Block 5 Flashcards
What is a bronsted acid (2 definitions)
A substance that has a proton (H+) which can be taken by a base. A molecule with a H- Electronegative atom.
Are acids electrophiles or nucleophiles
electrophiles because they don’t have any lone pairs/ - charge and therefore are electron deficient.
Are bases electrophiles or nucleophiles
Nucleophiles because they have more electrons than protons so want some +
What is a bronsted base
A substance that can take a proton from an acid (meaning it has a lone pair of electrons)
What does triprotic, diprotic mean when referring to acids
how many protons the acid has available to take off.
Give examples of Bronsted acid
HCl, H2SO4,
Give examples of Bronsted base
NH3
What is Kw
equilbrium constant for pure water, 10^-14 = [H30+][OH-]
How do you calculate [H3O+] from pH
10^-ph = [H30+]
Strongness/ Weakness of an acid depends on what
The position of equilibrium: Far to the right= completion = strong and a single arrow. Weak is close to the middle/left=reactants>products = double arrow
What is Ka
Acid dissociation constant : [products]/ [reactants (excl.water]
How does size of Ka relate to strength of acid
A big Ka means that reaction lies to the right, so acid is strong. A small Ka means that reaction lies to left so acid is weak.
A big Ka means what size pKa
a small pKa–> stronger acid
Relate pKa , strength of acidity and structure together
A strong acid is something that has a weaker H bond because that makes it easier to pull off.
What are factors that make a weaker H - bond- therefore easier to pull off a M
making bonds longer - bulkier atoms. Decreasing electron density in the bond between electronegative atom and H by having other electron withdrawing groups that help to pull even more charge away from H.
Decreasing the amount of charge in the molecule (+ harder to leave a M-)
Delocalisation (resonance) which spreads charge through resonance which makes the base form still stable
when pH=pKa what species will there be
Half of acid has reacted to form half of base. Equal concentrations of [HA] and [A-]
when pH < pKa what species will there be
more [HA] than [A-]
when pKa < pH what species will there be
more [A-] than [HA]
In an amino acid, there are two functional groups: NH3+ and NH2 and COOH and COO-
What species is the acid species relating to pKA for both functional groups
NH3+ and COOH
The pKa1 of COOH is smaller than pKa2 for NH3+ . When the pH is smaller than pKa (cooh) what species are on the amino acid
Acid form for both: NH3+ and COOH
The pKa1 of COOH is smaller than pKa2 for NH3+. When the pH is greater than the pKa2, what species are on the amino acid
Base form for both: NH2 and COO-
When the pH is above pKa1 (COOH) but not higher than the pKa2 (NH3) what species are on the amino acid
COO- (base form) and NH3+ (acid form). This is an overall neutral species called Zwitterion.
Both species of conjugate acid and base will be present in the solution when the pH is what
Within 1 unit of the pKa of the acid form
What the assumptions we make when doing strong acid calculations involving pH and pKa. When is this not true
A strong acid completely dissociates in water: [H30+]= [HA], assuming only the strong acid contributes to the H30+. This is not true if pH>6. Also in very dilute solutions bc water contributes H30+
What the assumptions we make when doing strong base calculations involving pH and pKa. When is this not true
A strong base completely dissociates in water: [OH-]=[B], assuming only the strong base contributes to [OH-] This is not true if the pH is <8. Use Kw
What the assumptions we make when doing weak acid calculations involving pH and pKa.
- The dissociation of [HA] is small compared to the original amount, most of the reactant hasn’t dissociated so [HA] at the time of the reaction is pretty much the same as it was initially.
- [HA]>[A-] so the pH should be on the acid side of pKa.
- the only source of H30+ comes from dissociation of HA so [A-]=[H30+]
What the assumptions we make when doing weak base calculations involving pH and pKa.
- The only source of [OH-] is from the [B] so [OH-]= [HB]
- The amount of [B-] is pretty much the same as it was initially as only a small amount of [B-] react.
The pH should be on the base side of the pKa
What is a formula that relates pKa and pKb / pOH and pH
pKa or pH+ pKb or pOH = 14
What is a formula that relates ka & kb or H30+ and OH–
Ka * kb = kw = 10^(-14) = [H30+][OH-]
What the assumptions we make when doing amphoteric molecule calculations involving pH and pKa.
Amphoteric species acts as both a weak acid & base. It has two pKas for the two reactions.
1.The concentrations of H3O+ or OH- are much less in water than the concentratio of the amphoteric species so the only reaction the amphoteric species undergoes is with itself.
