CHEMISTRY COMMUNITY

David S

Looks like a concentration cell to me. If we want our concentration cell to do work, Ecell has to be positive. Since Ecell = -(RT/nF)lnQ (E° = 0 for conc. cell), and the ln of numbers less than 1 is negative, we know that we want a Q that is less than one. For Q to be less than one, [P]<[R]. Thus, m...

David S

You first set up ice table and Ka expression for the diprotic acid, and then you solve for [H+]and [HA-]. You then set up separate ice table and Ka2 expression for the dissociation of HA- into A2- and H+ using the [H+] and [HA-] found in part 1. Solve for new [H+] and then turn it into pH. Note that...

David S

The total heat involved in the process = heat needed to melt the ice into liquid (q=n∆H melting) + heat needed to raise temperature of liquid from 0°C to 20°C (q=nC∆T)

David S

I got 0.3 and here is how: 1) K=\frac{(P_{Br_{2}})(P_{Cl_{2}})}{(P_{BrCl})^{2}} 2) After setting up ice table and finding expressions for equilibrium pressures in terms of x, you get: K = \frac{x^{2}}{(3.30-2x)^{2}} 3) multiply both sides by denominator and then simpl...

David S

You can only use the approximation when equilibrium constant K is notably small. In class, lavelle told us that in general, when K < 10^-3, we can assume the amount of reactant lost to form product (x) is small enough to ignore. After using this approximation to solve for x, you have to check it usi...

David S

The adjective "spontaneous" is specifically used in this case to describe a reaction with a negative delta G (Greactant > Gproduct). The activation energy has to do with kinetics, which only has to do with rates of reaction, so you dont have to pay attention to it when determining whether ...

David S

I'm assuming you are referring to balancing redox reactions in acidic or basic conditions. When you are balancing equations, you want to make sure that the same amount of atoms are on both sides of the equation right? This applies to hydrogen as well, and when you are balancing redox reactions that ...

David S

Looking at Professor Lavelle's learning outcomes for Electrochemistry (you can access this on the 14B class website), I do not see any explicit emphasis on those types of electrodes so you don't have to really know about it in detail. However, you should know about inert electrodes (e.g. Platinum, G...

David S

Delta G, or the change in Gibbs free energy, represents the change in the max amount of energy available to do non-expansion work at a given Temp and Pressure. In other words \Delta G = w_{max} @ const. T and P. Exergonic processes have negative Delta G values, which corresponds to the release of en...

David S

I believe so, because the oxidizing agent is the reactant in the reduction half reaction, and the reducing agent is the reactant in the oxidation half reaction. Since the total redox reaction is the simple sum of the balanced half reactions, both the oxidizing and reducing agents end up in the react...

David S

I don't believe we need to use Platinum as an electrode here because the copper and zinc can react with their solid electrodes to be reduced and oxidized respectively, making both metals decent conductors of electricity. We would need an inert Platinum electrode to act as an electricity conductor if...

David S

I think you use $\Delta G=\Delta G^{^{\circ}} + RTln(Q)$

David S

The Van't Hoff equation is meant to show the Temperature dependance of K, and while it is derived from an expression that relates G to K, in its ultimate for the equation excludes G. Derivation: 1) Given \Delta G = -RTln(K) and \Delta G = \Delta H - T\Delta S : \Delta H - T\Delta S = -RTln&#...

David S

K < 1 implies that at a given temp, the reactants are more favored than products, and thus the forward reaction to form more products is unfavorable. This is confirmed by a Positive Delta G, which tells us that the forward reaction is not spontaneous (endergonic), and needs energy/work input to be d...

David S

We use this equation to calculate the equilibrium constant K for a reaction at a different, new temperature (K is dependent on temperature). Specifically, K1 is your currently known equilibrium constant at initial Temperature (T1). Then, given that you know the reaction enthalpy, you use this equati...

David S

The opposite would be the negative value of \Delta H_{vap} . This is because if you put in X amount of heat energy to get a certain liquid to become a gas, that gas must lose the same amount of energy to return to being a liquid. Losing energy => exothermic process => negative enthalpy for the proce...

David S

The degeneracy of a system is basically the number of ways you can arrange the components of the system (molecules, atoms, etc.) while keeping the energy level of the system constant. In the simple cases we covered in class (Atoms A and B being in the left or right flask), degeneracy (W) = (# of pos...

