Ch14 - acids and alkalis
Dilute acids
● Sour taste
● Can conduct electricity (contain mobile ions for carrying the current)
● React w reactive metals
● React w bases
● React w metal carbonates + hydrogencarbonates
● Mineral acids - derived from minerals, irritant
○ Dilute hydrochloric acid (HCl)
○ Dilute sulphuric acid (H2SO4)
○ Dilute nitric acid (HNO3)
Reaction w reactive metals
● Metal + dilute acid → salt + hydrogen
○ Metal + dil HCl → chloride + H2
○ Metal + dil H2SO4 → sulphate + H2
○ Normally, dil HNO3 produce NO (nitrogen monoxide) gas when it reacts w metals
● Hydrogen present in the acid is replaced by metal (above hydrogen in reactivity series)
to give salt
○ Reactive metals - K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb
○ Unreactive metals - Cu, Ag, Au
○ Unsafe to react w Na/ K ⇒ too reactive ⇒ explosion
● Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
○ H2 is released as bubbles in acid
○ Soluble MgCl2 ⇒ magnesium ribbon appears to dissolve in dil HCl
● Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g)
Reaction w bases (=neutralisation)
● Base - substance that reacts w an acid to form salt and water only
● oxide/ hydroxide of metal + dilute acid → salt + water
● CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
○ CuO - insoluble in water
○ CuSO4 - blue solution
●
Zn(OH)2 + 2HCl(aq) → ZnCl2(aq) + 2H2O(l)
○ Zn(OH)2 - insoluble in water
Reaction w metal carbonates + hydrogencarbonates
● Metal carbonate/ hydrogencarbonate + dilute acid → salt + water + carbon dioxide
○ Metal carbonate/ hydrogencarbonate + dil HCl → chloride + H2O + CO2
○ Metal carbonate/ hydrogencarbonate + dil H2SO4 → sulphate + H2O + CO2
● CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
○ CO2 is released as bubbles in acid ⇒ effervescence
○ Test for CO2 - limewater initially turns milky, but then becomes clear again if
excess CO2 is passed through limewater
○ CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l) - milky
○ CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq) - clear again
● 2NaHCO3(s or aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) + 2CO2(g)
Summary
Dilute HCl
Dilute H2SO4
React w reactive metal
Salt + hydrogen
React w base
Salt + water
React w metal carbonate
and hydrogencarbonate
Salt + water + carbon dioxide
Fe + 2HCl → FeCl2 + H2
Ca(OH)2 + 2HCl → CaCl2 + 2H2O
MgCO3 + 2HCl → MgCl2 + H2O + CO2
Dilute
sulphuric
acid
React w zinc
Zinc sulphate + hydrogen
Zn + H2SO4 → ZnSO4 + H2
React w copper(II)
oxide
Copper(II) sulphate + water
CuO + H2SO4 → CuSO4 + H2O
React w potassium
carbonate
Potassium sulphate + water
+ carbon dioxide
K2CO3 + H2SO4 → K2SO4 +
H2O + CO2
React w sodium
hydroxide solution
Sodium sulphate + water
2NaOH + H2SO4 → Na2SO4 +
2H2O
Pure acids
● Solid (crystal) - citric acid, oxalic acid
● Liquid - sulphuric acid, nitric acid, ethanoic acid
● Gas - hydrogen chloride (hydrochloric acid when dissolved in water)
Test
Citric acid crystals/ solid
Aqueous solution of citric acid
Dry blue litmus paper
Remain blue
Turn red
Magnesium ribbon
X reaction
Colorless gas bubbles (H2 gas)
Sodium carbonate
X reaction
Colorless gas bubbles (H2 gas)
Citric acid + water → hydrogen ion + citrate ion
● Citric acid crystals dissolve in water
● Citric acid molecules dissociate/ ionise to produce hydrogen ions (dissociation)
● Ionisation - formation of ions from atoms/ molecules
● Citric acid + water → hydrogen ion + citrate ion
HCl(g) + water → H+(aq) + Cl-(aq)
● Hydrogen chloride gas dissolves in water
● Hydrogen chloride molecules dissociate to produce hydrogen ions
H2SO4(l) + water → 2H+ + SO42-(aq)
HNO3(l) + water → H+(aq) + NO3-(aq)
Hydrogen ion in aq solution
● Acid produces hydrogen ions when dissolved in water
● Hydrogen atom loses its only electron to form hydrogen ion - proton
● Hydrogen ion is too reactive to exist by itself in aq solution
● Hydrogen ion combines w water molecule via dative covalent bond
● Produce hydronium ion - H3O+ - represented by H+(aq)
●
Summary
● Acid - compound which produces hydrogen ions (H+(aq)) as the only positive ions when
dissolved in water
● Water must be present for acid to exhibit its typical properties
Sodium carbonate + oxalic acid powder + water - observation?
