Chapter 20 - Acids, Bases and pH Flashcards
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?
A pair containing 2 species that can be interconverted by the transfer of a proton
What is the formula for a hydronium ion?
H3O +
What is a monobasic acid?
An acid that releases one proton (H+) per acid molecule
What is a dibasic acid?
An acid that releases 2 protons (H+) per acid molecule
What is a tribasic acid?
An acid that releases 3 protons (H+) per acid molecule
What is the ionic equation for neutralisation?
H+ (aq) + OH- (aq) → H2O (l)
What happens when an acid reacts with a metal (what type of reaction is this)?
Acid + metal → salt + hydrogen
Redox reaction
What happens when an acid reacts with a carbonate (what type of reaction is this)?
Acid + carbonate → salt + water + carbon dioxide
Neutralisation reaction
What happens when an acid reacts with an alkali/base/metal oxide (what type of reaction is this)?
Acid + base/metal oxide/alkali → salt + water
Neutralisation reaction
Who was the founder of pH?
Soren Sorenson
What is the mathematical relationship between pH and [H+ (aq)], what is the reverse of this?
pH = -log[H+ (aq)]
[H+ (aq)] = 10^-pH
Why is pH more convenient measurement for acid concentrations than [H+]?
pH makes numbers manageable as [H+] deals with negative indices over a very wide range
Why is the pH scale a logarithmic scale?
A change of one pH number is equal to a 10 times difference in [H+ (aq)]
How can you calculate the concentration of H+ ions in strong mono, di and tribasic acids?
Mono: [H+] = [HA]
Di: 2[H+] = [HA]
Tri: 3[H+] = [HA]
What is a strong acid?
An acid that completely dissociates in aqueous solution
What is a weak acid?
An acid that only partially dissociates in aqueous solution
How can you show the partial dissociation of a weak acid?
HA (aq) ⇌ H+ (aq) + A- (aq)
What is the acid dissociation constant, Ka?
Ka = [H+][A-] / [HA]
Units are always mol dm-3
What information is provided by Ka values?
Strength of acid/extent of dissociation
What does the value of Ka suggest?
The larger the numerical value of Ka, the further the equilibrium is to the right. Therefore, the larger the Ka value, the greater the dissociation and the greater the acid strength
What is the mathematical relationship between Ka and pKa?
pKa = -logKa
Ka = 10^-pKa
What is the relationship between weak acid strength, Ka and pKa?
The stronger the acid, the larger the Ka value and the smaller the pKa value.
The weaker the acid, the smaller the Ka value and the larger the pKa value
How can you simplify the Ka expression?
Ka = [H+]^2 / [HA]
[H+] at equilibrium, [HA] at start
What two assumptions must be made to simplify the Ka expression?
- HA dissociates to produce equilibrium concentrations of H+ and A- that are equal
[H+] ≈ [A-] - Only a small proportion of HA dissociates, so we can assume that [HA]»_space; [H+] therefore [HA] start ≈ [HA] end
How can Ka for a weak acid be determined experimentally?
Prepare a standard solution of the weak acid of known concentration, and then measure the pH of the standard solution using a pH meter
What is the expression for Kw?
Kw = [H+][OH-]
What is Kw?
The ionic product of water
What is the importance of Kw?
It sets up a neutral point in the pH scale, it changes with temperature but controls the concentration of H+ and OH- in aq solutions
How can we show a solution being acidic, alkaline and neutral through Kw?
Acidic: [H+] > [OH-]
Alkaline: [H+] < [OH-]
Neutral: [H+] = [OH-]
How can you calculate the pH of a strong base?
- the concentration of the base
- the ionic product of water, Kw
What is a monobasic base?
Where 1 mol of the base releases 1 mol of OH- ions
How do you calculate the pH of weak bases?
Do not need to do at A level - only partially ionise
