Chemistry 3/4 2013 Thread

[lolipopper]

Question 10 - VCAA 2008 Sample Exam

Some Gaseous PCl₅ is placed in an empty container.
PCl₅ PCl₃ + Cl₂        K_eq = 2.5 M
When equilibrium is reached, the mass of the gas mixture, compared to the initial mass of PCl₅, is

A   Halved.
B   Unchanged.
C   One and a half times greater.
D   Doubled.

Can someone please tell me the answer?

B. unchanged. this is due to law of conservation of mass.

[lolipopper]

Question: (I can do part a. but not b.)

An important step in the manufacture of sulfuric acid by the Contact process is the oxidation of sulfur dioxide
according to the equation

2SO2(g) + O2(g) <=> 2SO3(g)

a). In an experiment to investigate this reaction at 1000°C, 1.00 mol of SO2 was mixed with 1.00 mol of
     O2 in a 1.00 L vessel. When the mixture reached equilibrium the vessel contained 0.172 mol of SO2.
     Calculate the value of the equilibrium constant, K, for the reaction at 1000°C.

b). To improve the yield of SO3 in this reaction, should the temperature be raised or lowered? Explain
    your choice.

my troubles are that how can i tell whether the reaction is endo or exothermic? or is this supposed to be presumed knowledge? BTW this is taken from NEAP 2007 Unit 3 exam

[barydos]

Question: (I can do part a. but not b.)

An important step in the manufacture of sulfuric acid by the Contact process is the oxidation of sulfur dioxide
according to the equation

2SO2(g) + O2(g) <=> 2SO3(g)

a). In an experiment to investigate this reaction at 1000°C, 1.00 mol of SO2 was mixed with 1.00 mol of
     O2 in a 1.00 L vessel. When the mixture reached equilibrium the vessel contained 0.172 mol of SO2.
     Calculate the value of the equilibrium constant, K, for the reaction at 1000°C.

b). To improve the yield of SO3 in this reaction, should the temperature be raised or lowered? Explain
    your choice.

my troubles are that how can i tell whether the reaction is endo or exothermic? or is this supposed to be presumed knowledge? BTW this is taken from NEAP 2007 Unit 3 exam

I believe the textbook (Heinemann) does teach that the reactions in the sulfuric acid contact process are exothermic.

Either way, this isn't specifically tested in the exam.

Spoiler

"All outcomes in Units 3 and 4 will be examined. All of the key knowledge that underpins the outcomes in
Units 3 and 4, and the set of key skills listed on page 12 of the study design are examinable except
• specific details related to the study of a selected chemical (one of: ammonia, sulfuric acid or nitric acid)."

_{http://www.vcaa.vic.edu.au/Documents/exams/chemistry/chem-specs-samp-w.pdf}

[thushan]

No...that's the effect, not the cause.

The actual cause is the weak attraction alkali metals have for their lone valence electrons, leading to a low activation energy and hence a fast and vigorous reaction.

Note that activation energy is not the same as reducing strength; lithium reacts very slowly with water and in a rather boring matter because its valence electrons is held more strongly due to its proximity to the nucleus, leading to a higher activation energy, but it is more "reactive" than sodium.

That's correct for showing why they react quickly; the violent and exploding nature of the reaction is absolutely got to do with H2 production rate - the rapidly produced H2 causes a buildup of pressure in the reacting medium and the release of that pressure would be sudden, basically an explosion.

[Scooby]

From TSFX Unit 3 2008

Question 13
Which of the following statements regarding monosaccharides is incorrect?
A. The functional group in monosaccharides is the hydroxy group
B. Monosaccharides behave as weak reductants
C. Monosaccharides cannot be hydrolysed to produce smaller units
D. Monosaccharides can be oxidised to produce smaller units

The right answer is B. Doesn't that contradict D though? I mean, glucose is oxidised in cellular respiration... unless it's a strong reductant?


"A weighed mass of hydrated sodium carbonate will contain more sodium carbonate than expected"

This isn't right, is it?



And states for organic reactions...

The answers say that aqueous but-1-ene reacts with gaseous hydrogen bromide, producing aqueous 1-bromobutane. And that in reacting 1-bromobutane with sodium hydroxide, all the reactants and products are aqueous... are these states valid?

[Holmes]

The question asked to circle the peptide bond, and the answer's say that it's wrong if we circle the atoms around the peptide bond as well. Seems a bit dodgy, is this what we have to do for VCAA as well - circle the bond only and not the surrounding atoms?

[thushan]

The question asked to circle the peptide bond, and the answer's say that it's wrong if we circle the atoms around the peptide bond as well. Seems a bit dodgy, is this what we have to do for VCAA as well - circle the bond only and not the surrounding atoms?

I think that's a bit anal; I remember VCAA asking a similar question once and they were ok with circling the atoms.

[Jaswinder]

Methanol is reacted with oxygen in a fuel cell to produce electrical energy. Write a half equation for the oxidation of methnol using an acidic electrolyte in a fuel cell.

what i got: 2CH3OH + 3O2 -> 2CO2+4H2O

what the answer says: CH3OH + H20 -> CO2 + 6H+ + 6e-

Also with questions like, Hydrogen can be produced by electrolysis of a dilute solution of potassium chloride. Give the overall equation for this reaction. what are your thought processes, and how would you answer it?

