Y13 Acids, bases and buffers Flashcards
Bronsted-Lowry acid definition
Proton donor
Bronsted-Lowry base definition
Proton acceptor
Alkali definition
Soluble base which dissolves in solution to release OH- ions
Monobasic acid definition
Can release 1 proton per molecule e.g. HNO3
Strong acid definition
Proton donor which fully dissociates in aqueous solution
Weak acid definition
Proton donor which partially dissociates in aqueous solution
What is formed when an acid reacts with a base?
Salt + water
What is formed when an acid reacts with a carbonate?
Salt + water + carbon dioxide
What is formed when an acid reacts with a metal?
Salt + hydrogen
What is formed when an acid reacts with ammonia?
Ammonium salt
Which of the 4 acid + x reactions is NOT neutralisation?
Acid + metal = redox because metal is oxidised and H+ ions are reduced
Conjugate acid-base pair definition
A pair of 2 species which transform into each other by the gain or loss of a proton
Label the acid-base pairs in this reaction: NH4+ + CO3(2-) –> NH3 + HCO3-
NH4+ = CO3(2-) –> NH3 + HCO3-
A1 B2 B1 A2
pH definition
-log [H+]
How do you calculate [H+] from pH?
[H+] = 10^(-pH)
How can you calculate pH when given the acid concentration of a STRONG acid (HA)?
pH = -log [HA]
What are the 3 stages to calculating pH after a dilution?
- Dilution factor: new volume / original volume
- New concentration: old conc / dilution factor
- pH = -log [HA]
How is a Ka expression written?
Ka = ([H+] x [A-]) / [HA]
How is pKa calculated from Ka?
pKa = -log (Ka)
How is Ka calculated from pKa?
Ka = 10^(-pKa)
What does a smaller pKa value mean?
Stronger weak acid
Which acids will act as the conjugate acids in a conjugate-base pair?
The weaker acids
What 2 approximations are made when calculating the pH of a weak acid and what is ignored/assumed?
- [H+]eqm = [A-]eqm Ignored: dissociation of H2O
- [HA]eqm = [HA]start Assumed: dissociation is very small
What is the expression for Kw?
Kw = [H+] [OH-]
What is the value of Kw at 25°C?
1.00 x 10^-14
What is a neutral solution?
A solution where [H] = [OH-]
What is the equivalence point on a pH curve?
The mid point of the vertical section on the curve
Where is the equivalence point on the 3 different pH curves (that we need to know)?
- Strong acid-strong base: at pH 7.00
- Strong acid-weak base: below pH 7.00
- Weak acid-strong base: above pH 7.00
How must you write pH values?
To 2 decimal places
What are the 4 steps to sketching a pH curve?
- Find the start and end points using calculation
- Find the volume added (calculating moles of substance in flask, using this value to find volume of substance added in burette)
- Shape: weak/strong and the mid point
- Proportion: 2 pH gradual increase, vertical line minimum of 3 pH
What is the end point of a reaction?
When there are equal amounts of the weak acid and conjugate base present
How is a suitable indicator chosen for a reaction?
pH range of the indicator matches the vertical section of the pH curve
Buffer solution definition
Mixture which minimises pH changes by the addition of small amounts of acid or base
What are the 2 ways of making a buffer solution?
- Weak acid with its conjugate base
- Excess of a weak acid with a base (partial neutralisation)
Explain how a buffer solution works
Added acid: [H+] increases which reacts with A- to produce HA which shifts equilibrium to the left hand side
Added alkali: [OH-] increases which reacts with H+ to form H2O. Equilibrium shifts right to increase [H+] which was used up when forming H2O
(Include equations!)
What is the equation for calculating [H+] of a buffer solution?
[H+] = Ka x ([HA] / [A-])
How is the pH of a buffer solution made through PARTIAL NEUTRALISATION calculated?
- Moles of initial HA
- Moles of base
- Moles of excess HA (moles of HA - moles of base)
- [H+] = Ka x ([HA] / [A-])
- pH = -log [H+]
What pH range does blood plasma need to have?
pH 7.35-7.45
