CHAPTER 16 ACIDS AND BASES

I. INTRODUCTION A) The Oldest Theory is the Arrhenius Theory 1) You should be familiar with this one from your earlier chemistry courses and chapter 3.

2) Arrhenius looked at the substances which were called acids. Some of these substances were known from even before the days of alchemy. They taste sour, turn blue litmus to red, neutralize bases, release hydrogen gas when added to an active metal and release carbon dioxide when added to a carbonate. 3) Arrhenius said these properties were due to the production of H + ions when acids dissolve in water.

Today we know that a bare proton does not exist in water, it forms a chemical bond with a water molecule forming the H 3 O +. This ion is called the hydronium ion. Why should you know this? What do you know about the structure of the water molecule?

4) He looked at the properties of bases. They taste bitter, feel slippery (soapy), turn red litmus blue and neutralize acids. 5) Arrhenius said these properties were due to the production of OH- ions when bases are dissolved in water. 6) Neutralization is H + + OH -  H 2 O or TODAY H 3 O + + OH -  2 H 2 O

7) This acid base theory is limited to water solutions. WATER must be the __________. B) Brønsted - Lowry Theory - Brønsted - Danish, Lowry - British working at the same time and independent of one another came up with the same theory i.e. two names on the theory. 1) A B/L acid is the species which donates a proton in a proton transfer reaction. What is meant by a proton here?

2) A B/L base is the species which _________. 3) A general acid - base reaction: HA (aq) + B (aq)  HB + (aq) + A - (aq) acid base conjugate acid conjugate base

4) A specific strong acid - base reaction: HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) HCl (aq) is the ___________________. H 2 O (l) is the ________________. H 3 O + (aq) is the ________________________. Cl - (aq) is the _________________________.

Memorize 6 Strong Acids HClO 4 ; HI; HBr; HCl; H 2 SO 4 ; HNO 3 Why do we call them strong acids? H 2 SO 4 is special because it is diprotic. There is an equilibrium constant for the removal of the 2 nd H +.

Every Brønsted - Lowry acid has a _________________ __________________, and every B/L base has a _______________ ______________. 5) For a specific weak acid reaction: With a weak acid, we have an equilibrium mixture of all four species. Weak acids do NOT dissociate 100%.

CH 3 COOH is the _____________________. H 2 O is the _________________________. H 3 O + is the ____________________. CH 3 COO - is the _____________________.

For a specific weak base reaction: OH - is the ____________________. NH 4 + is the _____________________. NH 3 is a weak base. There are not many ions in solution. The strong smell is due to __ NH 3 is the _________________________. H 2 O is the _____________________

What do you notice about water in the above examples?

Memorize 6 Strong Bases The strong bases are LiOH, NaOH, KOH, Ca(OH) 2, Sr(OH) 2, and Ba(OH) 2.

C) The Lewis Theory of Acids and Bases 1) Certain reactions have the characteristics of acid-base reactions, but do not fit the Brønsted - Lowry concept.

2) The Lewis acid is BF 3, it needs a pair of electrons to complete its shell, "need two from you." 3) The Lewis base is the NH 3, it has a pair of electrons to share, "have pair will share." 4) A Lewis acid is a species that can form a covalent bond by accepting a pair of electrons from another species. 5) A Lewis base is a species that can form a covalent bond by donating an electron pair to another species.

H > NH 4 + Zn H 2 O -----> Zn(H 2 O) 4 2+ Co Cl > CoCl 4 2-

II. Relative Strengths of Acids and Bases (B/L) A) Strong Acid No molecules of HA in water solution. B) Weak Acid An appreciable number of HB molecules exist in water solution. Only a few H 3 O + ions and B - ions exist in the solution.

C) An acid-base reaction normally goes in the direction of the weaker acid. You can use this fact to compare the relative strengths of any two acids. D) If we have 0.10 M of the following acids, we obtain the following results. For acetic acid: For hydrofluoric acid:

CH 3 COOH and HF are __________ acids than H 3 O +. HF is a __________ acid than CH 3 COOH. E) In water all the strong acids are essentially 100% ionized. It is difficult to rank order them in strength. Is HCl stronger or weaker than HI? Or are they equal? In water they are of equal strength since they both are 100% ionized. F) This is known as the leveling effect of water.

