CHEMISTRY COMMUNITY

KDang_1D

If you consider the graphs we often use to represent these processes, a catalyst lowers the peak of the activation energy (Ea). However, the initial and final energy values of the curve remain unchanged.

KDang_1D

K = [products]/[reactants]. Therefore, given H2O(l) --> H3O+(aq) + OH-(aq), Kw = [H3O+][OH-]. Because H3O+ and OH- are in a 1:1 molar ratio, their concentrations will be equivalent. Thus, Kw = x•x = x^2. As explained by Michelle, take the square roof Kw to solve for x. This is the concentration of [...

KDang_1D

When using the Ideal Gas Law typically, following units are used. However, they may be converted appropriately.
P = atm
V = L
n = moles
R = 0.08206 (Latm)/(molK)
T = K

PV=nRT is also often manipulated to become P=(n/V)RT = CRT, where C = concentration (M=mol/L)

KDang_1D

For gases, an increase in volume without an increase in moles would result in a decrease in concentration. Thus, increasing volume by an amount would be a similar type of problem to decreasing moles by a ratio. I believe there was one like this on Pizza Rolls. You would have to know the molar ratios...

KDang_1D

You could possibly argue that two solids would not be a homogeneous mixture as two aqueous solutions would be. However, I would take Charlyn's advice and use a comma if it comes up.

KDang_1D

Intensive properties do not change with changes to amount. For example, the density of water is always 1g/mL regardless of how much water you have.

Extensive properties do depend on amount. For example, mass and volume

KDang_1D

7D.1 The rate constant of the first-order reaction 2 N2O(g) + 2 N2(g) --> O2(g) is 0.76 s^-1 at 1000. K and 0.87 s^-1 at 1030. K. Calculate the activation energy of the reaction. The solutions manual seems to have manipulated the Arrhenius equation to ln\frac{k_{1}}{k_{2}} = \frac{E_{a}}{R}\cdot \f...

KDang_1D

For homework problem 7A.15, how do you know which experiments to use to calculate the order? For example for A, if you used experiments 1 and 3 instead of experiments 2 and 4, you would get a=3 instead of a=1.

KDang_1D

7A.7 Express the units for rate constants when the concentrations are in moles per liter and time is in seconds for (a) zeroth-order reactions; (b) first-order reactions; (c) second-order reactions.

Why do you divide by mol/L for first and second order reactions?

KDang_1D

Do you have an example of a problem? I don't believe you'll never have to come up with the Eº values on your own since they are experimentally derived values. The only one might be a hydrogen electrode, which is 0V since it's the baseline.

KDang_1D

Why can you use partial pressure and concentration together when calculating K or Q in the Nernst equation. Also, why can you use different units. 6N.3 combines atm and [ ], and 6N.5 combines bars and [ ]

KDang_1D

How do we know that there is elemental oxygen present? Usually we can only balance redox reactions with H2O, OH-, and H+

KDang_1D

6M.5 For each reaction that is spontaneous under standard con- ditions (that is, K . 1), write a cell diagram, determine the stan- dard cell potential, and calculate DG8 for the reaction: (a) 2NO3-(aq) + 8H1(aq) + 6Hg(l) --> 3 Hg2^2+(aq) + 2 NO(g) + 4 H2O(l) (c) Cr2O7^+2(aq) + 14 H+(aq) + 6 Pu3+(aq...

KDang_1D

5J.15 Calculate the equilibrium constant at 25 8C and at 150 8C for each of the following reactions, using data available in Appendix 2A. (a) NH4Cl(s) <--> NH3(g) + HCl(g) (b) H2(g) + D2O(l) <--> D2(g) + H2O(l) For these problems, why do you have to use ∆Hº and ∆Sº instead of directly using ∆G?

KDang_1D

(b) Ce4+(aq) + I-(aq) --> I2(s) + Ce3+(aq) The solution says, "An inert electrode such as Pt is necessary when both oxidized and reduced species are in the same solution." What does this mean? The iodine and the cerium are in separate electrodes of the cell. I understand that the two aque...

KDang_1D

Pt(s)|O2(g)|H+(aq)||OH-(aq)|O2(g)|Pt(s)

How would you reason through separating this cell into its half-reactions? How do you know where to include H2O?

