topic 8/18 - acids and bases Flashcards

Question

compare the conjugate base of a weak acid to that of a strong acid

Answer 1

the conjugate base of weak acid is stronger (has a higher affinity for a proton) than does the conjugate base of a strong acid

Answer 2

an effective proton acceptor that completely dissociates in water

Answer 3

A strong base is a base that ionizes completely in an aqueous solution. A weak base is a base that ionizes only slightly in an aqueous solution.

Answer 4

because it does not have the capacity to accept a proton.

Answer 5

in solution the hydroxide ion acts as a base, accepting a proton: OH- + H3O+ -> 2H2O

Answer 6

NH3 (aq) + h2O (l) <-> NH4+ (aq) + OH- (aq)

Answer 7

in a stronger acid/base, concentration of ions is greater, making it more conductive

Answer 8

acid - in a strong acid, there is a higher concentration of H+ ions, so lower pH base - in a strong base, there is a higher concentration of OH- ions, so higher pH

Answer 9

stronger acid/base means higher concentration of H+/OH- ions, leading to a higher rate

Answer 10

the neutralisation reaction removes ionised species from the dissociation reaction, so driving the reaction to completion

Answer 11

Vegetation: The acid (H+ ions) can displace metal ions from the soil that are consequently washed away (calcium, magnesium, potassium ions). Mg2+ ions needed to produce chlorophyll, so photosynthesis is stopped Acid rain also causes aluminium ions to dissolve from rocks, which damages plant roots and limits water growth. This can cause stunted growth and thinning/yellowing of leaves on trees. Lakes and rivers: Insect, larvae, fish and invertebrates cannot survive below pH 5.2. No life will survive below pH 4.0. Acid rain can also dissolve minerals like Al3+ from rocks, which can damage aquatic life (aluminium damages fish gills). Buildings: Limestone and marble are eroded by acid rain and dissolve away. This exposes a fresh surface to react with more acid. CaCO3 (s) + H2SO4 (aq)CaSO4 (s)+H2O (l) + CO2 (g) Human health Acids irritate mucous membranes and cause respiratory illnesses such as asthma and bronchitis Acidic water can dissolve heavy metal compounds releasing poisonous ions such as Cu2+, Pb2+ and Al3+ which may be linked to Alzheimer’s.

Answer 12

due to the presence of dissolved carbon dioxide, which forms weak carbonic acid, H2CO3

Answer 13

CO2 + H2O <-> H2CO3 H2CO3 <-> H+ + HCO3- HCO3- <-> H+ + CO32-

Answer 14

Fossil fuels are often contaminated with small amounts of sulfur impurities: - When these contaminated fossil fuels are combusted, the sulfur in the fuels get oxidised to sulfur dioxide - S (s) + O2 (g) → SO2 (g) - Sulfur dioxide may be further oxidised to sulfur trioxide - 2SO2 (g) + O2 (g) ⇌ 2SO3 (g) The sulfur dioxide and sulfur trioxide then dissolve in rainwater droplets to form sulfurous acid and sulfuric acid - SO2(g) + H2O (l) → H2SO3 (aq) - SO3 (g) + H2O (l) → H2SO4 (aq)

Answer 15

H2SO3 (aq) + H2O (l) <-> HSO3- (aq) + H3O+ (aq)

Answer 16

The temperature in an internal combustion engine can reach over 2000 °C Here, nitrogen and oxygen, which at normal temperatures don’t react, combine to form nitrogen monoxide: N2 (g)+ O2 (g) ⇌ 2NO (g) Nitrogen monoxide reacts further forming nitrogen dioxide: 2NO (g) + O2 (g) ⇌ 2NO2 (g) Nitrogen dioxide gas reacts with rain water to form a mixture of nitrous and nitric acids, which contribute to acid rain: 2NO2 (g) + H2O (l) → HNO2 (aq) + HNO3 (aq)

Answer 17

increased industrialisation and economic development in many parts of the world have led to rapidly increasing emissions of the nitrogen and sulfur oxides that cause acid rain

Answer 18

Pre-combustion: Hydrodesulfurization (hydrotreating) Heating crude oil fractions with hydrogen in the presence of a catalyst. This converts the sulphur to hydrogen sulphide (H2S), which can be removed from the reaction mixture by bubbling it through an alkaline solution. The H2S can be subsequently converted back into sulphur and sold to other companies to make sulfuric acid.

