Acids and bases Module 5 Flashcards
What are acids?
Proton donors
Release H+ ions when mixed with water in form of hydroxonium ions
HA + H2O = H3O+ + A-
What are bases?
Proton acceptors
When mixed with water taken hydrogen ions from water molecules
B + H2O = BH+ + OH-
What do you call acids in terms of how many protons they can release?
Monobasic, HCl
Dibasic H2SO4
Tribasic, H3PO4
Describe the equilibrium set up by an acid reacting with a base in terms of conjugate pairs?
HA(acid) + B(base) ⇋ BH+(acid) + A-(base)
In forward reaction HA acts as acid as donates a proton to B
In reverse reaction A- acts as base to accept proton from BH+
HA and A- are a conjugate pairs, HA being conjugate acid and A- being conjugate base
B and BH+ are a conjugate pairs, BH+ being the conjugate acid, and B being conjugate base
What are conjugate pairs?
Species linked by the transfer of a proton
Always on opposite sides of the equation
Water is a special case, what happens if you react it with an acid?
Forms a conjugate acid (H3O)+
Water is a special case, what do you get if you react it with a base?
Forms a conjugate base (OH-)
What do you get if react an acid with a reactive metal?
Hydrogen gas
What do you get if react an acid with carbonates?
CO2 and water
What are Alkalis?
Type of base which releases OH- ions in water
What do you get if react an acid with an alkali?
Water
What’s a metal oxide?
An insoluble base
What do you get if react an acid with a metal oxide?
Water
Formula for pH if you know it’s H+ concentration?
pH = -log10 (H+)
If you know the pH of solution how do you find it’s H+ concentration?
(H+) = 10^-pH
What’s an important rule for monobasic acids or bases?
H+ concentration = Concentration of acid
OH- concentration = Concentration of base
Equation for water dissociating
H2O ⇋ H+ + OH-
How do you find the ionic product of water Kw?
Kw = (H+)(OH-)
If water is pure then ratio between them is 1:1, so Kw is the same as (H+)^2
For a weak aqueous acid what equilibrium do you get?
HA ⇋ H+ + A-
What’s important about a weak acid?
Don’t fully ionise in solution so H+ conc isn’t the same as acid concentration
Also it’s concentration is the same at start and at equillibrium
Formula for Ka the acid dissociation constant?
Ka = ((H+) x (A-)) / (HA)
Or Ka = (H+)^2 / (HA)
Units for Ka?
mol dm^-3
Formula to find pKa?
pKa = -log10 Ka
Formula to find Ka when you know pKa?
Ka = 10^-pKa
What’s a buffer?
A solution that minimises changes in pH when small amounts of acid or base are added
What do acidic buffers contain?
A weak acid and it’s conjugate base
What are the 2 ways to form an acidic base?
Mix a weak acid with the salt of it’s conjugate base
Mix an excess of weak acid with a strong alkali
What is the equilibrium for a weak acid and it’s conjugate base?
HA ⇋ H+(aq) + A-
Eg CH3COOH(aq) ⇋ H+(aq) + CH3COO-(aq)
How do acidic buffers resist changes in pH?
If you add a small amount of acid the H+ ion increases,, meaning the position of equilibrium shifts to the left reducing H+ ion concentration. pH doesn’t change much
If Alkali is added OH- ion concentration increases, they react with H+ to form water, removing H+ ions from solution. Causing equilibrium to shift right so more CH3COOH dissociates, H+ ion concentration increases, pH doesn’t change much
What are the 2 buffer systems that occur in our blood?
H2CO3 ⇋ H+ + HCO3-
H2CO3 ⇋ H2O + CO2
How are levels of H2CO3 controlled in our blood?
Respiration, when we breah out CO2 it’s concentration reduces so less H2CO3 formed
How are levels of HCO3- controlled?
The kidneys, with the excess being excreted in urine
How to calculate the pH of a buffer solution?
Assume that initial concentration of salt is the same the equilibrium value of A-
HA initial concentration is the same as it’s equilibrium concentration
Use and rearrange the Ka formula if necessary
What can you use to measure the pH of a solution?
pH meter
How do you calibrate a pH probe?
Put in distilled water and adjust reading to 7
Do the same with pH 4 solution and pH 10 solution rinsing it with distilled water each time
What do titrations allow you to do?
Find out exactly how much alkali is needed to neutralise and acid
How to draw strong acid / strong base curve?
Starts near 0, ends near 14
vertical part in centre
How to draw strong acid/ weak base
Starts near 0 ends near 10
Vertical part 3/4 on x axis
How do you draw a pH plot?
pH on y axis
volume of alkali added on x axis
How to draw weak acid/strong base?
Starts near 5 ends near 14
Vertical part 1/4 way down x axis
How to draw weak acid/ weak base?
Starts near 5 ends round 10
No vertical part it’s S shaped
What’s the vertical part of a pH plot?
The equivalence point
The point where the acid is neutralised and (OH-)=(H+)
How do you chose which indicator to use for a titration?
The pH range of the indicator must lie completely within the vertical part of the pH plot
Describe features f Methyl orange indicator?
Colour at low pH is red
Colour at high pH is yellow
Approx pH of colour change is between 3 and 4.5
So useful for strong acid/ strong base and strong acid/ weak base
Describe phenolphthallein?
Colour at low pH is colourless
Colour at high pH is pink
Approx pH of colour change is 8 to 10
So useful for strong acid/ strong base and weak acid/ strong base
Why can indicators be thought as weak acids?
They have differently coloured conjugate pairs, so as H+ or OH- concs change the equilibrium positions also do
phenolphthallein-H (colourless) ⇋ phenolphthallein-(pink) +H+
Methyl orange-H (red) ⇋ Methyl orange- (yellow) + H+
