Acid and Base Strength Chapter 19.2

How can we test how strong an acid is? electrical conductivity can be used to determine the strength of an acid or base electricity is carried by charged particles in the solution (ions)

Electrical Conductivity both solutions are the same concentration (0.1M) concentration means amount of solute in the solution hydrochloric acid acetic acid

Electrical Conductivity the more ions in the solution the brighter the bulb on the conductivity meter HCl is a stronger acid (forms more H+ ions in solution) than HC 2 H 3 O 2 (acetic acid)

Vocabulary Word strong acids: acids that ionize completely and are good conductors of electricity

Vocabulary Word weak acids: produce fewer ions and are poor conductors of electricity

Figure 14.6 A Strong Acid (a) and a Weak Acid (b) in Water

Acid Dissociation Constant (K a ) HA(aq) + H 2 O(l)  H 3 O + (aq) + A  (aq) Where, K a is the acid dissociation constant. In dilute solution we can assume that the concentration of liquid water remains essentially constant when an acid is dissolved.

Equilibrium Constants equilibrium constants are a measure of an acid’s ionization the bigger the equilibrium constant, the stronger the acid K a > 1 means a strong acid K a < 1 means a weak acid

Strong Acids strong acids produce weak conjugate bases so the forward reaction happens more than the reverse reaction HX + H 2 O H 3 O + + X- strong acid weak conj. base

Weak Acids weak acids produce strong conjugate bases so the reverse reaction happens more than the forward reaction HY + H 2 O H 3 O + + Y - weak acidstrong conj. base

Figure 14.4 Graphic Representation of the Behavior of Acids of Different Strengths in Aqueous Solution

Strong Bases The conductivity of a base is also related to its strength The more OH- ions produced the greater the conductivity

Strong Bases Strong bases dissociate completely to metal ions and hydroxide ions NaOH (s) Na + (aq) + OH - (aq) strong base weak conj. acid

Weak Base Weak bases only ionize partially, so the reverse reaction is favored over the forward reaction CH 3 NH 2 + H 2 O CH 3 NH 3 + OH - weak base strong conj. base

Figure 14.5 Acid Strength Versus Conjugate Base Strength

Strength vs Concentration acid or base strength is about how much it ionizes (which reaction is favored: forward or reverse) concentration is about how much solute (moles) is dissolved in the solution