CHEMISTRY COMMUNITY

Tyra Nguyen 4H

The problem should provide us this information. If not, the slow step is the rate-determining step, thus the slow step will match the overall rate law. It honestly depends on the context but if you were just given the mechanism and nothing else I don't think you would be able to figure out because i...

Tyra Nguyen 4H

From page 628 of the 6th edition textbook

ln2 will simplify to 0.693, which is the equation given on the provided formula sheet on tests.

Tyra Nguyen 4H

Microscopic reversibility: The mechanism of a reversible reaction is exactly the same (but reversed) for both the forward and backward version of the reaction. The transition states for each mechanism step are identical regardless of reaction direction.

from the chem.ucla.edu site!

Tyra Nguyen 4H

15.63 The rate constant of the reaction between CO2 and OH - in aqueous solution to give the HCO3 - ion is 1.5 x 1010 L.mol -1 s -1 at 25 o C. Determine the rate constant at blood temperature (37 o C), given that the activation energy for the reaction is 38 kJ.mol -1 . I got the right answer, but I ...

Tyra Nguyen 4H

What does Dr. Lavelle mean when he refers to "reaction mechanisms?"

Tyra Nguyen 4H

Why is the standard reaction potential not flipped for 14.23 part a? The reaction is : 2NO 3 - (aq) + 8H + (aq) + 6Hg (l) --> 3Hg 2 2+ (aq) + 2NO (g) + 4 H 2 O (l). The given standard reaction potential for the anode is Hg 2 2+ (aq) --> Hg (l), with standard reaction potential of 0.79, but this is n...

Tyra Nguyen 4H

We are given standard reduction potentials for half-reactions. Do we flip the sign of these half-reactions when we want the result for an oxidation reaction, as the equation itself flips?

Tyra Nguyen 4H

I think it is similar to Au and Ag, which are also typically metals used as electrodes. I believe we only use Pt (s) when there is no given metal in the solid phase.

Tyra Nguyen 4H

14.15 Write the half-reactions and devise a galvanic cell (write a cell diagram) to study each of the following reactions: (a) AgBr(s) --> Ag + (aq) + Br - (aq) Why does the solution manual give an answer for the anode as "AgBr(s) --> Ag (aq) + Br - (aq)" I thought that the anode was where...

Tyra Nguyen 4H

Because Sn4+ and Sn2+ are apart of the anode reaction, this indicates that it is an oxidation reaction. An oxidation reaction must "release" an electron, as oxidation is loss of an electron. Thus, the oxidation half-reaction is Sn4+ --> Sn2+ + 2e-

Tyra Nguyen 4H

4.11 Write the half-reactions and the balanced equation for the cell reaction for each of the following galvanic cells: (d) Pt(s)|O 2 (g)|H + (aq)||OH - (aq)|O 2 (g)|Pt(s) Are O 2 (g) and H + (aq) the reactants of the oxidation half-reaction (as seen in solutions manual)? I am not sure how to go abo...

Tyra Nguyen 4H

I just don't really understand what this is trying to say? It appears to me that the molecule on the left has an oxidation number of -5, but why is it -7?

Tyra Nguyen 4H

Example 14.1 in the textbook says to balance MnO₄^-(aq) +H₂C₂O₄(aq) $\rightarrow$ Mn²⁺(aq) + CO₂(g) and for the reduction half reaction, it indicates that MnO₄^- $\rightarrow$ Mn²⁺. How do we know that the charge of O₄^- is -2, as stated in the textbook?

Tyra Nguyen 4H

Question 9.55 asks you to write a balanced chemical equation for the formation reaction of various gases. In part C, it asks you to do so for CO(g). Why do you use C(s, graphite) for the chemical equation and not C(s, diamond) or C(g)? Is C(s, graphite) the most stable form of carbon?

Tyra Nguyen 4H

When do I know when to use kJ or kJ.mol^-1 when talking about enthalpy, whether it be standard reaction enthalpy or standard enthalpy of formation, etc.?

Tyra Nguyen 4H

Why does adding an inert gas not affect the reaction?

Tyra Nguyen 4H

OF 2 (g) + H 2 O(g) ---> O 2 (g) + 2HF(g) and \Delta H = -318kJ What is the change in internal energy for the reaction of 1.00 mol OF2? What does the \Delta H = -318kJ mean in this particular problem? I know what the standard reaction enthalpy is but how does it differ from this question? Do I have ...

