General non-science questions and class announcements.

Chem_Mod
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1. ΔU = 310 J

2. ΔU = 0 kJ

3. w = –5.48 kJ
q = +5.48 kJ
ΔU = 0

4. +29.5 kJ⋅mol-1

5. N2H4(l) + H2(g) → 2NH3(g) –151 kJ⋅mol-1

6. 51.2 g

7. 2 and 3

8. no temperature

9. ΔG and ΔSsys

10. +0.683 J⋅K-1

11. 75.9 g

12. 2Fe2O3(s) + 3C(s, graphite) → 4Fe(s) + 3CO2(g)

301 kJ/mol

Sebastian
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### Re: Quiz 1 Preparation Answers

Could someone please explain #7 part 5? Thanks

Hue_Vo_1D
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### Re: Quiz 1 Preparation Answers

In an isolated system, neither matter nor energy is exchanged with the surroundings->no change in internal energy->deltaU=0

Got it! Thank you.
Last edited by Hue_Vo_1D on Sun Jan 29, 2017 3:58 pm, edited 1 time in total.

Jocelyn_Dao_2N
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### Re: Quiz 1 Preparation Answers

Hi I was also wondering if someone could explain #7 and #9 on quiz one?

Tyler Miao 1A
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### Re: Quiz 1 Preparation Answers

5kJ of energy is the internal energy itself, not the change in internal energy. The question wants to know how much the internal energy CHANGE is. Just because the starting internal energy is 5 doesn't mean anything in an isolated system, because there is no change, so the change of internal energy is 0.

Chem_Mod
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### Re: Quiz 1 Preparation Answers

Hue_Vo_1D wrote:For #2,

If NEITHER matter nor energy is exchanged with the surroundings in an isolated system, then why does the internal energy goes down to 0? It must have something to do with the long period of time, 100 years? But what happen to the 5kJ of heat as time goes by?

What's the main concept behind this question?

Please read the question. Their is no change in the internal energy, that is why ΔU = 0.

Chem_Mod
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### Re: Quiz 1 Preparation Answers

Sebastian wrote:Could someone please explain #7 part 5? Thanks

5) A glass of water loses 100 J of energy reversibly at 30°C.

This is a decrease in entropy as energy left the system.

Chem_Mod
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### Re: Quiz 1 Preparation Answers

Q9. Degeneracy at a minimum can be 1, never zero.

Changes in state functions of the system are zero when the system returns to the same initial conditions.

Xin Huang 3E
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### Re: Quiz 1 Preparation Answers

I am also wondering about #9, specifically about ΔS(surr), why is it not 0? Isn't it a state function, too? I know that ΔS(sys) + ΔS(surr) = ΔS(univ) > 0, but that is only for a spontaneous process? Why is #9 a spontaneous process?
Last edited by Xin Huang 3E on Sun Jan 29, 2017 4:05 pm, edited 1 time in total.

Chem_Mod
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### Re: Quiz 1 Preparation Answers

***All equations and constants are given in all my quizzes and exams.***

BrianaBarr2A
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### Re: Quiz 1 Preparation Answers

For number 6, I'm a little stuck on which equation to use. Could someone help me with this?

Chem_Mod
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### Re: Quiz 1 Preparation Answers

Xin Huang 3E wrote:I am also wondering about #9, specifically about ΔS(surr), why is it not 0? Isn't it a state function, too?

Please read carefully: Changes in state functions of the system are zero when the system returns to the same initial conditions.

Delta S (total) > 0 for this process. Therefore delta S (surroundings) > 0.

Diego Cortez 2J
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### Re: Quiz 1 Preparation Answers

Can someone show me the setup for #6? Please and thank you!

Hue_Vo_1D
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### Re: Quiz 1 Preparation Answers

Diego Cortez 2J wrote:Can someone show me the setup for #6? Please and thank you!

-Write out the equation for the combustion of propane and make sure to balance the eqn.
-From the given standard enthalpy of combustion (kJ/mol), figure out how many moles of propane is burned in order to release 2580. kJ of heat.(use stoichiometry).
-Convert moles to mass using MM.

Just let me know if you're still confused with any step, I can be more specific.

Hue_Vo_1D
Posts: 19
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Quiz 1 Preparation Answers

BrianaBarr2A wrote:For number 6, I'm a little stuck on which equation to use. Could someone help me with this?

Use the equation for the combustion of propane.
C3H8 + 5O2 => 3CO2 + 4H2O (balanced)

Diego Cortez 2J
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### Re: Quiz 1 Preparation Answers

Hue_Vo_1D wrote:
Diego Cortez 2J wrote:Can someone show me the setup for #6? Please and thank you!

