1.10 Acids, Bases and pH Flashcards
Bronsted-lowry acids are …
proton donors
Bronsted-lowry bases are …
proton acceptors
What are produced when H+ ions are released
Hydroxonium ions H3O+
How much do strong acids/bases dissociate in water?
Almost completely so the equilibrium lies to the right
How much do weak acids/bases dissociate in water?
Slightly so the equilibrium lies to the left
What is Kw
The ionic product of water
Kw = [H+][OH-} or in pure water Kw = [H+]^2
What is the formula for pH
pH = -log[H+] or [H+] = 10^-pH
What do you have to remember when calculating the pH of strong diprotic acids?
The [H+] is twice the concentration of the acid
How can you use Kw to calculate the pH of strong bases?
You will be given Kw and [OH-] = concentration of base so you can work out [H+]
What is Ka used for
Working out how much a weak acid dissociates
Ka =
[H+][A-] / [HA]
pKa =
-log(Ka) or Ka = 10^-pKa
Describe how to carry out a titration
- Measure out some acid of know concentration using a pipette and put it in a flask along with an appropriate indicator
- Carry out a rough titration by adding alkali to the burette and swill with the alkali. Find an estimate to the end point. Make sure the flask is regularly swirled to make sure the acid is mixed properly with the alkali
- Repeat this process to obtain an accurate titration. Run the alkali to within 2cm^3 of the end point then add drop by drop
- Record the amount of alkali required
For which type of titration is an indicator no appropriate and what should be used instead?
Weak acid/weak base as it has no clear end point
pH probe
Where do you find the equivalent point on a pH curve?
The mid point of the vertical section
What do you need to consider when choosing an indicator
It must change colour over a narrow pH range that lies entirely over the vertical section of the curve
What is a buffer?
A solution that resists changes in pH when a small amount of acid or alkali is added
What are the two types of buffers?
acidic and basic
What are acidic buffers?
Have a pH less than 7
Made of a mixture of a weak acid and one of its salts
Give an example of an acidic buffer and its equilibrium
ethanoic acid + sodium ethanoate
CH3COOH H+ + CH3COO-
What happens when an acid is added to an acidic buffer?
Adding acid increases [H+] so H+ combine with A- to form HA.
Equilibrium shifts to the left to reduce the H+ value
What happens when a base is added to an acid buffer
Adding base increases [OH-] which react with H+ to form water and remove H+ causing more HA to dissociate
Equilibrium shifts to right to increase the H+ value
What are basic buffers?
Have a pH greater then 7
Made of a mixture of a weak base and one of its salts
Give an example of a basic buffer and its equilibrium
Ammonia and ammonium chloride
NH3 + H2O NH4+ + OH-
What happens when an acid is added to a basic buffer?
[H+] increases and H+ react with OH- to form with water.
Equilibrium shifts to the right to replace the lost OH- ions
What happens when a base is added to a basic buffer?
[OH-] increases and OH- react with NH4+ to form NH3 and water.
The equilibrium shifts to the left to remove OH- ions
How do you find the pH of an acid buffer?
- Write out Ka of weak acid
- Rearrange for [H+]
- Solve to find [H+]
- Substitute value into pH equation
How do you find the pH of an acid buffer made by adding weak acid and alkali?
- Write out equation for the neutralisation reaction
- Work out moles of acid and base at the start of the reaction
- Work out the moles of acid and salt left at end of reaction
- Calculate concentration of acid and salt
- Use Ka to find [H+]
- Substitute value into pH equation
