CHEMISTRY COMMUNITY

Cody Do 2F

In essence, reaction mechanisms define the number of steps needed to form products. They offer an explanation to how a reaction occurs (the creation and subsequent consumption of intermediates). It also helps in identifying the rate of reaction based on the slowest step from all the elementary steps.

Cody Do 2F

When looking at a graphical representation, you can see that there are many places where the instantaneous rate of reaction can be calculated. However, when calculating the rate, we only look at the start of this graph. Essentially, we are ignoring the reverse reaction and focusing only on the forwa...

Cody Do 2F

What else was discussed today? I unfortunately couldn't make it because I was feeling ill. Expanding on the previous comment, as temperature increases, then K increases as well. This is because a higher temperature will increase the amount of collisions between reactants (as well as the energy in e...

Cody Do 2F

I agree with Tracy! My TA assigned a book problem that gave a table of concentrations and rates and then asked to solve for various things like the order of the reaction and k. As everything that is assigned as homework is seen as free game for the final (Dr. Lavelle pulls questions from the homewor...

Cody Do 2F

It might be helpful to know where the equations come from, but I believe the final/core equation will be provided on the equation sheet. Unless another equation is taught to us that requires the use of derivatives/integrations later on, I don't think it's absolutely necessary to learn these concepts...

Cody Do 2F

Knowing the order of a reaction will also help in either identifying or creating the appropriate graph for the reaction. So far, in class, we've gone through first order reactions, but we'll be covering zero order reactions and second order reactions in lecture this week. Though third and fourth ord...

Cody Do 2F

When an anode or cathode do not undergo a phase change, you're correct that you use a comma to separate the two instead of a bar. It is important to note that if there is a situation like this, you should place the oxidized element first and then the reduced element. Using your example, Fe 3+ (a...

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The reducing agent is the one who reduces another element while the oxidizing agent is the one who oxidizes another element. In order to reduce another element, the reducing agent must itself be oxidized. Likewise, in order to oxidize another element, the oxidizing element must itself be reduced.

Cody Do 2F

If you get confused, it helps to think about it like this: Much like a hiring agent at any firm is in charge of hiring other individuals, an oxidizing agent is in charge of oxidizing other elements and a reducing agent is in charge of reducing other elements. In order to oxidize other elements, the ...

Cody Do 2F

Isobaric means there's no change in pressure. As pressure is constant, use the formula w=-p*deltaV to find the work. Other words include Isochoric, which means there's no change in volume (AKA no work) and isothermic, which means no change in temperature (Delta U = 0).

Cody Do 2F

The acronym "PD TV HUGS" help with remembering the state functions. They are: pressure, density, temperature, volume, enthalpy, internal energy, gibbs free energy, and entropy.

Cody Do 2F

Gibbs Free Energy is a concept developed to explain the energy within a system that is readily available to do work. It can't be directly measured and thus must be ascertained through calculations. When discussing the change in Gibbs Free Energy, we can say that if the value is negative, then the re...

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The Third Law of Thermodynamics describes a theoretical situation (it doesn't really exist). Essentially, it's saying that a perfect crystal (an item that is composed of molecules/atoms that are exactly the same and has only one possible alignment between molecules) is at 0 degrees Kelvin (the lowes...

Cody Do 2F

It's probably a safer bet to know how to utilize PV=nRT to see if there is a change in pressure/volume/temperature, as that's what we did in one of the example questions in class yesterday. Essentially, just start marking off each aspect of the formula that remains constant in the reaction/situation...

Cody Do 2F

During one of Lavelle's lectures, he mentioned that intensive properties are more useful in the field of science due to the fact that they are properties intrinsic to the material (they don't depend on the amount).

Cody Do 2F

In reference to the plant question, any biological organism is considered an open system. This is because all organisms that we know of today interact with their environment in various ways. Plants take in matter through the air and ground, which is used to grow, and also utilize solar energy from t...

