Chapter 20 Acids, Bases And Ph

Question 1

What are Bronsted Lowry acids?

Answer 1

Proton donors
Release H+ ions when in solution
HA + H2O —> H3O+ + A-

Question 2

What are Bronsted Lowry bases?

Answer 2

Proton acceptors
When in solution, they accept H+ ions from water molecules.
B + H2O —> BH+ + OH-

Question 3

What are monobasic acids?

Answer 3

Donate one mole of H+ per mole of acid
Eg, HCl / HNO3 / CH3COOH

Question 4

What are dibasic acids?

Answer 4

Donate two moles of H+ per mole of acid
Eg, H2SO4 / (COOH)2

Question 5

What are tribasic acids?

Answer 5

Donate three moles of H+ per mole of acid
Eg, H3PO4

Question 6

State what is meant by salt?

Answer 6

Where the H+ of an acid is wholly or partially replaced by a metal ion or ammonium

Question 7

What are conjugate base pairs?

Answer 7

Pair of 2 species that can transform into each other by loss or gain of protons

HA + B <—> BH+ + A-

Question 8

H2SO4 + HNO3 <—> HSO4- + H2NO3+
State the conjugate base pairs of this equation

Answer 8

In the forward reaction:
H2SO4 acts as an acid donating a proton
In the reverse reaction:
H2SO4- acts as a base accepting a proton from H2NO3+ ion to form H2SO4
So H2SO4 and H2SO4- is a conjugate pair; H2SO4 is the conjugate acid of HSO4- and HSO4- is the conjugate base of H2SO4

In forward reaction:
HNO3 acts as a base accepting a proton to form H2NO3+
In reverse reaction:
H2NO3+ acts as a acid donating a proton
So HNO3 and H2NO3+ are conjugate pairs, HNO3 is the conjugate base of H2NO3+ and H2NO3+ is the conjugate acid of HNO3

Question 9

State what must be included in a conjugate base pair equation

Answer 9

-There must be an acid and base on each side of the equation
-Acid 1 and Base 1 are conjugate pairs
-Acid 2 and Base 2 are conjugate pairs

Question 10

How do acids react with metals?

Answer 10

Metals react with acids in a redox reaction releasing hydrogen gas.

Question 11

Write the ionic equation for Mg reacting with HCl

Answer 11

Mg + H+ —-> Mg2+ + H2

Question 12

How do acids react with carbonates?

Answer 12

Acids react with carbonates in a neutralisation reaction releasing Carbon dioxide and water
CuCO3+ 2H+ ——> Cu^2+ + H2O + CO2

Question 13

How do acids react with metal hydroxides?

Answer 13

Acids react with metal hydroxides in a neutralisation reaction to produce water
Na2O + 2H+ —> 2Na+ + H2O

Question 14

How do acids react with metal oxides?

Answer 14

Acids react with metal oxides in a neutralisation reaction to produce water
OH- + H+ —-> H2O

Question 15

What are strong acids?

Answer 15

Fully dissociate in solution donating H+ ions

Question 16

What is the concentration of H+ equal when a strong monobasic acid fully dissociates?

Answer 16

HA —> H+ + A-
Because of the ratio the:
[H+] = [HA]

Question 17

What is the concentration of H+ when a strong dibasic acid fully dissociates?

Answer 17

H2A —> 2H+ + A^2-
Because of the ratio:
[H+] = 2 x [H2A]

Question 18

State the equation used to calculate pH

Answer 18

PH = -log[H+]

Question 19

State the equation used to calculate [H+]

Answer 19

[H+]= 10^-pH

Question 20

When is an acid determined as strong?

Answer 20

Greater [H+] and smaller pH = stronger acid

Question 21

How does a change in pH of 1 change the concentration of [H+]

Answer 21

A difference of 1 in pH is a 10 fold difference in concentration

Eg, A-0.005 moldm^3 HNO3 = 2.3 pH
B- 0.05 moldm^3 HNO3 = 1.3 pH
So B is ten time more concentrated than A so it will have a higher [H+] and therefore have a lower pH of 1

Question 22

What are weak monobasic acids?

Answer 22

Partially dissociate in solution donating one H+ ion

Question 23

What is meant by Ka?

Answer 23

The acid dissociation constant that measures the extent of dissociation for a weak acid.

Question 24

What is the equation for Ka?

Answer 24

Ka = [H+] [A-]. OR Ka= [H+]2
————— ————-
[HA] [HA]

Question 25

State the approximations assumed when calculating the dissociation of weak acids

Answer 25

1) Since dissociation of weak acid is so small [HA] equilibrium ~ [HA] undissociated 2) Technically the [H+] is the combined dissociation of the acid and water in aqueous solution But water dissociates very little so: [H+] equilibrium ~ [H+] acid only 3) [H+] = [A-] due to 1:1

Question 26

How can Ka be calculated from pKa?

Answer 26

Ka= 10^-pKa

Question 27

State the values of Ka and pKa which show a stronger weak acid

Answer 27

The greater the Ka value the stronger the weak acid The smaller the pKa value the stronger the weak acid

Question 28

How can pKa be calculated from Ka?

Answer 28

PKa= -log(Ka)

Question 29

What is the ionic product of water?

Answer 29

Kw is the dissociation constant for water At 25 degrees = 1.00 x 10^-14

Question 30

What is the equation for Kw?

Answer 30

Kw = [H+] [OH-] Simplified Kw= [H+]^2

Question 31

How do you calculate [H+] from Kw?

Answer 31

[H+] = Square root of Kw

Question 32

State how [H+] and [OH-] change in an alkaline solution

Answer 32

Alkaline solution [H+] < [OH-]

Question 33

State how [H+] and [OH-] change in an acidic solution

Answer 33

Acidic solution [H+] > [OH-]

Question 34

State how [H+] and [OH-] change in a neutral solution

Answer 34

[H+] = [OH-]

Question 35

Describe how the value of Kw changes when the temperature of pure water increases Dissociation of water is endothermic

Answer 35

Therefore, Kw increases when temperature increases because equilibrium will shift in the forwards direction favouring endothermic reaction. The pH of the pure water will decrease as temperature increases as there is a greater [H+] being produced However, water is still neutral as [H+] = [OH-]

Question 36

How can the pH of a strong base be calculated?

Answer 36

-Using the concentration of the base -Using Kw [H+] = Kw/ [OH-]

Question 37

State and explain when the approximations assumed for the weak acid calculations may not work

Answer 37

-When there is a greater extent of dissociation of the weak acid greater than 5% dissociation Eg, the weak acid gets stronger Therefore, can no longer assume that [HA] equilibrium ~ [HA] undissociated