Acid and Bases Flashcards
What is a Bronsted Lowry acid?
A proton donor
What is a Bronsted Lowry base?
A proton acceptor
What is a Lewis acid (electrophile)?
An electron pair acceptor
What is a Lewis base?
An electron pair donor
Name the conjugate acid and base pairs in this equation
- HCl + NH3 ⇌ NH4+ + Cl-
Acid and Base 1- HCl and Cl-
Acid and Base 2- NH4+ and NH3
If there is a strong base will the conjugate acid be strong or weak?
Weak as the base has a high tendency to accept a proton, the conjugate acid will not get rid of this proton very easily
If the Ka of an acid is large will the pKa be large or small?
Small as the pKa is -logKa
How do you calculate the Ka from the pKa?
10^-pKa
What does pKa represent?
- The pH at which an acid is half dissociated
- A strong acid would have a very low pKa as they would normally be fully dissociated in water, there needs to be an extremely high proton concentration for half of the conjugate base to be associated with a proton
How would you distinguish between the strength of of HCl and HBr?
In water, HCl and HBr appear to be the same strength. To distinguish between them we use a different proton acceptor such as CH3COOH, it is much harder for these strong acids to lose a proton with this poor proton acceptor and therefore we see that HBr is a stronger acid as it has a lower pKa when using CH3COOH as the proton acceptor
What is the water dissociation constant?
Kw= 10^-14
Concentration of H+ and OH- is equal, they’re both equal to 10^-7 giving a PH of 7
How does Kw change with temperature?
- The dissociation of water is endothermic, pure water is always neutral so neutral pH depends on the temperature
- If temperature is increased, equilibrium shifts in the direction of the forward reaction and the concentration of H3O+ and OH- increases and therefore pH decreases
- Kw therefore increases with temperature and the neutral pH of water decreases with temperature
How do you calculate the pH of a weak acid when given the concentration of H3O+?
- Write out the Ka expression for the specific acid
- We assume [H3O+] = [Conjugate base of the weak acid]
- Then solve the equation by rewriting the top of the Ka expression as [H3O]+^2, you will have the Ka value and the concentration of the original weak acid and you can solve equation
What is the degree of dissociation?
- The ratio of dissociated ions to starting compound x 100 to obtain a percentage
- [H3O+] / [HA] x100
What is the expression for Kb?
- [BH+] [OH-] / [B]
- pKb = -logKb
How do you calculate Kw from the Ka and Kb of a conjugate acid base pair?
- Ka x Kb = Kw
- Log (Ka x Kb) = Log Kw
- Log Ka + Log Kb = Log Kw
- -Log Ka - Log Kb = -Log Kw
- pKa + pKb = pKw
What is the pKw of water at 298K?
14
What does the pKa + pKb of a conjugate acid base pair equal at 298K?
14
If an acid has a low pKa will its conjugate base have a high or low pKb?
High
Why is iodine the strongest acid of the hydrogen halides?
The H-I bond is the weakest, this allows a proton to dissociate more easily
Why is HF a very weak acid?
When F- dissociates from HF, it can form hydrogen bonds with H3O+, this effectively masks the H3O+ therefore HF is a weak acid
What is an oxo acid?
- An acid that contains a double bond between an oxygen and an element
- It also contains an OH group
What is the general formula for an oxo acid?
HmEOn where m is the number of hydrogens, E is an element and n is the number of oxygens
How do you work out the number of oxo groups that an oxo acid has?
n-m, n is the number of oxygens and m is the number of hydrogens
When comparing different oxo acids, what happens to the pKa as n-m increases?
The pKa decreases as the number of oxo groups increases. This is because there are more possible resonance forms and this makes the deprotonated ion more stable and therefore the oxo acid dissociates more readily
What is Bell’s rule?
pKa = 8-5(n-m)
Why is CF3COOH far more acidic than CH3COOH?
The conjugate base of CF3COOH has more resonance forms than the conjugate base of CH3COOH as the fluorine atoms can withdraw electron density. This means that CF3COO- is more stable and therefore CF3COOH dissociates more readily
What is an amphoteric metal oxide?
