Chemistry 12 UNIT 4 ACIDS AND BASES

Transcription

1 Chemistry 12 UNIT 4 ACIDS AND BASES CHAPTER 9 and 10 Recall the definitions from Unit 3: Strong electrolyte Strong electrolytic solution Weak electrolyte Weak electrolytic solution Non - electrolyte Acids and bases are both electrolytes RECALL FROM CHEMISTRY 11: Svante Arrhenius - Swedish Chemist 1884 ARRHENIUS THEORY OF ACIDS AND BASES: ACID: BASE: NEUTRALIZATION REACTIONS: SALTS:

2 CHARACTERISTICS OF ACIDS AND BASES: Acids BASES Both dissolve in water to give a solution that: -tastes sour -tastes bitter -conducts electricity (both are electrolytes) -causes certain indicators to change colour -produces or donates a proton -accepts a proton (reacts with a base) (reacts with an acid) -produces H2 (g) when reacting with -feels slippery metals eg. Mg(s) + 2HCl(l) MgCl2(aq) + H2(g) (note that some metals such as Na and K are nonreactive with acids) -turns litmus paper RED -turns litmus paper BLUE (red in acid) (Blue in Base) -loses the above properties when reacted with a base / acid (but the resulting solution conducts a current) All of the above characteristics can be used to identify acids and bases in a lab except taste!!!! There are several criteria to consider when deciding whether solutions are acidic or basic. The Arrhenius Theory does not consider all cases such as solutions that prove to be acidic even when there aren t H+ ions present. For that reason, there are other ways to classify Acids and Bases:

3 BRONSTED - LOWRY THEORY OF ACIDS AND BASES: Johannes Bronsted - Danish chemist 1923 and Thomas Lowry - English chemist ACID: BASE: Bronsted - Lowry Acids: HCl (g) + H2O (l) H3O + (aq) + Cl - (aq) STRUCTURALLY: Acid: HCl (donates H + ) Base: H2O (accepts H + ) HYDRONIUM ION: Free protons will always bond to water when in aqueous solution. The Symbol H + is often used because it is more convenient. This ion is acidic since it can donate a proton to a base. Bronsted - Lowry Bases: NaOH (s) Na + (aq) + OH - (aq) dissolve in water Example of a base that does not "fit" the Arrhenius Theory: NH3 (g) + H2O (l) NH4 + (aq) + OH - (aq) ammonia accepts a proton and is therefore a Bronsted base.

4 WEAK / STRONG ACIDS / BASES RECALL THE DEFINITIONS OF WEAK AND STRONG ELECTROLYTES. Acids and Bases are electrolytes because they conduct electric current in solution. Therefore: STRONG ACID: completely ionized in solution eg. Hydrochloric acid is a very strong acid. HCl (g) + H2O (l) H3O + (aq) + Cl - (aq) Even when you dilute HCl, it completely ionizes. Therefore even dilute HCl is a strong acid. WEAK ACID: partially ionized in solution; equilibrium CH3COOH (g) + H2O (l) H3O + (aq) + CH3COO - (aq) STRONG BASE: completely dissociates in water NaOH (s) Na + (aq) + OH - (aq) WEAK BASE: partially dissociates in water; equilibrium NH3 (g) + H2O (l) NH4 + (aq) + OH - (aq) Molecules that differ only by a proton are sometimes referred to as CONJUGATE ACID - BASE PAIRS In any neutralization reaction, there are always two acid-base pairs: NH3 (g) + H2O (l) NH4 + (aq) + OH - (aq) BASE ACID 1 ACID BASE 2 HCN (g) + H2O (l) H3O + (aq) + CN - (aq) BASE ACID 2 ACID BASE 1 AMPHIPROTIC SUBSTANCE: capable of acting as both an acid and a base, SEE ABOVE TWO EXAMPLES - water is an acid in the first example and a base in the second example.

5 Lewis theory: ACID: electron pair acceptor BASE: electron pair donor More on this in University level Chemistry... ph 7: There is approximately 6 x H + ions in a liter of water. Therefore: 6 x (hyrogen ions) x 1 = 1 mole 6 x particles (atoms / ions) 10 7 The notation ph 7 comes from: p: power of 10 H: hydrogen ions 7: the exponent on the power of 10 ACIDIC SOLUTIONS: 6 x (hyrogen ions) x 1 = 1 mole 6 x particles (atoms / ions) 10 1 ph 1 BASIC SOLUTIONS: 6 x (hyrogen ions) x 1 = 1 mole 6 x particles (atoms / ions) ph 14

6 ph ph is the measure of how acidic or basic a solution is. The ph scale runs from 0 to acid base neutral Consider pure water: H2O (l) + H2O (l) H3O + (aq) + OH - (aq) at 25 o C 10-7 M 10-7 M Kw = [H3O + ][OH - ] = (10-7 M) (10-7 M ) = 1 x the equilibrium constant for water ph of pure water: ph = -log[h3o + ] = -log 10-7 = 7 (recall log 10 x = x) therefore water is neutral ph = -log[h3o+] [H3O+] = 10 -ph H2O (l) H + (aq) + OH - (aq) -same equation as before, but writing proton instead of hydronium ion check: [H3O + ] = 10 -ph if water is neutral, then [H3O + ] = 10-7 What is the ph if the concentration of H3O + is 4.2 x 10-5 M? ph = -log[h3o + ] = -log (4.2 x 10-5 ) = 4.38 therefore acidic

7 If [H3O + ][OH - ] always equals at 25 o C, then calculate [OH - ] in the above problem. NOTE ALSO: CONSIDER: 2H2O (l) + kj H3O + (aq) + OH - (aq) As shown above, if temp increases, then Kw increases since it is an endo reaction. Therefore, if Kw = at a certain temperature, then calculate ph. NOTE MATHEMATICAL THEORY: [H+] = 3.16 x 10-6 M = 10.5 x 10-6 = ph = sig figs (because of 3.16) the 5 in front of the decimal does not count for the sig fig poh poh = -log[oh-] [OH-] = 10 -poh Consider: [H3O + ][OH - ] = ph x 10 -poh = take antilogs of both sides (-ph)+(-poh) = -14 multiply through by -1 ph + poh = 14

8 If the ph of a solution is 2.8, calculate: a) [H3O + ] b) [OH - ] c) poh d) Is the solution acidic or basic? Answers: a) [H3O + ] = 1.58 x 10-3 M b) [OH - ] = 6.33 x M c) poh = 11.2 d) acidic!!!! NOW DO WORKSHEET 4.1 FOR HOMEWORK

9 Wkst 4.1: ph and poh [H + ], M [OH - ], M ph poh Acidic/Basic? 1.0 x x x x x x

10 Wkst 4.1: ph and poh [H + ], M [OH - ], M ph poh Acidic/Basic? 1.0 x x Basic 5.0 x x Acidic 9.0 x x Acidic 1.3 x x Basic 3.0 x x Acidic 2.5 x x Acidic 3.2 x x Acidic 1.0 x x Neutral 1.8 x x Acidic 1.3 x x Acidic 3.2 x x Acidic 7.9 x x Acidic