CHEMISTRY COMMUNITY

Angela 1K

I believe it is because the Ag(s) is acting as an electrode. Although they are both solids, it would be difficult to tell which is the electrode if you just put a comma as they are both solids, so they put a line to differentiate between the electrode and the "solution". How do we know th...

Angela 1K

According to my understanding of Galvanic cell models, if two elements/molecules are in the same phase, they are put together between | | and separated by commas. However, the answer to this problem is Ag(s) | AgBr(s) | Br - (aq) || Ag + (aq) | Ag(s) Why isn't it Ag(s) , AgBr(s) | Br - (aq) || Ag + ...

Angela 1K

How are we supposed to know that the oxidation reaction is Au³⁺(aq) + 3e^- --> Au(s) if, in the reaction given Au and Au³⁺ are on the same side (products)?

Angela 1K

When writing a cell diagram, if two elements/molecules are in the same container (anode or cathode) and are in the same phase, does it matter which is written first? For example, in this reaction Ce 4+ (aq) + I - (aq) --> I 2 (s) + Ce 3+ (aq) Does it matter if I write this Pt(s) | I 2 (s) | I - (aq)...

Angela 1K

Do you think on a test, they would expect us to already know that I₂ is a halogen and can't conduct electricity, and thus we need Pt(s), or do you think they'd explicitly mention that on a test?

Angela 1K

I'm still having some trouble figuring out how to create a cell diagram. For example, in Test 2 #6, we were given a question about it. The question said: The following redox couple forms a galvanic cell which generates a current under standard conditions. O 3 /O 2 , OH - and O 3 , H + /O 2 (a) Ident...

Angela 1K

If given two reactions, is there a way to determine which reaction will go on the anode side vs. the cathode side by comparing their \Delta E ? And the \Delta E values I'm referring to would be determined using a chart like Appendix 2B. In other words, does a stronger (more positive) cell potential ...

Angela 1K

The equation that we're supposed to balance is H + (aq) + Cr 2 O 7 2- (aq) + C 2 H 5 OH(aq) --> Cr 3+ (aq) + C 2 H 4 O(aq) + H 2 O(l) According to the solution manual, the balanced oxidation half-reaction should be C 2 H 5 OH(aq) --> C 2 H 4 O(aq) + 2e - + 2H + (aq) I'm having difficulty figuring ou...

Angela 1K

I was working on homework problem 9.73, and it asks how \Delta S TOT could help assess spontaneity, and according to the solutions manual, the total change in entropy has to be positive. Can someone explain why, both mathematically and logically, a positive change in entropy signifies spontaneity? T...

Angela 1K

When can you tell if the entropy of the surroundings/system is supposed to be positive/negative? Like in problem 9.45, it asks to find \Delta S of the systems and surroundings for the following situations: (a) vaporization of 1.00 mol CH 4 (l) at its normal BP (b) melting of 1.00 mol C 2 H 5 OH(s) a...

Angela 1K

Then how come, for example, in question 9.15, which asks to calculate the entropy change for (a) the freezing of 1.00 mol H₂O(l) at 0.00 C and the vaporization of 50.0g of ethanol at 531.5K, the solutions manual uses the equation $\Delta S=\frac{q}{T}=\frac{\Delta H}{T}$ ? Why isn't it negative?

Angela 1K

Hi! Because it asks for the rate your body generates heat "in your surroundings" the formula has a -q. This because the system is releasing heat into the surroundings, or heat is flowing out of the system (with the system being the body). That makes sense, but then why is the final answer...

Angela 1K

I'm having difficulty differentiating between when to use the equation $\Delta S=\frac{qREV}{T}$ and $\Delta S=-\frac{\Delta H}{T}$ .

If someone could please explain, that would be appreciated.

Thanks!

Angela 1K

I guess this just proves how much I've forgotten about old material, but I'm having trouble on 9.1 The question asks the find at what rate does your body heat generate entropy in your surroundings, taken to be at 20. C? The body generates heat at a rate of about 100. W (J/s) I can't seem to figure o...

