CHEMISTRY COMMUNITY

Minie 1G

A is the pre-exponential factor. It is the fraction of molecules that would react if the activation energy was zero (if Ea is 0, e^0/RT would be 1 so your arrhenius equation would be k=a).

A is usually determined with experimental data, since it's different for different reactions.

Minie 1G

the oxidizing power increases as you go to the right and up (similar to electronegativity). The reducing power is the opposite, with elements like lithium being a strong one.

Minie 1G

you use the deltaG=-nFE equation.
Fe2+ + e- -> Fe2+ deltaG1 = -(1)(F)(0.77)
Fe2+ + 2e- => Fe(s) deltaG2 = -(2)(F)(-0.44)

Add them up, then solve for E.
deltaG1+deltaG2 = -(3)(F)(E).

Which is -0.037 V.

Minie 1G

LEO the lion says GER.

a loss of electrons is oxidation, so the oxidizing agent is what makes the loss of electrons happen, aka what gets more electrons by oxidizing something else.

The reducing agent is what causes the gain of electrons, so it gives up electrons to reduce something else.

Minie 1G

pre-equilibrium: basically when the process is not at equilibrium. Since the rate of reaction measures how fast the reaction progresses UNTIL equilibrium (before it reaches equilibrium)

Minie 1G

you can think about it this way... the equilibrium constant k is basically products/reactants. If the activation energy is super high, then it's harder to form products since we have to first overcome this barrier. So when the numerator is smaller, k is smaller.

Minie 1G

The rate determining step would be the slowest step of the reaction. You would calculate the rate based on the slower step.

Minie 1G

Well, we could possibly be asked to calculate the order of the reaction with respect to a certain substance given a table of concentrations.

Minie 1G

Also, as general comprehension of formulas, we should know the theory behind where they came from.

Minie 1G

Dang girl you are a 10.

On a pH scale.

Because you are basic.

Minie 1G

Since it 10^-3 mol is the same as 1 mmol, it's the same thing. Convention maybe just wanted the answer to be in standard moles.

Minie 1G

14.107 What range (in volts) does a voltmeter need to have to measure pH in the range of 1 to 14 at 25 C if the voltage is zero when pH = 7?

When using the formula E=RT/nF lnK, why is n = 1? Is it always this way, or is there something in this problem specifically that tells us we have one mol?

Minie 1G

I think by convention you put the anode first, because then when you read it from left to right your eyes follow the flow of electrons from anode to cathode. So, oxidation (anode) -> reduction (cathode) site.

Minie 1G

Yeah, what's given to us is the cell potential. The question is asking for the M2/M couple, which is the cathode. So rearranging the potential = cathode-anode will give us the correct answer, -0.689+0.34=-0.349 V.

Minie 1G

Water is only ice below 0 degrees, so we would use the specific heat of ice below that and then include the heat of fusion at 0 degrees then the specific heat of water above 0 degrees. So we use ice -> phase change -> water. Your delta temperature for the ice should have 0 as the final, and the delt...

Minie 1G

the NERNST equation can be written with both ln and log, there's just an added constant for log. So for ln: E = E-RT/nF(lnQ) but for log: E = E-2.303RT/nF(logQ)

Minie 1G

most of the times in calculations it's delta s, because we are measuring the entropy of some reaction and therefore take the entropy of the products minus reaction (thus a change in entropy).

Minie 1G

I don't know what "five steps of enthalpies we have to calculate" are but number 7 was about making iced-tea. So the enthalpy of fusion from ice melting is 30grams*(1mol/18.01grams)*(6010Joules/mol) which you add to the heat(q=mcDeltaT) of your now water to get the total q_ice. for referen...

Minie 1G

The answer is actually positive,
100/(20+273.15)=0.341 Joules/(Kelvin second)

Minie 1G

As given, is there no way to figure out what deltaG is? In order to do this problem, is the only way to search up the deltaG of the entire reaction by some appendix?

Minie 1G

So how would you tell whether the equations, A and B, are spontaneous or not?

Minie 1G

How can we determine if a reaction is spontaneous by just looking at it? Especially in the context of this case. 9.103 A scientist proposed the following two reactions to produce ethanol, a liquid fuel: C 2H4(g) + H2O(g) -> CH3CH2OH(l) (A) C 2H6(g) + H2O(g) -> CH3CH2OH(l) + H2(g) (B) Assume standard...

Minie 1G

Both probably, but it's always helpful to understand concepts before you can carry out computations.

