Chemistry 3/4 2013 Thread

[barydos]

So uhh, transesterification involves hydrolysis then esterification right?
is it 1. trigylceride + water --> glycerol + fatty acid molecules
2. fatty acid molecules + methanol (or something) --> fatty acid methylester (FAME) + water?

in the textbook it shows an overall equation, triglyceride + methanol (with KOH solution) --> glyceral + FAME ... so are they both correct?


Oh and what's the difference between AAS, UV-Vis and Colorimetry?

[Stick]

I just wanted to double-check something.

1. When a question tells you that a redox equation is predicted to occur based on the electrochemical series, but doesn't in reality, the reason has to do with reaction rate, NOT with non-standard conditions.

2. When a question tells you that a galvanic cell is predicted to result in a particular voltage, but the voltage is lower in reality, the reason has to do with non-standard conditions, NOT with reaction rate.

Are there any other reasons? Also, when is 'side reactions occur' a viable answer?

Thanks.

[lzxnl]

I just wanted to double-check something.

1. When a question tells you that a redox equation is predicted to occur based on the electrochemical series, but doesn't in reality, the reason has to do with reaction rate, NOT with non-standard conditions.

2. When a question tells you that a galvanic cell is predicted to result in a particular voltage, but the voltage is lower in reality, the reason has to do with non-standard conditions, NOT with reaction rate.

Are there any other reasons? Also, when is 'side reactions occur' a viable answer?

Thanks.

Generally, non-standard conditions don't affect the ordering too much. Look up "Nernst equation" for more details.
E_cell=E^o_cell+RT*ln(Q)/nF

E_cell is the electrode potential difference
E^o_cell is the standard potential difference, found by reading off the electrochemical series
where n is the number of electrons transferred per reaction, Q is the reaction quotient/concentration fraction, using pressures of gases in bars (100 kPa), concentrations in M and assigning values of pure liquids and solids of 1; F is 96500 C/mol, Faraday's constant, and R is the ideal gas constant.
If we have T=298 K, then the large number of symbols becomes 0.0257*ln(Q)/n

As you can see, Q has to increase incredibly for the actual cell potential to change by much.
To be honest, I still don't quite get though why at 5 M, the oxidation of chloride is preferred...this equation says that the reduction potential of chloride ion at 5 M relative to SHE would only increase to 1.25 V, still lower than that of water. I'm presuming that when saturated, activity coefficients are no longer close to one, so the activities in the reaction quotient may no longer be directly replaced with concentrations...

[Stick]

Generally, non-standard conditions don't affect the ordering too much. Look up "Nernst equation" for more details.
E_cell=E^o_cell+RT*ln(Q)/nF

E_cell is the electrode potential difference
E^o_cell is the standard potential difference, found by reading off the electrochemical series
where n is the number of electrons transferred per reaction, Q is the reaction quotient/concentration fraction, using pressures of gases in bars (100 kPa), concentrations in M and assigning values of pure liquids and solids of 1; F is 96500 C/mol, Faraday's constant, and R is the ideal gas constant.
If we have T=298 K, then the large number of symbols becomes 0.0257*ln(Q)/n

As you can see, Q has to increase incredibly for the actual cell potential to change by much.
To be honest, I still don't quite get though why at 5 M, the oxidation of chloride is preferred...this equation says that the reduction potential of chloride ion at 5 M relative to SHE would only increase to 1.25 V, still lower than that of water. I'm presuming that when saturated, activity coefficients are no longer close to one, so the activities in the reaction quotient may no longer be directly replaced with concentrations...

Thanks, but I'm still a bit confused. Does that mean that both of my numbered points are wrong? What are the right explanations that would be appropriate to provide in a VCE exam then? And also, what do you mean by "the ordering"?

[lzxnl]

I just wanted to double-check something.

1. When a question tells you that a redox equation is predicted to occur based on the electrochemical series, but doesn't in reality, the reason has to do with reaction rate, NOT with non-standard conditions.

2. When a question tells you that a galvanic cell is predicted to result in a particular voltage, but the voltage is lower in reality, the reason has to do with non-standard conditions, NOT with reaction rate.

Are there any other reasons? Also, when is 'side reactions occur' a viable answer?

Thanks.

Thanks, but I'm still a bit confused. Does that mean that both of my numbered points are wrong? What are the right explanations that would be appropriate to provide in a VCE exam then? And also, what do you mean by "the ordering"?

"Ordering" here refers to the order of the electrochemical series; you won't ever see sodium ions being weaker oxidants than lithium ions, for instance. The gap is too large.
Generally, the conditions have to be highly non-standard, like with different temperatures or even solvents, for the electrochemical series to be invalid. I think the explanation with reaction rates is fine. Take hydrogen peroxide as the classic example.

Lower voltages would ALWAYS mean non-standard conditions, assuming your voltmeter is accurate.
Reaction rate means current, not voltage.

[Stick]

Alright, thanks.

[Scooby]

Some questions from the multiple choice section of VCAA 2009 Exam 2

Question 13
Why does the oxidation reaction take place in region X rather than the reduction reaction?

