CHEMISTRY COMMUNITY

annabel 2A

For #8, can someone explain why deltaH is negative? Shouldn't the enthalpy change of the reaction be positive since the temperature of the solution increases? So the solution absorbed heat?

annabel 2A

For #5b, would the reaction only be reversible if deltaG=0?

annabel 2A

Intermediates are in the rate laws for elementary steps but not in the overall rate law of the reaction

annabel 2A

Although deltaH is positive, deltaS is also positive (water is less orderly than ice), and deltaS*T is probably a greater value than deltaH which makes deltaG=deltaH-TdeltaS negative

annabel 2A

For the integrated rate law worksheet #5a, I found the answer using
1/((7.0*10^9 M*s)*(120 sec) + 1/0.086M)
= 1/ (8.4*10^11 M*s^2 + 11.63 M^-1)
and I got the right answer but I'm confused by the units. How do the units work out to be M? And how do you add 8.4*10^11 M*s^2 with 11.63 M^-1?

annabel 2A

Why is it Fe2+ --> Fe3+ and not Fe2+ --> Fe or any of the other ones? Should we just assume that it will be Fe2+ --> Fe3+?

annabel 2A

I think that you're supposed to assume that HCl dissociates into its ions in an aqueous solution because it is a strong acid, so you're supposed to see the equation as Cl2(g) + H2(g) --> 2H+(aq) + 2Cl-(aq)

annabel 2A

When asked to calculate the equilibrium constant from deltaG°=-RTlnK, will we always be told what temperature to calculate it at?

For example, on Karen's Gibbs Free Energy worksheet #1d, how do we know what temperature to calculate the equilibrium constant at?

annabel 2A

The rate of formation is specific to each product, while the unique rate of a reaction is the same value for all reactants and products in the unique reaction.

annabel 2A

Include Pt as the electrode for any half reaction that doesn't have a solid metal.

annabel 2A

I think H2O is the solvent so you don't need to include it in the cell diagram. It's implied?

annabel 2A

If one of the half reactions does not include a solid metal, then use platinum

annabel 2A

But why is Cl-(aq) on the left and Cl2(g) on the right?

annabel 2A

| is used to separate phase changes and to indicate a porous barrier. Since Sn are both aqueous, they are separated by a comma.

annabel 2A

For 6th Edition Chapter 9 #55: Write a balanced chemical equation for the formation reaction of (a) NH3 (g); (b) H2O(g); (c)CO(g); (d) NO2(g). For each reaction, determine H,S, and G from data in Appendix 2A. When calculating the Grxn, can we use the equation "deltaG = deltaGformation(products)...

annabel 2A

Why can we use the same deltaH and deltaS to calculate deltaG at any temperature? (like in this problem: 9.53 Calculate the change in molar Gibbs free energy for the process NH3(l) --> NH3 (g) at 1atm and (a) 15.0 C; (b) 45. C (see Tables 8.3 and 9.1 for standard enthalpy and entropy of vaporization...

annabel 2A

The spontaneity of a reaction does not indicate the reaction rate, so not necessarily.

annabel 2A

Chem_Mod wrote:
005199302 wrote:for #6, shouldn't the work be -12.4kJ?

w=-PxdV = (-122atm/L)(101.33J/atmL) = -12.4kJ

Please see above answer to the same question

Why do we need to find a different pressure if pressure is constant?

annabel 2A

How do you calculate internal energy for #6 without knowing the temperature?

annabel 2A

Does that mean that water/substances can undergo phase change without input of heat?

annabel 2A

Also for this question why do we ignore the heat used to raise the temperature of the water? For reference, the question is: A calorimeter was calibrated with an electric heater, which supplied 22.5 kJ of energy as heat to the calorimeter and increased the temperature of the calorimeter and its wate...

annabel 2A

If you want to be really extra you can write out all the bonds in the reactants and all the bonds in the products. Then you can tell which bonds changed in the reaction.

annabel 2A

Dr. Lavelle will probably go over it in lecture this week if it's on the syllabus.

annabel 2A

We don't need to memorize them.

annabel 2A

I think it was supposed to be a really redundant obvious point... so I think you're right

annabel 2A

The enthalpies of the reactants and the enthalpies of the products.

annabel 2A

Can someone explain how to do Worksheet 1 #4d? My friend did it with initial concentration of NH3 as 0 and she got the right answer. Why doesn't the initial concentration of NH3=5.82*10^-3 ?

annabel 2A

What does the question say?

annabel 2A

What is an inert gas?

annabel 2A

As pressure increases, volume decreases, which causes the concentration of reactants and products to change. This change makes the system not in equilibrium, and therefore causes movement towards equilibrium.

annabel 2A

I think one of the TAs said that we won't need to know it. I think if it's on the test we'll probably be given the equation and the important values?

annabel 2A

What does the problem say? I think generally it tells you which to solve for.

annabel 2A

Also, Kp=Kc*RT^[(moles of products)-(moles of reactants)]

annabel 2A

I think BaO is basic because the O will have 3 lone pairs which will allow it to donate an electron pair?

annabel 2A

Can someone also explain why SO3 is acidic?

annabel 2A

For Week 9 worksheet #5, why cant H3O+ and H2O neutralize an added base? And why cant H2O neutralize an acid and be potentially amphoteric?

