CHEMISTRY COMMUNITY

Skyllar Kuppinger 1F

You use the integrated rate law when you're trying to find how much of a species will be remaining after a certain amount of time, or trying to find k or t based on the final and initial concentrations of a species. Here's an example from the book where you use the integrated rate law: "for the...

Skyllar Kuppinger 1F

Why is the "rate at which oxygen reacts" written as a positive number? Oxygen is a reactant. In what situation would we write the negative sign?

Skyllar Kuppinger 1F

I'm almost certain that this is an error in the textbook; I've done that same problem twice and I've gotten your answer twice. You may want to see Prof. Lavelle's updated solutions manual errors on his website to see if this is one of the errors. But I can assure you that your work is correct. Than...

Skyllar Kuppinger 1F

Can someone tell me what I'm doing wrong? I did 5G.13 right and it's identical to this one. The book says the answer is -27 but I'm getting -2.7. Why am I one decimal off?

Skyllar Kuppinger 1F

Unless it specifically asks for Q p or Q c , I don't think you need to convert anything. And typically you're going to be given partial pressures for gases and concentrations for aqueous solutions, which I believe was the case in the problem you provided as an example. Right but you can convert par...

Skyllar Kuppinger 1F

How can you have a Q value that contains both pressure and concentration?. I've been converting the pressures to concentrations this whole time Bc I thought you had to make it uniform , and didn't know why I was getting it wrong until someone sent me their solutions manual.

Skyllar Kuppinger 1F

The answer in the book is a typo! In the solutions manual, they put that n = 2 moles even though it should only be 1 mole of electrons. By using 1 mole, you get the correct answer of K = 1 x 10 2 rather than K = 1 x 10 4 ! Hope this helps! Thank you so much!!! I lost my solutions manual so I didn't...

Skyllar Kuppinger 1F

I got part A right but part B wrong even tho I did them the exact same way. The problem said to find K. The book says the answer is 1x10^4 but I got 107. Will someone please tell me what I'm doing wrong

Skyllar Kuppinger 1F

So it says to order the species in order of least reducing agent to most reducing agent. I know that the more negative the redox potential , the more reducing agent it is. However in the appendix there are multiple eqtns for each species. How do I know which one to use? For example for Cu there is a...

Skyllar Kuppinger 1F

Why is the circled Au a solid? Shouldn't it be aqueous bc the Au on the reactant side is aqueous not solid?

Skyllar Kuppinger 1F

For part C, you can split HCl into a positive hydrogen ion and negative chlorine ion. This will give you the two half reactions. For part D, Au is being oxidized to produce the 3+ ion and reduced to produce a solid. Since there is simply a transfer of electrons between two Au atoms, Au is both the ...

Skyllar Kuppinger 1F

I really don't understand what's being oxidized and whats being reduced at this anode/what reaction is occurring at this anode. Will someone please explain what's going on here?

Skyllar Kuppinger 1F

I also got that answer and I have no idea why. Do you think it's a typo?

Skyllar Kuppinger 1F

I think the C(gr) means graphite and it is being used as an electrode

Skyllar Kuppinger 1F

Parts A and B make sense but I have no idea what's going on w parts C and D. For part C: how are there 2 reactants and only 1 product?? Isn't a redox rxn supposed to happen in pairs? How do I know which way the electrons r being transferred?? For part D: how is one reactant being both reduced and ox...

Skyllar Kuppinger 1F

I think you can look at the redox pair: Fe3+ and Fe2+. Since there are 6 of Fe3+ that means the charge is 18+. There are 6 of Fe2+ so the charge is 12+. The difference between the two is 6 electrons. How come the the Cr pair doesn't work then? 2 of Cr3+ = 6+, but then you have Cr2O72- so you're goi...

Skyllar Kuppinger 1F

In part D I just have no idea what's going on in this cell. Both the cathode and anode are exactly the same except there's an H+ on the left and an OH- on the right. What is being reduced?? What is being oxidized??? Why are there 3 diff species on each side instead of 2???

Skyllar Kuppinger 1F

What is the point of the nitrate in this concentration cell?? (Image attached)What does it do? Does it keep the solution neutral? If so how?

