Acid & Bases Flashcards
What is a Bronsted-Lowry Acid
A proton donor
What is a Bronsted-Lowry base
- proton acceptor
What is a hydronium ion
H3O+
When acids and bases react…
The products formed also behave as either acids or bases
A strong acid is ………… dissociated
Completely
A weak acid is …………… dissociated
Partially
CH3COOH <-> H+ + CH3COO-
What is the ‘strength’ of an acid
The extent to which the acid disassociates
What is the concentration of an acid?
Number of moles of acid per unit volume
What is protocisity
Tells you how many protons are released when an acid disassociates
What is basicity?
- tells you how many OH_ are released
A strong base is…
Completely dissociated
NaOH -> Na+ + OH-
A weak base is…
Partially dissociated
NH3 + H2O <-> NH4+ + OH-
Write an equation to show the waters slight dissociation
H2O <-> H+ + OH-
Forwards reaction is endothermic
What is Kw?
Equilibrium constant of water
The ionic product of water
[H+][OH-]
What is Kw at 298K
Kw = 1X10^-14 mol2dm-6
What does is the effect an increase in temperature has on the dissociation of water
- forward endothermic reaction is favoured
- p.o.e shifts to RHS
- higher [H+][OH-]
- Kw increase
Why is pure water always neutral?
[H+] = [OH-]
How do you calculate the pH of pure water at different temperatures
Kw = [H+][OH-]
Kw = [H+]^2
Root of Kw = H+
PH = -log10[H+]
How do you calculate [OH-]
Kw/[H+]
What is the equation for the dissociation of a weak acid HA, in water?
HA + H2O <-> H3O+ + A-
Why is the concentration of water with a weak acid effectively constant?
- very few molecules are protonated as a weak acid is only partially dissociated
What is Ka
Acid disassociation constant
Kc * [H2O]
([H3O+][A-])/[HA]
How do you get pKa from Ka?
-logKa
What is Ka for weak acids and why?
X2/a
[H+][A-]/[HA]
- the extent to which HA dissociates is very small so X is very much smaller than a
- follows that the value a-x is essentially equal to a
How can the simplified expression for Ka be used to calculate the pH of weak acids?
√Ka[HA] = H+
What is the equivalence point?
- the point at which we have exactly the right number of moles of acid required to just neutralise the base
- the two solutions have been mixed together in exactly the right proportions
What is the half equivalence point?
The point at which we have added half of the amount of base required to completely neutralise the acid (or vice versa)
Give an expression for the acid disassociation constant at half equivalence
Ka= H+
A- = HA
What does pka equal at the half equivalence point
pKa = pH
Where can the equivalence point be found on a pH curve?
- centre of the vertical portion of the graph
What do pH probes measure?
- pH changes that occur when acids and bases react together
Describe an experiment which allows you to observe changes in pH when reagents are added together
pH probe calibration:
1. Rinse pH probe throughly with distilled water and shake gently to remove excess water.
2. Place probe in pH 7.00 solution and turned dial until pH reads 7.00 on the probe.
Strong acid/strong base
3. Rinse a burette with a 0.100mol dm–3 hydrochloric acid solution and fill this burette with solution. (Burette A)
4. Transfer 20cm3 of hydrochloric acid to a beaker
5. Rinse a second burette with 0.100mol dm–3 NaOH solution and fill this burette with this solution. (Burette B)
6. Rinse the pH probe with distilled water and clamp it so bulb is fully immersed in the hydrochloric acid within the beaker.
7. Titrate the hydrochloric acid with NaOH using 2cm3 at a time. Stir the solution with a clean dry glass rod and record the pH each time.
8. When end point is being reached add 0.50cm3 of NaOH each time
9. After this, continue adding 2cm3 of NaOH until it is in excess.
Strong acid/weak base
10. Rinse a burette with a 0.100 mol dm-3 ammonia solution and fill the burette with this solution. (Burette C)
11. Use Burette A to transfer 20cm3 of hydrochloric acid to a beaker
12. Repeat steps 6-9 and record data.
Weak acid/strong base
13. Measure out 20cm3 of 0.100 mol dm-3 ethanoic acid using a measuring cylinder and pour into a clean beaker
Use Burette B to repeat steps 6-9 (titrate ethanoic acid against NaOH)
What is a titration curve?
