CHEMISTRY COMMUNITY

Anne Cam 3A

Yes, I checked again and realized I put in the wrong number of electrons. Thank you both for your help!

Anne Cam 3A

It looks like you have the previous steps correct (making ΔE or hv negative because light was emitted, using the right constants, etc.) and all you need to do is solve for n. Multiply both sides by n 2 , so you will have 1 = (n 2 )(2.78x10 -2 ). Then divide both sides by 2.78x10 -2 to get 1/(2.78x10...

Anne Cam 3A

In other words, the question is asking for the maximum number of electrons that can be in the orbitals determined. (They would have the same quantum numbers, besides the magnetic spin, if located in the same orbitals). Because the orbitals are d orbitals, there are 5 of them in total and each can ho...

Anne Cam 3A

Question 4, part A of the 2010 final asks whether C 2 + or C 2 - has the stronger bond, and I determined C 2 + to be stronger since it has the higher bond order. But the answer says that C 2 - is stronger since the extra electron is in a bonding molecular orbital. Is this true? When I drew the MO di...

Anne Cam 3A

You would need to use [A-] for the deprotonation equation in that case, because the definition of deprotonation is the conversion of the acid HA to its conjugate base A-.

Anne Cam 3A

Yes, the conjugate base of a weak acid would be stronger than that of a strong acid, though it wouldn't be as strong a base as the ones listed in the course reader. In a buffer system it is optimal to have [HA]=[A-] to keep the pH constant, because if the amounts were different the addition of an ac...

Anne Cam 3A

This video involves calculating the equilibrium constant Kc, and finding equilibrium concentrations.

Quiz_3_Problem_8.m4v [ 160.79 MiB | Viewed 747 times ]

Anne Cam 3A

Buffers generally help stabilize the pH of the solution they are in. So I think that if a buffer was added to strong acid/base solutions, it would still help maintain the pH of those solutions.

Anne Cam 3A

A buffer system is made up of a weak acid and its conjugate base, or a weak base and its conjugate acid. If a generic weak acid was HA, its conjugate base is A-. - HA reacts with strong bases (such as NaOH) that are added, by donating its proton to form H2O and A- ion. - The A- ion reacts with stron...

Anne Cam 3A

Generally there is a trend, because as joannali1027 mentioned, after removing one proton (H+) from a polyprotic acid, the molecule has a negative charge (ex. H2PO4- from H3PO4 losing 1 proton). Negative charges attract positive charges, so the negatively charged molecule is more unlikely to let go o...

Anne Cam 3A

Yes, because their product is the constant Kw.

Anne Cam 3A

Acids produce conjugate bases when they dissociate in aqueous solution. For example, HCl(aq) + H2O(l) --> Cl-(aq) + H3O+(aq). The conjugate base is Cl- because it is an ion produced from the acid's dissociation, and it can accept protons to produce the acid again (although HCl is so strong that its ...

Anne Cam 3A

C6H5O- is a base because it accepts protons, not donates them. Since phenol is a weak acid, it would not produce C6H5O- as easily, and its dissociation reaction requires an equilibrium symbol: C6H5OH(aq) + H2O(l) <--> C6H5O-(aq) + H3O+(aq) C6H5O- would also be readily converted back to C6H5OH, makin...

Anne Cam 3A

The electron withdrawing effect is true for oxoacids. For example Cl-O-H is stronger than Br-O-H because Cl, being more electronegative than Br, draws electron density towards itself and stabilizes the negative charge on O. (So electronegativity would cause a greater electron withdrawing effect.) Th...

Anne Cam 3A

Nonmetals starting in group 3, like P, S, and Cl, have empty d orbitals that can be filled with extra electrons, giving them an expanded octet. So S can gain more than 8 electrons because it has spare 3d orbitals to accommodate them.

Anne Cam 3A

4s electrons are lost first only from group 1 and 2 metals like K and Ca, because there are no electrons in 3d orbitals. But with d-block transition metals, as the 3d orbitals gain electrons they become lower energy than 4s. Cr is [Ar]3d 5 4s 1 and Cu is [Ar]3d 10 4s 1 because they are more stable w...

Anne Cam 3A

To find the average molar mass of an element from the percentage of its isotopes, you would multiply the percentages (in decimals) by their respective molar masses. In (a), only the masses of 6 Li and 7 Li atoms are given, so you would need to convert them to g/mol. Multiply each mass value by 6.022...