Angela 1K

The question asks to find the final temperature of a solution when a piece of zinc of mass 8.5g is dropped into an apparatus containing 800.0mL of 0.500M HCl(aq). You're supposed to assume that the density and molar heat capacity of the HCl solution are the same as those of water. When looking at th...

Angela 1K

Hey everyone, I made a Google Doc Study Sheet that everyone can access via their UCLA emails (@g.ucla.edu). It has all the main topics listed by Dr. Lavelle in his chapter outlines and a section for other info for each chapter. I'll be adding to it as I go through studying (which is why it's empty r...

Angela 1K

I used the equation $ln\frac{[A]t}{[A]0} = -kt$ and got the same answer as the solutions manual. I think the two equations are equivalent because of the negative sign I used in my answer, when transferred to the other side, would flip the two concentrations.

Angela 1K

The derived rate laws are not on the constants sheet, and as Lavelle emphasizes, he never asks specifically the steps for derivation, but I assume it would still be helpful to understand the derivations and at least memorize the derived formulas and how they are used.

Angela 1K

The unique average rate can be applied to any of the molecules in the reaction by multiplying it by the coefficient in front of the molecule, if that helps!

Angela 1K

Adrian Lim 1G wrote:Does anyone know which homework problem 15.6 would go up to?

I believe it is up to question 39

Angela 1K

The question asks to find the amount of time it would take in the reaction of A --> 2B + C, when [A] 0 = 0.15M, for the concentration of B to increase to 0.19M, given that k = 0.0035L/mol*min in the rate law for the loss of A. I was wondering why the solutions manual solves this equation as a second...

Angela 1K

As always, I don't believe we'll need to know the specific steps of the derivations, but it is always helpful to understand how to get from one equation to another. However, the derived equations (I believe, please correct me if I'm wrong) are not included in the constants sheet.

Angela 1K

I was wondering which textbook questions would correspond to the test this upcoming week so I know which ones to focus on and practice. Thanks!

Angela 1K

The question asks to put the following metals in order of increasing strength as reducing agents for species in aqueous solutions. Cu, Zn, Cr, Fe According to the solutions manual, Zn is weaker than Cr. When looking at appendix 2B, the E o of Cr 3+ + 3e - —> Cr is -0.74 V. However the E o of Cr 2- +...

Angela 1K

What happens in this reaction to the potassium and the chloride though? And are we supposed to assume that permanganate will always dissociate into Mn²⁺ and Fe²⁺ into Fe³⁺?

Angela 1K

The question asks to write cell diagram for this equation Cl 2 (g) + H 2 (g) —> HCl(aq) According to the solutions manual, it looks like Pt(s)|H 2 (g)|H + (aq)||Cl - (aq)|Cl 2 (g)|Pt(s) I’m confused as to why, on the cathode side, Cl - comes before Cl 2 if, according to the reaction, Cl 2 is the rea...

Angela 1K

How come when figuring out the reaction E, we don’t manipulate the potentials? For example, for homework problem 14.11, the question involves the half reactions oh Ni(s) + 2e - —> Ni with E = -0.23V and Ag + + e - —> Ag with E = +0.80V. In order to make the reaction, we must reverse the first reacti...

Angela 1K

There are a couple standard rules that will help determine the oxidation states of certain atoms in molecules. Hydrogen always has a +1 oxidation state. Oxygen almost always has a -2 oxidation state (unless its with H 2 O 2 ). Let's find out the oxidation state of oxygen in H 2 O 2 as an example. We...

Angela 1K

This is what I did for the entire question: P 4 (s) --> H 2 PO 2 - (aq) + PH 3 (g) 0 +1 +1 -2 -3 +1 (respective oxidation states of the atoms as they appear in the equation For the oxidation half reaction, we have P 4 (s) --> 4H 2 PO 2 - (aq) I added the coefficient 4 in front of H 2 PO 2 - because ...

