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Confused about mechanism for the reaction between magnesium and HCl

Hi, I've found experimentally that the rate equation is rate = k[H+]2 (Mg is not included nor investigated for its rate order as it is a solid) and I've come up with a mechanism:

Step 1. 2HCl -> 2H+ + 2Cl-
Step 2. 2H+ + Mg -> H2 + Mg2+
Step 3. Mg2+ + 2Cl- -> MgCl2

But the thing is, if my rate equation is " rate = k[H+]2 " then which one is my rate determining step? Any step which has things not included in the rate equation can't be in the rate equation... That's what I'm confused about, my mechanism (which I can't think of an alternative mechanism) is saying one thing and then my rate equation is contradicting it :frown:
(edited 8 years ago)
Original post by stevepanchal
Hi, I've found experimentally that the rate equation is rate = k[H+]2 (Mg is not included nor investigated for its rate order as it is a solid) and I've come up with a mechanism:

Step 1. 2HCl -> 2H+ + 2Cl-
Step 2. 2H+ + Mg -> H2 + Mg2+
Step 3. Mg2+ + 2Cl- -> MgCl2

But the thing is, if my rate equation is " rate = k[H+]2 " then which one is my rate determining step? Any step which has things not included in the rate equation can't be in the rate equation... That's what I'm confused about, my mechanism (which I can't think of an alternative mechanism) is saying one thing and then my rate equation is contradicting it :frown:


You can't have the first step as dissociation of the acid as it is already dissociated.

If your rate equation has two hydrogen ions then you must use one to produce an intermediate that is needed for the second (RDS).

example

H+ + Mg --> H + Mg+
H+ + Mg+ --> H + Mg2+ ......... RDS
H + H --> H2

In this (possible) mechanism you look at the RDS and see one H+ ion. BUT there is also an Mg+ needed. So you must add in the components from step 1 needed to make the Mg+, i.e. another H+ ion

Finally to check the mechanism validity you add all the steps together and cancel out common terms - this should give you the balanced equation for the reaction.

H+ + Mg --> H + Mg+
H+ + Mg+ --> H + Mg2+ ......... RDS
H + H --> H2
---------------------------------------------------------- add together
2H+ + Mg --> Mg2+ + H2
Reply 2
Original post by charco
You can't have the first step as dissociation of the acid as it is already dissociated.

If your rate equation has two hydrogen ions then you must use one to produce an intermediate that is needed for the second (RDS).

example

H+ + Mg --> H + Mg+
H+ + Mg+ --> H + Mg2+ ......... RDS
H + H --> H2

In this (possible) mechanism you look at the RDS and see one H+ ion. BUT there is also an Mg+ needed. So you must add in the components from step 1 needed to make the Mg+, i.e. another H+ ion

Finally to check the mechanism validity you add all the steps together and cancel out common terms - this should give you the balanced equation for the reaction.

H+ + Mg --> H + Mg+
H+ + Mg+ --> H + Mg2+ ......... RDS
H + H --> H2
---------------------------------------------------------- add together
2H+ + Mg --> Mg2+ + H2


I see... I didn't realise that if there were two H+ then I needed an intermediate stage (if I decide to use H3O+ theoretically then I shouldn't need to use an intermediate as H3O+ + H3O+ is possible whereas H+ + H+ is not - am I right on this understanding?).

However, I've read in multiple books and online that there is a rule that the RDS must involve what is in my rate equation and nothing else (i.e. The RDS must only include H+ and not Mg)...

In my example, if the RDS is " H+ + Mg+ --> H + Mg2+ ", this contradicts what is in my rate equation " rate = k[H+]2 " where it only has H+ on the left hand side and not other ions/molecules (in this example, Mg+)

Thank you
(edited 8 years ago)
Original post by stevepanchal
I see... I didn't realise that if there were two H+ then I needed an intermediate stage (if I decide to use H3O+ theoretically then I shouldn't need to use an intermediate as H3O+ + H3O+ is possible whereas H+ + H+ is not - am I right on this understanding?).

However, I've read in multiple books and online that there is a rule that the RDS must involve what is in my rate equation and nothing else (i.e. The RDS must only include H+ and not Mg)...

In my example, if the RDS is " H+ + Mg+ --> H + Mg2+ ", this contradicts what is in my rate equation " rate = k[H+]2 " where it only has H+ on the left hand side and not other ions/molecules (in this example, Mg+)

Thank you


Read through my post again more carefully. It explains all of your questions.

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