Acids bases and pH Flashcards
What is the Arrhenius definition of an acid?
An acid dissociates in water to produce H⁺ ions.
What is the Arrhenius definition of a base/alkali?
A alkali dissociates in water to produce OH⁻ ions.
What is the Bronsted-Lowry defintion of an acid
A proton donor
What is the Bronsted-Lowry definition of a base
A Proton acceptor
Define a conjugate acid base pair
A conjugate acid base pair contains two species that can be interconverted by the transfer of a proton.
Name the acid base pairs in:
HCl + OH⁻ ⇌ H₂O + Cl⁻
Pair 1:
acid = HCl base = Cl⁻
Pair 2:
acid = H₂O base = OH⁻
Name the acid base pairs in:
HCl + H₂O ⇌ H₃O⁺ + Cl⁻
Pair 1:
Acid=HCl Base=Cl⁻
Pair 2:
Acid=H₃O⁺ Base=H₂O
Define monobasic acids
Acids capable of donating one proton per molecule in an acid base reaction
Define dibasic acids
An acid capable of donating two protons per molecule in an acid base reaction.
Define tribasic acids
An acid capable of donating three protons per molecule in an acid base reaction.
Write the ionic equation for:
2HCl + Mg → MgCl₂ + H₂
2H⁺ + Mg → Mg²⁺ + H₂
Write the ionic equation for a neutralisation reaction
H⁺ + OH⁻ → H₂O
(aq) (aq) →(l)
What is Kₐ
The acid dissociation constant same as k𝒸 but measures the extent of acid dissociation
Write the equation to convert proton concentration [H⁺] to pH
pH = -log[H⁺]
Write the equation used to convert Kₐ to pKₐ
pKₐ = -log [Kₐ]
Does a strong acid have a large or small Kₐ value?
Large
When using weak acids what is the simplified Kₐ equation?
(using HA)
Kₐ= [H⁺]² / [HA]
Why can weak acids use a simplified Kₐ equation?
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]»_space; [H]
[HA] equi = [HA] start - [H⁺] equi
resulting in:
[HA] start = [HA] equilibrium
Why cant ‘stronger’ weak acids use the simplified equation for Kₐ?
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.
What is Kᴡ called?
The ionic product of water
What is a buffer solution?
A buffer solution is a system that minimises pH changes when small amounts of acid or base are added.
Write the two methods used to prepare a buffer solution, giving exmaples
-A weak acid and its salt
CH₃COOH, CH₃COONa
-Excess of a weak acid and a strong alkali
CH₃COOH, NaOH
Describe what happens when acid is added to a buffer solution.
(weak acid and its salt)
-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.
Describe what happens when alkali is added to a buffer solution.
(weak acid and its salt)
-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.
How can an acid buffer solution pH be calculated?
Rearrange the Kₐ equation to get [H+] and then take -logs of both sides, [A-] and [H+] do not have the same conc!
What pH must blood be maintained at?
7.35 to 7.45
What buffer system is used to maintain blood pH?
(name and equation)
Carbonic acid - hydrogen carbonate system
H₂CO₃ → H⁺ + HCO₃⁻
What is the equivalence point? (for a pH titration curve)
The equivilance point is at the centre of the vertical section. The moles of acid and base are stoichometrically equivalent.
What is an acid-base indicator made of?
A weak acid
What must be different with a weak acid and it’s conjugate base to be an indicator?
The acid and it’s conjugate base must have distinct different colours.
What happens to the indicator at the end point of a pH titration?
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.
Describe what occurs when methyl orange is added to a basic solution. (colours and process)
-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.
Describe what occurs when methyl orange is added to an acidic solution. (colours and process)
-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.
How can an indicator be choosen for a pH titration.
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.
What type pH titrations are no indicators used for and why?
Weak acids and weak bases, as there is no vertical section.
How is the pH measured for a pH titration graph?
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.
