CHEMISTRY COMMUNITY

Emily Mei 1B

Unique rate is 1/a*(d[A]/dt)

Emily Mei 1B

In question 9.103 it asks A scientist proposed the following two reactions to produce ethanol, a liquid fuel: C2H4 (g) + H20 (g) -> CH3CH2OH (l) C2H6 (g) + H20 (g) -> CH3CH2OH (l) + H2 (g) Assume standard-state conditions and determine if either reaction is thermodynamically spontaneous. Does anyone...

Emily Mei 1B

Can anyone explain why kinetics controls reactions at lower temperatures, but not higher ones? The explanation doesn't make sense in the book (reference 15.79).

Emily Mei 1B

It's a good example of a substitution reaction.

Emily Mei 1B

Probably just with one temperature since that's all we've discussed in class (and that's all that's on the constants sheet now).

Emily Mei 1B

Either way, I'm pretty sure the 2nd order half life equation is on the constants and equations sheets on our tests.

Emily Mei 1B

The first few homework problems are also solid practice for determining reaction order.

Emily Mei 1B

Also you can think about k as the slope of the graph for (in this case) ln[A] vs t. The slope of the line will be -k.

Emily Mei 1B

For a 1st order reaction, k units = s^-1
2nd order reaction, k units = L/(mol*s)
and increasing the powers of L and mol as you go up in reaction order so that rate will be in unit of mol/(L*s)

Emily Mei 1B

Any of the integrated rate laws relate reaction rate to time.

Emily Mei 1B

Josh Moy 1H wrote:A proper first order graph is just a straight decreasing line?

Yes, if you're graphing ln[A] vs time. All the "proper" graphs are going to look like straight lines, but the order depends on what's on the y-axis.

Emily Mei 1B

^And because we assume this, we also assume that products are being removed from the system in order to ignore the reverse reaction.

Emily Mei 1B

Half reactions have 6 electrons, which is 6 mols of electrons, so n=6.
Red: 6Fe3+ + 6e- --> 6Fe2+
Ox: 2Cr3+ --> Cr2O7 2- + 6e-

Emily Mei 1B

The skeleton equation was Au+(aq) + --> Au(s) + Au3+(aq). Can someone explain why the the anode half reaction is Au3+(aq) + 3e- -> Au(s)?

Emily Mei 1B

Seems like it. In the textbook when they give us skeletal equations, they're also not balanced for atoms.

Emily Mei 1B

They're different -- q is heat, and deltaH is change in enthalpy, so they shouldn't be interchangeable.

Emily Mei 1B

Not sure what you mean by "stable", but larger compounds are typically heavier, which means they'd have a larger degeneracy and therefore more entropy. So Fe3O4 should have a higher entropy.

Emily Mei 1B

If you're calculating heat, I think you're always supposed to include the phase change?

Emily Mei 1B

It is saying a has more molecules, and more molecules in a container means more entropy change.

Emily Mei 1B

If there are more molecules, there can be more movement, so more entropy change.

Emily Mei 1B

Which would you expect to have a higher molar entropy at T = 0, single crystals of BF3 or of COF2? Why?
I don't really understand the explanation at the back of the book for this one.

Emily Mei 1B

The 2nd law of thermodynamics relates entropy change to heat and temperature, and part of what we now know is that if heat is constant, entropy change will be greater at lower temperatures than at higher ones.

Emily Mei 1B

Example 9.2 on page 322 of the textbook also talks about this if you want a textbook reference!

Emily Mei 1B

For 8.99, why do we have to calculate the limiting reagent? The problem is as follows: 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 ini...

Emily Mei 1B

Also, when we discuss enthalpies, we're talking about heat released or absorbed in chemical reactions.

Emily Mei 1B

Also for reference, an open system can exchange both matter and energy with its surroundings (e.g. a car engine).

Emily Mei 1B

In 14A the conversions were on the constants sheet given to us on tests, though I can't say whether we'll have to memorize them for this class.

Emily Mei 1B

We can use bond enthalpies to calculate the deltaH of a reaction as well.

