Acids and Bases Flashcards
What is an Arrhenius acid?
A substance that ionises in water to give H⁺ ions and anions
What is an Arrhenius base?
A substance that ionises in water to give hydroxide ions and cations
What reaction happens when an acid and a base react?
Neutralisation which produces water and a salt
What are the two main limitations of the Arrhenius definition?
They only apply to aqueous solutions but acid-base reactions can occur in other solvents too
It is difficult to see how the base ammonia can function as it does not contain an OH group
What is a Brønsted-Lowry acid?
A proton donor
What is a Brønsted-Lowry base?
A proton acceptor
What is solvation?
When a compound dissolves, the individual ions or molecules interact with the solvent molecules
Hydration is a specific type of solvation where an ionic compound dissolves in water
What is a conjugate base?
The product formed by the loss of a proton from the acid
What is a conjugate acid?
The product formed by the addition of a proton to the base
In the following reaction, decide which is a conjugate base, a conjugate acid, an acid and a base
HCN + H₂O ⇌ CN⁻ + H₃O⁺
HCN - Acid
H₂O - Base
CN⁻ - Conjugate Base
H₃O⁺ - Conjugate Acid
In the following reaction, decide which is a conjugate base, a conjugate acid, an acid and a base
H₂O + NH₃ ⇌ OH⁻ + NH₄⁺
H₂O - Acid
NH₃ - Base
OH⁻ - Conjugate Base
NH₄⁺ - Conjugate Acid
What strength does the conjugate base have when it is formed by a strong acid?
It is weak
What strength does the conjugate acid have when it is formed by a strong base?
It is weak
What is the difference between strength and concentration?
Strength is a measure of the extent to which an acid can donate H⁺or the extent a base can accept H⁺
Concentration is a measure of the amount of substance is in a given volume
Why is the concentration of water in the following reaction essentially constant?
HA + H₂O ⇌ A⁻ + H₃O⁺
The reaction takes place in a dilute solution
What is the equation to turn pKₐ into Kₐ?
pKₐ = log₁₀(Kₐ)
What do a high Kₐ value and a low pKₐ value mean?
It is a strong acid
What do a low Kₐ value and a high pKₐ value mean?
It is a weak acid
What is solvent levelling?
Since strong acids are completely ionised in water, they appear to be of equal strengths
To compare the strength of these acids, a solvent that is weaker proton acceptor than water e.g. ethanenitrile must be used as ionisation is not complete
Changing the solvent will however affect the acid dissociation constant e.g. the pKₐ for HCl is -7 in water, but only 8.9 in ethanenitrile
What is the equation for pH?
pH = -log₁₀[H₃O⁺]
What is an acidic solution defined as?
[H⁺] > [OH⁻]
What is an alkaline solution defined as?
[H⁺] < [OH⁻]
What are the two assumptions that can be made when working out the pH of a weak acid?
[H₃O⁺] = [A⁻]. This neglects the [H₃O⁺] present in the solution from the ionisation of water, but this is usually very small in comparison
[HA] at equilibrium is equal to the concentration of HA used to make the solution. This neglects the small proportion of HA molecules that dissociate
Calculate the pH of 0.105 mol dm⁻³ of ethanoic acid at 298K. The Kₐ is 1.7 x 10⁻⁵ mol dm⁻³
pH = 2.9
What is the reaction showing the self-ionisation of water?
H₂O + H₂O ⇌ OH⁻ + H₃O⁺
Why can the denominator of the equilibrium constant for the self-ionisation of water be removed?
It is effectively constant as only a very small proportion of water molecules are ionised, creating a new constant, the self-ionisation constant (Kw)
What is the equation for the self-ionisation constant?
Kw = [H₃O⁺][OH⁻]
What are the units for the self-ionisation constant?
mol² dm⁻⁶
What is the value of the self-ionisation constant at 298K?
1 x 10¹⁴ mol² dm⁻⁶
Why does the value of Kw increase at higher temperatures? How does this affect the pH of pure water?