Question 20
I get that iron is a stronger reductant than chloride and would be oxidised preferentially, but why does that matter? Aren't we just trying to extract lithium metal?

[SocialRhubarb]

Q13. The concentration of oxygen dissolved in water is much less than the concentration of oxygen in air, so there is not enough oxygen to support the reduction of oxygen and water to form hydroxide ions, so the reaction occurring in region X must be oxidation of iron.

Q20. The question asks for the 'best choice'. The iron rod isn't so much a 'wrong' choice as a worse choice than carbon.

[lolipopper]

for this question i dont understand what i did wrong.

i did the following:
1. worked out total coulombs of electricity possible.
2. because it is only 75% efficient, i took out 75% of the total coulombs.
3. i used the Q= n(e-) x F to work out moles of electrons.
4. there was a given equation of "CH4(g) + 2H2O(l) → CO2(g) + 8H+(aq) + 8e-" and so the moles of CH4 had to be 1/8th the moles of e-.
5. then i used n=v/24.5 rule for SLC to work out the volume.

PLEASE HELP ME!

[lzxnl]

for this question i dont understand what i did wrong.

i did the following:
1. worked out total coulombs of electricity possible.
2. because it is only 75% efficient, i took out 75% of the total coulombs.
3. i used the Q= n(e-) x F to work out moles of electrons.
4. there was a given equation of "CH4(g) + 2H2O(l) → CO2(g) + 8H+(aq) + 8e-" and so the moles of CH4 had to be 1/8th the moles of e-.
5. then i used n=v/24.5 rule for SLC to work out the volume.

PLEASE HELP ME!

What's the original question?

I think you misinterpreted the 75% efficient.
Let's say that you need 10000 moles of electrons. If it's 75% efficient, then 10000 moles of electrons IS 75% of the total amount of electricity. You divide by 0.75

[Edward21]

What's the original question?

I think you misinterpreted the 75% efficient.
Let's say that you need 10000 moles of electrons. If it's 75% efficient, then 10000 moles of electrons IS 75% of the total amount of electricity. You divide by 0.75

With these you just gotta think it out logically, if you need a certain amount, but the process is less than 100% (almost everything..) then you'll need more of the reactants or whatever just to get that amount, so multiply by 100/75 (or divide by 0.75) as we want 100% but we only have 75% (percentage conversion or efficiency)

[Jaswinder]

Which molecule would you expect to have the shortest retention time in the chromatography column?
 Explain your answer.

2-chloro-3-methylpentane, would move quicker through the column due to a lower mass when compared to 2-chloro-3-methylhexane.

the answers say 2-chloro-3-methylpentane because it has smaller molecules/lower molecular mass and so has weaker dispersion force attraction to the stationary phase and travels faster through the column.

Would I still get the mark or do I have to be more precise?

Also the attached multiple choice 

[brightsky]

unfortunately I don't think you'd get full marks. yes 2-chloro-3-methylpentane may have a lower molecular mass than 2-chloro-3-methylhexane (and be careful how you word it, it's lower MOLECULAR mass, not just mass), but so what? WHY does lower molecular mass mean lower retention time? you have to mention stationary phase in your response, and talk about how stuff of low molecular mass would form weaker forces of attraction with the stationary phase, and hence move faster, and hence have a lower retention time.

the answer is A. you just have to know the name. they're obviously not carbohydrates (because they involve ring systems and what not), obviously not carboxylic acids (because they don't have a carboxyl group, only ester group) and obviously not monoglycerides (which is basically like triglyceride except you react glycerol with only 1 fatty acid, instead of 3).

[lzxnl]

Oh this exam. I did that one yesterday actually.

They're esters...and they have a CH3 group, so methyl ester seems fine for the name.

As for the chromatography question, it's more that the higher molecular mass means more electrons, and with more electrons comes greater instantaneous dipoles and thus dispersion forces (I wonder what would happen if VCAA asked what dispersion forces were on the exam...that would be interesting). It's more that these dispersion forces are the ONLY way the non-polar alkanes can bond to anything.

[brightsky]

Oh this exam. I did that one yesterday actually.

They're esters...and they have a CH3 group, so methyl ester seems fine for the name.

As for the chromatography question, it's more that the higher molecular mass means more electrons, and with more electrons comes greater instantaneous dipoles and thus dispersion forces (I wonder what would happen if VCAA asked what dispersion forces were on the exam...that would be interesting). It's more that these dispersion forces are the ONLY way the non-polar alkanes can bond to anything.

okay this is a minor point but does higher molecular mass necessarily entail higher electron count? molecular mass is tied to number of nucleons, and while number of protons = number of electrons, surely higher molecular mass does not automatically imply higher electron count, especially when you move down the periodic table, where the elements have a lot of neutrons than protons. vcaa, and the textbook, seems to think that higher molecular mass = greater dispersion forces, but my teacher skewered them for that...higher electron count = greater dispersion forces. there seems to be a subtle difference?