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Acids & bases. How does pH value affect our environment?. Recall Questions. What is an acid? What is a base/alkali?. A substance which produces hydrogen ions (protons) when dissolves in water. A base refers to substances like metal oxides and metal hydroxides. - PowerPoint PPT Presentation
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ACIDS & BASESHow does pH value affect our environment?
Recall QuestionsWhat is an acid?What is a base/alkali?
•A substance which produces hydrogen ions (protons) when dissolves in water.•A base refers to substances like metal oxides and metal hydroxides.•A substance which reacts with acid to form salt and water only•An alkali is a soluble base which in solution produces hydroxide ions. •Most bases are insoluble in water. 3 soluble bases are NaO/NaOH,KO/KOH,CaO/Ca(OH)2
Recall QuestionsWhat causes acidity?
•It is the hydrogen ions that give an acid its acidic properties when they dissolve in water and dissociate into ions. E.g. HCl gas is a covalent compound. When dissolves in water, it forms HCl acid which dissolciate to form ions.
Recall QuestionsWhat is basicity(proticity)?
Basicity• refers to the no.of H atoms in one molecule of acid that acn be repleced by a metal.•refers to the no. of H+ that can be replaced by one molecule of that acid. E.g. HCl (monobasic),H2SO4(dibasci),H3PO4(tribasic)
Bronsted-Lowry theory
An acid is defined as a molecule or ion that acts as a proton donor (H+).
A base is defned as a molecule or ion that acts as a proton acceptor (H+).
HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)
Hydrogen chloride gas dissolved in water
HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)
The equation can be split into(i) HCl(aq) Cl-(aq) + H+(aq) acid conjugate base
(ii) H2O(l) + H+(aq) H3O+(aq) base conjugate acid
Ammonia gas dissolved in water
NH3(g) + H2O(l) NH4+(aq) + OH-
(aq)
What does ammonia and water act as in the reaction above?
Competition between acid/base and its conjugate(i) HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) acid base conjugate acid conjugate
base
(ii) CH3COOH(l) + H2O(l) H3O+ (aq) + CH3OOO-(aq)
acid base conjugate acid conjugate base
(i) Water is a much stronger base than chloride ion and has a stronger tendency to accept proton.The equilibrium shifts more to the right.
(ii) Ethanote ion is a much stronger base than water molecule. The equilbrium shifts to the left.
Gas-phase acid-base reaction HCl(g) + NH3(g) NH4Cl(s)
The Bonsted-Lowry model can be extended to gas-phase acid-base reaction.
It involves the transfer of hydrogen ion from hydrogen chloride to ammonia.
(i) HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) acid base conjugate acid conjugate base
(ii) CH3COOH(l) + H2O(l) H3O+ (aq) + CH3OOO-
(aq) acid base conjugate acid conjugate
base
Strong acids produce relatively weak conjugate bases in aqueous solutions.
Weak acids produce relatvely strong conjugate bases in aqueous solutions.
Common acids & conjugate bases in order of strengths
Acids that have single proton to donate – monoprotic (monobasic).
E.g. HCl(aq), HNO3(aq), HNO2(aq) Acids that have 2 protons to donate –
diprotic E.g. H2SO4(aq), H2SO3(aq), H2CO3(aq) H3PO4(aq) is triprotic.
Lewis theory A Lewis acid is defined as a substance
that can accept a pair of electrons from another atom to form a dative (coordinate) covalent bond.
A Lewis base is defined as a substance that can donate a pair of electrons to another atom to form a dative (coordinate) covalent bond.
B: H+ +BH
Examples
Reaction between ammonia and proton
H3N: H+ +NH4 Reaction between a water molecule and
proton
H2O: H+ H3O+
Lewis bondingIn complex ions formed by transition metals
The 6 water molecules, each donate a lone pair electrons from oxygen of their water molecules to the empty 3d orbitals of iron.
What does each water molecule and iron(III) ion act as in the reaction above?
