Acids bases and pH

Question 1

What is the Arrhenius definition of an acid?

Answer 1

An acid dissociates in water to produce H⁺ ions.

Question 2

What is the Arrhenius definition of a base/alkali?

Answer 2

A alkali dissociates in water to produce OH⁻ ions.

Question 3

What is the Bronsted-Lowry defintion of an acid

Answer 3

A proton donor

Question 4

What is the Bronsted-Lowry definition of a base

Answer 4

A Proton acceptor

Question 5

Define a conjugate acid base pair

Answer 5

A conjugate acid base pair contains two species that can be interconverted by the transfer of a proton.

Question 6

Name the acid base pairs in:
HCl + OH⁻ ⇌ H₂O + Cl⁻

Answer 6

Pair 1:
acid = HCl base = Cl⁻
Pair 2:
acid = H₂O base = OH⁻

Question 7

Name the acid base pairs in:
HCl + H₂O ⇌ H₃O⁺ + Cl⁻

Answer 7

Pair 1:
Acid=HCl Base=Cl⁻
Pair 2:
Acid=H₃O⁺ Base=H₂O

Question 8

Define monobasic acids

Answer 8

Acids capable of donating one proton per molecule in an acid base reaction

Question 9

Define dibasic acids

Answer 9

An acid capable of donating two protons per molecule in an acid base reaction.

Question 10

Define tribasic acids

Answer 10

An acid capable of donating three protons per molecule in an acid base reaction.

Question 11

Write the ionic equation for:
2HCl + Mg → MgCl₂ + H₂

Answer 11

2H⁺ + Mg → Mg²⁺ + H₂

Question 12

Write the ionic equation for a neutralisation reaction

Answer 12

H⁺ + OH⁻ → H₂O
(aq) (aq) →(l)

Question 13

What is Kₐ

Answer 13

The acid dissociation constant same as k𝒸 but measures the extent of acid dissociation

Question 14

Write the equation to convert proton concentration [H⁺] to pH

Answer 14

pH = -log[H⁺]

Question 15

Write the equation used to convert Kₐ to pKₐ

Answer 15

pKₐ = -log [Kₐ]

Question 16

Does a strong acid have a large or small Kₐ value?

Answer 16

Large

Question 17

When using weak acids what is the simplified Kₐ equation?
(using HA)

Answer 17

Kₐ= [H⁺]² / [HA]

Question 18

Why can weak acids use a simplified Kₐ equation?

Answer 18

Two apporximations are made:
1. The concentration of [H⁺] and [A⁻] are the same, the additional [H⁺] for the dissociation of water is negligable.
2. The dissociation of weak acids is so small that the difference in [HA] from the start to equilibrium is negligable.
[HA]&raquo_space; [H]
[HA] equi = [HA] start - [H⁺] equi
resulting in:
[HA] start = [HA] equilibrium

Question 19

Why cant ‘stronger’ weak acids use the simplified equation for Kₐ?

Answer 19

The approximation that [HA] sᴛᴀʀᴛ = [HA]ᴇǫᴜɪʟɪʙʀɪᴜᴍ can no longer be true as the dissociation of [H⁺] becomes significant.
For weak acids where Kₐ > 10⁻².

Question 20

What is Kᴡ called?

Answer 20

The ionic product of water

Question 21

What is a buffer solution?

Answer 21

A buffer solution is a system that minimises pH changes when small amounts of acid or base are added.

Question 22

Write the two methods used to prepare a buffer solution, giving exmaples

Answer 22

-A weak acid and its salt
CH₃COOH, CH₃COONa
-Excess of a weak acid and a strong alkali
CH₃COOH, NaOH

Question 23

Describe what happens when acid is added to a buffer solution.
(weak acid and its salt)

Answer 23

-The concentration of H⁺ ions increases.
-The conjugate base reacts with the H⁺ ions.
-The equilibrium position shifts to the left as most of the H⁺ ions are removed.
-maintaing pH.

Question 24

Describe what happens when alkali is added to a buffer solution.
(weak acid and its salt)

Answer 24

-The concentration of OH⁻ increases.
-The H⁺ ions react with the OH⁻ions to produce water.
-The weak acid dissociated to produce more H⁺ ions, shifting the equilibrium position to the right, to control the pH.

Question 25

How can an acid buffer solution pH be calculated?

Answer 25

Rearrange the Kₐ equation to get [H+] and then take -logs of both sides, [A-] and [H+] do not have the same conc!

Question 26

What pH must blood be maintained at?

Answer 26

7.35 to 7.45

Question 27

What buffer system is used to maintain blood pH?
(name and equation)

Answer 27

Carbonic acid - hydrogen carbonate system
H₂CO₃ → H⁺ + HCO₃⁻

Question 28

What is the equivalence point? (for a pH titration curve)

Answer 28

The equivilance point is at the centre of the vertical section. The moles of acid and base are stoichometrically equivalent.

Question 29

What is an acid-base indicator made of?

Answer 29

A weak acid

Question 30

What must be different with a weak acid and it’s conjugate base to be an indicator?

Answer 30

The acid and it’s conjugate base must have distinct different colours.

Question 31

What happens to the indicator at the end point of a pH titration?

Answer 31

The indicator contains equal parts weak acid and conjugate base, resulting in a colour between that of the weak acid’s and conjugate base’s.

Question 32

Describe what occurs when methyl orange is added to a basic solution. (colours and process)

Answer 32

-OH- ions from the basic solution react with H+ ions from the indicator to form water.
-The equilibrium position shifts to the right as the weak acid dissociates to make more H+ ions.
-The colour changesto orange then yellow, as equilibrium shifts to the right.

Question 33

Describe what occurs when methyl orange is added to an acidic solution. (colours and process)

Answer 33

-The H+ ions from the acidic solution react with the conjugate base.
-The equilibrium shifts to the left as more HA is made, to reduce the increase in H+.
-The colour shifts to orange to red.

Question 34

How can an indicator be choosen for a pH titration.

Answer 34

An indicators colour must change coinciding with the vertical section of the pH titration.
-ideally the inidcators end point and the titration’s equivalence point would intersect.

Question 35

What type pH titrations are no indicators used for and why?

Answer 35

Weak acids and weak bases, as there is no vertical section.

Question 36

How is the pH measured for a pH titration graph?

Answer 36

A pH meter is used.
-First the pH meter is placed into a conical flask containing an acidic solution.
-Using a burette 1cm³ of base is added, into the conical flask, swriling the solutin then reading the pH (using pH meter), and noting down result.
-This continues untill a large increase in pH is met, the base is then added drop wise from the burette, until pH increase is no longer large then back to 1cm³ increments.
-Stop after pH barely changes.