Reaction Rates

[shinny]

Okaaaay I'll get to the point; does increasing the volume of something whilst maintaining constant concentration increasing reaction rate? e.g. Adding another 100 mL of 1M HCl to a mixture of 100mL 1M HCl+Magnesium

Originally I would have thought no, but the TSFX lecturer yesterday said otherwise but I didn't completely understand it...something about how when the HCl is being consumed, the concentration reestablishes itself faster since theres more particles to disperse (but more volume to disperse throughout? hmph) and something about unit per time which she kept repeating but which I didn't really get. Secondly, even if it IS true, do we need to know it for VCE? Whilst it's not covered in any textbook, that doesn't necessarily mean we don't need to know it so yeh...any ideas?

[Collin Li]

Hm.. I think this may be correct because as the reaction proceeds, the amount of hydrochloric acid drops. This effect on the rate of reaction (concentration) is less in 200 mL, than in 100 mL.

However, if the system is already in equilibrium, then nothing will happen.

[Toothpaste]

Okaaaay I'll get to the point; does increasing the volume of something whilst maintaining constant concentration increasing reaction rate? e.g. Adding another 100 mL of 1M HCl to a mixture of 100mL HCl+Magnesium

Originally I would have thought no, but the TSFX lecturer yesterday said otherwise but I didn't completely understand it...something about how when the HCl is being consumed, the concentration reestablishes itself faster since theres more particles to disperse (but more volume to disperse throughout? hmph) and something about unit per time which she kept repeating but which I didn't really get. Secondly, even if it IS true, do we need to know it for VCE? Whilst it's not covered in any textbook, that doesn't necessarily mean we don't need to know it so yeh...any ideas?

Jack the famous Springvale tutor said no. ....... (to adding more volume to increase rate of reaction)

The Mg(s) would be exposed to the same amount of HCl no matter how much you still pour in. (without taking into consideration what happens over time + equilibrium)

*scratches head*

I'm going to ask a few chem teachers tomorrow(forgot, tuesday!) to see what they say...
Better confirmed than not.

[shinny]

Hmm...I'll try using numbers and see what happens:
1L of 1M HCl=1 mol of HCl
Consider a case where 0.5 mol of Mg is used up, hence 0.5mol of HCl is used up, 0.5 mol remains.
Therefore final concentration remaining is 0.5M

Now;
2L of 1M HCl=2 mol of HCl
0.5 used up, 1.5 remains
Therefore final concentration is 0.75M

Well there you go I guess o_O
With this, I guess its safe to assume a similar difference occurs during the reaction, hence faster rate of reaction.

EDIT: Guess I'll elaborate on what I can deduce and someone clarify if this is correct;

• INITIAL rate of reaction is the same, because concentration is the same

BUT

• rate of reaction DURING the reaction is higher because the concentration remains higher due to the same amount of whatever being consumed
• FINAL net rate of reaction is higher than what it previously was, but would be negligible since the backwards reaction occurs just as fast?
  

[kj_]

See, what I believe is when you increase the volume while maintaining constant concentration, the amount of molecules per volume unit remains constant - and so if according to the collision theory, the rate should not be affected as there is no change to the amount of fruitful collisions.

Take for example A(aq) + B(s) <--> C + D,
and say initially you have 1 Litre, 1M of A reacting with a set amount of B.
since we assume that A is already in excess, it will have a set reaction rate.

now considering 2 Litres, 1M of A reacting with the same set amount of B.
as with the situation before, A is already in excess, and thus adding more should pose -no- change in the reaction rate

..if that makes sense

Maybe, I'm not sure, maybe I'm completely wrong  :buck2:

[shinny]

I think I explained it all in my post just above =T The INITIAL concentration is the same, so the initial reaction rate is the same. However, as you can see from those calculations, as the reaction progresses, having less volume makes the concentration decrease at a faster rate, so the reaction rate during the reaction is going to be faster.

[bturville]

From kinetics, reaction rates can be calculated by using rate equations, like , where k is a rate constant, and the rest are concentrations. From that, you can see that its the concentration of HCl or Mg changing that affects the rate, not the number of moles in the flask (or whatever). Hope that makes sense.

[Also the surface area of Mg makes a difference as you would probably guess.]

[shinny]

Is everyone totally ignoring what I've just said ._. I know that its the concentration that affects the rate of reaction, but I've just proved that at any time, by having a higher volume of stuff, the concentration of excess reactant will be decreasing at a slower rate, and hence be reacting at a faster rate of reaction DURING the reaction. Gee, someone get coblin or mao since clearly my word doesn't stand around here =P

[Collin Li]

Is everyone totally ignoring what I've just said ._. I know that its the concentration that affects the rate of reaction, but I've just proved that at any time, by having a higher volume of stuff, the concentration of excess reactant will be decreasing at a slower rate, and hence be reacting at a faster rate of reaction DURING the reaction. Gee, someone get coblin or mao since clearly my word doesn't stand around here =P

Yep. That's exactly what I said shinjitsuzx, but I'm still unsure about it.

Hm.. I think this may be correct because as the reaction proceeds, the amount of hydrochloric acid drops. This effect on the rate of reaction (concentration) is less in 200 mL, than in 100 mL.

However, if the system is already in equilibrium, then nothing will happen.

[shinny]

Yeh I just went further to prove that the decrease in concentration over time is definitely less in 200mL than 100mL in one of my above posts, so I see no reason why the rate of reaction won't be higher...

[Collin Li]

For the students who are becoming confused, this is unlikely to be examined, but understand the difference between what you already knew (that concentration, not amount or volume, affects rates), and what is being asked.

The only reason why volume matters is because it is affecting the way that concentration changes (which consequently affects the rate). It is through what you already knew, which is why this works. You might ask: why does the concentration change differently? How come I never thought about this before?

It's because we usually work with homogenous systems, like all-aqueous species, or all-gaseous species. This effect won't happen to those sorts of systems.

However, since this is a system where HCl is in excess and contact with a somewhat constant surface area of metal, the rate at which the amount of HCl decreases is constant. This means that if there is more volume, the concentration decreases more slowly - hence changing the relative rates as the reaction proceeds.

In a homogenous system, all particles are free to bump into other particles, so an increase in concentration also increases the amount of collisions per unit time, and so this effect (of "changing the relative rates as the reaction proceeds") shouldn't happen - I think.

[bec]

Yeah I follow the same logic as shinjitzu and coblin, I agree with you.
From memory the VCAA dotpoints only include homogeonous systems (is that right?) so IF that's the case, we definitely don't need to know this.

[shinny]

I think the TSFX lecturer said that this concept was tested in the sample exam, and definitely had quite a few other concepts which you'd need to derive with principles such as the one we just did since they're not mentioned in ANY textbook. I'm not quite up to doing the sample and I'm not prepared to browse through it, so would anyone be willing to confirm this? =T

[Pandemonium]

The TSFX lecturer was so crap, what she said was pretty much on spot but like... I left after the first break.

Anyway, I'm here to agree with shinjutsux and coblin - the concentration does decrease more slowly in the larger volume solution therefore as the reaction progresses in the solution with the larger volume will react faster than the one with less.

Andrew, don't doubt your own calculations. You gonna do well anyway. Better than me. ;d

[bec]

I've got another (much simpler) question about rates...


The above reaction would achieve equilibrium most rapidly under the conditions of
HIGH/LOW temperature and a HIGH/LOW flow rate over the catalyst.

I put low temp and high flow initially but now I'm thinking it should be high temp, because this makes the reaction rate faster. What does everyone else think the answer would be?