CHEMISTRY COMMUNITY

Angus Wu_4G

I actually don't think we will need to know any calculus, as long as you memorize or understand how many of the equations are derived. In fact, I think almost of the equations are in the equation sheet, so you won't be expected to derive them on the test. (I think).

Angus Wu_4G

You would also use an ice box for weak acid/base calculations. These acids and bases will also have their own K values.

Angus Wu_4G

I think the mistake you did was using a wrong value in the equation, instead of using 0.257/2, you should've used 0.0592/n, where n is the number of mole of electrons.

Angus Wu_4G

At the ends of the cell diagram, you put your solid metal electrodes, or Platinum if your species that is reduced or oxidized is a liquid or a gas. The you use a single vertical line to separate chemical species of different states, and then put in your chemical species that is oxidized. Then you us...

Angus Wu_4G

An electrolytic cell is a cell that has a spontaneous reaction that generates energy to drive another reaction that is non-spontaneous. The quote "The potential supplied to an electrolytic cell must be at least as great as that of the cell reaction to be reversed" basically means that the ...

Angus Wu_4G

It means that the reduction half reaction requires energy and is not spontaneous, therefore the oxidation half reaction is spontaneous and will generate electrical energy.

Angus Wu_4G

The reduction reaction does not necessarily occur at the electrode with the higher concentration, but it can. The point is that concentration doesn't really matter, what matters is the E value of each of the half reactions, that will determine which reaction is the reduction half reaction, and oxida...

Angus Wu_4G

Can someone please explain the difference between balancing a redox reaction in an acidic vs basic solution? What is the difference if there is a different way to approach both the different types of problems? Specifically, what is the difference in the problems between 6K.3 and 6K.5?

Angus Wu_4G

In a galvanic cell, Ecell is by definition positive, and deltaG is by definition negative. A galvanic cell is always spontaneous. In cells that are not spontaneous like voltaic cells, the Ecell is negative and therefore it is not spontaneous. However, if you input energy into the cell, the reaction ...

Angus Wu_4G

In an electrolytic cell, the reaction is spontaneous and favorable, but it is paired with a nonspontaneous reaction so that the energy released from the electrolytic cell is driving the nonspontaneous reaction. Some applications of electrolytic reactions are in mining, or in metal plating.

Angus Wu_4G

Dr. Lavelle gave many examples of electrolytic cell uses in class, one in which he covered in depth was the application in mining and plating, how electrolytic cells are used in mining to purify ores to yield precious metals like magnesium, and electrolytic cells are also used to plate materials and...

Angus Wu_4G

You need to look at the values of E in each of the half reactions. If the question tells you that the reaction is spontaneous and it is a galvanic cell, flip the half reaction that will give you the positive Ecell, or the most positive Ecell value. If the reaction is nonspontaneous and it is a volta...

Angus Wu_4G

In part D, I would say chlorine is being reduced since it is going from a neutral state to -1 in HCLO. I don't know what to do about the Cl2 at the products side though, it seems counterintuitive and would just cancel out with the Cl2 in the reactants.

Angus Wu_4G

To identify what is being reduced in the equation, you need to compare each elements before and after charge. For part A, oxygen got reduced and bromine got oxidized because Br went from -1 to +5

Angus Wu_4G

*Correction to my previous post: Actually yes, the electrons always flow from anode to cathode since the anode is oxidizing and generates electrons, and the cathode is being reduced. What I meant to say is that depending what your E values are for the half reactions and what half reaction is the ano...

Angus Wu_4G

Electrons don't always necessarily move from anode to cathode, but if you want a spontaneous reaction that generates energy (galvanic cell), then the anode will flow to the cathode. If you input energy into the system instead, you can reverse the direction of electron flow and create a voltaic cell....

Angus Wu_4G

I'm not too sure, but I think it depends on the temperature at which this reaction is occurring. In general though, I would imagine this reaction is endothermic because it takes a lot of energy to break the bonds, and while you do release energy when the bond is broken, I think overall it takes more...

