[s3a]

Question:

In each of the following pairs of reactions,choose the one that will be the slower.

A) The reaction of 2.5g of powdered zinc metal, Zn(s), with 1mol/L HCl(aq) at 50°C
B) The reaction of 5g of powdered zinc metal, Zn(s) with 1 mol/L HCl(aq) at 20°C.

Answer: B
Reasoning: The concentration of the zinc is the same. The reaction will be slower at 20°C due to fewer effective collisions with sufficient energy.

What I don't get is it says the concentration is the same but how is it the same if the first is 2.5g while the second is 5g? The temperature does not confuse me at all, it's just that I don't know what has a greater effect. Having double the concentration or 2.5 times the temperature.

Any help would be greatly appreciated!
Thanks in advance!

[Borek]

Looks to me like the reasoning behind is wrong. While activity of the metal will be identical in both cases (thus the same equilibrium) speed at which gas evolves is directly proportional to the metal surface - and as such it will be larger in the case of 5 g of zinc.

[s3a]

You say that the surface is a factor in increasing the rate of a reaction. The thing is I already know this and I also know that the rate of a reaction is directly proportional to a kelvin temperature. Again, what I am confused about is how to determine which one will be faster when comparing an increase of mass with a decrease in temperature and vice versa. How do you know whether the mass or temperature has a greater effect than the other?

Thanks!

[s3a]

Wait I think I just got it....is the reason why the solution guide says they have equal concentrations BECAUSE the 1 mol/L HCl solution is NOT a specific value and that it can mean half the mass but also half the HCl solution hence equal concentrations but different temperatures? And of course the higher temperature means more particle collisions.

Can someone please confirm if this is the proper logic behind this question.
I would REALLY appreciate it!
Thanks in advance!

[Borek]

How do you know whether the mass or temperature has a greater effect than the other?

You don't. You can check experimentally. As of now question can't be answered IMHO. Unless someone tries to answer assuming that activity of the solid is always 1, so amount of solid doesn't matter. I suspect that's what the person asking the question had on mind, but that's wrong assumption.

BECAUSE the 1 mol/L HCl solution is NOT a specific value and that it can mean half the mass but also half the HCl solution hence equal concentrations but different temperatures?

Sorry, I have no idea what you mean.