Angela 1K

The atoms being oxidized and reduced actually aren't both present in the same molecule in the products. The oxidation half reaction is O3 --> O2 and the reduction half reaction is Br- --> BrO3- I don't understand how O 3 is being reduced to O 2 if the oxidation state of both oxygen atoms are 0.

Angela 1K

To expand,

It doesn't matter, but if you add H₃O⁺ we have to continue to balance the reaction with more H₂O molecules because of the oxygen atoms present.

Thus, in order to save time, it's easier to just add H⁺ ions to balance out the presence of H atoms.

Angela 1K

To expand on that answer: You only add H 2 O when you need to balance out the oxygens in an equation. In the end (your answer), it doesn't matter which side the water molecules are on. We can add these water molecules because we assume that all these reactions occur in aqueous solutions, and so wate...

Angela 1K

Can someone please walk me through how to do this problem?
It's asking to balance the equation O₃(aq) + Br^-(aq) --> O₂(g) + BrO₃^-.
I'm just confused on how to balance this when the atoms being oxidized and reduced are both present in the same molecule in the products.

Angela 1K

In C 2 H 2 O 4 , we know that the H has an oxidation state of +1 (hydrogen always has an oxidation state of +1, unless I'm mistaken) and the O has an oxidation state of -2 (oxygen almost always has an oxidation state of -2, unless in situations as H 2 O 2 , where it has an oxidation state of -1, but...

Angela 1K

We use the equation q=mC\Delta T+m\Delta H to find the q ice cream because we have to account for the fact that it changes from solid (frozen) to liquid (melted). The mC\Delta T accounts for the ice cream changing and then m\Delta H accounts for the phase change, where \Delta H=\Delta Hfusion .

Angela 1K

Make sure you account for the phase change of the ice to liquid in your q_ice

Angela 1K

The third law of thermodynamics says that the total entropy is always increasing, so delta S total must be positive. The standard enthalpy for a reaction can be positive or negative depending on the reaction. For example, building polymers from monomers has a negative delta S (the complexity is inc...

Angela 1K

Can anyone else list any more?

Angela 1K

Can someone please walk me through how to find \Delta S for the reaction? The question goes: You have a system consisting of 0.60 moles of an ideal gas contained in a 50.0L container at 1.0 atm. You just love chemistry to a fault, so you perform a series of steps to the system. First, you perform an...

Angela 1K

I'm trying to come up with a composite list of all the special situations that can occur under certain conditions. For example, I believe that under isothermal conditions, \Delta U = 0 . If anyone could add to this list, that would be appreciated! And also, perhaps adding an explanation as to why ea...

Angela 1K

For isothermal reactions, $\Delta U=0$ and so $q=-w$

Angela 1K

Are you referring to delta u in an isothermal reaction? This is because for an ideal gas U = 3/2 nRT. Therefore, if the moles of gas stay the same, n is constant, R is the gas constant, and if T is constant (which is what isothermal means) then U is constant which means delta U = 0. Sorry, do you m...

Angela 1K

I believe Kelly is talking about the direction of the arrows. If it is a double-sided arrow (like in an equilibrium reaction), it is reversible. If it is one-sided, it is irreversible.

Angela 1K

Yes! As seen through stoichiometry.

Angela 1K

Would someone be able to briefly explain the difference between isobaric, isochoric, and isothermal? I have these definitions written down and I've read through the textbook, but I feel as though I've memorized the differences and don't fully grasp the concept. Thanks! As Dr. Lavelle explained in h...

Angela 1K

It is negative because the q of the calorimeter is going to be the negative of the q of the reaction, which is what we use in the equation.

Hope that makes sense.

Angela 1K

You use this equation in problems regarding calibrating a calorimeter because there's no "mass" to take into account. Rather, it would be the specific heat of a single calorimeter.