Angus Wu_4G

For a galvanic cell, a battery is also dead when you run of out metal in the anode. If Zn has to give off 2 electrons to become Zn2+ to power the battery, then your battery will die when you run out of Zn metal, since all you are left with is Zn2+ ion.

Angus Wu_4G

The salt bridge simply allows the battery to continue to operate since it balances the charges on both sides, and keeps both sides negative. If you didn't have a salt bridge, electrons build up on your cathode and that high concentration of negative charge will eventually cause the electrons to no l...

Angus Wu_4G

I could probably answer your question better if you had a specific problem or example, but they way I see it, unless they tell you otherwise through the problem or scenario, it is safe to assume that the pressure is constant (1atm) and that temperature is 25C, since that this the temperature and pre...

Angus Wu_4G

Water, depending on solid, liquid, or gas state, has different specific heats. 4.184J/gC is the specific heat capacity for liquid water. For ice, solid water, the specific heat capacity is 2.03J/gC

Angus Wu_4G

It really depends on the problem, some problems might straight up give you the delta H and delta S, or some problems may have multiple parts where you need to find delta H, and then use your calculated values to find delta G. You won't be expected to memorize delta G values, or bond enthalpies, spec...

Angus Wu_4G

Are there any homework problems where we have to use the integral instead of the simplified formula? We use -p(delta)v when the pressure is constant, because it allows us to factor out the variable P from the integral to end up with just integrating the constant "1", which simplifies the ...

Angus Wu_4G

If you check the equation sheet online in Dr.Lavelle's website, that specific value is given to you. (101.325J=1 LXatm) You can also derive this conversion if you divide the 2 gas constants R, ((8.314J(K)^-1,(mol)^-1) / (0.08206 Latm (K)^-1(mol)^-1). You will find that the units cancel out to give y...

Angus Wu_4G

Here's a simple website that I used that helped me understand the concept, I hope it helps:

http://hyperphysics.phy-astr.gsu.edu/hb ... ntrop.html

Angus Wu_4G

Degeneracy, or W, is just the total number of energy states a system can achieve. You can think of entropy as chaos and a gradual decline to disorder. The higher the degeneracy, there are more possible states for the system to be, and the more possible states there are, the more potential for disord...

Angus Wu_4G

We use -p(delta)v when the pressure is constant, because it allows us to factor out the variable P from the integral to end up with just integrating the constant "1", which simplifies the integral into a simple -p(delta)v. If the pressure were not constant, then you cannot factor the P var...

Angus Wu_4G

S=Rln2 only when the it is a simple 2 state system. For systems other than 2 state, the equation would be S=RlnX, where X is the number of states.

Angus Wu_4G

An open system allows for the loss and gain of heat and matter, so like an open container or beaker. A closed system allows for the loss and gain of heat, but not matter, so like a closed container. An isolated system does not allow for the loss and gain of hear and matter, so like a sealed hydrofla...

Angus Wu_4G

In high school I was always taught that H20 was in the gas state because the fire in the combustion reaction itself was hot enough to evaporate the water, and sometimes the water condenses back to into liquid. I'm not sure, but I would just follow what Dr. Lavelle's examples say.

Angus Wu_4G

Lavelle gave that extra example in class, because if you had calculated the pH of 1X10^-11 M HCl, you would actually get a pH of greater than 7, which is incorrect since an acid will not make the solution more basic. Essentially, your acid is so diluted that it is negligible, and that you have to re...

Angus Wu_4G

You can use the approximation if the K value is less than 1X10^-3. After you obtain your final H30+ concentration, you should also make sure the final H30+ concentration is less than 5% of the initial concentration of acid. If it is less than 5%, your approximation is probably fine. If your final co...

Angus Wu_4G

When the pH is negative, it just means that it is a very very very strong acid, and the H30+ concentration is extremely high. The pH level will be negative when you have a H30+ concentration greater than 1.0M. The pH range of 0-14 is only in general, there will be acids that can be less than zero pH...

