CHEMISTRY COMMUNITY

George Hernandez 3I

Thank you for being my UCLA professor, Dr. Lavelle. You've made chemistry such an interesting subject for me to explore and you dedicate so much time into helping us do well. You're an inspiration for me to advance deeper into the fields of science. :)

George Hernandez 3I

Zero order means that even if you add more reactant, the rate will stay the same.

George Hernandez 3I

Speaking on STP, yes the Kw is always 14

George Hernandez 3I

Its basically the same as adding pH and pOH to get 14. The logarithmic properties allow you to add the pKa and pKb values to get pKw. But the concentrations themselves must be multiplied.

George Hernandez 3I

It all depends on what values are present in the problem. The constant aims to cancel out most of them

George Hernandez 3I

Outlines and sapling review problems are very useful

George Hernandez 3I

Add the order numbers of the reactants to get the order of the reaction.

George Hernandez 3I

The bomb calorimeter is an example of a real isolated system. But I doubt it is a completely perfect system.

George Hernandez 3I

Like the previous reply mentioned, its based on experiments. If you refer back to Lavelle's monday week 9 lecture, he shows how experimentation determines the order of the reaction.

George Hernandez 3I

Gases are applied to this law only

George Hernandez 3I

Deep breaths. As long as you know you've done your best within the quarter to learn the material, you should be able to achieve peace of mind before, during, and after the exam.

George Hernandez 3I

I've been having similar problems trying to complete it as well. Thanks for the tip!

George Hernandez 3I

It would be safe to assume that a bomb calorimeter is a near perfect system because of its incredibly insulated and protective mechanisms.

George Hernandez 3I

Reverse the lesser value. Your overall voltage can never be negative.

George Hernandez 3I

As the charge begins to build up, the cathode gets more negative and repulsion starts to take place when more electrons attempt to enter. By using a salt bridge, some ions can move from the cathode to the anode and allow for more electrons to be transferred.

George Hernandez 3I

The acidic solutions includes H+ in the equations to balance it, while basic solutions include OH-

George Hernandez 3I

The oxidizing agent is the one that is being reduced. It takes the electron and oxidizes the element, which makes it the oxidizing agent.

George Hernandez 3I

I think of the anode as negatively charged which is giving electrons to the more positively charged cathode. So the anode is becoming positive while the cathode is becoming negative with the movement of electrons, If that makes sense. Thats how I interpret it at least.

George Hernandez 3I

Oxidation is becoming positive by losing electrons, reduction is becoming negative by gaining electrons.

I use LeO says GeR like the previous post mentioned as well. All it comes down to is practice.

George Hernandez 3I

Knowing the oxidation numbers of common elements would be useful, but I don't think you'll need to know all of them

George Hernandez 3I

Greater temperature and greater space allows more possibility for disorder, which increases entropy. The opposite of both of these states will decrease it.

George Hernandez 3I

Its classified as disorder regarding number of possible varying positions it can be in

George Hernandez 3I

They are closed, insulated environments. I would say theyre always isolated.

George Hernandez 3I

State functions are primarily focused on start and end states. The "path" taken is irrelavent.

George Hernandez 3I

Kelvin and celcius are essentially the same in terms of their step size (+5 C° also means +5° K exactly). The only difference is that they are about 273 units apart. So to calculate from C to K, just add 273.

George Hernandez 3I

An easy way to distinguish these terms is by understanding what they represent. Enthalpy is a value which measures the energy in a system. That energy can be transferred to another system by means of 'heat' and/or 'work.' Energy transfer through means of heat is done where the system releases energy...

George Hernandez 3I

Negative means it 'releases' to its surroundings while positive means it 'absorbs' from its surroundings. That's how I try to remember these types of things in thermodynamics.

George Hernandez 3I

Closed system means that you cannot add anything else in it, as in other chemicals. But I believe a piston adding pressure to a system still counts as a closed system since no other matter is being added.

George Hernandez 3I

Yes, a negative change in heat (- delta H), means that heat is released from the system and into the environment. The opposite occurs when there is a positive change in heat. Enthalpy and change in heat generally mean the same thing.

George Hernandez 3I

It all depends on the problems. Some discuss certain units so you would use the constant that include those said units.

George Hernandez 3I

When dealing with equilibrium equations, water is always the solvent, so H2O is always removed from the equilibrium equation. If you remember from one of Lavelle's lectures, the 'concentration of water in water' is 55.5 mol/L which is wayyy larger than any concentration of acid or base that we deal ...

