Acids and Bases Flashcards
What is a Bronsted lowry acid
Proton donor
What is a Bronsted lowry base
Proton acceptor
How do H+ ions exist in water
As H3O+ ions
What is the general equation for the reaction of an acid with water
HA(aq) + H2O(l) —> H3O+(aq) + A-(aq)
What is the general equation for the reaction of a base with water
B(aq) + H2O(l) —-> BH+(aq) + OH-(aq)
What is a strong base
A base which fully dissociates into its ions in solution
What is a weak base
A base which partially dissociates into its ions in solution
What is a strong acid
An acid which fully dissociates into its ions in solution
What is a weak acid
An acid which partially dissociates into its ions in solution
Is CH3COOH a strong or weak acid
Weak acid
What is the equation for the dissociation of ethanoic acid
CH3COOH ⇌ CH3COO- + H+
In general, are carboxylic acids weak or strong
Weak acids
Give 3 examples of strong acids
HCl
H2SO4
HNO3
Why are carboxylic acids weak
Backwards reaction favoured and so only partial dissociation occurs, meaning not many H+ produced (equilibrium lies to the left)
What is the equation for the dissociation of HCl
HCl ⇌ H+ + Cl-
Why are HCl, H2SO4, HNO3 strong acids
Forwards reaction strongly favoured and full dissociation occurs, meaning many H+ produced
(equilibrium lies to the right)
Give 2 examples of strong bases
NaOH
KOH
What is the equation for the dissociation of NaOH
NaOH ⇌ Na+ + OH-
Why are NaOH and KOH strong bases
Forwards reaction strongly favoured and full dissociation occurs, meaning many OH- produced
(equilibrium lies to the right)
Give an example of a weak base
NH3
What is the equation for the dissociation of ammonia
NH3 + H2O ⇌ NH4+ OH-
Why is NH3 a weak base
Backwards reaction favoured and so only partial dissociation occurs, meaning not many OH- produced
(equilibrium lies to the left)
What does ammonia require to produce OH-
The presence of water
What ion does NH3 form
NH4+
What is the general equation for when acids and bases react
HA(aq) + B(aq) ⇌ BH+(aq) + A-(aq)
What is the role of water when it reacts with an acid
Water acts as a base
What is the equation for pH
pH = -log[H+]
What is the equation for [H+]
[H+] = 10⁻ᵖᴴ
What is the equation for the dissociation of H2O
H2O ⇌ H+ + OH-
What is the equation for Kᵥᵥ
Kᵥᵥ = [H⁺]²
This is due to
Kᵥᵥ = [H⁺][OH⁻] and [H⁺] = [OH⁻] in the equation:
H2O ⇌ H+ + OH-
So we can write Kᵥᵥ = [H⁺]²
What is the value of Kᵥᵥ at 25 degrees celcius
1x10⁻¹⁴
What is a monoprotic acid
Monoprotic acids dissociate to produce one H+ ion for every acid molecule
Give 2 examples of monoprotic acids
HCl
HNO3
What is a diprotic acid
Diprotic acids dissociate to produce two H+ ion for every acid molecule
When we have the concentration of a diprotic acid, how do we get pH
Multiply concentration of acid by 2, then put into log equation
Give an example of a diprotic acid
H2SO4
Which equations do we use to calculate the pH of strong bases
Kw equation to get H+ conc
pH equation to get pH
What equation do we use to calculate the pH of strong acids
Kw equation to get H+ conc
pH equation to get pH
What do we use Kₐ for
Calculating H+ conc for weak acids
Why do we use Kₐ and now Kw for weak acids
Weak acids only partially dissociate and so the [H+] we would get from the Kw equation is not equal to the concentration of the acid
What is the equation for Kₐ
What is the units for Kₐ
moldm-3
What is the dissociation equation for weak acids
HA ⇌ H⁺ + A⁻
What is the weak acid approximation equation for Kₐ
What is the equation for pKₐ
-logKₐ
What is the equation for Ka if we know pKa
10⁻ᵖᴷᵃ
If the value of Ka is smaller, what does this mean about the strength of a weak acid
It is a weaker, weak acid
If the value of pKa is smaller, what does this mean about the strength of a weak acid
It is a stronger, weak acid
If the value of Ka is larger, what does this mean about the strength of a weak acid
It is a stronger, weak acid
If the value of pKa is larger, what does this mean about the strength of a weak acid
It is a weaker, weak acid
Draw the pH curve for a strong base being added to a strong acid
Draw the pH curve for a weak base being added to a strong acid
Draw the pH curve for a strong base being added to a weak acid
Draw the pH curve for a weak base being added to a weak acid
What is the equivalence point on a pH curve
The point at which an acid/base has been neutralised by a base/acid respectively
Where is the equivalence point on a pH curve
At the start of when the curve becomes completely vertical
What is the half neutralisation point on a pH curve and what do we use it for
It is the half way point between the equivalence point and 0
We can find the pH at this point to calculate pKa of a WEAK ACID
Explain the relationship between pKa and pH for weak acids at the half neutralisation point on a pH curve
How do we correctly determine which indicator to use from a pH curve
The indicator must change colour entirely within the vertical part of a pH curve
What are the colour changes of methyl orange and what reactions would this indicator be suitable for
Red at low pH
Yellow at high pH
Suitable for:
Strong acid/strong base
Strong acid/weak base
What are the colour changes of phenolphthalein and what reactions would this indicator be suitable for
Colourless at low pH
Pink at high pH
Suitable for:
Weak acid/strong base
What is Methyl orange’s colour change range
Methyl orange’s colour change range is from red to yellow over a pH range of 3 to 4.4
What is phenolphthalein’s colour change range
Phenolphthalein’s colour change range is from red to yellow over a pH range of 8.3 to 10
Why is there no indicator suitable for a weak acid - weak base titration
Weak acid - weak base titrations have no sharp pH change, therefore we have to use a pH meter
Why do diprotic acids have two equivalence points in their pH curves
The 2 protons are released from the molecule separately (they don’t leave at the same time)
All the molecules release the same proton on the same part of the molecule first, once all are done, then second proton comes off
What is a buffer solution
A solution that resists changes in pH on addition of small amount of acid, base or on dilution with water
What is the dissociation equation for method 1 (acidic buffer) of making a buffer
What is the equation for calculating the H+ concentration of a buffer
Then -log(H+)
What is the dissociation equation for a buffer made by method 2 (basic buffer)
What are the 5 steps in calculating the pH of a buffer made from method 2 (basic buffer) (assuming we know the concentration and moles of HA & A-)
1 - Calculate the moles of HA(start) (C =n/V)
2 - Calculate the moles of OH- (C=n/V of base, but check ratio of base to OH)
3 - Calculate moles of HA(end) (nHA(start) - nOH-)
4 - Write down nA- (nA- = nOH-)
5- Use buffer equation to find concentration of H+, then put into log equation
What are the steps in calculating the pH of a buffer made by method 1 (acidic buffer)
Then -log(H+)
What is the equation to calculate H+ concentration of a buffer solution after adding an acid
What is the equation to calculate H+ concentration of a buffer solution after adding a base
Which way does equilibrium shift when we add an acid to a buffer solution, what effect does this have on the concentration of HA
Upon addition of an acid, equilibrium shifts left, and HA concentration increases
Which way does equilibrium shift when we add a base to a buffer solution, what effect does this have on the concentration of HA
Upon addition of a base, equilibrium shifts to the right and HA concentration decreases
Give 2 uses for buffers
Shampoo
Washing powder