Emily Mei 1B

Also if you find that you're missing more than one variable, it's possible that the specific heat value can be found in a table in the reading (possibly 8.2).

Emily Mei 1B

Another example of an isolated system from the textbook is a hot liquid poured into a thermos/container with insulated walls.

Emily Mei 1B

Because it's asking for the [OH-] after the person diluted the 5.00mL of the stock solution to 500 mL.

Emily Mei 1B

Does anyone have any tricks for remembering VSEPR shapes?

Emily Mei 1B

Do I use mole ratio when solids are in equation to determine which way it will favor. Example, A(s) <--> B(g) + C(g) If volume is decreased, would I say the reaction favors reactants because mole ratio is (0:2) I would say that if the volume is decreased, it means the molarity of the products goes ...

Emily Mei 1B

I recall in Friday lecture, Dr.Lavelle was talking about how if the concentration of H3O+ is less than 10^-7, the solution then the solution is considered neutral. He brought up numbers like 0.0000001 M H3O+ and 0.0000000001 M H3O+ and he said that if you take the negative log of the smaller number...

Emily Mei 1B

In question 11.37, eq.3 = 1/2eq1, and the K of eq.3 = (K of eq.1)^1/2. Eq.4 = 2eq.1, and K of eq. 4 = (K of eq. 1)^2. Is this pattern actually a rule?

Emily Mei 1B

In lecture we learned that K remains constant unless the temperature changes, so its graph would look like a straight line. You might be able to plot Q over time and compare the values to K at each given time.

Emily Mei 1B

The roman numeral is the oxidation state of the transition metal. You determine that based on the overall charge of the compound, and on the individual charges of everything else in the compound.

Emily Mei 1B

In class we assumed that the initial ADP and Pi concentrations were ≈0, and ignored the sig figs. How would taking the initial concentrations into account change the way we did the problem?

Emily Mei 1B

The question asks out of 3 sample molecules which one has the largest dipole moment. How do we determine relative dipole magnitudes based on structure without numbers?

Emily Mei 1B

@Leah No, it's not. So it's asking us to consider that the work functions of metal are unique to each metal, and that increasing the frequency still may not overcome that specific metal's work function.

Emily Mei 1B

The shape is trigonal planar because you determine molecular shape by the number of bonding pairs on the central atom. When you then factor in the lone pairs, they bonded e- and lone pair e- repel each other and the molecular shape becomes T-shaped, changing the angle to less than 90º.

Emily Mei 1B

Not sure about the types of nodes, but in one of the review sessions I remember one of the TAs saying something about just knowing what the electron density around a node is.

Emily Mei 1B

I think trigonal planar

Emily Mei 1B

In the 9/4 Fundamentals review session, we went over problem M.22 from the textbook, which reads: A compound produced as a by-product in an industrial synthesis of polymers was found to contain carbon, hydrogen, and iodine. A combustion analysis of 1.70 g of the compound produced 1.32 g of CO2 and 0...

Emily Mei 1B

In class today, we said the sulfate ion with the double bonds was the best representation based on formal charge. Why was this?

Emily Mei 1B

What is important/what do we need to know about nodal planes in the atomic orbitals?

Emily Mei 1B

As you go down and left across the periodic table, atomic radius will increase. I think the reason the radius increases as you go down is because quantum numbers increase, and therefore valence electrons will begin to occupy higher levels. As the quantum number increases, electrons are held further ...

Emily Mei 1B

Would anyone be able to explain/go through the derivation of the De Broglie equation? I know we don't necessarily need to know it but it would help my understanding.

Emily Mei 1B

If molybdenum is irradiated with 194 nm light, what is the maximum possible kinetic energy of the emitted electrons? We're given in the last problem that that Molybdenum metal absorbs radiation with minimum frequency of 1.09x10^15 s^-1. I also calculated the minimum energy (E=hv) is 7.22 x 10^-9 J. ...

Emily Mei 1B

The numbers involved in Rydberg's formula v = R{(1/n1^2)-(1/n2^2)} make sense to me, but, in words, what does the formula tell you?

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