Properties of acids and basesConstruct a concept map to show the
physical and chemical properties of acids and bases
Preparation of SaltsConstruct a concept map to show a
summary of the preparation of salts.
Strong and weak acids and bases
Strong acid When strong acid
dissolves, virtually all acid molecules react with the water to produce hydronium ions In general for a strong acid HA
HA + H2O(l) H3O+(aq) + A-(aq) or HA H+(aq) + A-(aq) 0% 100% 0% 100%
Examples : HCl, H2SO4,HNO3, HClO4
Strong and weak acids and bases
Weak acid When a weak acid
dissolves in water, only a small % of its molecules (typically 1%) react with water molecules to release hydrogen or hydronium ions. The equilibrium lies on the left-hand side of the equation.
HA + H2O(l) H3O+(aq) + A-(aq) or HA H+(aq) + A-(aq) 99% 1% 99% 1%
Examples : CH3COOH, aqueous carbon dioxide
Distinguish between strong and weak acids
Base on the information above, how do we distinguish betwee strong and weak acids of the same concentration (e.g. HCl and CH3COOH)?
0.1 mol dm-3 HCl(aq)
0.1 mol dm-3 CH3COOH (aq)
[H+(aq)] 0.1 mol dm-3 - 0.0013 mol dm-3 pH 1.00 2.87Electrical conductivity high lowRelative rate of reaction with magnesium
fast slow
Relative rate of reaction with calcium carbonate
fast slow
How to distinguish between strong and weak acids? A weak acid has a lower concentration of hydrogen ions
and hence a higher pH than a stronger acid of the same concentration.
A weak acid, because of its lower concentration of hydrogen ions, will have much poorer electrical conductor than a stronger acid of the same concentration.
Weak acids react more slowly with reactive metals, metal oxides, metal carbonates and metal hydrogencarbonates than strong acids of the same concentration.
Strong and weak acids can also be distnguished by measuring and comparing their enthalpies of neutralisation.What is the difference between the strength (strong and weak)
and the concentrated (concentrated or dilute)?
Strong and weak acids and bases
Strong base A strong base undergoes almost 100%
dissociation/ionisation when in dilute aqueous solution.
BOH B+(aq) + OH-
(aq) 0% 100%
Examples : NaOH, KOH, Ba(OH)2
Strong and weak acids and bases
Weak base All bases are weak except the hydroxides of
groups 1 and 2. Weak bases are composed of molecules that react
with water molecules to release hydoxide ions. In general for a weak molecular base, BOH
The equiibrium lies on the left side of the equation.
BOH + (aq) B+(aq) + OH-
(aq)
Examples : aqueous ammonia, ethylamine, caffeine, bases of nuclei acids
The pH indicator scale that measures the strength of an
acid and alkali. pH of a substance is measured when it is
dissolved in water. pH stands for “power of hydrogen” [H+] = 1 x 10-n moldm-3 ( n = pH number)
The pH Scale
pH probe and meterAn accurate method of measuring pH value.A pH probe is dipped into the solution being
testedand the pH value is then read directly from
the meter.
pH Calculation pH is a measure of the concentration of H+ ions in a
solution. pH = -log10[H+(aq)]
Example:
If the concentration of H+ is 2.50 x 10-3 moldm-3 , what is the pH?
pH = -log (2.50 x 10-3) = 2.60
Example:
Calculate the concentration of H+ of a solution that has a pH = 3.2.
-log[H+] = 3.2log [H+] = -3.2 [H+] = 6.31 x 10-4
Example:
(a) What is the pH pf 10cm3 of 0.1 moldm-3 HCl? pH = -log (0.1) = 1
(b) If 90cm3 of water is added to the acid, what happens to the pH? Total volume = 100cm3 In 10cm3 solution, concentration of H+ is 0.1 moldm-3
In 100cm3 solution, concentration of H+ is 0.01 moldm-3
pH = -log (0.01) = 2
(c) If the solution from (b) is diluted by a factor of 105 , what is the approximate pH?The pH will increase by 4 to 6.