CHEMISTRY COMMUNITY

William Chan 1D

Extensive properties change with the amount of substance available. Intensive properties do not change with the amount of substance available. For example, if I had 10g of Fe and 100g of Fe, then the amount of heat it takes to heat up 1 g or 1 mol of that substance doesn't change.

William Chan 1D

By regular, do you mean a constant-pressure calorimeter? This would be like an insulated styrofoam cup with an open top.

A bomb calorimeter has constant volume, so it isn't open. This is usually considered an isolated system, for no heat or matter can be exchanged with the environment.

William Chan 1D

If there is solid metal in the redox reaction (like Cu), then that can be used as the anode or cathode. If there is no solid metal (like Fe3+ to Fe2+), then a nonreactive metal like C (graphite) or Platinum should be used.

William Chan 1D

I think it is rare, but if you increase the concentration of certain reactants, then the overall reaction rate will decrease.

I found an example of the conversion of ozone to oxygen here: https://www.quora.com/What-are-negative-order-reactions-And-what-are-the-examples

William Chan 1D

First-order reactions have a linear relationship between time and ln([A]).

William Chan 1D

My TA dropped it off at her mailbox at Young Hall. Ask your TA about it

William Chan 1D

It is Faraday's constant and should be given in the equations sheet.

William Chan 1D

Yes, I think the slow step is determined experimentally in real life. Then a mechanism is proposed to match it.

William Chan 1D

"n" is the number of electrons that are transferred as you balance the respective reduction and oxidation reactions.

William Chan 1D

There has to be some difference in the cell potential of both cells for it to do work.

William Chan 1D

The grades are up on MyUCLA, but I would email your TA if you want more specific information.

William Chan 1D

Having zero entropy is theoretical. The substance must be in a perfect crystal structure at absolute zero. This would eliminate any positional entropy as well as any entropy from movement.

William Chan 1D

Gibbs free energy is defined by a combination of enthalpy, temperature, and entropy. If Gibbs free energy is negative, that means the reaction is spontaneous, and if it is positive, the reaction will not be spontaneous. Note, spontaneity does NOT mean that the reaction will just proceed quickly, jus...

William Chan 1D

Basically, if the change in energy of the system is 0, then that means no energy is lost or gained in the system. That means that any heat is used to do work or vice versa.

William Chan 1D

The pressure of a system is not completely defined by the number of molecules in a system, but also their average kinetic energy, or temperature. Pressure can be conceptualized by molecules hitting a surface, and "pushing on it," in a way. The more collisions that there are, and the higher...

William Chan 1D

Kelvin. If the reaction happened at below 0°C, then the subsequent equation would be negative, having some weird consequences (i.e., negative pressure, negative volume).

William Chan 1D

The answer would be b. The amount of mols HCL and NaOH reacting would is the highest in this problem, as well as the lowest volume of liquid. Because there is less liquid in the beaker, the temperature difference would be higher.

William Chan 1D

Using Le Chatlier's principle to simplify things, we can imagine heat as either a reactant or a product. In an exothermic reaction, heat can be treated as a product because heat is given off during an exothermic reaction. By increasing temperature, we are essentially increasing the heat available in...

William Chan 1D

The "°" in the equation refers to standard conditions. This means 25°C, 1 atm, 1M. If it isn't present, then the property may not be at standard conditions.

William Chan 1D

^above, the w stands for water. kw is the equilibrium constant for water at 25°C. For the actual question, from pka, we can determine ka. Using ka, we can use an ice table, using the initial concentrations of the reactants and products to determine the [H+] concentration, and using [H+], we can dete...

William Chan 1D

The general rule is that, if k is smaller than 10^-4, then it should be safe to assume that the change in concentration is negligible and that x can be ignored. Of course, you can always check that the difference is less than 5% to make sure.

William Chan 1D

If the reactants and products are given in units of pressure (bars, atmospheres, etc.), then use kp. If reactants and products are given in units of concentration (mol/L), then use kc.

William Chan 1D

You can use an ICE table to help you figure out the concentrations in terms of known concentrations and changes in concentrations (usually determined in terms of 'x'). Depending on the size of k (if k is larger), then you may have to use the quadratic formula to solve for x. If the change in concent...

William Chan 1D

Yes. Intermediates are molecules that are formed in one step of an overall reaction and consumed in one of the following steps. It will not show up in the overall reaction as it will cancel out once you add up the steps of the equation.

William Chan 1D

Technically, yes the rate order could be determined if you know the units of k. The units for k do change for different order equations. If you check to see that the units cancel out and are equal on both sides of the equation, you should be okay.

