CH.16 - Acid/Base Equilibria

If the pH changes by 1 unit, what is the corresponding change in concentration?

-log (1)

What is a "p" scale? What does "p" mean?

-log (of whatever follows)

You should be able to calculate the pH of a weak acid solution when given the initial concentration of acid (using an ICE table).

1. Find Ka 2. ICE table: -HA + H2O A- + H3O+ -Solve for x([H3O+]) 3. pH = -log[H3O+]

You should be able to calculate the pH of a strong acid solution when given the initial concentration of acid.

1. [Acid]I = [H3O+]e 2. pH = -log[H3O+]

You should be able to calculate the pH of a strong base solution when given the initial concentration of base.

1. calculate [OH-] based on the # of OH in the chemical formula 2. two options: -1. pOH = -log[OH-] pH = 14 - pOH -2. [H3O+] = Kw/[OH-] pH = 14 - pOH

What are the trends in oxoacid strengths? If I give you two oxoacids, you should be able to tell me which one is the stronger acid.

1. when comparing oxacids w/ the same # of oxygens on the central atom, acid strength increases w/ the electronegativity of the central atom 2. when comparing oxacids w/ different # of oxygens bonded to the central atom, acid strength ???

You should be able to calculate the pH of a weak base solution when given the initial concentration of base (using an ICE table).

1.Find Kb 2.ICE table -A- + H2O HA + OH- -Solve for x([OH-]) 3. Two options: -1. pOH = -log[OH-] pH = 14 - pOH -2. [H3O+] = Kw/[OH-] pH = 14 - pOH

What is the acid ionization constant? What is its symbol? How is it related to the strength of an acid? (Which is a stronger acid: the acid with a low Ka or the acid with a high Ka?)

Acid strength is measured by the size of the equilibrium constant when the acid reacts with H2O Ka: acid dissociation constant the larger the Ka, the stronger the acid

What are the basic characteristics of acids and bases?

Acids have a sour taste, react with active metals to produce H2, have a pH <7 Bases have a bitter taste, feel slippery, have a pH >7

You should be able to identify the conjugate acid and base in any Bronsted-Lowry acid-base reaction.

An acid base pair (HX and X-) that differ only in the presence or absence of a proton are called a conjugate acid-base pair Every acid has a conjugate base (Formed by removing H+) Every base has a conjugate acid (Formed by adding an acid)

What is an Arrhenius acid? An Arrhenius base? You should be able to look at a chemical formula and determine if it is an Arrhenius acid or an Arrhenius base.

Arrhenius acids are substances that have H in their formulas & when you put the substance on water, it dissociates to give H+ (hydrogen ion or proton) Arrhenius bases are substances that contain OH- in their formulas & when you put them in water, they dissociate to give OH- as one of the products (1 step)

You should memorize the equilibrium expression for the autoionization of water (and its value at 25 oC).

At 25°C, KW = 1.0 x 10^-14

What is the base dissociation constant? What is its symbol? How is it related to the strength of a base? (Which is a stronger base: the base with a low Kb or the base with a high Kb?)

Base strength is measured by the size of the equilibrium constant when the base reacts with H2O Kb: base dissociation constant

What were some of the problems with the Arrhenius theory of acids and bases?

Does not explain why molecular substances, like NH3, dissolve in water to form basic solutions - even though they do not contain OH- ions Does not explain how some ionic compounds, like Na2CO3 or Na2O, dissolve in water to form basic solutions - even though they do not contain OH- ions Does not explain why molecular substances, like CO2, dissolve in water to form acidic solutions - even though they do not contain H+ ions Does not explain acid-base reactions that take place outside aqueous solution

What is a Bronsted-Lowry acid? A Bronsted-Lowry base? You should be able to look at a chemical formula and determine if it is a Bronsted-Lowry acid or a Bronsted-Lowry base.

Emphasis on proton transfer Applies to both reactions that do and do not occur in aqueous solution. An acid is a substance that can donate a proton A base is a substance that can accept a proton

What is the hydronium ion? How is it formed? Why do scientists often use the terms (and formulas) for hydrogen ions and hydronium ions interchangeably?

H+ + H2O ⇌ H3O+ Hydrogen ion + water ⇌ Hydronium ion

Strong Acids

HCl, HBr, HI, HNO3, HClO4, HNO3, H2SO4

Strong Bases

NaOH, KOH, LiOH, RbOH, CsOH, Ca(OH)2, Ba(OH)2, Sr(OH)2

What is the relationship between the hydronium ion concentration and the hydroxide ion concentration for a neutral solution? For an acidic solution? For a basic solution?

