CHEMISTRY COMMUNITY

Sanjana K - 2F

https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Balancing_Redox_reactions Here's a pretty helpful link! In an acidic solution, the reaction is balanced after you've added your H+ to balance the hydrogens and el...

Sanjana K - 2F

Remember that it's the number of electrons after you balance your equations (so the number of electrons transferred in the cathode half reaction should equal the number of electrons transferred in the anode half reaction, which should equal your value of n).

Sanjana K - 2F

The values in the table are always standard reduction potentials, so since the anode is being oxidized and not reduced, the sign gets flipped.

Sanjana K - 2F

I think that might be an error in the manual because I solved the problem without subtracting 0.59 and I still got 2.72 x 10^(10) as my final answer. Also, if you subtract 0.59, you actually end up with (k'/k) = 0.588, which isn't right.

Sanjana K - 2F

https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Redox_Chemistry/Balancing_Redox_reactions This link was super helpful to me! The only big difference between balancing in an acidic vs. basic solution is: in an acidic solution, ...

Sanjana K - 2F

The electrons are moving from the anode to the cathode because the anode is being oxidized (losing electrons) while the cathode is being reduced (gaining electrons). When you look at the standard reduction potentials for the half-reactions, the reaction in the cathode half cell should have a more po...

Sanjana K - 2F

It's an equation that helps you find the cell potential under non-standard conditions & relates the Q value to the cell potential.

Sanjana K - 2F

Yeah! A concentration cell has the same ion on both sides, but different concentrations. I'm not sure how it can be compared to a galvanic cell though, sorry. I guess it could be compared to a galvanic cell because like a galvanic cell, it doesn't require an external source plugged to an outlet and...

Sanjana K - 2F

One thing to remember is that you should always take the balanced number of moles of electrons for the reaction as your value of n.

Sanjana K - 2F

The double vertical lines represent separation of the cells, with your anode on the left and cathode on the right. On the anode side, let's say you have both Fe (s) and Fe2+ (aq) present because you're trying to show that Fe (s) --> Fe2+ (aq) + 2 e- and on the cathode side you have Cu(s) and Cu2+ (a...

Sanjana K - 2F

If there’s no solid conducting metal in one half of the cell diagram, then that half will have a platinum electrode.

Sanjana K - 2F

I feel like it's just a lot of practice with the homework problems he assigned since there's just so much math involved. Also, going through the book and writing down important equations (and when to use them/what conditions) also helps.

Sanjana K - 2F

Rafsan Rana 1A wrote:Isn't the equation G = Gnot + RTlnQ ?

Yes, it should be delta G = delta G(naught) + RTlnQ.

Sanjana K - 2F

You'd use it to relate the reduction potential (under non-standard conditions) to the standard electrode potential. At any point in the reaction, you can find the cell potential using this equation.

Sanjana K - 2F

This diagram is pretty helpful too.

Sanjana K - 2F

S just means entropy, rather than change in entropy that should be denoted as ΔS. Entropy is almost never 0 (the entropies of all perfect crystals approach zero as the absolute temperature approaches zero; "perfect crystal" refers to a substance in which all the atoms are in a perfectly o...

Sanjana K - 2F

In addition to the answer above, you can also tie this to the first law of thermodynamics (energy is neither created nor destroyed but rather transferred from one form to another). So, this would be an example of the conversion.

Sanjana K - 2F

Individual tests aren't curved :(

Sanjana K - 2F

Although reactions being exothermic/endothermic and spontaneous/nonspontaneous are closely related, you should also keep in mind that not all exothermic reactions will be spontaneous. When checking for spontaneity, look at the delta G sign (meaning you need the sign of delta H AND the sign of delta ...

Sanjana K - 2F

Isn't it assumed to be at constant temperature as well as pressure?