● Colorless gas is given out
● When water is added ⇒ oxalic acid dissolves ⇒ its molecules dissociate to produce
hydrogen ions
● The hydrogen ions are responsible for the typical properties of an acid ⇒ giving out
carbon dioxide gas w sodium carbonate
Reaction between magnesium and dilute hydrochloric acid
●
●
●
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Mg(s) + 2H+(aq) + 2Cl-(aq) → Mg2+(aq) + 2Cl-(aq) + H2(g)
Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g)
Reaction between sodium hydrogencarbonate and dilute sulphuric acid
● 2NaHCO3(s or aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) + 2CO2(g)
● 2Na+(aq) + 2HCO3-(aq) + 2H+(aq) + SO42-(aq) → 2Na+(aq) + SO42-(aq) + 2H2O(l) + 2CO2(g)
● 2HCO3-(aq) + 2H+(aq) → 2H2O(l) + 2CO2(g)
● HCO3-(aq) + H+(aq) → H2O(l) + CO2(g)
●
●
●
Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g)
Fe(s) + 2H+(aq) + SO42-(aq) → Fe2+(aq) + SO42-(aq) + H2(g)
Fe(s) + 2H+(aq) → Fe2+(aq) + H2(g)
●
●
●
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
CaCO3(s) + 2H+(aq) + 2Cl-(aq) → Ca2+(aq) + 2Cl-(aq) + H2O(l) + CO2(g)
CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)
Effervescence occurs when lemon juice is added to sodium carbonate powder
● 2HCit(aq) + Na2CO3(aq) → CO2(g) + H2O(l) + 2NaCit(aq)
● 2H+(aq) + 2Cit3-(aq) + 2Na+(aq) + CO32-(aq) → CO2(g) + H2O(l) + 2Na+(aq) + 2Cit3-(aq)
● 2H+(aq) + CO32-(aq) → CO2(g) + H2O(l)
● OR: Na2CO3(s) + 2H+(aq) → 2Na+(aq) + H2O(l) + CO2(g)
Basicity (of acid) - maximum number of hydrogen ions that can be produced by one molecule of
the acid
Hydrochloric acid (HCl) - monobasic acid
● HCl(aq) → H+(aq) + Cl-(aq)
● Every hydrogen chloride molecule produces 1 hydrogen ion in water
Ethanoic acid (CH3COOH) - monobasic acid
●
●
Only the hydrogen atom bonded to the -COO group can be released as hydrogen ion in
water - the only ionisable hydrogen atom
Sulphuric acid (H2SO4) - dibasic acid
● H2SO4(aq) → H+(aq) + HSO4-(aq)
● HSO4-(aq) ⇌ H+(aq) + SO42-(aq)
Phosphoric acid (H3PO4) - tribasic acid
● H3PO4(aq) → H+(aq) + H2PO4-(aq)
● H2PO4-(aq) ⇌ H+(aq) + HPO42-(aq)
● HPO42-(aq) ⇌ H+(aq) + PO43-(aq)
Acids
Chemical formula
Basicity
Hydrochloric acid
HCl
1
Nitric acid
HNO3
1
Ethanoic acid
CH3COOH
1
Carbonic acid
H2CO3
2
Sulphuric acid
H2SO4
2
Phosphoric acid
H3PO4
3
Identify all the ionisable hydrogen atoms in tartaric acid (dibasic acid)
Bases and alkalis
● Base - substance that neutralises an acid to form salt and water only
● Alkalis - bases which are soluble in water (most hydroxides)
○ Sodium hydroxide (NaOH)
○ potassium hydroxide (KOH)
○ Calcium hydroxide (Ca(OH)2)
○ Ammonia (NH3)
● Insoluble bases (most oxides)
○ copper(II) oxide (CuO)
○ Magnesium oxide (MgO)
○ lead(II) oxide (PbO)
○ iron(III) hydroxide (Fe(OH)3)
● Alkali - base which releases hydroxide ions (OH-(aq)) when dissolved in water
Sodium hydroxide (NaOH)
● Alkali (soluble base)
● Ionic solid - when dissolved in water ⇒ crystal lattice is broken down ⇒ ions spread
throughout the solution
● NaOH(s) + water → Na+(aq) + OH-(aq)
Ammonia (NH3)
● When ammonia gas dissolved in water ⇒ some ammonia molecules react w water
molecules ⇒ hydroxide ions are produced
