Module 5: 20 & 21 Flashcards Preview

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Flashcards in Module 5: 20 & 21 Deck (67)
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What is a Bronsted-Lowry acid?

A proton donor


What is a Bronsted-Lowry base?

A proton acceptor


What is a conjugate acid-base pair?

Contains 2 species that be interconverted by transfer of a proton

HCl + H2O ⇌ H3O+ + Cl-
NH3 + H2O ⇌ NH4+ + OH-


What are some common acids and their conjugate bases?

HCl -> Cl-
HNO3 -> NO3-
H2O -> OH-
NH4+ -> NH3
H3O+ -> H2O


Why is water special?

Can act as an acid or a base (accept and donate protons)
- Amphoteric


What is the type of acid linked to?

The number of hydrogen ions that can be replaced per molecule in an acid-base reaction

Monobasic acid: HCl & CH3COOH
Dibasic Acid: H2SO4
Tribasic acid: H3PO4


What is the general redox reaction involving acids?

Acid + metal -> salt + hydrogen gas (MASH)


What are the 3 neutralisation reactions involving acids?

Acid + alkali -> salt + H2O
Acid + carbonate -> salt + H2O + CO2
Acid + metal oxide -> salt + H2O


How can molecules accept protons?

Lone pairs of electrons -> dative covalent bonds


What is a strong acid?

HA -> H+ + A-
- strong tendency to donate H+ ions
- fully dissociate in aqueous solution
- equilibrium lies to the right


What does the pH scale show?

the relationship between pH and hydrogen ion concentration [H+(aq)]


How is pH calculated?

pH = -log[H+(aq)]

- give all answers to 2 decimals places


How can the pH of a strong acid be calculated?

- directly from the conc of the acid because the acid completely dissociates into ions
- H+ ion conc is equal to the conc of the strong acid in question

HA(aq) -> H+(aq) + A-(aq)
[HA(aq)] = [H+(aq)]


How is the pH changes on dilution calculated (1)?

diluted concentration = original moles / volume after dilution


How is the pH changes on dilution calculated (2)?

diluted concentration = original conc x (volume before dilution / volume after dilution)


What is a weak acid?

HA ⇌ H+ + A-
- weaker tendency to donate H+
- partially dissociate in aqueous solution
- equilibrium lies on the left
- strength of a weak acid is given by the acid dissociation constant (Ka)


How is Ka calculated?

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

units: moldm-3


What is Ka?

The acid dissociation constant


What do the Ka values represent?

- The larger the value of Ka, the stronger the acid
- The smaller the value of Ka, the weaker the acid


When does Ka change?

Changes with temperature (standard temp is used, 298K)


How is pKa calculated?

pKa = -log(Ka)


How is Ka calculated with pKa values?

Ka = 10^-pKa


How is [H+(aq)] calculated from Ka?



What do the pKa values indicate about the acid?

- the larger the pKa value, the weaker the acid
- the smaller the pKa value, the stronger the acid


What happens if 2 acids are in the same reaction?

- Weaker acids act as a base and accept the proton
- Stronger acid acts as an acid and donates the proton


What 2 approximations does the weak acid pH calculation require?

1. [H+] = [A-] are equal at equilibrium as the dissociation of water is negligible (produces a very small amount of H+ ions)

2. [HA]start = [HA]eqm since dissociation eqm lies on the left, little HA dissociates
- the conc of the undissociated acid is much greater than the [H+] at equilibrium


What happens to the Ka equation when the 2 approximations are applied to it?

Ka = [H+]^2 / [HA]

- concentrations are equal
- little water dissociates so we can ignore any decrease from dissociation


How can Ka of a weak acid be determined experimentally?

- preparing a standard solution of the weak acid of a known concentration [HA]
- measuring the pH of the standard solution using a pH meter [H+] = 10^-pH


When do the approximations break down in validity?

1. breaks down for very weak acids or very dilute solutions (pH>6) as the dissociation of water starts to become significant

2. breaks down for stronger acids with Ka>10^-2


Equation for the ionisation of water?

H2O ⇌ H+ + OH-