CHEM1101 Exam HELL (1 Viewer)

[bobbie212]

sorry...need help again, both these questions are on 2003-N-11

You are a member of a research team of industrial chemists who are discussing the operation of an ammonia plant. Ammonia is formed from nitrogen and hydrogen according to the following equilibrium reaction

N2(g) + 3H2(g)<------> 2NH3(g)

The plant operates close to 700 K, at which Kp is 1.00 × 10–4 atm–2 and employs the stoichiometric ratio 1:3 of N2:H2. At equilibrium the partial pressure of NH3 is
50 atm. Calculate the partial pressures of each reactant and hence the total pressure under these conditions.

and

Ammonium carbamate (NH2COONH4) is a salt of carbamic acid that is found in the
blood and urine of mammals. At 250 ºC, Kc = 1.58 × 10–8 M3 for the following
equilibrium:

NH2COONH4(s)<------> 2NH3(g) + CO2(g)

If 7.81 g of NH2COONH4 is introduced into a 0.500 L evacuated container, what is the
total pressure inside the container at equilibrium at 250 ºC?

 

[tennille]

yea,i kind of know what you are saying

sorry didnt put up the whole question

the question was 100ml of NAOH is mixed with 100ml of HCL , both 100ml and 2M. the temp went from 24.6-38 deg.

assuming a perfect calorimeter, determine the standard enthalpy change for neutralisation reaction.

so

4.18*100*-13.4=5601(j) (same for both solutions)

5601/2+5601/2=5601(j/M)
right?
and so 5.6 kJ/M but the answer was 56kj per mol
i dont know what i am missing?

You don't divide by the concentration, you divide by the amount of moles, which is 2*0.1=0.2

 

[allstarr69]

ok im getting a bit stressed here... lots of easy shit i still dont know...

Ive been goin through the november 2004 paper.

Can anyone tell me how to do these?

n-3 part 2. I worked out the wavelength, but how do you get the unknown element?

n - 4 part 2. What is the longest wavelength of light taht the ground state of C2+ ion will absorb? (NO Idea)

n - 5. Total number of (put symbol here) bonding and nin bonding electron pairs on atom. How do you work these out? Also how do you know for sure what shape it is? In the answer doesnt chlorine end up having 10 electrons when it should only have 8 in its out shell? How to know when to allow for this?

n - 5 part 2. Explain why the first ionisation energy of an atom of oxygen is slightly lower than that of an atom of nitrogen, despite being further across the period.


Can someone kindly help me with these? At the moment i reckon im gonna fail. Thanks Alot

 

[tennille]

ok im getting a bit stressed here... lots of easy shit i still dont know...

Ive been goin through the november 2004 paper.

Can anyone tell me how to do these?

n-3 part 2. I worked out the wavelength, but how do you get the unknown element?

n - 4 part 2. What is the longest wavelength of light taht the ground state of C2+ ion will absorb? (NO Idea)

n - 5. Total number of (put symbol here) bonding and nin bonding electron pairs on atom. How do you work these out? Also how do you know for sure what shape it is? In the answer doesnt chlorine end up having 10 electrons when it should only have 8 in its out shell? How to know when to allow for this?

n - 5 part 2. Explain why the first ionisation energy of an atom of oxygen is slightly lower than that of an atom of nitrogen, despite being further across the period.


Can someone kindly help me with these? At the moment i reckon im gonna fail. Thanks Alot

I can't work out n-3 and n-4. To work out the shape in n-5, look at the Silberberg book at about page 370. You have to take into account the amount of lone pairs. If you have a trigonal bypyramidal arrangement (5 paired electrons) and there is one lone pair (one that isn't invovled in bonding), then the molecule is see-saw shaped. Silberberg explains this really clearly.

When forming a Lewis dot structure, they don't have to follow the octet rule. So the chlorine can have 10 electrons.

 

[bobbie212]

with the dot structures

grp 5A (P,As, Sb) can share 10electrons

grp 6A (S, Se, Te) can share 12

grp 7A (Cl, Br, I) can share 14

 

[Templar]

n - 5 part 2. Explain why the first ionisation energy of an atom of oxygen is slightly lower than that of an atom of nitrogen, despite being further across the period.

