Acids And Bases Flashcards by Barney Burden

What is a Bronsted acid / base?

Bronsted acid is a proton donor

Bronsted base is a proton acceptor

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How do you measure the strength of Bronsted acids?

Through acidity constant Ka

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How do you find the Ka?

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How do you find pKa?

-log [ Ka ]

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How does a high/low pKa value relate to acidity?

Strong acid = pKa < 0

Weak acid = pKa > 0

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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

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What is the value for Kw?

Kw = 1.00 x 10 x 10^14

(At 25˚C)

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How do you calculate Kb?

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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

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Give an example of a polyprotic acid, and explain what one is

H3PO4 - an species with more than 1 acidic H

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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

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What are oxo-acids?

They are acid that contain oxygen, more specifically the acidic H is bound by a hydroxyl group

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How does the strength increase with oxo-acids?

Strength of oxoacids increases with the number of unprotonated Oxygen atoms

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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

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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

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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

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What are buffer solutions?

A solution of a weak acid or base and its conjugate salt

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What equation is used when working with buffers?

The Henderson-Hasselbach equation

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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

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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