Acids and bases Flashcards
What is meant by a Bronsted-Lowry acid?
Proton donor
What is meant by a Bronsted-Lowry base?
Proton acceptor
What is meant by a strong acid?
Fully dissociates
What is meant by a weak acid?
Partially dissociates
Definition of PH
pH = -log10[H+]
Definition of [H+]
[H+] = 10-pH
Definition of the ionic product of water (Kw)
Kw = [H+][OH-]
What equation should be used when calculating the pH of a strong acid?
pH = -log10[H+]
What equation should be used when calculating the pH of a weak acid?
Ka = [H+]2 / [HA]
Rearrange to get [H+]
What equation should be used when calculating the pH of a buffer?
Ka = [H+][A-] / [HA]
Rearrange to get [H+]
metal + acid ……
salt + hydrogen
e.g. HCl (aq) + Na (s) —> NaCl (aq) + 1/2 H2 (g)
hydrochloric acid + sodium —> sodium chloride + hydrogen
metal oxide + acid ……
salt + water
e.g. 2HNO3 (aq) + K2O (aq) —> 2KNO3 (aq) + H2O (l)
nitric acid + potassium oxide —> potassium nitrate + water
acid + metal hydroxide …..
salt + water
e.g. H3PO4 (aq) + 3NaOH (aq) —> Na3PO4 (aq) + 3H2O (l)
phosphoric acid + sodium hydroxide —> sodium phosphate + water
acid + metal carbonate …..
salt + water + carbon dioxide
e.g. H2SO4 (aq) + Na2CO3 (s) —> Na2SO4 (aq) + CO2 (g) + H2O (l)
sulfuric acud + sodium carbonate —> sodium sulfate + carbon dioxyde + water
ammonia + acid ….
salt
e.g. NH3 (aq) + HCl (aq) —> NH4Cl (aq)
ammonia + hydrochloric acid —> ammonium chloride
e.g. NH3 (aq) + H3PO4 (aq) —> (NH4)3 PO4 (aq)
What is a monoprotic acid?
only one H+ ion released e.g. HCl
What is a diuretic acid?
two H+ ions released e.g. H2SO4
What does smaller pH mean in terms of [H+]
Higher concentration of [H+]
What are the 4 main examples of strong acids?
- HCl
- H2SO4
- HNO3
- H3PO4
Where does the position of equilibrium for strong acids usually lie?
entirely to right side
Where does the position of equilibrium for weak acids usually lie?
More to the left side
How to find pKa from Ka?
pKa = -log10Ka
How to find Ka from pKa?
Ka = 10-pKa
What is the Kw value for water?
1 x 10-14 mol2dm-6
Why is water always neutral?
[OH-] = [H+]
How can [H+] be found from Kw?
Kw = [H+]2
How can Kw] be found from [H+]?
[H+] = square root of Kw
Strong bases…
- fully dissociate
- to release OH- ions
Where does the position of equilibrium for strong bases usually lie?
entirely on right side
Equation to find [H+] of strong alkalis
[H+] = Kw / [OH-]
Suggest why pure water is not alkaline
[H+] = [OH-]
Explain why [H2O] is not shown in the Kw expression
[H2O] is (almost) constant
A student has an unknown weak acid of unknown concentration and a bottle of 0.5 mol dm-3 NaOH.
Explain, using brief practical details, how you would determine the Ka of the weak acid. [6]
- Measure 25 cm3 of acid into a beaker/conical flask using volumetric pipette.
- Use a pH probe to measure the initial pH
- Add 1cm3 NaOH from a burette swirl and record the pH
- Repeat until the NaOH is in excess
- Plot a graph of pH against vol NaOH added in cm3
- Use the vertical section to find the volume of NaOH needed for neutralisation
- The half neutralisation point is half the volume of neutralisation.
- Read off the pH at the half neutralisation point.
- At half neutralisation ka = [H+]/pKa=pH
- Ka = 10-pH
write equation to show how ethanoic acid behaves as a weak acid in it’s reaction with water
CH3COOH —> CH3COO- + H+
write equation when pure ethanoic acid reacts with pure nitric acid where ethnic acid acts as a base
CH3COOH + HNO3 —. CH3COOH2+ + NO3 2-
explain why chloroethanoic acid is stronger than ethanoic acid
- electronegative chlorine withdraws electrons
- weakens O-H bond
explain why data books do not usually contain Ka values for strong acids
- strong acids completely dissociate
- ka for strong acids is very large
state how buffer solution made
add excess weak acid to alkali
state how buffer solution resists changes in PH
CH3COO- from salt reacts with H+