Angus Wu_4G

I think it depends on the actual PLF too, but I think you'll be fine as long as you attend the topics that you are looking for some help on.

Angus Wu_4G

You only need to find the enthalpy of formation for carbon dioxide and water. The enthalpy of formation for oxygen and nitrogen gas is zero.

Angus Wu_4G

A state function is any function in which the value is not dependent on the path taken. If a value were dependent on its path, then it is a path function. A common example of path vs state function is like displacement vs distance traveled. You can travel 5 meters East and 5 meters West, your displa...

Angus Wu_4G

Keep in mind it is important to distinguish between the pressures caused by different gases. If you added a lot of noble gas into a reaction vessel, even though the pressure of the reaction will increase, the equilibrium will not shift because the noble gas does not react with the reactants or produ...

Angus Wu_4G

Changing of pressure will induce the reaction to shift equilibriums. When pressure increases, the reaction will try to compensate for the pressure increase by shifting the reaction to the side with less moles of gas. When pressure decreases, the reaction will try to compensate for the pressure decre...

Angus Wu_4G

Sometimes you are given the value for Ka, but the reaction is basic and you need Kb. You can find Kb from Ka by using the relationship that Ka X Kb = 1.0X10^-14.

Angus Wu_4G

Temperature also changes the equilibrium constant, equilibrium constants are usually standardized at 25 degrees Celsius. Unless the problem specifies otherwise, you can usually assume the reaction is room temperature (25C).

Angus Wu_4G

When temperature increases, it depends on whether your reaction is exothermic or endothermic. If the reaction is endothermic, requiring energy to proceed, then increasing temperature will create more product to use up the heat. Decreasing the temperature will create more reactant to regenerate the l...

Angus Wu_4G

I think the reaction that is more thermodynamically stable is the reaction that produces more product. So for part C I think all you do is compare the 2 products and the one with more product is the correct answer.

Angus Wu_4G

The ideal gas law fails when the temperature is extremely low and when the pressure is extremely high. When the ideal gas law fails, you can no longer use the ideal gas law equation, you need to use the Van Der Waals Equation, which is quite a bit more complicated than the ideal gas law equation.

Angus Wu_4G

Unless you are given the precise change, you would just use x as an placeholder. For reactants, the change will be negative x, and for products the change will be positive x. Be sure to also account for the coefficients in the balanced chemical equation, if there is a coefficient of 2, then your ch...

Angus Wu_4G

It is called partial pressure because when there are multiple gases, each gas is contributing to the overall pressure, and therefore the contribution of each individual gas to the overall pressure is called the partial pressure. According to Dalton's law of partial pressures, if you add up all the p...

Angus Wu_4G

Ammonia and Acetic Acid can also undergo autoprotolysis.

2NH3 ⇌ NH2- + NH4+

2CH3COOH ⇌ CH3COO− + CH3COOH2+

Angus Wu_4G

Unless you are given the precise change, you would just use x as an placeholder. For reactants, the change will be negative x, and for products the change will be positive x. Be sure to also account for the coefficients in the balanced chemical equation, if there is a coefficient of 2, then your cha...

Angus Wu_4G

There is no such thing is solvent concentration. When you have a concentration of salts, acid, or any compound, concentration is usually measured in moles per L, or M. The key here is that the moles per liter is moles per liter of water (or whatever your solvent is). It is not possible to have the w...

Angus Wu_4G

For the net ionic equation, you should separate the ions in all compounds and cancel out ions that appear in both the reactants and products side. For this particular problem, both Na+ and NO3- will be cancelled out, leaving the equation as just 2Ag + 2OH- yields Ag20 + H2O. Then write your equilibr...

Angus Wu_4G

The double-bonded oxygens do not count as ligand bonding sites because those oxygens can't serve as lewis bases. Lewis bases are electron donors and will donate a pair of electrons to form a coordinate covalent bond. Since the double-bonded oxygens already have 2 bonds and since oxygen does not form...