KDang_1D

6Fe^3+(aq) + 2Cr^3+(aq) + 7H2O(l) --> 6Fe^2+(aq) + Cr2O7^2-(aq) + 14H+(aq), Ecellº=-1.29V I tried calculating the Ecellº on my own for practice and didn't get -1.29V. From the Appendix: Fe^3+(aq) --> 6Fe^2+(aq), Eº=+0.77V Cr2O7^2-(aq) + 14H+(aq) --> 2Cr^3+(aq) + 7H2O(l), Eº=+1.33 V Wouldn't Eºcell ...

KDang_1D

O3(aq) + Br-(aq) --> O2(g) + BrO3-(aq) in basic solution

Why isn't O3 reduced to BrO3- if the oxidation number is changing from 0 to 2-? O3 to O2 is still 0 to 0.

KDang_1D

Cl2(g) --> HClO(aq) + Cl2(g)

The oxidation half-reaction is the easiest to see: Cl2 has an oxidation number of 0. The Cl in HClO has an oxidation number of +1.

What is the reduction half of the reaction?

KDang_1D

Hopefully these diagrams will show with an arrow the direction of the flow of electrons. The side that the electrons are moving towards are gaining electrons. Thus this side would be the anode/reduction.

KDang_1D

Why do we need to measure potential difference with reference to hydrogen? Why can't we measure the flow of electrons from the anode to the cathode as a value on its own?

KDang_1D

In galvanic cells, the metal that is being oxidized will start to dissolve. As the element's oxidation number increases from 0 to a +#, the atom becomes an ion. These ions then enter the solution rather than being solid metal on the probe. In contrast, the metal that is being reduced will experience...

KDang_1D

He hasn't explained how to do this, so unless you find something in the textbook, I don't think so! Just for sure know that platinum is the most common.

KDang_1D

I believe it is in included in the Hg2Cl2 half-reaction since it's part of the solid and relevant to Hg's chemical change. For the H half-reaction, the Cl- is completely dissociated from HCl and therefore is not affecting the H reaction.

KDang_1D

Since Dr. Lavelle has been emphasizing left/right so much, yes I believe we can safely assume that. However, I would recommend checking if you can. A quick way to do this is to see which direction the electrons are flowing as it is often included in the diagram. The side that is G aining electrons w...

KDang_1D

What is meant by "a charge buildup" regarding the salt bridge? Why does this prevent electron transfer?

KDang_1D

"... First you perform an isobaric compression of the container to 10.0L...."

Why is ∆U=0 for the first part of the problem if the process is isobaric, not isothermic?

Also, why is ∆S=0? Wouldn't a decrease in volume cause a decrease in number of possible positions?

KDang_1D

As used in Pizza Rolls 5, how do you know when you can substitute w=-nRTln(T2/T1) into ∆S=qrev/T? Why is this process considered reversible for you to do this?

KDang_1D

4I.9 Initiallyanidealgasat323Koccupies1.67Lat4.95atm.The gas is allowed to expand to 7.33 L by two pathways: (a) isothermal, reversible expansion; (b) isothermal, irreversible free expansion. Calculate DStot, DS, and DSsurr for each pathway. How do these two processes differ? Why is no work done by...

KDang_1D

Why does the trans isomer of a MX2Y4 molecule have 3 different orientations? If the radial atoms are all identical, wouldn't this be the only possible configuration?

KDang_1D

Calculate the change in entropy of a block of copper at 25ºC that absorbs 65J of energy from a heater.

Why can we assume that this is a reversible process?

KDang_1D

As applied to ∆S=∆H/T, for example, what does it mean that temperature remains constant as enthalpy changes? Wouldn't an increase/decrease in energy cause the temperature to go up/down?

KDang_1D

Our TA explained that substances in their standard state have a standard enthalpy of formation of 0. In lecture, Dr. Lavelle did an example in which we calculated the standard enthalpy of the combustion of methane. He stated that gases are in their standard state at 1atm and pure liquids are always ...

KDang_1D

4B.9 In an adiabatic process, no energy is transferred as heat. Indicate whether each of the following statements about an adia- batic process in a closed system is always true, always false, or true in certain conditions (specify the conditions): (a) ∆U=0; (b) q=0; (c) q<0; (d) ∆U=q; (e) ∆U=w. Why...

KDang_1D

As used in 4B.5, how do you convert Torr(mL) to J?

KDang_1D

I guess I'm asking, how would this piston function in practice/how could you model this experimentally?

KDang_1D

Today (Monday Week 5), Dr. Lavelle talked about isothermic reversible reactions. The external pressure decreases gradually, allowing heat to instantaneously enter the system as it does work. What would this process look like for reversible expansions for which we use the formula -\int_{V_{1}}^{V_{2}...