annabel 2A

An amphoteric compound can act as both an acid and a base, so it can donate and accept protons.

annabel 2A

There are 3 Br because one bromide ion has a charge of -1, and since the compound is neutral, you need to balance out the +3 charge on the Cobalt. Therefore you need 3 Bromide ions 3*(-1)=-3 to balance out the +3 charge of the Cobalt.

annabel 2A

I think we're supposed to memorize the ligand names.

annabel 2A

On Hannah Chew's Week 6 Worksheet #4a [Rank the following from weakest intermolecular forces to strongest a) H2S, H2Te, H2Se] why is the strength of london dispersion forces more important than the dipole-dipole forces? Doesn't H2S have the strongest dipole-dipole forces of the three molecules?

annabel 2A

On the Week 6 Worksheet #4a [Rank the following from weakest intermolecular forces to strongest a) H2S, H2Te, H2Se] why is the strength of london dispersion forces more important than the dipole-dipole forces? Doesn't H2S have the strongest dipole-dipole forces of the three molecules?

annabel 2A

I thought that completing the octet was more important than minimizing formal charges?

annabel 2A

is it more favorable to have formal charges spread out or to have them concentrated? For example, for N3- is it more favorable for one of the formal charges to be [2-, 1+, 0] or [1-, 1+, 1-]? or are neither of them the best one?

annabel 2A

If there is a double bond will there always be 1 sigma and 1 pi bond?

annabel 2A

The shape is trigonal pyramidal and the lewis structure is
'''''''''''F
'''''''''''|
F — Se — F
'''''''''''. .

annabel 2A

A lone pair is also more delocalized than a bonding pair, so it will have greater ability to move around and repel other electrons.

annabel 2A

The VSEPR model is the Valence-Shell Electron-Pair Repulsion model, which explains experimentally observed shapes of molecules.

annabel 2A

I think there are 11 we need to know:
linear, trigonal planar, angular, tetrahedral, trigonal pyramidal, trigonal bipyramidal, see-saw, t-shaped, octahedral, square pyramidal, square planar

annabel 2A

Why can the Cl in ClO4- have 7 bonds? I thought that 6 was the limit for Cl? Can it access more than the 4s and 3d orbitals?

annabel 2A

Also, in 3.57b) the solutions show a perchlorate ion where Chlorine has 7 bonds. How/why can Chlorine have 7 bonds?

annabel 2A

The question asks: 3.57 Write the Lewis structure, including typical contributions to the resonance structure (where appropriate, allow for the possibility of octet expansion, including double bonds in different positions)... and the solutions include the Lewis structure that obeys the octet rule, b...

annabel 2A

For the Lewis Structure of BrO-, does O carry the formal charge because it's more electronegative?

annabel 2A

How do we know how many resonance structures to draw?

annabel 2A

Across a period (from left to right), first ionization energy increases. This makes sense since across a period, the number of protons are increasing while the valence electrons are being added to the same energy shell (so they aren't further from the nucleus). This means the nucleus can hold onto t...

annabel 2A

When and why does hydrogen exist as hydride?

annabel 2A

Is there a specific way that the electron configuration will change when the electron is excited?

annabel 2A

For question 7 on worksheet 2, why is it not possible to use the eqn in the book that says v= R(1/ni ^2 - 1/nf^2). I tried this just to see if I got the same answer and I didn't. Why doesn't this work as well, because we are using frequency to get n initial? I think that it works as long as you add...

annabel 2A

Can someone explain what the statement in #9 of worksheet 3 means? "It is impossible to determine both the speed and position of an electron with absolute uncertainty." Why is it absolute uncertainty and not absolute certainty?

annabel 2A

I think we do need to memorize the prefixes and be able to convert between them.

annabel 2A

do we need to memorize the conversion factor between Joules and eV?

annabel 2A

https://lavelle.chem.ucla.edu/wp-conten ... hedule.pdf

^those are the exam dates and times

annabel 2A

Here is the problem (6th edition Example 1.7): Estimate the minimum uncertainty in (a) the position of a marble of mass 1.0 g given that its speed is known to within ±1.0 mm s^-1

In the solution, the speed is multiplied by 2. Is it because of the ±? Can someone please explain this!!!

annabel 2A

I would recommend going to the workshops! They help you a lot with foundational skills and are very helpful if you are totally lost on a particular concept. I think that they are just as helpful as TA office hours and that I feel more comfortable asking really basic clarifying questions during the w...

annabel 2A

When doing problems using the Rydberg Equation, how do you know which energy level is n1 and which is n2?

Sample question -- 6th Edition Chapter 1 #13: Use the Rydberg formula for atomic hydrogen to calculate the wavelength of radiation generated by the transition from n=4 to n=2.

annabel 2A

For the test, do we only have to show the amount of intermediate steps shown in the solutions manual?

annabel 2A

Where is the workshop going to be?

annabel 2A

For 6th Edition Chapter F #21, why don't you begin the problem by finding mass percentage? When are you supposed to calculate the mass percentage? This is the problem: "In 1978, scientists extracted a compound with antitumor and antiviral properties from marine animals in the Caribbean Sea. A s...

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