Angela 1K

Yes, that's correct. Because the equation is asking for the internal energy for the production of 1.00 mol H 2 , you should divide the entire equation by 3 in order to get 1 as a coefficient of H 2 on the right-hand side. Thus, your equation becomes 1/3 CH 4 (g) + 1/3 H 2 O(g) --> 1/3 CO 2 (g) + H 2...

Angela 1K

Can someone please walk me through the steps of this problem? You have a system consisting of 0.60 moles of an ideal gas contained in a 50.0L container at 1.0 atm. You just love chemistry to a fault, so you perform a series of steps to the system. First, you perform an isobaric compression of the co...

Angela 1K

I think it'll definitely to helpful to understand which types of internal energy contribute most to entropy.

Angela 1K

It is also helpful to remember the reversible systems do not occur in real life. Rather, this equation is used to find the maximum work available for a system.

Angela 1K

I know that in some previous question, the $\Delta H$ _vap has been given if you're asked to solve for something like $\Delta S$ _vap of a substance and also given the boiling point.

Angela 1K

I'm not sure where you got this problem from, but I just did it through and wanted to check my answers to see if it was correct. I got 75.84 g H 2 O. My work is as follows: 49.7g PbO \cdot \frac{1 mol PbO}{223.2 g PbO}\cdot \frac{106.9 kJ}{1 mol PbO}=23.8 kJ q=mc\Delta T -23,800J=m(4.184 J/gC...

Angela 1K

I just want to clarify, that in an isothermal system/reaction, $\Delta U=0$ , correct?

Thanks.

Angela 1K

Just to clarify, if $\Delta G$ is negative, the compound is stable/thermodynamically favored right? And the more negative $\Delta G$ is, the more stable the compound is.

Angela 1K

I agree with Justin!

The total entropy is equal to the sum of the entropy of the system and the entropy of the surroundings. Thus, when one process has a higher entropy of the surroundings, it'll have a higher total entropy than the other.

Angela 1K

According to Trouton's rule, the entropy of vaporization at the boiling point is approximately 85 J/kmol. This is because approximately the same increase in positional disorder occurs when any liquid is converted into vapor, so we can expect the change in entropy to be much the same in each case. Su...

Angela 1K

Why is the total entropy of a reversible reaction = 0?

Angela 1K

I was wondering where the solutions manual got 75.3 J/Kmol as the C_p,m of water from.

Thanks.

Angela 1K

When trying to do 9.25 in the homework, I was trying to input (1.381\cdot 10^{-23})ln(6^{6.02\cdot 10^{23}}) in my calculator but it showed an overflow error. I'm assuming that the number was too big for my scientific calculator to handle. I'm wondering what we would do during a test...

Angela 1K

The question asks: Which would you expect to have a higher molar entropy at T = 0, single crystals of BF 3 or of COF 2 ? Why? The answer key says COF 2 , but I don't really understand its explanation for why. Could anyone rephrase the explanation or perhaps explain it in a different way in hopes tha...

Angela 1K

I'm confused about the equations that we use in 9.7 vs the equation used in 9.13. 9.7 asks to calculate the entropy change associated with raising the temperature of 1.00 mol of ideal gas atoms reversibly from 37.6 C to 157.9 C at (a) constant pressure and (b) constant volume and to assume that the ...

Angela 1K

I understand that, according to \Delta S=\frac{-\Delta H}{T} , temperature and entropy are inversely related. So, an increase in temperature would result in a decrease in entropy. But I don't understand that because, how I see it, and increase in temperature results in an increase in the speed of th...

Angela 1K

The question asks: Calculate the entropy of vaporization of acetone at 296 K with an external pressure of 1 bar. The molar heat capacity of liquid acetone is 127 J/Kmol, its boiling point is 329.4 K, and its enthalpy of vaporization is 29.1 kJ/mol. I understand the first two steps of finding entropy...

Angela 1K

The question asks: A sample of nitrogen gas of volume 20.0 L at 5.00 kPa is heated from 20. C to 400. C at constant volume. What is the change in the entropy of the nitrogen? The molar heat capacity of nitrogen at constant volume, C V, m is 20.91 J/Kmol. The textbook solves this question by using \D...