Tyra Nguyen 4H

8.27 Calculate the work for each of the following processes beginning with a gas sample in a piston assembly with T = 305 K, P = 1.79 atm, and V = 4.29 L: (a) irreversible expansion against a constant external pressure of 1.00 atm to a final volume of 6.52 L; (b) isothermal, reversible expansion to ...

Tyra Nguyen 4H

8.3 Air in a bicycle pump is compressed by pushing in the handle. If the inner diameter of the pump is 3.0 cm and the pump is depressed 20. cm with a pressure of 2.00 atm, (a) how much work is done in the compression? (b) Is the work positive or negative with respect to the air in the pump? (c) What...

Tyra Nguyen 4H

I believe that we will typically use heat the specific heat capacity. Unless we deal specifically with moles, I would not use molar heat capacity to make it easier in terms of units. The formula to calculate the heat released is q = m*C* \Delta T. To calculate the heat capacity, C, for a constant pr...

Tyra Nguyen 4H

We do not go in the order of the textbook. This is what was on the outlines located on Dr. Lavelle's website: Outline 1: 7th Edition: Focus 5G-J (Omit 5G.3, 5G.4, and pages 432-433) Outline 2: 7th Edition: Focus 6A.4; 6B; 6C.1-6C.2; 6D; 6E.1; Box 6E.1 Outline 3: 7th Edition: Focus 4A-4D.5; 4E3 Outli...

Tyra Nguyen 4H

When the work of a system is positive, it means that the system is doing work. This can be seen as the gas in a container expanding, pushing a piston. The system is expanding, thus is doing work.

Conceptually, positive work means that work is being done by a system.

Tyra Nguyen 4H

I believe that typically when a molecule contains a nitrogen, it will accept a proton and create the hydroxide ion. If not, then the molecule it will donate the proton to create a hydronium ion. You should probably memorize the few strong acids though, like HCl. Secondly, you don't have to include t...

Tyra Nguyen 4H

"Chemical Equilibrium, Acid and Base Equilibria, Thermochemistry, Thermodynamics to the end of entropy" which is essential outlines 1 through 4!

Tyra Nguyen 4H

The volume should be 4.29L, not the final volume, since you want to find the number of moles at the start of the expansion. I plug this in and my answer is 3.068 x 10 -5 mols. Plugging in the rest of the numbers, I get the work to be -0.3J which is very off from the -326 it is supposed to be. What ...

Tyra Nguyen 4H

8.31 Calculate the heat released by 5.025 g of Kr(g) at
0.400 atm as it cools from 97.6 ^oC to 25.0 ^oC at (a) constant pressure and (b) constant volume. Assume that krypton behaves as an ideal gas.

I don't really know where to start with this problem or what equations to even use.

Tyra Nguyen 4H

8.27 Calculate the work for each of the following processes beginning with a gas sample in a piston assembly with T = 305 K, P = 1.79 atm, and V = 4.29 L: (a) irreversible expansion against a constant external pressure of 1.00 atm to a final volume of 6.52 L; (b) isothermal, reversible expansion to ...

Tyra Nguyen 4H

I don't really understand why work done during a reversible expansion of gas is the maximum expansion work possible. I understand the difference between irreversible expansion and reversible expansion but how does this relate to "maximum expansion work"?

Tyra Nguyen 4H

Right so you have the equation U = q + w, then moving the variables around to solve for work you have w = U - q. Since the internal energy is -2573kJ, you have w= -2573kJ - q. Because the cylinder surrounding the fuel is absorbing the energy, that means q = -947kJ, because the system, fuel, loses he...

Tyra Nguyen 4H

The question is as follows: 8.9 An ideal gas in a cylinder was placed in a heater and gained 5.50 kJ of energy as heat. If the cylinder increased in volume from 345 mL to 1846 mL against an atmospheric pressure of 750. Torr during this process, what is the change in internal energy of the gas in the...

Tyra Nguyen 4H

So the first step is to take off a Cl from the compound, because it's an ionic compound and ionic compounds fully dissociate. So for NH 4 Cl, it would turn into NH 4 + . Then do the chemical equation if you added H 2 O to the ion. The products should be NH 3 and H 3 O + . From there you can do an I...