Sanjana K - 2F

For the example in class on Friday ((H+) + (MnO4)- + 5(Fe 2+)) --> Mn2+ + 5(Fe)3+ + 4H20), we know that oxygen always has an oxidation number of -2 so in Mn(O4)-, 4(-2) = -8 so Mn + (-8) = -1 meaning that Mn has to have +7 charge on the reactants side. On the products side, we're told that the overa...

Sanjana K - 2F

I think they'll be provided on the formula sheet but here are the formulas you'd use anyways:
Ideal gas: Cp = (5/2)R
Ideal gas: Cv = (3/2)R

Sanjana K - 2F

Yes, I believe you would use this equation for calorimetry problems. Can someone explain to me why you don't use mass?

Sanjana K - 2F

I think you'll be given this value, but just for reference 1 L times atm = 101.325 J.

Sanjana K - 2F

Yes, enthalpy will only equal heat of a reaction at constant pressure.

Sanjana K - 2F

Here's a graphical representation of both. One thing to note is that the work done under a reversible process will always be greater than that done under irreversible, as you can see.

Sanjana K - 2F

Examples: energy, pressure, volume, temperature, density, heat capacity
Non-examples: work, heat

Sanjana K - 2F

Also, in addition to what is being said above, something to remember: whenever you're dealing with phase changes, you have to calculate the heat required in 2 different steps. For example, if you're trying to go from a block of ice at 0 degrees to water at 50 degrees, you first need to find the heat...

Sanjana K - 2F

I don't think it'll be posted online because only the midterm answers were posted for Chem 14A last quarter, but you can ask your TA to go over it with you. Or you can post a specific question here.

Sanjana K - 2F

We haven't gone over kinetics yesterday but he mentioned it a little in class to emphasize that delta G could tell us about a reaction's spontaneity (but a spontaneous reaction isn't necessarily a fast one because to find out about a reaction's speed/rate you'd have to consider kinetics and not ther...

Sanjana K - 2F

For the example he gave us in class, we had 2 possible states (represented by the two circular ends) and 2 particles (A and B), so we do (# of states)^(# of particles) = 2^2 = 4 microstates possible.

Sanjana K - 2F

I feel like I studied for the tests, midterm, and final the same way though. Even if they are structured a little differently, they're basically all word problems so doing the book problems and going over notes is still the best way to study.

Sanjana K - 2F

Could someone also explain why heat is not a state property? Thanks! If it's heat given off under constant pressure, then it's basically enthalpy so yes, it would be a state property because the path DOES matter as you take into account what intermediate steps where needed to get to the final tempe...

Sanjana K - 2F

That's really cool! I like how you can see the rhenium atoms travelling around and then coming into the carbon nanotubes -- makes the concept of bond formation and breaking easier to visualize on a microscopic level.

Sanjana K - 2F

Also, what is the difference between intensive properties and state properties? State properties don't really take into account the "path" of how the substance was prepared (like altitude, pressure, volume, temperature). I think you mean to ask about the difference between intensive and e...

Sanjana K - 2F

The change in heat per celsius is the same as the change in heat per kelvin, so you could use either.

Sanjana K - 2F

You can also look at bond enthalpies or use Hess's Law to figure out the total energy on the reactants and products side or the energy needed to form/break the bonds and if delta H < 0, it's exothermic and if delta H > 0, it's endothermic.

Sanjana K - 2F

It might help if you view heat as another reactant (for an endothermic reaction) or as another product (for an exothermic reaction). If you add more of a reactant, according to Le Chatelier's, the effect of the change will need to be minimized so the reaction will proceed to the right. So in an endo...

Sanjana K - 2F

The ICE table is just supposed to help you write your equilibrium constant expression and to track changes in the concentrations of the reactants and products. Your final Kc expression won't include solids and liquids, so you wouldn't have any values for them in your ICE table.

Sanjana K - 2F

The easy way to tell what changes will occur is to see how the moles of gas on the reactants and products sides differ. If the volume (moles of gas) is lower on the products side, then the reaction shifts to the right but if the volume (moles of gas) is higher on the products side, the reaction shif...

