Chapter 20 Acids, Bases And Ph Flashcards
What are Bronsted Lowry acids?
Proton donors
Release H+ ions when in solution
HA + H2O —> H3O+ + A-
What are Bronsted Lowry bases?
Proton acceptors
When in solution, they accept H+ ions from water molecules.
B + H2O —> BH+ + OH-
What are monobasic acids?
Donate one mole of H+ per mole of acid
Eg, HCl / HNO3 / CH3COOH
What are dibasic acids?
Donate two moles of H+ per mole of acid
Eg, H2SO4 / (COOH)2
What are tribasic acids?
Donate three moles of H+ per mole of acid
Eg, H3PO4
State what is meant by salt?
Where the H+ of an acid is wholly or partially replaced by a metal ion or ammonium
What are conjugate base pairs?
Pair of 2 species that can transform into each other by loss or gain of protons
HA + B <—> BH+ + A-
H2SO4 + HNO3 <—> HSO4- + H2NO3+
State the conjugate base pairs of this equation
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
State what must be included in a conjugate base pair equation
-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
How do acids react with metals?
Metals react with acids in a redox reaction releasing hydrogen gas.
Write the ionic equation for Mg reacting with HCl
Mg + H+ —-> Mg2+ + H2
How do acids react with carbonates?
Acids react with carbonates in a neutralisation reaction releasing Carbon dioxide and water
CuCO3+ 2H+ ——> Cu^2+ + H2O + CO2
How do acids react with metal hydroxides?
Acids react with metal hydroxides in a neutralisation reaction to produce water
Na2O + 2H+ —> 2Na+ + H2O
How do acids react with metal oxides?
Acids react with metal oxides in a neutralisation reaction to produce water
OH- + H+ —-> H2O
What are strong acids?
Fully dissociate in solution donating H+ ions
What is the concentration of H+ equal when a strong monobasic acid fully dissociates?
HA —> H+ + A-
Because of the ratio the:
[H+] = [HA]
What is the concentration of H+ when a strong dibasic acid fully dissociates?
H2A —> 2H+ + A^2-
Because of the ratio:
[H+] = 2 x [H2A]
State the equation used to calculate pH
PH = -log[H+]
State the equation used to calculate [H+]
[H+]= 10^-pH
When is an acid determined as strong?
Greater [H+] and smaller pH = stronger acid
How does a change in pH of 1 change the concentration of [H+]
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
What are weak monobasic acids?
Partially dissociate in solution donating one H+ ion
What is meant by Ka?
The acid dissociation constant that measures the extent of dissociation for a weak acid.
What is the equation for Ka?
Ka = [H+] [A-]. OR Ka= [H+]2
————— ————-
[HA] [HA]
State the approximations assumed when calculating the dissociation of weak acids
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
How can Ka be calculated from pKa?
Ka= 10^-pKa
State the values of Ka and pKa which show a stronger weak acid
The greater the Ka value the stronger the weak acid
The smaller the pKa value the stronger the weak acid
How can pKa be calculated from Ka?
PKa= -log(Ka)
What is the ionic product of water?
Kw is the dissociation constant for water
At 25 degrees = 1.00 x 10^-14
What is the equation for Kw?
Kw = [H+] [OH-]
Simplified
Kw= [H+]^2
How do you calculate [H+] from Kw?
[H+] = Square root of Kw
State how [H+] and [OH-] change in an alkaline solution
Alkaline solution
[H+] < [OH-]
State how [H+] and [OH-] change in an acidic solution
Acidic solution
[H+] > [OH-]
State how [H+] and [OH-] change in a neutral solution
[H+] = [OH-]
Describe how the value of Kw changes when the temperature of pure water increases
Dissociation of water is endothermic
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-]
How can the pH of a strong base be calculated?
-Using the concentration of the base
-Using Kw
[H+] = Kw/ [OH-]
State and explain when the approximations assumed for the weak acid calculations may not work
-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