Nitrogen has all 2p electrons with the same spin. Oxygen has 3 up and one down. Removing that one down decreases the repulsion between different spins, hence it is easier to remove than electrons in nitrogen.

 

[fush]

do we need to know detailed info on all the batteries? like the anode and cathode reaction for the different types?


all i know is that there are 3 different types, primary (dry cell, alkeline cell), secondary (lead-acid cell, lithium cell) <-----can be recharged, and fuel cells where the reactans are fuels

 

[allstarr69]

thanks you three

 

[allstarr69]

do we need to know detailed info on all the batteries? like the anode and cathode reaction for the different types?


all i know is that there are 3 different types, primary (dry cell, alkeline cell), secondary (lead-acid cell, lithium cell) <-----can be recharged, and fuel cells where the reactans are fuels

hmm i think we have to know the equations for them all.. in some of the past papers there were questions on them

 

[tennille]

If the reactants of N2 + 3H2 --> 2NH3 were in a 1:6 ratio, what would the partial pressures of the reactants be if Kp=1*10^-4 and the partial pressure of NH3 is 50 atm?

I did a question similar to this, which was in the past exam paper, but the ratio is 1:3. I'm not sure how to do this, because I used exactly the same method, but changed the ratio, and my answer is wrong.

 

[tennille]

hmm i think we have to know the equations for them all.. in some of the past papers there were questions on them

I didn't see any question based on that. Only the lead-acid one but we didn't have to know the equation.

 

[bobbie212]

can some1 please help me wit those questions i asked above

 

[llamalope]

it's over! it's over now! WOOOOOHOOOOOOOOO

that exam was pretty good. I was seriously stressing because i had to work all weekend, and didn't have much time to study, but as soon as i got into the exam, the ink was flowing from my pen so fast, i didn't know what hit me..... I even knew what to do (I think) in the calculations. I'm so happy it's over... on to introductory microeconomics

 

[fush]

yep chem is OVER! it was actually pretty easy i thought
i left bout 25 mins early

 

[Steven12]

its over... i wana comfirm some of my answers


for wavelength of boron question. 75nm
for aluminium using electrolysis 82.9kg
for standard cell potential between zn and cr i think i got 0.02?

For heat of formation of NO, i got like 90 or something
for Enthalpy of change of N2h2, i got like-1101 or something(think i got it wrong)

Multiple choice
what was electron density proportional to?

 

[tennille]

its over... i wana comfirm some of my answers


for wavelength of boron question. 75nm
for aluminium using electrolysis 82.9kg
for standard cell potential between zn and cr i think i got 0.02?

For heat of formation of NO, i got like 90 or something
for Enthalpy of change of N2h2, i got like-1101 or something(think i got it wrong)

Multiple choice
what was electron density proportional to?

I think electron density is proportional to the wave function squared. For the heat of formation of NO, I got like 180.7. Were you given 2 equations for that? Do you do normal or advanced?

 

[Steven12]

normal. but dont you have to half them ? for its for NO and not 2NO

 

[tennille]

normal. but dont you have to half them ? for its for NO and not 2NO

I never thought of that. But when you leave it as 2NO, that's how it's naturally formed. I dunno. You're probably right.

 

[Steven12]

na , you could be right, not sure cos i know question did ask for heat of formation for NO. but i still not sure if i should have halved it ,

but when you normal Enthalpy of Change , if the reactants are 2(h2o) , you have to times that by 2 to get the right result, ,,, but i dont know , what did you get the the question after that,

the n2h2 question

 

[tennille]

na , you could be right, not sure cos i know question did ask for heat of formation for NO. but i still not sure if i should have halved it ,

but when you normal Enthalpy of Change , if the reactants are 2(h2o) , you have to times that by 2 to get the right result, ,,, but i dont know , what did you get the the question after that,

the n2h2 question

Nah, you're right. I looked it up. I can't believe I lost easy marks. Far out. I do advanced, I didn't get that question.