Sanjana K - 2F

Yes, Na2O (s) + H2O (l) ⇌ 2Na+ (aq) + 2OH- (aq) is the correct reaction

Sanjana K - 2F

If you don't use the quadratic equation, you still want to justify your reason for ignoring the "-x" terms in your equilibrium by saying that since x is less than 5% of the initial concentration, its change is negligible.

Sanjana K - 2F

Yes, and you observe the same thing with H+ concentration and pH as well. The higher the H+ concentration, the lower the pH and the stronger the acid. It's just in the nature of natural logarithms.

Sanjana K - 2F

pKa is dependent on Ka so if the Ka is greater than 10^-7, then the pKa would be less than 7 but if Ka is less than 10^-7, then the pKa would be greater than 7.

Matt Sanruk 2H wrote:Wouldn't pKa give an integer thats less than 7?

Sanjana K - 2F

In class, he also said that if your Ka is less than 10^(-3), then you can generally approximate by ignoring the "-x" term but you might want to justify it on the test by explaining that since x is less than 5% of the initial concentration, it's considered a negligible difference so it can ...

Sanjana K - 2F

It generally won't tell you exactly what R value to use, but all the constants are given in the formula sheet so depending on whether pressure is reported in atm or kilopascals, you'll know which R-value to use. Lavelle gives you 4 different values for the R constant on his formula sheet during test...

Sanjana K - 2F

If you start with a higher reactant pressure, you'll have a lower reactant volume (because increasing pressure tends to compress volume) so the equilibrium will actually shift left to make up for that loss in volume, according to Le Chatelier's Principle. So, with the reaction shifting left, the equ...

Sanjana K - 2F

5% Rule: If x/(initial concentration) < 5%, then you can ignore the "-2x" term but if not, then you'd need to use the quadratic formula. Make sure you write out the rule to justify ignoring the "-x" terms whenever you have this type of problem

Sanjana K - 2F

Basically, it's telling you to ignore spectator ions like Li+, Na+, K+, Rb+, Sr2+, Ba2+, Cl-, Br-, I-, NO3-, ClO4-, SO4(2-), etc. because these ions occur in the same form on the left and right side of the chemical equation. So when you're writing your equilibrium constant expression they'd cancel a...

Sanjana K - 2F

Yes, so for example if you have 2NO2 in equilibrium with N2O4, your left hand side of the ICE table would be (initial concentration of 2NO2) - 2x and your right hand side would be (initial concentration of N2O4) + x.

Sanjana K - 2F

Whenever you make an ICE table, you should be listing the concentration values, not the number of moles.

Sanjana K - 2F

Here's an example:
K2CrO4
we know that K has a charge of +1 so 2(+1) = +2 and O has a charge of -2 so -2(4) = -8. The molecule is neutral and we're trying to find the oxidation number of the transition metal so (+2) + Cr - 8 = 0 so Cr = +6.

Sanjana K - 2F

In hybridization, you basically have atomic orbitals mixing into new hybrid orbitals in order to better pair electrons to form chemical bonds in VSEPR (affecting molecular geometry and bonding properties). And yes, this does lower the energy of the molecule, so hybridized molecules are more stable.

Sanjana K - 2F

If, in the presence of water, a salt gains an H+ (becoming an acid) and produces OH- ions, the solution will be basic. If, in the presence of water, a salt loses an H+ (becoming a base) and produces H+ ions, the solution will be basic. ex: C2H3O2 (-) (aq) + H2O (l) ---> HC2H3O2 (aq) + OH- (aq) so ba...

Sanjana K - 2F

Also, generally speaking, anions will act as Lewis bases (donate an electron pair) while cations act as Lewis acids (accept an electron pair)

Sanjana K - 2F

For a very weak acid, the Ka can be an extremely small number, so to deal with such numbers, we use the pKa, which converts the Ka values to larger numbers we're more familiar with. It also allows for convenience with graphing.