KDang_1D

Why doesn't the calorimetry formula q=-C∆T include mass like q=mC∆T. And why is the q value negative if the calorimeter is gaining the energy lost by the reaction?

KDang_1D

4A.5 A piston confines 0.200 mol Ne(g) in 1.20 L at 25 8C. Two experiments are performed. (a) The gas is allowed to expand through an additional 1.20 L against a constant pressure of 1.00 atm. (b) The gas is allowed to expand reversibly and isothermally to the same final volume. Which process does ...

KDang_1D

What is heat capacity?

KDang_1D

The information should be given, possibly in a phase change graph. I believe it would be the difference in y-value between two of the horizontal lines (the height of the diagonal) as this would be the energy into the system to bring the substance from one state up to the next.

KDang_1D

You can think of enthalpy as the energy stored in the bonds (technically there's heat but don't get it mixed up with the T, temperature). So a certain amount of energy is needed to break the energy of bonds (+∆H), and energy is released when energetically favorable bonds are formed (–∆H).

KDang_1D

What do the variables w and U stand for?

KDang_1D

6A.19 Calculate the molar concentration of OH2 in solutions with the following molar concentrations of H3O1: (a) 0.020 mol/L-1; (b) 1.0x10^-5 mol/L-1; (c) 3.1 mol/L-1. c. Why do you divide \frac{1.0\times 10^{-14}}{3.1\times 10^{-3}} , not \frac{1.0\times 10^{-14}}{3.1} , since [H3O+] = 3.1 M (like...

KDang_1D

5I.25.) A reaction mixture is prepared by mixing 0.100 mol SO2, 0.200 mol NO2, 0.100 mol NO, and 0.150 mol SO3 in a reaction vessel of volume 5.00 L. The reaction SO2(g) 1 NO2(g) ∆ NO(g) 1 SO3(g) is allowed to reach equilibrium at 460 8C, when Kc 5 85.0. What is the equilibrium concentration of each...

KDang_1D

Dr. Lavelle went over it in lecture, so I believe it would be a fair question.

You would need to apply the ideal gas equation: PV=nRT. Divide V over to get P = (n/V)RT. (n/V) is concentration, which should be given, and RT are constant so just solve for P, your partial pressure

KDang_1D

I think we should include the subscript just to be safe! Especially for reactions involving gases. If given a unitless value for K, the reader would need to know if it's for pressure or concentration and they need to convert

KDang_1D

This state is called a dynamic equilibrium. Even though it seems like the reaction has stopped, the forward and reverse reactions continue to proceed, just at equivalent rates so it is unnoticeable. But why certain reactions favor specific ratios, however, is beyond what we know in 14B

KDang_1D

I'm not sure how it would be possible to increase the pressure of one type of gas in the same reaction vessel as the other. However, assuming that this change in pressure essentially causes a change in concentration, the other reactant would only be affected as a result of LeChatellier's principle. ...

KDang_1D

Yes, that expression refers to "percent protonation." Take note that these are the concentrations of the "primary" compounds involved in the reaction. For example, regarding NH3 + H2O --> NH4+ + OH-, you would only use [NH4+] formed /[NH3] initial, not including [H2O] and [OH-] A...

KDang_1D

For some problems (i.e. 5I.25), you have to calculate the Q value first and then compare it with K. If Q<K, the reaction will move forward, and if the Q>K, the reactions will move in reverse. This is all relative to how the chemical reaction is written. Then, set up your Ka expression and ICE table ...

KDang_1D

In lecture, Dr. Lavelle did an example using Ba(OH)2 to show that Kw, a special value of Ka, can be used to determine [H3O+]. Why is this possible? Don't all aqueous reactions have different Ka values, so shouldn't Ba2+ affect the value of Ka/Kw?

KDang_1D

Removing some of the product would decrease its concentration/partial pressure value. Mathematically the ratio [P]/[R] would be smaller. Since Q<K, the forward reaction will be favored until Q=K again

KDang_1D

One of the most well-known applications of Le Chatelier's Principle is the production of NH3 for fertilizer. Dr. Lavelle mentioned it in lecture and one of the TA's talked about it in discussion N2(g) + 3H2(g) --> 2NH3(g) Chemist Fritz Haber highly pressurized the NH3 gas (with a catalyst) so that i...

KDang_1D

How does what we're learning about equilibrium relate to what we learned in 14A about limiting reagents? Wouldn't those calculations be invalid since we were assuming that the reactions ran to completion?

