Chapter 20 - Acids, Bases and pH Flashcards
What is the Arrenhius model of acids and bases ?
- Acids dissociate and release H+ ions in aqueous solution
- Alkalis dissociate and release OH- ions in aqueous solution
- H+ ions are neutralised by OH- ions to form water
What is an alkali ?
An alkali is a soluble base
What is a bronsted lowry acid ?
A proton donor
What is a bronsted lowry base ?
A proton acceptor
What is a conjugate acid-base pair ?
Two species that can be interconverted by transfer of a proton
In the dissociation of HCl to H+ and Cl-, identify the conjugate acid and base.
- HCl releases a proton so it is a conjugate acid
- Cl- accepts a proton therefore it is a conjugate base
In an aqueous solution what does dissociation require ?
It requires a proton to be transferred from an acid to a base
When water is the base, what is formed ?
- The hydronium ion
- H3O+
Why is the hydronium ion important ?
It is the active ingredient in any aqueous solution
What do the terms monobasic, dibasic and tribasic refer to?
The total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction.
How many hydrogens in a monobasic acid ?
1
How many hydrogens in a dibasic acid ?
2
How many hydrogens in a tribasic acid ?
3
How can redox reactions be simplified ?
Remove the spectator ions
What is the word equation for the reaction of an acid and a metal ?
Acid + metal → salt + hydrogen
What is the word equation for the reaction of an acid and a carbonate ?
Acid + carbonate → salt + water + carbon dioxide
What is the word equation for the reaction of an acid and a base ?
Acid + base → salt + water
What is the word equation for the reaction of an acid and an alkali ?
Acid + alkali -> salt + water
What is the relationship between pH and concentration of H+?
- Low value [H+] = high pH
- High value [H+] = low pH
What is the equation for working out pH from [H+]?
pH = -log[H+] (base 10)
How is the pH calculated for a strong acid?
Assume it fully dissociates therefore [H+] = [HA]
How is the new pH calculated for a strong acid on dilution?
- Work out change in concentration of HA and therefore [H+]
- Then put it back into pH = -log[H+]
What does a pH less than 7 show ?
It shows increasing acidity
What does a pH more than 7 show ?
It shows increasing alkalinity
What is a pH of 7 ?
Neutral
What is a change in one pH number equal to ?
It is equal to a 10 times difference in [H+]
What is the general formula for the dissociation of a weak acid ?
HA -><- H+ + A-
How is the acid dissociation constant calculated ?
[H+][A-] / [HA]
What are the units for the acid dissociation constant ?
moldm^-3
What is the equilibrium like the larger the numerical value ?
- The greater the dissociation
- The greater the acid strength
Why do we use pKa ?
- Because it is difficult to use the Ka values that have a negative indices
- We convert the Ka value into a negative logarithm called pKa
What is the equation for pKa ?
pKa = -log(10)Ka
How do we get from pKa back to Ka ?
Ka = 10^-pKa
What are the pKa and Ka values like for a strong acid ?
- Large Ka value
- Smaller pKa value
What are the pKa and Ka values like for a weak acid ?
- Small Ka value
- Large pKa value