Sanjana K - 2F

Here's an example of a problem involving coordination numbers: In Co(NH3)6, the coordination number is 6 because you have 6 ligands attached to the central atom Co. CO3 (2-) has a coordination number of 3 because of the 3 oxygens surrounding the carbon atom. You can relate coordination numbers to mo...

Sanjana K - 2F

For example, if you want to find the oxidation number for Cr in CrCl3, you have to start off by figuring out the charges you DO know (the charges of the non-TM ions). Group 17 elements have an oxidation number of -1 so 3(-1) = -3. The charge of the overall molecule is 0 and we only have one Cr atom,...

Sanjana K - 2F

It will be helpful (later on, in Chem 14B) to know all the strong acids because there are only 7 and any acid that isn't one of those 7 will automatically be weak. So when we do more acid-base equilibria that involve math, it'll be pretty useful. Here they are: 1) HClO4 - perchloric acid 2) HClO3 - ...

Sanjana K - 2F

So the Bronsted acid/base theory deals with PROTON transfer (Bronsted acids donate a proton while Bronsted bases accept a proton). The Lewis acid/base theory deals with ELECTRON PAIR transfer (Lewis acids accept an electron pair while Lewis bases donate an electron pair). Generally, all Bronsted aci...

Sanjana K - 2F

In addition to the previous answer, you can look at the following examples: - Acetic acid (CH3COOH) is a Bronsted acid because it donates a hydrogen atom to water -- it's a proton donor basically, so you're left with CH3COO- (the conjugate base). And in this case, water would be the Bronsted base be...

Sanjana K - 2F

Also, if you're dealing with water, the shape is also bent but the bond angle would be 104 degrees due to the presence of 2 lone pairs, which pushes the bonded atoms even closer together, further reducing the bond angle.

Sanjana K - 2F

SBr4 is a nonpolar molecule because the central atom (S) is surrounded by 4 identical (Br) molecules that are equivalent in their electronegativity, so the dipoles cancel each other out, and as a result, there is no dipole-dipole interaction. The individual bonds can still be polar but the whole mol...

Sanjana K - 2F

The presence of lone pairs causes a higher repulsion since in terms of repulsion, lone pair---lone pair > lone pair---bonded atom > bonded atom---bonded atom, so the lone pair will take the place of a bonded atom and essentially exert greater repelling force against the other atoms, pushing them clo...

Sanjana K - 2F

You'll only be expected to name the shape molecularly (not electronically). So in your case, you'd name the molecules as T-shaped because trigonal bipyramidal only tells you that there are 5 regions of electron density (assuming that atoms also count as electron-rich areas). 5 regions of electron de...

Sanjana K - 2F

Hydrogen bonds will always be stronger than other dipole-dipole interactions because hydrogen bonds occur between a partially positive hydrogen atom and a highly electronegative atom (N, O, F), so the dipole will automatically be at one of its strongest in a hydrogen bond, and therefore, the polarit...

Sanjana K - 2F

Oxygen's ionization energy is lower than that of nitrogen because nitrogen has 3 electrons in the p-shell so it's half-full but oxygen has 4 electrons so there will be one set of paired electrons and 2 unbonded electrons, making it less stable. To be more stable, we're more likely to pull out the ex...

Sanjana K - 2F

Seesaws: 1 LP, seesaw molecular shape, trigonal bipyramidal electron arrangement, bond angles of 90 and 120 degrees Tetrahedral: 0 LP, tetrahedral molecular shape, tetrahedral electron arrangement, bond angles of 109.5 degrees Trigonal pyramidal: 1 LP, trigonal pyramidal molecular shape, tetrahedral...

Sanjana K - 2F

Adding on to those answers above, since Cl is a larger atom than H (larger radius) and its electron density is also higher, it'll tend to push the H atom inward more, reducing the angle further (so it's less than 109.5 degrees). You can observe the same with CH3F too.

