Acids And Bases

Question 1

What is an acid?

What is a base?

Answer 1

A proton donor

A proton acceptor

Question 2

Define pH scale

What is the formula for pH

Answer 2

The measure of the hydrogen ion concentration in a solution

pH= -Log10[H+]

Question 3

Define a Monoprotic acid

Answer 3

Each molecule of the acid will release one proton when it disassociates

Question 4

Define diprotic acid

Answer 4

Each molecules of acid releases 2 protons when it dissociates

Question 5

What is the equation for the ionic product of water?

What happens if you increase the concentration of H+? Why?

Answer 5

Kw = [H+] [OH-]

OH- decreases because Kw is dependent on temperature

Question 6

What types of acid and base dissociate completely in water?

What types dissociate slightly?

Answer 6

Strong

Weak

Question 7

How do you find the pH of a strong acid?

Answer 7

Use Kw = [H+] [OH-] and rearrange for H+

Use pH = -Log10 [H+]

Question 8

How do you find the pH of a weak acid?

Answer 8

Use Ka = [H+] [A-] / [HA] where the concentrations are at equilibrium

Use pH = -Log10 [H+]

Question 9

In the equation for Ka, what is the rule for the concentration of the acid reactant? Why?

Answer 9

Concentration at the start is the same as the concentration at equilibrium because it only dissociates slightly

Question 10

What is the equation for pKa?

What is the relevance of a lower relative value of pKa?

Answer 10

pKa = -Log10 (Ka)

The acid is stronger

Question 11

What is the equivalence point of a pH curve?

Answer 11

The steepest point in the graph, where the acid = alkali and the acid has been neutralised

Question 12

What is an indicator

At what point does it change colour

Answer 12

A substance that changes colour at the end of a titration

At the steepest point on the graph, the equivalence point

Question 13

What is used instead of a indicator when a weak acid and weak base react?

Why is it used?

Answer 13

A pH meter

Because the colour change of an indicator would be too gradual as the gradient is not steep. No sharp change in pH

Question 14

How do you find the vol of base required to neutralize an acid? (Using a pH curve)

Answer 14

Find equivalence point. This is the vol of base required

Question 15

Define a buffer

Answer 15

A solution that resists small changes in pH, despite the dilution or addition of small amounts of acid or base

Question 16

How is the addition of an acid resisted when using an acid buffer?

Answer 16

Hydrogen ion concentration increases
Excess hydrogen ions bond to A-
A- and H+ conc decreases
Eq shifts to left

Question 17

How is the addition of a base resisted when using an acid buffer

Answer 17

OH- increases
Bonds to H+ to form H2O causing H+ to decrease
More HA dissociates to prod H+
Equilibrium shifts to the right

Question 18

How is acid resisted when using a basic buffer?

Answer 18

H+ increases so H+ + OH- > H2O
OH- decreases

So equilibrium shifts to the right so more OH- is produced

Question 19

How is base resisted when using a basic buffer?

Answer 19

OH- conc increases
Excess OH- reacts with BH+ to form B + H2O
Equilibrium shifts to the left to counteract change

Question 20

Why does H2O not affect pH

Answer 20

H2O > H+ + OH-
H+ and OH- cause the pH to change by equal but opposite amounts
No overall change in pH occurs

Question 21

What can buffers be used for?

Answer 21

Shampoo - acidic pH maintained because alkali makes hair rough
Bio washing powders - so enzyme function and don’t denature

Question 22

What is an alternative way of producing an acidic buffer?

Answer 22

Excess weak acid + alkali
All of the alkali will neutralize some of the acid to produce a salt
Some of the weak acid will still remain un-neutralized