VCE Chemistry Question Thread

[jyce]

I'm really lost with this (Q1 spectra 1 from 2015 Lisachem exam) . I was under the impression that the peak (unless wary of isotopes) that has highest m/z is molecular ion, and hence is molar mass of compound? I thought I remembered being tricked by a tiny peak before, so I was looking at the 76 m/z or even 75. But why is the molecule 74g.mol? Maybe I'm over thinking this, but I'm really not sure.

Unfortunately I am unable to attach anything (nor see anybodies attachments anymore) but to best describe the last few peaks of the spectra- a large one at 74 m/z, quite a small one at 75 and a barely visible one at 76.

When do I assume that the highest m/z are isotope caused? Thanks

What you're describing are three molecular ion peaks, all due to a different combination of isotopes. If they asked for one only molar mass, I would use the largest of the three peaks as this is the most common molar mass of the sample. So, to answer your question, you would assume that you have multiple molecular ion peaks when they're really close together and some of them are quite small. Besides, a molar mass of 74 g mol^-1 would be propanoic acid, and you'd be struggling to somehow make that into a molar mass of 75 or 76 g mol^-1.

[Alter]

Spoiler

Q7: No idea

It's A. Double the kJ/mol for the equation 2CuO(s) → Cu2O(s) + ½O2(g) as you're using double mol for everything.

Q8: can you explain why the concentration of hydrogen gas is higher than Iodine? and why is it that the concentration of HI increase more than all the products?

We went through this question in class once, but it's still giving me trouble now. I'd say you're meant to look mainly at the fact that HI has increased more compared to H2 and I2 concentrations and that this was the distinguishing factor between B and C. This is due to a higher initial concentration of the HI.

Q12: Still massively struggling with pH. Someone save me please, how can I get better at pH stuff?

Always bring it back to your mathematical understanding of what pH is, I suppose. That is, the concentration of H+ ions in a solution. You know concentration is mol/vol. "I" is wrong because less pH = more acidic, which would imply more H+ ions. This isn't the case. III is just wrong because you know weak bases still partially ionise in water. Therefore, it must have at minimum some propanoate ions.

Q16: I chose C but idk why its D?

I think the examiner's report explains this fairly well. The product at the cathodes can't be the same if you go through step-by-step. Cathode = site of reduction, reduction is gain of electrons. If there's an aqueous solution, you must factor in water being a reactant, so this is what makes the difference between molten and aqueous.

Q18: Chose A, answer is D. Isnt it in galvanic cells, the negative electrode = oxidation and positive electrode = reduction?

Yeah, that is definitely true for galvanic cells. Maybe you got the order of reductant strength reversed? That is, you said P was the strongest reductant instead of R. That's the only way you could get A, I think.

Q19: the last three options all produce 2 moles of CO2 per mole of the fuel?

Yes, but this is still for 1 mol of the respective fuels. I'm not sure what you're trying to say here :/
If you divide the mol of CO2 by electrons, you get 6/6/7/5 respectively for A/B/C/D. Obviously 5 and 7 stand out, but you want D because you desire a greater amount of CO2 at the end of the day (from the same amount of electricity).

edit: absolutely late but still posting because i typed it all out

[Bruzzix]

If you haven't done the 2014 VCAA exam, don't read:

Spoiler

For the last question (question 12) where it was about the effect of different catalysts on the enthalpy of a reaction I wrote the following for experimental outcomes and I'm wondering if I would be awarded the mark:
"Catalysts provide an alternative energy pathway of lower activation energy and thus increase the likelihood of H₂O₂ particles decomposing to H₂O and O₂ at any given moment. This therefore increases the rate of reaction and since the reaction is exothermic, heat will accumulate at a faster rate. Different catalysts increase the rate of reaction to different extents so it is expected that the heat will accumulate faster in a beaker with a catalyst that lowers the activation energy more than another catalyst in another beaker."

