Unit 3.4.3 - Acids and Bases Flashcards
What is a bronsted-lowry acid?
A proton donor
What is a bronsted-lowry base?
A proton acceptor
What is the general equation for when a bronsted-lowry acid dissociates in a solution?
HA ⇋ H⁺ + A⁻
What is the general equation for when a bronsted-lowry base dissociates in a solution?
B + H₂O ⇋ BH⁺ + OH⁻
What is the general equation what a bronsted-lowry base and acid are together in solution?
HA + B ⇋ BH⁺ + A⁻
What is the general word equation what a bronsted-lowry base and acid are together in solution?
Acid + Base ⇋ Conjugate acid + Conjugative base
What does BH⁺ stand for?
Conjugative acid
What does A⁻ stand for?
Conjugative base
What is a conjugative acid?
A species produced when a base gains a proton
What is a conjugative base?
A species produced when an acid loses a proton
What is a conjugative acid-base pair?
Two species related to one another by the presence or absence of a proton
What is the general equation for when an acid is added to water?
HA + H₂O ⇋ H₃O⁺ + A⁻
In this reaction HA + H₂O ⇋ H₃O⁺ + A⁻, what does water act as and why?
A bronsted lowry base as it accepts a proton
What is the pH scale?
A measure of the hydrogen ion concentration in a solution
pH = ?
pH = -log[H⁺]
How many decimal places must pH be calculated to?
2
[H⁺] = ?
10-pH
How do you find the pH of a strong acid?
You use the concentration given, if its monoprotic leave it, if its diprotic double it, and then sub it into pH = -log[H⁺]
What doe monoprotic mean?
Each molecule of acid releases one proton when it dissociates
What does diprotic mean?
Each molecule of acid releases two protons when it dissociates
Name three examples of strong acids?
HCl, HNO₃, H₂SO₄
What does water weakly dissociate into?
Hydroxium ions and hydroxide ions
What are the two equations for how water weakly dissociates?
2H₂O ⇋ H₃O⁺ + OH⁻ OR H₂O ⇋ H⁺ + OH⁻
Why is water incorporated into Kc to make Kw?
Water only dissociates a tiny amount so the position of equilibrium lies far to the left, so there is a lot of water compared to H⁺ and OH⁻ that it is considered to have a constant value and incorporated into Kc to make Kw
What stand Kw stand for?
The ionic product of water
Kw = ?
Kw = [H⁺][OH⁻]
What is the value of Kw at 298K?
1 x 10⁻¹⁴
What can you simplify Kw = [H⁺][OH⁻] to for pure water? And why?
Kw = [H⁺]² - because there is one H⁺ ion for each OH⁻ ions so it can be simplified
How does increasing the temperature effect the pH of pure water?
Increases it
Why does pure water have a higher pH at higher temperatures?
Dissociation is an endothermic reaction and so increasing the temperature means the system seeks to oppose the increase in temperature by favouring the forward reaction and shifting the position of equilibrium to the left producing more H⁺ ions and OH⁻ ions and so increasing the value of Kw and pH
Name two examples of strong bases.
NaOH and KOH
How do you find the pH of a strong base?
You use the concentration given, if its monoprotic leave it, if its diprotic double it, and then sub it into Kw = [H⁺][OH⁻] and then use pH = -log[H⁺]
Why can you use the concentration of the base for [OH⁻] in strong bases?
They fully dissociate and so you get one mole of OH⁻ ions per mole of base (if its monoprotic)
What is meant by a weak base or acid?
They only partially dissociate
Why can’t you just use the concentration of the acid or base to find pH if its weak?
There is only partially dissociation so you don’t get one mole of OH⁻ ions or H⁺ ions per mole of base or acid
What does Ka stand for?
The acid dissociation constant
Ka = ?
What can you simplify the equation for Ka to for weak acids? And why?
You can assume that [H⁺] and [A⁻] are the same
What are the relative values for Ka for weak and strong acids? And why?
Weak - small Strong - large
Ka shows the position of equilibrium so small value means there isn’t much dissociation so weak acid and large value means there is a lot of dissociation so strong acid
Why is pKa often used instead of Ka?
The values of Ka vary so much so it is easier to use a logarithmic scale
pKa = ?
pKa = -log(Ka)
Ka = ?