Sanjana K - 2F

Lone pairs in particular will occupy equatorial positions because that's where they face the least repulsion (more energetically favorable). For example, for a trigonal bipyramidal structure, if you have the lone pair on the axial plane, it'll be 90 degrees away from 3 regions of electron density bu...

Sanjana K - 2F

You might be able to use this table to help you conceptualize the models

https://web.gccaz.edu/~kimld88531/VSEPR%20handout.pdf

Sanjana K - 2F

N,O, and F tend to bond with H in a hydrogen bond and since they're more electronegative, they'll result in a higher polarity with the positive hydrogen so the bond will also be stronger than a normal dipole-dipole.

Sanjana K - 2F

I really like this one!

https://web.gccaz.edu/~kimld88531/VSEPR%20handout.pdf

Sanjana K - 2F

For hydrogen bonds and dipole-dipole bonds, you're looking at the strength of the bond between partially + and partially - atoms. Since N,O, and F usually bond with H in a hydrogen bond and they tend to be more electronegative, they'll result in a higher polarity against the positive hydrogen so the...

Sanjana K - 2F

Hybridized orbitals tend to have lower energy states than unhybridized ones, so it's mostly for stability. It also helps predict the molecular geometry better by VSEPR (so bonds are more accurate in distance/angles).

Sanjana K - 2F

It's basically a measure of how polarizable a system is. It'll tell you how the + and - electrical charges are spread with the equation p = q*d where q = magnitude and d = distance between a pair of opposite charges.

Sanjana K - 2F

The radius increases as you go down a group (because you're adding more shells) and decreases as you go to the right across a period (because electrons are added to the same shell and the electrostatic attraction increases L-->R so the increasing nuclear charge draws them inwards and radii length de...

Sanjana K - 2F

So you'd use it for atomic spectra (since the energy at a specific shell is denoted by -hR/n^2) and you could find the frequency emitted from one level to another by doing E(final) - E(initial). You'd apply En = -hR/n^2 to the two different n values (or shells) and then subtract the two to find the ...

Sanjana K - 2F

If you can't narrow it down, you might want the negative formal charge to be on the more electronegative atom. For example, you can refer to #12 on the Dino Nuggets Midterm, where we ultimately chose our structure to be N --- N - O instead of N -- N -- O because we'd rather have the -1 charge on the...

Sanjana K - 2F

Ionization energy is the amount of energy you need to remove an electron from an atom/ion while electronegativity is how likely an atom is to gain/attract an electron.

Sanjana K - 2F

They might ask you to identify which rule is being violated when filling shells (like they did for #9j on the Dino Nuggets practice midterm). So it would help to know that Pauli's Exclusion Principle says you can only have 2 electrons per orbital that must be spin-paired and that Hund's Rule says th...

Sanjana K - 2F

Delocalized electrons are basically the electrons that aren't fixed to any particular atom or covalent bond. They're the ones that you see go back and forth, from one atom to another, resulting in the resonance structures we see for molecules like benzene (where the double bonds constantly change po...

Sanjana K - 2F

I've noticed that for molecules like carbonate and benzene, they may have you draw out all the resonance forms OR you can use dotted lines to represent the bonds that change from one structure to another. Here's an example.

Sanjana K - 2F

1. Fe^+ has the same config as Mn. 2. Fe^- has the same config as Co. 3. Fe^+ means that Fe lost an electron. 4. Fe^- means that Fe gained an electron. You can think of Fe as initially a neutral molecule [Fe]. But by adding a minus sign after it, as [Fe]^-, you're saying that it has one additional e...

Sanjana K - 2F

Also, just some general rules for finding the lowest energy resonance structure:
- most atoms with FC = 0
- lower FC in general (negative is good too!)
- resonance forms with opposite sign charge on adjacent atoms
- most electronegative atoms
- most number of octets

Sanjana K - 2F

It also helps you keep track of the electrons and can help you predict the reactivity of the molecule. It assumes that all electrons are shared equally so you don't have to worry about bond polarity contributing to differences in potential. It'll help more when we do coordinate covalent complexes la...

