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