Tyra Nguyen 4H

Not sure how to approach this problem, as the K_b values aren't given in any tables?:

Calculate the pH of each of the following solutions: (a) 0.19 m NH4Cl(aq); (b) 0.055 m AlCl3(aq).

Tyra Nguyen 4H

(a) When the pH of 0.10 m HClO2(aq) was measured, it was found to be 1.2. What are the values of Ka and pKa of chlorous acid? (b) The pH of a 0.10 m propylamine, C3H7NH2, aqueous solution was measured as 11.86. What are the values of Kb and pKb of propylamine? I am stuck on this question. I calculat...

Tyra Nguyen 4H

Hi! So using the 17.78 M OH from part b, you can use that and the balanced equation to work backwards. Because NaOH is a strong base, it will completely dissociate, meaning that [OH] = [Na] = [NaOH]. If we have 17.78 M [OH], the we have 17.78 M [NaOH]. The equation is Na2O + H2O ----> 2NaOH If we h...

Tyra Nguyen 4H

What's the difference between atm and bar when calculating the equilibrium constant with pressure?

Tyra Nguyen 4H

Tyra Nguyen 4H

I don't think that solids can be in equilibrium? The molar concentration of a solid does not change, thus they are just represented as 1 in the equilibrium constant.

Tyra Nguyen 4H

The question is as follows: A student added solid Na2O to a 200.0-mL volumetric flask, which was then filled with water, resulting in 200.0 mL of NaOH solution. 5.00 mL of the solution was then transferred to another volumetric flask and diluted to 500.0 mL. The pH of the diluted solution is 13.25. ...

Tyra Nguyen 4H

The question states: Consider the reaction 2NO(g) \rightleftharpoons N 2 (g)+O 2 (g). If the initial partial pressure of NO(g) is 1.0 bar, and x is equilibrium concentration of N 2 (g), what is the correct equilibrium relation? (a) K=x 2 /(1.0-x) (b) K=x 2 (c) K=x 2 /(1.0-2x) 2 (d) K=4x 2 /(1.0-2x) ...

Tyra Nguyen 4H

Can someone please explain why solids and liquids are not included in the calculation of an equilibrium constant? The textbook is a little confusing in its explanation.

Tyra Nguyen 4H

There is a table earlier in the chapter which is needed for reference in this problem. The table will give you the equilibrium constant, K, at various temperatures for specific equilibrium reactions, such as H 2 (g)+Cl 2 \rightleftharpoons 2HCl (g). Knowing the specific equilibrium constants of thes...

Tyra Nguyen 4H

There would be different concentrations of O 3 and O 2 because there are different amounts of starting moles (.10-mol compared with 0.5-mol). The difference in starting moles while keeping the same liter size yields a difference in concentration, as concentration is calculated with moles/liter. the ...

Tyra Nguyen 4H

When looking at equilibrium reactions where there are products and reactants in a gas state, how do I know when to use molar concentration (with square brackets in equilibrium constant equations) or the partial pressure (P in equilibrium constant equations)? I understand that it often depends on the...

Tyra Nguyen 4H

The question comes with an image of 4 flasks illustrating the 4 stages towards the dissociation of a diatomic molecule, X 2 , over time until the reaction reaches equilibrium. Part c of the question asks, "Assuming that the initial pressure of X 2 was 0.10 bar, calculate the value of K for the ...

Tyra Nguyen 4H

What are the difference between the two?

Tyra Nguyen 4H

How many questions are on this final?

Tyra Nguyen 4H

If a ligand was tridentate, does that mean that the complex would have a coordination number of 3? Similarly, if a ligand was bidentate, would its coordination number be 2?

Tyra Nguyen 4H

I'm still confused on how to determine dipole moments, especially within a polyatomic molecule. For example, the referenced question asks whether or not the molecule CH₂Cl₂ is polar or non-polar. How do I know which direction the dipole moments are pointing in?

Tyra Nguyen 4H

Drawing the Lewis Structure usually helps! Once you draw the structure you can see if there is a net dipole moment - if there is one then it is polar. A helpful tip though is that C bonded to H is always nonpolar. Also, if the Lewis Structure is symmetric, it is usually nonpolar because the dipoles...

Tyra Nguyen 4H

I understand part a, where the molecular shape SOCl 2 is trigonal pyramidal. However, how are OSCl and ClSCl also trigonal pyramidal? Part C of question 7 asks the values of their bond angles, giving the answer to be less than 109 o , but there are only 3 atoms thus I thought the answer would be les...