Answer 19

Post-combustion: Commonly used in coal-fired power stations Involves passing the exhaust gases from the furnace through a vessel where SO2 can react with alkalis/bases like calcium oxide, carbonate, or hydroxide. CaCO3 (s) + SO2 (g) → CaSO3 (s) + CO2 (g)

Answer 20

CaCO3 (s) + H2SO4 -> CaSO4 + CO2 + H2O

Answer 21

electron pair acceptor

Answer 22

electron pair donor

Answer 23

a coordinate bond

Answer 24

Lewis base

Answer 25

Lewis acid

Answer 26

HA + H2O <-> A- + H3O+ Ka = [A-][H3O+]/[HA]

Answer 27

B + H2O <-> BH+ + OH- Kb = [BH+][OH-]/[B]

Answer 28

- the larger the Ka - the weaker the conjugate base - the smaller the Kb of the conjugate base

Answer 29

- the larger the Kb - the weaker the conjugate acid - the smaller the Ka of the conjugate acid

Answer 30

endothermic

Answer 31

- a rise in temperature will result in the forward reaction being favoured - thus increasing concentration of hydroxide and hydrogen ions - increase in magnitude of Kw - decrease in pH

Answer 32

hydrochloric acid, Hal nitric acid, HNO3 sulphuric acid, H2SO4

Answer 33

lithium hydroxide, LiOH sodium hydroxide, NaOH potassium hydroxide, KOH barium hydroxide, Ba(OH)2

Answer 34

ethanoic acid, CH3COOH carbonic acid, H2CO3 phosphoric acid, H3PO4

Answer 35

ammonia, NH3 ethylamine, C2H5NH2

Answer 36

a solution that resists changes in pH when small amounts of acid or alkali are added to itt

Answer 37

acidic buffer solutions basic buffer solutions

Answer 38

made from a mixture of weak acid and the salt of its conjugate base eg. ethnic acid, CH3COOH (aq) and sodium ethanoate, CH3COONa (aq), a salt of the conjugate base

Answer 39

made from a mixture of a weak base and the salt of its conjugate acid eg. ammonia, NH3 (aq), a weak base, and ammonium chloride, NH4Cl, a salt of the conjugate acid

Answer 40

CH3COOH, a weak acid, dissociates only partially: - equilibrium 1: CH3COOH (aq) <-> CH3COO- (aq) + H+ (aq) CH3COONa, the salt of the conjugate base, is soluble in water so it dissociates completely: - reaction 2: CH3COONa (aq) -> CH3COO- (aq) + Na+ (aq) the buffer solution will contain a high concentration of CH3COO- ions, produced in reaction 2, meaning that the position of equilibrium 1 lies very far to the left, resulting in a high concentration of CH3COOH acid, undissociated, and a relatively low conc of H+ ions 1. adding H+ ions: almost all of the ions will react with the reservoir of CH3COO- ions and equilibrium 1 shifts to the left. the conc of H+ ions hardly changes and, therefore, pH remains almost constant 2. adding OH- ions; almost all of the OH- ions react with the low concentration of H+ ions present to form water. equilibrium 1 then shifts to the right, replacing the H+ ions and therefore pH remains almost constant.

Answer 41

NH3 + H2O <-> NH4+ + OH- NH4Cl -> NH4+ + Cl- when acid (H+) is added, it will react with OH- to form water, causing the equilibrium to shift to the right. when alkali (OH-) is added, it will react with NH4+ and cause the equilibrium to shift to the left.

