U8: Acids and Bases

○ An adjective that describes a chemical that can accept a proton from another molecule, or donate a pair of electrons. Anything above 7 on the pH scale

○ Any substance that will cause the hydroxide ion (OH-) concentration to increase.

○ Bitter taste

○ Turn Litmus Blue

○ Slippery feel

○ Ex. Ammonia, baking soda, lye, antacid

● 7 on the pH scale is neutral, and an example would be water

Calculating pH

● 1.0 * 10-14 is the equilibrium constant for the autoionization of water (Kw)

● Equilibrium occurs in all solutions (acid, base, neutral)

○ All aqueous solutions have a delectable concentration of both hydronium and hydroxide ions

○ The product of all of these ion concentrations is always Kw

○ Kw = H3O+ * OH+ * 1.0 * 10-14

● If concentration of hydroxide ion is known, to determine the pOH:

○ pOH = 14 - pH

● If pOH is known, to determine pH:

○ pH = 14 - pOH

● If the hydronium concentration is known, to determine the hydroxide ion concentration :

○ (OH-) = 1.0 * 10-14/ (H3O+)

● Calculate pH from the hydronium concentration:

○ pH = log (H3O+)

● If pH is known, to determine the hydronium concentration:

○ (H3O+) = 10-pH

Buffer Solutions

● A weak acid or base mixed with its conjugate

● It resists a change in pH when H+ or OH- is added

● when H+ or OHis added to the buffer:

○ Determine which buffer component reacts

○ Work out the stoichiometry to find the new concentrations of bummer components

○ Use equilibrium constant to determine the new pH

● Ways to make buffers

○ Weak acid + Conjugate Base

○ Weak Base + Conjugate Acid

○ Excess Weak Acid + Strong Base

○ Excess Weak Base + Strong Acid

○ Excess Salt + Strong Acid or Base

● The acid-dissociation equilibrium constant, which measures the tendency of an acid to disassociate and is described with the Buffer Equation:

○ H+ = Ka(HA)/(A-)

○ For a buffer to be effective you must have a high concentration of acid/conjugate base compared to H+ or OH- added, so that H+ = (HA)/(Ka(A-))

○ Note that (H) depends on the ratio of (HA) to (A). If that ratio stays fairly consistent then (H) doesn't change much.

● Determine the exact amount of acid and conjugate base needed to make the buffer for a certain pH through the Henderson-Hasselbach equation

ICE Tables

● Initial, Change, Equilibrium tables are very helpful when trying to understand equilibrium and for calculating pH of a buffer solution

○ Use initial concentrations of reactants and products

○ Evaluate changes they undergo during the reaction and their equilibrium concentrations

Titration Curves

● Plots of pH vs standardized acid or base added

● These curves will have difference according to what is being titrated

○ Strong Acid - strong base

■ The equivalence point’s pH is 7.0

○ Weak acid - strong base

■ The equivalence point is basic

○ Strong Acid - weak Base

■ The equivalence point is acidic

■ Base has been neutralized but its conjugate acid remains

● Titrate is added incrementally and pH is determined each step

● Summary of Titration Curves

○ Start: Find the pH from dissociation of acid/base

○ Until equivalence point: Use buffer equation after stoichiometry

○ At equivalence point: pH is dictated by disassociation of conjugate acid/base

○ Past equivalence point: pH is dominated by excess H+ or OH-

Acid-Base Reaction

● A type of chemical process where there’s an exchange of one or more hydrogen ions, H+, between solutions that may be neutral or electrically charged ions

Acid Base Indicators

● Most are weak bases that change colors when they lose their H+

● Ka/H+ = (In-)/(HIn)

● In general, indicators change color at pH values close to their pKa

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