Date

A.P. Biology Ch. 3

DefinitionsAcid-- “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 BaseWater is able to act as a weak acid and a weak

baseH2O + H2O (H3O+) +

(HO-)[H3O+] hydronium, [HO-] hydroxide ionReaction occurs instantaneously both ways, in

equilibrium under normal conditions

Importance of AcidsAcids:

increases [H+]starts interfering with intermolecular and

intramolecular bonds disrupts hydrogen bonds that are inside and

outside of moleculesExamples: Citric acid, stomach

acid/digestive acid

Importance of BasesBases:

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 ScalePower of Hydrogen (pH)

Based off of concentrations of [H+] found in solutionsConverts negative base 10 exponents into single digit scale

10-1 10-14

pH 1 pH 14

Acidic Basic

Lots of H+ Very Little H+

Finding pHExample: A solution has a [H+] of 0.0001

0.0001 = 1.0 x 10-4

Absolute value of exponent = 4pH of solution is 4 (acid or base?)Does this solution have a lot of H+ or little

H+?

pH ScaleNote 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 EquilibriumChemical Equilibrium:

Equilibrium is reached in a reaction when forward and backwards process occur at a set ratio

No net gain/loss in a chemical equilibriumIf one side becomes unequal, reaction will shift

and “drive” in opposite direction Question: How can water be neutral pH if it always produces H+

BuffersBuffer--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 baseH2CO3 HCO3- + 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 H2CO3 will drive reaction forward.

Carbonic Acid(Weak Acid, H+ donor)

Bicarbonate ion(Weak Base, H+ acceptor

Status Check Questions1. One definition of an acid is that it

A. Always donates a protonB. Always donates an electronC. Always donates a neutronD. Always donates hydroxide ion (OH-)

Status Check Questions2. If the pH of a solution drops, then

A. The concentration of H+ dropsB. The concentration of H+ risesC. No net change change occursD. Solution will move towards equilibrium

Status Check Questions3. If the pH of a solution increased from pH 7

to pH 9, this means thatA. 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 Questions4. A substance that is “neutral pH” means that

A. It has no H+ in itB. It has no OH- in itC. It has equal H+ and OH- in itD. It is a buffer solution

Status Check QuestionsUse 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?

NH3 + H+ NH4+