Hello, I am trying to do a school lab on the reaction rate constant "k" on the addition of Mg to HCl. I know that the rate law law for a first order
rxn is ln[A]=-kt+ln[Ao] and the second order rxn (1/[A])=(1/[Ao])+kt. I am trying to solve for k. I am ale to change the concentration of the HCL
solution but I was wondering how I would change the concentration of the Mg to determine if the reaction was first or second order?Galinstan - 27-3-2014 at 07:32
to determine the rate with respect to the magnesium just change the mass of magnesium used while keeping the concentration of HCl the same and measure
the time it takes for all the Mg to dissolve. while at the same time measure the volume of H2 gas evolved to work out the rate at which the magnesium
is being dissolved. hope this helpsSteam - 27-3-2014 at 07:41
That does.
Could the rate that the Mg is dissolved be accurately determined by the change of PH in the HCl solution?DraconicAcid - 27-3-2014 at 07:57
You cannot change the concentration of magnesium in this reaction. The rate of the reaction will not depend on the mass of magnesium present, but on
its surface area, which will not remain constant as the reaction proceeds (particularly if you're using magnesium ribbon or turnings).Steam - 27-3-2014 at 07:58
so then this reaction has to be a first order reaction?DraconicAcid - 27-3-2014 at 08:23
so then this reaction has to be a first order reaction?
No- I don't know what the order will be with respect to the acid. It may be first order, it may be second order, it may be more complicated. But the
variation in surface area of your magnesium is going to make this experiment very tricky indeed. You would need to use a large piece of clean
magnesium to make sure you have the same surface area in all of your trials.Steam - 27-3-2014 at 08:49
Could varying the amount of mg turnings added determine the rate law with respect to the Mg?DraconicAcid - 27-3-2014 at 09:02
Could varying the amount of Mg turnings added determine the rate law with respect to the Mg?
Not unless you can measure the surface area of the turnings. This sounds tough, but it's even harder if the turnings aren't perfectly free of oxide
coatings.
It would be much easier to work with a one-phase reaction, with everything in solution, rather than attempting to find the rate law for a two-phase
reaction.Steam - 27-3-2014 at 10:32
Got it, I think I am just going to do the rate in terms of the HCl concentration.
