McMurry (Kap.2,8) Flashcards
What decides the strength of an acid?
Stronger acids (HCl) react almost completely with water.
Weaker acids (CH3CO2H) reacts only slightly with water.
In other words, stronger acids completely dissociate (give out all their H+) whereas weaker acids partially dissociate (give out some of their H+).
What is the acidity constant (Ka)?
The exacts strength of a given acid in a water solution is described using this constant, for the acid dissociation equilibrium.
Why is the concentration of the solvent (opløsningsmiddel - typisk vand) ignored in the equilibrium expression?
Because the amount of for example water doesn’t change throughout the reaction. It is the same on both sides of the equilibrium and does not attribute to the reaction, therefore can be ignored.
What does it mean if Ka is big or small?
Big Ka means stronger acid, smaller Ka means weaker acid.
Stronger acids have their equilibrium towards the right, and weaker acids have their equilibrium towards the left.
What are acid strengths usually expressed with?
pKa value, which is the negative logarithm of the Ka value.
What does a big pKa indicate, and a small pKa indicate?
Big pKa = weaker acid.
Small pKa = stronger acid.
What is the ion product constant of water?
The ion product constant of water (Kw) is a measure of how much water dissociates into hydrogen ions (H⁺) and hydroxide ions (OH⁻). At 25°C, its value is 1*10^-14.
Kw = [H3O+] * [OH-]
This only happens when water reacts with itself, as an acid and a solvent.
What is the inverse relationship between the acid strength of an acid, and the base strength of its conjugate base?
A strong acid has a weak conj. base, and a weak acid has a strong conj. base.
A strong acid loses a H+ easily, which means that its conj. base holds the H+ weakly, and therefore is a weak base.
A weak acid loses a H+ with difficulty, meaning that its conjugate base holds the proton tightly and therefore is a strong base.
Why does a strong acid have a weak conj. base, and a weak acid have a strong conj. base?
Because when the strong acid dissociate, the conj. base that forms is stable.
Stable base = weak base.
When the weak base dissociate, the conj. base that forms is unstable.
Unstable base = strong base.
