3.9 Acids & Bases

Question 1

Lowry-Bronsted theory??

Answer 1

the Lowry-Bronsted theory states that acid-base equilibria involves the transfer of protons bet substances and substances can be classified as acids or bases depending on their interaction w/ protons

Question 2

define a Lowry-Bronsted acid and give an example:

Answer 2

A Lowry-Bronsted acid is a proton donor e.g. Ammonium ions (NH4+)

Question 3

define a Lowry-Bronsted base and give an example:

Answer 3

A Lowry-Bronstead base is a proton acceptor e.g. Hydroxide ions (OH-)

Question 4

describe the diff bet a strong acid & a weak acid??

Answer 4

a strong acid dissociates almost completely in water meaning nearly all H+ ions are released
a weak acid only partially dissociates in water so only a small number of H+ ions released

Question 5

describe the diff bet a strong base & a weak base??

Answer 5

a strong base dissociates almost completely in water so nearly all OH- ions are released
a weak base only partially dissociates in water so only a small number of OH- ions released

Question 6

expression of pH in terms of [H+]

Answer 6

pH = -log[H+]
[H+] = 10^-pH

Question 6

rs bet pH & hydrogen ion conc, [H+]??

Answer 6

the pH scale is a measure of hydrogen ion conc, the lower the pH the higher the conc of hydrogen ions

Question 7

give examples of strong acids & state the pH range which indicates a strong acid??

Answer 7

examples: HCl, H2SO4, HNO3
pH range of strong acids 0-3

Question 8

give examples of weak acids & state the pH range which indicates a weak acid??

Answer 8

examples: CH3COOH, hydrogen sulfide(H2S) any organic carboxylic acid
pH range of weak acids: 4-just below 7

Question 9

give examples of strong bases & state the pH range which indicates a strong base??

Answer 9

examples: NaOH, KOH, Ca(OH)2)
pH range of strong base: 12-14

Question 10

give examples of weak bases & state the pH range which indicates a weak base??

Answer 10

examples: ammonia(NH3), methylamine(CH3NH2)
pH range of weak bases: just above 7 to 11

Question 11

what is the acid dissociation constant Ka?

Answer 11

the acid dissociation constant Ka is a measure of how strong an acid is in a soln

Question 12

formula used to calc Ka for rctm of form HA(aq) <–> H+(aq) + A-(aq)

Answer 12

Ka = [H+][A-]/[HA]

Question 13

units for Ka??

Answer 13

moldm^-3

Question 14

how does strength of acid relate to value of Ka??

Answer 14

Ka is the equilibrium constant for the dissociation of an acid: HA <–>H+ + A-
the stronger the acid, the equilibrium lies to the RHS so higher conc of prod causing Ka to increase

Question 15

why is Ka used to find the pH of a weak acid??

Answer 15

weak acids only partially dissociate in water so conc of [H+] ions is not the same as the acid conc(as w/ strong acids) meaning the pH cant be found using [H+] so Ka is used

Question 16

formula used to find Ka of a weak acid??

Answer 16

for a weak acid assume that all H+ ions in soln come from the acid so [H+] = [A-] & you can assume that [HA]equilibrium = [HA]start
so for weak acid: Ka =[H+]^2/[HA]

Question 17

diff bet describing an aid/base as ‘conced’ compared to ‘strong’?

Answer 17

‘conced’ implies there are many moles per dm^3
‘strong’ relate to dissociation of substance implies the acid/base almost completely dissociating in water

Question 18

formulas used to convert bet Ka & pKa?

Answer 18

pKa = -log(Ka)
Ka = 10^-pKa

Question 19

eqn for the ionic prod of water??

Answer 19

Kw = [H+][OH-]

Question 21

pH of pure water at room temp??

Answer 21

7

Question 22

how does the pH at which water is neutral ([OH]=[H+]) change as the temp increases??

