AQA A-Level Chemistry: acids and bases

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What is pKa, why is it used?

-log(Ka)

pH

-log[H+]

Assumptions for Ka

1.)Weak bases and acids dissociate only slightly in aqueous solution 2.) [H+] from dissociation of HA not water 3.)[A-] comes from HA unless salt present

What is the value of Kw @ r.t.p.

1x10^-14

Basic buffer equation

B + H+ <-> BH+

Diprotic base example

Ba(OH)2

Example of a weak acid and weak base

Ethanoic acid is weak, ammonia is a weak base.

Dibasic acid example

H2SO4,

Acidic buffer equation

HA <-> H+ + A-

Example of a strong acid and strong base

HCL strong acid, NaOH strong base.

Monobasic acid example

HCl, HNO3, HBr

Kw

Kw = [H+] [OH-]

How can you manipulate the Kw equation if using pure water? Why does this work?

Kw = [H+]^2 Because OH- and H+ concentrations are equal.

Indicator for Strong acid - Weak base

Methyl orange

Indicator for Strong acid - Strong base

Methyl orange/phenolphthalein

What do you have to do when calculating pH of a dibasic acid?

Multiply the concentration of the acid by 2

Monoprotic base example

NaOH

Indicator Weak Acid - Weak Base

No suitable indicator

What is a monobasic acid?

One mole of acid reacts with one mole of base

What is a dibasic acid?

One mole of acid reacts with two moles of base

What is a monoprotic base?

One mole of base reacts with one mole of acid

What is a diprotic base?

One mole of base reacts with two moles of acid

Indicator Strong base- Weak acid

Phenolphthalein

What is a Bronsted-Lowry base?

Proton acceptor

What is a Bronsted-Lowry acid?

Proton donor

Explain why the value of Kw increases as temperature increases

Reaction is endothermic, Therefore reaction shifts to right to decrese temperature change

What does an acidic buffer solution contain? + Example

Solution of weak acid and its salt e.g. ethanoic acid + sodium ethanoate

What does a basic buffer solution contain? + Example

Solution of weak base and its salt e.g. ammonia + ammonium chloride

How are strong and weak acids and bases different?

Strong acids/ bases completely dissociate into ions when in solution. Weak acids/ bases only partially dissociate.

Why is a logarithmic scale used for pH?

The concentration of hydrogen ionsin aqueous solution covers a very wide range

Why doesn't Kw include concentration of water?

[H2O] is constant [H2O] is very high compared to [OH-] and [H+]

What is a buffer?

a solution that resists changes in pH on addition of small amounts of acid or base

What does Kw stand for?

The ionic product of water


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