CHEMISTRY COMMUNITY

Chem_Mod

Answer: Yes, pure solids or pure liquids will not effect the equilibrium.

Chem_Mod

Question: When we write an equilibrium expression, do we only include the compounds that are in the (g) or (aq) forms and drop the solids and the liquids?

Chem_Mod

The reaction could form products or reactants when K>1, not having a tendency for either. See page 368 in the text. "Large values of K (>10^3), favors product. Intermediate values of K (10^-3 -- 10^3), neither reactants nor products are strongly favored. Small values of K (<10^-3), reactants ar...

Chem_Mod

What does the value of K tell us about which reaction is prefered?

Chem_Mod

To be amphoteric means that the oxide can react either with acid or base. In the case of As2O3 we have: As2O3 + 6HCl --> 2AsCl3 + 3H2O (reaction with acid) NaOH + As2O3 + 3H2O --> 2Na[As(OH)4] (reaction with base) Something similar would apply to Bi2O3. Bi_{2}O_{3} dissolves in both acidic and basic...

Chem_Mod

How can you tell that As2O3 or Bi2O3 is amphoteric?

Chem_Mod

For this question, first draw the Lewis structures of each acid. You will see that for H3PO3 one of the hydrogens is bonded directly to phosphorus. This is not an acidic hydrogen and therefore H3PO3 is diprotic. On the other hand, H3PO4 is triprotic, all hydrogens bonded to oxygens. Looking at the p...

Chem_Mod

How is H3PO4 is a weaker acid than H3PO3? Doesn't H3PO4 have more electron withdrawing atoms than H3PO3 and therefore be a stronger acid, just like HClO4 would be stronger than HClO3?

Chem_Mod

In order to find Kb based on the reaction shown for the base, you would need to know the concentration of [OH-]. You are initially given data about the pH of your solution, but that does not tell you information about [OH-]. But using, Kw = [H3O+][OH-] = 1E-14, we can calculate [OH-]. This then woul...

Chem_Mod

How can pH be used to find Kb?

Chem_Mod

Answer: For a titration, you begin with an unknown solution, in this case a base. To titrate it, you use a known solution, in this case an acid. The initial point on your titration curve is the initial pH of your unknown solution. As you add more acid to titrate it, the pH begins to decrease. The s...

Chem_Mod

Question: Can you give an outline for finding the pH in a titration problem?

Chem_Mod

Answer: H2SO3 has 2 H+ ions to donate. In order to fully react both H+ ions, you would need 2 NaOH to provide the needed 2 OH- ions. Therefore you would need 2 moles of NaOH to neutralize 1 mole of H2SO3.

Chem_Mod

Question: Why is it that 2 mol NaOH = 1 mol H2SO3?

Chem_Mod

Based on the Ka of lactic acid (Ka=8.3E-4), the value is quite small, so you would assume very little products should form. On this assumption, you can think that (x+y)~x and (2x-y)~2x since x>>y. After finishing the calculations, you can then (and should do as good practice), check if the calculate...

Chem_Mod

When we try to solve for x or y from the acidity constant y(y+x)/(2x-y) = Ka, why can it be assumed that y is negligible and Ka just becomes (y)(x)/(2x).

Chem_Mod

One approach is to draw the Lewis structures. From this you can see that there is a difference in polarity between the two acids. The nitrous acid is more polar and therefore more acidic.

Chem_Mod

How would one know that the conjugate base of HNO2 is weaker than the conjugate base of H2CO3 without looking at the table? Since both are comparatively weak acids, would H2CO3 would be a stronger acid because of its 3 oxygens, thus resulting in a weaker conj. base

Chem_Mod

Answer: For the given solutions, at the stoichiometric point, you will need to add the same number of moles of CH3COOH as NaOH. Since the molarities are the same, then that means you will have to add the same volume of both solutions to have equal number of moles at the stoichiometric point. Becaus...

Chem_Mod

Question: For a titration of 0.20M CH3C00H with 0.2M NaOH, assuming that no volumes are given, how do we approach the problem?