Anne Cam 3A

The textbook does this assuming that [H3O+]=[A-], because according to the standard weak acid dissociation equation, HA(aq) + H2O(l) <--> H3O+(aq) + A-(aq), the mole ratio of H3O+ and A- is 1:1. Thus H3O+ and A- would be interchangeable in the deprotonation equation.

Anne Cam 3A

For part (b), write the dissociation reaction for HClO₄ in water: HClO₄(aq) + H₂O(l) --> H₃O⁺(aq) + ClO₄^-(aq).

Because the mole ratio of HClO₄ to H₃O⁺ is 1:1, [H₃O⁺]=6.0x10^-5 M. Then use pH=-log[H₃O⁺] to get your answer.

Anne Cam 3A

The compounds involved are salts, which dissociate in water. For example, NH 4 Br would become NH 4 + and Br - in water. NH 4 + is the only ion that reacts with water, donating a proton to form H 3 O + ions in solution. Br - is too weak (the conjugate base of HBr) to have any reaction, and it appear...

Anne Cam 3A

How would you account for the fact that 2,4,6-trichlorophenol is a stronger acid than phenol, if only given their Lewis structures and not their Ka values? Phenol: https://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Phenol-2D-skeletal.png/80px-Phenol-2D-skeletal.png 2,4,6-trichlorophenol: http...

Anne Cam 3A

The stronger the acid, the higher the Ka. The reverse is true for pKa: the stronger the acid, the lower the pKa. So according to the given values, HIO3 is the stronger acid because it has the lower pKa. HIO3 is stronger because in an oxoacid, when there are more O atoms bonded to the central atom (t...

Anne Cam 3A

I think you would generally assume [H3O+]<10 -7 as a guideline to consider the solution neutral. Page 145 of the course reader also says that autoprotolysis of water results in 10 -7 M H3O+. I also found a website that might be of some help: the Dilute Weak Acids and Bases section of http://chemwiki...

Anne Cam 3A

CH3NH3Cl is an ionic compound, consisting of CH3NH3+ and Cl- ions. The overall salt does not donate protons, the CH3NH3+ ion does (to form H3O+) when the salt is dissociated in water. Cl- is a very weak conjugate base so its basicity is negligible. Therefore the salt is acidic because of CH3NH3+, a ...

Anne Cam 3A

For (a), you would write the dissociation reaction (with the equilibrium arrows) for CH3COOH in water, then do an ICE box for CH3COOH, CH3COO- and H3O+, with initial [CH3COOH] = 0.20 M. The change would be -x, +x and +x respectively because everything is at a mole ratio of 1:1. Then you would set up...

Anne Cam 3A

From what I remember, the fundamentals problems with net ionic equations involved salts which dissociate in water. So for those, you would have to recognize that a compound is made of positive and negative ions. For example, NH 4 I contains NH 4 + and I - ions. But there is a guideline sheet for rec...

Anne Cam 3A

You can also find a list of common acids and bases (including strong/weak acids and bases) on page 151 of the course reader.

Anne Cam 3A

You would need to separate out the ions by writing a net ionic equation. The (am) subscript means the reaction occurs in ammonia solution, but the ions dissociate the same way. NH 4 I, KNH 2 and KI are all ionic compounds. So you would write NH 4 + (am) + I - (am) + K + (am) + NH 2 - --> K + (am) + ...

Anne Cam 3A

Acidity and basicity would vary with the definition you use. According to the Arrhenius definition: - acids produce H+ in aqueous solution - bases produce OH- in aqueous solution. For the Bronsted-Lowry definition, - acids are proton donors - bases are proton acceptors. For the Lewis definition, -ac...

Anne Cam 3A

According to the Arrhenius theory, acids produce H+ ions in solution while bases produce OH- ions in solution. For the Bronsted-Lowry theory, acids donate protons while bases accept protons. But for compounds like NaOH, the Arrhenius theory would be more relevant because it dissociates to form Na+ a...

Anne Cam 3A

The halfway point of a titration is when the concentrations of the acid and its conjugate base are equal, or the pKa = pH. I assume that by "volume added at the stoichiometric point" you mean the volume added to reach the stoichometric point? Then you could follow the method in page 163 of...

Anne Cam 3A

Salts with conjugate acids/bases are used because these conjugates are usually in the form of ions and would not be found freely in solution (unlike the weak acid/base which is already a compound). When a salt with the required conjugate acid/base ion is mixed with the buffer solution, it would diss...

