CHEMISTRY COMMUNITY

Ashley Fang 2G

Do you have 11 protons?
Because you're sodium fine!

Ashley Fang 2G

I heard that Oxygen and Magnesium were going out and I was like OMg

Ashley Fang 2G

The Arrhenius Equation is k=Ae^-Ea/RT
So if you take the ln of that you will get lnk = -Ea/RT + lnA
If you have 2 reactions, subtracting lnk₁ from lnk₂ will get you:
lnk₂ - lnk₁ = -Ea/RT₂ -(-Ea/RT₁) + lnA - lnA
Which will simplify to:
ln(k₂/k₁) = -Ea/R (1/T₂ - 1/T₁)

Ashley Fang 2G

I don't believe so, just know the Arrhenius Equation in regards to chemical kinetics:
K=Ae^-Ea/RT where A is the frequency factor or pre-exponential factor (indicates # collisions with correct orientation)

Ashley Fang 2G

Microscopic reversibility means that the reaction has the same intermediates (same pathway) in both the forward and reverse directions.

Ashley Fang 2G

PranaviKolla2B wrote:Does someone have a good video explaining how to find a first order reaction vs a second?

These videos explain reaction orders pretty well!

https://www.youtube.com/watch?v=2LMdj91x2HA
https://www.youtube.com/watch?v=VXfbzwAv2Dc

Ashley Fang 2G

What are characteristics that indicate a first order reaction If the reaction rate increases proportionally with the reactant concentration, then it's a first order reaction (assuming there's only 1 reactant) because the exponent is 1 in the rate equation. Rate = k[A] n For example, if the reaction...

Ashley Fang 2G

It depends on the rate equation.
Rate = k[A]ⁿ[B]^m
So if the overall rxn order is 1 (n+m), then k would be in units of 1/s since you're trying to get the rate units to equal mol/L-s.
If the overall rxn order is 2, then k would be in units of L/mol-s, etc.

Ashley Fang 2G

Both of them are correct but the first one is preferred because the generally accepted unique rate equation is -1/a d[A]/dt = 1/b d[B]dt = 1/c d[C]/dt
In the second one it's harder to see the relationship between the coefficients.

Ashley Fang 2G

Given multiple experiments with the initial concentration of reactants and the initial rate listed, you can divide one experiment over the other to determine how changes in concentration affect the rate. For example, if doubling the concentration of one reactant doubles the rate, then we know that t...

Ashley Fang 2G

Yes, work (max) equals delta G under constant temperature and pressure.

Ashley Fang 2G

The n comes from the equation ΔG = -nFE which is used to derive the nernst equation.
Since F is in units of C/mole, multiplying that by n (moles) would cancel out the units.

Ashley Fang 2G

Therefore, it's a property of matter that does not change even if the amount of matter changes.

Ashley Fang 2G

O2 + 4H+ + 4e- -> 2H2O E(anode)=1.23V
O2 + 2H2O + 4e- -> 4OH E(cathode)=0.40V
Reverse the first equation since it's the anode and add the two equations together
You should get 4H2O -> 4H+ + 4OH- which simplifies to
H2O -> H+ + OH-
E(cell) will be 0.40V - 1.23V = -0.83V

Ashley Fang 2G

Yes. By definition, the oxidation number of an atom is the charge on that atom.
I don't believe there is a difference, it's just that we conventionally use the term "oxidation number" when talking about redox reactions.

Ashley Fang 2G

You can see how entropy increases with an increase in volume in the equation ∆S = nRln(V2/V1)
Similarly, entropy also increases with an increase in temperature in the equation ∆S = nCln(T2/T1)

Ashley Fang 2G

You would use delta H = nC(delta T) for each of the reactants or products.

Ashley Fang 2G

For monoatomic gases, Cv would be 3/2 R and Cp would be 5/2 R.
For linear/diatomic molecules, Cv would be 5/2 R and Cp would be 7/2 R.
For nonlinear molecules, Cv would be 3R and Cp would be 4R.

Ashley Fang 2G

To clarify, for phase transitions you use the equation q=n(deltaH) and for heating you use the equation q=mC(deltaT).

Ashley Fang 2G

To further elaborate on that,
1) the enthalpy change of the ice melting would be n(delta H)
2) the enthalpy of the rising the ice temperature from 0C to 20C would be mC(deltaT)

Ashley Fang 2G

You can also alter the concentration of products to reactants in order to change the G value. if you increase the amount of product, the lnK part of the expression will become greater, making the G value more negative. Delta G will be negative, thus the reaction will be more spontaneous. If you decr...

