Acids, Bases and pH Flashcards
What are Bronstead-Lowry acids?
They are proton donors- they release hydrogen ions when they’re mixed with water. You never get H+ ions by themselves in water as they are always combined with water to form hydroxonium ions.
What categorises a strong acid?
Strong acids dissociate almost completely in water- nearly all the H+ ions will be released. Hydrochloric acid is a strong acid.
What categorises a strong base?
Strong bases ionize almost completely in water
What categorizes a weak acid?
Weak acids dissociate only slightly in water- so only small numbers of H+ ions are formed. An equilibrium is set up which lies well over to the left.
What categorizes a weak base?
Weak bases only slightly ionise in water. Just like with weak acids, the equilibrium lies well over to the left.
How does water dissociate slightly?
Water dissociates into hydroxonium ions and hydroxide ions.
How do you calculate pH?
pH= -log10 [H+]
How do you calculate hydrogen ion concentration?
[H+] = 10^-pH
What is the range of the normal pH scale?
0-14 but it can be negative
7 is considered neutral
What are monoprotic acids?
Monoprotic acids release one proton when it dissociates per each molecule. This means one mole of acid produces one mole of hydrogen ions so the H+ concentration is the same as the acid concentration.
What are diprotic acids?
Each molecule of a strong diprotic acid releases 2 protons when it dissociates. So diprotic acids produce 2 mol of hydrogen ions for each mol of acid.
How do you calculate the ionic product of water?
Kw = [H+][OH-]
What is the value of Kw at room temperature?
1x10^-14
What is the equation for Kw in pure water?
Kw = [H+]^2
What is Ka?
Ka is the acid dissociation constant
How do you calculate Ka for weak acids?
Ka = [H+]^2/[HA]
What are the two equations involving pKa?
pKa = -log10 Ka
Ka = 10^-pKa
What does the graph look like for a strong acid/strong base?
The pH starts around 1, as there’s an excess of strong acid. It finishes up around pH 13 when you have an excess of a strong base.
What does the graph look like for a strong acid/weak base?
The pH starts around 1, as there is an excess of strong acid. It finishes up around pH 9, when you have an excess of weak base.
What does the graph look like for a weak acid/strong base?
The pH starts around 5, as there’s an excess of weak acid. It finishes up around pH 13, when you have an excess of strong base.
What does the graph look like for a weak acid/weak base?
The pH starts around t, as there’s an excess of weak acid. It finishes up around pH 9, when you have an excess of weak base.
What is the equivalence/end point on a base/pH graph?
The part of the graph that is vertical.
What should you consider when choosing an indicator for your titration?
You need it to change color exactly at the end point of the titration. So you need to pick one that changes color over a narrow pH range that lies entirely on the verical part of the pH curve.
What indicators do you use for a strong acid/strong base?
both
What indicator do you use for a strong acid/weak base?
methyl orange
What indicator do you use for a weak acid/strong base?
phenolphthalein
What indicator do you use for a weak acid/weak base?
There is no sharp pH chance so neither indicators work so you should just use a pH meter.
What is the pH range and color change of phenolphthalein?
8.3-10
colourless in acid and pink in alkali
What is the pH range and color change of methyl orange?
3.1-4.4
red in acid and yellow in alkali
In what cases do you get a pH curve with two equivalence points?
This happens when you titrate a diprotic acid.
What is a buffer?
A buffer is a solution that resists changes in pH when small amount of acid or base are added, or when it is diluted.
How can you make an acidic buffer?
Acidic buffers have a pH of less than 7- they’re made by mixing a weak acid with one of its salts. E.g. ethanoic acid and sodium ethanoate.
How does an acidic buffer work when acid is added?
If you add a small amount of acid, the H+ concentration increases. Most of the extra H+ ions combine with the ethanoate ions to form the acid. THis shifts the equilibrium to the left, reducing the H+ concentration to close to its original value so the pH doesn’t change.
How does an acidic buffer work when base is added?
If a small amount of base is added, the OH- concentration increases. Most of the extra H+ react with H+ ions to form water- removing H+ ions from the solution. This causes more acid to dissociate to form H+ ions and shifting the equilibrium to the right. The H+ concentration increases until it’s close to its original value, so the pH doesn’t change.
How can you make a basic buffer?
Basic buffers have a pH greater than 7- and they’re made by mixing a weak base with one of its salts. A solution of ammonia and ammonium chloride acts as a basic buffer.
What are some of the uses of buffers commercially?
- they are used in shampoo to prevent human hair from becoming rough when exposed to alkaline conditions
- they are used in biological washing powders to keep the pH at the right level for the enzymes to work most efficiently
- The blood is also maintained at pH 7.4 which is an example of a buffer system.