George Hernandez 3I

You should check it everytime. It only takes a second to divide and find out. Better to be safe than sorry.

George Hernandez 3I

The main rule is that K can never be changed except for temperature, so that statement would certainly be false.

George Hernandez 3I

Unfortunately you need to memorize the strong acids and bases. There is only several of each. Any other acid/base that you come across that isn't part of that 'strong' should automatically be deemed weak.

George Hernandez 3I

It should be important to know the relationships between pressure, temperature, and volume as well as being able to find each variable (including moles) by using this equation.

George Hernandez 3I

Simply put, an ionic compound (one with a cation and an anion) will be a salt. An acid usually begins with an H while a base usually ends with an OH. However sometimes it can be a little more complicated than that with certain compounds like NH3 (which is a base since it accepts a proton). I would r...

George Hernandez 3I

Strong acids yield weak conjugate bases and vice versa. Strong bases yield weak conjugate acids and vice versa. A salt is comprised of a conjugate acid and conjugate base. If the two conjugates are the same strength, the salt is neutral, however if one is derived from a strong acid (it would be a we...

George Hernandez 3I

Like the others have said, its the negative log of Ka. Whenever you see "p" behind a unit denomination (Im talking about ka, kb, H, OH) then it means youre taking the negative log of that original value.

George Hernandez 3I

The coefficients determine how many x's are added or subtracted on each cooresponding side of the chemical equation.

George Hernandez 3I

So my understanding is that Ka times Kb equals Kw (1*10^14), but [H+] times [OH-] also equals Kw. Yet [H+] and [OH-] don't mean the same as Ka and Kb respectively. How is it possible that each pair of values both equal Kw when multiplied, but Ka and [H+] aren't the same and Kb and [OH-] aren't the s...

George Hernandez 3I

K is known as the equilibrium constant of products to reactants. This means that for the value of K, you have K times the concentration of reactants as you do products. For instance, lets say at equilibrium, you have 4 M of products and 2 M of reactants. Your K value in this instance would be 2 sinc...

George Hernandez 3I

One of the sapling question feedbacks for week 1 said that you could even use moles for the ice box. It really doesn't matter but for simplicity's sake, if you can do everything in mol/liter, that would probably be best to save a lot of headaches :)

George Hernandez 3I

Always have something to look forward to. Encourage yourself to complete your work in pursuit of a reward afterwards (i.e. getting to watch a movie, play videogames, go out with friends, etc.)

George Hernandez 3I

If you're fidgety when you're stressed, try investing in a high-quality stress ball. You won't regret it :)

George Hernandez 3I

The pH of a solution is determinant of the amount of H3O+ ions there are. A high concentration of a weak acid can outperform the acidity of a low concentration of a strong acid, simply because the former will protonate more water molecules simply due to its amounts, unlike the latter. So that's why ...

George Hernandez 3I

You can look up the formula, it essentially can be looked at the same as a regular concentration ratio.

George Hernandez 3I

So essentially, you have to look at each ion and reference what kind of conjugate acid/base is it from it's counterpart base/acid. If the ion comes from a strong acid, then that makes it a weak conjugate base. If it came from a weak acid, then it is a strong conjugate base. The same is said for base...

George Hernandez 3I

At that point, you're going to need the equilibrium constant, K, and the reaction quotient, Q, and compare your scenario from there. If the reaction quotient is less than K, then that means the current ratio of products to reactants is too reactant favored and will shift to the right, and vice versa...

George Hernandez 3I

Smaller than 10^-3 to be safe. A value that incredibly small will have pretty much zero effect on yoyr approximations

George Hernandez 3I

Like the others said, solving for x means finding the unknown changes to the concentrations. Remember that K will always stay the same, with the exception of temperature change. So by adding more product, some of that product will decompose/form to more reactants until it reaches the product/reactan...

George Hernandez 3I

I could be wrong but shouldn't you multiply by Avogadro's number if you're trying to go from moles to molecules/atoms (i.e. 5 moles of carbon * 6.022*10^23 = number of atoms of carbon).

George Hernandez 3I

The empirical formula is the smallest whole number ratio of the atoms within the compound. When you solve for the chemical equation (or if you simply already have it), you can find the empirical formula by finding dividing by the lowest common multiple of each of the number of atoms in the compound.

George Hernandez 3I

Yeah, like the others have said, think of the arrow as the direction of pull. The arrow points towards the more electronegative atom, as it has the stronger negative charge.

George Hernandez 3I

Expanded octets are the norm, not the exception. Mostly any atom past the third row can utilize expanded octets to create unique bonding shapes.

