CHEM12 - unit 4A - acids + base part 1

part one - acids and base part 1

arrhenius theory

• acid

• base

• salt

acid: formula starts with H → releases H+ in solution

base: formula ends with OH → releases OH- in solution

salt: ionic compound produced from a neutralization reaction

• HA + BOH → BA + H2O

acid properties (irl)

• turns litmus paper red

• corrosive

• sour

• react w/ metals

what do H+ form in water

since positive charge of H+ strongly attracted to negatively charged regions of water… forms H3O+ (hydronium ions)

• water is polar (H+ forms a bond at one of O’s lone pairs)

what do we call H+ also

protons since its made of one proton and nothing else

is

• HCl (g) → H+ (aq) + Cl- (aq)

the true ending? is it fully done?

no, bc H+ forms H₃O+ with water so… its really:

• HCl (g) + H2O (l) → H₃O+ (aq) + Cl- (aq)

when do we need to add water as a part of an reactant?

• whenever H+ will be one of the products in the “original”

bc if H+ will be one of the products… it will react with water !!

Bronsted-Lowry Theory

- consider what

• acid

• base

-not all bases contain OH-

-consider eqb equations

• acids: proton donors (give up H+)

• base: proton receives (receive H+)

hydrolysis reactions def

eqb eqs involving acid or base reacting with water

• acids react w/ water to produce H₃O+

• bases react w/ water to produce OH-

define:

• monoprotic acids

• diprotic acids

• polyprotic acids

• monoprotic acids = can donate 1 proton

• diprotic acids = can donate up to 2 protons

• polyprotic acids = can donate >1 protons

amphiprotic substances def

• eg

species that can act as either acid or base depending on the circumstance

• water and polyprotic acids that have lost at least 1 proton (at least one H and at least one negative charge)

• eg. HCO₃ -

on both sides of equilibrium eq will have what

-will have both acid + base on both sides

• they will switch as which is which bc eqb

-overall charge is same

define:

• conjugate acid + base

• conjugate acid

• conjugate base

-pair of chemical species that are the SAME EXCEPT for 1 proton

• conjugate acid = the one with the extra proton (protonated)

• conjugate base = the one without the extra proton (non-protonated)

how to tell if species are ONLY ACIDS or ONLY BASES

• egs

  • HCl

  • CN-

  • -COOH

HCl : has proton to donate but no neg charge to gain a proton = ACID ONLY

CN- : has a negative charge to gain a proton but no proton to donate = BASE ONLY

- carboxylic acids, organic acids that end in -COOH: has proton to donate but no neg charge to gain a proton = ACID ONLY

organic acids that end in -COOH

• what are they

ACID ONLY bc no negative charge to gain a proton

each eqb reaction has 2 different conjugate pairs

eg. SO₃ ^2- + H₂O ⇌ HSO₃ ^- + OH-

1. identify which acid which base

2. identify the two different conjugate pairs

1. SO₃ ^2- + H₂O ⇌ HSO₃ ^- + OH-

   B A ⇌ A B

2. one: SO3 2- & HSO3-

   two: H2O & OH-

strong and weak acids and bases def

• SA + SB

• WA + WB

• SA + SB = fully ionize/ionize 100%, one-way arrow

• WA + WB = do not fully ionize/ionize <100%, eqb arrow

notes about Nitrogen about acid or base

• -NH2 or -NH is BASE ONLY

• how to tell if NH3 is the acid or base at start..

  • *note that N is not stable with two lone pairs

  • NH3 (one lone pair) cannot be acid and turn into NH2-

strong vs weak is NOT SAME as concentrated vs dilute

• eg. 0.0001M HNO3 vs. 10M HF

0.0001M HNO3: dilute bc small [ ]; strong bc one-way arrow

10M HF: concentrated because big [ ]; weak bc eqb arrow

-acids produce what in solution

-what happens if the acid is really strong

-produce H₃O+ in sol.

-the stronger (eqb arrow; top of arrow) the acid, the greater [H₃O+]

-bases produce what in solution

-what happens if the base is really strong

-produces OH- in sol.

-the stronger (eqb arrow; bottom of arrow) the base, the greater [OH-]

spectator ions

• who

• exceptions

• the 5 “bases” at the top of table are conjugate bases of strong acids

  • but too weak to act as bases = spectators

• HSO₄ -: can’t act as a base since conjugate base of strong acid BUT also a weak acid = NOT spectator

• exceptions: if also a weak acid

-strongest acid

-how is acid strength measured

• strongest acid is H₃O+ ; all strong acids ionize completely to form H₃O+

• acid strength is a measure of [H₃O+]

Cl- is what

• conjugate base of strong acid (one way arrow)

• spectator ion since its only that ^

levelling effect def

all strong acids and strong bases are EQUALLY strong (all ionize 100%… cant be more than that)

salts that produce O^2- (metal oxides) are

• eg?

metal oxides are strong bases (makes O^2-)

