CHEMISTRY COMMUNITY

Hannah Chew 2A

There's 2 carbons that undergo a change in oxidation state, so you need 2 e-. I prefer to look at it as a balance of charge. You need the 2 H+ ions on the right side to balance the hydrogens, and since the left hand side is neutral and right hand side is +2, you need to add 2e- to the right hand sid...

Hannah Chew 2A

We multiply reaction 3 by 3 for the same reason that we multiply reaction 2 by 3. The question asks for 3 mol of NADH, so reaction 2 needs to be multiplied by 3 since the given reaction is for 1 mol of NADH. However, by multiplying by 3, the number of electrons transferred is multiplied by 3. Reacti...

Hannah Chew 2A

I think it's easier to start with part b and c b) Remember that q=moles (molar heat capacity) (delta T). You have all the information, so just substitute to find q. a/c) Delta U = q + w So all that is left is you need to solve for work since q was solved above. The work is from the volume change. In...

Hannah Chew 2A

An extensive property is dependent on size (or mass), and like you said, entropy = q/T, and q in itself is dependent on the mass, so therefore, it is extensive. Mass is an extensive property. An example of an intensive property would be density of water. No matter how much water you have, the densit...

Hannah Chew 2A

I did problems 1-39, 83, and 99 because those are the ones without reaction mechanisms/catalysts/activation energy components.

Hannah Chew 2A

In example 15.4, they give you the experimental data of concentration versus time. To figure out if it is first order, you can take the natural log of all the concentration data on your scientific calculator and plot that data. You can also tell it's linear because the ln[concentration] decreases re...

Hannah Chew 2A

Hi! The steps you wrote are exactly what I did, but it's hard to tell where you went wrong without listing the exact numbers. Maybe you can figure out which numbers differ from mine?

Here are the #'s I got:
[A] initial = 4.15x10^-5 mol/L
[A] = e ^ -10.3 = 3.22 x 10^-5 mol/L
grams of [A] = 10.9g

Hannah Chew 2A

While the general reaction is definitely needed to see what's happening in the bigger picture, we specifically look at the slowest step to determine the rate law because the slowest step determines the reaction rate. In class, Dr. Lavelle used the example of baking brownies; it doesn't matter how fa...

Hannah Chew 2A

Since k, the constant, is always positive, we must change the sign in front of k to denote the slope. So for zero order reactions, if we plot [A] vs time, the slope is (-), meaning that slope = (-)k. For first order reactions, if plot ln[A] vs time, the slope is also (-), so we set that equal to (-)...

Hannah Chew 2A

The definition of a second-order reaction is that rate is proportional to the second power (ie. squaring) of the concentration. By doubling the reactant concentration, you would be actually quadrupling the reaction rate since 2 squared is 4. If it was a first-order reaction, rate is proportional to ...

Hannah Chew 2A

I asked the UA this, and he said it was definitely unclear, but you should always go with the half reaction that is most (-), even if there are multiple listed half reactions involving the same element.

Hannah Chew 2A

I had the same question because gold solid is not a reactant in the overall reaction. However, when you balance the half reactions (with gold solid being the reactant of the second half reaction) and sum them, the overall reaction matches the reaction stated in the problem.

Hannah Chew 2A

The oxidized form should be before the reduced form if both are the same phase. But can anyone answer my original question?

Hannah Chew 2A

Can anyone explain the order of the cell diagram? I thought it was reactant first and product second, but in several problems, I found it to be inconsistent. Ex) 14.11 b) Cl- is written first but is a product. 14.11 e) Cl- / Hg2Cl2 / Hg is written like this in the book, but Cl- and Hg are both produ...

Hannah Chew 2A

Calculate the potential of a cell constructed with two silver electrodes. The electrolyte in one compartment is 1.0 m AgNO3(aq). In the other compartment, NaOH has been added to a AgNO3 solution until the pH 12.5 at 298 K. Answer from back of book: At pH 12.5, pOH = 1.5 and [OH-] =10 ^ -1.5 = 0.032 ...

Hannah Chew 2A

Given this cell diagram, write the chemical equation. Hg(l) / HgCl2(s) / HCl(aq) // Hg2(NO3)2 (aq) / Hg(l) I checked with the answers in the back of the book, and I don't understand 1) why does Hg2(NO3)2 become 2Hg(l) and 2NO3- instead of 2Hg+ (aq) and 2NO3- ? 2) why does the overall reaction show 2...

Hannah Chew 2A

^ 1) But what is the difference between standard molar entropy and residual entropy?
2) In the solution manual, 9.55 looks at the difference between formation and molar entropy, so what is the difference?

