1.12-Acid and bases Flashcards
Define conjugate base
The species formed by the removal of a proton from an acid
Define Conjugate acid
The species formed by a base accepting a proton from an acid
Why is the kw expression
- Kw = [H+ (aq) ][OH- (aq) ]
And not
- Kc= [H+ (aq)][OH- (aq)]/[H2O(l)]
As the equilibrium lies to the LHS because water only very slightly ionises
The concentration of water is very large in comparison to the concentration of the ions
The concentration of water can be regarded as constant
We can therefore have a new constant: Kw
Kw is the ionic product of water
What are the “ steps to success” for working out the pH of a strong base strong acid mixture.
1) Calculate moles of acid and base
2) Calculate moles of H+ and OH-
3) Calculate which is in excess and the Calculate the conc of the excess reagent
4) Calculate pH, if OH- is in excess use kw equation
How does the Ka value tell you which acid is the strongest?
[H+] is proportional to Ka so biggest Ka means highest concentration of H+ and therefore the strongest acid
What is a Buffer Solution?
A buffer solution is a solution that resists changes in pH when small amounts of acid, alkali or water are added to it. (2 Marks)
What is the difference between acidic buffers and basic buffer solutions?
Acidic buffer solutions have a pH less than 7.00 Basic buffer solutions have a pH more than 7.00
Components of an acidic buffer?
A weak acid
A salt of a weak acid
[HA] and [A-] are much higher than [H+]
What are the 2 ways to make an acidic buffer?
1 – Directly dissolving a salt of the weak acid (A-) in a solution of the acid (HA)
2 – Adding a strong base like sodium hydroxide to an excess of a weak acid.
What are the steps to Calculate the pH of an acidic buffer solution formed from an acid and a salt of the weak acid?
Calculate moles of salt Calculate the moles of A- Calculate the conc of A- Calculate the conc of HA ( ONLY NEEDS TO BE RECALCULATED IF IT IS A SALT SOLUTION BECAUSE THE TOTAL VOLUME HAS CHANGED) - DOES NOT NEED TO BE RECALCULATED FOR SOLID SALT Rearrange Ka for [H+] Use pH equation
What are the steps to Calculate the pH of an acidic buffer solution formed from a weak acid and a base?
Step 1 :Calculate the moles of HA
Step 2 : Calculate the moles of OH-
Step 3: Check moles of HA is XS
Step 4: Use the chemical equation and work out moles before and
after reaction for HA and A-
Step 5: Calculate [HA] left over and [A-] formed
Step 6: State the Ka expression. Calculate [H+] by rearranging Ka and substituting the values
Step 7: Use pH = -log[H+] to calculate pH
What happens when small amounts of acid are added to an acidic buffer solution? (3 marks)
On addition of a small amount of acid, some A- will react with the added acid.
This shifts the equilibrium to the left, and [H+] is restored to its original value.
The ratio [HA]/[A-] remains ALMOST constant, since they are both in large excess, so [H+] remains ALMOST constant.
So the pH remains ALMOST unchanged.
What happens a when small amount of base is added to a buffer solution? (3 marks)
On addition of a small amount of base, some H+ will react with the added base. The equilibrium shifts to the right, and [H+] is restored to its original value.
The ratio [HA]/[A-] remains almost constant, since they are both in large excess, so [H+] remains almost constant.
So the pH remains almost unchanged.
Suggest why chloroethanoic acid is a stronger acid than ethanoic acid? (2)
M1Cl is (more electronegative so) withdraws electrons
OR negative inductive effect of Cl
Ignore electronegativity.
Ignore chloroethanoic acid has a lower Ka value.
Allow Cl reduces +ve inductive effect of methyl group.
1
M2Weakens O–H bond
OR O–H bond is more polar
OR reduces negative charge on COO−
OR stabilizes COO− (more)
State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement? (1)
Over time / after storage meter does not give accurate readings
Do not allow ‘to get an accurate reading’ or ‘reading drifts’ on its own.
Allow ‘temperature variations affect readings’.
Describe how you would obtain the pH curve for the titration.(5)
In the conical flask - 25.0 cm3 of a 0.100 mol dm–3 carboxylic acid solution
I’m the burette - 0.100 mol dm–3 sodium hydroxide solution.
Any five from:
Ignore references to the use of the pipette, the filling of the burette and the calibration of the pH meter.
•Measure pH (of the acid)
•Add alkali in known small portions
Allow 1 – 2cm3.
•Stir mixture
•Measure pH (after each addition)
•Repeat until alkali in excess
Allow 27 – 50cm3.
•Add in smaller increments near endpoint
Allow 0.1 – 0.5cm3
What does a bigger Ka value mean?
Stronger acid
What does a bigger pKa value mean?
Weaker acid.
