unit 8: acids and bases

Bronsted Lowry definitions:

• Acid: molecule that donates hydrogen ions

• Based: molecule that accepts hydrogen ions

pH scale for strong acids and bases:

• Above 7: basic

• 7: neutral

• below 7: acidic

HOWEVER, you CAN have pH values that are above and below 14 (common misconception is otherwise)

equations:

• pH= -log [H+] (also can be written as H3O+)

  • to find the concentration of H+ ions use: 10^-pH value

Kw: the equilibrium constant for water

• Kw= [H3O+][OH-] (both concentrations)

• dependent upon temperature

  • as temp. increases, the Kw increases as well

  • as temp. decreases, the Kw decreases as well

• additionally, when the temp increases/decreases, the [H3O+] and [OH-] concentrations change in the same manner

strong acid mnemonic (all + an H+ ion):

SO4

I

Brought

NO3

Clean

ClO4thes

strong bases:

• metal hydroxides from groups 1 and 2

weak acids:

• all acids that are not composed of the six strong acid molecules are considered “weak acids”

• when a weak acids is involved in a reaction, only a certain portion of the acid is dissolved

  • this is completely different to the way strong acids/bases react, completely dissolving unlike weak acids

• equilibrium constant: Ka

  • since the Ka is so small, the pKa is usually recorded instead (later used)

• the weaker the acid, the less ions are created

weak bases:

• all acids that are formed outside groups 1 and 2 are considered “weak bases”

• when a weak base is involved in a reaction, only a certain portion of the base is dissolved

• equilibrium constant: Kb

weak acid and weak base equations:

weak acid standard equation:

HA (aq) + H2O (l) ⇌ H3O+ (aq) + A- (aq)

• HA is the weak acid

• A- is the conjugate base created with the release of the H+ ion

• H3O+ is the main ion byproduct

weak base standard equation:

B(aq) H2O (l) ⇌ HB+ (aq) + OH- (aq)

• B is the weak base

• HB+ is the conjugate acid created with the acceptance of the H+ ion

• OH- is the main ion byproduct

percent ionization:

• the measure of the extent of ionization of an acid

  • strong acids have a 100% percent ionization

  • weak acids, however, do not

    • this formula is used to calculate how much of the initial concentration of the acid has become H+ ion

percent ionization= ([H3O+] at equilibrium)/([HA] initially) x 100

different types of acid-based reactions:

• strong acid+ strong base:

  ex. H+ (aq) + OH- (aq) ⇌ H2O (l)

  • ALL of the acid and base will react

  • to find the pH, find the limiting reagent then solve for pH using the moles of either acid or base left over

• weak acid + strong base:

  ex. HA (aq) + OH- (aq) ⇌ A- (aq) + H2O (l)

  • if the weak acids is in excess, the solution is a BUFFER solution

    • use the Henderson-Hasselbach equation to determine the pH

  • if the strong base is in excess, calculate the same way you calculate when there is a strong acid + strong base

• weak base + strong acid:

  ex. B (aq) + H3O+ (aq) ⇌ HB+ (aq) + H2O (l)

  • if the weak base is in excess, the solution is a BUFFER solution

    • use the Henderson-Hasselbach equation to determine the pH

  • if the strong acid is in excess, calculate the same way you calculate when there is a strong acid + strong base

• weak acid + weak base:

  ex. HA(aq) + B(aq) ⇌ A- (aq) + HB+ (aq)

acid-base titration:

• typically, the known concentration of a base is slowly added to the unknown concentration of an acid

  • then the unknown is calculated for

titration of a strong acid using a strong base:

• the steepest point on the graph corresponds with the equivalency point

• equivalency point: the point in which the moles of H+ ions = OH- ions

titration of a weak acid using a strong base:

• the steepest point on the graph corresponds with the equivalency point (same as strong+strong titration)

• when a strong base is added to a weak acid, the pH will be greater than 7 as conjugate base will be formed to create a basic solution

• half equivalency point:

  • the point at which:

    • pKa=pH

    • [HA]=[A-]

    • the solution is half way to equivalency

  • once you move past the half equivalency point, you have more conjugate base than weak acid in the solution

titrations involving polyprotic acids:

• titrations using acids that donate more than one hydrogen

  • results in multiple equivalency points

the “strength” of an acid or base refers the the Bronsted-Lowry definition:

• the more H+ ions an acid donates, the stronger it is

• the more H+ ions a base accepts, the stronger it is

*****IMPORTANT RELATIONS*****

molecular trends/ structure of acids and bases:

• acids + acids strength:

  • as acids have longer + weaker bonds, they become stronger acids

    • this is because they have a higher tendency to release H+ ions and dissociate

  • as acids contain more and more oxygens, the increase in strength

    • increased electronegativity

    • more stable conjugate base

Henderson- Hasselbach Equation:

pH = pKa + log [𝐴−]/[𝐻𝐴]

things to consider when choosing the correct indicator:

• strong acid + strong base: eq point = 7

• weak acid + strong base: eq point > 7

• strong acid + weak base: eq point < 7

buffer: a solution that contains a conjugate acid-base pair

• used to limit the change in pH of a solution when acids or bases are added

• pH maintenance

  • if an acid is added to a buffered solution, the conj. base will react w the acid to counteract change

  • if a base is added to a buffered solution, the conj. acid will react w the base to counteract change

signs of a buffer solution:

• a strong acid + a weak base

• a weak acid + a strong base

• an acid or base + its conjugates

buffer capacity: the amount of acid or base that can be added to a buffer solution without changing the pH by more than 1 pH unit

• when to comes to capacity, the high the pH the more resistant to change from an acid

• in the same token, the lower the pH, the more resistant to change from a base

the advantage of having more moles of a buffer is that there are more moles to react the acid/base added, meaning more moles to stay within the buffer capacity