Answer 22

in the equil of water the forward rctn is endothermic & so is favoured when temp of water is increased, so pH at which water is neutral(when conc of H+ & OH- is same) decreased when temp increased.

Question 23

pH curve??

Answer 23

graphs which plot pH against vol of acid/base added are called pH curves & can be used to help identify the point of neutralisation of a soln

Question 24

what is the equivalence point on a pH curve??

Answer 24

equivalence point also called the end point at the point the pH curve is vertical where solution has been neutralised

Question 25

pH curve to identify type of reactants involved in rctn if pH starts at 1 of ends up very high

Answer 25

strong base added to strong acid and pH intitially at around 1 as strong acid is in excess, pH ends up very high as a strong base is in excess

Question 26

pH curve to identify type of reactants involved in rctn if pH starts at jus below 7 of ends up very high

Answer 26

strong base added to a weak acid pH starts about 5 where there’s an excess in weak acid and finishes w/ high pH where there’s an excess of a strong base

Question 27

pH curve to identify type of reactants involved in rctn if pH starts just above 7 and ends jus below7

Answer 27

weak acid is added to weak base pH starts abt 8-9 where there’s excess weak base, finishes w/ pH around 5 when there’s excess of weak acid

Question 28

buffer soln

Answer 28

a buffer soln is a soln which is able to resist changes in pH when small vols of acid/ base are added
a buffer soln commonly formed from a weak acid & a its salt or a weak base & its salt, prods a mixture containing H+ ions & a large pool of OH- ions which helps to resist any change in pH

Question 29

how is an acidic buffer soln made my mixing sodium ethanoate w/ ethanoic acid??

Answer 29

ethanoic acid is a weak acid so will partially dissociate leaving lots of undissociated ethanoic acid molecules remaining in soln, the sodium ethanoate will fully dissociate prod lots of ethanoate ions CH3COO-. equil set up which will move to counteract changes in pH, CH3COOH(aq) <–> H+(aq) + CH3COO-(aq)

Question 30

consider the buffer soln made by mixing sodium ethanoate w/ ethanoic acid, how does the pH change when a small amnt of acid is added??

Answer 30

equilibrium is set up w/in the buffer soln: CH3COOH <–> H+ + CH3COO-
is a small amount of acid is added the conc of H+ increases, most of the extra H+ ions combine w/ CH3COO- ions to form CH3COOH so the equilibrium shits to LHS reducing conc of H+ near its OG value so pH doesnt change

Question 31

explain the significance of buffer solns in nature??

Answer 31

buffer solns are common in nature in order to keep systems regulated, enzymes in living organisms often req an optimum pH & this can be maintained by a buffer soln

Question 32

why are buffers used in industrial processes??

Answer 32

industrial processes use buffer solns to maintain the optimum rctn conds for large scale manufacturing

Question 33

examples of where buffer solns are used in industrial processes??

Answer 33

buffers are used in fermentation, buffer solns added before fermentation begins to prevent the soln from becoming too acidic
theyre also used in fabric dyeing processes & to perform chemical analysis

Question 34

salt hydrolysis??

Answer 34

a rctn where one of the ions from a salt reacts w/ water to form an acidic or basic soln

Question 35

what needs to be considered when selecting an indicator for a titration??

Answer 35

indicator chosen for a titration must change colour at exactly the end point of the titration, the indicator should change colour over a narrow pH range which coincides w/ the vertical section of pH curve

Question 36

examples of indicators which can be used in a titration & describe colour changes??

Answer 36

phenolphthalein low pH colourless, pH of colour change 8.3-10, high pH pink
methyl orange low pH red, pH of colour change 3.1-4.4, high pH yellow

Question 37

why should a pH meter be used instead of an indicator in weak acid/ weak base titrations??

Answer 37

in weak acid/weak base titrations there is no sharp pH change so very diff to successfully use an indicator to get an accurate point of neutralisation, a pH meter used to increase accuracy & precision