Ashley Fang 2G

Adding on, a reaction can be spontaneous even when delta S is negative if delta H is negative enough, as long as the delta G value is negative.
delta G = delta H - T(delta S)

Ashley Fang 2G

We need to know the concept of Gibbs free energy because it tells us whether a reaction will be spontaneous or not. If delta G is negative, the reaction is spontaneous, and the products will have less free energy than the reactants. If delta G is positive, the reaction is NOT spontaneous, and the pr...

Ashley Fang 2G

Also, it's 3R for a nonlinear molecule at constant volume.
At a constant pressure, it's 5/2R if monoatomic, 7/2 if diatomic, and 4R if the molecule is nonlinear.

Ashley Fang 2G

Heat capacity is the amount of heat energy required to raise the temperature of a substance by 1 degrees C. It is extensive because it depends on how much mass there is. The units are in J/C. Specific heat capacity is the heat capacity divided by a certain amount of mass. It is intensive because the...

Ashley Fang 2G

Here's my work if that helps!

Ashley Fang 2G

You use the integral in a pressure vs volume graph to find the work of expansion Since W=-PdeltaV, calculating the integral would give the area under the line (W) For irreversible expansions, just calculating the rectangular area would be enough But for reversible expansions, you have to use the int...

Ashley Fang 2G

Work off of the variables you're given. As long as the units cancel out, you should be fine!

Ashley Fang 2G

The Boltzmann Equation is: S = K_BlnW
It shows the relationship between entropy and the number of ways the atoms or molecules of a thermodynamic system can be arranged.

Ashley Fang 2G

The first law of thermodynamics is the conservation of energy. The second law of thermodynamics is that the entropy of any isolated system always increases. The third law of thermodynamics is that the entropy of a pure crystalline substance at absolute zero is 0. I think just keep these laws in mind...

Ashley Fang 2G

I'm not sure, but here's the equations sheet if you want to take a look at it!
https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

Ashley Fang 2G

I did heat released by metal = -heat absorbed by water
20.0 x 0.39 x (100-T) = -50.7 x 4.18 x (T-22)
and got T = 24.8 C or about 25 C

Ashley Fang 2G

Other examples of diatomic molecules that have zero standard enthalpy include H2, N2, F2, (O2), I2, Cl2, Br2 (Have No Fear Of Ice Cold Beer)

Ashley Fang 2G

To add to that, yes going down a phase is exothermic because from gas to liquid, water would release a great deal of energy. This is the reason for severe steam burns that Dr. Lavelle was talking about last week in class. It's also why sweating (evaporation of liquid water to gas) cools you down bec...

Ashley Fang 2G

Examples of combustion would occur in this format:

Ashley Fang 2G

It makes sense that as temperature increases, so does enthalpy if we refer back to the equation:
delta G = delta H - T(delta S)

Ashley Fang 2G

Yes, if a reaction is exothermic, it will always release energy and delta H will always be less than 0.

Ashley Fang 2G

I believe it's shown in this diagram

Ashley Fang 2G

When delta H is positive, is it endothermic or exothermic? And if H is negative? If delta H is positive, that means the sum of the enthalpy of reactants is LARGER than the sum of the enthalpy of products. This means that the reaction is endothermic, or uses heat. If delta H is negative, the sum of ...

Ashley Fang 2G

There is a general rule that thermodynamic properties that are extensive are written in capital letters: V (volume), C (heat capacity), etc. Properties that are intensive are written in lower case. Exceptions are temperature and pressure which are generally represented by upper case letters T and P,...

Ashley Fang 2G

I don't think so either, but in case we do it's this:

Ashley Fang 2G

We never use pure solids (s) or liquids (l) in calculating equilibrium, but we do use gaseous (g) and aqueous (aq) substances. Maybe he was referring to the liquid concentration in the aqueous substance.

Ashley Fang 2G

First find the initial concentrations of A and B by dividing moles of A and B by the overall volume (6L). Then plug this information along with the equilibrium concentration of A into the ICE table. Assuming they give you the equilibrium constant, you should then be able to find x, which will in tur...

Ashley Fang 2G

When the pressure of a system is decreased, a quick way to determine which direction the reaction will shift is seeing which side has the greater number of moles. The side with the greater number of moles will be favored. Conversely, when the pressure of a system is increased, the side with the fewe...

Ashley Fang 2G

DesireBrown1J wrote:For c, would the 2nd container with 0.50 mol O₃ have a larger ratio of P_O2/P_O3 because it has a greater overall partial pressure?