Tyra Nguyen 4H

How do you know which shape is which? For example, SO₃^2- has an electron arrangement that is tetrahedral, but its molecular shape is trigonal pyramidal. Why is it not trigonal planar?

Tyra Nguyen 4H

There is a greater extent of orbital overlap in a sigma bond, thus it is a stronger bond that requires more energy to break.

Tyra Nguyen 4H

Polarization is the process where an electron cloud of an anion is distorted due to a small, highly charged cation being close to a larger anion. Cations with a small radius, thus being highly charged have high polarizing power, meaning that they can cause the largest distortions of a polarizable an...

Tyra Nguyen 4H

But how do you know which corner it is pointing to? Does this relate to the dipole moments?

Tyra Nguyen 4H

Was reading the textbook and didn't understand what this meant: "(a) An s-orbital and a p-orbital hybridize into two sp hybrid orbitals that point in opposite directions, forming a linear molecular shape. (b) An s-orbital and two p-orbitals can blend together to give three sp 2 hybrid orbitals ...

Tyra Nguyen 4H

My RA said that last year Dr. Lavelle set the average as a B and went from there.

Tyra Nguyen 4H

Most likely the bond angles too! Dr.Lavelle said we would have to know how SO₃^2- has a repulsion strength of less than the expected trigonal pyramidal shape of 109.5^o due to its lone pair repulsion, so I'm assuming we also have to know the other basic bond angles.

Tyra Nguyen 4H

I believe Boron is an exception to the octet rule! Just something to memorize:)

Tyra Nguyen 4H

I was reading the textbook (6th Edition, page 90, section 3.12), and it described the resonance hybrid structure of a Cl 2 molecule. The textbook states that "the ionic structures make only a small contribution to the resonance hybrid; the bond is almost purely covalent" but I don't really...

Tyra Nguyen 4H

The electron configuration of Ni²⁺ is [Ar]3d⁸, so why is the number of unpaired electrons 2?

Tyra Nguyen 4H

I do not understand this. Please help me understand! Thanks

Tyra Nguyen 4H

Professor Lavelle's slide said that the question on the midterm would be from homework, chemistry community, and online assessments -- do you guys think that the midterm questions are restricted to only these?

Tyra Nguyen 4H

I understand how Cu⁺ is [Ar]3d¹⁰, but I don't understand parts b, c, and d? I think the f-block throws me off, but can anybody explain them? Thanks!

Tyra Nguyen 4H

Anybody know how to do question 1.55, part c? The wording of the question is slightly confusing to me, where it says "absorbing at 3500 cm^-1".

Tyra Nguyen 4H

Why is electron density important?

Tyra Nguyen 4H

We just need to know it for UV and visible light (I went to Lavelle's office hours and that was what he said :))

Tyra Nguyen 4H

The only part where significant figures matter is the answer.

Tyra Nguyen 4H

My TA said by Sunday night at the end of the week (may differ from TA to TA because it depends on when they check I guess).

Tyra Nguyen 4H

Is there another variable that is used to represent frequency? It can be confusing when E=hv and E_k=(1/2)m_ev_e² and the v is also velocity.

Tyra Nguyen 4H

What is an Angstrom and how is it applied in the lab?

Tyra Nguyen 4H

"In an experiment, 4.14 g of phosphorus combined with chlorine to produce 27.8 g of a white solid compound. (a) What is the empirical formula of the compound? (b) Assuming that the empirical and molecular formulas of the compound are the same, what is its name?" Anybody know how to figure ...

Tyra Nguyen 4H

During Professor Lavelle's lecture, he also said that he would use M to represent molarity concentration. However, in the textbook (at least in the 6th edition), molarity concentration is represented as the variable "c".

Tyra Nguyen 4H

Question: Nicotine has mass percentage composition 74.03% C, 8.70% H, and 17.27% N, and a molar mass of 162.23 g/mol. Determine the molecular formula. I'm struggling to answer this question, as I keep getting the ratio of C:H:N to be 5.7:7.9:1 and cannot solve for the empirical formula. None of the ...

Tyra Nguyen 4H

By the end of Week 2, we are supposed to have completed the "Photoelectric Effect", "Atomic Spectra and the Bohr Frequency Condition", "Wave Properties of Electrons and the De Broglie Equation", and "Heisenberg Uncertainty Equation" modules!

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