A metal oxide that dissolves in both acid and base eg. Al2O3
What is fluoroantimonic acid?
A mixture of SbF5 and HF that is a superacid, it can protonate methane to form CH5+
What does a buffer solution consist of?
A weak acid and its conjugate base or the weak base and its conjugate acid
How is a buffer solution formed?
- Small reservoir of salt of conjugate base, the reservoir is small because it fully dissociates to form a large reservoir of the conjugate base
- Large reservoir of the weak acid, this dissociates to form a small reservoir of the conjugate base and H3O+
- As there is already a large reservoir of the conjugate base from the dissociation of the salt, there is a large reservoir of the weak acid and its conjugate base. This enables the buffer to resist changes in pH
How does a buffer solution resist changes in pH?
- When OH- is added, it reacts with the weak acid by removing a proton, this forms the conjugate base and therefore equilibrium has shifted right and pH remains the same
- When H+ is added it reacts with the conjugate base to form the weak acid, equilibrium has shifted left and change in pH is resisted
What is the Henderson Hasselbach equation?
pH = pKa + log ( [A-] / [HA] )
What is the relation between pKa and pH as shown by the Henderson Hasselbach equation?
- When [A-] = [HA], log ( [A-] / [HA] ) = 0, this means that pH = pKa when [A-] = [HA]
- Therefore when the acid is half dissociated, pH = pKa, this means pKa is the pH at which an acid is half dissociated
What is the equation to determine pH change when adding a base to a strong acid?
[H3O+] = [ (Original amount of acid in moles) - (Amount of base added in moles) ] / [ (Volume of acid) + (Volume of base) ]
[OH-] = [ (Original amount of base in moles) - (Amount of acid added in moles) ] / [ (Volume of acid) + (Volume of base) ]
What does pH + pOH =
14
Why is the equivalence point for a weak acid strong base titration above 7?
As the salt that is produced dissociates and the dissociated base reacts with water to form OH-
In a titration what does the pKa of the acid tell us?
The pH at which the acid is half neutralised
What is the formula for calculating the pH of a weak acid?
pH = -log(Ka x conc of weak acid)^1/2
When does an indicator change colour?
When [HIn] = [In]
How do you decide which indicator is suitable for a titration?
Select an indicator that has a pH range of colour change that is within the vertical section of the titration curve
What is the equation for calculating the pH of a buffer solution in a titration?
pH = pKa + log [ (moles of conjugate base) / (moles of acid - moles of base added) ]
How do you work out the end point of the first titration of a polyprotic acid when you have the first 2 pKa values ?
Add the two pKa values together and divide them by 2
What is a speciation curve for a polyprotic acid?
- A set of curves that demonstrate that at the pKa values for the different ions formed from the polyprotic acid, there is half of the original ion and half of the deprotonated ion
- For example the second pKa value of H3PO4 is 7.21, this means at pH 7.21 there is half H2PO4- and half HPO4^2-
What is the general structure for an amino acid?
NH2CRHCOOH
At a neutral pH, will an amino acid be in a neutral form?
- No, an amino acid will never be in a neutral form, it will always have at least one charge on the molecule
- Although the molecule will always have a charge on it, the overall charge of the molecule can be neutral as the molecule can contain a positive and negative charge which would give an overall neutral charge
What is a zwitterion?
An amino acid that has a charged amino group and a charged carboxyl group but no overall charge on the molecule
What is the isoelectric point of an amino acid?
The pH at which the amino acid has no overall net charge
How do you work out the isoelectric point when given the 3 pKa values of an amino acid?
Determine which two values the amino acid would be neutral between. Add these two values together and divide by 2 to give the isoelectric point (pI)
Which is a stronger base out of NH3 and CH3NH2?
NH2 as the CH3 group pushes electron density towards the NH2 group making it more delta negative than before, this means it attracts H+ more easily
How does electrophoresis work?
- There is a gel body where the amino acids are inserted into
- One side of the gel is positively charged and the other side is negatively charged
- You can set the pH of the gel, you can determine which molecules are which from how far they move to one side
- Changing the pH will cause the molecules to change charge and they will move in the gel
