in excess ? RATE OF REACTION?

[matt123]

hey guys.
Lets just say... for e.g.
We have a reaction between HCL and Cac03 ...

And the HCL is in XS.

If we add MORE hcl ONLY . does the rate increase even though its in excess? ..my answer was YES . as there are more particles to colide with .. but this exam i did ( 2004 neap booklet i think) says that
"NO , it will not increase the rate as the Hcl is already in excess"

what do you guys think?

[jasonn93]

hmm..I would have gone with yes as well..

[JinXi]

No. Volume does not increase the rate of reaction. Concentration does.

[Martoman]

No. Volume does not increase the rate of reaction. Concentration does.

but if you add more you are adding more *mol* so there are more particles to collide with. I think NEAP are getting confused with the amount evolved overall won't change.

[fady_22]

No. Volume does not increase the rate of reaction. Concentration does.

but if you add more you are adding more *mol* so there are more particles to collide with. I think NEAP are getting confused with the amount evolved overall won't change.

You are adding more, but the amount of HCl in a given volume is the same (as is the concentration). So the rate would not change.

[Martoman]

we assume it stays the same because its VCE chemistry?

[fady_22]

we assume it stays the same because its VCE chemistry?

Yep.
Don't overanalyse the questions... you might actually use your brain.
And god forbid that to happen.

[Martoman]

 :angel:

[jasonn93]

ah, so it would only be 'yes' if they state that the volume stays constant, and they put in more HCl?

[matt123]

ah, so it would only be 'yes' if they state that the volume stays constant, and they put in more HCl?

Yeah im a little confused.

the way i see it.
Add more HCL = more moles = more particles to collide with

what is wrong with that?? lol i dont get why its "no" still.
I mean .... If you add more HCl .. are you saying .. the Concentration stays the same?.

[fady_22]

Yes, concentration stays the same. So the rate should also stay the same.

[3Xamz]

Yeah, even though you are adding more HCl, you are increasing the volume too. Hence they negate each other so to speak, and the rate stays the same. However, this is assuming that the added HCl is the SAME concentration of the original HCl in the sample.

If you have a say 100ml 0.5M HCl solution reacting with some mass of CaCO3 and then you add 100mL of 10M HCl, obviously the rate is going to change.
Simply because original concentration was 0.5M, new concentration is;

((0.1L * 0.5M) + (.1L * 10M) ] / (0.2) = 5.25M

[matt123]

Yeah, even though you are adding more HCl, you are increasing the volume too. Hence they negate each other so to speak, and the rate stays the same. However, this is assuming that the added HCl is the SAME concentration of the original HCl in the sample.

If you have a say 100ml 0.5M HCl solution reacting with some mass of CaCO3 and then you add 100mL of 10M HCl, obviously the rate is going to change.

Yeh true.
Already so let me get this straight.
hmm adding more hcl wont change the rate.
What if it WAS NOT in excess?
because
I remember .. " adding more reactants" increases the rate of reaction?
is this not the same thing? adding more reactant HCL .. .??

[JinXi]

Yeah, even though you are adding more HCl, you are increasing the volume too. Hence they negate each other so to speak, and the rate stays the same. However, this is assuming that the added HCl is the SAME concentration of the original HCl in the sample.

If you have a say 100ml 0.5M HCl solution reacting with some mass of CaCO3 and then you add 100mL of 10M HCl, obviously the rate is going to change.

Yeh true.
Already so let me get this straight.
hmm adding more hcl wont change the rate.
What if it WAS NOT in excess?
because
I remember .. " adding more reactants" increases the rate of reaction?
is this not the same thing? adding more reactant HCL .. .??

If it's not in excess then yes. The rate will increase as you add more HCl. Like fady said above. Don't overthink VCE. I know for a fact that many things taught in VCE chem is incorrect and changes when we get to uni chem.

[3Xamz]

Well of course, if it WASNT in excess then you are fundamentally changing the reaction rate. As you are providing it with something that it already needs.

If you add it when its already in excess the CaCO3 simply goes 'eh, don't really need you' because there was much of it around already.