-Write out the equation for the combustion of propane and make sure to balance the eqn.
-From the given standard enthalpy of combustion (kJ/mol), figure out how many moles of propane is burned in order to release 2580. kJ of heat.(use stoichiometry).
-Convert moles to mass using MM.

Just let me know if you're still confused with any step, I can be more specific.

I balanced the equation, but I'm still confused on how to get the moles.

Hue_Vo_1D
Posts: 19
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Quiz 1 Preparation Answers

Diego Cortez 2J wrote:
Hue_Vo_1D wrote:
Diego Cortez 2J wrote:Can someone show me the setup for #6? Please and thank you!

-Write out the equation for the combustion of propane and make sure to balance the eqn.
-From the given standard enthalpy of combustion (kJ/mol), figure out how many moles of propane is burned in order to release 2580. kJ of heat.(use stoichiometry).
-Convert moles to mass using MM.

Just let me know if you're still confused with any step, I can be more specific.

I balanced the equation, but I'm still confused on how to get the moles.

2220 kJ/1 mol=2580 kJ/X mol

Solve for X

Hue_Vo_1D
Posts: 19
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Quiz 1 Preparation Answers

Hue_Vo_1D wrote:
Diego Cortez 2J wrote:
Hue_Vo_1D wrote:
-Write out the equation for the combustion of propane and make sure to balance the eqn.
-From the given standard enthalpy of combustion (kJ/mol), figure out how many moles of propane is burned in order to release 2580. kJ of heat.(use stoichiometry).
-Convert moles to mass using MM.

Just let me know if you're still confused with any step, I can be more specific.

I balanced the equation, but I'm still confused on how to get the moles.

Use stoichiometry.
2220 kJ/1 mol=2580 kJ/X mol

Solve for X

Diego Cortez 2J
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### Re: Quiz 1 Preparation Answers

Hue_Vo_1D wrote:
Hue_Vo_1D wrote:
Diego Cortez 2J wrote:I balanced the equation, but I'm still confused on how to get the moles.

Use stoichiometry.
2220 kJ/1 mol=2580 kJ/X mol

Solve for X

Thanks

Hogan Irwin 3A
Posts: 5
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Quiz 1 Preparation Answers (#11)

How many grams of water can be heated from 25.0 degrees Celsius to 100 degrees Celsius by the heat released from converting 49.7g of PbO to Pb?

The converting reaction is: PbO(s) + C(s) --> Pb(s) + CO(g) deltaH = -106.9kJ

How do I go about setting up this problem in order to solve it?

Coleen_Leslie_2L
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### Re: Quiz 1 Preparation Answers

What formulas will be included on the formula sheet for Quiz 1?

Martha Xuncax 3G
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### Re: Quiz 1 Preparation Answers

The same formulas from the back of your periodic table I believe.

csebastiani_1B
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### Re: Quiz 1 Preparation Answers

Why doesn't process 1 in #7 lead to an increase in entropy?

Posts: 9
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### Re: Quiz 1 Preparation Answers

Hue_Vo_1D wrote:
Diego Cortez 2J wrote:
Hue_Vo_1D wrote:
-Write out the equation for the combustion of propane and make sure to balance the eqn.
-From the given standard enthalpy of combustion (kJ/mol), figure out how many moles of propane is burned in order to release 2580. kJ of heat.(use stoichiometry).
-Convert moles to mass using MM.

Just let me know if you're still confused with any step, I can be more specific.

I balanced the equation, but I'm still confused on how to get the moles.

2220 kJ/1 mol=2580 kJ/X mol

Solve for X

Hi, I'm also stuck on this problem. I've been trying to solve this but I keep getting a larger number than the given answer. I've done it five times now and get the same answer every time. Is this for sure the way to solve the problem? I don't understand where that ratio is coming from.

Kira_Maszewski_1B
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### Re: Quiz 1 Preparation Answers

What equation do we use for work in #3?

Alina_Mitchell_2C
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### Re: Quiz 1 Preparation Answers

Kira_Maszewski_1B wrote:What equation do we use for work in #3?

you use the equation w = -nRT ln (V2/V1) because the reaction is reversible and isothermal

IssaelG_3E
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### Re: Quiz 1 Preparation Answers

Can anyone explain how they calculated the enthalpy of vaporization for question 4?

Claire_Zhou_1A
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### Re: Quiz 1 Preparation Answers

I read previous replies and still confused about #9. Could someone explain to me? I know that △S(system) is 0 because it's a state function, but why is △S(surroundings)＞0? Also, w and q are not state functions, but the processes in the question seem to be the opposite, shouldn't the q for each step just cancel out and give w=0?

Natalie Hunt 1E
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### Re: Quiz 1 Preparation Answers

Can anyone explain how they calculated q in #3? I was able to calculate w, and I know how to calculate deltaU if I can determine q. Do we assume that deltaU = 0, which would make q=-w? Thank you!

