CHEMISTRY COMMUNITY

skyeblee2F

Why is the half-life for first order reactions (t1/2=(ln2/k) independent of the initial reactant concentration [A]0, while the half-life of zero order and second order reactions are dependent? ([A]0 is included in both of their t1/2 formulas)

skyeblee2F

The electrolytes in a salt bridge need to be relatively unreactive with other chemicals in the cell and have cations and anions with similar migratory speed. KCl is often used because the ions K+ and Cl- are similar in size and transport numbers (.49 and .40, respectively). During the long time of o...

skyeblee2F

It is always true for ideal gasses. An ideal gas has no interactions between particles or intermolecular forces, so pressure change at constant temperature does not change internal energy.

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Free expansion is an irreversible process in which a gas expands into an insulated evacuated chamber. Because it's insulated, there is no change in temperature and q=0. Because there is no external pressure and gas is moving freely, w=0 too.

skyeblee2F

When dealing with an ideal gas (which we pretty much always are in this class), the ∆U is 0 for every isothermal process. In the case os isothermal reversible expansion, ∆U=q+w=0 so q=-w

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Correct. If they are involved in the overall reaction they would be included in the cell diagram.

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You can tell by calculating the formal charges of each bond ([# of valence electrons on atom] – [non-bonded electrons + number of bonds]). A molecule is most stable when the formal charge of as many of its bonds as possible are at or near 0.

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In both instances, they are approaching equilibrium with their surrounding temperature. It would require no input of energy for a cup of hot coffee to cool down (when it is hotter than its outside environment) or an ice cube to heat up (when it is cooler than its outside environment). Thus, both are...

skyeblee2F

Exothermic/endothermic indicates a transfer of energy, but not always in the form of heat, so the temperature does not necessarily change.

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You don't account for solids or liquids in your equilibrium.

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It's used to understand electron transfer when conditions aren't standard (so not occurring at 25ºC)

skyeblee2F

Yes. Bomb calorimeters are used as closed systems for constant-volume experiments.

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Yes, they are both written in terms of concentrations

skyeblee2F

It was for Test 2 so I'd strongly assume yes.

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The source of the activation energy needed to push reactions forward is typically heat energy from the surroundings.

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haha yea

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skyeblee2F

Galvanic cells and electrolytic cells are the same thing. Problems would have to do with oxidation and reduction half reactions.

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They're the same equation rearranged. You'd use one over the other depending on the information your given and the information you need to find.

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Think of it as two reactions occurring, where the ice (gains heat and) is undergoing a phase change to water and water (loses heat and) is undergoing a phase change to water. Thus you would have two identical equations equal to each other except for the specific heat and enthalpies. ice: q(gain) = m...

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Based of the Gibbs free energy equation (∆G = ∆H - T∆S) a spontaneous reaction will always occur when ∆H is negative and ∆S is positive, and a reaction will always be non-spontaneous when ∆H is positive and ∆S is negative.

skyeblee2F

Because it's a spontaneous reaction that converts electrical energy to chemical energy.

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The question asked to rank Pt, Pb, and Cu in order of increasing power going from their neutral to second oxidation state. I assumed this meant we had to switch the signs of each Eº, considering all of their reactions were listed backward. And since reducing power increases with a more negative Eº, ...

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For part (d) of #8.67 (6th edition) the problem is the same as part (c) except we take into account resonance. I would've assumed that we use the average of the bond enthalpies for a C--C bond (348 kJ/mol) and C==C bond (612 kJ/mol) and multiply that by 6 moles rather than add up 3 of each. However,...

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How do we know when to use the work function with -P or -nRT?

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The problems for chapter 8 consist of a lot of conversions between units (i.e. 1 Torr = 133.3 Pa). Should we be memorizing all of these conversions, some of them, or would they all be provided?

skyeblee2F

The varieties of word table lists ∆V as being in units of m^3 but #8.3 calculates it in terms of L. Which unit should we try to use?

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Could someone explain which elements don't affect pH and why?

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Why is electronegativity proportional to acidity?

skyeblee2F

If the temperature and volume is the same in two containers, and they are undergoing the same reaction, their equilibrium constants will be the same even if they have different concentrations of reactant. If you are just looking at the ratio of product/reactant (not K, which takes into account stoic...

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For problem 11.13 in the 6th edition, in the answer booklet, the quotients for (a) and (c) are written in terms of partial pressure (so P with the subscript). The answer for (b), however, is written with the molecules placed inside brackets. Is there a reason why (b) is formatted differently? Or is ...

skyeblee2F

For pH, one sig fig corresponds to one decimal place (ex: 1.5) and two sig figs correspond to two decimal places (ex: 1.53) and so on. On the test, we will likely use three sig figs as per usual so answers would look like something like 1.532

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sorry, I meant trigonal bipyramidal, not pyramidal

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Acids are stronger if they can more readily release H+ ions. Acetic acid is stronger because it has more to release than formic acid.

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Each atom would have their own coordination number, just like how each have their own VSEPR formula, shape, or formal charge. Any question you'd get would specify which atom it wants you to provide the coordination number for.

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It always helps to draw the Lewis structure for each of these molecules first. If there appears to be multiple lone pairs that a single metal ion could attach to, then it is polydentate. I've attached a drawing of what the first one would look like, which is tridentate.

