CHEMISTRY COMMUNITY

Aaron Kwan 3B

Another way I found in order to solve this problem if you can't remember the special Arrhenius equation variant is this: From the Arrhenius equation: any change in the rate constant due to temperature doesn't depend on the pre-exponential factor (A). In other words, it doesn't matter what A is- the ...

Aaron Kwan 3B

Hello! One trick I use to figure out the reaction order with respect to one reactant, given you know the reaction rates of the other two is: If you know the reaction order of [A] and [C], you can construct a reaction rate equation, then set up a system of equations. For example, since we know the re...

Aaron Kwan 3B

In terms of chemical kinetics, k represents the rate constant of the forward reaction, while k' represents the rate constant of the reverse reaction. These two are related to the equilibrium constant K by k/k' = K. As we can see, when the rates of the forward and reverse reactions are equal, the rea...

Aaron Kwan 3B

V just stands for volts- know that the equation Ecell=E* - (0.0592/n)logQ is giving the answer in the unit of Volts.

Aaron Kwan 3B

If you think of a substance being an arrangement of stuff, in different orientations or spaces- the bulk property of possible states does not depend on the actual mass of the sample. A perfect crystal of mass 500 tons still has no residual entropy, while a single ammonia molecule can have some entro...

Aaron Kwan 3B

One situation where this holds true is of a gas expanding into a vacuum. There is no work being done (no molecules being pushed aside), nor heat exchange- yet it is still spontaneous. This is because the entropy will increase through this process.

Aaron Kwan 3B

Because G=H-TS, even if H is positive, as long as TS is greater than it (entropy and temperature should be high), G can be negative and the reaction can still be spontaneous.

Aaron Kwan 3B

Entropy increases with the amount of possible states (with equal energy) a system can have.

Aaron Kwan 3B

With regards to the T in the equation delta S = delta H/T, we know that if T (normally a constant) is higher, then the entropy change must be less. We can think of it as this: shouting in a quiet library will seem alot louder than shouting in a busy street. The temperature (and thus entropy) is alre...

Aaron Kwan 3B

Equilibrium usually just means max entropy. This is because equilibrium is the natural tendency for a reaction to occur in, governed by Gibb's free energy. A negative Gibb's free energy means more entropy.

Aaron Kwan 3B

Yeah! so we know deltaS=q(rev)/T. If we want the rate (say, per second), we divide this whole equation by the number of seconds. Then, we get: deltaS=q(rev)/time*T Also, deltaS (surroundings) is always the negative of deltaS (system) in a reversible reaction. Thus, we get that deltaS (surroundings)=...

Aaron Kwan 3B

And if you need entropy/heat per mole, you don't have to multiply or know the moles at all- just use Cp!

Aaron Kwan 3B

Remember that sometimes you need Cp and sometimes you need Cp_mol! It depends on what units the problem is asking for. If total entropy/heat, you need Cp_mol (accomplished by multiplying Cp by number of moles).

Aaron Kwan 3B

Also keep in mind that entropy is a state function! Thus, you can use a combination of all of the above equations, and add them together to find the total entropy change.

Aaron Kwan 3B

Agreed with above- if the process is irreversible, either work or heat gets released to the surroundings, thus, changing the entropy.

Aaron Kwan 3B

From the equation delta S = delta H/T, we know that if T (normally a constant) is higher, then the entropy change must be less. We can think of it as this: shouting in a quiet library will seem alot louder than shouting in a busy street. The temperature (and thus entropy) is already high in the busy...

Aaron Kwan 3B

An example of an substance with no residual entropy would be a perfect crystal- there is no variation in the arrangement of the molecules within.

Aaron Kwan 3B

even when a substance is at 0 kelvin, most substances will have some possible different arrangements of the molecules (corresponding to some level of entropy). This is called residual entropy.

Aaron Kwan 3B

q_rev stands for q from an entirely reversible process! q in the real world (where processes are partly irreversible) is going to be less.

Aaron Kwan 3B

An easy way to remember this is to think of work being an object, like money. Next, think of the system as your bank account. A withdrawal of money would look like a negative number on your bank account- work (money) is leaving the system. If someone deposited work (money) into your system (bank acc...

