Acids and Bases – Acid Strength and K a

The Nature of Acids Acids – Have a sour taste – Have the ability to dissolve many metals – Turns red litmus paper blue – Neutralize bases – Found in food, used to manufacture fertilizers, glues and dyes. Common acids: – Hydrochloric acid: HCl – Sulfuric acid: H 2 SO 4 – Nitric acid: HNO 3 – Acetic acid: HC 2 H 3 O 2  this is a carboxylic acid (an organic acid that contains the formula COOH. These are substances derived from living organisms.) – Citric Acid: H 3 C 6 H 5 O 7 – Carbonic acid: H 2 CO 3 – Hydrofluoric acid: HF – Phosphoric acid: H 3 PO 4

The Nature of Bases Bases – Have a bitter taste – Feel slippery – Turns red litmus paper blue – Have the ability to neutralize acids – Are used in petroleum and cotton processing, and in soap and plastic manufacturing. They are also found in medication, in products such as Drano, and in the kitchen (an example, baking soda). Common base: – Sodium hydroxide: NaOH – Potassium hydroxide: KOH – Sodium bicarbonate: NaHCO 3 – Sodium carbonate: NaCO 3 – Ammonia: NH 3

Definitions of Acids and Bases There are three definitions of acids and bases, that together correctly defines them. – 1.) Arrhenius’s Definition – An acid is a substance that produces hydrogen (H + ) ions in solution, while bases are substances that produce hydroxide (OH - ) ions in solution. – 2.) Brønsted-Lowery’s Definintion (this theory is the focus in this course! On the regents exam, this is known as the alternate acid-base theory) – Acids are proton (H + ) donors and they become conjugate bases, and bases are proton (H + ) acceptors, and they become conjugate acids. – 3.) Lewis’s Definition (This will not be covered on the AP exam) – Acids are electron pair acceptors and bases are electron pair donors.

Examples or Arrhenius Acids and Bases HCl is an Arrhenius acid because it produces H + ions in solution: HCl(aq)  H + (aq) + Cl - (aq) The hydrogen ions are very reactive, and they react with the water molecules that form the aqueous solutions. Hydrogen ions always associate with water molecules to form hydronium ions (H 3 O + ). NaOH is a base because it produces hydroxide (OH-) ions in solution: NaOH(aq)  Na + (aq) + OH - (aq). Arrhenius acids and bases combine to form water, neutralizing each other in the process: H + (aq) + OH - (aq)  H 2 O(l)

Examples of Brønsted-Lowery Acids and Bases According to Brønsted-Lowery, HCl is an acid because in solution it donates protons: HCl(aq) + H 2 O(aq)  H 3 O + (aq) + Cl - (aq) According to Brønsted-Lowery, NH 3 is a base because in solution, it accepts protons from water. NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq) In the previous equation, if you look at it in reverse, we can see that NH 4 + donates an proton (it is an acid) and OH- accepts a proton (it is a base). The substance that was a base (NH 3 ) becomes an acid (NH 4 + ). The same this goes for the substance that was an acid. These are called conjugate acid-base pairs. NH 4 + an NH 3 are a conjugate acid-base pair. (YES, YOU NEED TO KNOW THIS FOR THE AP EXAM!) As you can see, in the two equations above, water acted both as an acid in one and a base in the other. For this reason, water is considered amphoteric (or amphiprotic) – meaning that it can act as either an acid or a base

Let’s Try a Couple of Practice Problems! In the reaction below, identify the Brønsted-Lowery acid, and base, and the conjugate acid, and the conjugate base. C 5 H 5 N(aq) + H 2 O(l) C 5 H 5 NH + (aq) + OH - (aq) H 2 O is the acid, OH - is the conjugate base, C 5 H 5 N is the base, and C 5 H 5 NH + is the conjugate acid. Which pair is not a conjugate acid-base pair? a.) (CH 3 ) 3 N; (CH 3 ) 3 NH + b.) H 2 SO 4 ; H 2 SO 3 c.) HNO 2 ; NO 2 - b.) H 2 SO 4 ; H 2 SO 3 is not a conjugate acid-base pair because they are both acids.

Acid Strength and the Acid Ionization Constant (K a ) Just as strong and weak electrolytes were defined earlier in the year, a similar definition applies to strong and weak acids. Strong acids are those whose H + ions dissociate fully (they ionize fully), and weak acids consist of H + ions that only partially dissociate (they partially ionize). Let’s look at the following equation: HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) HA is a general formula for an acid! If the equilibrium lies far to the right, the acid is strong (it completely ionizes). If it lies to the left, it is weak (only a small percentage ionizes).

Strong vs. Weak Acids A single arrow  indicates a strong acid (full ionization), while a double sided arrow indicates a weak acid. Examples: HCl(aq) + H 2 O(l)  H 3 O + (aq) + Cl - (aq) HCl is a strong acid, so 1.0 M HCl would yield 1.0 M H 3 O + HF(aq) + H 2 O(l) H 3 O + (aq) + F - (aq) HF is a weak acid, so 1.0 M HF would yield a considerable smaller amount of H 3 O +. Here [H 3 O + ] is much less that 1.O M If the attraction between HA is strong, you have a weak acid. If the attraction between HA is weak you have a strong acid. Generally, the stronger the acid the weaker the conjugate base, and vice versa. Both HCl and HF are considered monoprotic acid, because they consist of one ionizable proton. Polyprotic acids, those with two ionizable protons are considered diprotic, and those with three are triprotic.

Acid Ionization Constant (K a ) Let’s looks at the following two equations: HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) HA(aq) H + (aq) + A - (aq) [H 3 O + ] = [H + ] [H 3 O + ][A - ] [H + ][A - ] So, K a = = [HA] [HA] The smaller the ionization constant, the weaker the acid!

Let’s Try a Practice Problem! Consider these two acids and their K a values: HF K a = 3.5X10 -4 HClO K a = 2.9X10 -8 Which acid is stronger? HF is stronger since its K a value is larger (it has a larger acid ionization constant).

pgs #’s 34, 36, 42, 43 and 44 (For the questions that have multiple parts, only do a and b) Read pgs