Skyllar Kuppinger 1F

I am having the same problem. Apparently it is a typo and instead of Cl2 on the right side , it should say Cl-

Skyllar Kuppinger 1F

It says to identify the oxidizing agent and the reducing agent in this rxn: Cl2(g)->HClO(aq) + Cl2(g) In the half rxn Cl2(g)->HClO, I see that Cl is being oxidized and the balanced half rxn is 2H20 + Cl2->2HClO + 2H+ + 2e- (correct me if this is incorrectly balanced). But then isn't the other half r...

Skyllar Kuppinger 1F

So the oxidation half rxn is the oxidation of bromide and the reduction half that needs to be balanced is this: O3->O2 It's in BASIC solution. So here's what I was doing, based on the steps in the book: Step 1= balance oxygen by adding one water to the right side: O3->O2 +H2O Step 2= balance H by ad...

Skyllar Kuppinger 1F

I understand that isobaric means same pressure, but how is it possible to compress a gas and not have its pressure change, unless you are taking some of the gas out of the container?

Skyllar Kuppinger 1F

Since both cases are isothermal, you can assume that delta U is 0. Work is calculated differently in these two scenarios because during an irreversible expansion, there is a large difference between the internal pressure and the external pressure, so expansion occurs quickly. This is why we can use...

Skyllar Kuppinger 1F

it is because more mass = more electrons. electrons have so many possible states, so when there are more electrons there are even more possible states. so basically entropy has to do with not only the arrangement of atoms, but also their subatomic particles.

Skyllar Kuppinger 1F

it is based on the size and how complicated the structure is. If there are two molecules of equally complex structure then the larger one has higher residual entropy bc more atoms=more electrons and electrons are always moving around so they have many different possible states. if its 2 diatomic mol...

Skyllar Kuppinger 1F

There was a problem from the review session that asked which things are state functions, and it said that heat capacity was a state function. Why is this the case? Isn't heat capacity an inherent property of a substance? I understand that entropy , for example , is a state function because change in...

Skyllar Kuppinger 1F

When the gas is expanding it is losing energy in the form of work. However if the expansion is reversible, then this is happening slowly. This means there is time for heat from the surroundings to enter the system. So basically the energy lost by work is replaced by heat. Here is the math: Delta u= ...

Skyllar Kuppinger 1F

The first one is for when you're given C as the molar heat capacity. The second one is for when you're given C as the specific heat capacity.

Skyllar Kuppinger 1F

That is just a convention thing. Enthalpy densities are always written as positive because a density cannot be a negative value.

Skyllar Kuppinger 1F

Those are combustion equations. The combustion of a hydrocarbon is: the hydrocarbon + oxygen = water + carbon dioxide. The combustion of just hydrogen is hydrogen + oxygen = water. So you write those combustion equations and balance them by putting the right coefficients in front of each molecule.

Skyllar Kuppinger 1F

If it asks you for the standard enthalpy of a reaction then it will be in kj bc you can't really have moles of a reaction. If it's the standard enthalpy of FORMATION then it is kj/mol bc that is the enthalpy change that occurs when one mole of that molecule is formed.

Skyllar Kuppinger 1F

Yes, the negative sign is always there. You're right that when the volume is decreasing, it makes the work positive. It's a convention thing-- when the system is being compressed, we say the work is positive. and when the system is expanding against something, we say the work is negative.

Skyllar Kuppinger 1F

I believe it means the most stable form of that molecule/element. For example, the standard state of oxygen is O2 because this is the most stable way for oxygen to exist and is thus the most common, or "standard" form.

Skyllar Kuppinger 1F

"calculate the heat that must be supplied to a copper kettle of mass 400.0g containing 300.0g of water to raise its temperature from 20.0 degrees Celsius to the boiling point of water, 100.0 degrees Celsius." the answer manual just does q=mcdeltaT for both copper and water and adds them up...

Skyllar Kuppinger 1F

"Water expands when it freezes. How much work does 100.g of water do when it freezes at 0 degrees Celsius and pushes back the metal wall of a pipe that exerts an opposing pressure of 1070 atm? the densities of water and ice at 0 degrees Celsius are 1.00g/cm^3 and 0.92g/cm^3, respectively."...