I'm not completely sure as I think we would need more information in order to calculate the ratio P_O2/P_O3

Ashley Fang 2G

To add to that, by solving for Q and comparing it to K, you can see what direction the reaction will proceed.
If Q<K, the reaction will proceed to the right (products side).
If Q>K, the reaction will proceed to the left (reactants side).
If Q=K, the reaction is at equilibrium.

Ashley Fang 2G

Bryce Barbee wrote:Can someone please explain to me how to ask a question on Chem Community. I cannot figure it out. Thanks

Go back one page and scroll to the top. You should see a "new topic" button with a star next to it

Ashley Fang 2G

For gases, we can use either pressure or concentration (Kp or Kc), but for aqueous solutions, we can only use concentration to find Kc.

Ashley Fang 2G

The chemical activity of a compound is equal to [compound]/[compound] ° where ° stands for its standard value. This usually cancels out to equal 1. Therefore, we just leave out the chemical activity of a compound when writing out the Kc equation and just use the values for concentration (ignoring th...

Ashley Fang 2G

PO2/PO3 will be different because the ratio of the reactant to the product is different at equilibrium point for different amounts of starting reactant.
Only (PO2)^3/(PO3)^2 or (PO3)^2/(PO2)^3 are guaranteed to be the same because they are the equilibrium constant and the inverse of it.

Ashley Fang 2G

Chemmybear also has links to a bunch of resources, strongly recommend!

https://chemmybear.com/

Ashley Fang 2G

Similarly, conjugate acids of strong bases are weak.

Ashley Fang 2G

A conjugate acid is a base with a hydrogen ion added to it.
A conjugate base is what is left over after an acid has donated a proton during a chemical reaction.

Ashley Fang 2G

Polyprotic acids are acids that can lose more than one proton per molecule in acid-base reactions. Examples: H2SO4, H2CO3 Polyprotic bases are bases that can accept more than one proton per molecule in acid-base reactions. Examples: [SO4]2-, [CO3]2- In titration curves, they will have multiple equiv...

Ashley Fang 2G

It can only act as an acid or a base one at a time.
An example would be water either accepting or donating a hydrogen ion.

Ashley Fang 2G

An amphoteric compound can be either an acid or a base, but only one at a time.
It can either accept or donate a hydrogen ion.

Ashley Fang 2G

Here's a diagram to make it easier to visualize

Ashley Fang 2G

To determine its shape from using VSEPR, you would just have to count the number of bonding pairs and lone pairs it has and match it to its corresponding molecular geometry.

Ashley Fang 2G

It can also be defined as half of the difference between the number of bonding and antibonding electrons.

Ashley Fang 2G

Yes, this is because octahedral structures have 6 electron domains

Ashley Fang 2G

In the case of CS2, the S atoms have 1 double bond with C and 2 lone pairs, so it would also have 3 electron domains and yes it would be hybridized to sp2 as well.
As for terminal atoms that are halogens, it just depends again on counting the number of electron domains the atoms have.

Ashley Fang 2G

Think like a proton and be positive!

Ashley Fang 2G

hahaha

Ashley Fang 2G

What kind of dogs do chemists have?

Laboratory retrievers!

Ashley Fang 2G

also it's not cumulative nor will it include hybridization

Ashley Fang 2G

I believe notating it as <109.5 degrees is fine, but if asked you should still know that H2O has a smaller bond angle because of its 2 lone pair-lone pair repulsions, compared to just 1 in NH3.

Ashley Fang 2G

I'm taking chem 14B as well next quarter!
But I heard 14BL is better taken with 14C or after 14C if possible because of the level of knowledge you're expected to know.

Ashley Fang 2G

To add to that, the arrow always points to the lowercase delta negative sign, and a line strikes through the arrow at the lowercase delta positive sign (making a cross or a "+" to help you remember it)

Ashley Fang 2G

If anything, the electronegativity values will be given if on a test.
The trends are increasing as you go up a column and also to the right across a period.

Ashley Fang 2G

To add to that, the dipoles can get closer together due to the greater surface area in rod-shaped molecules than spherical-shaped molecules, thus increasing the intermolecular forces.

Ashley Fang 2G

The greater the molecular mass, the greater the electron cloud distortion and polarizability. London dispersion forces tend to be stronger between molecules that are easily polarized. Thus, with a higher intermolecular force, the melting point/boiling point will also tend to be higher because more e...

Ashley Fang 2G

Lanthanum is actually the first element of the 5d orbital, cerium through lutetium would be part of the f block

Ashley Fang 2G

HCl has a greater ionic character than HI because hydrogen and chlorine's electronegativity difference is greater than that of hydrogen and iodine.
Remember that electronegativity increases right across a period and up a group!