Anne Cam 3A

Alkaline earth metals past calcium are considered heavier because of their molar mass. The most likely reason for radium not being part of the base-forming metals is its reactivity; its compounds are very short-lived and unstable. Beryllium and magnesium are lighter in comparison, but beryllium form...

Anne Cam 3A

You would not need to use the ICE box method. Just write the dissociation reactions for each compound in water; then you will be able to find [H 3 O + ] from its mole ratio with the acid, and thus the pH. For instance, in (a), you could write HCl(aq) + H 2 O (l) --> H 3 O + (aq) + Cl - (aq). Because...

Anne Cam 3A

If Ka₂ is significantly less than Ka₁, that means the concentration of products is less than that of the reactants for the Ka₂ compared to Ka₁. In other words, the second deprotonation of the acid is not favored and thus unlikely to occur.

Anne Cam 3A

Are zinc oxide and zinc hydroxide both amphoteric compounds or can they act as bases? If so, under what circumstances? Would they be strong or weak bases?

In Fundamentals J.7 part b, zinc hydroxide reacts with nitrous acid to produce zinc nitrite. Could zinc oxide do the same thing as well?

Anne Cam 3A

For the final, will we need to know examples of biologically important buffers and how they work?

Anne Cam 3A

Dissociation energy is the energy required to break a bond, which is always positive. The shorter the bond, the higher the dissociation energy (because it becomes harder to break shorter/stronger bonds). In the course reader, examples are bonds in F 2 , O 2 and N 2 , which increase progressively in ...

Anne Cam 3A

Because there are only two atoms in a diatomic molecule there cannot be resonance. A fractional bond order such as 1.5 indicates that the molecule in question is less stable than another with a higher bond order like 2. However, it would still be more stable than a molecule with a BO of 1, which has...

Anne Cam 3A

In addition, electrons in antibonding MO produce more repulsions than those in bonding MO, making the molecule less stable.

Anne Cam 3A

Generally the closer you get to F (the most reactive/electronegative atom) the more electronegative a given element would be. The farther you get from F the less electronegative the given atom (except H which is much more electronegative than the group 1 metals). So PF5 would have a stronger covalen...

Anne Cam 3A

It's fairly clear that KOH is a base because it is an alkaline metal (first group) hydroxide and it dissociates completely in water. But when writing its reaction in water (KOH + H₂O --> K⁺ + OH^- + H₂O), how would that show whether KOH accepts a proton as a Bronsted base?

Anne Cam 3A

If you look at the compound's reaction in water you should be able to see whether it accepts or donates a proton. For example, NH3 in water produces NH4+ and OH- ions; because NH3 accepted a proton from water, it is a Bronsted base. It would also be a Lewis base because it technically "donated&...

Anne Cam 3A

You would use bis-, tris-, and tetrakis- when the ligand already has prefixes like di- and tri- in its name, or if you know for sure it is polydentate (includes bidentate, tridentate, hexadentate or generally anything over monodentate). If you were naming 2 dien ligands, you would write bis(dien) or...

Anne Cam 3A

Adding solids does not affect the equilibrium, because Kc only includes aqueous and/or gas components, and Kp only includes gases. Also Le Chatelier's principle applies to concentration, pressure and temperature changes. When the concentrations of products/reactants changes, the reaction shifts in o...

Anne Cam 3A

The text uses some different symbols from the ones we use and the equations are more advanced which can be intimidating, but it has a lot of useful background information in the actual text portion. The bolded words are also relevant most of the time.

Anne Cam 3A

The units of pressure would need to be given. If pressure is in atm, then use R = 0.08206 L*atm/K*mol to cancel out. If pressure is in bar, the unit for R would be L*bar/K*mol, and for torr it is L*torr/K*mol.

Anne Cam 3A

Sulfato is listed as generally monodentate but can also be bidentate. Under what circumstances would it be the latter?

Anne Cam 3A

How can you tell if a "ring" is formed around the central metal atom as is characteristic of chelating ligands? Can this be determined from the Lewis structure or is the molecular shape necessary? In other words is the ring usually three dimensional? Porphyrins don't seem to do this accord...

Anne Cam 3A

How can you tell the number of bonds a transition metal can make, based on its oxidation number? Or will it only be necessary to determine the coordination number from the formula of the coordination complex?