Minie 1G

Free energy is like all the energy in the universe available to be released or consumed through reactions taking place. The Gibbs free energy is a measure of "usable" energy in the system. Basically how much energy you have available to do work. So when we have deltaG, we are looking at th...

Minie 1G

it tells us to assume ideal gas conditions, which means we assume 1 mol of gas.

Minie 1G

not all isothermal reactions are reversible. for a thermodynamic process to be reversible, it must maintain equilibrium with its surroundings at all instances. So, the process must progress slowly without acceleration. The expansion of an ideal gas into a vacuum is isothermal. It is expanding into e...

Minie 1G

Here's one proof, scroll down to the second half of the page for Cp (the first half is Cv) http://physics.bu.edu/~redner/211-sp06/class24/class24_heatcap.html and also you might ask where the Energy = 3/2 * nRdeltaT comes from, which is explained conceptually in the following video: https://www.khan...

Minie 1G

An open system freely exchanges energy and matter with its surroundings. The simplest example of this is boiling water, where both water can leave in the form of vapor and it gives off heat. A closed system only exchanges energy with its surroundings, so this is like boiling water with a lid on. You...

Minie 1G

basically, nothing about the isothermal reversible reaction calculations.

Minie 1G

Heat transfers from a higher point of temperature to a lower one. So look at a problem and determine if the system losing heat or gaining heat.

Minie 1G

isolated systems always go towards thermodynamic equilibrium, and that is a state with maximum entropy. An example is if you have an ice cube in a room, and the room is an isolated system, the total entropy inside the room will increase as the ice melts. The system is exchanging heat within itself a...

Minie 1G

mercury in a thermometer: no matter can escape, but heat can. We can see that when the temperature is hot the mercury expands, causing the thermometer to have a higher reading. The mercury is affected by the heat of the surroundings. That's a simple explanation. But no matter escapes because as we s...

Minie 1G

Also, Dr. Lavelle noted that for our calculations we can kind of equate atm and bar since they are so similar in value.

Minie 1G

Or, you can always do dimensional analysis and see which units cancel out when calculating.

Minie 1G

Yeah, also stable bonds like staying as such so it would take more energy to break them. A bond with a weak enthalpy would be able to dissolve without much input.

Minie 1G

There is a question on the final practice test, number 22, that asks how many O2 molecules can myoglobin bind and how many myoglobins form a hemoglobin. I know biologically the answer is each myoglobin can bind one oxygen and hemoglobin is composed of 4 myoglobins, but what is the chemical explanati...

Minie 1G

I think you might be mistaken, HCl is a stronger acid than HF.

There are 7 common strong acids and HCl is on that list, HF is not.

Minie 1G

For part a, the conjugate base to HClO3 is ClO3-. The lewis structure has a central Cl atom, connected to two oxygen atoms with double bonds and a third oxygen with a single bond. The double bonded oxygens have 2 lone pairs each while the single bond connected oxygen has 3 lone pairs (6 total lone e...

Minie 1G

K is Kp, the equilibrium constant based on the partial pressures. The relationship between Kc and Kp is Kp = Kc(RT)^delta(n). This can be derived from the ideal gas equation (PV=nRT) by letting P = nRT/V (and n/V is the concentration, so this is P = [gas]RT) and plugging that in to the Kp equation (...

Minie 1G

You just have to use the same constant for all your calculations. Since the units eventually cancel out to render K unitless, it doesn't really matter as long as it's all the same (so don't use atm for one molecule and bar for another, but if you use atm for all then all the atms cancel anyways). Si...

Minie 1G

Ferrous means iron. Ever notice that the symbol for iron is Fe instead of like... Ir (actually iridium)? It's derived from latin. Another example is Copper, which you might expect to be Co (that's actually cobalt) but Copper is actually Cu... stands for cupric, also latin for copper. Tin is another ...

Minie 1G

Polydentate ligands is basically when you bind a bunch of these given molecules to a central metal atom or ion. So how many bonds does each molecule make with the metal? I'll give the easiest example, part c, which is water. This is monodentate because only the oxygen atom is a donor atom that has e...

Minie 1G

Aren't you all glad we have this chemistry community forum? It makes 14A less of a cheMYSTERY for us all :)

Minie 1G

You know that the Si-O bonds are polar because of their location on the periodic table. We know that C-O bonds are polar, since oxygen is more electronegative than carbon, and Silicon is in the same group as carbon -> Si-O bonds are also expected to be polar.