Cody Do 2F

In class today, we spoke about residual entropy mostly in terms of absolute cooling (where the temperature is 0 K, or rather, as close to 0 K as we can presently reach). In molecules where degeneracy > 1 exist, even in the event that 0 K were reached, some level of residual entropy would exist due t...

Cody Do 2F

In essence, degeneracy refers to how many states available for one energy level. For example, in class, we briefly spoke about the molecule CH 3 Cl. As the Cl atom can be in one of four different locations (as the CH 3 Cl molecule is tetrahedral), it has a degeneracy of 4. However, the molecule CH 4...

Cody Do 2F

Delta T is the change in temperature (Final Temperature - Initial Temperature) while Delta H is the change in Enthalpy.

Cody Do 2F

According to Lavelle, intensive properties are more useful in the field of science due to the fact that they are properties universal to the material (they don't depend on the amount).

Cody Do 2F

It's important to notice the state of each element as well! Le Chatelier's principle says there is no effect due to a change in pressure (which would cause a change in volume) as long as both sides have equal moles of gas.

Cody Do 2F

If anything, try to think of it in terms of Le Chatelier's Principle of trying to reduce any stress added to the system. For endothermic reactions, heat is absorbed. Thus, if the temperature is increased, the system has more heat, and this extra heat is offset by being absorbed through more endother...

Cody Do 2F

When temperature increases, if the reaction is endothermic, then the products are favored. Conversely, if the temperature is decreased for an endothermic reaction, then the reactants are favored. When temperature increases for an exothermic reaction, then the reactants are favored. Conversely, if th...

Cody Do 2F

The reason why the answers are given in three sig figs instead of two is due to the rules of significant figures. When you do the ICE table to find the x value, the x-value is in fact given to two sig figs due to the K c 's value of 0.56 (x is equal to 0.07). However, as you're asked to find the equ...

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K_w refers to the ionization constant of water. When we learned it specifically in class, K_w at 25 degrees Celsius is 1.0 x 10^-14. This means that at 25 degrees Celsius, the concentration of H₃O+ and OH- are equal. Thus, K^w = [H³O+][OH-] = [1.0 x 10^-7][1.0 x 10^-7] = 1.0 x 10 x ^-14.

Cody Do 2F

Yes, in ICE tables, you exclude the solids and pure liquids from the calculations. This makes sense as these values are occasionally used they are in the k calculation, which also excludes solids and pure liquids.

Cody Do 2F

Pressure can be increased by adding an inert gas into the container! However, the inert gas won't cause any changes in the equilibrium of the chemical equation. Additionally, pressure can also increase if you increase the temperature, as they are both directly proportional to each other (PV=nRT).

Cody Do 2F

For similar reasons, pure liquids are not used in the calculation for the equilibrium constant. Only gases and aqueous molecules are used in calculations for K.

Cody Do 2F

We know that for the equation N 2 + 3H 2 <-> 2NH 3 K= 41. When we write it out: K= P NH3 2 /(P N2 )(P H2 ) 3 = 41 For part a, the equation given is 2NH 3 <-> N 2 + 3 H 2 . Thus they are looking for K where: K= (P N2 )(P H2 ) 3 /P NH3 2 = 41 When you look at it, this is essentially the inverse of the...

Cody Do 2F

Something that helps me figure out which side of the equation is favored is to visualize the equation as a tub of water. If you add more water to the left side (adding more of a reactant), then the water will move to the right side (favor the products) in order to reach equilibrium. The opposite is ...

Cody Do 2F

A Bronsted acid is a proton donor and a Bronsted base is a proton acceptor. Conversely, a Lewis acid is an electron pair acceptor while a Lewis base is an electron pair donor. They both describe the same thing, the only difference is what is being focused on (the proton or the electron pair). For ex...

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The shape of the ligand (or rather, where each atom is placed) does affect whether a molecule is mono/bi/tridentate! For example, if you had a ring-shaped ligand that had a binding site on one end and another binding site on the complete opposite end, then the molecule is monodentate (This is becaus...