George Hernandez 3I

Ionic bonds are much stronger than any other force, which makes it more difficult to break apart, which in turn heightens the boiling point of the compound.

George Hernandez 3I

Instead of creating concentrated pairs of electrons, the atom instead spreads out its electrons over multiple orbitals in the d-subshell, creating multiple extra bonds to be used. More atoms are capable of expanded octets than those that aren't. It's the norm, not the exception. :)

George Hernandez 3I

It can be either or

George Hernandez 3I

If the salt in question was formed by a strong acid and strong base, then it will be a neutral salt (i.e. HCL + NaOH => H2O + NaCl), however if a weak acid or a weak base was used, then the resulting salt will be an acidic salt or a basic salt, respectively.

George Hernandez 3I

A salt cannot be strong or weak

George Hernandez 3I

pH is a form of reactivity for solutions. Different pHs cause for different reactions. pH means the log of H, meaning Hydrogen ions, on a scale of 1-14. As the others have said, more H ions means a more acidic solutions and vice versa.

George Hernandez 3I

Know that the 5s orbital is of a higher energy level than the 4d level. If the lower levels aren't filled completely first, then the higher energy levels will not be able to carry electrons in a stable state, as they will want to fall to a tighter level.

George Hernandez 3I

The l quantum number will determine the subshell. So n=3 l=0 is 3s while n=3 l=1 is 3p.

George Hernandez 3I

The problem should always tell you when an excess reactant is present in a reaction. So then you'll have to work with the limiting reactant to solve whatever it is the problem is asking. Oxygen will very typically always be in excess since it is unusual, in the real world, to run out of oxygen in an...

George Hernandez 3I

Trailing zeroes are never significant unless a decimal is present. So yes you are correct since it is for 40.

George Hernandez 3I

That's an interesting question. However the definition of the empirical formula is the whole number ratio of atoms within a molecular formula. So it would not make sense for an empirical formula to have more mass than its molecular formula.

George Hernandez 3I

A way to understand the concept of formal charge mentally is to comprehend whether an atom has gained or lost electrons. If you see an oxygen atom with 3 lone pairs and a single bond, for instance, you should be able to 'eyeball' that the atom has a formal charge of -1 because oxygen is supposed to ...

George Hernandez 3I

Pi and Sigma are greek symbols. Pi-bonds just share the same name as the mathematical pi value. It's simply a name.

George Hernandez 3I

In the science field, organic will 99% of the time refer to carbon, nitrogen, oxygen, and hydrogen (occasionally sulfur and phosphorus) and possess biological properties. And inorganic will typically refer to everything else with lack biological significance.

George Hernandez 3I

Also, the presence of H+ ions within a given solution will reflect the strength of an acid. A higher concentration means a stronger Acid. The same goes for OH- ions in the case of the strength of a base.

George Hernandez 3I

When energy is applied to the atom, the first electron that will be removed is always the outermost electron since they require the least amount of energy to take out.

George Hernandez 3I

By the looking at the structure, if the molecule is asymmetrical with bonding, electrons, etc, then it is always non-polar because that means that one side is slightly more positive/negative than the other.

George Hernandez 3I

Compounds with ionic bonds have a higher boiling point (meaning they are more difficult to boil) than molecules with h-bonds, or any other bonds for that matter.

George Hernandez 3I

Remember that all molecules have London dispersion forces. This example has dipole-dipole forces as well because it is polar. A molecule is polar based on the uneven shape that it exhibits, which is seen in its lewis structure. The uneveness creates a slight positive side, as well as a negative side...

George Hernandez 3I

Yes. Be sure to focus on electronegative atoms. Atoms like carbon aren't strong enough to create a hydrogen bond.

George Hernandez 3I

Interchangeable terms. You can also say, dipole-induced-dipole interactions. All mean the same.

George Hernandez 3I

Yes the excess energy is still classified as kinetic energy. If the work function is less than the energy of a photon, then the 'excess' kinetic energy of the electron will result as the difference. Since electrons have mass, this energy would be kinetic.

George Hernandez 3I

Ionic bonds give and take electrons (cations give away electrons while anions take electrons) and are expressed in lewis structures using dots. Covalent bonds share electrons and are expressed in lewis structures using lines.

George Hernandez 3I

I've always been turning in the homework Sunday evening (usually 8-10pm) and it's shown as graded. So yes, the date means Sunday night :)

George Hernandez 3I

A bond consists of two electrons, one from each of the bonded atoms. When you are discussing formal charge, the reason why you only count a bond as one electron is because you are only focused on a single atom, and only one electron in a bond belongs to that said atom. Hopefully that makes more sense.

