases across the periodic table. D orbitals have more energy than s orbitals when they are unfilled, but once they are filled, they have less energy. In the electron configuration, 3d comes...

Leah Savage 2F

I don't really understand why you use 10 for the uncertainty in position rather than 5. Can someone explain?

Leah Savage 2F

The periodic table follows quantum numbers, which makes it really easy to read it. You know that the row number corresponds to the quantum number which corresponds to the energy level, so like 1s, the first row, is the first energy level. This means that as you go down the periodic table, row to row...

Leah Savage 2F

You are always looking for either the uncertainty in position or the uncertainty in momentum (mass times velocity) so when the question asks for one of these, you use Heisenberg Indeterminacy. If you have momentum then you know the mass times velocity so you don't need to know the mass.

Leah Savage 2F

When they say "longest wavelength" they're referring to the longest wavelength that would have the threshold energy, or the minimum energy needed to remove an electron from a metal. That means that the kinetic energy would be 0, because there would be no excess energy as the incoming energ...

Leah Savage 2F

Remember that intensity means different things for waves and for particles. Intensity of a wave corresponds to the amplitude of a wave, whereas increasing intensity for particles is just increasing the numbers of particles. If you have 400 photons and none of them are ejecting electrons and you up t...

Leah Savage 2F

In terms of electromagnetic radiation and energy levels, quantized means that you can't jump from energy level 1 to energy level 1.5. You can only jump from 1 to 2, 1 to 3, 2 to 3, 2 to 4, etc. The energy levels are set in stone and you have to apply enough energy for an electron to jump an entire l...

Leah Savage 2F

Remember that all types of radiation have a range of wavelengths that are applicable. For microwaves, it is from 1 mm to 1 m. There is no one constant wavelength for a type of electromagnetic radiation.

Leah Savage 2F

A lot of times questions will ask for the percent composition before asking for the empirical/molecular formula. In this case, then it's obviously to your best advantage to find the empirical/molecular formula with the process you described, assuming 100 grams and then putting into moles and going f...

Leah Savage 2F

Molarity is the number of moles per liter. There are 2 atoms of Na for every 1 atom of Na2CO3, meaning that the molar ratio between Na and Na2CO3 is 2:1. This means that the molarity, also, should have a ratio of 2:1 if the volume is constant. A way to think about this problem is to find the molarit...

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