Acids And Bases Flashcards
What is a Bronsted acid / base?
Bronsted acid is a proton donor
Bronsted base is a proton acceptor
How do you measure the strength of Bronsted acids?
Through acidity constant Ka
How do you find the Ka?
How do you find pKa?
-log [ Ka ]
How does a high/low pKa value relate to acidity?
Strong acid = pKa < 0
Weak acid = pKa > 0
What is the autoionisation of water?
Where water splits into into conjugate acid and base pairs, this happens rapidly and at very low concentrations
Hence the value for Kw being so small
What is the value for Kw?
Kw = 1.00 x 10 x 10^14
(At 25˚C)
How do you calculate Kb?
What’s the relationship between a Bronsted acid and its conjugate base?
The stronger a Bronsted acid, the weaker is its conjugate base - and vice versa
Give an example of a polyprotic acid, and explain what one is
H3PO4 - an species with more than 1 acidic H
Why does order of acid strength decrease in polyprotic acids as more protons dissociate?
Harder to remove a proton from a negatively charged species than a neutral species
What are oxo-acids?
They are acid that contain oxygen, more specifically the acidic H is bound by a hydroxyl group
How does the strength increase with oxo-acids?
Strength of oxoacids increases with the number of unprotonated Oxygen atoms
What are Paulings rules for oxoacids?
1 for the oxoacid OpE(OH)q the pKa ≈ 8-5p
2 successive pKa values for polyprotic acids (q>1) will increase by ≈ 5 pKa units for each successive proton transfer
What are the 3 classes of Bronsted acids?
Aqua acids - coordinated with H2O molecules
Hydroxo-acids - OH with no neighbouring =O
Oxo-acids - OH with neighbouring =O
Why are buffer solutions important?
They resist change in pH
- essential in maintaining pH in living organisms
- used in lab to maintain correct pH
- used in food industry as acidity regulators
What are buffer solutions?
A solution of a weak acid or base and its conjugate salt
What equation is used when working with buffers?
The Henderson-Hasselbach equation
What the relationship between pKa and pKw? How’s this important?
pKa = pKw - pKb
Helps when using HH equation as you need pKa not pKb
What are Lewis acids and bases?
Lewis acid is a lone pair donor
Lewis acid is a lone pair acceptor
What are Lewis acids and bases in organic chemistry?
Lewis acid = electrophile
Lewis base = nucleophile
What is any Bronsted acid HA of Lewis acid H+ and Lewis base A-?
An adduct of the Lewis acid and base
Give an example of a Group 14 Lewis acid
SiF4 - electron precise compound
Si can become hypervalent and forms an octahedral [SiF6]2-
What must a Lewis acid have or be able to do?
A Lewis acid must have less than an octet of valence e-
Or be able to become hypervalent
Give an example of a Group 13 Lewis acid
BF3
Boron is electron deficient - requires 2 e- to complete its octet and so acts as a Lewis acid
What are the uses of BX3s?
BX3s are used as Lewis acid catalysts in synthetic chemistry
What relation does SbF5 have to ‘superacids’?
BF5 is already hypervalent but can still act as a powerful Lewis acid
SbF5 can accept a F from 2HF, creating SbF6- and H2F+ —> SUPERACID
What’s SbF5 relation to ‘magic acid’?
Why are AsX5 and SbX5 strong Lewis acids?
Because both Sb and As can become hypervalent, AND Cl and F are both very electronegative
Give an example of a powerful Group 16 Lewis acid
SO3 - electron deficient, needs 2e-
Powerful Lewis acid which forms an adduct with bases such as amines
Reaction with H2O forms H2SO4
Give an example of a group 15 Lewis acid
SbF5
AsX5
Give an example of a powerful Group 16 Lewis acid
SO3 - electron deficient, needs 2e-
Forms addicts with bases such as amines
Rwacvtion with H2O forms sulfuric acid
Why are solvents as acids/bases important?
Because most chemical reactions take place in solution
Acid/base properties can influence outcome of a reaction
Give an example of a polar aprotic solvent
Acetonitrile
DMSO
DMF
What is needed for Lewis base solvents?
For the solvent to have lone pairs
Can form adducts with Lewis acidic solutes
How do steric effects impact acid/base chemistry?
Steric effects can influence Lewis acid/base interactions
How do steric effects impact HMDS?
Weak Lewis base due to steric hindrance - weak e- pair donor
Powerful Bronsted base - very strong proton acceptor
What period of elements are Lewis acids if they have fewer then 8 e-?
2nd row
What do hard bases from stable adducts with?
With hard acids
What do soft acids form stable adducts with?
With soft bases
What’s the difference between hard and soft acids?
Hard acids/bases form predominantly ionic bonds/species
Soft acids/bases
An atom/ion is polarisable if…
It’s electron distrustion can be easily distorted
Give the characteristic of HARD acids/bases
Hard Lewis acids:
- cations have noble gas e- configurations
- small highly charged cations
- high oxidation states
Hard Lewis bases:
- very electronegative donor atoms
Give the characteristics of SOFT Lewis acids/bases
Soft Lewis acids:
- don’t have noble gas configurations
- heavy metals
- low oxidation states
Soft Lewis base:
- larger donor atoms
- less electronegative donor atoms
What are Lithophiles?
Occur as oxides/other ionic compounds, e.g. Ca, Na, Al, Ti
Cations are hard Lewis acids
- rock-loving
- remain on or near earths surface that readily form oxides that don’t sink to core
What are Chalcophiles?
Occur as sulphides, e.g. Cu, Ag, Hg, Pb
Cations are soft Lewis acids
- sulfur/chalogen loving
- remain close to earths surface readily forming sulfides that are denser than ore of lithophiles so sink but not to the core
Why is the HSAB concept useful?
Can be used to predict the direction of a reaction
When log K is bigger what doe this mean?
Stronger bond, hence more stable adduct bound
