Acids/Bases

Question 1

What is the Bronsted-Lowry model?

Answer 1

• states that acids are proton donators / bases are proton acceptors

Monoprotic - dones one H
Polyprotic - donates multiple H
- can be diprotic or triprotic
- the likelihood of H + donation at each stage decreases as the acids become weaker

Question 2

What are conjugate acids and bases?

Answer 2

Conjugate base:
- the product formed from an acid that has donated a proton (H+)

Conjugate acid:
- the product formed from a base that has accepted a proton (H+)

Acid and conjugate base –> conjugate pairs

Question 3

Whar are amphiprotic substances?

Answer 3

• substances that act as both a base and acid, meaning they can donate and accept protons
• e.g water

Question 4

Strong vs Weak acids

Answer 4

Strong acids:
- completely ionise in a solution, meaning they readily donate their H+
- HCl, H2SO4

Weak acids:
- do not completely ionise in a solution
- CH3COOH

Question 5

Strong vs Weak Bases

Answer 5

The strength of a base depends on its ability to accept H+ and therefore ionise

Strong:
- readily accept H+ from acids during a reaction
- NaOH, KOH

Weak:
- don’t readily accept H+ from acids
- NH3

Question 6

Strenght vs Conc.

Answer 6

Concentration:
- measures the number of molecules, both ionised and unionised, in a given volume

Strength:
- measures the percentage of molecules that ionise

Question 7

How does water self-ionise?

Answer 7

• it self-ionises to form hydronium (H30+) and hydroxide (OH-) ions
• H20 + H2O –> H3O(+) + OH(-)

Question 8

what is the ionisation constant?

Answer 8

• represents the product of the ions H3O and OH when water self-ionises
• the Kw value is constant, meaning any changes in the conc. of either ion means fluctuation in the conc of the other ion

Kw = [H3O+] [OH-] = 10^-14 at 25C

Question 9

What is pH and how is it calculated

Answer 9

• it measures the acidity of the solution

pH = - log [H3O+]
or
[H3O+] = 10^-pH

Question 10

What is the acid disassociation constant?

Answer 10

• Ka is a measure of the substances ability to reversibly disssociate in a solution
• can determine the extent of the disassociation of acids at a particular temp at eq.

Ka = [H3O+] [A-] / [HA]

Question 11

What is the base dissociation constant?

Answer 11

• Kb measures the ability of a base to dissociate

Kb = [BH+] [OH-] / [B]

Question 12

Strength based on Ka/Kb

Answer 12

• strong acids have a high Ka / weak acids have a low Ka
• strong bases have a large Kb / weak bases have a low Kb

Question 13

What are buffers?

Answer 13

• solutions that can resist a slight change in pH when an acid/base is added to a solution
• they work by neutralising the acid/base added

Made from weak conjugate pairs
- weak acid and/or salt containing the conjugate base
- weak base and/or salt containing the conjugate acid

Question 14

What salt is primarily used?

Answer 14

• salt is used as a sodium-based salt due to the solubility of sodium

Question 15

What occurs when a base is added with a buffer?

Answer 15

• the [OH-] ions react with the weak acids to form water and the conjugate base
• prevents pH from changing too much
• favours the forward reaction and shifts eq. to the right

Question 16

What occurs when an acid is added with a buffer?

Answer 16

• the [H+] ions react with the conjugate base to form water and the weak acid
• due to the inability of weak acids to dissociate fully, there is no large change in pH
• favours the backward reaction and shifts eq. to the left

Question 17

What is an acid-base indicator

Answer 17

• often weak acids/bases and indicate colour change based on the solution they are added to
• with the solution acidic and high in [H3O+], there is a shift to the left, causing a colour change
• with the solution basic and low in [H3O+], there is a shift to the right, causing a colour change

Question 18

Relationship between pH and pKa

Answer 18

Indicator will change colour when Ka = [H3O+]
- pKa ( -log of Ka) can be used due to the number being so small
- the smaller the pKa, the stronger the acid

Therefore, when pKa = pH –> there will be a colour change

Question 19

How does Volumetric Analysis work?

solutions of known conc.

Answer 19

• solutions of known concentrations (primary standards) are used to determine the conc. of unknown solutions

Question 20

How does Titration work?

Answer 20

• a solution with a known quantity is titrated against a solution of an unknown conc.

Question 21

Aliquot, Titrant, Analyte?

Answer 21

Aliqout:
- an exact amount of a substance

Titrant:
- substance of a known conc. in the burette

Analyte:
- substance of unknown conc. in the conical flask

Question 22

What is the equivalence point?

Answer 22

• point at which the substances in a reaction have reacted according to the stoichiometric ratio, as per the balanced chemical equation
• this is when neutralisation occurs

Question 23

What is the end point?

Answer 23

• point in an acid-base titration whwere a permanent colour change occurs

Question 24

When will the colour of the indicator change?

Answer 24

When pKind = pH

Question 25

The buffer point?

Answer 25

• within the buffer region, there is a half-equivalence point / buffer point
• it is where half the reactants have reacted –> [HA] = [A-]

Therefore, Ka = [H+]
or
pKa = pH

Question 26

Steps to calculating an unknown conc.

Answer 26

1. determine average titre of known solution
2. calculate the moles of known solution used
3. determine the moles of unknown solution used –> molar ratio
4. calculate the unknown conc

Question 27

Determining conc./vol before and after dilution?

Answer 27

C1 x V1 = C2 x V2

Question 28

How to find conc of original undiluted solution

Answer 28

Dilution factor in mL:
= volume of diluted solution / volume of concentrated sample

Then:
- you multiply the conc of the aliquot by the dilution factor to calculate the conc of the original concentrated sample