[Stevensmay]

Jaswinder,
You have written the overall equation for the reaction, not the half equation.
Half equation will show how the electrons move, oxidation or reduction.

[lolipopper]

Methanol is reacted with oxygen in a fuel cell to produce electrical energy. Write a half equation for the oxidation of methnol using an acidic electrolyte in a fuel cell.

what i got: 2CH3OH + 3O2 -> 2CO2+4H2O

what the answer says: CH3OH + H20 -> CO2 + 6H+ + 6e-

Also with questions like, Hydrogen can be produced by electrolysis of a dilute solution of potassium chloride. Give the overall equation for this reaction. what are your thought processes, and how would you answer it?

question 1:
what you wrote is the full reaction including that of the redution. methanol oxidation is going to be a redox half cell
starting with CH3OH as reactant and CO2 as product.
1. CH3OH -> CO2 
2. CH3OH + H20 -> CO2 (balance oxygens by adding water)
3. CH3OH + H20 -> CO2 + 6H+ (balance hydrogens by adding H+ ions)
4. CH3OH + H20 -> CO2 + 6H+ + 6e- (balance charges by adding electrons)

question 2:
well when i think of electrolysis i think anode positive and cathode negative. i also quickly look at the ECS to see the reactant species, searching for strongest oxidant and strongest reductant.
Answer TIME!
so your reacting species are K+ ions, Cl- ions and H2O. looking at the ECS strongest reductant and oxidant is water. (NOTE: the chlorine exception doesn't apply as weak dilute solution).
thus the half reactions that occur are: (written as on ECS)
i) O2 + 4H+ + 4e-  -> 2H2O
ii) 2H2O + 2e- -> H2 + 2OH-
now reverse the oxidation reaction (i), combine the two reactions and balance both sides.
6H2O -> 2H2 + 4OH- + 4H+ + O2

[jgoudie]

C) Hydrolysed means adding water to break it down, this can not happen with monosaccharides (thus correct answer)
D) Oxidised, well think react with oxygen (i know it really means lose electrons) this happens when you burn a monosaccharide which we can do.

Hope this helps.

From TSFX Unit 3 2008

Question 13
Which of the following statements regarding monosaccharides is incorrect?
A. The functional group in monosaccharides is the hydroxy group
B. Monosaccharides behave as weak reductants
C. Monosaccharides cannot be hydrolysed to produce smaller units
D. Monosaccharides can be oxidised to produce smaller units

The right answer is C. Doesn't that contradict D though? I mean, glucose is oxidised in cellular respiration... unless it's a strong reductant?


"A weighed mass of hydrated sodium carbonate will contain more sodium carbonate than expected"

This isn't right, is it?



And states for organic reactions...

The answers say that aqueous but-1-ene reacts with gaseous hydrogen bromide, producing aqueous 1-bromobutane. And that in reacting 1-bromobutane with sodium hydroxide, all the reactants and products are aqueous... are these states valid?

[Scooby]

C) Hydrolysed means adding water to break it down, this can not happen with monosaccharides (thus correct answer)
D) Oxidised, well think react with oxygen (i know it really means lose electrons) this happens when you burn a monosaccharide which we can do.

Hope this helps.

Sorry, I meant B was the right answer (that's why I thought it contradicted D)

[aestheticatar]

Heyyyya,
I don't really understand these questions from a chemology exam.
Detailed explanations would be great!

Ch33rz

[aestheticatar]

In regards to thermochemistry, if they ask 'what would be the energy change resulting from the reaction?', do we only need to include the magnitude (in kJ) or?


Can a pH value be less than 0 or above 14, particularly within VCE/VCAA guidelines? If I received an answer outside of the 0 to 14 range, should I be crapping my pants? I recently came across a question where the answer was 14.4pH.


Consider the following reaction:
CH3COOH(aq)+ NaHCO3 (s) -> NaCH3COO(aq)+ CO2 (g) + H2O(l)
Which of the following properties could best be used to measure the reaction rate?
A. the volume of CO2
B. the volume of H2O
C. the mass of CH3COOH
D. the surface area of NaHCO3

I chose A. However, why is the answer D?

[Limista]

In regards to thermochemistry, if they ask 'what would be the energy change resulting from the reaction?', do we only need to include the magnitude (in kJ) or?

Well energy change is delta H, so the units should be kJ/mol


Can a pH value be less than 0 or above 14, particularly within VCE/VCAA guidelines? If I received an answer outside of the 0 to 14 range, should I be crapping my pants? I recently came across a question where the answer was 14.4pH.

I have my doubts that this was a VCAA question?


Consider the following reaction:
CH3COOH(aq)+ NaHCO3 (s) -> NaCH3COO(aq)+ CO2 (g) + H2O(l)
Which of the following properties could best be used to measure the reaction rate?
A. the volume of CO2
B. the volume of H2O
C. the mass of CH3COOH
D. the surface area of NaHCO3

I chose A. However, why is the answer D?

increasing surface area of one of the reactants increases reaction rate.
A would have been correct, if CO2 was a reactant (because let's not forget that pressure and volume are inversely proportional).