5.3 Acids, Bases and pH

Question 1

What are acids ?

Answer 1

They dissociate H+ ions into an aqueous solution.

Question 2

What are bases ?

Answer 2

They dissociate and release OH- ions into an aqueous solution.

Question 3

What is ionic equation for water ?

Answer 3

H+ + OH- = H2O

Question 4

Are alkalis a soluble base ?

Answer 4

Yes

Question 5

What is a Bronsted-Lowry acid described as ?

Answer 5

Acid that acts as a proton donor.

Question 6

What is a Bronsted-Lowry base described as ?

Answer 6

Base that acts as a proton acceptor.

Question 7

What is a conjugate acid-base pair ?

Answer 7

Contains 2 species that can be interconverted by the transfer of a proton between the acid and base.

Question 8

What is the acid base equation for the hydronium ion ?

Answer 8

HCl + H2O = H3O+ + Cl-
A1 B2 A2 B1

Question 9

What does a mono basic, dibasic and tribasic acid refer to ?

Answer 9

Refers to the total number of hydrogen ions in the acid that can be replaced per molecules in an acid-base reaction.

Question 10

What is the reaction with an acid and a metal ?

Answer 10

Acid + metal = salt + hydrogen

Question 11

What is the ionic equation equation for the reaction between an acid and a metal ?

Answer 11

2H+ + Mg = Mg2+ + H2

Question 12

What is the reaction with an acid and a carbonate ?

Answer 12

Acid + carbonate = salt + water + carbon dioxide

Question 13

What is the ionic equation for the reaction with an acid and a carbonate ?

Answer 13

2H+ + CO3 = Cu2+ + H2O + CO2

Question 14

What is the reaction with an acid and a metal oxide ?

Answer 14

Acid + metal oxide = salt + water

Question 15

What is the ionic equation with an acid and a metal oxide ?

Answer 15

2H+ + MgO = Mg2+ + H2O

Question 16

What is the reaction of an acid with an alkali ?

Answer 16

Acid + alkali = salt + water

Question 17

What is the ionic equation of the reaction of an acid with an alkali ?

Answer 17

H+ + OH- = H2O

Question 18

What value of pH does a low value of H+ ions show ?

Answer 18

High pH value.

Question 19

What value of pH does a high value of H+ ions show ?

Answer 19

Shows a low pH.

Question 20

How do you work out pH using logarithms ?

Answer 20

pH = -log [ H+ ]

Question 21

How do you work out H+ ions concentration if you are given the pH ?

Answer 21

[H+] = 10* -pH

Question 22

What does a change in one pH number equal to in difference in H+ ions ?

Answer 22

Equal to 10x the difference in H+ ion concentration, so there is a large difference in hydrogen ion concentrations between pH 2 and pH 3.

Question 23

What is a strong acid ?

Answer 23

Acid that fully dissociates all of its H+ ions into an aqueous solution.

Question 24

What is a weak acid ?

Answer 24

Acid that partially dissociated some of its H+ ions into an aqueous solution.

Question 25

In a strong acid, what is the concentration of H+ ions equal to ?

Answer 25

[H+] = [HA]
So, pH of a strong acid can be calculated directly from the concentration of the acid.

Question 26

What is the acid dissociation constant, Ka ?

Answer 26

For any weak acid,
Ka = [H+] [A-] / [HA] mol dm-3

Question 27

Where does the equilibrium lie if there is a large value of Ka ?

Answer 27

The larger the value of Ka, the further the equilibrium lies to the right. Larger value means the greater dissociation and the greater the acid strength.

Question 28

What is pKa ?

Answer 28

pKa = -log (Ka)

Question 29

How do you work out Ka from pKa ?

Answer 29

Ka = 10* -pKa

Question 30

What effects does a strong acid have on Ka and pKa value ?

Answer 30

Stronger the acid, the larger the Ka value and the smaller the pKa value.

Question 31

What effect does a weak acid have on Ka and pKa value ?

Answer 31

The weaker the acid, the smaller the Ka value and the larger the pKa value.

Question 32

For weak acids, why is [H+] = [A-] in an equilibrium ?

Answer 32

HA dissociates to produce equilibrium concentrations of H+ and A- that are equal. There will also be a very small concentration of H+ from the dissociation of water but this will be extremely small and can be neglected compared with the H+ concentration from the acid.

Question 33

For weak acids, why is [HA]eqm = [HA]start - [H+]eqm ?

Answer 33

Concentration of HA is smaller than the undissociated concentration. As the dissociation is small, you can assume that [HA] at start is greater than [H+] and you can neglect any decrease in the concentration of HA from dissociation.

Question 34

What is the simplified Ka equation ?

Answer 34

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

Question 35

How can you calculate pH from H+ concentration ?

Answer 35

[H+] = square root of Ka x [HA]
-log [H+] = pH

Question 36

What is the ionisation of water ?

Answer 36

H20 + H20 = H3O+ + OH-
Acts as both an acid and a base, setting up the acid-base equilibrium.

Question 37

What is Kw ?

Answer 37

Is the ionic product of water, the ions in the water are multiplied together. Kw also varies with temperature.

Question 38

What is the equation for Kw ?

Answer 38

Kw = [H+] [OH-]

Question 39

Why is Kw important ?

Answer 39

Significance of having Kw at 1.00 x10*-14 at 25C means that the value sets up the neutral point in the pH scale. On dissociation it is neutral so produces same number of H= and OH- ions.

Question 40

According to Kw, what makes an aqueous solution acidic, neutral or alkaline ?

Answer 40

[H+] > [OH-]
[H+] = [OH-]
[H+] <[OH-]

Question 41

How do you calculate the pH of a strong base ?

Answer 41

Find [OH-]
Find [H+] using Kw and [OH-]
-log [H+] to find pH.

Question 42

What is pOH ?

Answer 42

Used as a convenient way to show [OH-].
pOH = -log [OH-]
[OH-] = 10* -pOH