Topic 8 Acids and bases 6 hours

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1 Topic 8 Acids and bases 6 hours Hydronium ion (H3O + ) = more stable form of hydrogen ion (H + ) H + + H2O H3O Theories of acids and bases 2 hours 1. Arrhenius H-X / M-OH ACID a substance that dissociates in water to produce one or more hydrogen ions (H + ) H-X, where X is any anion ex. hydrochloric acid HCl (aq) H + (aq) + Cl (aq) BASE a substance that dissociates in water to produce one or more hydroxide ions (OH ) M-OH metal hydroxides ex. sodium hydroxide NaOH(aq) Na + (aq) + OH (aq) Define acids and bases according to the Brønsted Lowry and Lewis theories. Discuss the value of using different theories to explain the same phenomenon. What is the relationship between depth and simplicity? Deduce whether or not a species could act as a Brønsted Lowry and/or a Lewis acid or base Deduce the formula of the conjugate acid (or base) of any Brønsted Lowry base (or acid). Students should make clear the location of the proton transferred, for example, CH 3COOH/CH 3COO rather than C 2H 4O 2/C 2H 3O Brønsted-Lowry ACID = PROTON DONOR BASE = PROTON ACCEPTOR Proton p + = H + hydrogen nucleus ACID a substance that donates a proton to another substance (PROTON DONOR) ex. acetic acid (vinegar) CH3COOH É CH3COO + H + carboxylic acids have a carboxyl group (COOH) that is stable with a H + removed BASE a substance that accepts a proton from another substance (PROTON ACCEPTOR) ex. ammonia NH3 + H + É NH4 + ammonia is a Brønsted-Lowry base 3. Lewis ACID = ELECTRON ACCEPTOR BASE = ELECTRON DONOR Gilbert Lewis based his definition on chemical bonding theory where pairs of valence electrons form bonds ACID a substance that can accept a pair of non-bonding electrons from a Lewis base (lone pair). BASE a substance that can donate a pair of non-bonding electrons to a Lewis acid

2 8.2 Properties of Acids and Bases 1 hour Outline the characteristic properties of acids and bases in aqueous solution. Bases that are not hydroxides, such as ammonia, soluble carbonates and hydrogencarbonates, should be included. Alkalis are bases that dissolve in water. Students should consider the effects on indicators and the reactions of acids with bases, metals and carbonates. Property ACID BASE Definition Senses Arrhenius form H + (H3O + ) ions form OH ions Brønsted-Lowry proton (H + ) donor proton (H + ) acceptor Lewis Lone pair acceptor Lone pair donor Feel soapy Taste sour bitter Measure ph LOW ph (ph < 7) HIGH ph (ph > 7) Indicators Litmus red blue Phenolphthalein clear pink Bromothymol Blue yellow blue Reactions Carbonates (baking soda,limestone) Active Metals (Mg, Al, Zn, Fe) carbon dioxide gas evolved (bubbles) MCO3(s) + HX(aq) MX(aq) + H2O(l) + CO2(g) hydrogen gas evolved (bubbles) M(s) + HX (aq) MX(aq) + H2(g) no reaction no reaction Strong complete (100%) ionization HCl, hydrochloric acid H2SO4, sulfuric acid HNO3, nitric acid NaOH, sodium hydroxide (lye) KOH, potassium hydroxide (lye) Weak partial ionization equilibrium H2CO3, carbonic acid All carboxylic acids: CH3COOH, acetic acid; HCOOH, formic acid Some amino acids: Asp, Glu NH3, ammonia (NH4OH) Amines (1º, 2º, 3º) Some amino acids: Arg, Lys carbonates (CaCO3 - chalk, limestone) hydrogencarbonates (NaHCO3 - baking soda) Oxides Non-metal oxides (CO2, SO3) Metal oxides (Na2O, CaO) Household citric acid, lactic acid, vinegar detergents, cleaners = IF AN ACID OR BASE TOUCHES YOUR SKIN, RINSE IMMEDIATELY WITH WATER! = NEVER TASTE CHEMICALS IN THE LAB!

