Urgent SL Chemistry Questions

[CeciliaL]

Working through some past Paper 1s and a couple questions make no sense to me! Any clarification to any (or all) would be most appreciated!

Here they are:

The temperature of a 2.0 g sample of aluminum increases from 25 °C to 30 °C . How many joules of heat energy were added? (Specific heat of Al = 0.90 J g−1 K−1 )

A. 0.36

B. 2.3

C. 9.0

D. 11

(Answer: C)

Separate solutions of HCl (aq) and H2SO4 of the same molar concentration and same volume were completely neutralized by NaOH (aq). X KJ and Y KJ of heat were evolved respectively. Which statement is correct?

a) X=Y

b) Y=2X

c) X=2Y

d) Y=3X

(Answer: B)

Thanks in advance

[lexie]

The first one, use the equation Q = mc(delta T)

So, energy = mass (specific heat) (change in temp)

So, Q = 2.0 (5) (0.9) = 9.0

For these q's, keep in mind to make sure the units of specific heat energy correspond to that of temp and mass, and that they could ask you to find joules or kilojoules.

anyway, the answer is 9.0, so C hope that helps.

[iber2011]

1st question: 9 x 0.90 x (change in temp that is 30-25) 5 = 9

[Bishup]

For the second question. Sulfuric acid is bimolecular in that it has 2 hydrogen atoms so the H ion concentration needs to be doubled and as the movement of free ions = kinetic energy = heat energy, then the sulfuric acid is double.

[lexie]

For the second Q,

the equations are:

HCl + NaOH --> H2O + NaCl

(1/2)H2SO4 + NaOH --> H2O + (1/2)Na2SO4

Anyone know what to do from there?

[Bishup]

look at my answer. It's very simple

[CeciliaL]

Thanks everyone! You guys are amazing

I was trying to figure out how I can attach files for the questions with equations that I couldn't copy onto here and I've finally figured it out! Please take a look and help me out again.

Answer to first (question 3): B

Answer to second (question 23): A

Cheers

[Drake Glau]

Hey guys,

I was trying to figure out how I can attach files for the questions with equations that I couldn't copy onto here and I've finally figured it out! Please take a look and help me out again.

Answer to first (question 3): B

Answer to second (question 23): A

Cheers

Law of conservation of mass? Mass+mass=end mass. There's only one product so all mass has to go to that "theoretically"

2nd quest, I don't know. It's not working in my head =/ I feel like they're all equal because of dissociation and the fact that it can go back the other way. Did this question happen to come with a Kc?

Edited May 7, 2011 by Drake Glau

[Bishup]

Ok let me start with the second question. Acid PH depends on the concentration of H+ ions that means that equilibrium is shifted either to the left or right depending on how much is added or taken away. Going from the same principle as my previous answer for the question last night. Sulfuric acid is bimolecular. When this strong acid dissociates it will produce twice as many H+ ions than any other acid for 2 reasons. 1 it has 2 hydrogens and 1 because it is a strong acid so therefore fully dissociates or almost. No equilibrium sign for a strong acid. Please when you embark on such a question analyse the nitty gritty and notice that ohhhh dude wow there are 2 hydrogens.

Edit : Sorry it's not Sulfuric and it doesn't look strong because its carbonic but my point still stands about being bimolecular.

I'll let someone else answer the 1st question.

[dessskris]

qn3)

Ar(Fe)=55.85

Ar(S)=32.06

n(Fe)=mass/Ar

=10/55.85

=0.179 mol

n(S)=mass/Ar

=10/32.06

=0.312 mol

So Fe is the limiting reagent.

n(FeS)=n(Fe)

=0.179 mol

mass(FeS)=n*Mr

=0.179*(55.85+32.06)

=(10/55.85)*(55.85+32.06)

=(10/55.85)*(87.91)

=87.91*10.0/55.85 (B)

[CeciliaL]

mass(FeS)=n*Mr

=0.179*(55.85+32.06)

=(10/55.85)*(55.85+32.06)

I'm sorry I'm not very good with Math! If you could please explain how you went from the step above to the following one...?

[dessskris]

Actually if the question asks you to just give the final numerical answer, you can just calculate 0.179*(55.85+32.06) but it's paper one, you cannot use calculator and the question wants you to give the rough working, so you need to get back those numbers. You initially got your 0.179 from 10/55.85 that's why you substitute it back in.

[CeciliaL]

Looks like I'll be trying to take advantage of the awesome brains around here as much as I can till my exam tomorrow!

Here's a redox equation answer and it's answer (attached).

Somehow when I balance it I get 4 of Ag not 3. Help??

[dessskris]

I feel exploited haha just kidding!

I think it's because the net oxidation number of NO₃^- is -1 and the oxidation number of 4H⁺ is +4. So on the left hand side the net oxidation no. is +3.

Look at the right hand side, NO and 2H₂O both give oxidation no. of 0, so the +3 must come from the Ag⁺. Hence the 3Ag⁺ on the right hand side, and the 3Ag on the left hand side.

Maybe someone else can explain this better?

Edited May 8, 2011 by dessskris

[CeciliaL]

I feel exploited haha just kidding!

I think it's because the net oxidation number of NO₃^- is -1 and the oxidation number of 4H⁺ is +4. So on the left hand side the net oxidation no. is +3.

Look at the right hand side, NO and 2H₂O both give oxidation no. of 0, so the +3 must come from the Ag⁺. Hence the 3Ag⁺ on the right hand side, and the 3Ag on the left hand side.