Aaron Kwan 3B

In an open system, heat exchange can also happen with the surroundings.

Aaron Kwan 3B

If you mean chemical equilibrium, not usually. In order to solve chemical equilibrium problems, the usual method is to construct an ICE table.

Aaron Kwan 3B

Yes, if the volume remains constant, the gas will be unable to do work!

Aaron Kwan 3B

Usually, a gas expansion will do work. The molecules of gas take up more volume, pushing other surrounding molecules- this is the idea of work. If you have 1 molecule of gas, and it splits into 2 molecules- suddenly, the entire mixture will expand- even if the number of atoms remains the same. Likew...

Aaron Kwan 3B

This equation can be used when a gas is expanding at a constant pressure. In this case, the reaction is irreversible. The units for the solution is given in L * atm: convert to joules by multiplying by 101.325.

Or, use w=-deltaN*R*T (Using the R value that gives J).

Aaron Kwan 3B

My bad! for the ideal gas law, R is actually 8.2*10^-2. This is because the equation gives units of L * atm * 1/K * 1/mol.

Then, the equation w=-nRTln(Vf/Vi) uses the R value of 8.314.

Aaron Kwan 3B

Hey Natalie!

I think that we can calculate the moles of gas using the ideal gas law (PV=nRT). We know P=2.48, V=4.11, R=8.314, and T=298.

Hope this helps!

Aaron Kwan 3B

I see- Thank you!!

Aaron Kwan 3B

Consider these reactions, where M represents a generic metal. 1. 2M(s)+6HCl(aq)⟶2MCl3(aq)+3H2(g). ΔH1=−585.0 kJ 2. HCl(g)⟶HCl(aq). ΔH2=−74.8 kJ 3. H2(g)+Cl2(g)⟶2HCl(g). ΔH3=−1845.0 kJ 4. MCl3(s)⟶MCl3(aq). ΔH4=−362.0 kJ Use the given information to determine the enthalpy of the reaction 2M(s)+3Cl2(g...

Aaron Kwan 3B

Think of work as helping another molecule, such as by moving it. When expanding in a vacuum, there are no other molecules, so you can't possibly help anything.

Aaron Kwan 3B

I think that for the scope of this class, we assume that substances stay at the same temperature during phase change.

In other words, superheating or supercooling, where the temperature can rise/drop a few degrees during phase change, is not assumed to happen.

Aaron Kwan 3B

An exothermic reaction gives off heat. When you touch an ice cube, it melts and your hand feels cold. The cold sensation is caused when heat flows from your hand to the cube (melting is endothermic). If we imagine the opposite, if your hand was the temperature of a freezer, heat would flow from the ...

Aaron Kwan 3B

As illustrated in Dr. Lavelle's lecture, steam holds around 5-6 times more heat energy than water at the same temperature. Because this energy is instantly given off in order to change steam to water, you receive many kJ of energy at once.

Aaron Kwan 3B

Generally, change in enthalpy is equal to change in internal energy of the system + P * delta V. In other words, if the reaction does not undergo expansion, then change in enthalpy is equal to change in internal energy. If the system underwent expansion (positive delta V), then change in enthalpy is...

Aaron Kwan 3B

After you get enough practice with ICE boxes, you can start to do them in your head!

Aaron Kwan 3B

Because delta H is less than zero, the change in energy to the system is negative. If the system lost energy, then the energy must have come out of the system (as heat), meaning the system is exo-thermic. Similarly, if delta H is more than zero, the change in energy to the system is positive. Thus, ...

Aaron Kwan 3B

If you wait long enough, the Q (ratio of activities of products to reactants) will eventually settle to equilibrium, and match K.

Aaron Kwan 3B

Isabella, Make sure you figure out whether you can use the approximation to calculate x or not. If x is less than 5% of the initial 0.026, then the approximation is valid.

Aaron Kwan 3B

If a base dissociates in water and leads to a concentration of OH- greater than 1M, calculating the POH through taking the -log leads to a negative number. Then, calculating PH through 14-POH leads to a number greater than 14.