● NH3(g) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
● Making aqueous ammonia
●
●
Inverted filter funnel - increase the area over which the gas can dissolve + prevent
sucking back
Uses of alkalis and bases
Drain cleanser
Sodium hydroxide (NaOH)
Glass cleanser
Ammonia (NH3)
Antacid tablets
Aluminium hydroxide (Al(OH)3)
Magnesium hydroxide (Mg(OH)2)
For reducing soil acidity
Calcium hydroxide (Ca(OH)2)
● Crops grow well in almost neutral soil
● Ca(OH)2 reacts w acid in soil
Dilute solutions of alkalis
● Bitter taste
● Feel slippery to skin (dilute alkalis convert oils in skin into soap)
● Can conduct electricity (contain mobile ions for carrying the current)
● React w solutions containing certain metal ions to form precipitates
● React w ammonium compounds
● Lab
○ Dilute potassium hydroxide solution (KOH) - corrosive
○ Dilute sodium hydroxide solution (NaOH) - corrosive
○ Calcium hydroxide (Ca(OH)2)
○ Dilute aqueous ammonia (NH3) - irritant
Reaction of dilute NaOH(aq) w solutions containing metal ions
Solution
containing
After adding a
few drops of
dilute
NaOH(aq)
Chemical
formula of
ppt formed
After adding
excess
dilute
NaOH(aq)
Ionic equations
Ca2+(aq)
White ppt
Ca(OH)2
Ppt does not
dissolve
Ca2+(aq) + 2OH-(aq) →
Ca(OH)2(s)
Mg2+(aq)
White ppt
Mg(OH)2
Al3+(aq)
White ppt
Al(OH)3
Pb2+(aq)
White ppt
Pb(OH)2
Mg2+(aq) + 2OH-(aq) →
Mg(OH)2(s)
Ppt
dissolves to
give
colorless
solution
⇒ due to
formation of
soluble
complex salt
Al3+(aq) + 3OH-(aq) → Al(OH)3(s)
Al(OH)3(s) + OH-(aq) →
[Al(OH)4]-(aq)
tetrahydroxoaluminate ion
Pb2+(aq) + 2OH-(aq) →
Pb(OH)2(s)
Pb(OH)2(s) + 2OH-(aq) →
[Pb(OH)4]2-(aq)
tetrahydroxoplumbate(II) ion
Zn2+(aq)
White ppt
Zn2+(aq) + 2OH-(aq) →
Zn(OH)2(s)
Zn(OH)2
Zn(OH)2(s) + 2OH-(aq) →
[Zn(OH)4]2-(aq)
tetrahydroxozincate ion
Fe2+(aq) + 2OH-(aq) →
Fe(OH)2(s)
Fe2+(aq)
Green ppt
Fe(OH)2
⇒ green ppt
of Fe(OH)2(aq)
turns brown
on prolonged
standing in air
due to
formation of
Fe(OH)3(aq)
Fe3+(aq)
Reddish
brown ppt
Fe(OH)3
Fe3+(aq) + 3OH-(aq) →
Fe(OH)3(s)
Cu2+(aq)
Pale blue ppt
Cu(OH)2
Cu+(aq) + 2OH-(aq) →
Cu(OH)2(s)
Ppt does not
dissolve
Reaction of dilute NH3(aq) w solutions containing metal ions
Solution
containing
After adding a
few drops of
dilute NH3(aq)
Chemical
formula of
ppt formed
After adding
excess
dilute
NH3(aq)
Ionic equations
Mg2+(aq)
White ppt
Mg(OH)2
Ppt does not
dissolve
Mg2+(aq) + 2OH-(aq) →
Mg(OH)2(s)
Al3+(aq)
White ppt
Al(OH)3
Al3+(aq) + 3OH-(aq) → Al(OH)3(s)
Pb2+(aq)
White ppt
Pb(OH)2
Pb2+(aq) + 2OH-(aq) →
Pb(OH)2(s)
Zn2+(aq)
White ppt
Zn(OH)2
Fe2+(aq)
Green ppt
Fe(OH)2
⇒ green ppt
of Fe(OH)2(aq)
turns brown
on prolonged
standing in air
due to
formation of
Fe(OH)3(aq)
Fe3+(aq)
Reddish
brown ppt
Fe(OH)3
Cu2+(aq)
Pale blue ppt
Cu(OH)2
Ppt
dissolves to
give
colorless
solution
⇒ due to
formation of
soluble
complex salt
Zn2+(aq) + 2OH-(aq) →
Zn(OH)2(s)
Ppt does not
dissolve
Fe2+(aq) + 2OH-(aq) →
Fe(OH)2(s)
Zn(OH)2(s) + 4NH3(aq) →
[Zn(NH3)4]2+(aq) + 2OH-(aq)
tetraamminezinc ion
Fe3+(aq) + 3OH-(aq) →
Fe(OH)3(s)
Ppt
dissolves to
give a deep
blue
solution
⇒ due to
formation of
soluble
complex salt
Cu2+(aq) + 2OH-(aq) →
Cu(OH)2(s)
Cu(OH)2(s) + 4NH3(aq) →
[Cu(NH3)4]2+(aq) + 2OH-(aq)