Skyllar Kuppinger 1F

"in the internal combustion engine in an automobile, hot compressed gas expands against a piston, turning the driveshaft that moves the vehicle along the road. If you are investigating the performance of such an engine, you might need to assess the work that each stroke of the piston can achiev...

Skyllar Kuppinger 1F

You can use an ice table with partial pressures too. You use them just as you would use concentrations. But if you are only given concentrations then you can still convert those into pressure by using PV=nRT. concentration is n/V, so rearrange the eqtn to get n/V= P/RT. if you have P and T then just...

Skyllar Kuppinger 1F

on problem 5.33, why wouldn't a catalyst produce the image shown in diagram 2? Wouldn't a catalyst cause more product to form? ( I understand why all the other options are correct/incorrect.)

Skyllar Kuppinger 1F

it is because you've copied down the problem incorrectly; the H2 is a product not a reactant :)

Skyllar Kuppinger 1F

I'm having the same problem. In the solution manual it says that kc= 1/6.1x10^-3. But according to the table, it should definitely be 1/6.1x10^23. But it seems that we are doing the process correctly. There must be an error somewhere, perhaps the value in the table is a typo or the value in the solu...

Skyllar Kuppinger 1F

graph a is showing an exothermic reaction. The Gibbs free energy starts high and ends low, meaning the net change in free energy is negative. we know that if delta G is negative, then the rxn is exothermic. this also means that the reaction lies to the right, so K > 1. Graph b is showing an endother...

Skyllar Kuppinger 1F

an example of when you would need to use that equation is if you are being asked to do some sort of problem involving an acid but are only give the Kb of its conjugate base (and vice versa)

Skyllar Kuppinger 1F

I have no idea why I’m getting this problem wrong. I’ve done it over and over. The problem word for word is “the pH of a 0.10M propylamine, C3H7NH2, aqueous solution was measured as 11.86. What are the values of Kb and pKb of propylamine?”
My work is attached. Thanks to anyone who can help

Skyllar Kuppinger 1F

When you’re given Kb of a base and you calculate for Ka, then that Ka is of the conjugate acid of that base. Similarly , if you’re given the Ka of an acid and calculate Kb, then that Kb is for the conjugate base of that acid. Hope this helps !

Skyllar Kuppinger 1F

correction-- in my previous reply I meant to say equilibrium partial pressures, not equilibrium concentrations.

Skyllar Kuppinger 1F

set up an ice table. it seems that 0.012 bar is simply the initial value for HCl (but they did word it kinda confusingly, at first I thought they were trying to say that 0.012 is the equilibrium value). the initial values of HI and Cl2 will be 0. ignore I2 bc it is a solid. Under the C row in the ic...

Skyllar Kuppinger 1F

The answer to 5.61 says that the system being compressed will have little or no effect on the system. I thought that the system being compressed would favor the formation of products bc there are 12 mol on the reactant side but only 7 on the product side. Why is my logic wrong?

Skyllar Kuppinger 1F

In 5.35 in the book , I keep getting the wrong answer for part B. Can someone tell me what I’m doing wrong? The answer is K= 1.54x10^-2 but I keep getting 1.63x10^-2
You’re supposed to use bar for pressure , right? I think that’s what the book said. I attached a pic of my work

Skyllar Kuppinger 1F

are you sure you're using the right K value? I made this same mistake at first-- the problem says to refer to a table, and the table gives both K and Kc. Make sure for this problem you're using Kc bc you are given mol and volume so it would be in terms of concentration not pressure.

Skyllar Kuppinger 1F

You can also use concentration for gases. This is called Kc (k with a subscript of c). So just make you write Kc= (insert answer) instead of just K= (insert answer)

Skyllar Kuppinger 1F

Make sure you are writing the balanced reaction. If they’ve given u a reaction in a problem, it might not be balanced already. Also, look out for any solids or liquids and make sure not to include them in the calculation. If you’re still getting it wrong then I would suggest posting a pic of your wo...

Skyllar Kuppinger 1F

Even if there is a limiting reactant, it is still possible to have more product than reactant. What happens is the majority of the reactant (limiting or not) is converted into product, meaning that more of the reacting molecules turned into product than has stayed as reactant.

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