Block 5: Acids And Bases Flashcards
Happens when an acid and alkali neutralise each other?
They form a salt and water (called a base) to
What does pH equal?
-log10 [H3O]
What is a neutral pH?
7
How do we calculate the concentration of H3O?
[H3O]= 10^-pH
Equation to calculate the concentration of H3O in a solution of HBr, pH 2.77?
[H3O] = 10^-2.77
Drawbacks of Arrhenius theory on salts?
Only works for substances dissolving in water
Doesn’t explain why some compounds containing H (like HCl) are acids, while others (like CH4) are not
Doesn’t explain why some compounds without OH can act as bases
What donates a proton?
Acids
What accepts a proton?
Bases
What’s the Bronsted equation?
HCl(aq)+ H2O (l) to H3O (aq) + Cl- (l)
What is the species that is left behind after a bronsted acid has transferred its proton called?
The conjugate base of the acid
What is the species that is formed after a Bronsted base has accepted a proton called?
The conjugate acid of the base
What is amphiprotic?
A species (like H2O) that can be an acid and a base
What happens if an acid can donate more than one proton (eg sulfuric acid H2SO4, phosphoric acid H3PO4)?
It’s show in two or more steps
When is there a position of equilibrium?
In proton transfer reactions
What is the equilibrium constant K equation?
HA(aq)+H2O(l) to A-(aq)+H3O (aq)
To
K= [A-(aq) x [H3O(aq)]/ [HA (aq)]
What is the standard concentration for species dissolved in solution?
1 mol L-1 (ie 0.1)
What is the proton transfer equilibrium constant and how is it calculated?
Ka
Same as K
What has a larger equilibrium constant?
Stronger acids
What is PKa?
-log10(Ka)
What has a smaller PKa?
Stronger acid
What would a Ka be for a species that appears on both sides?
1 (as it appears on both sides)
What affects the strength of an acid?
- The charge on A
- The polarity of the H-A bond
- The strength of the H-A bond
(Strength of bond is more important than strength of electronegativity)
What are oxoacids?
H3PO3 or H2SO4, when. It all the acids are necessarily acidic
What is Fw (equilibrium constant for water)?
Kw= [H3O(aq)]x[OH-(aq)]
What is the concentration of H3O in a solution of a strong acid?
0.01mol L-1 (as the acid is 100% dissociated (but needs to not be dilute)
How do you calculate the concentration of OH- for a strong base?
Calculate the concentration of OH-, then use Kw to get the concentration of H3O
When is the equilibrium not always all the way to the right?
Weak acids, where not all the acid is converted to H3O
Ka equation for solutions containing a weak acid and its conjugate base?
Ka = [A- (aq)] x [H3O(aq)]/[HA(aq)]
What is the pH of a solution containing a weak acid and its conjugate base?
pH= pKa + log10 {A-/ HA}
Called a Henderson-hasselbach equation
What is a titration?
A method of volumetric analysis used for determining the concentration pH an unknown solution by letting it react with another whose concentration is known
What is the equivalence point?
The volume at which the reaction is just completed
What is the concentration equation?
Amount= concentration x volume
N= c x v
What happens at the equivalence point in an acid/base titration?
The pH of solution changes rapidly
What is an indicator?
A substance that has different colours in acid and basic solution
What is the end point for the indicator?
The pH at which the colour change occurs (useful if the same as equivalence point)
What happens when a strong acid and base undergo a titration?
They’re both 100% dissociated
What is an indicator’s properties?
A weak acid, for which the acid and its conjugated base have different colours
Acidity constant equation for an indicator?
Kln = [ln-(aq)]x[H3O(aq)] / [HIn(aq)]
Equivalence point of a strong acid and a strong base?
7
Why do the acid and conjugate base forms have different colours?
Because of the extent of electron delocalisation difference of the two forms
What does the pH of an equivalence point in a titration depend on?
The acid (eg, a weak acid has a high pH)
What is that equation for pH in a weak acid/base titration?
pH= 7 + (1/2xpKa) + (1/2xlog10(c))
pKa is of the weak acid, and c is the concentration of the conjugate base at the equivalence point
Where is the acid and conjugate base equal to each other for a titration?
At volume halfway to the equivalence point, so, at this point, pH is equal to pKa
What is polyprotic?
Species capable of donating more than one proton (eg H2SO4)
For polyprotic species, which proton loss is easier?
The loss of the second proton
What is a buffer solution?
A solution whose pH changes much less than that of pure water as we add either acid or base
What does a buffer solution contain?
Either equal amounts of a weak acid and its conjugate base (if pH lower than 7) or a weak base and its conjugate acid (is pH is higher than 7)
Why can a buffer work?
Because it contains species that both accept and donate proteins
What does the pH change by in a buffer solution?
Only +1 unit or -1 unit
What makes a buffer solution more effective?
When there is a higher concentration of the two species
What is the human body buffer system like?
More conjugate base than acid, as likely waste products from the body are largely acidic
What does a raised pH in the body cause?
Light headedness
What is the pH at which the majority of species are zwitterions called?
The isoelectric point, which is denoted by pI, and depends on the relative strength of the acidic and basic parts of the molecule, and varies between amino acids
What is electrophoresis?
A technique which separates mixtures of amino acids
