8.1 EXPLAINING THE PROPERTIES OF ACIDS & BASES

SCH4U - Chemistry, Gr. 12, University Prep

Mr. Dvorsky

Common Properties of Acids & Bases

Property Acid Base

Taste Sour Bitter

Texture of Solution No characteristic texture Slippery

Aqueous Property of Oxides

Non-metal oxides form acidic solutions:

CO2(g) + H2O(l) H2CO3(aq)

Metal oxides form basic solutions:

CaO(g) + H2O(l) Ca(OH)2(aq)

Reaction with Phenolphthalein Colourless Pink

Reaction with litmus paper Blue litmus Red Red litmus Blue

Reaction with MetalsAcids react with metals above

H in the activity series to displace H2(g)

Bases react with certain metals (i.e. Al) to form H2(g)

Reaction with CO32- Form CO2(g) No reaction

Reaction with NH4Cl No reaction Form NH3

Reaction with Fatty Acids No reaction React to form soap

(saponification reaction)

Neutralization Reactions Acid + Base Water + Salt

Arrhenius Theory of Acids & Bases (Arrhenius, 1887)

Acids & bases are defined in terms of their structure and the ions produced when they dissolve in water.

Explains acid-base reactions and neutralization. ACID: dissociates in water to form H+

(aq)

HCl (hydrochloric acid), H2SO4 (sulphuric acid)

BASE: dissociates in water to form OH-(aq)

NaOH (sodium hydroxide), KOH (potassium hydroxide) LIMITATIONS:

hydrogen ion combines with water to form hydronium ion does not explain some bases (i.e. ammonia, salt

solutions) does not explain acid-base reactions without water (i.e.

gas)

)(3)(2)( aqlaq OHOHH

Brønsted-Lowry Theory(Johannes Brønsted & Thomas Lowry, 1923)

Recognizes an acid-base reaction as chemical equilibrium, have a forward and reverse reaction involving the transfer of a proton

ACID: substance from which a proton can be removed “proton-donor”

BASE: substance that can accept a proton “proton-acceptor”

proton = nucleus of a hydrogen atom (H+ ion)

)()(3)(2)( aqaqlaq ClOHOHHCl

acid base conjugate conjugate

acid base

Conjugate Acid-Base Pairs

dissociation is an equilibrium reaction because it proceeds in both directions H2O donates a proton in forward rxn acid OH- accepts a proton in reverse rxn

conjugate base If a substance acts as a proton donor

and a proton accepter, it is termed “amphoteric” (i.e. water)

)()(4)(2)(3 aqaqlaq OHNHOHNH

base acid conjugate conjugate

acid base

Strong Acids

Completely dissociates in water (equilibrium favours products, lies to the right) Binary acids

[HX(aq) where X = Cl, Br, I (not F)] Factors:

Across period: electronegativity Down a group: bond strength

Oxoacids (contain oxygen atoms) where the # [O] > # H (by 2 or more) Factors:

Number of oxygen atoms

Monoprotic only have single H atom that dissociates

Polyprotic have more than 1 H atom that dissociates Strength decreases as number of hydrogen

atoms that have dissociated increases

)()(3)(2)( aqaqlaq ClOHOHHCl

Strong Bases

Completely dissociates in water (equilibrium favours products, lies to the right) Oxides & Hydroxides of alkali metals (Group

1) and of of alkali earth metals (Group 2) below beryllium [e.g. NaOH sodium hydroxide, MgO magnesium oxide] Factors:

Metals with low electronegativity form ionic bonds with oxygen easily break bond with oxygen which reacts with water to form hydroxide ions

)()()(2)(2 22 aqaqlaq OHNaOHONa

)()(22)( 2 aqlaq OHOHO 2

)()()(2 2 aqaqaq ONaONa

Calculations that involve strong acids & bases

Strong acids/bases (and strong electrolytes) completely dissociates into ions in water [H3O]+

(aq) is equal to the [strong acid] [OH]-

(aq) is equal to the [strong base]

You cannot determine the concentrations of ions of weak acids/bases/electrolytes this way because they do not completely dissociate in solution (more next class)