12. Acids and Bases Flashcards
What is a Bronsted-Lowry acid?
A proton donor
What is a Bronsted-Lowry base?
A proton acceptor
When can protons only be given up by acids?
When there is a base that accept them
What do reactions between acids and bases involve?
Proton transfer
Where do most common acid-base reactions take place?
In aqueous solution
How are acids and bases generally defined? Why is this?
By the way they react with water - as most acid-base reactions take place in aqueous solution
What does how strong an acid/base is depend on?
How much they dissociate in water
How much do strong acids dissociate in water?
Almost completely dissociate
What is another phrase for ‘dissociates’?
Ionises
General equation for strong acids/bases dissociating in water?
HA ⇋ H⁺ + A⁻
How much do weak acids dissociate in water?
Only slightly dissociate
Example of a strong acid dissociating in water?
HCl ⇋ H⁺ + Cl⁻
Example of a strong base dissociating in water?
NaOH ⇋ Na⁺ + OH⁻
Example of a weak acid dissociating in water?
CH₃COOH ⇋ CH₃COO⁻ + H⁺
Example of a weak base dissociating in water?
NH₃ + H₂O ⇋ NH₄⁺ + OH⁻
Is the strength of an acid/base different to its concentration?
Yes
What does the acidity of a solution depend on?
The concentration of H⁺ (aq) ions
What is pH defined as?
-log₁₀[H⁺]
What type of scale is the pH scale? Why is this?
Logarithmic, as the concentration of hydrogen ions in aqueous solution covers a wide range
When there is a smaller pH, what will be true of the concentration of H⁺ ions?
Higher concentration of H⁺
What types of acid are there?
- monoprotic
* diprotic
What is a monoprotic acid?
One that releases one H⁺ ion per molecule
What is a diprotic acid?
One that releases two H⁺ ions per molecule
Is HCl monoprotic or diprotic?
Monoprotic
Is H₂SO₄ monoprotic or diprotic?
Diprotic
In strong acids, what is the moles of a monoprotic acid equal to? Why?
The moles of H⁺, therefore [H⁺] = conc. of acid, because strong acids fully dissociate
In strong diprotic acids, what are the moles of H⁺ equal to?
Double the moles of acid, therefore [H⁺] = double conc. of acid
What effect do diprotic acids have on pH compared to monoprotic?
Results in lower pH (stronger acid)
How many decimal places is pH given to?
2
How can pH be used to calculate the concentration of H⁺?
[H⁺] = 10⁻ᵖᴴ
If pH is used to find [H⁺] of a monoprotic acid, what is the concentration of the acid?
Conc. of H⁺ = conc. of acid
If pH is used to find [H⁺] of a diprotic acid, what is the concentration of the acid?
Conc. of H⁺ must be halved to determine conc. of acid
What effect will diluting a strong acid have?
- reduce its concentration
- hence concentration of H⁺ ions
- increasing pH slightly
How would you calculate the pH of the solution when 100 cm³ water is added to 50 cm³ of 0.1 moldm⁻³ HNO₃
[H⁺] original solution = 0.1
[H⁺] diluted solution = 0.1 x (original volume/total volume)
= 0.1 x 50/150 = 0.0333
pH = -log(0.0333) = 1.48
How would you calculate the pH when an original solution is diluted with water?
[H⁺] of diluted solution = [H⁺] of original solution x (original volume/total volume)
pH = -log(ANS)
What does it mean, that water is slight dissociated?
It exists not just as molecules but in equilibrium
What is the equation for dissociation of water?
H₂O ⇋ H⁺ + OH⁻
Do H⁺ ions exist in water alone?
No
What do H⁺ ions exist as in water?
Hydroxonium ions (H₃O⁺)
Equation for hydroxonium ions?
H₃O⁺
What equation can be used to show how water acts as a base?
2H₂O ⇋ H₃O⁺ + OH⁻
What is the equilibrium expression for the dissociation of water?
Kc = [H⁺] [OH⁻] / [H₂O]
Why is the Kc of water the same as Kw?
The concentration of water is constant
What is the equation for Kw?
Kw = [H⁺(aq)] [OH⁻(aq)]
What is Kw also called?
The ionic product of water
What is the value of Kw at 298K?
1 x 10⁻¹⁴
What is true of the concentration of H⁺ and OH⁻ in pure water?
They must be equal
What does it mean for the Kw equation, that H⁺ and OH⁻ concentrations are equal?
Expression simplified to Kw = [H⁺]²
What type of reaction is the dissociation of water?
Equilibrium
What would happen to the pH of water if the temperature was increased?
H₂O ⇋ H⁺ + OH⁻ (ΔH = 57.3 kJmol⁻¹)
- equilibrium shifts right in endothermic reaction to lower temperature
- which increases concentration of H⁺ ions
- so decreases pH
Why is water always neutral?
The number of H⁺ and OH⁻ ions are always equal, despite pH varying
Is water always neutral?
Yes
Is neutral always at pH 7?
No
What is the pH of water dependent on?
Temperature
What happens if a base is dibasic, when calculating the pH of a strong base?
[OH⁻] is twice the concentration of the base
When calculating the concentration of a strong base given the pH, what is the concentration of OH⁻ ions equal to?
- the concentration of the monobasic base
* dibasic base - concentration of OH⁻ needs to be halved to determine concentration of base
What are weak acids and bases?
Those that are only partially dissociated when dissolved in water
What does it mean, that weak acids and bases only partially dissociate in water?
Not all of the molecules split up into ions