Mod 6

Question 1

properties of acids and bases

Answer 1

acids –> sour, corrosive, conductive in solution, turns blue litmus red, ph< 7
bases –> bitter, caustic, conductive in solution, turns red litmus blue, ph> 7, soapy feel in solution

Question 2

indicator transition ranges

Answer 2

methyl orange –> red orange yellow, 3.1 - 4.4
bromothymol blue –> yellow green blue, 6.0 - 7.6
litmus –> red purple blue, 5.5 - 8.0
phenolphthalein –> colourless pale pink pink/magenta, 8.3 - 10.0

Question 3

antoine lavoiser

Answer 3

said acids were substances that contain oxygen
had many flaws cuz a bunch of metal oxides like CaO were basic
some acidic substances lacked oxygen like HF and HCl which are strong acids
said bases were things that neutralised acids

Question 4

humphrey davy

Answer 4

proposed acids had replacable hydrogens (partially or fully)
did not account for acids without hydrogen like acidic oxides CO2 NO2 SO2
couldnt account for basic nature of basic oxides
said bases were things that neutralised acids

Question 5

svante arrhenius

Answer 5

acids were substances that ionised to release h+ ions in aq solution
bases were substances which dissociated to release oh- ions in aq solution
able to explain difference in potency of acids (strong and weak) through complete and incomplete ionisation
worked for many but not all acids and bases (useful for hsc course)
does not recognise role of solvent in the strength of acid (HCl strong in water weak in diethyl ether)
couldnt explain why some oxides and carbonates are basic but dont contain oh- or release oh- in aq
couldnt explain some acidic and basic salts (NH4Cl, CH3COOK)

Question 6

bronsted lowry

Answer 6

acid is a substance that donates protons
base is a substance that accepts protons
explains how some ions or gaseous species act as acids/ bases
considers role of solvent
can explain behaviour in non-aqueous environment
explains how some substances are amphiprotic
explains non-hydroxide bases
cannot explain acids which don’t have a proton in its structure (to donate) such as PbO

Question 7

amphiprotic and amphoteric

Answer 7

amphoteric can act as acid or base
amphiprotic can act as acid or base specifically due to its ability to accept or donate a proton
all amphiprotic substances are amphoteric but not all amphoteric are amphiprotic
h2o, hso4-, h2po4-, hpo42-, hco3-, can be amphiprotic
PbO can be amphoteric but not amphiprotic due to lewis theory of acids

Question 8

Lewis theory

Answer 8

lewis acid accepts electron pair
lewis base donates electron pair

Question 9

proticity

Answer 9

monoprotic –> 1 hydrogen (HCl)
diprotic –> 2 hydrogen (H2SO4)
triprotic –> 3 hydrogen (H3PO3)

Question 10

acid strength

Answer 10

degree of ionisation is [H3O+] produced/ [HA] initial (ratio of total ions produced over total molecules)
strong acids have 100 percent ionisation as it completely ionised in aqueous solution (unidirectional arrow)
weak acid does not completely ionise (partially ionises in aqueous solution) (bidirectional arrow)

Question 11

strong acids

Answer 11

HCl hydrochloric
HBr hydrobromic
HI hydroiodic
HNO3 nitric
H2SO4 sulfuric (only first ionisation)
HClO4 perchloric

Question 12

conjugate acids and bases

Answer 12

when acid donates proton, it forms a conjugate base
when base accept proton, it forms a conjugate acid
conj base of weak acid is weak base and vice versa
conj base of strong acid is very weak base (almost neutral)
conj acid of strong base is very weak acid (almost neutral)

Question 13

arrows in neutralisation

Answer 13

one or 2 strong substances is full arrow
2 weak substances is equilibrium

Question 14

concentration

Answer 14

concentration is the ratio of the molecules solute to the solvent
higher concentration means more h3o+ or h+ ions in solution, decreasing ph

Question 15

antacid tablets

Answer 15

tablets that neutralise gastric acid/ stomach acid (hcl)
contain bases that neutralise acids in stomach that cause acid reflux
mg(oh)2 and al(oh)3 –> weak insoluble bases meaning non caustic when orally ingesting

Question 16

toothpaste

Answer 16

weakly alkaline –> no exothermic rxn
bacteria in oral cavity turn sugars into acids
toothpaste neutralises –> caco3 al2o3 mgco3

Question 17

industrial acid and base spills

Answer 17

acids and bases are caustic and corrosive so spills must be neutralised
need to be cheap safe and easy to store
weakly acidic or basic in excess so less dangerous
solid to contain spill (liquid spreads spill)
amphiprotic to neutralise acids and bases
nahco3 useful –> also creates bubbles when reacted to indicate process of rxn

Question 18

what is titration plus key terms (equiv, end, t error, titre, titrant, analyte, standard solution)

Answer 18

A titration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution
equiv point –> when equal moles of h3o+ and oh-mhave been reacted together
endpoint –> point at which sustained colour change is observed in rxn using appropriate indicator
titration error –> difference between equiv point and endpoint
titre –> minimum volume of added substance to reach endpoint of neutralisation rxn
titrant –> solution used to determine conc of unknown solution
analyte –> solution of which’s conc is to be determined
standard solution –> a solution with a known conc

