Topic 08 introduction
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Transcript of Topic 08 introduction
Introduction to
Acids and Bases
IB Chemistry Power Points
Topic 08
Acids and Baseswww.pedagogics.ca
In aqueous solutions, a proportion of the water molecules dissociate;
The ions formed are H+ or positively charged hydrogen ions and negatively charged hydroxide ions (OH-)
Technically
2 H2O(l) H
3O+
(aq) + OH-
(aq)
Equilibrium Constant Kw = [H + ][OH − ] = 1 x 10-14
Some chemical compounds contribute additional H+ to make the solution more acidic. Other compounds remove H+ ions.
A compound that increases [H+] is called an acid
Examples: HCl, H2SO4, HNO3, CH3COOH
A compound that removes H+ ions from an aqueous solution is called a base.
Often this is done by adding OH- ions for example NaOH, KOH, Ca(OH)2. Soluble bases are called alkalis.
Types of Neutralization Reactions
With hydroxidesacid + base water + a saltHCl + NaOH H2O + NaCl (aq)
• With metal oxidesacid + base water + a salt2 HCl + Cu2O H2O + CuCl2 (aq)
• With ammoniaacid + base a salt HCl + NH3 NH4Cl (aq)
The base is under assault!
Three theories of acidsArrhenius (most common)
Bronsted-LowryLewis
Arrhenius (most common): an acid dissociates to yield H+ and a base dissociates to yield
OH-
Hydrochloric acid H+ + Cl-
Sodium hydroxide Na+ + OH-
Bronsted-Lowryan acid is a proton (H+) donor
and a base is a proton acceptor
Amphoteric and Amphiprotic
Lewis Acid Lewis
Base
Lewis: An acid is an electron pair acceptor
and a base is an electron pair donor
A dative covalent bond is formed
This is a common example that is not an obvious acid/base rxn
Boron trifluoride acts as a Lewis Acid.
The boron has only 6 electron in valence shell so the lone pair of electrons forms a dative bond and fills up the valence shell of the boron
IndicatorsAcids and bases are substances with specific physical and chemical properties.
We can determine if substances are acidic or basic by testing their reaction with indicators.
Indicators are organic substances that change color in the presence of an acid or a base.
Some common indicatorsin acid in base
Litmus red bluePhenolphthalein colorless pinkMethyl orange red yellow
Reactions of acids – examples to knowReact with active metals (above copper in
reactivity series)2 HCl + Ca CaCl2 + H2
Reaction with carbonatesH2SO4 + Na2CO3 Na2SO4 + CO2 + H2O
Reaction with bicarbonatesHNO3 + NaHCO3 NaNO3 + CO2 + H2O
Acid/base properties of Period 3 oxides (topic 3)
Metal oxides Na2O and MgO react with water to form hydroxides (basic solutions)
Na2O + H2O 2 NaOH (aq)
Aluminum oxide is amphoteric (will react as a base with an acid or vice versa)
Al2O3 + 6 HCl 2 AlCl3 + 3 H2O
Other period 3 oxides (non-metal S, P, Cl oxides) react with water to form acidic solutions
SO3 + H2O H2SO4 (aq)
see page 15 in study guide
Acid/base properties of Period 3 chlorides (topic 13)
Chlorides across Period 3 become more acidic across the period
NaCl (aq) is neutral
MgCl2 (aq) is weakly acidic
Chlorides of Al, Si, P, S and Cl2 react with
water to produce HCl (aq) solutions
see Study guide page 16
Strong Acids vs Weak Acids
The strength of an acid or base depends on how easily it dissociates in water.
The dissociation of an acid or base is an equilibrium.
HA(aq) H+(aq) + A-
(aq)
BOH(aq) B+(aq) + OH-
(aq)Strong acids or bases dissociate (ionize) easily – the equilibrium favors the ionic products : kc >> 1
Strong vs Weak
How to tellStrong acids and bases are mostly ionized and therefore solutions are good electrolytes (high conductivity). The pH of the solution can also be measured.
Strong vs Weak
When the strength of an acid or base is discussed, it is very important NOT to confuse “strength” with “concentration”
A 5M acid solution contains 5 mol of acid per dm3 but its strength is determined by how much of that acid is ionized.
Strong acids : HCl, H2SO
4, HNO
3 (mono vs
diprotic)Strong bases : NaOH, KOH, Ba(OH)
2
Weak acids: CH3COOH, H
2CO
3, carbonic acid
CO2(aq)
Weak bases: NH3, ethylamine CH
3CH
2NH
2
Strong Acid
example HCl
HCl(aq) H+(aq) + Cl-(aq)
+ -[H ][Cl ]k = >> 1
[HCl]
• completely dissociated
• pH of 0.1 M soln = 1• strong electrolyte• reacts vigorously • note simplified “net
ionic” equation
Weak Acid
example CH3COOH
CH3COOH (aq) H+
(aq) + CH3COO-
(aq)
+ -3
3
[H ][CH COO ]k = << 1
[CH COOH]
• partially dissociated• pH of 0.1 M soln = 2.9• weak electrolyte• reacts slowly
What is the pH scale?
pH is a measurement of hydrogen ion concentration
It tells you how acidic or basic (or alkaline) something is
Ranges from 0 (most acidic) to 14 (most basic)
log[ ]pH H
How does scale work?
The scale is logarithmic. As you go up or down, the concentration is changed by a power of ten
Example pH 3 is 100 times more concentrated than pH 5
neutral
pH 10 is 100 times less concentrated than pH 8