Sanjana K - 2F

The reason the orbitals overlap more as you move away from the nucleus is because the distance between the shells decreases so as they move closer together, the orbitals overlap more. As a result, the potential energy drops and new electrostatic interactions and bonds between the orbitals form.

Sanjana K - 2F

It's basically the "building up" principle, which helps you determine electron configuration. It says electrons with lower energy levels with tend to fill up first. Here's a diagram of the order.

Sanjana K - 2F

This picture might help. Orbitals are arranged into shells and subshells. Shells are orbitals with the same value of n & subshells are orbitals with the same value of n and l. In the picture, you'll notice that anything from energy level "2" is of the same shell (so 2s and 2p belong to...

Sanjana K - 2F

So you want to base your shorthand off of the last element in the previous row (the last noble gas before your element). Chromium belongs in period 4 so you'd go to the previous row (period 3) and move to the last element (group 18), which is Argon. The "s" block consists of groups 1-2, th...

Sanjana K - 2F

The magnetic spin is used in the context of the spin magnetic number (the fourth quantum number that you use to describe an electron), so it is somewhat important. We denote the spin with either a +1/2 or a -1/2, which means the electron either spins clockwise or counterclockwise. No 2 electrons in ...

Sanjana K - 2F

You only really need to know about the electron spins in the context of the spin magnetic number (the fourth quantum number that you use to describe an electron). We denote the spin with either a +1/2 or a -1/2, which means the electron either spins clockwise or counterclockwise. No 2 electrons in t...

Sanjana K - 2F

I don't think you'll need to memorize the exact wavelengths, but you should have a good idea of where the different waves lie in relation to one another. It might also help to come up with examples for each. The "types of EM Radiation" picture in this site is pretty nice. https://www.brita...

Sanjana K - 2F

Use the "Ans" key on your calculator whenever possible. We're allowed to use scientific calculators but not graphing calculators, so you should be able to store values. Also, I generally leave at least 5 numbers after each decimal point (for each step) and apply sig figs at the very end.

Sanjana K - 2F

The state will usually be given to you in the problem (ex: 25ml of precipitate means solid, NaOH dissolved in water means aqueous). When we do acids/bases and solubility equilibria especially, you'll become familiar with which compounds dissociate in water (nitrates, acetates, compounds with group 1...

Sanjana K - 2F

Also, oxygen won't generally be present in a reaction as one oxygen atom alone. Oxygen is a diatomic molecule, so it'll only show up in a reaction as O2. Here's a list of other diatomic molecules (hydrogen, oxygen, fluorine, bromine, iodine, nitrogen, chlorine.. AKA HOFBrINCl).

Sanjana K - 2F

As Brian was saying, you want to make sure you're using the right number of sig figs first. Generally, if you're doing a lab and you're measuring amounts, it's better to use the whole number because it's easier to conceptualize 25ml than 2.5x10^(-2)L, but when you're answering a problem on paper, le...

Sanjana K - 2F

When performing calculations, you generally want to keep the powers (especially if you're dealing with a very small or large number). After calculations, you could express the powers in simpler units (just for convenience or when in a lab setting, where you can't measure negative exponents of 10). S...

Sanjana K - 2F

One thing to keep in mind is that when you're trying to find the molecular formula of the sulfide, you shouldn't just add an "S" to the metal. Based on the molar mass of the metal, you should try and identify it so that you can figure out its charge. If you know it's charge, you can then f...

Sanjana K - 2F

I think it's more important to distinguish between an empirical formula (which shows you the simplest whole number ratio of atoms of each element in the compound) and a molecular formula (which shows the actual number of atoms of each element in a compound). A lot of sources online are using the ter...

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