Neutral solutions have equal [H3O+] and [OH-] Acidic solutions have a greater [H3O+] than [OH-] Basic solutions have a greater [OH-] than [H3O+]

What is the autoionization of water?

One of the most important properties of water is its ability to act as either a Brønsted acid or base. One water can donate a proton to another water: H2O (l) + H2O (l) ⇌ OH- (aq) + H3O+ (aq) Keq: [OH-] [H3O+]/1

What is the difference between a strong acid/base and a weak acid/base? How much of a strong acid/base is ionized? How much of a weak acid/base is ionized?

Strong acids dissociate completely into ions in water -almost every acid molecules in a sample will dissociate HCl (g) + H2O (l) ⇌ H3O+ (aq) + Cl- (aq) K>>1 Weak acids dissociate only slightly into ions in water -only a few acid molecules in a sample will dissociate HCN (g) + H2O (l) ⇌ H3O+ (aq) + CN- (aq) K<<1 Strong bases completely dissociates in water -almost every molecule of base that is put into water breaks up to give OH- and a counterion NaOH (aq) ⇌ Na+ (aq) + OH- (aq) Kc >> 1 Weak base dissociate only slightly into ions in water -very few base molecules dissociate. Most remain in the molecular form. NH3 (g) + H2O (l) ⇌ NH4+ (aq) + OH- (aq) Kc << 1

How does the concentration of hydronium ion relate to the initial concentration of acid for a strong acid? For a weak acid?

The stronger the acid, the more molecule of a sample of acid that dissociate when you put them into water Strong acids donate [Acid]i = [H3O+]e Weak acids, only a few molecules in a sample will dissociate when you put them into water [Acid]i > [H3O+]e

How can you simplify equilibrium calculations? Under which conditions is this simplification valid?

When: 1. K must be relatively small 2. [ ]i must be relatively large We assume that x is small: [ ]i + x ~ [ ]i [ ]i - x ~ [ ]i In order to justify this assumption: [ ]i / K > 400, you can assume x is small

You should be able to write the chemical equation for the dissociation of any Arrhenius acid or base.

acid + water ⇌ hydronium ion + counterion

What are the trends in nonmetal hydride acid strengths? If I give you two nonmetal hydrides, you should be able to tell me which one is the stronger acid. (Note: "nonmetal hydride acid" is the same thing as "binary acid.")

acid strength increases across a row & down a column

What happens in a reaction between an Arrhenius acid and an Arrhenius base? You should be able to write the chemical equation for the reaction between any Arrhenius acid and any Arrhenius base

arrhenius acid + arrhenius base ⇌ H2O + salt

What is a Lewis acid? What are the requirements for a Lewis acid?

electron pair acceptors

What is a Lewis base? What are the requirements for a Lewis base?

electron pair donors

What types of particles can act as Lewis acids?

electron-deficient atoms molecules w/ polar double bonds metal cations

What is the pH for a neutral solution at 25 oC? For an acidic solution? For a basic solution?

pH < 7 is acidic pH > 7 is basic pH = 7 is neutral

You should be able to use the hydronium ion concentration to solve for pH (and vice versa).

pH = -log[H3O+]

You should be able to solve for pOH and pKa (or to use pOH and pKa to solve for [OH-] and Ka, respectively).

pH = -log[H3O+] pOH = -log[OH-] 14 = pH + pOH pKa = -logKa -log(Kw) = - log([H3O+] [OH-])

You should be able to classify an ionic compound (a salt) as producing an acidic, a basic, or a neutral solution.

salt = (positive ion) (negative ion) positive ion - comes from arrhenius base negative ion - comes from arrhenius acid

You should be able to write an equilibrium expression for the dissociation of any base.

strong base: [OH-] --> pOH = -log[OH-] pH = 14 - pOH weak base: If you are given an initial concentration of a base: -look up the Kb for that base -set up an ICE table -pOH = -log[OH-] pH = 14 - pOH

You should be able to use the relative strengths of acids and bases to predict the direction in which an acid-base reaction will proceed.

stronger acid --> weaker acid

How is pKa related to acid strength?

the larger the Ka, the stronger the acid

What is the name of the product of a Lewis acid-base reaction?

when a Lewis base gives electrons from a lone pair to Lewis acid, a covalent bond forms btwn the 2 the product is called an adduct