acids and bases Flashcards
What is a Bronsted-Lowry acid?
Protons donors- release H+ ions when they’re mixed with water
What is a Bronsted-Lowry base?
Proton acceptors
What does dissociate mean?
Break up into positively and negatively charged ions in water.
What do strong acids/bases do in terms of dissociation?
Fully dissociate/ionise in water
What do weak acids/bases do in terms of dissociation?
Partially dissociate/ionise in water so equilibrium lies well to the left
What is the ionic product of water at 25ºC?
1 x 10^-14 mol^2dm^-6
What is the equation for Kw?
Kw= [H+][OH-]
OR
Kw= [H+]^2
How do you calculate pH?
pH= -log [H+]
[H+] = 10 ^ -pH
What is the pH scale?
0 (very acidic) to 14 (very alkaline)
pH 7 is neutral
What does monoprotic mean?
Each molecule of an acid will release one proton when it dissociates
e.g HCl or HNO3
What does diprotic mean?
Each molecule of an acid will release two protons when it dissociates
e.g H2SO4
How do you calculate the acid dissociation constant?
Ka= [H+][A-] / [HA]
when dealing with weak acids [H+]=[A-] so Ka= [H+]^2 / [HA]
What does the pH curve look like for a strong acid/strong base reaction?
The pH starts around 1 as theres as excess of strong acid. It finishes around pH 13, when you have an excess of strong base
What does the pH curve look like for a strong acid/weak base reaction?
The pH starts around 1 as there’s an excess of strong acid. It finishes around pH 9, when you have an excess of weak base
What does the pH curve look like for a weak acid/strong base?
The pH starts around 5, as there’s an excess of weak acid. It finishes around pH 13 when you have an excess of strong acid
What does the pH curve look like for a weak acid/weak base?
The pH starts around 5 as there is an excess of weak acid. Finishes around pH 9, when you have an excess of weak base
What is the point of equivalence?
When the pH graph is almost vertical also known as end point
What is the range of pH for the colour change of phenalphthalein, and what is the colour change?
8.3-10
Colourless to pink
What is the range of pH for the colour change of methyl orange, and what is the colour change?
3.1-4.4
red to yellow
What is a buffer?
A solution that resists change in pH when small amounts of acid or alkali are added
What is an acidic buffer?
Have a pH of less than 7. Contain a mixture of weak acid with one of its salts
Why can acidic buffers resist an acid?
The large number of A- ions make sure that the buffer can cope with the added acid. If you add a small amount of acid, [H+] increases. Most of the extra H+ ions combine with the A- ions to form HA. This shifts the equilibrium to the left, reducing [H+] conc close to original so pH stays roughly the same
Why can acidic buffers resist a base?
If you add a small amount of base, [OH-] increases. Most of the extra OH- ions combine with the H+ ions to form water, causing more HA to dissociate into H+ ions, shifting equilibrium to the right. [H+] increases until close to original so pH stays roughly the same
What is a basic buffer?
Have a pH greater than 7, contain a mixture of a weak base with one of its salts.
How does a basic buffer resist an acid?
If you add a small amount of acid, [H+] increases. Some of the H+ ions react with OH- to form H2O. This shifts the equilibrium to the right, replacing the OH- ions. this will remove most of the added H+ ions so pH stays roughly the same
How does a basic buffer resist a base?
If a small amount of base is added, [OH-] increases making the solution more alkaline. Most of the OH- will react with the salt so the equilibrium shifts to the left removing OH- ions from solution. pH stays roughly the same
Why do buffers resist changes in pH when diluted by water?
If a small amount of water is added, the water dissociates slightly. The extra H+ and OH- ions push the equilibrium the same amount in both directions, leaving it unchanged
What are some real life applications of buffers?
Shampoos have a pH of around 5.5 to keep hair smooth and shiny as alkaline conditions make surface of individual hairs rougher
Bio washing powders contain buffers to keep pH at right level for enzymes to work