Chem_Mod

For [O2]+ there should be only one electron in the (pi*) orbitals. Total of 11 electrons in the molecule. BO = 2.5, paramagentic.

Chem_Mod

For [O2]+, how many electrons are there in 2p(pi*) orbital?

Chem_Mod

For arsenic acid, the H are attached to the O. Each of these can be protonated in solution. As for phthalic acid, C6H4(COOH)2, the H4 corresponds to four H connected to the benzene structure are connected to a C, which cannot be protonated. The remaining two H are connected to an O on the carboxyl g...

Chem_Mod

Why does arsenic acid have multiple proton donation steps until there aren't any hydrogens left, and why does phthalic acid only has one proton donation step with multiple hydrogens still left. Why is there a difference?

Chem_Mod

aquachlorobisethylenediaminechromium(III) chloride

Chem_Mod

What is the name of the coordinate compound: [Cr(en)2(H2O)Cl]Cl2?

Chem_Mod

Answer: When they say after neutralization, they are referring to when you have let your system equilibriate. At this point, your system is not at the stoichiometric point, and so you must calculate how much CH3COOH you have left. To calculate the equilibrium conditions, you can first assume all of...

Chem_Mod

Question: For a titration of 25.0 mL of 0.10 M CH3COOH with 10.0 mL of 0.10 M NaOH solution? How do you calculate the concentration of CH3COOH after neutralization?

Chem_Mod

If the temperature of the system is increased, assuming at equilibrium the pressure and volume of the system remain constant, then we know based on the ideal gas law that the number of molecules of the system must decrease. Therefore, since the product side has 2 moles of gas for every 1 mole of gas...

Chem_Mod

For H2O(g) + C(s) <=>H2(g) CO(g), what would happen to H2O if you were to increase the temperature?

Chem_Mod

Answer: The first protonation of H2SO4 is assumed to be that of a strong acid, meaning there is complete dissociation of the first H. Therefore, your solution will have at least a concentration of 0.10 M of [H+]. But the second protonation could also occur, which would then increase the [H+] past 0...

Chem_Mod

Question: How do we know that a solution labeled "0.10 M H2SO4" has a pH less than 1.0?

Chem_Mod

For a given acid that dissociates in water as follows: HA + H2O <=> A- + H3O+, there is an associated Ka value for this reaction. The Kb for that conjugate base is: A- + H2O <=> HA + OH-. If you want to use the Kb equilibrium expression, you will need to use the reaction for Kb. If you are used to y...

Chem_Mod

Is there a difference with the equation: A + H20 = HA + OH and HA + OH = H20 + A? Will they result in different answers when using the ICE box?

Chem_Mod

In this case you need to consider what the definition of electron affinity is, which in this case would be how much would that molecule like another electron. To do this, you need to consider how favorable is it to add an electron to the LUMO of each molecule. For example, adding an additional elect...

Chem_Mod

How do we determine the electron affinity using the Molecular orbital diagram?

Chem_Mod

Answer: The halfway to stoichiometric point is where your pH = pKa. At the stoichiometric point, the number of moles of your initial acid is equal to the number of moles of your strong base. Because of this, the dominate reaction would be: A- + H2O <=> HA + OH-. Using the Kb for this reaction, you ...

Chem_Mod

Question: Why is the significance of the point halfway to the stoichiometric point?

Chem_Mod

You need to place them in order according to the ligand, not the prefix attached to them. For [CoBr2(NH3)3(H2O)]+, the ligands are: bromo, ammine, and aqua. Placing these in order, it would be: ammine, then aqua, then bromo. Now you need to place prefixes for the number of ligands involved, so the n...

Chem_Mod

How do we name transition metal complexes?? Where should we put aqua? Is there a specific rule for it?

Chem_Mod

The Cl- anion is neutral, but the Al3+ is not. From the reaction AlCl3 + 3H2O <=> Al(OH)3 + 3HCl, HCl is a strong acid, so it will dissociate in water to form H+ and Cl-. As a result of this reaction, the solution will be acidic, with a pH < 7.

