IB Chemistry Power Points Topic 08 Acids and Baseswww.pedagogics.ca Introduction to Acids and Bases
In aqueous solutions, a proportion of the watermolecules dissociate; The ions formed are H+ or positively charged hydrogen ions and negatively charged hydroxide ions (OH-) Technically + - 2 H2O(l) H3O (aq) + OH (aq) + − -14 Equilibrium Constant Kw = [H ][OH ] = 1 x 10
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 hydroxides acid + base water + a salt HCl + NaOH H2O + NaCl (aq)• With metal oxides acid + base water + a salt 2 HCl + Cu2O H2O + CuCl2 (aq) • With ammonia acid + base a salt HCl + NH3 NH4Cl (aq)
Lewis:An acid is an electron pair acceptor Lewis Acid Lewis Baseand a base is an electron pair donorA dative covalent bond is formed
This is a common example that is not an obvious acid/base rxnBoron trifluoride acts as a Lewis Acid.The boron has only 6 electron in valence shell so the lonepair of electrons forms a dative bond and fills up thevalence shell of the boron
IndicatorsAcids and bases are substances withspecific physical and chemicalproperties.We can determine if substances areacidic or basic by testing their reactionwith indicators.
Indicators are organic substances thatchange color in the presence of an acid ora base.Some common indicators in acid in baseLitmus red bluePhenolphthalein colorless pinkMethyl orange red yellow
Reactions of acids – examples to know React with active metals (above copper in reactivity series) 2 HCl + Ca CaCl2 + H2 Reaction with carbonates H2SO4 + Na2CO3 Na2SO4 + CO2 + H2O Reaction with bicarbonates HNO3 + 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 AcidsThe strength of an acid or base depends on howeasily 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 WeakHow to tellStrong acids and bases are mostly ionized and thereforesolutions are good electrolytes (high conductivity). The pH ofthe solution can also be measured.
Strong vs WeakWhen the strength of an acid or base is discussed, it isvery important NOT to confuse “strength” with“concentration” 3A 5M acid solution contains 5 mol of acid per dm but itsstrength is determined by how much of that acid isionized. Strong acids : HCl, H2SO4, HNO3 (mono vs diprotic) Strong bases : NaOH, KOH, Ba(OH) 2 Weak acids: CH3COOH, H2CO3, carbonic acid CO2(aq) Weak bases: NH3, ethylamine CH3CH2NH2
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) [H+ ][CH3COO- ] k= << 1 [CH3COOH] • 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) pH log[ 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