3 8.3 Strong and weak acids and bases 2 hours Distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity. Although weakly acidic solutions are relatively safe, they still cause damage over long periods of time. Students could consider the effects of acid deposition on limestone buildings and living things State whether a given acid or base is strong or weak. Students should consider hydrochloric acid, nitric acid and sulfuric acid as examples of strong acids, and carboxylic acids and carbonic acid (aqueous carbon dioxide) as weak acids. Students should consider all group 1 hydroxides and barium hydroxide as strong bases, and ammonia and amines as weak bases Distinguish between strong and weak acids and bases, and determine the relative strengths of acids and bases, using experimental data. The following is a comparison using solutions of the same concentration (such as 1.0 mol dm 3 ) STRONG weak Ionisation molecule cation + + anion ACID reactions with water BASE reactions in water Conductivity based on total ion concentration Reaction Rate acids with reactive metals or carbonates based on concentration of reactive ions complete ionization (Kc > ) IONS ONLY - no molecular form HA + H2O A + H3O + H2SO4 + H2O HSO4 + H3O + HNO3, H2SO4, HCl MOH M + + OH KOH K + + OH NaOH, KOH, Ba(OH)2 HIGHER conductivity higher [ion] =[HA] or [MOH] faster initial rate [ions] collision frequency [H + ] = [HA] [OH ] = [MOH] 2 HCl + Mg MgCl2 + H2(g) HNO3 + NaHCO3 NaNO3 + H2O + CO2(g) bubbles faster molecule É cation + + anion partial ionisation (equilibrium) mostly molecular form, some ions HA + H2O É A + H3O + [H + ] << [HA] CH3COOH + H2O É CH3COO + H3O + carboxylic/organic acids, carbonic acid MOH É M + + OH NH4OH É NH4 + + OH NH3, amines lower conductivity small [ion] little ionisation [OH ] << [MOH] slower initial rate [ions] collision frequency 2 CH3COOH + Mg Mg(CH3COO) 2 + H2(g) CH3COOH + NaHCO3 CH3COO Na + + H2O + CO2(g) bubbles slowly Conjugate Acid Base Pairs (p ) Conjugate Acid Base Pair two molecules or ions that are related by the transfer of one proton Conjugate Acid = Base + 1 H + NH3 + H + NH4 + Base: NH3 Conjugate Acid: NH4 + Conjugate Base = Acid 1 H + H2SO4 HSO4 - + H + Acid: H2SO4 Conjugate Base: HSO4 - HSO4 - SO4-2 + H + Acid: HSO4 - Conjugate Base: SO4 2-

4 Monoprotic and Polyprotic Acids Monoprotic Acids Molecules that dissociate in water to produce only one H + (proton) ex. HCl, CH3COOH, HNO3 Polyprotic Acids molecules that dissociate in water to produce two or more H + (protons) ex. H2SO4, H2CO3, H3PO4 The first proton dissociates more (high Ka value) H2SO4 HSO4 - + H + Subsequent protons do not dissociate as easily (low Ka) HSO4 - SO4-2 + H + Functional Definitions Acid any substance that neutralizes a base Base reduces the amount of hydroxide ions in solution OH (aq) any substance that neutralizes an acid reduces the amount of hydronium ions in solution H + (aq) Amphiprotic Substance any substance that neutralizes both acids and bases can act as a proton donor (acid) or a proton acceptor (base) ex. Hydrogen sulfate ion (HSO4 ) HCl + HSO4 H2SO4 + Cl neutralizes acid (base) NaOH + HSO4 NaSO4 + HOH neutralizes base (acid) Hydrogen carbonate ion (HCO3 ) HCO3 H + + CO3 2 proton donor (acid) HCO3 + H + H2CO3 proton acceptor (base) Neutral Solutions ph = 7 1. Non-ionic substances elements, most molecules, all ionic compounds all organic compounds (except carboxylic acids and amines) 2. Water and Amino Acids equal amounts of H + and OH - produced Amino acids with equal numbers of COOH and NH2 groups

5 8.4 The ph scale 1 hour Distinguish between aqueous solutions that are acidic, neutral or alkaline using the ph scale Identify which of two or more aqueous solutions is more acidic or alkaline using ph values. Students should be familiar with the use of a ph meter and universal indicator State that each change of one ph unit represents a 10-fold change in the hydrogen ion concentration [H + (aq)]. Relate integral values of ph to [H+(aq)] expressed as powers of 10. Calculation of ph from [H+(aq)] is not required. TOK: The distinction between artificial and natural scales could be discussed Deduce changes in [H + (aq)] when the ph of a solution changes by more than one ph unit. Aim 8: A study of the effects of small ph changes in natural environments could be included.