BY MARTIN K I L P A T R I C K , JR. 4 X D J. H E N R Y RU.SIITOPi**
The present paper is a preliminary report of a part of a general investigation of the rate of solution of metals in acids, in aqueous and in non-aqueous solution, from the point of view of the extended theory of acids and bases as developed by Bronsted.' I n accordance with this view it will be shown that in aqueous solution the reaction is not wholly with the hydrogen ion, H,O-, but is in part with the other acids present. Bronsted defines an acid by the formal equation
A
i'
13
+ H.
where A is an acid, irrespective of charge, B ii: a base, and H' is a proton. This definition furnishes, as Hronsted has shown, a general basis for the consideration of acids and bases in aqueous and in non-aqueous solution. I n aqueous solution there exists a double acid-base equilibrium, due to the fact that the solvent itself is both an acid and a base. The reaction accornpanying the solution of hydrogen chloride in water may be written acid HCl
base
+ H20
acid H30-
base
+ C.'lk
Hydrogen chloride is such a strong acid that it readily gives u p its proton t o the base water, the equilibrium being displaced practically completely to thc right. The acid present, therefore, in n hydrochloric acid solution is the ion HaO+, or as we shall call it, the hydrogen ion. The concentration of the proton, HT, is so small as to be of no kinetic significance. The case above is a general illustration of the state of affairs existing when any strong acid is dissolved in water and explains why all strong acids appear of be of equal strength in aqueous solution. Consider now the weaker acids. I n a solution of, for example, formic acid in water there are present at appreciable concentrations two acids, the hydrogen ion and the formic acid molecule,
HCOOH + H20
H3O-
+ HCOO-.
If the concentration of the base HC'OO- is increased by the addition of sodium formate, the concentration of the €LOT is decreased and the solution is buffered with respect t o the hydrogen ion. *Contribution from the Laboratory of Physical Chemistry of the Ilniversity of PenIisglvania. **Paper presented at the ;9th meeting of the American Chemical Society in Atlanta, April 7-10, 1930. Bronsted: Rec. Trav. chim., 42, 718 (1923). See also Lowry: Chem. and I n d . , 42, 43 (1923).
2181
RATE O F SOLVTIOX O F MAGSESIUM IS A C I D S
I n aqueous solution of a strong acid there is, practically speaking, only one acid present-the hydrogen ion, HaO-and with this acid magnesium reacts according t o the stoichiometric equation
+
+
I '211g H30+ --t I '23lg++ 3- HpO I 'zH,. -4 formic acid solution contains two acids, as does a formate-formic acid
buffer solution ; the question therefore arises as to TThether magnesium in these solutions reacts with the acid Ha@', with the acid HCOOH, or with both. To test this point we measured the rate of solution of magnesium in solutions of a strong acid and in buffer solutions of various weak acids. The reaction was followed by measuring the volume of the gas evolved. The apparatus was similar to that used in a previous investigation' and consisted of a florence
Moles of HCI Per L i t e r
s HCI
G
+ 0.2 hl.NaC1
FIG.I
+ +o.j
?~I.llgCl,
0 HClOI
V +o.6M.NaCi
flask fitted with a mercury-seal stirrer with a n outlet attached t o a gas burette. A cylinder of magnesium of measured surface was attached to the shaft of the stirrer. I n all the experiments the solution was stirred at a rate of somewhat above 600 r.p.m. 1T-e found no appreciable change in rate of evolution of hydrogen when the speed of the stirrer was increased to 800 r.p.m. -111 experiments were carried out a t 2 j 0.01'. Our first experiment was in 0.01 11 hydrochloric acid solution, that is to say, in a solution 0.01 31 in hydrogen ion. Hydrogen was evolved at an initial rate of 0.j cc. per minute. We then made up a buffer solution I.; 11 in formic acid and 0.1 ?\I in sodium formate. The hydrogen ion concentration of such a solution is approximately 0.005 X I ; if the reaction were with the hydrogen ion alone a rate of about 0 . 2 j cc. per minute would accordingly be expected. .Is a matter of fact hydrogen was given off from this solution at a rate too rapid
+
Kilpatrick: J. .4m. Chem. SOC.,48, 2092 (1926).
MARTIS KILPATRICK, JR. ASD J. HESRY RUSHTOS
2182
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aa 1.8 1.b
1.4
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