VCAA's answer was way different to mine... they said:

Different catalysts will have no effect on the molar heat of reaction since the molar enthalpy of the
decomposition reaction of hydrogen peroxide is independent of the catalyst used.
 The same molar heat of reaction should be determined irrespective of the catalyst used, assuming all other
variables are controlled.
 A catalyst has no effect on the relative enthalpies of the reactants and products, hence does not affect the molar
heat of reaction.
 The same molar enthalpy should be determined for the decomposition of H2O2, irrespective of the catalyst
used.

I kind of disagree with their answers because although they are correct, the question asked for experimental outcomes and what I described is what I think would be a likely experimental outcome of using different catalysts, since one catalyst will speed up the reaction more than the other. By extension, over a specific time period, one beaker will have a higher temperature than the other as a result despite equal molar enthalpies. What do you think guys? 

[jyce]

If you haven't done the 2014 VCAA exam, don't read:

Spoiler

For the last question (question 12) where it was about the effect of different catalysts on the enthalpy of a reaction I wrote the following for experimental outcomes and I'm wondering if I would be awarded the mark:
"Catalysts provide an alternative energy pathway of lower activation energy and thus increase the likelihood of H₂O₂ particles decomposing to H₂O and O₂ at any given moment. This therefore increases the rate of reaction and since the reaction is exothermic, heat will accumulate at a faster rate. Different catalysts increase the rate of reaction to different extents so it is expected that the heat will accumulate faster in a beaker with a catalyst that lowers the activation energy more than another catalyst in another beaker."

VCAA's answer was way different to mine... they said:

Different catalysts will have no effect on the molar heat of reaction since the molar enthalpy of the
decomposition reaction of hydrogen peroxide is independent of the catalyst used.
 The same molar heat of reaction should be determined irrespective of the catalyst used, assuming all other
variables are controlled.
 A catalyst has no effect on the relative enthalpies of the reactants and products, hence does not affect the molar
heat of reaction.
 The same molar enthalpy should be determined for the decomposition of H2O2, irrespective of the catalyst
used.

I kind of disagree with their answers because although they are correct, the question asked for experimental outcomes and what I described is what I think would be a likely experimental outcome of using different catalysts, since one catalyst will speed up the reaction more than the other. By extension, over a specific time period, one beaker will have a higher temperature than the other as a result despite equal molar enthalpies. What do you think guys? 

Hi Bruzzix,

Spoiler

You wouldn't be awarded the mark for this, as this is not what they wanted. Time is not a consideration in this experiment: I would assume the student lets each of the two reactions go to completion, even if the reactions take different amounts of time which they probably would as you pointed out, and then compares the overall change in temperature. The overall change in temperature would be the same as catalysts do not change the energy of the reactants or the energy of the products. This is what VCAA wants. I think you've actually over analysed!

[Bruzzix]

Hi Bruzzix,

Spoiler

You wouldn't be awarded the mark for this, as this is not what they wanted. Time is not a consideration in this experiment: I would assume the student lets each of the two reactions go to completion, even if the reactions take different amounts of time which they probably would as you pointed out, and then compares the overall change in temperature. The overall change in temperature would be the same as catalysts do not change the energy of the reactants or the energy of the products. This is what VCAA wants. I think you've actually over analysed!

Thank you Jyce! My overall mark for this exam was 103-106/120. What study score do you think this would correlate to?

[jyce]

Thank you Jyce! My overall mark for this exam was 103-106/120. What study score do you think this would correlate to?

I'm not great at predicting study scores, and I don't even know what score I achieved on last year's exam in order to relate it to my SS. However, that's a lowish A+ and I imagine you'd be scoring somewhere between 40-45? That's definitely a guess, though!

[chekside]

Is it definite that they will never ask questions regarding the specifics of sulfuric acid/ethene or whatever was studied unless all the information is provided?

[jyce]

Is it definite that they will never ask questions regarding the specifics of sulfuric acid/ethene or whatever was studied unless all the information is provided?

Yes.

[bonjour-sarah]

Apologies more questions~
From Revision questions post a few years back~

Unit 3~
1. What further experiment (following a titration)  should be performed so that the exact concentration of compound X can be found? Consider HPLC, AAS, and Uv-Vis.