Chem_Mod

Is AlCl3 acidic or basic since Cl- anion is neutral?

Chem_Mod

Answer: Looking at H2CO3, you can see that it can be protonated twice. As a result there are 2 equilibria expressions, one for Ka1 and another for Ka2. In addition to this, if the total carbonate concentration is given, from the relation: [(CO3)2-]+[HCO3-]+[H2CO3] = total carbonate concentration. F...

Chem_Mod

question: When an acid is protonated twice, as with H2CO3, what are the concentrations of [(CO3)2-], [HCO3-], and [H2CO3]?

Chem_Mod

The concentration is doubled because your initial molecule is Ca(CH3CO2)2, which has 2 moles of (CH3CO2) per 1 mole of the molecule.

Chem_Mod

When trying to determine the concentration of CH3CO2- from Ca(CH3CO2)2, why is the amount doubled?

Chem_Mod

HCl and HF binary acids, and the other molecules are oxoacids, so different rules apply to the two groups of acids. Recall that for the halogens, acidity increases down the group.

Chem_Mod

When comparing acid strengths, which carries more weight: electronegativity or bond? For H2C03 vs. H2GeO3, C has a greater electronegativity so it is a stronger acid. But for HCl is stronger than HF, even though F has a greater electronegativity than Cl.

Chem_Mod

To determine if the pH was greater than, equal to, or less than 7, you would need to look at the given molecule and determine whether it is an acid, base, conjugate acid, or conjugate base. From this, you should be able to guess the reaction of the molecule in water. Take the NH4Br molecule for exam...

Chem_Mod

How can we determine if the pH is greater, equal to, or less than 7?

Chem_Mod

Answer: Since the wavelength decreases, the frequency increases. From an increase in the frequency, based on the equation E=hv, we would expect an increase in frequency to give an increase in energy. This increase in energy will give rise to an increase in velocity as well.

Chem_Mod

Question: If electrons emitted from a Na photocell decrease the wavelength of the emitted light from 435 to 405, would the velocity increase or decrease?

Chem_Mod

Question: When NH3 and H2O reacts with each other, how can I know for certain that water will as a acid instead of base? in other words, how do I know for certain that NH3 will act as a base?

Chem_Mod

Molecules such as HCl and HI are considered binary acids, and their acid strengths are explained in section 10.9 as being a result of weaker bonds. But to explain the acid strength of HClO and HIO, you need a different explanation, which is provided in section 10.10. It explains that the more electr...

Chem_Mod

HCl is a weaker acid than HBr and HI due to the greater radii of Br and I, but why is HClO more acidic than HIO? Because Cl is more electronegative, it is more acidic. Is the presence of oxygen responsible for this difference? Is there a rule/pattern that overshadows the other ones?

Chem_Mod

Answer: The Lewis structure would look like: O=As-O-As=O. There are 2 lone pairs of electrons on each O atom and As has 1 lone pair of electrons on each atom.

Chem_Mod

Question: How is the Lewis structure for As2O3 drawn?

Chem_Mod

Nearly all solids are more dense than their liquid phase. Thus increasing pressure induces formation of solid. Water is an exception, since the liquid phase is more dense, increasing pressure induces formation of liquid. If ice was more dense than water it would sink to the bottom of oceans, seas, l...

Chem_Mod

Can you explain the exception and clarify water's exception rules in freezing and melting? Why is the exception property of water so important?

Chem_Mod

Answer: Bromothymol blue was used to show the pH of water in the presence of carbon dioxide and calcium oxide. Since carbon dioxide forms carbonic acid which is a weak organic acid the solution turned yellow. Since calcium oxide gives rise to strongly basic solutions, here the solution turned blue.

Chem_Mod

Question: What is the concept behind the indicator demonstration work?

