[steelbird524]

I need help with this problem.  I'm suppose to come up with two mechanisms for this reaction.

CO2 + 2OH^-  ---> CO3^2- + H2O  

The rate law for the disappearance of CO2 is Rate = k[CO2][OH^-]

I came up with one mechanism, but I need help with the second.

CO2  +     OH      ----> HCO3
HCO3  +   OH      ----> CO3^2- + H20
CO2 +   2OH^-    ----> CO3^2- + H20

[Borek]

In water? Water is in excess.

[steelbird524]

yea this reaction aparently makes water

[Borek]

I am not talking about products. This reaction takes place in water probably - so the water is in excess and can be a part or mechanism.

Even if CO₂ reacts with solid hydroxide (like NaOH) water is there - although in small quantities at first.

[steelbird524]

ok so how would you write that mechanism?

[Albert]

I'd make CO₂ react with water. Then the product, an acid, reacts with a base (OH^-).

[AWK]

I think, these two reaction sufficiently explain mechamism of reaction"
CO2  +    OH^-      ----> HCO3^-
HCO3^-  +  OH^-      ----> CO3^2- + H20

It is known from another source carbonic acid exists, and is solid at RT, moreover it decomposes to water and carbon dioxide under catalytic influence of water. Proton exchange is a very fast process (second reaction), hence the limiting step is the first reaction, and kinetic equation is a pseudo-first order in relation to concentration of OH^-

[steelbird524]

yea thats one mechanism, but I have to come up with 2 mechanisms for this reaction.  I got the first one, and was wondering if someone could help with with a second mechanism?

[AWK]

Borek suggested the second one, the first step is the same as for CO2 in water
CO2 + 2H2O = HCO3^- +H3O^+
Then in the case of  a some excess of OH^-
H3O^+ + OH^_ = 2H2O
or
HCO3^-  +  OH^-      ----> CO3^2- + H20