CMH 121 Luca Preziati Chapter 8: Acids and Bases Acid = produces H + An acid is a compound that: 1. Has H somewhere 2. Has the tendency (is capable) of giving that H (as H + ) to another compound: Proton Donor Base = produces OH - in water A base is a compound that: 1. Not all bases have an OH group. 2. Has the tendency (is capable) of accepting H + from another compound: Proton Acceptor

CMH 121 Luca Preziati Chapter 8: Acids and Bases Acid = produces H + Base = produces OH -

CMH 121 Luca Preziati Chapter 8: Acids and Bases Strong acid: Strong acid: One that reacts completely with water to form H 3 O + ions. Strong base: Strong base: One that reacts completely with water to form OH - ions. Here are the six most common strong acids and the four most common strong bases:

CMH 121 Luca Preziati Chapter 8: Acids and Bases Weak acid: Weak acid: A substance that dissociates only partially in water to produce H 3 O + ions. Acetic acid, for example, is a weak acid. In water, only 4 out every 1000 molecules are converted to acetate ions: Weak base: Weak base: A substance that only partially reacts with water to produce OH - ions. Ammonia, for example, is a weak base:

CMH 121 Luca Preziati Chapter 8: Acids and Bases Conjugate acid-base pair: Conjugate acid-base pair: Any pair of molecules or ions that can be interconverted by transfer of a proton.

CMH 121 Luca Preziati Chapter 8: Acids and Bases

CMH 121 Luca Preziati Chapter 8: Acids and Bases monoprotic, diprotic, or triprotic Acids are classified a monoprotic, diprotic, or triprotic depending on the number of protons each may give up; examples are HCl, H 2 CO 3, and H 3 PO 4. Amphiprotic: Amphiprotic: A substance that can act as either an acid or a base. The HCO 3 - ion, for example, can give up a proton to become CO 3 2-, or it can accept a proton to become H 2 CO 3.

CMH 121 Luca Preziati Chapter 8: Acids and Bases When a weak acid, HA, dissolves in water The equilibrium constant expression, K eq, for its ionization is Because water is the solvent and its concentration changes very little when we add HA to it, we treat [H 2 O] as a constant equal to 1000 g/L or 55.5 mol/L. We combine the two constants to give a new constant, which we call an acid ionization constant, K a

CMH 121 Luca Preziati Chapter 8: Acids and Bases K a for acetic acid, for example is 1.8 x Because the acid ionization constants for weak acids are numbers with negative exponents, we commonly express acid strengths as pK a where: The value of pK a for acetic acid is 4.75.

CMH 121 Luca Preziati Chapter 8: Acids and Bases

CMH 121 Luca Preziati Chapter 8: Acids and Bases Chemical Properties of Acids and Bases: Neutralization Acid + Base  Water + Salt HCl + NaOH  H 2 O + NaCl Others : Reaction with metals Reaction with Metal Hydroxides Reaction with Metal Oxides Reaction with carbonates and bicarbonates Reaction with Ammonia and Amines

CMH 121 Luca Preziati Chapter 8: Acids and Bases pH pH = -log [H + ] Acidic solution: Acidic solution: One whose pH is less than 7.0. Basic solution: Basic solution: One whose pH is greater than 7.0. Neutral solution: Neutral solution: One whose pH is equal to 7.0.

CMH 121 Luca Preziati Chapter 8: Acids and Bases pH buffer: pH buffer: a combination of substances that act together to prevent a drastic change in pH A pH buffer is an acid or base “shock absorber.” A pH buffer is commonly referred to simply as a buffer. The most common buffers consist of approximately equal molar amounts of a weak acid and a salt of the weak acid; that is, approximately equal molar amounts of a weak acid and a salt of its conjugate base. For example, if we dissolve 1.0 mole of acetic acid and 1.0 mole of its conjugate base (in the form of sodium acetate) in water, we have an acetate buffer.

CMH 121 Luca Preziati Chapter 8: Acids and Bases How does an acetate buffer resist changes in pH? If we add a strong acid, such as HCl, added H 3 O + ions react with acetate ions and are removed from solution: If we add a strong base, such as NaOH, added OH - ions react with acetic acid and are removed from solution:

CMH 121 Luca Preziati Chapter 8: Acids and Bases The average pH of human blood is 7.4. Any change greater than 0.10 pH unit in either direction can cause illness. To maintain this pH, the body uses three buffer systems: Carbonate buffer: Carbonate buffer: H 2 CO 3 and its conjugate base, HCO 3 - Phosphate buffer: Phosphate buffer: H 2 PO 4 - and its conjugate base, HPO 4 2- Proteins Proteins.