Answer 42

acid: if a weak acid, such as ethnic acid, is partially neutralised by a strong base, such as sodium hydroxide: NaOH + CH3COOH -> CH3COONa + H2O all of the alkali reacts but some acid remains so the solution contains the weak acid, CH3COOH and its salt CH3COONa, so is a buffer solution. base: when a weak base (NH3) is partially neutralised by a strong acid (HCl) NH3 + HCl -> NH4Cl NH3 is excess, no HCl left, so solution contains NH3 and NH4+ (buffer)

Answer 43

using the Henderson hasselbalch equation

Answer 44

the pH of blood must stay between 7.35 and 7.45; controlled by a mixture of buffers. H2CO3 <-> H+ + HCO3- (H2CO3 <-> CO2.H2O) at high pH values the hydrogen carbonate anion can lose the remaining hydrogen as a proton to give a carbonate buffer: HCO3- <-> CO32- + H+

Answer 45

- strong acid/base - weak acid/strong base - weak base/strong base - weak base/weak acid

Answer 46

draw (p405) - starting point on the pH axis is an important feature of a pH curve as it is an indication of the relative strength of the acid. The strong acid gives an initial pH reading of =1 - there is a gradual rise in the pH as the titration approaches the equivalence point - the sharp rise in the pH at the equivalence point (pH=7) is described as the point of inflection of the curve - once there is no remaining acid to be neutralised, the curve flattens and finishes at a high pH reflecting the strong base eg HCl + NaOH -> NaCl + H2O

Answer 47

- weak acid gives an initial pH reading =3 - the initial rise is steep, as a strong base is being added to a weak acid so neutralisation is rapid - as the weak acid begins to be neutralised the strong conjugate base sodium ethanoate is formed, creating a buffer that resists change in pH. ethanoic acid is in equilibrium with the ethanoate ion: CH3COOH + H2O <-> CH3COO- + H3O+ - the continued addition of base to the solution uses up H+ ions, hence the forward reaction is favoured (=gradual change in pH) - half equivalence is the stage of the titration at which half of the amount of weak acid has been neutralised (pKa=pH) - there is a sharp rise in pH at equivalence point (pH>7), as a result of salt hydrolysis. - with no remaining acid to be neutralised, the curve flattens out. eg CH3COOH + NaOH -> CH3COONa + H2O

Answer 48

HCl + NH3 -> NH4Cl - weak base gives an initial pH reading =11 - as the weak base begins to be neutralised, the ammonium ion NH4+ (conjugate acid) is created resulting in a buffer that resists change in pH: NH3 + H2O <-> NH4+ + OH- - at the half equivalence point half of the amount of weak base has been neutralised. At this point, pOH=pKb - there is a gradual fall in the pH due to the buffering effect as the titration approaches the equivalence point - the pH falls sharply at the equivalence point (pH<7). the equivalence point is the result of salt hydrolysis. - with no remaining base to be neutralised, the curve flattens and ends at a low pH due to the presence of excess strong acid.

Answer 49

- weak base gives an initial reading =11.0 - the change in pH throughout the titration is very gradual - the point of inflection in the pH curve is not as steep as previous pH curves. the point of equivalence is difficult to determine, so this kind of titration has little/no practical use - with no remaining base to be neutralised, the curve flattens and ends at a pH that indicates the presence of a weak acid NH3 + CH3COOH -> CH3COONH4

Answer 50

usually weak acids (represented by HIn) in which the acid (H+) and its conjugate base (In-) are different colours HIn <-> H+ + In- colour a -> colour b when added to acid the equilibrium is pushed to the left and colour A is seen. when added to alkali the OH- combines with H+ so the equilibrium is pushed to the right and colour B is seen.

Answer 51

[H+][In-]/[HIn]

Answer 52

when [HIn] = [In-] at this point [H+] = Ka pH = pKa

Answer 53

the relative strengths of the acid and base and therefore the pH of the equivalence point. The midpoint of an indicator's colour change must correspond to the equivalence point of the titration.

Answer 54

phenol red pKa = 7.9 pH range: 6.8-8.4 acidic colour: yellow alkaline colour: red

Answer 55

methyl orange pKa = 3.7 pH range: 3.1-4.4 acidic colour: red alkaline colour: yellow

Answer 56

phenolphthalein pKa = 9.6 pH range: 8.3-10.0 acidic colour: colourless alkaline colour: pink

Answer 57

BOH (aq) <-> B+ + OH- colour a -> colour b