Angus Wu_4G

Not necessarily, you would have to look at the individual ligands and check if you have any bidentate, tridentate, or polydentate ligands. If you have ligands that are not monodentate and both the ligand binding sites are bonded with the transition metal central atom, then you have a chelate.

Angus Wu_4G

Then it wouldn't be possible, either they will give you the ligand, give you the name of the compound, or give you the chemical formula of the compound, otherwise it you would have insufficient information to find the coordinate number.

Angus Wu_4G

No I don't think so, but you should add "ate" to the end of the name of the transition metal if the overall coordination compound is negatively charged.

Angus Wu_4G

"The dissociation of water molecules into ions is bond breaking and is therefore an endothermic process (energy must be absorbed to break the bonds). Endothermic processes are favored by an increase in temperature and so as the temperature rises the equilibrium moves further to the right hand s...

Angus Wu_4G

No, I don't think so, because I don't think its possible for a molecule to be a bronsted acid and lewis base at the same time, and vice versa. Also, keep in mind that bronsted bases and acids are just special cases of lewis bases and acids when a hydrogen proton is involved. A bronsted base by defin...

Angus Wu_4G

According to the syllabus, the final exam is worth 180 points, while the class is worth 500 points. So the final exam is worth 36% of the final grade

Angus Wu_4G

Only structures with the best balance of formal charges count as resonance. Keep in mind that you want the most electronegative element in your molecule to carry the negative formal charge, and the least electronegative element to carry the positive formal charge. It is ideal if all the formal charg...

Angus Wu_4G

Yes, when determining hybridization only the electron density areas count, but the electron density does influence the specific shape.

Angus Wu_4G

I think all elements are capable of being ions if they interact with another atom that has significant electronegativity differences. If there are not enough electronegativity differences then a covalent bond is formed instead

Angus Wu_4G

Well most bases then to be very bitter in taste, and are generally very corrosive for the human body, even more so than acids. So from a food safety standpoint and a taste standpoint, I don't see why people would enjoy consuming basic foods.

Angus Wu_4G

A molecule has a permanent dipole moment if it is polar, whereas a nonpolar molecule can only have temporary dipoles (London Dispersion Forces, Van Der Waals, etc)

Angus Wu_4G

I believe we should just memorize all of them just in case, since you never know what could be on the test. Better memorize too much than memorize too little.

Angus Wu_4G

This is because when there is a lone pair, the lone pair occupies a larger space and causes electron-electron repulsion, forcing all the other regions of electron density closer to each other and farther away from the lone pair. This is why the bond angle in molecules with a lone pair is slightly le...

Angus Wu_4G

This is because the sulfur atom in the sulfite ion has a lone pair, and the lone pair present causes electron-electron repulsion, and therefore forces all the other bonds to be just slightly closer to each other. This is why the bond angle is 106, slightly less than the standard value of 109.5, whic...

Angus Wu_4G

Intermolecular forces means forces between multiple different molecules, hence the prefix "inter," similar to how international means between different countries, and interstate means between different states, intermolecular means between different molecules. Forces that are just in the mo...

Angus Wu_4G

Yes, hydrogen bonding is a special case of dipole dipole interaction in which the atoms involved are especially electronegative, like nitrogen, oxygen and fluorine

Angus Wu_4G

Here is an exhaustive list of all the shapes that I can think of: AX2 - Linear AX3 - Trigonal Planar AX2E1 - Bent AX4 - Tetrahedral AX3E1 - Trigonal Pyramidal AX2E2 - Bent AX5 - Trigonal Bipyramidal AX4E1 - Seesaw AX3E2 - T-shaped AX2E3 - Linear AX6 - Octahedral AX5E1 - Square Pyramidal AX4E2 - Squa...

Angus Wu_4G

Sometimes I go by the rule that the atom that is able to form the most bonds is the central atom of a lewis structure, though not an official rule, it has served me pretty well in the past.