KDang_1D

I believe both are correct if not specified. As long as you remember that partial pressure only applies to gases and not aqueous solutions

KDang_1D

Concentration and partial pressure can be converted using a form of the ideal gas equation: PV = nRT --> P = (n/V)R, where n/V is concentration. Equilibrium expressions using concentration (Kc) will contain brackets ("[X]") and expressions using partial pressures (Kp) will contain the vari...

KDang_1D

Regarding the Chemical Equilibrium Pt 1B Pre-Assessment, for reactions that involve liquid water as a product and H3O+(aq) or OH-(aq) as a product, are the H3O/OH included in the K calculation?

KDang_1D

How would you know that the coordinator for hydrated Al is 6?

KDang_1D

I was wondering how cisplatin binds to DNA since Cl doesn't typically form coordinating covalent bonds. Google says that the Cl is replaced with H2O. What drives this process?

KDang_1D

Is it correct to call CN "cyanido" or does it have to be "isocyanido"? (in reference to Table 9C.1)

KDang_1D

For 9C.3d "sodiumbisoxalato(diaqua)ferrate(III)", why is diaqua written in parentheses? Also, why does it come after the bisoxalato?

KDang_1D

Why doesn't H2SO5 have cis/trans isomers?

KDang_1D

In the radical HOCO, how do you know that the carbon would have the unpaired electron? Isn't oxygen more electronegative?

KDang_1D

Are all sp(1,2,3) hybridized orbitals in the 2nd energy level?

KDang_1D

Why does SO2 have two resonance structures? Doesn't the one with two double bonds have formal charges of 0, and the one with only one double bond have formal charges of +/-1?

KDang_1D

How does hybridization work for atoms with lone pair electrons? For example, HS2

KDang_1D

"p" means to take the -log of a value. Just as pH is the -log[H+], pKa is the -log(Ka).

For example, NH4+ has a Ka = 5.6x10^-10
pKa = -log(5.6x10^-10) = 9.25

KDang_1D

I'm sure it would have to be given in the problem. But if, for example, you were given an OH- concentration of 1x10^-8 M, you could calculate that the H3O+ concentration is 1x10^-6 M, since [H3O+] x [OH-] = 1x10^-14. You can also do this using pH values. The pOH for this solution would be 8. pH + pO...

KDang_1D

Solutions can fall outside the 0-14 range. Dr. Lavelle mentioned "superacids" which have a H3O+ concentration > 1x10^0 M and thus would have a negative pH. However, these are uncommon, especially in biological systems. I'm sure the same is true for bases.

KDang_1D

Concentrations for H3O+ (and OH-) are very small, so they will have a negative exponent in their molarity value. For example: [H3O+] = 1x10^-7 M. Since logarithms calculate the value of the exponent, log[H3O+] = -7. Negative numbers aren't as nice to work with as positives, so we just add the negati...

KDang_1D

Why do the halogens form the strong acids? Why would they have the weakest bonds if they have the smallest atomic radius in each period?

KDang_1D

In "diethylenetriamine," why does ethylene come before amine? Not A before E as would be for alphabetical order? In the other example "pentaamminechlorocobalt(II) chloride hydriate," "ammine" comes before "chloro" regardless of the prefix. *Also is NH3 spelled...

KDang_1D

In lecture, Dr. Lavelle used the example [Fe(CN)6]4-:

a. How is CN a ligand? Unless it's CN-, it would have an unpaired electron and wouldn't be able to form a coordinate covalent bond?
b. Why is the charge 4-?

KDang_1D

If interaction potential energy is given by the formula: Ep (is proportional to) $\frac{\alpha_{1} \alpha_{2} }{r^{6}}$ , then why does attractive force increase as atomic size increases?

KDang_1D

What other VSEPR structures do we need to know that Dr. Lavelle didn't go over in lecture?

KDang_1D

What does the pi bond's inability to rotate mean when determining VSEPR structure? How is this concept applied?

KDang_1D

The course outline says we need to be able to "distinguish pi bonds by their component orbitals." What are the component orbitals of pi bonds? Doesn't it depend on each individual molecule's central atom configuration?

KDang_1D

Why is CH2Cl2 a polar molecule? If drawn with the Cl's opposite of each other, wouldn't the dipoles cancel out?

KDang_1D

What is the VSEPR formula (AXE) for this?