Hannah Chew 2A

^ but 1) combustion involves liquid water, not gas and 2) the solution manual says that work is +3.72 kj/mol, so I am still confused.

Hannah Chew 2A

What is the difference between 1) standard molar entropies 2) standard molar entropies for formation 3) residual entropies?

Hannah Chew 2A

For part a) Calculate the work that must be done against the atmosphere for the expansion of the gaseous products in the combustion of 1.00 mol C6H6(l) at 25 C and 1.00 bar. Why is this considered expansion, and why is work being done against the atmosphere? The equation I had was: C6H6 (l) +15/2 O2...

Hannah Chew 2A

Hi! I'm not sure where I went wrong, but in 9.47, I tried substituting PV/T into nR in the equation delta S = nR ln (V2/V1). However, the answer I got was .0379 J/K instead of 3.84 J/K. Did I do something wrong, or am I mistaken in substituting PV/T into nR? My work: delta S = nR ln (V2/V1) = PV/T x...

Hannah Chew 2A

Why does oxygen (and other naturally occurring elements like I2, H2, K) all have 0 Gibbs free energy, even though there is some numerical value for molar entropy still listed in the Appendix? Doesn't delta G formation = delta H formation - T (delta molar entropy)?

Hannah Chew 2A

But why are the values different?

Hannah Chew 2A

What is the difference between delta Sm and delta Sf? Why are the values different? Thank you in advance:)

Hannah Chew 2A

Kb is Boltzmann's constant, which is given to us on exams. The question is essentially asking for the entropy based on how many ways can you arrange the molecules. For part a), there is only 1 way to arrange it because they are all the same molecules aligned the same way. Part b), there is 4^64 ways...

Hannah Chew 2A

Similar is the example Dr. Lavelle used in class. If you light a match in a large room, why doesn't the temperature increase? There is no significant increase because the heat of the match is too small to affect the entire room.

Hannah Chew 2A

Another common one is OIL RIG
Oxidation is loss (of e-)
Reduction is gain (of e-)

Hannah Chew 2A

^ You can keep the test.

Hannah Chew 2A

Well, there is no gas on the reactants side, but there is a gas on the products side. There is a net increase in gases, which results in a positive change in entropy. The molar entropy of a gas is much larger than that of solids and liquids, so we can assume that entropy's sign is positive without h...

Hannah Chew 2A

The diagram is of a heat reservoir, meaning that it is always at the same temperature. The ideal gas expands into the vacuum and does work, but to remain at the same temperature, heat is added. When it expands, it pulls up the mass. Delta U is still 0 because q + w = 0, where heat added is equal to ...

Hannah Chew 2A

^ but what is an example of work on the open system?

Hannah Chew 2A

This might be a silly question, but what are examples of how you can do work on an open system?

Hannah Chew 2A

Hi, I’m very confused about when we should use kj/mol and when we should use kj. The solution manual gives different units for each type of problem, some of them even contradicting each other. Can anyone give me a concrete answer when to use what? Here are a few examples of where I am confused. 1. I...

Hannah Chew 2A

You first need to write a chemical reaction between HCl and zinc. This is also a limiting reactant problem, so you need to keep that in mind. Afterwards, you need to solve for the enthalpy of reaction using enthalpies of formation in the book. The energy released by the reaction can be found by mult...

Hannah Chew 2A

delta T is the final temperature minus the initial temperature, so in this case, 100 degrees celsius -20 degrees celsius. The means that delta T = 80 degrees celsius. You can also write it as 80K because the difference in temperature can be expressed as either unit.

Hannah Chew 2A

Internal energy is a state function because it depends only on the current state of the system. For example, It doesn't matter whether you added 50J of heat or the surroundings did 50J of work on the system. Therefore, it is independent of how the system achieved the current state.

Hannah Chew 2A

To start off, you should write the reaction for the formation of N2O5, where N2O5 is on the right hand side with a coefficient of 1. Using the other reactions they give you, you want to rearrange the equations so that the sum of the 2 reactions would give you the overall reaction above. Using Hess's...

Hannah Chew 2A

Isn't molar heat capacity J/Kmol ? In this problem, why doesn't the Kelvin appear?

Hannah Chew 2A

Hi, I am confused about the heat capacity units in 8.43. The book says that the heat capacity for a solid is 30 J/mol and for a liquid, it is 60J/mol. What does "J/mol" exactly mean, and what does it say about the temperature? I thought that heat capacity was J/ unit of temperature, so why...

