Exam 3 (Ch. 13, 14, 15)

if exothermic, Kc decreases and reaction goes left

if endothermic, Kc increases and reaction goes right

Catalysts decrease Ea by an equal factor for both the forward and reverse reactions.

Composition of the equilibirum mixture remains constant.

a substance that dissociated in water to produce H+

ex. HCl > Cl₂ + H+

a substance that dissociates in water to give OH^-

ex. NaOH > Na+ + OH-

a substance that can transfer H, can act as a proton donor

ex. H₂CO₃ > H+ + HCO₃ -

a substance that can accept protons, usually with an available lone pair of electrons

ex. H+ + NH₃ > NH₄+

acid: HA > conjugate base: A- (take away H, and 1 charge)

base: B > conjugate acid : BH+ (add H, and 1 charge)

• completely dissociate in water
• strong electrolytes
• produce H₃O in solution
• large Kc values

the power of the hydrogens

pH = - log [H+]

[H+] = 10 ^-pH

pH = 0 - 6

[H] > [OH]

pH = 7

[H] = [OH]

pH = 8-14

[H] < [OH]

Kw = [OH] [H] = 1.01 E -14

On periodic table. .

- increase from left to right (due to electronegativity)

- increase from top to bottom ( due to hydrogen bond strength)

• use electrochemical sensors (electrode) to measure [H] as function of voltage
• electrodes are more sensitive, accurate, and precise than indicators
• portable, digital, can also measure other ions

chemists use pKa to compare strength of acids

pKa = -log (Ka)

as Ka increases the pKa will decrease

strong acids have lower pKa values

describe the reaction of a base in water and the accepting of the proton

pKb = - log (Kb)

describes the strength of bases, the ability to accept protons

stronger base have high Kb and lower pKb

The Ka and Kb are related by the equilibrium constant for water dissociation.

Ka * Kb = Kw

pKa + pKb = 14

dissociate / undissociate * 100

or

deprotinated form / protinated form * 100

monoprotic systems - involves transfer of one H

diprotic system - involves transfer of two H

polyprotic system - involves transfer of many H

1. strong acid & strong base
2. strong acid & weak base
3. weak acid & strong base
4. weak acid & weak base

(1-3 all have large equilibrium constants (Kn) and go to completion)

Kn = 1 / Kw

(an extension of Le Chateliers)

if an ion is added to a solution that already contains that ion, the equilibrium will shift accordingly

a conjugate acid / base pair in solution

resists change in pH when an acid or base is added to the system

most effective when used +/- pH unit from the pKa of the acid form

= buffer capacity

how resistive is the buffer to pH changes

the greater the concentrations of [HA] , [A] the greater the buffer capacity

pH = pKa + log ( [A-] / [HA] )

dependant on the ratio of conjugate pair concentration and not the total concentration

Describing a solid dissolving in a liquid solvent (usually water)

equilibrium constant = Ksp

• to calculate the concentration of dissolved ions in a saturated solution
• to calculate molar solubility which in turn can be used to calculate mass solubility

used to compare to Ksp and predict precipitation

IP > Ksp : precipitation will occur

IP = Ksp : equilibrium

IP < Ksp : precipitation will not occur

Ksp increases with temperature

common ions in solution will effect solubility

• if an acid is added > solubility will increase, equilibrium will shift to the right and dissolve more ions