Unit 4 Questions MEGATHREAD!

[Graphite]

I would go with A, how do you measure the mass of CH3COOH?

[Graphite]

The molar volumes of an ideal gas at STP (22.4L/mol) and SLC (24.5L/mol) are not 100% accurate but from suggested solutions of prac exams and stuff, seem to be the more acceptable way of calculating gas related things. Would we actually be penalised if we followed the general gas formula and did

101.325xV=n x 8.31 x 273 (or 298) instead? 

Thanks

EDIT: Inaccurate as in the solutions are sometimes a few digits out of place, don't want to lose SF marks and such now!

They should be the same thing though, the gas constants are derived from the universal gas equation simply by plugging in the 'constants'. So using either is fine

[b^3]

I would go with A, how do you measure the mass of CH3COOH?

Now that I look at it that does make more sense, as if we tried to measure the mass of the CH3COOH by weighing we will just end up meauring the water that we have made. Really it should be measuring the mass lost from the reactants, i.e. the mass lost by the CO2 escaping.

[Graphite]

Actually no, because heat loss can also result in mass loss. The best way would be to use a syringe and measure the volume of CO2 evolved.

[b^3]

I would go with A, how do you measure the mass of CH3COOH?

Now that I look at it that does make more sense, as if we tried to measure the mass of the CH3COOH by weighing we will just end up meauring the water that we have made. Really it should be measuring the mass lost from the reactants, i.e. the mass lost by the CO2 escaping.

Actually no, because heat loss can also result in mass loss. The best way would be to use a syringe and measure the volume of CO2 evolved.

I was agreeing with you and stating why my logic was wrong. Thats what the "Now that I look at it that does make more sense" part meant.

[Graphite]

I thought you meant you can measure the change in mass to calculate the amount of CO2 evolved. But in any case, some solutions may accept that.

[david10d]

The unit 4 checkpoints seems really dodgy and i think i should move away from it lol

Checkpoints wasn't that bad and it can be done in just over a week, but then I have expirenced the horror of methods checkpoints.

i'm speeding through it. it seems like the non-vcaa ones are a lot easier than what i would expect for external exams.

[b^3]

The unit 4 checkpoints seems really dodgy and i think i should move away from it lol

Checkpoints wasn't that bad and it can be done in just over a week, but then I have expirenced the horror of methods checkpoints.

i'm speeding through it. it seems like the non-vcaa ones are a lot easier than what i would expect for external exams.

Yeh that just over a week was after whole days of school, so it you know you stuff then now (in the holidays) it won't take that long

[Mao]

Actually no, because heat loss can also result in mass loss. The best way would be to use a syringe and measure the volume of CO2 evolved.

Heat loss? How does heat loss result in mass loss?
I don't think this reasoning is right.


Here are my thoughts:

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

To properly assess this reaction, let's assume somethings:
-ethanoic acid is in excess, and the concentration is quite high. (Let's say, 0.1M to 1M)
-we add solid pieces of sodium bicarbonate. Let's assume we add significant amount so a balance can detect it (say, about 1 g)
-the sodium bicarb is in powder form. rate of reaction is fast.

We can rule out measuring the volume of water (B). Because there are no easy way to measure the liquid volume, and I can think of several problems with this method.
Measuring the surface area of sodium bicarb doesn't make sense either (D). I can think of one way to do this (using BET isotherm, postgrad level technique), but it is very far fetched.

Measuring the volume of CO2 would be possible. This would require a good syringe that captures all of the gaseous product of the reaction, which could be challenging. The glassware required will be very expensive, and we haven't even got to the syringe yet. Anyhow, difficult.

The easiest method by far would be to measure the mass of the entire reaction vessel. We will simply expose the reaction vessel (beaker in this case) to air, so all CO2 escape into the atmosphere, the reaction vessel thus becomes lighter. Here, we are measuring the 'mass loss' due to CO2 bubbles. The final mass loss should be ~53% of the mass of sodium bicarb, which is reasonable.

Back to the question. We are not measuring the mass of CH3COOH, we're measuring the mass of the entire system, so C is technically incorrect. We are not directly measuring the volume of CO2 either so A is not right. But at least we are indirectly measuring the volume of CO2, so A seems the most 'reasonable' answer.

TL;DR, I would pick A.


This is spot on:

I would go with A, how do you measure the mass of CH3COOH?

Now that I look at it that does make more sense, as if we tried to measure the mass of the CH3COOH by weighing we will just end up meauring the water that we have made. Really it should be measuring the mass lost from the reactants, i.e. the mass lost by the CO2 escaping.

[Graphite]

If your system is not perfectly isolated then the heat loss will account for the mass loss from chemical transformations

[Mao]

If your system is not perfectly isolated then the heat loss will account for the mass loss from chemical transformations

via what mechanism? Heat doesn't have a mass, and is negligible in the chemistry context ('mass' loss via chemical transformations are always negligibly small. The only time when a closed system undergoes a meaningful mass change is when there is a nuclear reaction.)

[Graphite]

True that but I was just point out that such a thing happens in nature

[Asx4Life]

Can someone help me with  Heinemann unit 4 area of study 2, Question 20. 5CO+I2O5=I2+5CO delta h=-1175kJ. Decreasing T will increase the amount of I2O5 is what the answer says, but I thought decreasing T in exothermic reactions gives more forward reaction.

[Graphite]

Must be an error

[Christiano]

Does temperature increase the rate of movement of particles when talking about rate of reaction? Or does it only give reactant particles more energy to overcome the activation energy barrier?