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The extent we should know is that it's a hexadentate ligand that essentially forms a cage around metal ions by attaching to its 6 lone pair binding sites in an octahedral fashion.

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IF5 has a VSEPR formula of AX5E but has a square pyramidal shape. Why doesn't it have a trigonal bipyramidal shape? Wouldn't the electron densities be more spaced out this way?

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According to the textbook and solutions, benzyne (C6H4) is highly reactive because of the sp hybridization of two of the carbon atoms (there are two double bonds and one triple bond). Can someone explain to me how hybridization has an effect on how reactive a molecule is?

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Their difference in strength has to do with their overlap. Sigma bonds are characterized by end-to-end overlap while pi bonds (which occur between "leftover" p-orbitals) overlap in a side-to-side fashion. As you can see in the image below, the head-on contact allows much more overlap and t...

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When there is a lone pair present, its electron density will always cause a repulsion of the other densities, causing slightly smaller bond angles. A trigonal pyramidal molecule is essentially a tetrahedral (109.5º) where one of the bonds is replaced with a lone pair. This lone pair experiences repu...

skyeblee2F

The bond angles are all 90º. Your second guess is correct; when there are two lone pairs, the repulsions cancel out and the square planar also has 90º angles.

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The electron density around lone pairs are stronger than bonded pairs. Thus, the other pairs are repelled slightly from the lone pair, pushing them closer together and resulting in angles slightly less than the normal 109.5º that a tetrahedral has.

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The lone pair does affect the seesaw structure, actually. It pushes the linear pairs away and the trigonal planar pairs closer together. The resulting angles are slightly less than 90º and 120º.

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You got it! The larger an atom's radius, the longer its bond will be with another atom and the weaker the pull is between its protons and electrons.

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Smaller, more highly charged cations have greater polarizing power because they have a stronger pull on the electrons of ions. So you could generally say polarizing power increases up and across the periodic table in terms of cations. Polarizability increases as the anion gets larger and less electr...

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How is it possible that SO2 and SO3 have the same bond length, as implied by the solutions manual? Doesn't SO3 have more bonds between atoms and thus longer bonds, since bond length is inversely related to the number of bonds between atoms?

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One of the questions asks to draw the Lewis structure of a molecule but it gives the name and not the formula (i.e. ammonium chloride). Generally, will we need to know many formulas from their names? If so, which ones?

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Non-metals form anions. Their ionization energies are too high to form cations.

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When you are writing out the electron configuration for copper and chromium you'll notice their d-orbitals contain 9 electrons. It is much more stable to have the full 10 electrons, so we take away an electron from the valence s-orbital and add it to the d-orbital. Thus, instead of copper being writ...

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It would normally be expected for copper to have an electron configuration of [Ar]3d^9 4s^2, but since it would be a lot more stable to have a full d-orbital, it is rather written as [Ar]3d^10 4s^1. Thus, when an electron is taken away from the outermost orbital (s), it becomes a positively charged ...

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You should have a general idea of the EM spectrum:
UV light = 100-400 nm
Visible light = 400-700 nm
Infrared = 700 nm-1 mm

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There is no such thing as a negative wavelength or frequency. The negative sign you may calculate using Rydberg's equation is simply a representation of energy being either released or emitted and can be ignored in terms of frequency. Going from n=4 to n=2 emits energy, so the equation yields a nega...

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Basically... The Schrodinger Equation is used to understand the physical properties of a system. The wave function (psi) exists in three dimensions (x,y,z), so solving for psi yields three quantum numbers. These three numbers define the energy shell occupied by the electron, energy subshell, and ori...

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As explained by wave-particle duality, light exhibits characteristics of both waves and particles. In the case of the photoelectric effect, light acts as particles, while in the case of the DeBroglie theorem, light acts as a wave. In sum, "wave" and "particles" are terms that can...

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It tells us how much energy is being released (as light) when an electron shifts from one energy level to another, with "n" representing the energy level. We can expect the answer to be negative when energy is being released (and the electron moves closer to the nucleus) and positive when ...

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I noticed in the homework a lot of problems asking which part of the spectrum a certain number of nm fell into (after doing a problem where you calculate the wavelength of something). For tests and such, will we need to have the spectrum memorized? Like, will I need to know that 340 nm falls in the ...

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I'm replying to my own question because I just noticed what the issue was. Though it is not said in the manual, Avogadro's number and the photon energy are also being multiplied by 1.00 mol. This cancels out with the mol^-1.

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It asks to find how much energy is emitted by 1.00 mol of sodium atoms with a wavelength of 589 nm. The solutions manual says to simply multiply Avogadro's number by the energy it takes to emit a single photon of that wavelength. The units do not seem to add up, though. We just want an answer in jou...

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I believe you always answer with 3 sig figs in it's scientific notation form. So if it's 1.201 * 10^(-1), then you would answer 12.01 for the molar mass.

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For this problem, it asks to find the mass of HCl that can neutralize Mg(OH)2. In the solutions manual, it appears that we have to come up with our own formula, based on the given formula with CaCO3. How do I come up with this on my own? As in how do I know what pairs up with what?

skyeblee2F

I agree with Anushi. Whatever the given mass is in the problem is the unit you should give in your answer.

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