Angus Wu_4G

To my knowledge, there are generally no exceptions to the rule that stronger bonds means that something is more likely to be tightly packed together (solid or liquid) state than something with weaker bonds, when the temperature is the same. If there are any exceptions, unless Dr.Lavelle mentions it ...

Angus Wu_4G

Because C4H90H has a hydroxyl group, which means it is capable of forming hydrogen bonds, whereas C2H50C2H5 is only capable of forming dipole-dipole interactions, not hydrogen bonds.

Angus Wu_4G

I don't think so, but I think the experiments that Dr.Lavelle went over in class would be in the midterm, like the photoelectric effect and stuff

Angus Wu_4G

Yes, if we get significant figures wrong be will be docked points. As for converting SI units, I would say if the question specifies a unit for the answer, then go for the specific unit. If the question doesn't, then as long as it is an SI unit, (meter, kilogram, second, etc) then I think its fine.

Angus Wu_4G

An induced dipole-induced dipole interaction is different from dipole-induced dipole interactions in that in an induced dipole-induced dipole interaction both the molecules are nonpolar. If the atoms were polar then there would be a dipole. An induced dipole-induced dipole interaction forms when ele...

Angus Wu_4G

Electronegativity (from greatest to least): F, O, N, Cl, S, P Atomic Radius (from greatest to least) P, S, Cl, N, O, F Electron Affinity (from most negative to least) Cl, F, S, O, P, N Ionization Energies (from greatest to least) F, N, O, Cl, P, S There are generally very predictable trends for each...

Angus Wu_4G

The periodate ion is just IO4- and the hydrogen phosphate ion is just HPO4(2-). Those are just different names for some chemical compounds.

Angus Wu_4G

One of the oxygens needs to have a single bond in order for the entire molecule to have an overall negative charge since the molecule is ClO4-. The single bond is given to the oxygen so that oxygen would have the negative formal charge. Recall that oxygen is more electronegative than chlorine, and t...

Angus Wu_4G

You do not need electronegativity values to calculate the dipole moment, the equation for dipole moment is the charge multiplied by the distance between the atoms. But I think we won't be asked to calculate dipole moments in the tests.

Angus Wu_4G

I'm just gonna assume that by "probability area" you mean the likelihood of finding an electron in a given area In an ionic bond, since one atom is electronegative enough to completely steal an electron, you should expect that the "probability area" has shifted towards the anion,...

Angus Wu_4G

We use the formal charge equation to find the most stable electron sharing pattern for a molecule. You want to maximize the number of zero formal charges that you can, but if it is impossible to assign every atom a zero formal charge, then ideally you would put the negative formal charge with the mo...

Angus Wu_4G

Another exception is that although hydrogen is all the way to the left of the periodic table, it actually has a very similar electronegativity to carbon.

Angus Wu_4G

You would want to use double or triple bonds to make your formal charges zero, if possible. Generally there are some patterns though. Carbon tends to form 4 bonds, nitrogen tends to form 3 bonds, oxygen tends to form 2 bonds, and halogens tend to only form 1 bond. There are exceptions of course, but...

Angus Wu_4G

If you cannot avoid assigning a negative formal charge to an atom, then always draw your structures so that the most electronegative element has the negative formal charge. Ideally though you would want every atom to have zero formal charge, but there are times when that is impossible to achieve.

Angus Wu_4G

The quantum number, or n, tells us which quantum level the electron is in and gives us restrictions for the other 3 quantum numbers. a quantum number of n=1 tells us that the electron is in the 1st quantum level, and n=2 for the second quantum level, and etc. In general, L must be one less than n, a...

Angus Wu_4G

The unrealistic answer tells us that the electrons cannot be located at the nucleus of the atom, because the value we used for the uncertainty in position for the calculation was the diameter of the hydrogen nucleus. This tells us that electrons cannot be located in the nucleus, and therefore are or...