The self-ionisation of water is endothermic, leading to the pH of pure water to be less than 7 above 298K and greater than 7 below 298K
Write the basicity constant (Kb) for the following reaction:
B + H₂O ⇌ BH⁺ + OH⁻
Kb = ([BH⁺][OH⁻]) / [B]
Why can water be removed from the equilibrium constant expression to form Kb?
The concentration of water is effectively a constant
How strong is the conjugate acid of a strong base?
It is weak
What happens when you multiply the equations for Ka and Kb?
It cancels out to give the expression for Kw
What is the equation when you take -log₁₀ of both sides of the following expression?
Ka x Kb = Kw
pKa + pKb = pKw
What is a buffer solution?
A solution that resists changes in pH, even when small amounts of acids or alkalis are added
The pH does not change when the solution is diluted also
What is contained within a buffer solution?
They are either made from a weak acid and its conjugate base, or a weak base and its conjugate acid
What two assumptions can you make about the species present at equilibrium in the buffer?
All of the A⁻ ions come from the salt - HA supplies very few A⁻ ions in comparison with the salt which is an ionic compound that dissociates almost completely
Almost all of the HA molecules put into the buffer solution remain unchanged - this is because it dissociates very little
What happens in a buffer solution when a small amount of acid is added?
The extra H₃O⁺ ions disturb the position of equilibrium. Some A⁻ ions from the salt react with H₃O⁺ ions and the position of equilibrium moves to the left to minimise the effect of the change
What happens in a buffer solution when a small amount of alkali is added?
The OH⁻ ions react with H₃O⁺ ions in the buffer solution, disturbing the position of equilibrium. The weak acid HA reacts with H₂O and the position of equilibrium moves to the right to produce more H₃O⁺ ions to minimise the effect of the change
What is the Henderson-Hasselbach equation?
pH = pKₐ + log₁₀ x ([A⁻] / [HA])
Why can’t the ratio of conjugate base to weak acid be greater than 1:3, or 3:1?
It changes the [H₃O⁺] by a factor of 9, altering the pH by approximately one unit
This should not be broken as there will not be enough HA or A⁻ for the buffer to act as both a source and a sink
Why does the pH of a buffer not change when you dilute it?
When adding water, the concentrations of the acid and the conjugate base are reduced equally so that the ratio of their concentrations remains the same
How can the starting pH of a strong acid-strong base titration be calculated?
By working out the pH of the strong acid
How can the pH be calculated before neutralisation in a strong acid-strong base titration?
[H₃O⁺] = ((Conc. of acid x Vol. of acid) - (Conc. of base x Vol. of base added)) / (Vol. of acid) + (Vol. of base)
What is the equivalence point?
When all of the acid has been neutralised - in a strong acid-strong base titration, this happens at pH 7
How can the pH be calculated after neutralisation in a strong acid-strong base titration?
The concentration of OH⁻ needs to be known by calculating the moles of OH⁻ and dividing it by the total volume of acid and alkali added
The pOH can then be calculated by -log[OH⁻], and the pH can be calculated by pH + pOH = 14
20.0 cm³ of 0.240 mol dm⁻³ nitric acid is transferred to a conical flask and titrated against 0.400 mol dm⁻³ sodium hydroxide solution. What is the pH of the solution when 8.0cm³ of the base has been added?
1.24
Why is the same amount of volume of base needed to neutralise a strong acid compared to a weak acid of the same concentration?
One mole of base will react with one mole of acid, regardless of whether the acid is strong or weak
Why will the initial pH for a strong acid-strong base titration be higher than in a weak acid-strong base titration?
The weak acid is only partially ionised
Why does a weak acid-strong base titration create a buffer solution?
Both the acid and conjugate base are present, leading to a slow change in pH before neutralisation
What is the pH of the equivalence point in a weak acid-strong base titration?
It is greater than 7 as the salt hydrolyses (reacts with water) so the neutralised solution is alkaline
What is pH equal to at the point of half-neutralisation?
pH = pKₐ as the [A⁻] = [HA]
What part of the titration curve must the pH range cover for it to be an effective indicator?
