Chemistry – Chapter 19

Properties of Acids and Bases Acidic solutions taste sour Ex: lemon juice Basic solutions taste bitter and feel slippery Ex: soap Pure water is a nonconductor of electricity, but the addition of an acid or base makes it a conductor Ions In Solution Aqueous solutions contain hydrogen ions (H + ) and hydroxide ions (OH - ) and the relative amounts of the two ions determine whether the solution is acidic, basic, or neutral

Acidic solution – contains more H + than OH - ions Basic solution – contains more OH - ions than H + ions A neutral solution contains equal numbers of H + and OH - ions Ex: water Arrhenius Model– states that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in aqueous solution; a base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in aqueous solution HCl H + + Cl - NaOH Na + + OH - This model does not account for all bases

Bronsted-Lowry Model – an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor When a molecule of acid dissolves in water it donates a H + ion to a water molecule; the water molecule acts as a base and accepts the H + ion HX + H 2 O H 3 O + + X - On accepting the H+ ion, the water molecule becomes an acid H 3 O + X - is a base b/c it has a negative charge and can accept a positive H + ion Conjugate acid – species produced when a base accepts a H + ion from an acid The base H 2 O accepts a H + ion from the acid HX and becomes the conjugate acid H 3 O + Conjugate base – species that results when an acid donates a H + ion to a base The acid HX donates its H + ion and becomes the conjugate base X -

Hacky Sack game Amphoteric – substances that can act as both acids and bases Ex: water Substances classified as acids and bases by the Arrhenius model are classified as acids and bases by the Bronsted-Lowry model; however some substances not classified as bases by the Arrhenius model are classified as bases by the Bronsted-Lowry model

Strengths of Acids and Bases Strengths of Acids Strong acid – acid that ionizes completely Produce the maximum number of ions Good conductors of electricity Ex: hydrochloric acid (HCl) Weak acid – acid that ionizes only partially Some of the acid remains in molecular form Ex: acetic acid (vinegar) Using the Bronsted-Lowry model, if the reactant acid is strong, its conjugate base is weak

Acid ionization constants Weak acids produce an equilibrium mixture of molecules and ions in aqueous solutions The equilibrium constant, K eq, provides a quantitative measure of the degree of ionization of the acid Acid ionization constant (K a ) – value of the equilibrium constant expression for the ionization of a weak acid Value indicates whether reactants or products are favored at equilibrium Weakest acids have the smallest K a values

Strengths of Bases Strong base – dissociates entirely into metal ions and hydroxide ions Conductivity of a base depends upon the extent to which the vase produces hydroxide ions in aqueous solutions Some metallic hydroxides (i.e., CaOH) have low solubility and thus are poor sources of OH - ions; however CaOH and other slightly soluble metallic hydroxides are considered strong bases because all of the compound that does dissolve is completely dissociated Weak base – ionizes only partially in dilute aqueous solution to form the conjugate acid of the base and hydroxide ion

Base ionization constants Weak bases also form equilibrium mixtures of molecules and ions in aqueous solutions Base ionization constant (K b ) – value of the equilibrium constant expression for the ionization of a base The smaller the value of K b, the weaker the base Strong/weak, concentrated/dilute? For acids and bases, weak/strong and dilute/concentrated have different meanings Weak or strong refers to the degree to which the acid or base separates into ions

What is pH? Ion Product Constant for Water H 2 O H + + OH - Ion product constant for water (K w ) – value of the equilibrium constant expression for the self-ionization of water K w = [H + ][OH - ] K w = 1.0 x If the concentration of H + ions increases, the concentration of OH - ions decreases

pH and pOH pH – negative logarithm of the hydrogen ion concentration pH = -log [H + ] pH of zero is strongly acidic pH of 14 is strongly basic pH of 7 is neutral pH scale is logarithmic, therefore change in one pH unit represents a tenfold change in ion concentration pOH – negative logarithm of the hydroxide ion concentration pOH = -log [OH - ]

pH + pOH = When a strip of pH paper is dipped into an acidic or basic solution, the color of the paper changes; then the color of the paper is compared to a standard on a chart

Neutralization Reaction Between Acids and Bases Neutralization reaction – an acid and a base react in aqueous solution to produce a salt and water Salt – ionic compound made up of a cation from a base and an anion from an acid Titration – method for determining concentration of a solution by reacting a known volume of the solution with a solution of known concentration To find the concentration of an acid solution, titrate the acid solution w/ a solution of a base of known concentration To find the concentration of a basic solution, titrate a base of unknown concentration w/ an acid of known concentration

Acid-base indicators – chemical dyes whose colors are affected by acidic and basic solutions End point – point at which the indicator used in a titration changes color Some titrations have equivalence points 7; it depends on the relative strengths of the reacting acids and bases Salt Hydrolysis – anions of the dissociated salt accept hydrogen ions from water or the cations of the dissociated salt donate hydrogen ions to water

Buffered Solutions Control of pH is important in your body Blood pH must be maintained at an average of 7.4 Buffers – solutions that resist changes in pH when limited amounts of acid or base are added Buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid Buffer solution’s capacity to resist pH change can be exceeded by the addition of too much acid or base Buffer capacity – amount of acid or base a buffer solution can absorb w/o a significant change in pH