Minie 1G

The bond angle for something is the angle between your central atom and the two directly attached to it. For example, in water the bond angle is the angle created by H-O-H. For a trigonal bipyramidal molecule like PF5, the bond angles are either 120 or 90 degrees. See the attached graphic... It's ea...

Minie 1G

And when drawing the arrows for dipole movements, it always goes from the less electronegative atom -> more electronegative atom. It's a measure of polarity, and the large electronegativity difference, the larger dipole movement force.

Minie 1G

When waves run into each other they either contructively build or destructively become weaker. So when we measure "how much" wave we get at each location, we get a diffraction pattern that shows where waves added up and where they canceled out. The following gif representation of how diffr...

Minie 1G

You can look for either lone pair electrons or polar bonds. If we take two molecules, CO2 and H2O which may both seem to be linear in shape because they have 3 atoms each, we find that actually CO2 is linear while H2O is bent. Because the lone pair electrons on water repel the hydrogens and force a ...

Minie 1G

Many chemical properties are related to size of an atom or how many protons/electrons it contains. For two elements diagonal to each other, they differ by one group (vertical column of periodic table) and share similar characteristics. Size decreases when you move from left to right, and increases w...

Minie 1G

Yeah, add or subtract the amount of electrons (add one electron for 1-, add two for 2-, remove one for 1+, remove 2 for 2+, etc.) then put your whole molecule in brackets [these things] and put the charge in the corner so ammonium would be [NH4]+ (except it would be nitrogen as the central atom with...

Minie 1G

With the loss of an electron, the remaining electrons in a cation are drawn closer to the nucleus. With the gain of an electron, the electron cloud of anions grows larger because electrons repel each other. Generally, increasing electrons (increasing negative charge) leads to increased ionic size an...

Minie 1G

[Ar](3d^5)
You lose electrons from the largest number shell first (4th shell) here
and if you had even less electrons like it was Cr3+ then it would be [Ar](3d^3)

Minie 1G

A double derivative is the same as a second derivative. Find the derivative of your function. Then, take another derivative -> you now have a double derivative.

Minie 1G

Where can I find Bruincast for 14A? I know that they are on CCLE under the media tab but the CCLE links to Lavelle's website not the CCLE chem14A website... so anyone know where it is??

Minie 1G

Also the f orbital. So if the p orbital is two lobes, aka two ellipses, the d orbital would be (generally) like a double p with 4 lobes. And the f is (generally) like a triple p with 6 lobes (because one p has 2 lobes, so triple that would be 6 total). It's also arranged similarly, 6 ellipsoids orie...

Minie 1G

Yeah, the wavelength exists but is very small and imperceivable. The entire concept of De Broglie's is to propose that all matter has wave-like properties, such as wavelengh. It is usually applied to describe the wave-like nature of an electron, but can still be used for larger objects. Just doesn't...

Minie 1G

You can still turn in questions from chapter 1 as long as they're not the same as the ones you turned in last week. Since the email Lavelle sent out told us specifically which hw problems would be covered on the test, doing the hw from this section would be good preparation.

Minie 1G

Back to your original question, I understand it like this. You cannot use the Ek = 0.5mv^2=hc/lambda because we are calculating the wavelength for an electron, and there is a given velocity (3.6x10^3 km/s, in the problem) for this. Electrons don't travel at the speed of light (the constant c, in the...

Minie 1G

If we used 250 mL of stock solution as you said, we would have 250 mL of 2.00 M NaOH. This is not what we want. What we want is 250 mL of 0.650 M NaOH. The stock solution is the solution calculated in Q2A. What you know about this stock is that it is 2.00 M NaOH, as given in the problem. So given th...

Minie 1G

Yes, I think you're right. I mean, would you want to (or could you) read under either UV or infrared light rays? They aren't even visible to the human eye.

Minie 1G

When calculating for the limiting reactant, you need to not only find how many moles of each (cause usually you're given the reactants in grams of mass) you have, but how many moles of one reactant is needed in relation to the other reactants. For example, if your equation is X + 3Y -> 2Z then every...

Minie 1G

Often, the problem will ask how much product is formed given two amounts of reactant. In this case, straight up calculating and comparing how much product would be formed from using all of each reactant would simultaneously give you the answer to how much product you actually get. Then, you can simp...

Minie 1G

I see that the syllabus says all tests and exams must be written in pen. But for the 4 HW problems that will be collected each week in the discussion, does anyone know if these can be done in pencil? I already did them in pencil, so I want to know if I have to transcribe them over to pen...

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