Chem_Mod

Answer: It is always a good rule to use the same units in a calculation. However if there are units that cancel then it does save time to not convert them. Since the calculation has mL/mL they cancel. To convince yourself that the same answer is obtained, calculate the answer using mL/mL and then c...

Chem_Mod

Question: A stoicheiometric problem was given as follows: 8.99E-3 mol/L x (1.000mL/500mL) x (10mL/250mL)= 7.2E-7 mol/L. Why are the mL units not first converted to L? Isn't it impossible to calculate an equation when the units don't match.

Chem_Mod

Answer: 1st check that the reaction is balanced (which it is). 2nd, calculate the # of moles of urea produced (grams urea/molar mass urea). 3rd look at the molar ratios of reactants & products. Since 1mol CO2 --> 1mol urea, you have the # of moles of CO2. 4th use ideal gas eq: PV=nRT to calc V.

Chem_Mod

Question: How do you calculate the number of moles of an element when just given the mass of the compound and the reaction? For example, Urea, CO(NH2)2, is made by the reaaction: CO2(g) + 2NH3(g) --> CO(NH2)2(s) + H2O(g). What volumes of CO2 and NH3 at 200 atm and 450C are needed to produce 2.50 kg...

Chem_Mod

STP (Standard Temperature & Pressure) is 0 degrees C and 1 atm and an ideal gas has a molar volume of 22.41 L/mol under STP conditions.

Chem_Mod

What are STP conditions?

Chem_Mod

Answer: The answer is that Al^3+ is more like a metal as it will form complexes, but since it has a very high charge density (3+ and small size), the anions that it binds too (as a complex), are unlikely to dissociate when in solution. Therefore it is different from metals like NaCl and MgCl2 which...

Chem_Mod

Question: Regarding electron affinities of the metalloids, what type of characteristics does Al^3+ display? Does it act as a metal even though it is isoelectric with neon?

Chem_Mod

Answer: For the elements after Xe the valence orbitals become closer in energy, and therefore small changes in their values results in different orbitals being occupied. The order in which these orbitals are filled depends on the number of e- (screening & e-/e- repulsion). Cerium is a difficult...

Chem_Mod

Question: Cerium has the configuration [Xe]4f15d16s2. Why do they fill this way? Doesn't the 4f-orbital fill before the 5d-orbital?

Chem_Mod

Knowing the number of bonds between two atoms is made possible by calculating the bond order. Bond Order = 1/2[(no e- in bonding molecular oribitals) - (no e- in antibonding molecular oribitals)]. e- in bonding molecular oribital stabilizes the molecule (bond). e- in antibonding molecular oribital d...

Chem_Mod

What is the concept behind bond orders?

Chem_Mod

Same rules for Lewis Structures. The Lewis base is the same because it had a lone pr. The Lewis acid would be different as it now has an "additional e- pr".

Chem_Mod

For Lewis Acid-Base reactions, if an acid and a base are put together structurally(Lewis Structure), how should they be combined and how is the shape determined? Would you just follow the same rules for Lewis Structures( would they be symmetrical?)

Chem_Mod

Look at the product to see new bonds. Then look at reactants to see which bonds need to be broken. In your example: the C double bond & the H-Br bond will be broken (requires energy to break bond, +ve values). Product has a new C-C bond, C-H and C-Br bond (energy released with bond formation, -v...

Chem_Mod

If given a chemical reaction and asked what kind of reaction it is, how would we start to break up bonds to calculate the bond enthalpies? i.e. CH(2)=CH(2) + H-Br ... CH(3)-CH(2)Br. How would we know what bonds to break such as is a H-H bond broken and a C-H or a C-C? Also, are the reactancts always...

Chem_Mod

No, it terms of the effect on a chemical reaction. It is true that decreasing the volume of a sealed container increases the pressure. Also since concentration is moles/volume (usually liters), decreasing volume results in increasing concentration and hence has an effect on equilibrium composition. ...

Chem_Mod

What is it mean by "increasing the pressure"(isn't it same as compression since increasing pressure decreases volume?) by introducing an inert gas has no effect of the equilibrium composition?

