LECTURE 9 - Acids and Bases

Transcription

1 LECTURE 9 - Acids and Bases Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Acids & Bases, slide 1 here Acids The classical definition of an acid is a substance that dissociates to yield free protons: Acids & Bases, slide 2 here 9-1 Later it was realized that existing in water was impossible for a bare proton (no electrons). Instead the "hydronium" ion is formed: 9-2 In reality the hydronium ion is not only H 3 O +, but rather, some hydrated form with the general formula H 2n+1 O n +. Nitric acid is a stronger acid than water. Therefore nitric acid acts as an acid in the presence of water by donating a proton to water and forming the hydronium ion. Brønsted defined an acid to be a "proton donor." Acids & Bases, slide 3 here Lewis suggested an alternative definition of an acid. The Lewis definition of an acid is any substance that accepts a lone pair of electrons (see figure 9-1). For most purposes, this definition is not important in geology; only in the study of catalysts does it apply. 1

2 Acids & Bases, slide 4 here In geology the term "acidic rock" has long been used. The term means any rock containing an excess of nonmetallic oxide (principally silica) over metallic oxides. Many nonmetallic oxides (CO 2, SO 2, SO 3, NO 2, etc.) will dissolve in water to yield acids (H 2 CO 3, H 2 SO 4, etc.) and this is the reason that nonmetallic oxides were originally called acidic. However, SiO 2 is almost insoluble in water and therefore the term, acidic, applied to silica is very misleading. Acids & Bases, slide 5 here An effort to replace the term acidic by the word felsic occurred. The older literature still contains references to acidic rocks, however. Many geologists continued to use the term acidic. So the word has now been redefined as weight percent silica content, not referring to the ph of any solution in contact with the rock. Bases Bases, classically, are substances that yield OH - (hydroxide ion) to solution. Figure 9-1 Acids & Bases, slide 6 here 9-3 Brønsted modified this definition to be that of a proton acceptor (see equation 9-4). 9-4 Here, the base or proton acceptor, KOH, has accepted a proton from the acid (H 2 O) and split, leaving K + and H 2 O; the original water molecule has become an OH -. Lewis defined a base as a lone pair donor, again unimportant in geology. Acids & Bases, slide 7 here 2

3 The terms basic and ultrabasic rocks also exist in the older literature. These terms describe rocks with an excess or a large excess of metallic oxides (MgO, CaO, FeO, etc.) over nonmetallic oxides. Today these terms are replaced by the words mafic and ultramafic. The terms basic and ultrabasic may be used with the silica weight percent classification of igneous rocks, however. Alkaline was and is used for rocks with an excess of Na 2 O or K 2 O over SiO 2. ph Water can be thought of as either an acid or a base. When pure it will dissociate as follows: Acids & Bases, slide 8 here 9-5 Water's dissociation or ionization constant is: 9-6 Dissociation constants are another example of a specific type of ionization constant. The value listed in equation 9-6 is temperature dependent and is strictly applicable only around room temperature. In a neutral solution: 9-7 Acids & Bases, slide 9 here The ph is the negative log of the hydrogen ion concentration. If [H + ] = 10-7 : In acidic solutions [H + ] increases and [OH - ] decreases, to maintain the value of for the product. This means that the value of ph decreases in acidic solutions and increases in basic solutions. Acids & Bases, slide 10 here 3

4 A strong acid dissociates completely in water. Thus a 0.1M solution of nitric acid will have [H + ] = 0.1 = Acids & Bases, slide 11 here A strong base will dissociate completely to yield OH - ions. Thus a 0.01M solution of NaOH will have [OH - ] = 0.01 = Acids & Bases, slide 12 here An analogous function is p(oh) defined as: 9-14 So if [OH - ] = 10-2, 9-15 If [H + ] = 0.1 then ph = 1 and p(oh) = 13. Normally, ph is perfectly adequate for describing a system, but the p(oh) form is sometimes used in the literature. Acids & Bases, slide 13 here Ionization Constants Many substances do not dissociate completely in water including weak acids, weak bases, and salts. For these substances an ionization constant similar to that for water is needed. An example is hydrofluoric acid. K HF = Acids & Bases, slide 14 here 4

5 Many acids and bases have more than one functional group (acid or base) and thus have more than one dissociation constant. Examples: Phosphoric acid is H 3 PO 4. Acids & Bases, slide 15 here Acids & Bases, slide 16 here The second ionization requires pulling a positive ion (H + ) from a negative cation (H 2 PO 4- ) which is much more difficult: Acids & Bases, slide 17 here The third ionization is more difficult yet:

6 9-23 Five species (H +, PO 4 3-, HPO 4 2-, H 2 PO 4 -, H 3 PO 4 ) will be present at equilibrium, in amounts given by the above equations. To specify the concentration of each of these five species, more equations are needed. Acids & Bases, slide 18 here These can be of the type, (charge balance) or, Acids & Bases, slide 19 here if the substance is totally dissolved and the total concentration of phosphate is known. If the substance did not dissolve totally then a solubility product equation could be set up. Acids & Bases, slide 206 here The same sort of situation holds for a base: Solving sets of simultaneous equations is possible, such as the five for H 3 PO 4, if the number of equations equals or exceeds the number of variables. However the procedure is tedious and approximations may be tried. Any approximations made must be checked against the results to be sure they are valid. 6

7 Acids & Bases, slide 21 here For weak acids or bases, K 2 is weaker than K 1 by a factor of Thus the total concentration of H + or OH - is, for practical purposes, given by K 1 alone Acids & Bases, slide 22 here 9-29 For a 0.01M solution of H 3 PO 4 : Acids & Bases, slide 23 here If K 1 is large ( 10-2 ) this approximation begins to fail because [H + ] is not small in comparison to [H n A]. On the other hand if the concentration is too low then the [H + ] calculated will be less than that for pure water and this approach fails again. Acids & Bases, slide 24 here The same types of multiple ionization constants apply to some salts:

8 The solution of a salt in water may give a neutral solution. This will happen when the salt is the product of a strong acid and a strong base, Acids & Bases, slide 25 here or The ions of such salts show no tendency to react with either H + or OH -, so the resulting solution is neutral. Acids & Bases, slide 26 here On the other hand, the salt of a strong acid and a weak base or a strong base and a weak acid often produce an acidic or a basic solution, respectively. Example: 9-41 As Li 2 CO 3 dissolves, it gives Li + and CO 3 2- ions. The Li + will not interact with either H + or OH - ions; therefore, Li + ions do not affect the ph of the solution. 8

9 Acids & Bases, slide 27 here CO 3 2- can and does interact with H + : 9-42 The CO 3-2 ions unite with some H + ions. Water dissociation equilibrium must be maintained so the [OH - ] goes up as [H + ] goes down. Therefore, the ph increases and the solution is basic. Acids & Bases, slide 28 here Ammonia is a weak base: This reduces the [OH - ] and makes the solution acidic. This type of process is known as hydrolysis Certain salts will produce either acid or basic solutions. Soluble sulfides (S 2- ) or carbonates (CO 3 2- ) produce basic solutions. Soluble salts of heavy metal ions will produce acidic solutions. Acids & Bases, slide 29 here 9

10 Acids & Bases, slide 30 here If the hydrolysis reaction constant is needed, it may be calculated from the ionization constant of the weak acid or base and K w (K w must be raised to the coefficient of H + or OH - if greater than 1). Example: Acids & Bases, slide 31 here Acids & Bases, slide 32 here

11 9-55 Reference Konrad B. Krauskopf, Introduction to Geochemistry, Second edition, McGraw-Hill, New York, LN.09_PP_F16 pdf September 27,