Natalie Hunt 1E
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### Re: Quiz 1 Preparation Answers

IssaelG_3E wrote:Can anyone explain how they calculated the enthalpy of vaporization for question 4?

deltaH sublimation = deltaH vaporization + deltaH fusion so by manipulating the equation to make deltaH vaporization = deltaH sublimation - deltaH fusion, you can then calculate the enthalpy of vaporization given the values of enthalpy of sublimation
and enthalpy of fusion.

Chem Student
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### Re: Quiz 1 Preparation Answers

If entropy is a state function, why is DeltaSurr not equal to zero when DeltaSys is?

Nayeon Kang 2M
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### Re: Quiz 1 Preparation Answers

How can you solve #11? Is it right to use H=ncT here??

melissapasao1B
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### Re: Quiz 1 Preparation Answers

Can someone please explain how I should approach #11?

Colleen_VeraCruz_3A
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### Re: Quiz 1 Preparation Answers

Could someone please explain how you get the answer for #11(How many grams of H20 can be heated from 25 degrees to 100 degrees by the heat released from converting 49.7g of PbO to Pb)? What equation(s) are we supposed to use?

Lily_Dermendjian_2B
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### Re: Quiz 1 Preparation Answers

melissapasao1B wrote:Can someone please explain how I should approach #11?

You divide 49.7 by the molar mass of PbO to get the number of moles, then multiply that by the deltaH to get the heat released by the reaction of that amount of PbO. You then use q=mCdeltaT and solve for m.

49.7g/(223.2g/mol)=0.223mol; 0.223mol*-106.9kJ=-23.8kJ
m=q/CdeltaT=(23.8*10^3 J)/((4.184 J/Cg)(75 C))=75.9 g H2O

Veronica_Giap_1C
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### #11 approach

#11 approach

First you want to find the amount of heat released by the PbO reaction
-you do that by converting PbO grams -> mols
-set up ration of (-106.9kJ/1mol)=(q_p kJ/n mols of PbO)

With q_p that you just found, that is the amount of heat you have to heat m grams of H2O from 25C -> 100C
-find mass using q_p=mC[delta]T

Tara_Shooshani_3N
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### Re: Quiz 1 Preparation Answers

For #8, I don't understand why the reaction will not be spontaneous at any temperature.

ahlam_ahmed_1d
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### Re: Quiz 1 Preparation Answers

Can someone help me understand number 7?

Emma_Green_2C
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### Re: Quiz 1 Preparation Answers

Tara_Shooshani_3N wrote:For #8, I don't understand why the reaction will not be spontaneous at any temperature.

If you plug in the given values into the equation deltaG= deltaH-T*deltaS and set delta G to zero, you will get a negative number for the temperature (-985.5K). You cannot have a temperature less than 0K, so a temperature less than -985.5K (which you would need for delta G to be negative and spontaneous) is impossible.

Simone Seliger 1C
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### Re: Quiz 1 Preparation Answers

Can someone verify that the equation needed to solve #12 is the one we learned for calculating standard free energy change? I am getting caught up in the wording: "Calculate the standard Gibbs free energy". Should I be using a different equation (even though we're given standard change in enthalpy of formation and standard entropy)? I am not getting the correct answer.

Myra_Zhan_2N
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### Re: Quiz 1 Preparation Answers

Simone Seliger 1C wrote:Can someone verify that the equation needed to solve #12 is the one we learned for calculating standard free energy change? I am getting caught up in the wording: "Calculate the standard Gibbs free energy". Should I be using a different equation (even though we're given standard change in enthalpy of formation and standard entropy)? I am not getting the correct answer.

You would use ΔG^o=ΔH^o−TΔS^o after you find ΔH^o (rxn) and ΔS^o (rxn) values from the tables provided. Make sure your units match. You will need to convert ΔS^o to KJ for the final answer.

Noelle Min-1N
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### Re: Quiz 1 Preparation Answers

Does anyone have any quizzes from previous year's coursereaders for extra practice?

Vivian Nguyen 2A
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### Re: Quiz 1 Preparation Answers

Natalie Hunt 1E wrote:Can anyone explain how they calculated q in #3? I was able to calculate w, and I know how to calculate deltaU if I can determine q. Do we assume that deltaU = 0, which would make q=-w? Thank you!

We can assume that deltaU = 0 because we are looking at an ideal gas that expands isothermally and reversibly.
Thus, like you stated, 0 = q + w and q = -w.

Posts: 23
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### Re: Quiz 1 Preparation Answers

Did anyone else notice that the initial conditions to Problem #3 are not consistent with PV = nRT?
(3atm)(6L) ≠ (2mol)(0.0826L-atm/mol-K)(300K)
18L-atm ≠ 49.56L-atm

Anna_Kim_2E
Posts: 31
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### Re: Quiz 1 Preparation Answers

Where do you find the prep quiz?
I only see the midterms from past years.

