Topic 21: Acids, bases and buffers

Question 1

Brønsted Lowrey acid

Answer 1

Proton donor

Question 2

Brønsted Lowry base

Answer 2

Proton acceptor

Question 3

pH

Answer 3

-log[H+]

Question 4

Strong acids

Answer 4

An acid that completely dissociates to ions when in solution with pH 0-1

Question 5

[H+] of strong acids

Answer 5

Will dissociate fully so if monoprotic acid same moles so same conc. of acid as H+ ions.

Question 6

[H+] from pH

Answer 6

10^-pH

Question 7

Water dissociation equation

Answer 7

H2O <===> H+ + OH-

Question 8

Kw meaning

Answer 8

Ionic product of water

Question 9

Kw equation

Answer 9

Kw = [H+][OH-]

Question 10

Why use Kw instead of Kc

Answer 10

Don’t include water in Kw equation because the eq’m shifted far to the left meaning [H2O] almost a constant so is incorporated into the constant of Kw

Question 11

Water at 298K pH

Answer 11

7 because [H+]=[OH-]

Question 12

pH of water at high temps acidic, basic, neutral why?

Answer 12

pH decreases at high temps as eq’m shifts so Kw increases.
Still neutral as proportion of [H+]=[OH-]

Question 13

Working out [H+] from [OH-]

Answer 13

Either [H+]=Kw/[OH-]
or
pH+pOH= 14 — do this if you can, saves so much time!!!

Question 14

Buffer def

Answer 14

A solution which can resist small changes in pH

Question 15

What does an acid buffer contain?

Answer 15

Weak acid and its salt

Question 16

Examples of strong acids

Answer 16

-HCl
-H2SO4
HNO3

Question 17

Examples of weak acids

Answer 17

Any carboxylic acid, organic acids tend to be weak

Question 18

Examples of weak bases

Answer 18

NH3, amines

Question 19

Examples of strong bases

Answer 19

-KOH
-NaOH
-Ba(OH)2

Question 20

Dissociation of weak acid (ethanoic acid)

Answer 20

CH3COOH <===> CH3COO- + H+

Question 21

Adding acid to acidic buffer

Answer 21

Weak acid in the buffer solution partially dissociates (hardly)
Adding an acid to buffer the H+ react with the anions (A-) of the weak acid to make more weak acid (HA). The salt fully dissociates so the supply of A- doesn’t run out. The H+ ions are “used up” so pH doesn’t really change.

Question 22

Adding alkali to acidic buffer

Answer 22

The OH- react with HA producing H2O and A-
OH- removed so pH mostly the same

Question 23

What do an alkaline buffer contain?

Answer 23

Weak base and its salt

Question 24

Adding acid to basic buffer

Answer 24

H+ react with base
NH3 + H+ —> NH4+

Question 25

Adding alkaline to alkaline buffer

Answer 25

OH- react with salt NH4+ + OH- ---> NH3 + H2O

Question 26

Ka equation

Answer 26

([Y-][H+])/[HY] Y- is salt - assume complete dissociation of salt HY is weak acid assume no dissociation of weak acid

Question 27

Amphoteric

Answer 27

A substance able to react as a base and as an acid

Question 28

Equivalence point

Answer 28

Point at which sufficient base has been added to just neutralise the acid (vice versa).

Question 29

Half neutralisation point

Answer 29

Half way between zero and the equivalence point

Question 30

Using the half neutralisation point to find Ka

Answer 30

Half the HA has been converted to A- and half remains so they cancel out in the equation meaning Ka = [H+]

Question 31

At half neutralisation point

Answer 31

pH = pKa