Aaron Kwan 3B

Make sure you figure out whether you can use the approximation to calculate x or not. If x is less than 5% of the initial 0.026, then the approximation is valid.

Aaron Kwan 3B

Does it also suffice to calculate percent protonation using the initial value of B?

Aaron Kwan 3B

I see, thanks!

Aaron Kwan 3B

Hello all, The Ka of a monoprotic weak acid is 0.00611. What is the percent ionization of a 0.105 M solution of this acid? I've set up a ICE table for this, and have obtained the equation x^2=0.00611(0.105-x). From this, I obtained x=0.022, and then percent ionization is 0.022/(0.105-0.022), and I g...

Aaron Kwan 3B

Hello,
The equilibrium constant is found after calculating the activities of the reactants and products at equilibrium. From the table, where do you see that the solution is at equilibrium (IE: the concentrations of product and reactant are not changing anymore)?

Aaron Kwan 3B

How would we use the approximation strategy we learned in class here?

Aaron Kwan 3B

The above are very helpful for starting this problem. An additional step with some problems is to balance the equation, making sure the reactant and product atoms are equal on both sides.

Aaron Kwan 3B

The ICE table is just a tool to help organize a system of expressions. In the end, you have a number of expressions that deal with the final concentration of the reactants and products. The ICE table helps you construct these expressions, which you then combine together in a form that represents the...

Aaron Kwan 3B

Depending on if you are solving for the K or the Kc value, you might have to convert back and forth between the (given) partial pressures/concentrations of gases in the problem. Kc uses concentrations, while K uses partial pressures.

Aaron Kwan 3B

If you are working with non-ideal gases, you can use Van der Waals equation: (p+V2n2a)(V−nb)=nRT!

Aaron Kwan 3B

Be careful- the problems in the textbook can sometimes give pressures in terms of pascals or kpascals, and you need to convert that to bars!

Aaron Kwan 3B

Yes, standard temperature and pressure is 25 C, and 1 bar!

Aaron Kwan 3B

Keep in mind that Thermodynamic tables use Kc, so make sure to convert gas partial pressures to moles when needing to use them!

Aaron Kwan 3B

You can use the Van der Waals equation ((p+V2n2a)(V−nb)=nRT) if you want to calculate characteristics for a non-ideal gas!

Aaron Kwan 3B

Yeah, try chrome instead of safari, that also helped for me!

Aaron Kwan 3B

Make sure you have 51 by week 10 to be safe. I know that sometimes the number of posts showing next to your profile is 1 off.

Aaron Kwan 3B

I believe Lavelle sent an email last week with a link to the manual!

Aaron Kwan 3B

For one of my math classes, even the discussion on Wednesday is canceled. I think it varies class-to-class!

Aaron Kwan 3B

You can view TA office hours here:
https://ccle.ucla.edu/course/view/21F-CHEM14A-1

Aaron Kwan 3B

You can view your TA's office hours here:
https://ccle.ucla.edu/course/view/21F-CHEM14A-1

Aaron Kwan 3B

It depends on if Lavelle still offers the remote learning option!

Aaron Kwan 3B

Do we know if the class will be entirely remote next quarter yet or not?

Aaron Kwan 3B

Hello! LS30B, Math 3B, and Math 31B are instead co-requisites- this means you can take them at the same time you take 14b! I am taking LS30B & Chem14B next quarter.

Aaron Kwan 3B

much appreciated!

Aaron Kwan 3B

I believe that there will only be a lecture on Monday! Will someone else please clarify though?

Aaron Kwan 3B

How many similar bonds to these ^ can be created?

Aaron Kwan 3B

Yeah, Lavelle said in class that hybridization wasn't on the midterm.

Aaron Kwan 3B

But isnt the ionization energy of Oxygen higher than fluorine?

Aaron Kwan 3B

Be careful not to mix up the atom numbers and charges!

Aaron Kwan 3B

Try calculating formal charges on each of the models and figuring out which most acts like an ionic bond (lots of positive/negative, negative charge on the Cl). This is because ionic bonds have oxidation numbers.