Do I have to buy a separate booklet or is it in the course reader? Then what page is it in?

Posts: 23
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### Re: Quiz 1 Preparation Answers

Quiz 1 Preparation, pg 113

Jazmin_Morales_3J
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### Re: Quiz 1 Preparation Answers

Can someone please explain why on #5 the answer has 2NH3 on the product side? When I did the question and canceled everything, I divided 2NH3 by 2 so that it would be the same as in the target equation N2H4 + H2 = NH3. This gave me an enthalpy of -105.1kJ/mol, instead of -151 kJ/mol. Thanks

Hogan Irwin 3A
Posts: 5
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### Re: Quiz 1 Preparation Answers

Jazmin_Morales_3J wrote:Can someone please explain why on #5 the answer has 2NH3 on the product side? When I did the question and canceled everything, I divided 2NH3 by 2 so that it would be the same as in the target equation N2H4 + H2 = NH3. This gave me an enthalpy of -105.1kJ/mol, instead of -151 kJ/mol. Thanks

I believe that is a typo. The 2 should not be in front of the NH3. Try the problem without the 2 and see what you get.

Hue_Vo_1D
Posts: 19
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Quiz 1 Preparation Answers

Jazmin_Morales_3J wrote:Can someone please explain why on #5 the answer has 2NH3 on the product side? When I did the question and canceled everything, I divided 2NH3 by 2 so that it would be the same as in the target equation N2H4 + H2 = NH3. This gave me an enthalpy of -105.1kJ/mol, instead of -151 kJ/mol. Thanks

I don't think it's a typo. It's just that you are supposed to balance the equation.

N2H4 + O2 => 2NH3

This will get you the right answer.

Kira_Maszewski_1B
Posts: 17
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### Re: Quiz 1 Preparation Answers

Chem_Mod wrote:
Xin Huang 3E wrote:I am also wondering about #9, specifically about ΔS(surr), why is it not 0? Isn't it a state function, too?

Please read carefully: Changes in state functions of the system are zero when the system returns to the same initial conditions.

Delta S (total) > 0 for this process. Therefore delta S (surroundings) > 0.

What are q and w equal to in this case? What equations would I to solve or these values?

Kira_Maszewski_1B
Posts: 17
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Quiz 1 Preparation Answers

Lily_Dermendjian_2B wrote:
melissapasao1B wrote:Can someone please explain how I should approach #11?

You divide 49.7 by the molar mass of PbO to get the number of moles, then multiply that by the deltaH to get the heat released by the reaction of that amount of PbO. You then use q=mCdeltaT and solve for m.

49.7g/(223.2g/mol)=0.223mol; 0.223mol*-106.9kJ=-23.8kJ
m=q/CdeltaT=(23.8*10^3 J)/((4.184 J/Cg)(75 C))=75.9 g H2O

Why does the q go from -23.8 to 23.8?

Simone Seliger 1C
Posts: 30
Joined: Fri Sep 25, 2015 3:00 am

### Re: Quiz 1 Preparation Answers

Myra_Zhan_2N wrote:
Simone Seliger 1C wrote:Can someone verify that the equation needed to solve #12 is the one we learned for calculating standard free energy change? I am getting caught up in the wording: "Calculate the standard Gibbs free energy". Should I be using a different equation (even though we're given standard change in enthalpy of formation and standard entropy)? I am not getting the correct answer.

You would use ΔG^o=ΔH^o−TΔS^o after you find ΔH^o (rxn) and ΔS^o (rxn) values from the tables provided. Make sure your units match. You will need to convert ΔS^o to KJ for the final answer.

That is the equation I am using. And I remembered to convert J to kJ. Still got the wrong answer.

TramHo2G
Posts: 31
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Quiz 1 Preparation Answers

For #12, what do I plug in for deltaS?
Is deltaH 2*(-824.2)?
Last edited by TramHo2G on Mon Jan 30, 2017 8:39 pm, edited 1 time in total.

Kira_Maszewski_1B
Posts: 17
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### Re: Quiz 1 Preparation Answers

Myra_Zhan_2N wrote:
Simone Seliger 1C wrote:Can someone verify that the equation needed to solve #12 is the one we learned for calculating standard free energy change? I am getting caught up in the wording: "Calculate the standard Gibbs free energy". Should I be using a different equation (even though we're given standard change in enthalpy of formation and standard entropy)? I am not getting the correct answer.

You would use ΔG^o=ΔH^o−TΔS^o after you find ΔH^o (rxn) and ΔS^o (rxn) values from the tables provided. Make sure your units match. You will need to convert ΔS^o to KJ for the final answer.