George Hernandez 3I

Aluminum has a charge of +3. This means that in order for the formal charge to be zero, it needs to lose three electrons. Hydrogen has a charge of +1. It can either lose or gain an electron to be stable. In this scenario, it is best for the Aluminum to bond with three hydrogens as it will satisfy it...

George Hernandez 3I

For the photon, you would calculate the energy using e=hv. For the electron, use e=1/2mv^2. The reason why you use two different equations is because one involves mass while the other involves simply frequency. The photon's energy is calculated by converting it's frequency to the corresponding energ...

George Hernandez 3I

Remember that the wavelength and the corresponding frequency is a characteristic of the photon, and that having more of these photons will not affect this individual aspect. Think of it like horses pulling a wagon. If you add more horses, the 'intensity' of the wagon being pulled increases, but the ...

George Hernandez 3I

All wavelength equations would require you to convert to meters since that is an S.I. unit.

George Hernandez 3I

They have the shortest frequency of all waves. Which means they have the highest frequency of all waves as well. Remember that they are inversely related to each other.

George Hernandez 3I

Trailing zeroes are not significant unless there is a decimal place. Otherwise, yes they are.

George Hernandez 3I

The ability to pass through metal is a property that waves possess. So it would support the wave argument.

George Hernandez 3I

Anything that has mass must be less than the speed of light, otherwise it theoretically requires an infinite amount of energy for it to get it up to exactly the speed of light.

George Hernandez 3I

It is impossible for anything to surpass the speed of light. The speed of light is approximately 3.0*10^8 meters per second, so just remember this and you'll be good. :)

George Hernandez 3I

You have everything you need to plug it into the De Broglie equation and solve for the velocity, (Wavelength, planck's constant, mass of electron). Solve for the velocity and use E=0.5mv^2 to get the energy.

George Hernandez 3I

So remember that the wavelength and frequency is a characteristic of the photons. However, the energy emitted is based on the intensity/amount of photons. So the energy is the only factor that is affected in this scenario.

George Hernandez 3I

Yeah but it's not exactly how an atom looks visually (no one knows exactly how an atom looks, to my knowledge) but the model essentially simplifies atoms and their electron levels to work with on paper. It's still very usable, it's just not a model to represent visually how an atom looks in real life.

George Hernandez 3I

Question... I thought before we saw in the Photoelectric Effect that increasing intensity does not work to eject electrons because it is not acting like a wave model. Here, are we talking about the intensity of a wavelength then? Because in the most recent lecture, we see that increasing light inte...

George Hernandez 3I

In the ideal scenario (i.e. the problems in the homework), then yes, a singular wave of light of a particular frequency will have the exact same energy for all photons.

George Hernandez 3I

Assuming that the wave has enough energy, then yes. A higher intensity light would yield more photons, therefore more electrons being released.

George Hernandez 3I

If you do it at the end, it'll be easier for you because you'll be closer to the true answer. Otherwise, you could be quite far off and have to make estimations, especially when doing empirical formula problems.

George Hernandez 3I

If it's not too confusing, try to use the most exact values as you possibly can and round at the end. This way, you won't have much dispairity to the actual answer. If you round your values throughout the problem, as you work through it, you may expect a solid whole number at some point but actually...

George Hernandez 3I

A simple way to think about it is that a molecule is one unit, and a mole is 6.02*10^23 units. A molecule of water is H2O (two hydrogen atoms and one oxygen atom). It is unreasonably small to weigh and measure for most of our calculations, so we use the mole. A mole of water (6.02*10^23 atoms of wat...

George Hernandez 3I

Remember that one mole of two reactants will not always equal one mole of two products. You may be confused with comparing mass on both sides. The mass on both sides of the equation must always be the same because of the law of conservation of mass, but the moles can be completely different. One mol...

George Hernandez 3I

Hey, I actually just came from my discussion doing this question so I'll try my best to explain it. Hopefully others can fill in any gaps I leave out :) Note that I'll be using exact numbers from the periodic table, while solving this, to get as close to the approximate answer as possible. So the fa...

George Hernandez 3I

I find, especially when I'm trying to figure out the empirical formula, that using the entire value makes finding the approximate mole ratio a lot easier. For instance, I was in my discussion earlier and I found that if I used the entire mass without rounding, the mole ratio for the elements in the ...

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