Acids And Bases

Question 1

What is an acid base reaction?

Answer 1

A reaction that involves the transfer of protons/hydrogen ions

Question 2

What is an acid?

Answer 2

A substance that donates H+ ions

Question 3

What is a base?

Answer 3

A sibstance that accepts H+ ions

Question 4

What is the difference between a strong and weak acid?

Answer 4

A strong acid completely dissociates in a solution, a weak acid only partially dissociates in a solution

Question 5

What is an alkali?

Answer 5

A base that releases hydroxide ions in aqueous solution

Question 6

What is an acid base equilibria reaction?

Answer 6

A reversible reaction that involves the transfer lf protons in both directions

Question 7

What is the equation to calculate the pH of a colution?

Answer 7

-log[H+]

Question 8

Using pH, how do you calculate the conc. of hydrogen ions?

Answer 8

10^-pH

Question 9

What does a low/ high pH indicate?

Answer 9

A low pH means there is a large conc. of hydrogen ions
A high pH means there is a low conc. of hydrogen ions

Question 10

At 298K, what does a pH of 7,< or > mean?

Answer 10

pH= 7- neutral
pH>7- alkaline
pH<7- acidic

Question 11

What is a monoprotic acid?

Answer 11

An acid that only donates one hydrogen ion per mole

Question 12

When calculating the pH of monoprotoc acids, what can you assume?

Answer 12

Assume complete ionisation, so the conc. of hydrogen ions is equal to that of the acid

Question 13

Why do weak acids form a reversible reaction?

Answer 13

Because they only partically dissociate, so the undissociated molecules are in equilibrium with its ions.

Question 14

What is Ka? What is it measured in?

Answer 14

Ka is the acid dissociation constant for weak acids, measured in moldm-3

Question 15

What is the equation for Ka?

Answer 15

Ka= [H+][A-]/ [HA]

Question 16

What does a high Ka indicate?

Answer 16

A stronger acid

Question 17

What is pKa and its equation?

Answer 17

pKa is used to measure the relatove strength of weak acids.
= -log Ka

Question 18

What is Kw and its equation?

Answer 18

It is the equilibrium constant for the dissociation of water.
Kw= [H+][OH-]

Question 19

What is Kw at 298K?

Answer 19

1 x10^14

Question 20

What is teh equation for Kw in pure water, and why?

Answer 20

Kw= [H+]^2
Because the conc. of hydrogen and hydroxide ions are equal

Question 21

Explain the effect of an increased temperature on Kw. Why does the water always remain neutral when this happens?

Answer 21

The dissociation of water is an endothermic process, as energy is required to break bonds.
If temp increases, equilibria shifts to the right to favour the endothermic reaction (to oppose change in temp)
Therefore conc. of H+ increases, so pH decreases.
Yet it is still neutral because there are always equal amounts hydrogen and hydroxide ions.

Question 22

What do acidic and alkali solutions indicate about H+ and OH- ions?

Answer 22

Acidic- [H+]>[OH-]
Alkali- H+]<[OH-]

Question 23

What are stromg bases?

Answer 23

Bases tha5 fully dissociate in aqueous solutipn to produce OH- ions

Question 24

What is teh equation for Ka at equilibrium?

Answer 24

[H]^2 /[OH-]

Question 25

What is a buffer?

Answer 25

A solution that maintains an approximately constant pH when a small amount of acid or base is added

Question 26

What is an acid buffer solution?

Answer 26

A solution of a weak acid and a salt of the acid. It maintains a pH below 7

Question 27

What is a basic buffer solution?

Answer 27

A solution containing a weak base and a salt of the base. It maintains a pH above 7

Question 28

Describe the composition of an acid buffer

Answer 28

The weak acid partially dissociates into H+ and A- (forms a reversible reaction)
The salt of the acid completely dissociates into A- and salt+

There is excess weak acid and excess A-

Question 29

What happens when an acid is added to an acid buffer solution?

Answer 29

The acids H+ ions react with the A- in the buffer solution.
Equilibrium shifts to the left to counter the addition of the H+ ions

Question 30

What happens when an alkali is added to an acid buffer solution?

Answer 30

The conc. of OH- increases
The H+ ions in the buffer solution react with H+ to form water.
Equilibrium shifts to the left to counter the loss of hydrogen ions

Question 31

What is the equilibrium reaction formed in a basic buffer solution?

Answer 31

Excess weak base + H2O <—> OH- + (base +H+)

Question 32

What is the equivalence point of a titration/ pH curve?

Answer 32

The point at whoch the acid and base are in the correct molar ration (the correct stoichiometric amounts)

Question 33

What is the end point of a titration?

Answer 33

The point at which the indicatpr changes colour

Question 34

What is the pH range of methyl orange?

Answer 34

3.4-4.4

Question 35

What is the pH range of Methyl red?

Answer 35

4.2-6.3

Question 36

What is the pH range of Phenol red?

Answer 36

6.8-8.4

Question 37

What is the pH range of pheolphthalein?

Answer 37

8.2-10

Question 38

How do you choose an indicator when carrying out a titration?

Answer 38

The pH range of the indicator must correspond to the region of rapid pH change (the verticle area of the pH curve)

Question 39

How can a pH curve be used to calculate Ka

Answer 39

At the half equivalence point, half the acid has been neutralised. So HA = A-
Therefore Ka= H+
So pKa = pH at the half equivalence point

Question 40

Why do buffer solutions have a constant pH even when diluted?

Answer 40

The conc. of HX and X- are so large they are effectively constant, so HX/X- is constant