For some reason I am getting -301.49 kJ/mol... Do you have any idea why?

EmilyLeibovitch2E
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Joined: Wed Nov 18, 2015 3:00 am

### Re: Quiz 1 Preparation Answers

Is there a PDF that someone could post with exactly how to solve each problem on Quiz 1 Prep?

Katrina_Domingo_3G
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Joined: Wed Sep 21, 2016 2:59 pm

### Re: Quiz 1 Preparation Answers

Hello,

Can someone please explain Question #9 on the quiz prep?

A sample of 1 mol of gas initially at 1 atm and 298 K is heated at constant pressure to 350 K, then the gas is compressed isothermally to its initial volume and finally it is cooled to 298 K at constant volume. Which of the following values is 0?
DeltaSsurr and DeltaG

DeltaSsys and DeltaSsurr

q and w

W

DeltaG and DeltaSsys

Can someone please explain why the other choices are not 0 and why Delta G and Delta Ssys have a value of 0?

Thank you so much!

Raul Hernandez
Posts: 28
Joined: Fri Jun 17, 2016 11:28 am

### Re: Quiz 1 Preparation Answers

For number 3, what equation do we use since there seems to be a change in pressure? I can't seem to get the answer posted here by any method I've tried.
Thanks.

RitaChang_2C
Posts: 10
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Quiz 1 Preparation Answers

104450116 wrote:For number 3, what equation do we use since there seems to be a change in pressure? I can't seem to get the answer posted here by any method I've tried.
Thanks.

Use w=-nRT ln V2/V1
So then you'd get -(2.00 mol)(8.314 J/K mol)(300 K)(ln 18.00/6.00) to get -5480 J or -5.48 kJ.

Posts: 9
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### Re: Quiz 1 Preparation Answers

Simone Seliger 1C wrote:
Myra_Zhan_2N wrote:
Simone Seliger 1C wrote:Can someone verify that the equation needed to solve #12 is the one we learned for calculating standard free energy change? I am getting caught up in the wording: "Calculate the standard Gibbs free energy". Should I be using a different equation (even though we're given standard change in enthalpy of formation and standard entropy)? I am not getting the correct answer.

You would use ΔG^o=ΔH^o−TΔS^o after you find ΔH^o (rxn) and ΔS^o (rxn) values from the tables provided. Make sure your units match. You will need to convert ΔS^o to KJ for the final answer.

That is the equation I am using. And I remembered to convert J to kJ. Still got the wrong answer.

I also do not understand this question. The answer I am getting is not even close to 310, are we sure the equation is right?

Lauren Trent 2A
Posts: 21
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### Re: Quiz 1 Preparation Answers

Hogan Irwin 3A wrote:
Jazmin_Morales_3J wrote:Can someone please explain why on #5 the answer has 2NH3 on the product side? When I did the question and canceled everything, I divided 2NH3 by 2 so that it would be the same as in the target equation N2H4 + H2 = NH3. This gave me an enthalpy of -105.1kJ/mol, instead of -151 kJ/mol. Thanks

I believe that is a typo. The 2 should not be in front of the NH3. Try the problem without the 2 and see what you get.

The equation given in the question is not balanced; once you balance the equation there will be a product of 2NH3.

Myra_Zhan_2N
Posts: 25
Joined: Fri Jul 22, 2016 3:00 am

### Re: Quiz 1 Preparation Answers

Simone Seliger 1C wrote:
Myra_Zhan_2N wrote:

You would use ΔG^o=ΔH^o−TΔS^o after you find ΔH^o (rxn) and ΔS^o (rxn) values from the tables provided. Make sure your units match. You will need to convert ΔS^o to KJ for the final answer.

That is the equation I am using. And I remembered to convert J to kJ. Still got the wrong answer.

I also do not understand this question. The answer I am getting is not even close to 310, are we sure the equation is right?

Following the equation: ΔG^o=ΔH^o−TΔS^o, I got ΔG^o=467.87 kJ/mol−(298K) (0.55832 kJ/K*mol) = 301 kJ

RitaChang_2C
Posts: 10
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Quiz 1 Preparation Answers

Simone Seliger 1C wrote:
Myra_Zhan_2N wrote:

You would use ΔG^o=ΔH^o−TΔS^o after you find ΔH^o (rxn) and ΔS^o (rxn) values from the tables provided. Make sure your units match. You will need to convert ΔS^o to KJ for the final answer.

That is the equation I am using. And I remembered to convert J to kJ. Still got the wrong answer.

I also do not understand this question. The answer I am getting is not even close to 310, are we sure the equation is right?

Make sure to multiply each delta G by the number of moles in the balanced chemical equation. So multiply delta G for Fe by four, CO2 by three, and so on.

ariana_cruz_1C
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### Re: Quiz 1 Preparation Answers

Can someone explain why in #11 does q go from -23.8 to 23.8?