Hannah Chew 2A

8.11 asks which process does more work, so this question specifically tells us to look at work from the point of view of the system itself, not the surroundings. So using the formula, we would still conclude that reversible expansion produces a more negative work than irreversible expansion. Since w...

Hannah Chew 2A

In part a, you need heat to increase the temperature of both the stainless steel in 8.20 (or copper in 8.19) and the water. Using the formula q = mass x specific heat capacity x temperature change, you would need to add q of water and q of stainless steel (or copper). These should be separate sums, ...

Hannah Chew 2A

Work acts on the system, so if the volume is compressed, then work is being done on the system. Therefore, work is positive. However, if the volume expands, then work would be negative.

Hannah Chew 2A

The equation and constants sheet will be given.

Here is a link to the same question, but answered by Chem Mod.
viewtopic.php?f=129&t=25577

Hannah Chew 2A

If you really wanted to, you can also look at the reaction in terms of carbon, which would be 358.8kj/4mol C. You can then perform stoichiometric conversions to convert moles of S8 to Carbon and whatnot, but because the problem gives the reaction in terms of S8, it would be much more advantageous to...

Hannah Chew 2A

Dr. Lavelle said all we had to do was change our username/alias from 14A, so having previous posts are ok since those posts are associated with the same account.

Hannah Chew 2A

Unfortunately, there is no schedule that corresponds lectures to textbook sections, and the outline groups everything together. However, at the end of each chapter in the book, there's a "skills you should have mastered" section that shows which sections align with which topics. I would lo...

Hannah Chew 2A

Molar heat capacity measures how much heat is needed to increase the temperature of one mole of that substance by one degree while specific heat capacity measures how much heat is needed to increase the temperature of one gram by one degree. I believe specific heat capacity is used in calorimeter pr...

Hannah Chew 2A

Yes, you would just change the sign to indicate whether heat is being absorbed or released. When solid becomes liquid, the sign is positive since heat must be added to break bonds. If liquid froze to become solid, then the sign must be negative since heat must be released to form bonds.

Hannah Chew 2A

So this isn't a hw question, but this is more from the lecture notes. Why is HI a really strong acid, and HOI not as strong? I understand that HI has a long bond length and is long, and I understand that having more oxygens stabilizes the resulting anion. However, when combining these concepts, it w...

Hannah Chew 2A

I know this question has been asked several times, but I am still very confused about why formic acid is stronger than acetic acid. The solution manual and previous forums talk about electron donating properties and electron withdrawing. I'm not sure what these mean. 1) Why does the CH3 group in ace...

Hannah Chew 2A

If H2O is outside the coordination compound, then it is called hydrate. We include its Greek prefix. Is H2O the only compound where we would have a prefix even if it's outside the brackets?

Hannah Chew 2A

If classes after noon are cancelled, will the 11am and 1pm lectures be cancelled as well? And if the 11am is not cancelled, will the 11am lecture be "ahead" of the 1pm lecture?

Hannah Chew 2A

I'm assuming you are referring to 12.43 instead of 12.45, but I would use the "withdrawing power" or electronegativity differences to explain why 2,4,6 trichlorophenol has a larger Ka. You would use the Ka to identify which is stronger, but to "account for the difference in acid stren...

Hannah Chew 2A

Name the compound [CoBr(NH3)5]SO4

The back of the book states that the answer is pentaaminebromidocobalt(II) sulfate. I understand everything except for the charge of cobalt. Why is the Roman numeral II, rather than III?

Hannah Chew 2A

First identify which is a reactant and which is a product. Since the partial pressure of A decreases, it must be a reactant. Conversely, the partial pressures of B and C increase, so they must be products. To figure out the coefficients, you have to look at the numerical values of each partial press...

Hannah Chew 2A

The solution manual states that "Because the equilibrium constant is small, assume that x <<0.22." I understand that the change in concentration is pretty much negligible because the equilibrium constant is small, but in general, what is the "cutoff" value for K so that we can as...

Hannah Chew 2A

You can illustrate the reaction progress on a graph where molar concentration is on the y axis and time on the x axis. As the reaction proceeds, the concentration of reactants decreases while the concentration of products increases. Figure 11.1 shows this also.

Hannah Chew 2A

At equilibrium, the concentrations of reactants and products are constant, so they don't have to necessarily be equal. In this case, the concentration of "single" and "double" molecules do not change from the third to the fourth flask because the number of each remains constant. ...

Hannah Chew 2A

Can anyone explain why SiO2 is polar without knowing what the electronegativity values are? This topic was in another forum, but I'm still confused. Dr. Lavelle specifically said in class that we will not need to know the electronegativity values and that we can determine the difference in electrone...