David S

An open system allows both matter and energy to enter/leave the system. Think of a cup of tea with no lid on; the water is leaving the cup in the form of vapor, and the tea cools over time because there is heat transfer from the system to the surroundings. A closed system on the other hand is when e...

David S

Imagine if you are to climb up a mountain. Let's say I ask you for the distance you traveled to get to the top, which is 400 meters above the starting point altitude wise. If you follow a trail the distance to the top might be 10 miles. But if you take a bunch of shortcuts, you might get there after...

David S

This is because in reality, the strength and energy of a bond between individual atoms is influenced by other electrostatic forces in more complex molecules, and so therefore there is no single enthalpy value to easily describe most bonds. Take for example a bond enthalpy for O-H. Not all O-H bonds ...

David S

In theory, with enough reference tables, you could use whatever method, but in certain problems and situations, given info will make one method more preferable than another. For example, If I want to calculate the enthalpy of a reaction, and that reaction is a sum of other reations whose enthalpies ...

David S

Yes, because by increasing the concentration of any one reactant, the denominator (i.e. the product of reactant concentrations) of the quotient Q will become greater than the denominator of K at equilibrium. Thus, Q < K, and reaction will proceed to balance this larger denominator out by creating mo...

David S

Homeworks are due on whatever day you have discussion, and you turn it in during discussion

David S

Pure solids and liquids are considered to have an activity level of 1, which roughly equates to them having no effect on equilibrium concentrations

David S

HPO4^2- is an amphiprotic derivative of H3PO4. We know that with polyprotic acids, after the first proton is lost, subsequent amphiprotic acids formed are substantially harder to deprotonate (removing an H+ from an increasingly negative anion molecule is harder); the increase in pKa with each additi...

David S

No, but along with what others mention, just know that higher Ka --> lower pKa = stronger acid, higher Kb --> lower pKb = stronger base.

David S

Resonance definitely plays a role as a form of charge delocalization. To explain, I'll use the fact that when an acid has proton transfer reaction with water, it produces a conjugate base (can be anion like ClO4- or neutral molecule like NH3). If the conjugate base is strong, then a majority of the ...

David S

Another example is Na[Fe(OH2)_2(C2O4)_2]. The ligand C2O4^2- is oxalate, which is bidentate. Since we have two of these polydentate anions, we use bis- prefix to describe them in the full name (even though the name of the anion doesn't contain di-, tri-, etc.): Sodium diaqua bis oxalatoferrate(III)

David S

The oxidation state of the TM is often given in the name of the compound. If you only have the formula and not the name, then here are the steps. 1) If you aren't given the net charge of the coordination compound (everything inside the brackets), figure out the net charge of the coordination compoun...

David S

Coordinate covalent bonding is a key underlying concept in acid and base chemistry, as acids and bases interact via coordinate covalent bonds (think Lewis definition). Understanding the mechanisms of acids and bases allows us to better understand the way acids and bases behave in biological systems,...

David S

Yes, hydrogen bonds typically occur when a hydrogen is attached to a very electronegative atom (N, O, F) because this results in a strong partial positive charge on the hydrogen. When a molecule containing this sort of H to N, O, F bond encounters another such molecule, a hydrogen bond will form if ...

David S

By ratios, I'll take it that you mean a particular orbital in question doesn't strictly exist between two directly bonded atoms. You can definitely have delocalized π orbitals that spread over entire portions of molecules, and not belonging wholly between two directly bonded atoms. You will have who...

David S

Resonance essentially means that the actual model of a molecule is the hybrid of equally favorable lewis structures. One attribute of hybrid structures is that they have characteristics lying in between the attributes of the constituent structures. Let's take sulfate ion for example. In sulfate ion,...

David S

Lone pairs remain closer to the central atom because they are not being pulled by another atom into a central bonding region and occupy a slightly more diffuse space. This icreased penetration by the lone pair makes it a better repeller (like how internal, non valence electrons are better at shieldi...

David S

Electron pair geometry accounts for lone pairs and bonding pairs. Since it discounts the differences in strength of repulsion between the different types of electron density regions, the electron pair geometry gives us the idealized (equal repulsion) bond angles around a designated central atom; bon...

David S

It's simply a matter of convention. Sure, you can name it "square bipyramidal," and that can make sense, but somebody along the way decided to call it octahedral, and that name became the popular choice among scientists.

David S

A Dipole refers to a molecule that has a true and constant Dipole moment. Polar molecules, due to electronegativity differences between constituent atoms, will have permanent partial positive and partial negative regions. When these partial charge regions are oriented such that the resulting dipoles...