Angela 1K

What are some examples in which work is being done on a system?

In an example where volume and pressure both change, is work still being done on the system?

In a system that expands against a vacuum, is work being done?

Angela 1K

You can't necessarily measure just the enthalpy (H) of a system. Rather, when we calculate the heat that is transferred during an equation, we want to compare the heat before and after the reaction has occurred, thus we would always use $\Delta H$ in terms of measuring the enthalpy change.

Angela 1K

$\Delta U=\Delta H$ occurs when the volume is constant. If you want to derive this, the equation for $\Delta U$ is $\Delta U=\Delta H-P\Delta V$ . Thus, when there is no change in volume, $\Delta V=0$ and so $P\Delta V=0$ . Thus, $\Delta U=\Delta H$ . Not 100% sure, but I hope this helped.

Angela 1K

The question reads: Hydrochloric acid oxidizes zinc metal in a reaction that produces hydrogen gas and chloride ions. A piece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800.0 mL of 0.500 M HCl(aq). If the initial temperature of the hydrochloric acid solutions is 25C, what is...

Angela 1K

Does a lowering in molar energy of a molecule mean it's more stable?

Angela 1K

So after drawing benzene without resonance, what's the difference between CRC bonds and C-C bonds?

Angela 1K

For part (a), do we just assume that anytime we use bond enthalpies, we have to subtract the enthalpy of vaporization?

And can you please show me how the molecular structure of benzene without resonance would look?

Angela 1K

The question asks to estimate the enthalpy of formation of compounds in their liquid states (a) H 2 O (b) CH 3 OH (c) C 6 H 6 (without resonance) (d) C 6 H 6 (with resonance) Regarding (a), I understand how to use the bond energies of H 2 and O 2 to find the formation enthalpy of H 2 O, but I don't ...

Angela 1K

I was wondering why the reaction we are looking for is N 2 (g) + 5/2O 2 (g) --> N 2 O 5 (g) and why we can't solve the problem like this: 2(2NO(g) + O 2 (g) --> 2NO 2 (g)) \Delta H=2(-114.1 kJ) 4NO 2 + O 2 (g) --> 2N 2 O 5 (g) \Delta H=-110.2 kJ 4NO(g) + 2O 2 --> 4NO 2 (g) \Delta H=-228.2 kJ...

Angela 1K

But that ultimately give you a negative work, whereas work done ON a system should be positive???

Angela 1K

The question asks: (a) At its boiling point, the vaporization of 0.579 mol CH 4 (l) requires 4.76 kJ of heat. What is the enthalpy of vaporization of methane? (b) An electric heater was immersed in a flask of boiling ethanol, C 2 H 5 OH, and 22.45 g of ethanol was vaporized when 21.2 kJ of energy wa...

Angela 1K

This question explicitly asks to calculate the heat released. I was wondering how you know to find molar heat capacity (and thus you have to change the mass into number of moles, as shown in the solutions manual) rather than keeping the Kr in terms of grams.

Thanks.

Angela 1K

The questions asks the estimate the enthalpy of the reaction between bromine and propene to form 1,2-dibromopropane. The enthalpy of vaporization of Br 2 is 29.9 kJ/mol, and that of CH 3 CHBrCH 2 Br is 35.61 kJ/mol. The reaction is Br 2 (l) + CH 3 CH=CH 2 --> CH 3 CHBrCH 2 Br(l) In the first step on...

Angela 1K

What is the difference between these two types of calorimeters and what types of calculations would each infer (i.e. if a question said that it used one or the other, what types of things should we keep in mind during our calculations)?

Angela 1K

The question asks: Calculate the final temperature and the change in internal energy wen 500. J of energy is transferred as heat to 0.900 mol O 2 (g) at 298 K and 1.00 atm at (a) constant volume; (b) constant pressure. Treat the gas as ideal. I understand how the textbook determine C V,m and C P,m ,...

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