Unit 4~
1. How does reaction rate change with respect to time if the reaction is a) exothermic b) endothermic
(Not sure i understand what this question was asking. Is there a difference of normal reaction rate? Doesn't rate both just decrease as time goes on?  (was under 'Reaction rates' section, not anything specific))

2. Explain why pure water has pH 7

3. Using LCP, explain effect of adding water if water is also a reactant, to an equilibrium system? (what factor takes priority? dilution, addition of reactant, volume?)

4.Difference between concentration fraction and equilibrium constant?

5.Difference between the term 'renewable' and 'sustainable' energy sources

6. Difference between heat capacity and heat of combustion

7. Two ways actual value of calibration factor could be lower/ higher than calculated value

8. Three ways a calorimeter can be calibrated??

9. Define potential difference?

10. Explain the properties that an electrode must have (just conductor and inert?)

11. What is a faraday? / ->Faradays laws (Are these on course?)

12. Why are fuel cells considered more efficient than power stations?

[jyce]

Apologies more questions~
From Revision questions post a few years back~

Unit 3~
1. What further experiment (following a titration)  should be performed so that the exact concentration of compound X can be found? Consider HPLC, AAS, and Uv-Vis.

Unit 4~
1. How does reaction rate change with respect to time if the reaction is a) exothermic b) endothermic
(Not sure i understand what this question was asking. Is there a difference of normal reaction rate? Doesn't rate both just decrease as time goes on?  (was under 'Reaction rates' section, not anything specific))

2. Explain why pure water has pH 7

3. Using LCP, explain effect of adding water if water is also a reactant, to an equilibrium system? (what factor takes priority? dilution, addition of reactant, volume?)

4.Difference between concentration fraction and equilibrium constant?

5.Difference between the term 'renewable' and 'sustainable' energy sources

6. Difference between heat capacity and heat of combustion

7. Two ways actual value of calibration factor could be lower/ higher than calculated value

8. Three ways a calorimeter can be calibrated??

9. Define potential difference?

10. Explain the properties that an electrode must have (just conductor and inert?)

11. What is a faraday? / ->Faradays laws (Are these on course?)

12. Why are fuel cells considered more efficient than power stations?

Unit 3
1. This would depend on the analyte. However, I imagine UV-visible spectroscopy would be useful in many cases: it is able to analyse atoms, ions and molecules, and it is far more accurate than volumetric analysis which only uses laboratory equipment and is highly subject to human error.

Unit 4
1. From my understanding, the reaction rates of both exothermic and endothermic reactions will decrease over time as there will be fewer and fewer reactant particles.

2. At 25^oC, pure water has a pH of 7 because [H⁺] = 10^-7 M. At other temperatures, the pH of pure water is not 7 because its percentage ionisation changes, although pure water is still neutral at other temperatures since [H⁺] still = [OH^-].

3. If water is in the liquid state, you would ignore water's presence in the equation and consider the impact of the dilution. If water is in the solid or gaseous state, then you would consider the increase in concentration of water.

4. The equilibrium constant, at a given temperature, is the value of the concentration fraction when the system is at equilibrium.

5. A renewable energy source is one that is easily replenished in a short period of time. A sustainable energy source is one that is able to meet the energy demands of society into the future without depletion. Therefore, by definition, sustainable energy sources are renewable.

6. Specific heat capacity is the amount of energy needed to raise the temperature of 1 g of a substance by 1 ^oC. Heat of combustion refers to the amount of energy released when 1 mol of a substance is combusted.

7. The calibration factor would be affected by: changing the insulation, changing the solution in the calorimeter, and changing the volume of solution in the calorimeter, but NOT by changing the source of energy used for calibration.

8. I can only think of two main ways to calibrate a calorimeter: using electrical energy from a heater as your known amount of energy, or using a reaction with a known enthalpy change.

9. Potential difference is basically a measure of the amount of electrical energy transformed into some other form of energy, per coulomb of charge.

10. Electrodes obviously need to be conductors, or how would electricity flow through them? They may or may not be inert, depending on the particular cell. Also, electrodes in fuel cells are usually porous and catalytic.

11. One faraday = one mole of electrons. And yes, Faraday's laws are examinable in this course. Faraday's first law is basically just that the greater the electrical charge in a cell, the greater the amount of products. Faraday's second law basically just describes the fact that electrons react in mole ratios. These are really simple, and I doubt VCAA will actually ask you to define either of them.