Ashley Fang 2G

Naji Sarsam 4G wrote:2.) The charge of the central atom especially should either be 0 or negative (because it is the most electronegative atom); preferably 0 though

The central atom should be the least* electronegative atom

Ashley Fang 2G

The greater the electronegativity/electron affinity, the greater the pull on the electrons by the positively charged nucleus. Since fluorine is more electronegative than chlorine, the H-F single bond will be shorter than the H-Cl single bond. And as you increase the # of bonds (double, triple), you ...

Ashley Fang 2G

It can be calculated through FC = [# of valence electrons on atom] – [(# of lone pair electrons) + (# of shared electrons)/2]

Ashley Fang 2G

Do you remember if he mentioned it will be covered in the midterm?

Ashley Fang 2G

Picture yourself on top of a tree: you have high potential energy and are in a very unstable state.
Similarly, atoms don't like having a high potential energy; they'd rather create a bond and be stable at their lowest potential energy state.

Ashley Fang 2G

Ionization energy increases as you go up a column and to the right of a period. It's the energy required to remove an electron.

Ashley Fang 2G

Adding to that, ignoring noble gases, fluorine also has little shielding effect since its electrons are close to the nucleus in the n=2 shell. Thus, the protons in the nucleus have a stronger pull on the electrons than in any other element.

Ashley Fang 2G

On the contrary, I think that an atom that is isoelectronic with another atom of a fewer number of protons will attract more electrons, thus be more electronegative. Take for example N and F^2+. Although they both each have 7 electrons, fluorine will obviously attract more electrons because there is...

Ashley Fang 2G

Yes! An example of a polar covalent bond would be water (H2O), where most of the negative charge is from the oxygen on one side of the molecule and the positive charge of the hydrogen atoms is on the other side of the molecule. An example of a nonpolar covalent bond would be carbon dioxide (CO2), wh...

Ashley Fang 2G

Ashley Fang 2G

Hahahah

Ashley Fang 2G

I think that "ejected" just means that the electron is released from the atom. The detector that is used to measure the KE of the ejected electron has a slightly positive charge, thus causing it to move.

Ashley Fang 2G

Or, you could use the combined equations of E=hv and c=v(wavelength) to get (wavelength)=hc/E and solve for wavelength!
Hope that helps :)

Ashley Fang 2G

Yupp, his constants and equations sheet can be found on his website or here

https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

Ashley Fang 2G

Count me in! :)

Ashley Fang 2G

KayleyW_3L wrote:In addition to wondering when the Test 1 grades will be posted, I am also curious whether there is partial credit on the exams?

I asked my TA and there's definitely partial credit given on exams if you show your work!

Ashley Fang 2G

Generally in step-up sessions, what's the usual TA to student ratio and about how much time do they spend with each student trying to understand the concepts?

Ashley Fang 2G

Do you guys find the textbook readings to be more helpful for studying or other resources such as videos/lectures? I'm asking because I don't know if I'm missing any important information that's covered in the textbook that's not covered in Dr. Lavelle's videos or lectures. I feel like what tends to...

Ashley Fang 2G

Another tip is to generally start out balancing the element that shows up the least often in the equation and then work from there, eventually getting to elements in diatomic molecules such as H2 or O2 that stand alone and are easy to balance by themselves (just add a coefficient in front)

Ashley Fang 2G

Hohoho

Ashley Fang 2G

Yes, I am positive!

Ashley Fang 2G

The one true bro

Ashley Fang 2G

Chemistry jokes are sodium funny!
I slapped my neon that one!

Ashley Fang 2G

Personally I find that every time I read the textbook, I tend to fall asleep because it's just that long and boring, especially when you're new to the material. What I usually will do however is take notes of the summary and important formulas/equations. Once I have grasped the basic concepts, I wil...

Ashley Fang 2G

Speed of light is a constant and will be given during tests/exams.
The link of the constants and equations sheet is on his website but I'll link it here!

https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

Ashley Fang 2G

Honestly chemmybear has a lot of great resources for high school chemistry topics.
I also remember watching a lot of Bozeman videos, he explained the topics in a very easy-to-understand way.
Here are the links below!

http://chemmybear.com/
http://www.bozemanscience.com/ap-chemistry/

Ashley Fang 2G

Convert the units first if you're going to be canceling them out with other units. You want to make sure that like units cancel out like units. For example, you would not do 5kg x 1mol/18.02g, you would change the 5kg to 5000g first to make sure you're left with only moles when you cancel out the gr...

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