William Chan 1D

They should be given on the equation sheet, so the derivation isn't necessary. Knowing how to apply it will be most likely all you need to know, but it doesn't hurt to know how to derive it.

William Chan 1D

Half-life reaction rates are just a convenient way of measuring the time it takes for half the substance to decay. We could as easily have "quarter-life reaction rates," but this is more conventional. Most likely, it will just ask us to calculate the half-life of a certain substance of a g...

William Chan 1D

A 0 order reaction occurs when the concentration of the reactant doesn't matter. This most often happens when there is a catalyst present, and when the catalyst is catalyzing the reaction at "max" rate already, changing the concentration won't affect the overall reaction rate.

William Chan 1D

An intermediate will appear in one of the step and be used up in a subsequent step. It will not show up in the overall reaction.

William Chan 1D

I personally find that taking practice tests (like Lyndon's) helps a lot with the application of these concepts, so doing problems off of the topic outlines would definitely be helpful.

William Chan 1D

I think we'll be given the other components, and solving for just one of those variables.

William Chan 1D

Yes, you reverse it for the oxidation reactions.

William Chan 1D

I think the salt bridge has a semi-permeable membrane that allows ions to pass through it. If the ions don't flow into the cells, then the charge wouldn't be balanced and the flow of electrons would stop almost immediately.

William Chan 1D

The anode is where the oxidation reaction takes place. The cathode is where the reduction reaction takes place. Typically, in cell diagrams, the anode is written on the left and the cathode is written on the right. An(ode)Ox(idation)Red(duction)Cat(ode) is an helpful tool to remember.

William Chan 1D

If a reaction is exothermic, you can treat heat as a "reactant," where increasing the overall temperature will shift the equilibrium to the left, and decreasing the overall temperature will shift the reaction to the right.

William Chan 1D

pH is simply -log([H₃O⁺]). When the molarity of H₃O⁺ is higher than 1 M, then the pH will begin to become negative. Acids aren't typically this strong, and there's actually a special category for such strong acids called superacids, but they aren't common.

William Chan 1D

Inert gases are gases that don't react with any of the reactants or products. Noble gases are often used because they are notoriously unreactive.

William Chan 1D

When using a bomb calorimeter, we are usually measuring a change in the surroundings, and since q(system) = -q(surroundings), it'll be the opposite sign you measure in the surroundings. For example, for an exothermic reaction, you measure an increase in q of the surroundings, so q be positive for th...

William Chan 1D

It is proton transfer between two of the same molecule. For example, if we have two water molecules, one can donate a proton to the other, and result in OH- and H30+ molecules.

William Chan 1D

That's a vague question, but a titration is a procedure to determine the concentration of an unknown acid or base using titrant of a known concentration. The titrant is added until the equivalence point is reached. Using the information on how much titrant was used, we can solve for the concentratio...

William Chan 1D

Solids and liquids are essentially non-compressible, so their concentrations don't really change. Gas concentrations or ion concentrations will change though, those are the values we calculate for.

William Chan 1D

If a reaction is endothermic, it means it absorbs heat when the reaction proceeds forward. Adding more heat allows more forward reactions to occur, shifting the equilibrium to the right. It something helps to think of heat as a reactant in this case, where, according to Le Chatlier's principle, addi...

William Chan 1D

Under what conditions does the ideal gas law deviate and why? Do we need to memorize why?

William Chan 1D

The "quick" way refers to looking at the moles of gas on each side of the equation, and when pressure changes, let's say increased, then the reaction will shift toward the side with fewer moles of gas because it "relieves" some of the pressure.

William Chan 1D

I don't think we'll be expected to memorize k values, and will most likely be given by the problem.

William Chan 1D

If Q = K, then the reaction is at equilibrium.

William Chan 1D

They are both units of pressure and can be interchanged through unit conversions (like pounds and kilograms).

William Chan 1D

It applies to changes in temperature, pressure (only when it changes the concentration from a change in volume), or a direct change in concentration (from adding reactants/products).

William Chan 1D

K is a constant which doesn't change. Q is the reaction quotient, which is calculated the same as K, but the key difference is that k is calculated when the reaction is at equilibrium, while q doesn't have to be. q can be used to determine whether or not the equation will proceed forward or backward.

William Chan 1D

K doesn't have units technically because they all cancel out, and it comes out to be just a number ratio.

William Chan 1D

Ice tables are just to help us keep track of the molar concentrations before and after reactions occur. I stands for Initial concentration C stands for Change in concentration E stands for Equilibrium concentration We denote the change in concentration in terms 'x' usually, and solve for it to deter...

