Date A.P. Biology Ch. 3

Definitions Acid-- “proton donor;” molecule that is able to give up protons (H + ) Strong acids release H + easily Weak acids do not give up H + so fast Base--”proton acceptor;” molecule that is able to bind H + Strong bases always accept H + (may even “rip” H + off of other molecules); Can also produce OH - (hydroxide ions) Weak bases may bind H+, but it can come off just as easily

Water as an Acid and Base Water is able to act as a weak acid and a weak base H 2 O + H 2 O (H 3 O + ) + (HO - ) [H 3 O+] hydronium, [HO-] hydroxide ion Reaction occurs instantaneously both ways, in equilibrium under normal conditions

Importance of Acids Acids: increases [H+] starts interfering with intermolecular and intramolecular bonds disrupts hydrogen bonds that are inside and outside of molecules Examples: Citric acid, stomach acid/digestive acid

Importance of Bases Bases: Decreases [H+] Can literally change the polarity of a molecule (rips H off of nonpolar molecules, leaving behind unbonded atoms, which are “very unhappy”) Turns non-polar molecules into polar molecules (becomes soluble) Examples: Bleach/ammonia, Peptobismol, Baking Soda

pH Scale Power of Hydrogen (pH) Based off of concentrations of [H+] found in solutions Converts negative base 10 exponents into single digit scale pH 1 pH 14 Acidic Basic Lots of H+ Very Little H+

Finding pH Example: A solution has a [H+] of = 1.0 x Absolute value of exponent = 4 pH of solution is 4 (acid or base?) Does this solution have a lot of H+ or little H+?

pH Scale Note that pH refers to [H+], but [HO-] can be calculated Neutral pH is 7, both [H+] & [HO-] are equal A decrease in pH means and INCREASE in [H+] Lowering pH by one number means a 10x increase in [H+]

Chemical Equilibrium Chemical Equilibrium: Equilibrium is reached in a reaction when forward and backwards process occur at a set ratio No net gain/loss in a chemical equilibrium If one side becomes unequal, reaction will shift and “drive” in opposite direction Question: How can water be neutral pH if it always produces H +

Buffers Buffer--substance that can resist changes in pH (both increases or decreases) by acting as proton donor AND acceptor Buffers usually consist of a weak acid and its base H 2 CO 3 HCO H+ **Buffers work by keeping equilibrium; If one side of equation increases, buffer will shift to keep equation in equilibrium **More H+ will drive reaction in reverse, More H 2 CO 3 will drive reaction forward. Carbonic Acid (Weak Acid, H+ donor) Bicarbonate ion (Weak Base, H+ acceptor

Status Check Questions 1. One definition of an acid is that it A. Always donates a proton B. Always donates an electron C. Always donates a neutron D. Always donates hydroxide ion (OH - )

Status Check Questions 2. If the pH of a solution drops, then A. The concentration of H+ drops B. The concentration of H+ rises C. No net change change occurs D. Solution will move towards equilibrium

Status Check Questions 3. If the pH of a solution increased from pH 7 to pH 9, this means that A. There was a 10 fold increase in H+ B. There was a 100 fold increase in H+ C. There was a 10 fold decrease in H+ D. There was a 100 fold decrease in H+

Status Check Questions 4. A substance that is “neutral pH” means that A. It has no H+ in it B. It has no OH- in it C. It has equal H+ and OH- in it D. It is a buffer solution

Status Check Questions Use the equation to answer the following questions: A. What compound is the acid? (hint: “donor”) B. What compound is the base? (hint: “acceptor”) C. What would happen if there were more H+ added to the reaction? D. What would happen if the pH of the reaction increased? NH 3 + H + NH 4 +