David S

While it is true that we probably should memorize the bond angles pertaining to the different electron geometries, you dont have to (and probably shouldn't memorize) molecular geometries. Overall it's best I think to memorize the electron geometries (linear, trigonal planar, tetrahedral, trigonal bi...

David S

If you are talking about 4 bonds between two atoms in a molecule, a quadruple bond, is pretty rare and usually only happens with transition metals that have valence d orbitals that can combine to form delta bonds. Quadruple bond consists of one sigma bond, two pi bonds, and 1 delta bond. If you are ...

David S

In this case, the key here is that Oxygen and the other elements in it's group have just one p orbital with a paired electron. Since paired electrons experience more electron-electron repulsion, they are less tightly held by the atom and thus the ionization energy is lower (it is easier to remove th...

David S

The nomenclature suggests that hydrogen is acting more like an anion in this case (hence the -ide ending in hydride) because Hydrogen is more electronegative than aluminum (2.1 vs 1.5). That being said, Aluminum is acting like a +3 cation in this case. Since each hydrogen is basically a -1, we need ...

David S

Think of mass density, where it's the amount of mass occupying a space divided by the volume of a space. When an object has higher mass density, that means there is more mass packed within the same sized space compared to something that is less dense. With probability density, it works the same way....

David S

First, we must recognize that emitted radiation wavelengths in the Balmer series correspond to electron transitions from n_i > 2 to n_f =2. Next, remember that a larger transition -> higher energy radiation ∝ smaller wavelength radiation. This means that you are going to be calculating a wavelength ...

David S

What you need to know about the Schrodinger equation is: ⋅ It is a fundamental basis for Quantum Mechanics, integrating DeBroigle's concepts and Heisenberg's uncertainty principle. Some describe it as the "Newton's Laws" of Quantum Mechanics ⋅ The wavefunction, \psi , h...

David S

Effective nuclear charge, Z_{eff} , describes the nuclear charge experienced by an electron (we usually focus on the valence electrons when talking about it), and is often less than the actual nuclear charge because it accounts for electron-electron repulsion. Take for example, Carbon, which has 6 p...

David S

Within the same energy level, the order goes s<p<d< f. If you want to predict order the orbitals for electron configurations in order of filling (without periodic table), a little trick is to compare the sum of n and l for two different orbitals. The one with a smaller sum goes first. If the sum is ...

David S

The shrodinger equation is conceptually important in chemistry, since we can use the equation to produce the mathematical model for orbitals (the wavefunction squared helps model a probability density for where electrons are likely to be found). In our class, we DO NOT need to know how to ccompute s...

David S

The presence of a nodal plane (a zone where probability of finding an electron is zero) has imporant impacts on electrostatic attractions between electrons and the nucleus. In the case of p and d orbitals, for example, the nodal plane(s) pass through the nucleus. This means that electrons in these o...

David S

n_1 refers to the initial energy level of the electron, and n_2 refers to the final energy level of the electron. n_1 may be higher or lower than n_2. The negative sign vs positive sign doesn't matter in this case because we are solving for the frequency of the light emitted/absorbed. The signs only...

David S

The difference is that a molecule consists of a fixed number (at least 2) atoms bound together by covalent bonds (e.g glucose C6H12O6). So when I say I have 2 glucose molecules, I mean two separate, discrete pieces of the same C6H12O6; I'm not saying I have C12H24O12, which would be an entirely diff...

David S

Also, note that free electrons are at a higher energy state than bound electrons. While an electron is bound to an atom (via electrostatic forces) the system is most stable when the bound electrons are at their lowest energy levels allowable. That's why excited, bound electrons emit radiation to fal...

David S

Since the answers of this question all have to do with energy gained/absorbed as a result of electron movement between energy level shells, we will use the following formula to determine the right answer: E = \frac{hc}{\lambda} = -R_{h}(\,\frac{1}{n_{i}^{2}}\, - \, \frac{1}{n_{f}^{2}} ) , wh...

David S

The mathematical way is the following: If 1 meter = 1,650,763.73 wavelengths, then divide both sides by 1,650,763.73. You should get 6.058 x 10^-7 meters = 1 wavelength. Looking at the answers, this means that A is out the window (which intuitively makes sense). To determine which region of the EM s...

David S

You can use the full number if you want as long as your final answer is with the correct number of sig figs. Or you can round to the number of sig figs in the problem and then solve, but I think the first way will get you a more accurate answer.

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