Angus Wu_4G

Yes! Please add me too

My email is awu0113@g.ucla.edu

Angus Wu_4G

To my knowledge, you are not required to know the particle in the box experiment for the midterm. I wish I could explain the experiment, but I don't really understand it either

Angus Wu_4G

This is a very complicated question that I don't really comprehend, but I'll try my best to explain. The Lyman series, Balmer series, and Paschen series are both specific to only the hydrogen atom, so just because hydrogen has no higher energy level for electrons to reach, doesn't mean that all atom...

Angus Wu_4G

Heisenberg's Uncertainty Equation is (deltaP)(deltaX) is greater than h/(4pi) X in this case represents position, P represents momentum, and the delta represents the uncertainty. Keep in mind that momentum is equal to mass times velocity, but since the mass is the same and does not vary in the probl...

Angus Wu_4G

When a problem says that the energy level has increased, it is referring only to a specific electron. That electron has absorbed a certain amount of energy to have increased its energy and moved to a higher quantum level, (n=1 to n=3). Conversely, when the energy level decreases, the electron drops ...

Angus Wu_4G

No, I believe in the kinetic energy equation 0.5mv^2, the v corresponds to the velocity of the electron, not the velocity of the photon that strikes the electron. In the question, 3.6x10^3km/s is the speed of the electron after it has been struck by the photon, not the speed of the photon itself.

Angus Wu_4G

You would expect the transition from n=5 to n=1 to release more energy than the transition from n=4 to n=2 because going from 5 to 1 is going through more levels. Using the equation E=(hc)/(wavelength), we can see that the longer the wavelength, the lower the energy is. Therefore, the transition fro...

Angus Wu_4G

Professor Lavelle wants us to use the formula E=(-hR)/(n^2) but this formula can be used to derive the Rydberg Equation, so its ok with you use the Rydberg Equation. Essentially, you would use the formula E=(-hR)/(n^2) with n=1 to find the initial energy of the electron. Then you would use the 102.6...

Angus Wu_4G

The image in the lecture was basically a visual model proving way electron energy levels are quantized (n=1, n=2, etc) and not n=1.76 etc. In order for an electron to orbit the atom, the circular standing wave that the electron creates must be stable, in that the waves need to be able to connect, de...

Angus Wu_4G

I answered this question in a different discussion thread, but I'll repost my explanation below again: You would first use the wavelength you have and find the energy change using E=(hc)/wavelength. Once you have the energy change, use the equation E=(-hR)/n^2 with n=1 to find the energy of the elec...

Angus Wu_4G

You would first use the wavelength you have and find the energy change using E=(hc)/wavelength. Once you have the energy change, use the equation E=(-hR)/n^2 with n=1 to find the energy of the electron at n=1. Once you have the energy change and the energy of the electron at n=1, you can set up this...

Angus Wu_4G

We are given the fact that the lamp is shined for 2 seconds, and the light is a 32W light. Therefore, in those 2 seconds, the lamp emitted 64 joules of energy. By using the E=(hc)/(wavelength) equation you found the energy released by a single photon. Essentially, now you just have to find how many ...

Angus Wu_4G

Typically I do not apply significant answers until I get to the very end, because if you apply significant figures in the middle of the problem sometimes the small change can snowball enough in the end to throw off your final answer, especially if there are a lot of steps to the problem.

Angus Wu_4G

I think as long as you stick with a metric unit and it is equivalent to the answer, you should be fine

Angus Wu_4G

For the first example you listed, the answer would be "You would need 4.5mL of solution with concentration (something). " For the second example you listed, you would say "You can prepare the required solution by adding 12mL of pure water to the original molarity sample." I hope ...

Angus Wu_4G

In this question, the 4.14g does not translate into 4.14%. Since 4.14g of phosphorus combined with chlorine to form a 27.8g compound, we know that therefore the mass of chlorine must be 27.8 - 4.14 = 23.66g. Now that we know the mass of the chlorine used, 23.66g, and the mass of the phosphorus, 4.14...

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