Acids, Bases and pH

Question 1

What are Bronstead-Lowry acids?

Answer 1

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.

Question 2

What categorises a strong acid?

Answer 2

Strong acids dissociate almost completely in water- nearly all the H+ ions will be released. Hydrochloric acid is a strong acid.

Question 3

What categorises a strong base?

Answer 3

Strong bases ionize almost completely in water

Question 4

What categorizes a weak acid?

Answer 4

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.

Question 5

What categorizes a weak base?

Answer 5

Weak bases only slightly ionise in water. Just like with weak acids, the equilibrium lies well over to the left.

Question 6

How does water dissociate slightly?

Answer 6

Water dissociates into hydroxonium ions and hydroxide ions.

Question 7

How do you calculate pH?

Answer 7

pH= -log10 [H+]

Question 7

How do you calculate hydrogen ion concentration?

Answer 7

[H+] = 10^-pH

Question 8

What is the range of the normal pH scale?

Answer 8

0-14 but it can be negative
7 is considered neutral

Question 9

What are monoprotic acids?

Answer 9

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.

Question 10

What are diprotic acids?

Answer 10

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.

Question 11

How do you calculate the ionic product of water?

Answer 11

Kw = [H+][OH-]

Question 12

What is the value of Kw at room temperature?

Answer 12

1x10^-14

Question 13

What is the equation for Kw in pure water?

Answer 13

Kw = [H+]^2

Question 14

What is Ka?

Answer 14

Ka is the acid dissociation constant

Question 15

How do you calculate Ka for weak acids?

Answer 15

Ka = [H+]^2/[HA]

Question 16

What are the two equations involving pKa?

Answer 16

pKa = -log10 Ka
Ka = 10^-pKa

Question 17

What does the graph look like for a strong acid/strong base?

Answer 17

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.

Question 18

What does the graph look like for a strong acid/weak base?

Answer 18

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.

Question 19

What does the graph look like for a weak acid/strong base?

Answer 19

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.

Question 20

What does the graph look like for a weak acid/weak base?

Answer 20

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.

Question 21

What is the equivalence/end point on a base/pH graph?

Answer 21

The part of the graph that is vertical.

Question 22

What should you consider when choosing an indicator for your titration?

Answer 22

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.

Question 23

What indicators do you use for a strong acid/strong base?

Answer 23

both

Question 24

What indicator do you use for a strong acid/weak base?

Answer 24

methyl orange

Question 25

What indicator do you use for a weak acid/strong base?

Answer 25

phenolphthalein

Question 26

What indicator do you use for a weak acid/weak base?

Answer 26

There is no sharp pH chance so neither indicators work so you should just use a pH meter.

Question 27

What is the pH range and color change of phenolphthalein?

Answer 27

8.3-10 colourless in acid and pink in alkali

Question 28

What is the pH range and color change of methyl orange?

Answer 28

3.1-4.4 red in acid and yellow in alkali

Question 29

In what cases do you get a pH curve with two equivalence points?

Answer 29

This happens when you titrate a diprotic acid.

Question 30

What is a buffer?

Answer 30

A buffer is a solution that resists changes in pH when small amount of acid or base are added, or when it is diluted.

Question 31

How can you make an acidic buffer?

Answer 31

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.

Question 32

How does an acidic buffer work when acid is added?

Answer 32

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.

Question 33

How does an acidic buffer work when base is added?

Answer 33

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.

Question 34

How can you make a basic buffer?

Answer 34

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.

Question 35

What are some of the uses of buffers commercially?

Answer 35

- 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.