To observe a difference in the strengths of the strong acids you need a different solvent than water, one in which they are all not ______ ionized. G) Strong acids form conjugate bases which have no measurable strength in water. Within a series of acids, the stronger the acid, the weaker the conjugate base, and the reverse is also true. The _______ the acid the stronger the ________ base. What does this mean with respect to these particles in solution?

III. What structural properties cause a molecule to behave as an acid or a base? A) There are two main factors that determine whether a molecule containing an HX bond will behave as a Br  nsted – Lowry acid, and whether or not it will be strong. 1. The strength of the HX bond is important. The stronger the HX bond, the less easily the substance will be ionized, the weaker the acid.

Going down a column of elements on the periodic table, the size of the atom increases. As the size of atom X increases, the strength of the HX bond decreases, and the strength of the binary acid increases. The order of acid strength is: ____ < ____ < _____ < _____

2) The second important factor is the polarity of the bond. The more polar the bond, the more readily ionized, the stronger the acid. Going across the chart the electronegativity increases and the acid strength increases. As the polarity increases across the table, the acid strength __________________.

B) Oxyacids have the general formula HOY. C) For the compound to be an acid the bond between the H and O atom must break. 1) The more electronegative Y is, the weaker the O to H bond, the stronger the acid. Properties of Y which operate to increase its electronegativity are ________, _______________, and ________________. HOCl > HOBr > HOI Why is this the order?

2) The more O's, the stronger the acid. HClO 4 > HClO 3 > HClO 2 > HClO As the number of oxygen atoms increase, the attraction for the hydrogen's shared electron increases and this makes it easier for the H + to leave.

Another way of looking at the situation is to focus on the Cl atom. As the number of O atoms increases, the Cl atom increases in oxidation number, making it more positive, giving it more attraction for available electrons.

IV. pH and the Self Ionization of Water A) In any sample of water, there are some ions which result from the dissociation of water itself. Remember even the H 3 O + isn't really an accurate symbol as water molecules with H + come in clusters: H 5 O 2 +, H 9 O 4 +, etc.

We will keep it as simple as possible. For us: Experiments show that in pure water at 25 o C, the [H 3 O + ] = [H + ] = [OH - ] = 1.0 X M which is what we call a neutral solution.

B) Water solutions in which the [H + ] > [OH - ] are acid solutions. Water solutions in which the [OH - ] > [H + ] are basic solutions.

You must keep in mind that in water solutions of acids and bases there are always OH - and H 3 O + ions due to the self ionization of water. These ions from water do not add many ions to reasonable concentrations of acid and base solutions, but you must remember that they are there. They could become an important addition to the [ ] of the species in which we are interested.

C)In water at 25 o C, the [H 3 O + ] = [H + ] = [OH - ] = 1.0 X M. K w at 25 o C = [H 3 O + ] [OH - ] = (1.0 X ) 2 = 1.0 X It is very important to remember that this relationship holds for ANY water solution at 25 o C, not matter where the water comes from. If [H 3 O + ] is 1.0 x M then what is the [OH - ] ?

D) Since [H 3 O + ], [H + ], and [OH - ] are so small, scientists use the concept of p when dealing with very small numbers, where p stands for - log. pN = - log N pH = - log [H 3 O + ]= - log [H + ] pOH = - log [OH - ] pK w = - log K w pK a = - log K a

The pH Scale

E) pH of a strong acid in water solution 1) You have learned 6 strong acids, for 5 of them, excluding H 2 SO 4, we can calculate the pH of dilute solutions which will match a measured value with little error.

2) HCl + H 2 O  H 3 O + + Cl - The pH of a 0.10 M HCl solution is - log (0.10) = the number of decimal places reported in the log of N = the number of sig. figs. in the original number.

pH of a M HCl solution is 2.00 pH of a M HCl is 3.0

3) What is the pH of a solution which has a concentration of 3.40 X M NaOH?

4) Be able to go in the reverse direction. What is the[H 3 O + ] in a solution with a pH of 4.64?