Hannah Chew 2A

The bond angle is the angle between XAX (X being the surrounding atoms, and A being the central atom). Here is a useful chart that shows you what the angles are for any molecular shape. http://mmstcchemistry.weebly.com/upload ... ometry.pdf

Hannah Chew 2A

It is just to indicate the specific orbital within a sub shell. Normally, you would just write p3, but if you wanted to specify that there are three different orbitals lying along the x,y,z axis, then you would write px py pz. Both are acceptable unless otherwise stated.

Hannah Chew 2A

This is from the outline on Lavelle's website.

"Name and explain the relationship of each of the four s-, p-, and d-orbitals to the properties of electrons in these states."

I'm confused about how to answer this. Any help is appreciated!

Hannah Chew 2A

The difference between 3.11 and 3.9 is that 3.11 is talking about a 3+ cation whereas 3.9 is talking about a 2+ cation. For 3.11A, we need to find an element with the configuration [Ar]3d74s2. A parent atom loses 3 valence electrons to form a cation, so going from a cation to a parent atom means tha...

Hannah Chew 2A

An electron can have either spin value, ms = -1/2 or 1/2. Since 3d9 and 3d10 are in the same orbital, they must have opposite spin. It could be either 1) 3d9 is -1/2 and 3d10 is 1/2 or 2) 3d9 is 1/2 and 3d10 is -1/2. So therefore, 3d10 can have either spin number if 3d9’s spin number is not specified.

Hannah Chew 2A

The electron configurations tell us whether an atom is in an excited state or ground state. In a ground state, electrons fill orbitals to have the lowest energy, so electrons must occupy the lowest orbitals possible. In an excited state, not all electrons are expected to fill the lowest orbitals pos...

Hannah Chew 2A

Oxygen has a lower ionization energy than fluorine, as fluorine is farther to the right on the periodic table and follows the trend that IE increases as you go right. However, the anomaly is that oxygen has a lower ionization energy than nitrogen. If we look at the electron configurations, oxygen is...

Hannah Chew 2A

Well the previous reply is not always true because sometimes the ions doesn’t lose or gain enough electrons to subtract or add an energy level. Sometimes you can remove one electron and it would still have the same principal quantum number. Cations are smaller because cations have less electrons tha...

Hannah Chew 2A

So it makes sense that oxygen has a lower ionization energy than fluorine, as fluorine is more right on the periodic table and follows the above trend. However, the anomaly is that oxygen has a lower ionization energy than nitrogen. If we look at the electron configurations, oxygen is p4 while nitro...

Hannah Chew 2A

3.6 x 10^3 is km/s but the SI unit should be meters and not kilometers. Therefore, you must convert km/s to m/s so that all the units cancel out in the final equation.

Hannah Chew 2A

So we want to find how long one wavelength of krypton 86 is. The problem gives that there are 1,650,763.73 wavelengths in one meter. To find how long one wavelength is, we must divide 1m by the total number of wavelengths in one meter. An analogy is: there are 4 laps in a mile. How long is each lap?...

Hannah Chew 2A

This is the set of constants and equations given every test from the class website. https://lavelle.chem.ucla.edu/wp-content/supporting-files/Chem14A/constants_equations.pdf If there are any additional numerical constants specific to a problem, they should be given on the sheet (Example: 1ev= 1.602x...

Hannah Chew 2A

Avogadro's constant can be used to find the moles of anything really. Yes, you can use it to find the quantity of atoms, but you can also apply it for molecules, formula units, photons, etc.

Hannah Chew 2A

Hello, a) No, speed is constant at 3.00 x 10^8 m/s b) No, wavelength increases. Frequency and wavelength are inversely proportional since c = (lambda)(freq) c) Yes, the electric field oscillates in strength and direction. If the frequency decreases, then the field oscillates less. d) No, energy is d...

Hannah Chew 2A

"IF LAST DIGIT IS 5: ROUND TO NEAREST EVEN NUMBER (2 sf then 2.35 is 2.4 and 2.65 is 2.6)" I was reading through the "Everything You Want to Know about Sig Figs" link on the class website, and I read the part above. I've never heard of this rule and have been sticking to the 0-4,...

Hannah Chew 2A

I actually missed this too, but I realize now that the question asks for the net ionic equation. This means that spectator ions like sodium wouldn't be part of the equation unless it was asking for the complete ionic equation.

Hannah Chew 2A

There’s no really strict guideline to choosing which to balance first. What you said is correct; you balance either hydrogen or phosphorus first since oxygen appears in both reactants and thus harder to balance. While hydrogen and phosphorus have equal number of atoms, I would start with phosphorous...

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