804748473
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### Re: Quiz 1 Preparation Answers

csebastiani_1B wrote:Why doesn't process 1 in #7 lead to an increase in entropy?

This is because the pressure is doubled, meaning the system does not expand and thus takes up less space and the overall disorder in the system is not increased.

Nikhita_Jaaswal_2H
Posts: 31
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Quiz 1 Preparation Answers

Diego Cortez 2J wrote:Can someone show me the setup for #6? Please and thank you!

(2580) /(2220)= value
Multiply this value by the molar mass of propane

Alyssa_Hsu_2K
Posts: 55
Joined: Wed Sep 21, 2016 3:00 pm

### Re: Quiz 1 Preparation Answers

Hi, I was wondering why the sig figs for number 10 is 3 instead of 2? Doesn't the 20 degrees celsius mentioned in the problem only have 2 sig figs?

804748473
Posts: 21
Joined: Fri Oct 07, 2016 3:02 am

### Re: Quiz 1 Preparation Answers

If number 11 was on a quiz, would we be given the specific heat capacity of water?

ariana_cruz_1C
Posts: 19
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Quiz 1 Preparation Answers

Why in #5 do we need to flip the reaction 2H2 + O2 = 2H2O and not the other ones ?

ariana_cruz_1C
Posts: 19
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Quiz 1 Preparation Answers

804748473 wrote:If number 11 was on a quiz, would we be given the specific heat capacity of water?

It's on our formula sheet so I assume so!

kgiesch1N
Posts: 9
Joined: Fri Jul 15, 2016 3:00 am

### Re: Quiz 1 Preparation Answers

Can someone please explain why #8 is "no temperature"?

Kiara1F
Posts: 64
Joined: Fri Sep 25, 2015 3:00 am

### Re: Quiz 1 Preparation Answers

RitaChang_2C wrote:
104450116 wrote:For number 3, what equation do we use since there seems to be a change in pressure? I can't seem to get the answer posted here by any method I've tried.
Thanks.

Use w=-nRT ln V2/V1
So then you'd get -(2.00 mol)(8.314 J/K mol)(300 K)(ln 18.00/6.00) to get -5480 J or -5.48 kJ.

how do you then find the heat to find the internal energy?
With q=nCdeltaT ..?
What would be the value of C?

Julia Hwang 3G
Posts: 22
Joined: Wed Sep 21, 2016 3:00 pm

### Re: Quiz 1 Preparation Answers

Kiara1F wrote:
RitaChang_2C wrote:
104450116 wrote:For number 3, what equation do we use since there seems to be a change in pressure? I can't seem to get the answer posted here by any method I've tried.
Thanks.

Use w=-nRT ln V2/V1
So then you'd get -(2.00 mol)(8.314 J/K mol)(300 K)(ln 18.00/6.00) to get -5480 J or -5.48 kJ.

how do you then find the heat to find the internal energy?
With q=nCdeltaT ..?
What would be the value of C?

Rather than actually doing a new calculation, you analyze your results with the equation delta U = q + w.
Since work was equal to -5.48 kJ, you can use w = -q to know that q = +5.48 kJ. q and w then cancel each other out and delta U (change in energy) is 0.

904826427
Posts: 16
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Quiz 1 Preparation Answers

ariana_cruz_1C wrote:Can someone explain why in #11 does q go from -23.8 to 23.8?

You cannot have negative mass so it's in thermodynamics that when it is transferred the energy is now positive
Last edited by 904826427 on Tue Jan 31, 2017 1:09 pm, edited 1 time in total.

Anna_Kim_2E
Posts: 31
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Quiz 1 Preparation Answers

ariana_cruz_1C wrote:Can someone explain why in #11 does q go from -23.8 to 23.8?

Because -23.8kJ is negative because it was released from converting PbO to Pb.
Then, that amount of heat is transferred into the system, which is water. When heat is put into an system, it is an endothermic process and the sign should be positive.

John_Parks_3D
Posts: 6
Joined: Wed Sep 21, 2016 2:58 pm

### Re: Quiz 1 Preparation Answers

Can someone please explain why 3 is an answer for #7? I understand why number 2 is an answer.

Ara Yazaryan 1E
Posts: 44
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Quiz 1 Preparation Answers

Could someone explain to me how to get #10 please? I am confused because we do not know the final temperature.

BetcyGaspar_3C
Posts: 3
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Quiz 1 Preparation Answers

Ara Yazaryan 1E wrote:Could someone explain to me how to get #10 please? I am confused because we do not know the final temperature.

I do believe we use the equation deltaS=qrev/T,by plugging in 200J/(273+20) you will get the answer.

