Aqueous Solutions and Chemical Equilibria:

Transcription

1 Aqueous Solutions and Chemical Equilibria: Acidbase titration Complexometric titration Separations Electrochemistry Etc., etc. Illustration: acidbase titrations Concepts and determination of ph during course of titration Acidbase titrations involve several simultaneous equilibria. To fully understand changes in ph during titrations involving strong acids/strong bases, weak acids/strong bases, multiprotic acids/strong bases, etc. we must be able to treat the following cases: Solution ph of strong acid or base Solution ph of weak acid or base Solution ph of salts Solution ph of buffers Four possible levels of approximation apply depending upon whether or not we can 1) ignore autoprotolysis of water and 2) assume the concentration of the acid or base in solution is either zero (very strong acid or base) or the same as the analytical concentration of the acid or base (very weak acid or base). We will first deal with the estimation of ph for these various cases and apply these approaches to understanding the shapes of titration curves.

2 Compounds that dissolve in water to form solvated ions are called electrolytes:

3 Solubility is determined by the energies associated with disrupting solute bonds and solvent bonds, the formation of new solventsolute bonds, and entropy. Entropy always favors disorder (dissolving).

4 Acids and Bases We will use the BrønstedLowry definition of acids and bases: Acid = proton donor Base = proton acceptor H 2 O A OH HA Acid 1 Base 1 Base 2 Acid 2 Acid 1 (H 2 O) and Base 2 (OH ) are called a conjugate acid/base pair. H 2 O is the conjugate acid of OH and OH is the conjugate base of H 2 O. A is the conjugate base of HA and HA is the conjugate acid of A. All proton transfer reactions involve two conjugate acid/base pairs. H 2 O HA H 3 O A Base 1 Acid 1 Acid 2 Base 2 Amphiprotic molecules, like H 2 O, can act as either an acid or as a base.

5 Amphiprotic solvents undergo autoprotolysis: H 2 O H 2 O H 3 O OH Acid 1 Base 1 Acid 2 Base 2 NH 3 NH 3 NH 4 NH 2 Acid 1 Base 1 Acid 2 Base 2 CH 3 OH CH 3 OH CH 3 OH 2 CH 3 O Acid 1 Base 1 Acid 2 Base 2 The autoprotolysis behavior of a solvent plays a very important role in acid/base chemistry in the solution. It can serve as a differentiating solvent, in which differences in acid strengths are readily apparent, or as a leveling solvent, in which differences in inherent acidities are not readily apparent. e.g., HClO 4 dissociated 5000X more than HCl in glacial acetic acid; both are 100% dissociated in H 2 O

6 HA solvent solventh A Whenever solvent is a much stronger base than A, HA is completely dissociated. H 2 O is a much stronger base than either Cl or ClO 4. Therefore, HCl and HClO 4 are both strong acids ( 100% dissociated). HC 2 H 3 O 2 is much more basic than ClO 4 is not much more basic than Cl Hence, in glacial acetic acid, HClO 4 100% dissociated HCl << 100% dissociated

7 H 3 O is referred to as the hydronium ion Why H 3 O? Remember the special characteristic of H, it has a very large charge density due to its small size.it will stick to virtually anything and therefore is always solvated. Proton affinity of compound X = energy to remove a proton from XH proton affinity of water is 165 kcal/mol Sodium ion affinity of compound X = energy to remove a proton from XNa sodium ion affinity of water = 25 kcal/mole Acids produce H 3 O, the hydronium ion, in aqueous solution. In reality, the proton is even more heavily solvated H 3 O NH 3 H 2 O NH 4 Acid 1 Base 1 Base 2 Acid 2

8 Whether the solvent water acts as an acid or a base depends upon the solute. Proton donors that dissociate nearly 100% are referred to as strong acids... H O : H H X H O X: : [ H] H [ ] :.. e.g., HCl is a strong acid and is essentially fully dissociated in water.. Note: strong acids have weak conjugate bases and strong bases have weak conjugate acids Proton donors that dissociate less than 1% are referred to as weak acids. H 2 O HA H 3 O A Base 1 Acid 1 Acid 2 Base 2 e.g., HCN is a weak acid

9 Organic acids: e.g. CH 3 COOH H 2 O CH 3 COO H 3 O acetic acid acetate anion (vinegar) H O H H C C H O H H2O O Acidic H H 3 H C H O C O Acetic acid is an example of a carboxylic acid: O C OH Carboxyl group

10 HCl(aq) is a monoprotic acid.some acids are polyprotic: H 2 SO 4 (aq) = sulfuric acid is diprotic H 3 PO 4 (aq) = phosphoric acid is triprotic Polyprotic acids are associated with multiplycharged anions: e.g. H 2 SO 4 2H 2 O 2H 3 O SO 4 2 H 3 PO 4 3H 2 O 3H 3 O PO 4 3 In practice, polyprotic acids are dissociated in stepwise fashion: e.g. 1 H 2 SO 4 H 2 O H 3 O HSO 4 HSO 4 H 2 O H 3 O SO 4 2 net reaction H 2 SO 4 2H 2 O 2H 3 O SO 4 2 e.g. 2 H 3 PO 4 H 2 O H 3 O H 2 PO 4 H 2 PO 4 H 2 O H 3 O HPO 4 2 HPO 4 2 H 2 O H 3 O PO 4 3 net reaction H 3 PO 4 3H 2 O 3H 3 O PO 4 3