Maybe someone else can explain this better?

Blown away! Genius as usual Thanks!

part b) i) of this newly posted question anyone? I can't see how the final temperature is 73 and the initial 48.2...?

Nov 2010 (dragged).pdf

[dessskris]

Ah thank you

Look at the point where the time is 100 s (that is when the reaction takes place).

Final temp is 73.0°C (by looking at the graph or by using the equation of the straight line given). Initial temp is 24.8°C (from the table). So the temp change is 73.0-24.8=48.2°C.

[CeciliaL]

Ah thank you

Look at the point where the time is 100 s (that is when the reaction takes place).

Final temp is 73.0°C (by looking at the graph or by using the equation of the straight line given). Initial temp is 24.8°C (from the table). So the temp change is 73.0-24.8=48.2°C.

I must be so annoying, I understand! But it's crunch time and I'm super stressed Another paper 1 question attached... (Answer: C)

Your help is much appreciated :D

[dessskris]

No you're not a lot of people have helped me and it's time for me to pay it forward my semester exams are in 1 week anyway so I need to revise too

I think the kinetic energies are all the same because the temperature is the same. They are put in one same container, right? Therefore the kinetic energy must be the same. I am not sure though. I don't know! I think it's because of the same temperature but I'm not so sure.. I would wait till somebody else can give better answer.

[Drake Glau]

Looks like I'll be trying to take advantage of the awesome brains around here as much as I can till my exam tomorrow!

Here's a redox equation answer and it's answer (attached).

Somehow when I balance it I get 4 of Ag not 3. Help??

I think Desy pointed out why you were getting 4 already but...

Ag+NO₃^-->Ag⁺+NO

NO₃^-->NO

Balance oxygen with water

NO₃^-->NO+2H₂O

Balance hydrogens with H⁺

4H⁺+NO₃^-->NO+2H₂O

balance charges with electrons (I think this is where you messed up and got four )

3e^-+4H⁺+NO₃^-->NO+2H₂O

Ag->Ag⁺

Nothing to balance but electrons

Ag->Ag⁺+e^-

3e^-+4H⁺+NO₃^-->NO+2H₂O

3(Ag->Ag⁺+e^-)

When you combine them the electrons need to completely cancel out so you need to multiple the silver equation by 3

3e^-+4H⁺+NO₃^-->NO+2H₂O

3Ag->3Ag⁺+3e^-

From here you can start cancelling (which is only the electrons in this case)

3e^-+4H⁺+NO₃^-->NO+2H₂O

3Ag->3Ag⁺+3e^-

Combine it all together and make sure your original stuff given goes before the added water and electrons (don't ask why, IB likes it...)

NO₃^-+3Ag+4H⁺->NO+3Ag⁺+2H₂O

I hope I didn't miss anything. Too much bbc code

_____________________________________________________________________

Ah thank you

Look at the point where the time is 100 s (that is when the reaction takes place).

Final temp is 73.0°C (by looking at the graph or by using the equation of the straight line given). Initial temp is 24.8°C (from the table). So the temp change is 73.0-24.8=48.2°C.

I must be so annoying, I understand! But it's crunch time and I'm super stressed Another paper 1 question attached... (Answer: C)

Your help is much appreciated :D

A

Average kinetic energy would be the kinetic energy/number of particles

1mol is 6.02*10^23

2mol is 2(6.02*10^23)

3mol is 3(6.02*10^23)

The kinetic energy is the same since temperature is the average kinetic energy of the system.

So CO₂ would have (ideally) 1/3 the kinetic energy because it ends up being divided by the ratio of 3. Hope that made sense also...

Edited May 8, 2011 by Drake Glau

[CeciliaL]

I think Desy pointed out why you were getting 4 already but...

Ag+NO₃^-->Ag⁺+NO

NO₃^-->NO

Balance oxygen with water

NO₃^-->NO+2H₂O

Balance hydrogens with H⁺

4H⁺+NO₃^-->NO+2H₂O

balance charges with electrons (I think this is where you messed up and got four )

3e^-+4H⁺+NO₃^-->NO+2H₂O

Ag->Ag⁺

Nothing to balance but electrons

Ag->Ag⁺+e^-

3e^-+4H⁺+NO₃^-->NO+2H₂O

3(Ag->Ag⁺+e^-)

When you combine them the electrons need to completely cancel out so you need to multiple the silver equation by 3

3e^-+4H⁺+NO₃^-->NO+2H₂O

3Ag->3Ag⁺+3e^-

From here you can start cancelling (which is only the electrons in this case)

3e^-+4H⁺+NO₃^-->NO+2H₂O

3Ag->3Ag⁺+3e^-

Combine it all together and make sure your original stuff given goes before the added water and electrons (don't ask why, IB likes it...)

NO₃^-+3Ag+4H⁺->NO+3Ag⁺+2H₂O

I hope I didn't miss anything. Too much bbc code

A

Average kinetic energy would be the kinetic energy/number of particles

1mol is 6.02*10^23

2mol is 2(6.02*10^23)

3mol is 3(6.02*10^23)

The kinetic energy is the same since temperature is the average kinetic energy of the system.

So CO₂ would have (ideally) 1/3 the kinetic energy because it ends up being divided by the ratio of 3. Hope that made sense also...

Thanks for clarification of the first question. I understand it perfectly now

However, for the second question the correct answer is C not A...