Ka = 10-pKa
What do pH curves show?
The pH of the titration mixture against the amount of base or acid that has been added as the titration goes on
What is the equivalence or end point on a pH curve?
The midpoint of the vertical section
Which pH curves don’t have vertical sections?
Weak acid with weak bases
What are used instead of indicators to see the end point of a titration for weak acid and weak bases?
pH meter to measure the pH
Why are pH meters used instead of indicators in titrations of weak bases against weak acids?
There is no sharp change in pH, so the colour of the indicator would change very gradually and so it would be difficult to see where the end point is
What is an indicator?
Weak acids which have a different colour to their conjugate base
How do you choose which indicator to use for a titraion?
One that changes colour over a narrow pH range that lies entirely on the vertical part of the curve
How can you see how much alkali is needed to neutralise a quantity of acid from a pH curve?
The volume at the equivalence point
Why do pH curves for diprotic acids and bases looks different?
They release or accept two protons, so the neutralisation happens in two stages as the protons are released or accepted separately
What is a buffer?
A solution which resits small changes in pH when small amounts of acid or alkali are added
What are the two types of buffers you can get?
Acidic and basic
What pH do acidic buffers have?
Less than 7
How do you make an acidic buffer?
Mixing a weak acid with one its salts (conjugate base)
How does an acidic buffer work when you add an acid (H⁺ ions) ?
The excess H⁺ ions combine with the A⁻ shifting the position of equilibrium to the left, meaning the [H⁺] decreases close to its original value and so the pH doesn’t change that much
How does an acidic buffer work when you add a base (OH⁻ ions)?
They combine with the H⁺ ions causing the [H⁺] to decrease, shifting the position of equilibrium to the right to replace the lost H⁺, this means the [H⁺] increases close to its original value and so the pH doesn’t change much
Why does the concentrations of HA or A⁻ not change in an acidic buffer when an acid or base is added?
There are large reservoirs of them and so there concentrations stay pretty much constant
What pH do basic buffers have?
Greater than 7
How do you make a basic buffer?
Mixing a weak base with one of its salt (conjugate acid)
How does a basic buffer work when you add an acid (H⁺ ions)?
They combine with the OH⁻ ions, causing the [OH⁻] to decrease, this shifts the position of equilibrium to the right to replace the lost OH⁻ ions, this means the [OH⁻] increases close to its original value and so the pH doesn’t change very much
How does a basic buffer work when you add a base (OH⁻ ions)?
The excess OH⁻ combine with the BH⁺ and shifts the position of equilbrium to the left, this means the [OH⁻] decreases close to its original value and so the pH doesn’t change much
Why does the concentrations of B or BH⁺ not change in an basic buffer when an acid or base is added?
There are large reservoirs of them and so there concentrations stay pretty much constant
Name three things that contain a buffer.
Shampoo, biological washing powder, and blood
What pH buffer does shampoo have and why?
pH 5.5 buffer to counteract the alkaline soap int he shampoo
Why does biological washing powder have a buffer?
To keep the pH at the correct level for the enzymes to work best
Why does blood contain a natural buffer?
To keep the pH at the correct level for all the biological reactions to happen
What do you use to find the pH of an acidic buffer when you know the concentrations?
What is the other way to make an acidic buffer?
Take a weak acid and add a small amount of alkali, so that some of the acid is neutralised to make salt (conjugate base) but some is also left un-neutralised
How do you find the pH of an acidic buffer made by adding a small amount of alkali to a weak acid?
1.) Calculate the number of moles of acid and base at the start of the reaction and figure out which one is in excess 2.) If the acid is in excess then assume the moles of base at the start of the reaction is the same as the moles of salt at the end (look at the molar ratios) 3.) To find moles of acid at the end of the reaction you do the moles at the start - the moles used up (same as the moles of salt produced) 4.) Calculate the concentration of the acid and salt in the buffer solution (using the total volume of the solution) 5.) And then you can calculate pH using the Ka and pH formulas
What type of titraion does this pH curve show?
Strong acid and strong base
What type of titraion does this pH curve show?
Strong acid and weak base
What type of titraion does this pH curve show?
Weak acid and strong base
What type of titraion does this pH curve show?
Weak acid and weak base