It must cover the equivalence point
Give three examples of oxoacids
HNO₃, H₂SO₄ and H₃PO₄
What is the general formula for an oxoacid?
HₘXOₙ
How many OH groups does an oxoacid have?
m
How many doubly bonded oxygen atoms does an oxoacid have?
(n - m)
Why is the second pKₐ for the ionisation of sulfuric acid higher than the first pKₐ?
It is a weaker acid as it is harder to remove a proton from an anion rather than a neutral molecule
How does the number of doubly bonded oxygen atoms impact the strength of the acid?
The more there are, the stronger the acid
What are the two reasons that the number of doubly bonded oxygen atoms impacts the strength of the acid?
There is an inductive effect with the electronegative oxygen atoms
The more resonance forms a conjugate base has
Explain the inductive effect of the electronegative oxygen atoms in considering the strength of an oxoacid
The electronegative oxygen atoms draw electron density away from the hydrogen atoms in the O-H bonds
This weakens the O-H bond so that the proton is more easily removed
The more doubly bonded oxygen atoms there are, the greater the inductive effect that occurs
Explain how the number of resonance forms impacts the strength of the oxoacid
The more resonance forms there are, the greater the delocalisation of the electron so the more stable the anion
This higher stability makes it easier to form
Why is H₃PO₃ a diprotic acid and not a triprotic acid?
It only has two ionisable hydrogens as one is directly bonded to the Phosphorous atom
What do the terms ‘acidic’ and ‘basic’ oxides relate to?
Their reaction with water forms either H₃O⁺ (acidic oxide) or OH⁻ (basic oxide)
Define the term ‘amphoteric’
An oxide that reacts with both acids and bases
Give examples of some elements that are amphoteric
Be, B, Al, Si, Ga, Ge, As, Sn, Sb, Pb
Write equations to show Aluminium Oxide acting as an acid and as a base
Al₂O₃ + 6H₃O⁺ + 3H₂O → 2[Al(H₂O)₆]³⁺
Al₂O₃ + 2OH⁻ + 3H₂O → 2[Al(OH)₄]⁻
What is a Lewis acid?
An electron pair acceptor
What is a Lewis base?
An electron pair donor
What is an ‘adduct’?
The product of a Lewis Acid / Lewis Base reaction where a dative covalent bond is formed
What is a superacid?
An acid that is stronger than 100% sulfuric acid
Why are superacids so strong?
The Brønsted-Lowry acid donates a lone pair to the Lewis acid, breaking the H-X bond and the proton becomes attached to another molecule of the Brønsted-Lowry acid
What are two example reactants that can form a superacid?
SbF₅ and HSO₃F, forming magic acid that can dissolve candle wax
Order the Hydrogen Halides in order of their acidic strength
HI > HBr > HCl»_space; HF
Why do the Hydrogen Halides decrease in acidity as you go down the group?
The H-X bond gets weaker as you go down the group
Why does HF have a considerably lower acidity level than HCl?
There are very strong hydrogen bonds between H₃O⁺ and F⁻, making them tend to ‘stick’ together to make H₃O⁺F⁻ which is not acidic
What are amino acids?
Major components of bodies
Proteins are made of them
There are 20 naturally occurring amino acids in human proteins
All are chiral except glycine
They do not exist as neutral compounds
What is a zwitterion?
An amino acid that is charge neutral
At low pH, what charge does an amino acid have?
A positive charge as RNH₃⁺ is present
At high pH, what charge does an amino acid have?
A negative charge as COO⁻ is present
At the isoelectric point, what charge does an amino acid have?
It is neutral (a zwitterion is present)
Why does glycine have 2 pKₐ values?
It has one for the protonation of NH₂ and another for the deprotonation of COOH
What is electrophoresis?
An analytical method for amino acids
There is a plate with a positive charge on one side and a negative charge on the other, and the pH is ‘sweeped’ and movement is looked for
There should be no movement when the isoelectric point has been reached as the charge is neutral
How can the isoelectric point be calculated?
By averaging out the pKₐs on either side of the reaction