How do you calculate the pH of an amphoteric molecule and what do you need to check for
pH = 1/2 (pKa1 + pKa2) . Check that the conc of [H30+] is much less than the original conc of the amphoteric species
What is the henderson/hasselbach equation
pH= pKa + log (base/acid)
A buffer is a solution that resists changes in pH. When does a buffer work best/ what are the key characteristics of a good buffer?
When the weak acid HA and A- are of similar concentration, the log term is very small and the pH is approximately equal to the pKa of the acid form being added +or -1. It is also more effective with higher conc of the two species.
When adding one solution to another solution what has changed and what has stayed the same
The concentration has changed because c= n/v and while n has stayed the same, v has changed.
When water is added to something what do you expect will happen to the pH
it will move closer to 7
How does the pH of a solution of strong acid vs the pH of a buffer, change with dilution
Adding water will make Strong acid solution pH change, but the pH of buffer solution doesn’t change much with addition of water
What are steps to determining the pH or pKa of buffer
Understand what species are involved and therefore which pKa you will use.
What happens when a strong acid/strong base is added to a weak acid/base buffer solution
strong acid/base will completely react with the weak base/acid reacting with H3O+/OH- ions released, and solution will get more acidic/basic. So (provided 1:1 ratio) the n(weak base) available will decrease/increase by the nH30+ and the n(weak acid) will increase/ decrease by nH30+.
What happens to pH of buffer solution when the all the conjugate base or acid is used up and there is left over strong acid or base that hasn’t reacted?
the pH’s main contributor is the strong acid/base, so the weak acid/base’s contribution can be ignored. pH = -log [H30+] based on the leftover n[strong acid/base] / the total volume at the end of adding them together.
What is a titration
A method of volumetric analysis used for determining the concentrationt of an unknown solution by letting it react with another whose concentration is known
What is the difference between equivalence point vs end point
Equivalence point is the volume at which the reaction is perfectly completed- all of the moles of reactant have been used up. We can find this relationship experimentally using c1v1=c2v2. However the end point is where we decided to stop the reaction, - ideally at the equivalence point
What are the 4 parts of a titration curve you need to know
The beginning, the buffer region, half equivalence, and equivalence point.
What are the species present at the beginning of titration
only the weak acid/base in the flask at the bottom
What are the species present at the buffer region
the weak acid/base and its conjugate in a buffer solution
What are the species present at half equivalence point
The reaction is half completed, half of the volume needed to reach equivalence point has been added. Therefore, there are equal amounts of acid and conjugate base present, so pH=pKa
What are the species present at equivalence point (s) and how does this affect pH of ep.
only the conjugate of the original weak acid/base is present as the reaction is 100% completed. This means that the pH is either slightly higher than 7 (if remaining species is conj. base) or lower than 7 (if conj. acid)
What are the species after equivalence point and how does this affect the pH of the solution
After all the original base has been protanated or the original acid has been deprotanated, it becomes a mixture of a strong base + weak base or strong acid + weak acid, so only the strong one need to worry about, and can use the excess amount of strong thing to calculate the pH
What are indicators
compounds that are different colours in their acid and base form. Thus the colour of an indicator is dependent on the pH - the amount of H3O+ in solution
How do you choose the right indicator for a reaction
choose an indicator with a pH range of colour change within 1 of the pKa of the Indicator
For polyprotic acids, which acid is stronger - the first step or the conjugate base of the first step used as the acid for the next step
The first step has the stronger acid, because it is harder to remove a proton from something that has lost one already so the conjugate base of the first one that becomes the acid of the next step is weaker that the first step.
How can you find how many equivalent points there are for a polyprotic acid titration
Look at how protons it loses- number of deprotanation step where it reacts with OH-
What is the volume to get to each equivalence point in a polyprotic acid titration ?
the same for each reaction because the number of moles of polyprotic acid hasn’t changed and it can only remove one H from all the molecules at a time.
How can you find the pKa of each deprotanation step
look at the pH on the graph at the half equivalence point.
When you want to find the pH of a weak base, what must you use
pKb.
How do you find the pH at the final equivalence point of a polyprotic acid
only weak base exists, must use pKb. Although n(X3-)= (H3X), the volume has changed due to the base being added so concentration must be recalculated.
How do you calculate the pH at the equivalence points for polyprotic acid
the same as calculating for amphoteric molecule, knowing that the equivalence points lie between the 1/2 eq. points for the reactions where pH=pKa1 and pH=pKa2. So you can use 1/2 (pKa1 +pKa2)