William Chan 1D

K, the equilibrium constant, is a constant, which at some set conditions, is a constant at equilibrium. Q is calculated the same way but is calculated to see whether the reaction proceeds forward or backward to reach equilibrium.

William Chan 1D

You can use VSEPR to determine the shape, some of which can more easily lead to polarization than others. In general, though, you need to know the electronegativity of the atoms.

William Chan 1D

Trichloroacetic acid has 3 chlorine molecules, which pull harder on the electrons because chlorine has a higher electronegativity than hydrogen does. The stability of the anion determines the strength of the acid. When either trichloroacetic acid or acetic acid loses a hydrogen ion, the oxygen is le...

William Chan 1D

Remember that many chemical equations are at equilibrium, where there are forward and backward reactions happening all the tine. If a reaction is "favored to the right," the reactants will react/decay to form more products. If the reaction is "favored to the left," then the produ...

William Chan 1D

I think it's based on the sulfur content. The higher the sulfur content of the coal, the "dirtier" it is because, when burned, it'll react with the air to create acid rain.

William Chan 1D

If there's more than one H in front of the anion, let's say,

H2SO4, or H3PO4

It'll be polyprotic because there is more than 1 hydrogen in front.

William Chan 1D

How can we identify lewis acids and bases?

William Chan 1D

Will we have to figure out the pH of a weak acid/strong base interaction at equilibrium?

William Chan 1D

In most circumstances, we measure pH of a solution. It makes things easier/more standard, but we can also measure pOH. Just as pH is defined by -log[H+], pOH is defined by -log[OH-]. Because of the equilibrium constant of water, pH and pOH have an inverse relationship. pH = 14 - pOH, and pOH = 14 - ...

William Chan 1D

A Bronsted acid is simply defined as a proton donor.

A polyprotic acid is an acid can give multiple H+ ions to a solution, so yes, it is considered a Bronsted acid.

William Chan 1D

pH is a measure of the concentration of H+ ions, or more accurately, H3O+.

It's measured using the -log[H+].

The normal pH range is 1 - 14, but if the H+ concentration is higher (super acid) or lower (super base), it'll be outside that range.

William Chan 1D

Should we memorize the structure of cisplatin and transplatin? And what about hemoglobin and myoglobin?

William Chan 1D

In class, we did discuss cisplatin and hemoglobin/myoglobin, but I think he was discussing these more to show the prevalence of coordination compounds in biological systems. It certainly wouldn't hurt to understand a little bit about how these molecules function, but I don't think we need to know th...

William Chan 1D

What are some examples of when we would need to use this naming convention?

William Chan 1D

Ligands that are attached to the central ion (transition metal) would be considered inside the coordination sphere.

William Chan 1D

They can be thought of as the electron pair donors. They attach to the transition metal to form the coordination compouds.

William Chan 1D

The coordination sphere is made up of the ligands directly attached to the central ion.
If the coordination sphere has a net negative charge, it has the potential to attract cations to help neutralize the charge.

William Chan 1D

I can't say for certain but I believe that the number of electrons is more important than the shape. Whether it's linear or not will affect the IMF's, but I believe that, if anything, we will only have to compare molecules with the same molecular formula but different shapes, not something like this.

William Chan 1D

Yes, know bond angles for shapes.
Some key numbers are: 180°, 120°, 109.5°, and 90°
Lone electron pairs will push bonds closer together, making them slightly less than what those angles would be.

William Chan 1D

Lavelle mentioned that hybridization wouldn't be on Test 2.

William Chan 1D

All of the mentioned shapes have 6 pairs of electrons around the central atom. What differs is how many lone pairs and bonding pairs each has. Octahedral structures have 6 bonding pairs and 0 lone pairs. Square pyramidal structures have 5 bonding pairs and 1 lone pair. This one, as the name suggests...

William Chan 1D

When an atom has lone pairs of electrons, the bond angles are often written with a "less-than" symbol because lone electron pairs have a slightly higher repulsion than normal bonding pairs do, so they push them away more lone pairs, and thus a little closer to other bonding pairs.

William Chan 1D

There are differences. Dipole-dipole interactions are present in polar molecules that have a side with a partial negative and a side with a partial positive charge. These partial charges create attractions between the molecules. Induced-dipole induced-dipole interactions are present in all molecules...

William Chan 1D

The boiling point of a molecule is determined by that molecule's IMF's, or intermolecular forces. The stronger the IMF's the higher the boiling point, and the lower the IMF's the lower the boiling point. CH4 and CCl4 are similar atoms, but the key difference is that chlorine has many more electrons ...