tetraamminecopper(II) ion
Reaction w dilute NaOH(aq) w ammonium compounds
●
●
●
Heating ammonium nitrate (s or aq) w dilute NaOH(aq) produces ammonia gas, sodium
nitrate and water
NH4NO3(s or aq) + NaOH(aq) → NH3(g) + NaNO3(aq) + H2O(l)
NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)
●
●
Warm the mixture of NaOH(aq) and ammonium compounds
Ammonium compound gives a gas (ammonia) that can turn moist red litmus paper blue
●
Ammonia gas was collected by
upward delivery
○ Ammonia is less dense than
air
●
Water containing universal indicator
solution was sucked into flask
○ Ammonia is very soluble in
water
○ When ammonia gas dissolved,
a partial vacuum formed ⇒
atmospheric pressure would
force the water in the trough
to inject into flask through
glass tube
Observation of universal indicator
○ Water in the flask turned from
green to blue
○ NH3(aq) is alkaline
A flask was filled w dry ammonia gas
produced from the reaction between
ammonium chloride and calcium hydroxide
●
Several cm3 of water were injected into the
flask from syringe
The water containing universal indicator
solution was then automatically sucked into
flask through glass tube
Dilute
NaOH(aq)
+ aluminium
sulphate
solution
White ppt
(aluminium
hydroxide)
Al3+(aq) + 3OH-(aq) → Al(OH)3(s)
Al(OH)3(s) + OH-(aq) → [Al(OH)4]-(aq)
Al(OH)3(s) dissolves in excess dilute NaOH(aq)
due to formation of soluble complex salt
+ iron(III)
chloride
solution
Reddish
brown ppt
Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)
Heat w
ammonium
chloride
solution
Gas
(ammonia)
NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)
+ iron(II)
sulphate
solution
Green ppt
Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)
Distinguish between a solution containing zinc ions vs lead(II) ions
● Dilute aqueous ammonia (NH3(aq))
● Solution containing zinc ions
○ White precipitate (zinc hydroxide) w dilute aqueous ammonia
○ Precipitate dissolves in excess aqueous ammonia
● Solution continuing lead(II) ions
○ White precipitate (lead(II) hydroxide) w dilute aqueous ammonia
○ Precipitate does not dissolve in excess aqueous ammonia
Concentrated acid
● Ordinary conc HCl - 35% hydrogen chloride by mass
○ 350g of dissolved hydrogen in 1000g of solution
● Ordinary conc H2SO4 - 98% sulphuric acid by mass
● Ordinary conc HNO3 - 70% nitric acid by mass (corrosive, oxidising)
○ Tend to decompose to brown NO2 gas ⇒ light can speed up the decomposition
○ Conc nitric acid is often kept in a brown bottle
Diluting concentrated acids
● Dilute concentrated acid by adding it slowly to a large amount of water while stirring
● DO NOT add water to concentrated acid ⇒ heat released during the reaction may cause
the mixture to violently splash out of container + sputter onto one’s face
● Safety precautions
○ Wear safety goggles during dilution
○ Wear protective gloves during dilution
○ Carry out dilution inside fume cupboard
Corrosive
● Concentrated acids and alkalis - corrosive
● Cause chemical burns upon contact with the skin
● Cause complications when ingested
● Safety precautions - wear safety glasses and protective gloves
+ dilute
NaOH(aq)
White
ppt
formed?