Aaron Kwan 3B

This image might help.

Aaron Kwan 3B

This is because axial (corner) lone pairs usually repel 3 or more atoms.

Aaron Kwan 3B

Keep in mind that if its just one lone pair, it defaults to equatorial (middle) so it only repels 2 atoms.

Aaron Kwan 3B

Do Lewis Acids differ from regular acids?

Aaron Kwan 3B

so how many lone pairs do they have?

Aaron Kwan 3B

thanks!

Aaron Kwan 3B

Can someone give me a fact about lewis acids?

Aaron Kwan 3B

Make sure to keep track of units (mg, g, L, m)!

Aaron Kwan 3B

Thank you!

Aaron Kwan 3B

Is it a good idea to take classes during the summer to avoid overworking during the school year?

Aaron Kwan 3B

does Hund's rule mention anything about the maximum number of electrons in an orbital?

Aaron Kwan 3B

are there certain keywords I need to watch out for with regards to converting from energy to moles?

Aaron Kwan 3B

Great question! something called the strong nuclear force (attraction) overpowers electromagnetic force (repulsion) at small distances

Aaron Kwan 3B

This relationship can be measured by election affinity- it increases left to right across the periods and increases upwards as well.

Aaron Kwan 3B

It also helps if you imagine pulling objects away from a black hole with lots of gravity. If you want to get from the closest energy level (1) to the next closest (2), it takes a lot of force. If the object is already far away, at a energy level such as (5), then moving it further will require less ...

Aaron Kwan 3B

It seems that naturally, no election will change the magnitude of its spin or its magnetic moment.

Aaron Kwan 3B

Each election will always have its own unique set of quantum numbers! Usually two electrons in the exact same suborbital have different spins (ms).

Aaron Kwan 3B

Hello!

Wavelength and frequency dont change with more photons, so it stays the same. Energy, however, is additively added depending on the photon count.

Hope this helps!

Aaron Kwan 3B

If you do want to calculate the energy however, the formula E=hc/(wavelength) works!

Aaron Kwan 3B

Hello, you can start with this: The enthalpy of fusion of water at 0 ∘C is 333.6 J/g. This means that it requires 333.6 J of heat energy to melt 1 g of ice. You can multiply this by the total grams of ice to find the total energy required. Next, E=hc/(wavelength) gives the energy given per photon, d...

Aaron Kwan 3B

Hello! You can start here:

- v=R(1/n^2_1−1/n^2_2)

R is 3.29×10^15 Hz, n_1 is final, n_2 is initial.

Aaron Kwan 3B

Yes, be careful with converting units. Plank's constant gives units in kg instead of g, and m instead of cm...

Hmm, Vivien your plank's constant is not right, it should be 6.626 *10^-34!

Aaron Kwan 3B

It seems the actual energy per second of light depends on both frequency and intensity then. Higher frequency but lower intensity of each photon yield the same energy as lower frequency but higher singular intensity.

Aaron Kwan 3B

I believe that Dr. Lavelle will review them quickly when we get to them!

Aaron Kwan 3B

You got this! I am also slightly rusty in chem, being a freshman rn, took AP chem close to 2 years ago (when i was a sophmore in HS). My tips would be to skim through the textbook and write down any formulas, to jog your memory.

Aaron Kwan 3B

I found that trailing zeros to the right of the decimal are usually significant. If its a whole number, the trailing zeros are not significant (unless there is a decimal shown).

Aaron Kwan 3B

What helps me most when converting from moles is to think of each mole as one molecule. That way, you can work with simple numbers such as 3.4 moles while converting.

Aaron Kwan 3B

These are all bad chemistry jokes... all the good ones Argon

Aaron Kwan 3B

Whats the biggest rule in chemistry?

never lick the spoon.

Aaron Kwan 3B

H2O is water and H2O2 is hydrogen peroxide. What is H2O4?

Drinking.

Aaron Kwan 3B

Did you know that, you can be cooled to -273.15 C and still be 0k?

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