Question 19

titration steps

Answer 19

pick out primary standard (water soluble, high purity, definite chem composition, stable, hgih molecular weight, inexpensive and readily obtainable, non-volatile and safe) pick primary standard opposite to analyte. also avoid weak acid weak base titration due to vague endpoint. Acidic –> hydrated oxalic acid cooh . h20, Basic –> Na2CO3
prepping standard solution: dry primary standard in dessicator. weight approx amount in beaker prev cleaned with distilled water. dissolve measured mass in distilled water. transfer to volumetric flasked cleaned with distilled water. rinse out beaker 3 times with distilled water into volumetric flask to ensure all primary standard is transferred. fill volumetric flask with distilled water (meniscus bottom to gradation line). stopper the top and invert 20 times to homogenise.
select appropriate indicator
rinse all glassware with distilled water. rinse pipette and burette with substance that they will contain to avoid distilled water diluting it
doing titration: transfer 25 ml of standard solution to conical flask using pipette and pipette bulb. first rinse with distilled water then with substance of aliquot. then empty aliquot into conical flask (dont blow into pipette –> contamination. instead touch tip to side of flask for 5 seconds to get all droplets out.). add few drops of indicator to conical flask. place flask in front of white background.
rinse burette with distilled water then with the titre 3 times by filling and rolling liquid to sides. repeat 3 times. (remove bubbles by flicking glass and open tap for 2 seconds) (tip of burette instead neck of conical flask and record the titre volume in burette.
perform rough titration quickly and slow down when colour starts to change (once colour change lasts for more than 15 seconds titration is done)
perform 3 precise titrations after knwoing the approx rough value of colour change and then do calcs

Question 20

acid and base dissociation constant ka kb

Answer 20

is ka is large, equil lies to right and degree of ionisation is high
if ka is small, equil lies to left and degree of ionisation is low
therefore acids with lower ka are less acidic (weaker)
when ka&raquo_space;> 1, pka «< 0, strong acid
when ka < 1, pka > 0, weak acid
when ka «< 1, pka&raquo_space;> 0, very weak acid
for kb, when kb is higher, base is stronger
ka x kb = kw = 1.0 x 10^-14
therefore conjugate acid base pairs can be used to find each others ka/kb

Question 21

acetylsalisylic acid (ASA)

Answer 21

active ingredient in aspirin hc9h8o4
weak monoprotic so naoh or strong base used for titration
in direct titration with naoh, it is in powder form and can clump, hence, it is first back titrationed in ethanol solvent as it is more soluble in ethanol than water
a blank (titration with no aspirin… ethanol and naoh only) and back titration is done and difference in titre is volume of naoh required to completely neutralise ASA
back titration used for solid powders and slow neutralisation
asa reacts slowly with naoh so treating tablets with excess naoh, then titrating further excess naoh with hcl gives true quantity of ASA in tablet

Question 22

aboriginal acid base stuff

Answer 22

ochres and clays contain hydroxides of transition metals –> feo(oh) . nh2o (yellow), fe2o3 (red), al2(si2o5)(oh)4 (kaolin or white ochre)
would ingest ochres and clays for remedy for stomach discomfort (acid reflux) to neutralise stomach acid
OH- in ochre + HCl(aq) –> H2O + Cl-
used leaves to neutralise stings becuase stings are usually acidic (insect, marine sting/bite, snake)
Pig Face –> WA –> Extract juice from leaves –> Marine animal stings
Goat’s Foot –> NT and NSW –> Crush and heat leaves –> Stingray and Stonefish stings
Hop Bush –> QLD –> Chew leaves and extract juice –> Stingray and Stonefish stings

Question 23

Buffers

Answer 23

A buffer is a solution that can resist pH change upon the addition of an acidic or basic components
composed of comparable/similar concentration of weak acid and conj base, or weak base and conj acid
similar quantities of HA and A- are combined in single aq solution coming to equilibrium
h2co3 + h2o <=> hco3- + h3o+
if add acid (h3o+) equilibrium shifts left thus overall there is minimal change to conc and therfore ph (LCP minimises change) (acid consumed by hco3-)
if add base (oh-) equilibrium shifts right thu overall there is minimal change to conc and therfore ph (LCP minimises change) (base consumed by h2co3)

Question 24

how to explain buffers in exam

Answer 24

definition
2 equations stating conj base and acid/vice verse
lcp to explain minimising change
say what happens when acid added and base added (substance is consumed by other reactant on same side)

Question 25

buffers in human blood

Answer 25

ph of blood regulated to 7.35 - 7.45 which serves homeostasis (natural equilibrium of body)
body created co2 during respiration as byproduct, reacting with blood to form h2co3
h2co3 converted to hco3- using carbonic anhydrase enzyme, creating an equilibrium with conj acid and base of similar conc
if ph of blood becomes acidic and h3o+ rises, equilibrium shifts left to produce more co2, buffering blood pH (rate of respiration increases and co2 breathed out increases)
co2 + h2o <=> h2co3
h2co3 + h2o <=> hco3- + h3o+
hco3- + h2o <=> h2co3 + oh-
during hyperventilation, more co2 is used so oh- rises, therfore, equil is shifted like (eq3) and hco3- rises, buffering blood pH (rate of respiration decreases) (could faint)