acids and bases
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62 Terms
What is a Brønsted-Lowry acid?
A proton (H+) donor.
What is a Brønsted-Lowry base?
A proton (H+) acceptor.
What is a conjugate acid-base pair?
Two species that differ by one proton (H+).
What is the conjugate base of HCl?
Cl−.
What is the conjugate acid of NH3?
NH4+.
Why are acid-base reactions considered equilibria?
They involve reversible proton transfer.
Define a strong acid.
An acid that completely dissociates in aqueous solution.
Define a weak acid.
An acid that only partially dissociates in aqueous solution.
Define a strong base.
A base that completely reacts with water or completely dissociates in solution.
Define a weak base.
A base that only partially reacts with water.
Does acid strength depend on concentration?
No. Strength refers to the degree of dissociation, not concentration.
What does pH measure?
The concentration of hydrogen ions in a solution.
What is the equation for pH?
pH = −log10[H+].
What is the equation for hydrogen ion concentration?
[H+] = 10^(−pH).
What happens to [H+] when pH decreases by 1?
[H+] increases by a factor of 10.
What happens to [H+] when pH increases by 1?
[H+] decreases by a factor of 10.
What is the ionic product of water?
Kw.
What is the expression for Kw?
Kw = [H+][OH−].
What is the value of Kw at 25°C?
1.0 × 10^−14 mol² dm^−6.
What is the pH of pure water at 25°C?
7.
Why does the value of Kw change with temperature?
The dissociation of water is endothermic.
What happens to Kw as temperature increases?
Kw increases.
What is meant by a weak acid equilibrium?
The partial dissociation of a weak acid in solution.
What is Ka?
The acid dissociation constant.
What does Ka measure?
The strength of a weak acid.
What is the expression for Ka?
Ka = [H+][A−]/[HA].
What does a large Ka value indicate?
A stronger acid.
What does a small Ka value indicate?
A weaker acid.
What is pKa?
−log10Ka.
What is the relationship between acid strength and pKa?
The lower the pKa, the stronger the acid.
What is the relationship between Ka and pKa?
As Ka increases, pKa decreases.
What is the equation linking pH and Ka?
pH = pKa + log([A−]/[HA]).
When is pH equal to pKa?
When [HA] = [A−].
What is a buffer solution?
A solution that resists changes in pH when acids or alkalis are added.
What is an acidic buffer made from?
A weak acid and its conjugate base.
Give an example of an acidic buffer.
Ethanoic acid and sodium ethanoate.
How does a buffer remove added H+ ions?
The conjugate base reacts with H+.
How does a buffer remove added OH− ions?
The weak acid reacts with OH−.
What is the purpose of a pH titration curve?
To show how pH changes during a titration.
What is the equivalence point?
The point where the acid and alkali have reacted in exact proportions.
What is the shape of a strong acid-strong base titration curve?
A steep vertical section around pH 7.
What is the pH at the equivalence point for strong acid-strong base titrations?
Approximately 7.
What is the pH at the equivalence point for weak acid-strong base titrations?
Greater than 7.
Why is the equivalence point above pH 7 for a weak acid-strong base titration?
The salt formed hydrolyses to produce OH− ions.
What is an indicator?
A substance that changes colour over a specific pH range.
What must be considered when choosing an indicator?
Its colour change range must fall within the steep vertical section of the titration curve.
Which indicator is suitable for a strong acid-strong base titration?
Phenolphthalein or methyl orange.
Which indicator is suitable for a weak acid-strong base titration?
Phenolphthalein.
What is the half-neutralisation point?
The point where half the acid has been neutralised.
What is true about the half-neutralisation point?
pH = pKa.
Why is the half-neutralisation point useful?
It allows pKa to be determined from a titration curve.
What species predominates in a strong acid solution?
H+ ions and conjugate base ions.
What species predominates in a weak acid solution?
Mostly undissociated acid molecules.
What happens to acid dissociation when water is added?
The equilibrium shifts to the right.
State Le Chatelier's principle.
A system at equilibrium responds to oppose a change imposed on it.
What is meant by monoprotic acid?
An acid that donates one proton per molecule.
What is meant by diprotic acid?
An acid that can donate two protons per molecule.
Give an example of a strong acid.
HCl, HNO3 or H2SO4.
Give an example of a weak acid.
CH3COOH.
Give an example of a weak base.
NH3.
Why is pH a logarithmic scale?
Because hydrogen ion concentrations cover a very large range.