• eg. CaO

strongest base is ____ + forms from what in water

• strongest base is OH-

• formed from O^2- or NH- in water

  • NH- + H₂O → NH₃ + OH-

  • O^2- + H2O → 2OH-

how to find [OH-] in other species

• eg. in 4.0M Na₂O

two ways

1. multi step way

   Na₂O → 2Na + O^2-

   O^2- + H2O → 2 OH-

   = [OH-] = 8.0M

2. adding up (net eq?) way

   Na₂O → 2Na + O^2-

   O^2- + H2O → 2 OH-

- + -—-———————————-

Na₂O + H₂O → 2NaOH

= [OH-] = [NaOH] = 8.0M

how do we know if base needs adding water

if it forms O^2- which can’t be stable on own

how to decide which of amphiprotic reactants are acting as acid and base

• eg. H2C6H5O7 + HPO4 ⇌ HC6H5O7 + H2PO4

-determining the act as acid: whichever is the STRONGER weak acid (high on arrow; eqb arrow) acts as the acid/donates H+

-the other acts as base

• eg. H2C6H5O7 + HPO4 ⇌ HC6H5O7 + H2PO4

  • since H2C6H5O7 is higher on arrow/stronger weak acid… will donate H+

how to determine if reactants or products are favoured

• eg. H2C6H5O7 + HPO4 ⇌ HC6H5O7 + H2PO4

-compare the 2 weak acids in the reactions (will be on opposite sides)

-the STRONGER (look at arrow) weak acid will ionize more therefore pushing it to other way more ∴ other side w/ weaker weak acid is favoured

• eg. H2C6H5O7 + HPO4 ⇌ HC6H5O7 + H2PO4

  • since H2C6H5O7 is higher on list/stronger weak acid than H2PO4… its stronger/push more… favours products (think like make more)

• when products favoured, more [ ] of those are made

for a reaction between strong base and strong acid.. whats the net ionic eq

H+ + OH- → H2O

-bc water is the new substance formed

-the salt is aq so not in net eq

kw

• what is it

• formula

• value at 25* or room temp

equilibrium constant for water

• Kw = [H3O+] [OH-]

• 1.00 × 10^-14

since Kw is very small, what does this tell us

-water does not ionize to a large extent

• not a lot is not made?

-concentration of [H3O+] and [OH-] in pure water is very small + always equal

• think that [H3O+] and [OH-] concentration = 1.00 × 10^-7

what happens to value of Kw if at diff temp

• Kw will be diff since temp. dependent

  • therefore [ ] of H3O+ and OH- will be diff too

what happens if strong acid or strong base added to water

• eg. HCl + H2O → H3O+ + Cl-

• will impact ionization of water eq

  • 2H2O ⇌ H3O+ + OH-

eq scenario:

• adding strong acid (adding H3O+) ∴ shift left

• ∴ most of OH- will be consumed during shift BUT NOT ALL

*see graph

will all of OH- or H3O+ be consumed during shifts from adding strong acid or strong base

NO.

• will always be some OH- present in acid sol.

• will always be some H3O+ present in basic sol.

since weak acids + weak bases don’t ionize 100%… they__-

-form eqb ∴ can define an eqb expression

• this is Keq expressions for behaviour of acid and base in water

-larger the value of Ka..

-larger the value of Kb

-Ka and Kb tells us what

-the MORE the weak acid ionizes ∴ stronger the weak acid

-the MORE the weak base ionizes ∴ stronger the weak base

-Ka/Kb is the measure of acid/base STRENGTH for WEAK acids/bases

is there Ka/Kb for strong acids/base ?

no bc strong acid/base fully ionize (one-way arrow) ∴ don’t have eqb

Kw formulas

Kw = [H3O+] [OH-]

Kw = (Ka)(Kb)

since Kb not in table… how to find

• eg. find Kb for HCO^3-

use Kw = (Ka)(Kb)

• HCO³ + H2O ⇌ H2CO + OH-

• Kb = (Kw)/Ka_{(of base’s conjugate acid/on product side)}

• Kb = Kw / Ka(of H2CO)

pH neutral at ___ (what number) and at ____ (what temp.)

7 at 25*

pH formula

pOH formula

together formula

[H3O+]

[OH-]

pH = -log[H3O+]

pOH = -log[OH-]

pKw = pH + pOH

[H3O+] = 10^-pH

[OH-] = 10^-pOH

if need to find pH but from eq of a base

• find [OH-]

• use Kw =[H3O+][OH-] to find [H3O+]

• pH = -log[H3O+]

sf for pH / pOH

number of decimal places is the sf

value of pKw at 25*

14.000 (3 sf)

why is pKw = pOH + pH

Kw = [H3O+][OH-]

log both sides

logKw = log[H3O+][OH-]

*multi log rule = logAB = logA + logB

-logKw = -log[H3O+] + -log[OH-]

pkW = pH + pOH

word problems of making solutions of a certain pH

• eg what mass of Sr(OH)2 is needed to make 1.25L of sol. with pH =13.30

• eg. what vol of water should be added to 1.00L of HCl with pH=2.25 to increase pH to 2.75

• eg1.

  • find equivalent pOH since given is a base

  • find [OH-] needed

  • dissociation eq to find how much [ ] of OH- needed within given Sr(OH)2

  • mol ratio to find mass (start w/ 1.25L)

• eg2.

  • find initial [H3O+]

  • find final needed [H3O+]

  • find final vol will have (sol.) by dilution (M2V2 = M1V1)

  • subtract initial vol of sol. from final vol will have of sol. = vol. of water needed to be added

when do we know if [H3O+] and [OH-] change equally or not

depends on scenario…

1. if in pure water only and there’s a temp. change… then will change equally

2. if add in an acid OR base… then only that [H3O+] or [OH-] will increase and then consumed (unequally)