William Chan 1D

Nitrogen, oxygen, and fluorine have a relatively high electronegativity. When bonded with a hydrogen atom, these atoms will typically have a stronger partial negative charge, leaving hydrogen with a stronger partial positive charge. This makes the IMF's of these molecules stronger.

William Chan 1D

Dipole-dipole interactions are attractions between polar molecules with partial charges. Hydrogen bonding is similar to dipole-dipole interactions but is considered stronger because of the presence of hydrogen, and another strongly electronegative atom (N, O, or F) For practical purposes, Lavelle sa...

William Chan 1D

All molecules have London forces. These forces happen as a result of the random, uneven electron distribution around molecules. The more electrons an atom has, the stronger these forces will be.

William Chan 1D

Filling the octet refers to the idea that most elements want to complete their valence shell.

The valence shell (for 2nd period, usually for elements after carbon) will be the s and p subshells, which will have 8 electrons total.

William Chan 1D

I think that it's better to put one dot on each side

William Chan 1D

Longer bonds generally mean they're weaker, and shorter bonds generally mean they're stronger.

William Chan 1D

Dr. Lavelle talked about how fluorine, F2, has a weaker than expected bond strength because of lone electron pairs on neighboring atoms.

Why do lone electron pairs on neighboring atoms weaken the bonds?

William Chan 1D

Cations are atoms that have lost an electron and therefore have a positive charge. Anions are atoms that have gained an electron and therefore have a negative charge. Cations and anions are important to understand because they tell us why certain things like ionic salts can form, and also has applic...

William Chan 1D

In high school, we would fill out the electron configuration with the order they would fill in... for example bromine: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5 ... but in Lavelle's lecture, he said that we should fill them in with respect to their energy levels... bromine: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5 A...

William Chan 1D

In lecture today, Lavelle showed that sulfur can have 6 bonds (2 double bonds, 2 single bonds). Wouldn’t that be more than a full shell (of 8 e-)? Why is this allowed?

William Chan 1D

What kind of electron configuration exceptions are we expected to know? I know that Chromium and Copper will take an electron from the s subshell to stabilize the d subshell, but is there anything else we need to know?

William Chan 1D

Velocity and speed are often used interchangeably. None of the calculations we make in this class will necessarily require us to know the direction of light, so we just focus on the speed instead.

William Chan 1D

I believe you mean when the hydrogen electron gets excited. The lowest energy level is n=1, and it goes up from there, n = 2, 3, etc. Let's say we have an electron at n =2. When a photon with the right amount of energy hits an electron, it will excite it to a higher energy level, let's say n = 3. It...

William Chan 1D

It will be on the reference sheet. You won't need to memorize them.

William Chan 1D

Do we need to know magnetic spin for the test? Also, why is it + or - 1/2, and not a whole number?

William Chan 1D

Electrostatic potential energy is essentially the force that charged particles, like protons and electrons, will "feel." Like charges will repel each other and opposite charges will attract. In multielectron systems, the net force on electrons in the outer shells is reduced due to shieldin...

William Chan 1D

Does the rydberg constant only work for hydrogen atoms? Or can you use it for every element to solve spectral line problems?

William Chan 1D

The question is asking for the angular quantum number, or l. In the question, they give us that n=7. n is the principal quantum number. The textbook goes more in depth about n, but for this question, they're asking for l. Given n, we can determine all the possible angular quantum numbers using the e...

William Chan 1D

h bar, or ℏ, is h/2π.

William Chan 1D

It's not h, it's h bar, or ℏ, which is equal to h/2π, or about 1.054457 x 10^34

William Chan 1D

The equation is ΔpΔx ≥ 1/2 ℏ.

Not that ℏ is equal to h/2π, so in essence, ΔpΔx is equal to h /π, or 1/2 ( h/2π ). I think this is where the other 1/2 came from.

William Chan 1D

Usually leaving the answer in scientific notation (in powers of 10) is sufficient.

William Chan 1D

As far as I know, there isn't really any shortcut to determining the limiting reactant other than first converting the masses to moles, then comparing those numbers to the balanced chemical equation. I'm not really that efficient, I just trial and error, but this process shouldn't take too long.

William Chan 1D

It appears that both chemical equations are balanced, which makes things a bit easier. The first step is to convert both masses to moles. Given 0.036g of methane, and that the molar mass of methane is about 16g/mol, we can determine that we have about 2.25 x 10^-3 mols of methane. Given 0.108 grams ...

William Chan 1D

Yes, in addition to knowing Avogadro's number, which is ~6.02 x 10^23, you would also need to take into account that the atom's radius is 144 pm, but its diameter would be 288 pm.

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