Anne Cam 3A

In the course reader EDTA is listed as a hexadentate molecule, but could it also be tetradentate? If so, when would each of these forms occur? Do they have anything to do with the charge on the EDTA molecule?

Anne Cam 3A

Is it possible to determine cis or trans forms of coordination compounds from their names (or if we write the formula from the name)? Would this be necessary on quiz 3?

Anne Cam 3A

So what would happen if inert gases were added to the reaction? Would that have no effect on the equilibrium whatsoever?

Anne Cam 3A

For the equation 2SO 3 (g) <---> 2SO 2 (g) + O 2 (g) which requires heat, it makes sense that increasing the temperature would favor SO 2 production. But why is it that low pressure (instead of high) also favors production of SO 2 ? Is it related to how many types of gases are present on each side o...

Anne Cam 3A

The course reader states that -ate should be added at the end of the metal name if the complex has a negative charge, and shows an example with a charge at the end. Does this rule also apply when there are cations attached to the complex to cancel out the charge? Also do we just write the metal name...

Anne Cam 3A

In the workbook an equilibrium problem listed the pressures of three gases as 1.08 bar, 4.9 bar and 7.6 bar, but this unit isn't mentioned in the course reader. Do need to know it for the quiz, and how do we convert it to other units like torr and atm?

Anne Cam 3A

In the course reader Kc is used for most of the icebox examples involving gases. Would we ever need to use Kp in an equilibrium problem? When is Kp generally used in problems with gases? Also, when asked to write expressions for equilibrium constants, would writing either Kp or Kc be acceptable for ...

Anne Cam 3A

For coordination compounds, do we need to know how to differentiate between cis and trans forms of the same compound? If so, how would we do this?

Anne Cam 3A

When writing oxidation numbers for transition metals, do we write them with the sign first? For instance, if cobalt has an oxidation number of 3, would we write it as 3+ or +3? Is there any difference between the two?

Anne Cam 3A

In the diatomic molecule HF, the fluorine atom F holds its atomic orbitals closer than most atoms, so its 2s and 2p orbitals are lower energy than the hydrogen's 1s orbital. For other diatomic molecules involving F, is this the same case? On another note, would we need to draw MO diagrams for ionic ...

Anne Cam 3A

When determining the MO diagram, why are the \pi orbitals divided into \pi _{x} and \pi _{_{y}} and their corresponding antibonding components? In lecture Professor Lavelle said that the \sigma _{z} was for sigma bonding on the z axis. So are the pi bonding orbitals also divided according to bonds f...

Anne Cam 3A

How do you determine bonding and antibonding orbitals and which electrons go into the sigma and pi orbitals?

Anne Cam 3A

Does the number of a particular bond type formed by a central atom affect the average bond length?

For example in the textbook question 85b, both SO3 and SO2 have all double bonds. Would the fact that S formed more double bonds in SO3 make the average bond length shorter in SO3?

Anne Cam 3A

What is the correct Lewis structure for P4 (white phosphorus)?

Anne Cam 3A

Why is the Lewis structure double bonded instead of triple bonded? Does it matter which atom has the formal charge?

Anne Cam 3A

Do you know which way of drawing it would be most correct? I thought the first one might have implied covalent bonding since it used lines.

Anne Cam 3A

When writing the Lewis structure of the ionic compound $K_{3}P$ , or any similar ionic compound, how should the cations be placed around the anion?

Anne Cam 3A

If an atom in a Lewis structure has at least two lone pairs, as in NH2-, where should the lone pairs be located? Should we take into account molecular shape when locating the lone pairs?

Anne Cam 3A

When drawing a Lewis structure where a central atom is surrounded by at least two different types of atoms, does it matter how the structure is oriented? For example, with COCl2, can the positions of Cl and O vary as long as the bonds and lone pairs are correct?

Anne Cam 3A

P, S, and Cl can have expanded octets because they have extra 3d orbitals. Can this also happen with any atom in higher periods, since they will have 3d orbitals too? Do metals like Al, K, Ca and the transition metals follow the octet rule exactly?

Anne Cam 3A

In the course reader oxoanions "are polyatomic anions that contain oxygen" and includes ions like carbonate. Does this mean carbonic acid would then be an oxoacid? Also, would acetic acid be considered an oxoacid, since acetate also contains oxygen? If not, what is the criteria for determi...

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