Yes
+ excess
dilute
NaOH(aq)
Ppt
Yes
dissolved?
Al3+(aq)
Pb2+(aq)
Zn2+(aq)
No
Ca2+(aq)
Mg2+(aq)
Flame test
Ca2+ - brick-red flame
Mg2+ - no characteristic
flame color
No
What
color?
Green
Fe2+(aq)
Reddish
brown
Fe3+(aq)
Pale blue Cu2+(aq)
Ammonia gas (NH3) formed when mixture is
warmed?
NH4+(aq)
Identify carbonate ions
● Dilute HCl reacts w carbonate to produce carbon dioxide gas
● Carbonate gives carbon dioxide w dilute hydrochloric acid
Distinguish between Na2CO3(s) and NaNO3(s)
● Add dilute HCl to each solid separately
● Na2CO3(s) - effervescence
● NaNO3(s) - no observable changes
Distinguish between CaCl2(aq) and ZnCl2(aq)
● Add dilute NaOH(aq) to each solution until in excess
● CaCl2(aq) - white precipitate which does not dissolve in excess dilute NaOH(aq)
● ZnCl2(aq) - white precipitate which dissolves in excess dilute NaOH(aq)
OR
● Flame test
● CaCl2(aq) - brick-red flame
● ZnCl2(aq) - no characteristic flame color
Hygroscopic
● Volume of concentrated sulphuric acid increases when it is left in air
● Acid absorbs water vapour from air ⇒ hygroscopic
Deliquescent
● When anhydrous calcium chloride is left in air
● It absorbs water vapour from air + forms very concentrated solution ⇒ deliquescent
Drying agents
● Hygroscopic and deliquescent substances - drying agents
● Drying agents - widely used in food, electronic products and manufacturing industries
● Obtain a dry sample of gas - pass the gas through a drying agent
●
Drying agents in lab
○ Concentrated sulphuric acid (H2SO4)
○ Anhydrous calcium chloride (CaCl2)
○ Calcium oxide (CaO)
○
●
●
●
●
Concentrated sulphuric acid and anhydrous calcium chloride CANNOT be used to dry
ammonia gas ⇒ both react w ammonia gas
Calcium oxide is used to dry ammonia
Calcium oxide CANNOT be used to dry acidic gases (eg CO2, SO2)
Drying agents are used in desiccators ⇒ maintain dry environment for chemicals
●
●
●
●
End of delivery tube for the incoming gas should be immersed into the concentrated
sulphuric acid ⇒ incoming gas should pass through the drying agent
Calcium oxide CANNOT be used to dry HCl(g) ⇒ calcium oxide reacts w HCl(g) ⇒ should
use anhydrous calcium chloride
Correct diagrams
●
Distinguish between ammonium sulphate, magnesium chloride and potassium carbonate
● Test with dilute sodium hydroxide solution
○ Add dilute sodium hydroxide solution to solution of the solid
○
○
●
●
●
Solution of magnesium chloride gives a white precipitate.
Solutions of the other two solids (i.e. ammonium sulphate and potassium
carbonate) give no observable change
Warming the mixture
○ Warm the mixture obtained in the first test.
○ Solution of ammonium sulphate gives a gas that turns moist red litmus paper
blue.
○ Solution of potassium carbonate gives no observable change.
OR Test with dilute hydrochloric acid
○ Add dilute hydrochloric acid to the solid.
○ Potassium carbonate gives effervescence.
○ The other two solids (i.e. ammonium sulphate and magnesium chloride) give no
○ observable change.
Test with silver nitrate solution
○ Add silver nitrate solution to solution of the solid.
○ Solution of magnesium chloride gives a white precipitate.
○ Solution of ammonium sulphate gives no observable change.