Bronson_Barretto_2C
Posts: 11
Joined: Wed Nov 18, 2015 3:00 am

### Re: Quiz 1 Preparation Answers

kgiesch1N wrote:Can someone please explain why #8 is "no temperature"?

The reasoning is based on the chart (pg. 37 in the course reader) Dr. Lavelle presented in lecture that according to the +/- values of ∆H and ∆S, we can assume the +/- of ∆G, negative being spontaneous and positive being non-spontaneous. For #8, the ∆H value is positive and the ∆S value is negative; based on the chart, these values give a ∆G that will NEVER be spontaneous. As such, the answer to the question is then none or n/a.

Helena Vervaet 1N
Posts: 26
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Quiz 1 Preparation Answers

Hue_Vo_1D wrote:
Diego Cortez 2J wrote:
Hue_Vo_1D wrote:
-Write out the equation for the combustion of propane and make sure to balance the eqn.
-From the given standard enthalpy of combustion (kJ/mol), figure out how many moles of propane is burned in order to release 2580. kJ of heat.(use stoichiometry).
-Convert moles to mass using MM.

Just let me know if you're still confused with any step, I can be more specific.

I balanced the equation, but I'm still confused on how to get the moles.

2220 kJ/1 mol=2580 kJ/X mol

Solve for X

Your explanation helped a bunch - Thanks! I was just wondering if anything would be different about this process if there had been a stoichiometric coefficient with the propane? Right now it's balanced with a 1, but if there had been a 2 instead would there be another calculation involved?

Diana_Visco_1l
Posts: 31
Joined: Fri Jul 15, 2016 3:00 am

### Re: Quiz 1 Preparation Answers (#11)

Hogan Irwin 3A wrote:How many grams of water can be heated from 25.0 degrees Celsius to 100 degrees Celsius by the heat released from converting 49.7g of PbO to Pb?

The converting reaction is: PbO(s) + C(s) --> Pb(s) + CO(g) deltaH = -106.9kJ

How do I go about setting up this problem in order to solve it?

q = (n)ΔH

(49.7 g PbO/22 g PbO ) (-106.9) = -23.8 KJ
-23.8 KJ x (1000 J / 1 KJ) = -23800 J

q= mcΔT

-23800 J = m (4.184) (100°-25°)

m= 75.9 g H20

Chanvir_Singh_3E
Posts: 2
Joined: Sat Jul 09, 2016 3:00 am

### Re: Quiz 1 Preparation Answers

ariana_cruz_1C wrote:Why in #5 do we need to flip the reaction 2H2 + O2 = 2H2O and not the other ones ?

We need to flip the reaction because we need 2H2O on the reactant side so we can cancel it with the 2H2O on the product side in the given reaction N2H4 + O2 = N2 + 2H2O.

Xin Huang 3E
Posts: 40
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Quiz 1 Preparation Answers

Chem Student wrote:If entropy is a state function, why is DeltaSurr not equal to zero when DeltaSys is?

Because Entropy of surrounding is not a state function as entropy of the system does. And since deltaTotal is always > 0 (since this is the universal trend haha the world is always tending to confusion) while deltaTotal = deltaSurr + deltaSys and deltaSys = 0; => therefore, deltaSurr > 0.

Ara Yazaryan 1E
Posts: 44
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Quiz 1 Preparation Answers

BetcyGaspar_3C wrote:I do believe we use the equation deltaS=qrev/T,by plugging in 200J/(273+20) you will get the answer.

Thank you so much!

Jaime_Chamberlain_3G
Posts: 23
Joined: Sat Jul 09, 2016 3:00 am

### Re: Quiz 1 Preparation Answers

Could someone please explain number 6?

Minu Reddy
Posts: 31
Joined: Sat Jul 09, 2016 3:00 am

### Re: Quiz 1 Preparation Answers

Hi Everyone,
I have a question on #10.

So when I did the problem, I divided 200/20, but I was wondering if someone can please explain the conversion factor to get the answer on the key?

Thank you

Brian Huynh 3D
Posts: 20
Joined: Wed Sep 21, 2016 2:58 pm

### Re: Quiz 1 Preparation Answers

Will we need a calculator for this test?

Sharlene Reyes 2N
Posts: 16
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Quiz 1 Preparation Answers

Brian Huynh 3G wrote:Will we need a calculator for this test?

Yes, unless you're fine with the pain of doing all the calculations by hand. No graphing calculators though, so I scientific calculator or a more simple calculator.

Angela_Park_1F
Posts: 18
Joined: Fri Jun 17, 2016 11:28 am

### Re: Quiz 1 Preparation Answers

Alyssa_Hsu_2K wrote:Hi, I was wondering why the sig figs for number 10 is 3 instead of 2? Doesn't the 20 degrees celsius mentioned in the problem only have 2 sig figs?

deantuazon2G
Posts: 13
Joined: Fri Jun 17, 2016 11:28 am

### Re: Quiz 1 Preparation Answers

So for number 11, do we just assume that the deltaH given is per mole?