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As you said, a normal ligand is simply a molecule that binds to the central atom and sticks off one of its sides. However, a chelating ligand is where ONE molecule can bond to more than one side of a metal atom. Essentially, the way the chelating ligand is formed of multiple single bonds that allow ...

Cody Do 2F

Since polarizability depends on the size of the atom (large atoms = highly polarizable), its trend would follow that of atomic radius. Thus, an atom's size (and it's polarizability) increases as you go down a group and decreases as you go from left to right on a period.

Cody Do 2F

The strength of ionic character in a covalent bond can be determined by the difference in electronegativity between the atoms! A larger difference means greater ionic character. Electronegativity increases as you go from left to right on a period and decreases as you go down a group (Fluorine is the...

Cody Do 2F

Yeah, the overall charge of -4 is given by the superscript placed after the end square bracket! [Fe(CN) 6 ] -4 would mean that the entire [Fe(CN) 6 ] compound has a overall charge of -4 and thus Fe would have an oxidation state of +2. If it were [Fe(CN) 6 ] -3 instead, then the entire [Fe(CN) 6 ] co...

Cody Do 2F

An atom is highly polarizable if it readily undergoes a large distortion (in layman's terms, if the atom's electron cloud is easily distorted). An atom can be expected to be highly polarizable if it's large as the outer electrons are not as strongly attracted to the positive nucleus and thus not hel...

Cody Do 2F

Hey Tamera! We know that the entire compound [Fe(CN)6] has a overall charge of -4. We also know that CN has a charge of -1 (Simply from making the lewis diagram and counting all the charges, etc.). The compound [Fe(CN)6) has 6 CN molecules, thus giving it a charge of -6. Now we ask ourselves what va...

Cody Do 2F

Hey, Layla! For lone pairs, you have to see them as extremely repulsive in nature (they push everything, including bonds and other lone pairs, away from them). A linear molecule is flat and linear because the two atoms connected to the central atom want to be as far away from each other as possible....

Cody Do 2F

Using the Formal Charge formula, you'd calculate the formal charge for each atom. You want the resonance structure that has the appropriate charge for the molecule (neutral, positive, or negative) AND that has each atom with a formal charge closest to 0. If things are looking quite similar to each o...

Cody Do 2F

I'm sure there's a way to investigate the exact effect that a lone pair has on each bond angle (either through mathematical calculations or experimental observation), but I think that's well beyond the scope of this class. Lavelle said multiple times in class that, as long we understand that there i...

Cody Do 2F

Pretend that the nucleus of each atom has a large circle around it. This is the electron cloud, the region where electrons could be found (we can't say where electrons are exactly so we kinda just create an area where they're most likely to be found—hence the electron cloud). Now say you have sodium...

Cody Do 2F

As for the trend in electronegativity, it typically increases as you go across a period and decreases as you go down a group. This is because as you go down a group, the outer electrons are in higher energy levels and thus farther away from the positive nucleus' attractive forces. As you go across t...

Cody Do 2F

Hey Avery! Both the dotted and solid lines are notations for interactions! A solid line represents a covalent bond between two elements, a type of interaction in which the elements share electrons. A dotted line, however, represents interactions between two molecules (not elements!) due to their sli...

Cody Do 2F

A coordinate covalent bond is simply a form of covalent bond where both electrons come from the same atom!

Cody Do 2F

First, count the amount of electrons present! One carbon has 4 valence electrons, so two carbons will give us 8 electrons. As the C 2 has a negative charge associated with it, add an extra electron. Thus, there is a total of 9 electrons. When drawing the Lewis Structure, draw the two Carbons with a ...

Cody Do 2F

First, set up a chemical equation to help see where everything lies: C x H x O x + O 2 --> CO 2 + H 2 O. Then, you divide the molar mass of CO2 (44.011g) by that of Carbon (12.01g). This will give you about .2729. This value represents that 27.29% of the CO2 produced is carbon. Multiply .2729 by the...