- a graph showing how pH changes when base is added to an acid
How can you tell using a titration curve whether an acid is being added to a base or base added to acid?
The initial pH
How do you determine the initial pH of the solution in the conical flask
Strong acid: [H+] = [Acid] * prototicity pH = -log10H+
Weak Acid: Ka = [H+]^2 / HA. PH = -log10[H+]
Strong base: [H+] = (1*10^-14) / [OH-] PH= -log10[H+]
How do you interpret where the volume at which the equivalence point occurs using a titration curve?
- Write an equation for the acid-base reaction
- Calculate moles of acid or bases in the flask using n= (cv)/1000
- Use the ratio to determine number of moles of base/acid needed to react completely
- Calculate the volume using v = (n*1000)/c
How do you interpret the pH at the equivalence pointv from a titration curve
- depends on the species present in the conical flask at the equivalence
How do you figure out the half equivalence point when a strong base is added to a weak acid?
At the point where PH = pka
What does a strong acid - strong base curve look like?
What does a strong acid - weak base curve look like?
What does a weak acid - strong base curve look like?
What does a weak acid - weak base curve look like?
Why is the pH at the equivalence point for the reaction where CH3COOH is added to NAOH alkali?
Because CH3COO- is a proton acceptor (base)
Why is the pH at the equivalence point for the reaction between HCl and NH3 acidic?
- NH4+ is a proton donor
What is the end point of a titration?
- when the concentration of the acid (H+) and bases (In-) forms of the indicator are equal
What do we think about when choosing an indicator for a given acid-base titration?
- The pH range over which the indicator changes colour
- The pH range of the verticals portion of the titration
- rapid change in ph around the equivalence point must be in the pH range for the change of colour for the indicator
Why is only one or two drops of indicator best
So the [H+] of the solution will then not be affected by the presence of indicator
What is a buffer
A solution that resists changes in pH when small amounts of strong acid or strong base are added, or if it is diluted with water
What are the two types of buffers?
- acidic
- basic
How do buffers work when a base is added to the disassociation of a weak acid (ethanoic acid)
- The OH- will react with the H+ that comes from disassociating
- reaction will work to replace them through more disassociation
- [H+] and pH is constant
- weak acid solution is initially buffered against OH
What happens when acid is added to the disassociation of ehtanoic acid?
- H+ ions react with CH3COO- ions
- however this process is limited as [CH3COO-] is low (weak acid that is only very slightly dissociated and the Poe is far to the LHS)
- therefore weak acid is not buffered against H+ and will not remain constant
How do you allow the weak acid solution to be buffered against H+?
- increase [CH3COO-]
- by adding a sodium salt of the acid [CH3COO-Na+]
What makes up an acidic buffer?
- an aqueous solution of a weak acid
- aqueous solution of a salt of the weak acid
What happens when small amounts of acid is added to an acidic buffer?
- added H+ reacts with CH3COO- to make CH3COOH
- changes in concentration are insignificant because the starting concentration are so high to being with and only small amounts are added
- so [CH3COOH] / [CH3COO-] is virtually constant so pH is constant
What is inside basic buffers
- solution of a weak base with an aqueous solution of the salt of the weak base
- NH3 and NH4Cl
What are the three methods of preparing an acidic buffer?
- Addition of a solid salt of the weak acid to a solution of the weak acid.
- Addition of a solution of the salt of a weak acid to a solution of the weak acid.
- Addition of sodium hydroxide solution to an excess of the weak acid.
Give the ideal explanation for how a buffer works?
- If H+ ions are added the PoE shifts to the left to remove the extra H+ ions
- If OH- ions are added then H+ + OH- —-> H2O and then the PoE shifts to the right to replace lost H+ ions
- We have a large excess of both HA and A- so changes in concentration are negligible and the ratio of HA:A- remains roughly constant
Why does pH stay the same when small amounts of acid are added to the buffer solution
Changes in concentration of HA and A- are insignificant as they are both present in large excess. The ratio of HA:A- remains roughly constant, so the pH remains roughly the same.
Why does pH stay the same when a small amounts of base is added to our buffer
Changes in concentration of HA and A- are insignificant as they are both present in large excess. The ratio of HA:A- remains roughly constant, so the pH remains roughly the same.
What is the mathematical expression used to calculate pH of a buffer
H+ = Ka * [HA][A-]