Jake_Susi_2J
Posts: 13
Joined: Sat Jul 09, 2016 3:00 am

### Re: Quiz 1 Preparation Answers

I know people have already posted how to find work (w) for #3 but what do we do to find heat (q)? I know the problem consists of an ideal gas, change in volume and pressure, etc.. but is it just a modified version of q=nC(delta_T)?

Angelica Nava-1E
Posts: 14
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Quiz 1 Preparation Answers

BetcyGaspar_3C wrote:
Ara Yazaryan 1E wrote:Could someone explain to me how to get #10 please? I am confused because we do not know the final temperature.

I do believe we use the equation deltaS=qrev/T,by plugging in 200J/(273+20) you will get the answer.

Where did you get the 273 from? I know q=200J and the temp. is 20 degrees C but where did the 273 come from?

Chem_Mod
Posts: 19157
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 827 times

### Re: Quiz 1 Preparation Answers

Angelica Nava-1E wrote:
BetcyGaspar_3C wrote:
Ara Yazaryan 1E wrote:Could someone explain to me how to get #10 please? I am confused because we do not know the final temperature.

I do believe we use the equation deltaS=qrev/T,by plugging in 200J/(273+20) you will get the answer.

Where did you get the 273 from? I know q=200J and the temp. is 20 degrees C but where did the 273 come from?

To convert degrees C to K.

chris2E
Posts: 14
Joined: Fri Jul 22, 2016 3:00 am

### Re: Quiz 1 Preparation Answers

For #1, shouldn't the answer be 310.J instead of 310J since both 254J and 564J have three significant figures?

For #10, shouldn't the answer be 0.7J/K instead of 0.683J/K since both 200J and 20°C have one significant figure?

BrianaBarr2A
Posts: 22
Joined: Wed Sep 21, 2016 3:00 pm

### Re: Quiz 1 Preparation Answers

Jake_Susi_2J wrote:I know people have already posted how to find work (w) for #3 but what do we do to find heat (q)? I know the problem consists of an ideal gas, change in volume and pressure, etc.. but is it just a modified version of q=nC(delta_T)?

From what I was told, for this q=-w so it's just the negative of whatever you got when solving for w

BrianaBarr2A
Posts: 22
Joined: Wed Sep 21, 2016 3:00 pm

### Re: Quiz 1 Preparation Answers

deantuazon2G wrote:So for number 11, do we just assume that the deltaH given is per mole?

Yes just assume it's per mole

BrianaBarr2A
Posts: 22
Joined: Wed Sep 21, 2016 3:00 pm

### Re: Quiz 1 Preparation Answers

Minu Reddy wrote:Hi Everyone,
I have a question on #10.

So when I did the problem, I divided 200/20, but I was wondering if someone can please explain the conversion factor to get the answer on the key?

Thank you

I'm not sure if the conversion factor is on the key for this one or not but you first have to convert degrees C to K so add 273.25 to 20

404639305
Posts: 15
Joined: Fri Sep 25, 2015 3:00 am

### Re: Quiz 1 Preparation Answers

John_Parks_3D wrote:Can someone please explain why 3 is an answer for #7? I understand why number 2 is an answer.

I think increasing temperature increases entropy because heat excites the molecules and causes more random movements.

Fayt Sarreal 1G
Posts: 25
Joined: Fri Jul 22, 2016 3:00 am

### Re: Quiz 1 Preparation Answers

For #11, do the units not have to cancel out?

Ashley Curtis 2O
Posts: 21
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Quiz 1 Preparation Answers

Natalie Hunt 1E wrote:
IssaelG_3E wrote:Can anyone explain how they calculated the enthalpy of vaporization for question 4?

deltaH sublimation = deltaH vaporization + deltaH fusion so by manipulating the equation to make deltaH vaporization = deltaH sublimation - deltaH fusion, you can then calculate the enthalpy of vaporization given the values of enthalpy of sublimation
and enthalpy of fusion.

Why does this work? Why would you not have to factor in all the temperature changes to get to a temperature of vaporization or so?

Valeria Quintana 1G
Posts: 11
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Quiz 1 Preparation Answers

#8 on Quiz 1 Preparation says, "For reaction: 2C+2H2=C2H4, DeltaH= +52.3 kJ/mol and DeltaS= -53.07 J/K*mol at 298 K. At what temperature will this reaction be spontaneous?"

Initially, I thought the formula needed to solve for the temperature was DeltaG= DeltaH - T*DeltaS but then I saw that the answer is "no temperature." Could someone explain why the answer is "no temperature"? Please and thank you.