Cody Do 2F

Our TA gave us a chart that told us how many bonds each molecule typically has, as well as the number of lone pairs they have. This apparently helps a lot in O-Chem too, so knowing it in advance will probably help tons. (# of Bonds, # of Lone Pairs) Carbon- 4, 0 Hydrogen- 1, 0 Oxygen- 2, 2 Nitrogen-...

Cody Do 2F

It's likely that we won't be needing blue books for the chem midterm. Dr. Lavelle never told us to get it in class and those who took the Chem 20A midterm didn't need blue books.

Cody Do 2F

I don't believe we've been taught a way to calculate whether an electron is +1/2 or -1/2. The important parts to remember regarding electron spin are: +1/2 is counterclockwise and -1/2 is clockwise Pauli Exclusion Principle: Only two electrons per orbital. If there are two electrons, then they spin ...

Cody Do 2F

The wave function is basically a math expression that holds information about an electron. It tells us the energy level (n), the angular momentum (l), and the magnetic quantum number (m l ). The probability density is just the square of the wave function, and it tells us the probability that an elec...

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The Bohr Frequency Condition says that when the frequency matches the energy needed for the electron to jump to a higher energy level, then the light will be absorbed. If the frequency doesn't match the energy needed, then the light won't be absorbed. In essence, it simply states that light needs to...

Cody Do 2F

For part (a) of question 33, you use the De Broglie equation in order to find the wavelength of the ejected electron. Thus, energy is not calculated in part (a). Part (b) provides to the frequency of light that hits the metal and ejects the electron. Thus you use the frequency to find the energy of ...

Cody Do 2F

Hi Jules! The correct answer is E: both C and D are correct! During Dr. Lavelle's lecture for the Photoelectric Effect, one of the key concepts he highlighted was that unless Energy of a Photon is greater than or equal to the Energy used to remove an electron, then no electron is emitted. In essence...

Cody Do 2F

Hi Jane! The test is usually based on the homework questions assigned and as questions 11 and 13 (in some part) of the sixth edition reference the series, it's definitely a possibility that the series could be on the test. It's probably better to be safe than sorry in this situation and try to learn...

Cody Do 2F

Hey Searra! I believe that Dr. Lavelle was simply trying to teach us that the orbitals were based on mathematical functions (Schrodinger's Equation). These functions, when graphed on the x-y-z plane, produce the orbital designs that we saw today in class. However—and correct me if I'm wrong—we won't...

Cody Do 2F

Rydberg's Constant is used in the equation E n =-hR/n 2 The equation is used to find the energy released (or absorbed) when an electron either drops in energy or jumps in energy level. *Keep in mind that the energy should be negative if the electron is losing energy (dropping in energy level) and po...

Cody Do 2F

Hey Tatiana! As the energy levels of an electron increase, then the electron actually moves away from the nucleus. This means that high energy levels are quite far from the nucleus. When using the equation E n =-hR/n 2 , a higher n would make E n closer to 0. This is because when the electron reache...

Cody Do 2F

When changing from one energy level to the next, the electron needs a specific amount of energy in order to jump. If the energy isn't present, then nothing happens. For example, if we say that we need 5 J to jump from one energy level to another. If a photon with only 4 J hits it, then no energy is ...

Cody Do 2F

The rule of significant figures should be used for the final answer! You'd want to try to use exact values throughout all your calculations because it minimizes the chance for serious error—rounding error from multiple values would add up and could significantly alter your final answer!

Cody Do 2F

Coefficients aren't included in finding the molar mass of the reactants or products because if the coefficients were included, then the molar mass of the molecule would be incorrect! Molar mass refers to the mass of ONE molecule/atom—including the coefficient in calculations would be like finding th...

Cody Do 2F

Yep! Since you have the mass percentage breakdown now, use that information to find the empirical formula. You do this by pretending that we have 100g of the substance (this makes it easier on us as the percentages can now just be used as grams). Divide each element by its molecular weight, which is...

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