KDang_1D

109.5 you'll just to memorize, but the others you can tell from their shape. Trigonal planar has three atoms spread out evenly on a single plane. 360/3 = 120 degrees Trigonal bipyramidal has 2 axial elements coming out from both sides of the trigonal planar shape, so you should be able to visualize ...

KDang_1D

No, and even better, the shape of an AXE molecule will always be the same, regardless of the atoms

KDang_1D

In today's Friday lecture, we also learned the organic chemistry prefixes cis- and trans-. I don't think we should need to know how to draw organic molecules, but I think we should be familiar with general polarity. "Cis-" means same and "trans-" means opposite, so in general, wh...

KDang_1D

Why wouldn't this be a trigonal planar shape with the electron clouds in the axial positions? Didn't we talk about the electron clouds wanting to be as far apart as possible to minimize repulsion and make the molecule more stable?

KDang_1D

The VSPER theory only gives qualitative, not quantitative descriptions of the shape of molecules, so although we know how the atoms are arranged, we cannot predict exactly what their angles will be. AX2E will be less than 120 degrees, and AX2E2 will be less than 109.5 degrees. Those are the expected...

KDang_1D

What’s the difference between “electrostatic force/potential energy” and “effective nuclear charge?

KDang_1D

"Draw the complete Lewis structure for glycine, H2C(NH2)COOH." The molecule is drawn H2–N-CH2-CO-OH (if that makes sense). How would you know that the nitrogen is on the end and not in between the two carbons? It's written in the middle of the given formula and usually they give structural...

KDang_1D

I think we only need to know the basics that we put the wave function ( $\psi$ ) into the "Hamiltonian operator" and get out energy (E) values that are interpreted to give the shapes of the s, p, d, etc. orbitals that we are familiar with.

$H\psi =E\psi$

KDang_1D

Multi-electron atoms must follow the Aufbau principle, which states that electrons fill the lowest-energy orbitals before the largest. So s<p<d<f within the same energy level. For us, be able to write and interpret quantum numbers (n, l, ml, ms) and electron configurations.

KDang_1D

How would we know this without a table with values? I'm pretty sure they've said we shouldn't have to remember specific values.

KDang_1D

Which bond has more ionic character, C–F or C–Br? Why?

C-F (look at electronegativity difference)

Why isn't the electronegativity difference greater between C and Br since Br is farther away on the periodic table?

KDang_1D

Why can't the molecule be drawn Cl-C-O-Cl rather than centered around C? The formal charges would all be 0 instead of +1/-1.

KDang_1D

This is referencing the concept of bond length that Dr. Lavelle explained during Friday's lecture using the example of the strong acids: HF > HCl > HB > HI. Bond strength decreases as atomic radius of increases because the atoms are farther apart and easier to pull apart. Thus, among CF4, CCl4, CBr4...

KDang_1D

Hmm, I think this is a little more detailed than necessary. I believe it's explaining that all processes require energy, and the formation of ionic bonds is driven by the cation in the compound. More than the electron affinity of the anion, the ionization energy of the cation wanting to lose its ext...

KDang_1D

Essentially, the question is "what is the kinetic energy of one potassium ion with a wavelength of 11.23 nm?" We used the mass 39.1 g/mol for the potassium ion. Does that mean that the mass of ions and neutral elements are the same? Why wouldn't we have to subtract 9.11x10-31kg from the ma...

KDang_1D

Why does tungsten have 4 electrons in the d-orbital? If element #57 is a lanthanide and the additional electron would go into the f-orbital, wouldn't tungsten have 3 electrons in the d-orbital?

KDang_1D

1E.25 Give the notation for the valence-shell configuration (including the outermost d-electrons) of (a) the alkali metals; (b) Group 15 elements; (c) Group 5 transition metals; (d) the "coinage" metals (Cu, Ag, Au). What is the "valence-shell configuration"? What does n and (n-...

KDang_1D

P, S, and Cl are the only elements that we need to know since we will only be covering elements from the first 3 energy levels, I believe.

KDang_1D

It has to do with the impact of detection on the position of the object being detected. When the photons from the measuring device reaches the electron, the electron is pushed off course, altering it's velocity and thus also momentum. Therefore, the more certain we are about the electron's position,...

KDang_1D

The Heisenberg equation is expressed as $\Delta p\Delta x\geq \frac{h}{4\pi}$ , meaning that the uncertainty is greater than or equal to that value. Thus, the "minimum uncertainty" would be when the uncertainty is equal to h/(4 $\pi$ ). So just change >/= to =.

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