fully dissociates in water Ka >> 1

Acids and Bases Flashcards by Lavi R

Arrhenius acid

a substance that dissociates in water to produce hydrogen ions, H+

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Arrhenius base

a substance that dissociates in water to produce hydroxide ions, OH-

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shortfalls of Arrhenius theory

works with many compounds in water, but misses some (particularly bases), and clearly doesn’t describe non-aqueous solutions

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bronsted Lowry acid

a substance that can give a hydrogen ion (proton donor)

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bronsted Lowry base

a substance that can take a hydrogen ion (proton acceptor)

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difference/similarity between BL bases and Arrhenius bases

D: B-L bases do not need to contain OH-
S: they can generate OH- when in water

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general equation for a Bronsted Lowry acid and base interacting

HA + B <—> BH+ + A-

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conjugate acid-base pairs

chemical species whose formulas differ by only one H+

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acid dissociation

HA + H2O <–> H3O+ + A-
- an equilibrium
- H2O solvent is an (almost) pure liquid so is not in equilibrium expression
- Ka only describes the reaction of an acid with the solvent H2O as the base
- the stronger the acid, the larger the Ka

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base dissociation

B + H2O <–> OH- + BH+
- stronger the base, the larger the Kb

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amphiprotic

can both donate and accept protons

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amphoteric

can act as both an acid and a base

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dissociation of water

H2O + H2O <–> H3O+ + OH-
Kw = [H3O+][OH-] = 1 x 10^-14

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pH =

-log[H3O+]

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[H3O+] =

10^-pH

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pOH =

-log[OH-]

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[OH-] =

10^-pOH

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basic solution
neutral solution
acidic solution

[H3O+]<[OH-], pH > 7
[H3O+] = [OH-], pH = 7
[H3O+]>[OH-], pH < 7

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strong acid

fully dissociates in water
Ka&raquo_space; 1

weak acid

partially dissociates in water
Ka = 1

inert acid

does not dissociate in water

conjugates:

strong acid <-> ‘inert’ base
weak acid <-> weak base
‘inert’ acid <-> strong base

compare strong acids/bases and weak acids/bases as electrolytes

strong acids and strong bases are strong electrolytes. we assume they ionise completely in water.

weak acids and weak bases are weak electrolytes. they ionise to a limited but detectable extent in water

The levelling effect

for a strong acid in water, the ‘active proton donor’ isn’t HA, its H3O+
for a strong base in water, the active proton acceptor is OH-
- different strong acids usually have different Ka’s
- however, in water, they exhibit the similar acidic properties, i.e. the acid strength of H3O+

pKa + pKb =

distinguish between pH, pKa and Ka

pH measures the acidity of the solution and depends on the absolute [H3O+] pKa and Ka reflect the strength of the acid molecule and depends on the relative concentration at equilibrium

percent dissociation

a common and useful description of a weak acid in solution % dissociation = [HA]dissociated/[HA]initial x 100

factors that affect acid strength

1. degree of polarity of H-A bond 2. strength of H-A bond 3. oxoacids

degree of polarity of H-A bond

depends upon the electronegativity of A the more polar the H-A bond, the stronger the aid

strength of H-A bond

depends on the size of the A atom. the larger the A atom, the longer/weaker the bond, the stronger the acid

along the period, the most important factor is

electronegativity

down the group, the most important factor is

the H-A bond strength

oxoacids

YOm(HO)n - eg H3PO4 - if same structure, different Y: acid strength increases as the electronegativity of Y increases - a more electronegative Y pulls electron density away from O-H bonds, making it easier for H+ to dissociate - the strength of oxoacids also increases with m, the number of lone oxygen atoms - the electronegative oxygen atoms pull electron density from the chlorine, making it more positive, which in turn weakens the O-H bond

salt solutions from conjugates of strong acids and strong bases are

neutral

strong acids

HClO4, H2SO4, HNO3, HCl, HBr, HI

strong bases

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

salts that are derived from a weak base and strong acid yield

acidic solutions

why does weak base + strong acid give acidic solutions?

forms weak conjugate acid of a weak base which then dissociates eg NH4Cl (see slides) - solutions of NH4Cl are acidic because the ammonium ion (NH4+, a weak acid) will dissociate while the chloride ion does not

salts, such as NaCN, that are derived from a strong base (eg NaOH) and a weak acid (eg HCN) yield basic solutions

see slides for example

Ka>Kb Ka

solution contains excess H3O+ ions so pH < 7 solution will contain excess OH- ions so pH>7

Lewis acid

a species that accepts an electron pair

Lewis base

a species that donates an electron pair

hydrated metal cations

small, highly-charged metal ions (eg Al3+) form complexes in water the resulting complexes are proton donors

acid rain

- pollutants such as sulfur oxides and nitrogen oxides dissolve in rain to form acids - buildings and monuments made of marble and limestone eroded by acid rain

state the 2 equations for acid rain

ions that do not react appreciably with water to produce either H3O+ or OH-

conjugate cations from strong bases: - alkali metal cations (group 1) - alkaline earth metal cations (group 2) conjugate anions from strong monoprotic acids - Cl-, Br-, I-, NO3-, ClO4- because they are inert