Ch15 - molarity, pH scale and strength
Indicators - dyes extracted from natural sources
Neutral (orginal color)
In acid (dilute HCl)
In alkali (dilute NaOH)
Litmus solution
Purple
Red
Blue
Litmus paper
Blue
Red
Red
Blue
Methyl orange
Red
Yellow
Phenolphthalein
Colourless
Pink
pH - measure of concentration of hydrogen ions in a solution
⇒ from 1 mol dm-3 to 10-14 mol dm-3
Universal indicator solution/ pH paper
● Acidic solutions - bright red color
● Alkaline solutions - violet color
pH meter
● When electrode is placed in a solution, the meter shows a reading of pH
pH sensor attached to data-logger
● Advantages - can plot graph, accurate
● Disadvantages - expensive, need calibration before use
Saliva
● pH 5-6
● Slightly acidic
Oven cleanser
● pH 13-14
● Very alkaline
Strong acids and weak acids
● Ethanoic acid - less acidic than hydrochloric acid of the same concentration
● Difference - concentration of hydrogen ions in the acid
● Strong acid - acid that dissociates almost completely in water to give hydrogen ions
● Weak acid - acid that dissociates partially in water to give hydrogen ions
● HCl(aq) → H+(aq) + Cl-(aq)
○ When hydrogen chloride gas dissolves in water, almost all hydrogen chloride
molecules dissociate to give hydrogen ions and chloride ions
○ HCl(aq) - strong acid
● CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)
○ When pure ethanoic acid dissolves in water, only a small fraction of the molecule
dissociate into hydrogen ions and ethanoate ions
○ CH3COOH(aq) - weak acid
● CO2(g) + H2O(l) ⇌ H2CO3(aq)
○ H2CO3(aq) ⇌ H+(aq) + HCO3-(aq)
○ HCO3-(aq) ⇌ H+(aq) + CO32-(aq)
● Strong acids - HCl, H2SO4, HNO3
Acid
Main types of particle present (besides water molecules)
Ethanoic acid (CH3COOH(aq))
H+, CH3COO-, CH3COOH
Carbonic acid (H2CO3(aq))
H+, HCO3-, CO32-, CO2, H2CO3
0.1 mol dm-3 H2SO4(aq)
●
●
●
Strong acid
Dibasic
Higher concentration of hydrogen ions than monobasic
acids ⇒ lower pH than monobasic acids
0.1 mol dm-3 HCl (aq)
●
●
Strong acid
Monobasic
0.01 mol dm-3 NaOH(aq)
●
0.1 mol dm-3 NaOH(aq)
More alkaline than 0.01 mol dm-3 NaOH
0.1 mol dm-3 HCl(aq)
0.1 mol dm-3 CH3COOH(aq)
[H+(aq)]
Concentrations of
hydrogen ions
High
Low
pH
Low (more acidic)
High
Electrical conductivity
High
Low
Relative rate of
reaction w magnesium
High (more rapid bubbling)
Low
Distinguish between weak monobasic acid and strong monobasic acid (both pH 2)
●
●
●
●
Dilute each acid 10 times and then 100 times
Strong acid - pH will first increase by 1 unit and then by 2 units
Weak acid - pH changes will be less
Strong alkalis and weak alkalis
● Strong alkali - alkali that dissociates almost completely in water to give hydroxide ions
● Weak alkali - alkali that dissociates partially in water to give small amount of hydroxide
ions
●
●
●
NaOH(aq) + water → Na+(aq) + OH-(aq)
○ Strong alkali - almost completely dissociate into ions in waters
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
○ Weak alkali - only a small fraction of molecules react w water molecules to form
hydroxide ions
Strong alkalis - NaOH, K2SO3, Ba(OH)2
0.1 mol dm-3 NaOH(aq)
0.1 mol dm-3 NH3(aq)
[OH-(aq)]
Concentrations of
hydroxide ions
High
Low
pH
13
11
Electrical conductivity
High (more alkaline)
Low
Temperature rise
when alkali is
neutralised by dilute
HCl of the same
concentration
High
Low
Concentration vs strength
● Concentrated - relatively large amount of pure acid is dissolved in a relatively small
volume of water
● 6 mol dm-3 of HCl - concentrated solution of strong acid
● 6 mol dm-3 of CH3COOH - concentrated solution of weak acid
● Strength of acid - how much it dissociates in water - almost completely vs partially
100 cm3 of 0.10 mol dm-3 HX(aq) vs 100 cm3 of 0.00010 mol dm-3 HY(aq) (both monobasic + pH
4) - which acid is stronger
Show NaOH(aq) is stronger alkali than NH3(aq) through experiment
●
●
●
Measure pH ⇒ NaOH(aq) is higher
○ Same concentration of NH3(aq) and NaOH (aq)
Measure electrical conductivity ⇒ NaOH(aq) is higher
○ Same concentration and volume of NH3(aq) and NaOH (aq)
Measure temperature rise when neutralised by dilute HCl of same concentration ⇒
NaOH(aq) is higher
○ Same concentration and volume of NH3(aq) and NaOH (aq)