Chem_Mod

Inorganic acids & bases are usually strong. Organic (containing Carbon) acids and bases are usually weak. Remember e- withdrawing groups stabilize an anion and can make a weak organic acid a strong one. Go through your class notes on strong acids (non-metal oxides) & bases (metal oxides) &am...

Chem_Mod

For acids and bases, how many different ways are we supposed to be able to tell if its strong or weak? I know that strong acids and bases completely dissociate in water, but aside from the obvious acids and bases that we have been studying so far, how can we tell? I know it has to do with all the di...

Chem_Mod

Answer: Yes it definitely does.

Chem_Mod

Question: The last few notes concerning the Heisenberg Equation stated that there was a limit on the accuracy to which the momentum and position of a particle can be known simultaneously. My question relates to an earlier note about the properties of electrons in QM. If electrons are supposed to ha...

Chem_Mod

Answer: To fully answer this question requires more details about quantum mechanics that is beyond the level of this course. Very briefly, the spin angular momentum can be measured and is found to be: S = [square root of s(s + 1)] divided by a constant. This equation can be solved when Ms is 1/2 or...

Chem_Mod

Question: While reading about the Ms quantum numbers, it stated that the two spins are either 1/2 or -1/2. Why is it labeled with 1/2 as opposed to 1?

Chem_Mod

Yes. When naming acids the suffix "ic" is added after the stem of the second element. E.g. HCl is hydrochloric acid

Chem_Mod

For naming simple binary compounds (acids), to name an acid (aq) is the suffix "ic" used after the stem of the second element.

Chem_Mod

Yes for both cases.

Chem_Mod

Do all equal energy resonance structures have the same number of atoms with the same formal charge?
if so, is it okay to have different formal charges on different atoms for different formal charges? For example, a formal charge of Oxygen on one resonance structure is -1 and 0 on the other.

Chem_Mod

Answer: Work out the formal charge and also think about the way an element bonds in other molecules. Cl usually has a single bond.

Chem_Mod

Question: For resonance structures, how do I know when the two elements can form double or triple bonds? For example, how do I know that N forms double bonds only with O, but not with Cl?

Chem_Mod

Answer: Yes, oxygen is very electronegative so it would not like a +1 charge. There are however cases where oxygen takes on a +1 charge. For instance, carbon monoxide :C---O: (where --- is a triple bond) has oxygen with a +1 charge. A structure with a +1 oxygen would not be a major contributor to t...

Chem_Mod

Question: When drawing resonance structure for oxygen, since it is more electronegative, would it prefer a negative charge?

Chem_Mod

Answer: Chlorine usually prefers to be single bonds, so putting the double bond between Nitrogen and Chlorine is unstable. Also, placing a double bond between N and F (assuming you left the N-O bonds as single bonds) gives them both a +1 formal charge and the Oxygen atoms both a -1 formal charge. W...

Chem_Mod

How should we approach the problem of drawing a resonance structure of ClNO2?
I am confused as to why the one oxygen has 3 pairs of electrons when it only has 6 electrons in its valence shell. Does the oxygen receive an extra electron from the Nitrogen to pair its normally unpaired electron?

Chem_Mod

Answer: The Lewis structures for ozone show a double bond and a single bond. These bonds switch for the two resonance structures. Averaging this effect, one gets that there is a 1.5 bond from either the outer oxygen to the central oxygen, so it is in effect only one bond length present.

Chem_Mod

Question: Regarding the bond lengths of ozone (O3) vs O2, why is there only one observed bond length?

Chem_Mod

Answer: Resonance involves delocalization of electrons over multiple bonds so they have both single and double bond character making them stronger than the single bond.

Chem_Mod

Question: Do resonance structures affect the strength of bonding? And if so, how?

Chem_Mod

Answer: In both cases you count the total number of valence e- for all the atoms in the molecule and any remaining e- (after giving 8e- to each atom) are placed on the central I atom. So one does not really know before hand, it just works out that way.

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