Acids and Bases (chem equilib continued)

Question 1

Neutralisation Reaction

Answer 1

An acid reacts with a base to form water and a salt.

Question 2

What does a

Soluble salt of a strong acid and base dissolve in water to

Answer 2

A neutral solution

Question 3

why will solution of a salt of a

strong base and weak acid have a high pH

Answer 3

The weak parent acid will react with and remove hydrogen ions from the solution. Eg sodium ethanoate

CH3COO- + H+ > CH3COOH

ethanoate ions have formed ethanoic acid

Question 4

Why will a water and salt aqueous solution with a

weak parent base be low pH

Answer 4

In the water equilibirum some hydroxide ions have been removed, weak parent base reacts and removes the OH- ions. Eg ammonium chloride

NH4+ + OH- > NH3 + H20

ammonium ions in ammonium chloride form ammonia

Question 5

Soaps

Answer 5

Soaps are salts of fatty acids (weak acids) and strong bases. Soap dissolves in water to form an alkaline dsolution.

Question 6

Monoprotic and Diprotic

Answer 6

Monoprotic- Only one proton H+ is formed eg. HCl
Diprotic- 2 protons are formed eg H2SO4

This means the conc of these acids will vary from the conc of H+ ions.

Question 7

Concentrations when an acid is

Mono/di/triprotic

Answer 7

For HCl, [H+] is equal to concentration of acid.
When an acids diprotic the [H+] is double the acid concentration.

(H2SO4) c= 0.1 mol l-1 , [H+]=0.2 mol l-1

HCl c=0.1 [H+]= 0.1

Question 8

pKa

Answer 8

A number that describers the acidity of a molecule, the higher it is the lower the acidity.

Question 9

Calculating pH of

Weak acids

Answer 9

Only a very small proportion of original molecules dissociate
pH=pKa - 1/2 log c

Question 10

Define

Buffer Solutions

Answer 10

Resists a change in pH when small amounts of acid or base are added.

pH remains approximately constant

Question 11

Acid buffer

(and example)

Answer 11

Consists of a solution of a weak acid and one of its salts

e.g. ethanoic acid and sodium ethanoate

Ethanoic acid only partially dissociates and exists as CH3COOH, while sodium ethanoate is fully dissociated salt, existing as CH3COO- + Na+

Question 12

in an acid buffer solution

what provides what

Answer 12

• Weak acid provides the hydrogen ions when theyre removed.
• Salt of weak acid provides conjugate base which absorbs excess hydrogen ions.

Question 13

Addition of an acid to an

Acidic buffer

of ethanoic acid and sodium ethanoate

Answer 13

H+ reacts with ethanoate( CH3COO-) ions present from sodium ethanoate. forming ethanoic acid!
CH3COO- + H+ > CH3COOH, [H+] remains unchanged

H+ react with parent weak acid of the salt

Question 14

Addition of OH- ions to an

Acid buffer

of ethanoic acid and sodium ethanoate

Answer 14

OH- reacts with ethanoic acid because it didnt really dissociate(CH3COOH), forming ethanoate ions and water, extra OH- is removed!

CH3COOH + OH- > CH3COO- + H2O

Hydroxide ions react with just the weak acid thats on its own

Question 15

defien

Basic Buffer

Answer 15

solution of a weak baseand one of its salts

Question 16

In a BASIC BUFFER solution

what removes what

Answer 16

• weak base removes excess hydrogen ions
• conjugate acid provided by salt supplies hydrogen ione when theyre removed

Question 17

What will be formed by loss/ gain of a proton

Answer 17

For every acid, conjugate base is foemed by the loss of a proton.
For every base, conjugate acid is formed by the gain of a proton

Question 18

How to calculate pH of a

Buffer solution

Answer 18

pH=pka - log [acid]/[salt]

Question 19

Indicators

Answer 19

• Are used in acid-base titrations as they change colour at the end point of a reaction
• Usually weak acids
• colour of acid is distinctly different from its conjugate base
• The colour is determined by the pH

Question 20

how to represent indicators

Answer 20

HIn, where In- is conjugate base.

Question 21

Applying equilibrium law to indicator equilibiria

Answer 21

HIn(aq) + H20(l) <>H30+(aq) +In-(aq)
(indicator) (conjugate base)

KIn= [H3O+] [In-] just like K !!
[HIn]

Question 22

indicators

Thepretical point at which colour changes

Answer 22

[HIn] = [In-], concentration of acid equals conjugate base, therefore KIn, ** indicator dissocation constant** (the turning point) is equal to concentration of Hydronium ions. KIn=[H3O+]

Howeveer this is too difficult to see and in practivce colour change visible when HIn and In- differ by a factor or 10

Question 23

pH range over which colour change occurs

Answer 23

Estimayted to be pH=pKIn +- 1

in other words, it changes colour over 2pH range

Question 24

Equivalence point

Answer 24

When a reaction is just complete, region of rapid pH change around the end point.

Question 25

I

Indicators for a weak acid + alkali

Answer 25

Not possible, as pH does not change rapidly enpugh at end point