12. Whenever energy is transformed (i.e. changed from one form into another), some of the energy is always lost in the form of heat. In fuel cells, chemical energy is directly transformed into electrical energy, so with only one transformation there is only one opportunity to lose energy. In power stations, on the other hand, there are three energy transformations (chemical --> thermal --> mechanical ---> electrical) and therefore more energy is lost.

[Bruzzix]

I had a few questions:

How do we use the Ka values given for each of the indicators in the data book and what are they there for?

If we have an extremely low concentration of H+ in solution, for example 10^-15M, the pH would be 15? How is this possible if pH only goes from 0 to 14? Likewise if we have a really high concentration such as 10M, then the pH is calculated as -1? Again how is this possible?

What is the pH at the equivalence point of the following titrations: weak acid-strong base, strong-acid-weak base, strong acid-strong base?

[jyce]

I had a few questions:

How do we use the Ka values given for each of the indicators in the data book and what are they there for?

If we have an extremely low concentration of H+ in solution, for example 10^-15M, the pH would be 15? How is this possible if pH only goes from 0 to 14? Likewise if we have a really high concentration such as 10M, then the pH is calculated as -1? Again how is this possible?

What is the pH at the equivalence point of the following titrations: weak acid-strong base, strong-acid-weak base, strong acid-strong base?

1. Acid-base indicators act as weak-acids. You would use acidity constants of indicators in the same way you normally use acidity constants. Take a look at VCAA 2007 Exam 1, Section B, Question 2 for an example.

2. As you have demonstrated, it is definitely possible to have a pH above 14 or below 0. But my understanding is that the pH scale becomes useless/unreliable outside the conventional 0-14 range and we're better off using the concentration of the acid/base itself, rather than placing it on a logarithmic scale. Acids with a pH below 0 and bases with a pH above 14 have very high concentrations, which are easy to work with.

3. Assuming 25^oC:
- weak acid + strong base = pH > 7, as there is some basic product
- strong acid + weak base = pH < 7, as there is some acidic product
- strong acid + strong base = pH of 7, since the products are neutral

[cosine]

For Chemistry exam Unit 4 2007:

Question 4c: CH_3OH + C_{17}H_{35}COOH -> CH_3OCOC_{17}H_{35} + H_2O would this be accepted? the only difference is the positioning of the OCO and theirs COO, and so my ester bond was OCO?

Question 5b:
I circled just a C-N bond, is this still a peptide bond or am I wrong? Answer:

[jyce]

For Chemistry exam Unit 4 2007:

Question 4c: CH_3OH + C_{17}H_{35}COOH -> CH_3OCOC_{17}H_{35} + H_2O would this be accepted? the only difference is the positioning of the OCO and theirs COO, and so my ester bond was OCO?

Question 5b:
I circled just a C-N bond, is this still a peptide bond or am I wrong? Answer: (Image removed from quote.)

4c. Ester linkages are normally represented semi-structurally as 'COO', so I think this would not be accepted.

5b. A peptide bond also includes the oxygen double bonded to the carbon and the hydrogen bonded to the nitrogen (i.e. CONH), so you are unfortunately wrong.

[cosine]

For Chemistry exam Unit 4 2007:


Question 7b: I thought that hydrogen gas was the stronger reductant because it was oxidised and released 2 electrons, as then why would there be a continuous supply of the gas? Why is the answer Cd?

Question 7ciii). I said 0 because the product at the half cell 1 is actually X(s) forming onto the electrode, hence its an element so the oxidation number would be zero? I know the diagram says the electron flow is going to the right, so that means the X2+ is donating electrons to become X3+ so its oxidation state would be 3, but if you read the stem of the question, it says that the molarity of X2+ was decreasing, so wouldnt that mean that X(s) is forming on the electrode as the concentration of X2+ decreases?



Also when you have something large like my previous question with the methanol and a fatty acid, how do you know how to write the chemical equation? Like i wrote OCO instead of COO because thats the configuration, like in order of the bonds on the molecule itself?