Chapter 20-21-Chapter 20-21- Acids, Bases, Buffers and neutralisation Flashcards
Brønsted-Lowry acid
A Brønsted-Lowry acid is a proton (H+) donor.
Brønsted-Lowry base
A Brønsted-Lowry base is a proton, H+, acceptor.
Alkali
An alkali is a base that dissolves in water forming OH-(aq) ions.
Neutralisation
Neutralisation is a chemical reaction in which an acid and a base react together to produce a salt and water.
Acid-base pair
An acid-base pair is a pair of two species that transform into each other by the gain or loss of a proton.
Strong acid
A strong acid is an acid that completely dissociates in solution.
Weak acid
A weak acid is an acid that partially dissociates in solution.
pKa
pKa=-log Ka
Ka using pKa
Ka=10-pKa
Ka using concentrations
[H+(aq)][A-(aq)]
Ka= ———————
[HA(aq)]
Ka for weak acids using concentration
[H+(aq)]2
Ka= ———————
[HA(aq)]
Kw
Kw is the ionic product of water.
Kw = [H+(aq)][OH-(aq)]
At 25 °C, Kw = 1.00 x 10-14 mol2 dm-6
Buffer solution
A buffer solution is a mixture that minimises pH changes on addition of small amounts of acid or base.
[H+]
[H+] = Ka ( [HA]/ [A-]) [H+] = 10^-pH
pH
pH= pKa + log [A-]
————-
[HA]
pH= -log([H+])
Equivalence point
The equivalence point is the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution (it depends on the solutions).
End point
The end point is the point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator. The colour at the end point is midway between the colours of the acid and conjugate base form (it depends on the indicator).
How can you calculate H^+ concentration using pH?
[H^+]= 10^-pH
Does a bigger Ka value mean the pH is more or less acidic?
The bigger the value the more acidic.
Does a bigger pKa value mean the pH is more or less acidic?
The bigger the value the less acidic.
How do you calculate the H^+ concentration for strong acids and weak acids?
strong acid- [H^+] = [HA]
weak acid- [H^+] = ([HA]Ka)^1/2
How would you calculate the pOH of these stronger alkalis?
Ca(OH)2
NaOH
Ca(OH)2= -log([OH^-]x2)
NaOH= -log([OH^-])
How can you deduce the pH using pOH?
14 = pH + pOH
Name the conjugate acid-base pairs in the following reaction…
HCl+ OH^- →H2O + Cl^-
HCl+ OH^- →H2O + Cl^-
A1 B2 A2 B1
What is a conjugate acid-base pair?
Contains two species that can be interconverted into each other by transfer of a proton.
What is the formula of a hydronium ion?
H3O^+
Write an equation to show the dissociation in HCl when added to water.
HCl + H2O → H3O^+ + Cl^-
←
The simpler version is typically written as...
HCl → H^+ + Cl^-
←What’s the formula of carbonic acid?
H2CO3
What’s the formula of hydrogen carbonate?
HCO3^-
What are the products of this equation?
acid + metal
salt + hydrogen
What are the products of this equation?
acid + carbonate
salt + water + carbon dioxide
What are the products of this equation?
acid + base
salt + water
What is an alkali?
dissociates to release OH^- ions in aqueous solutions
What are the products of this equation?
acid + alkali
salt +water
50cm^3 of 0.1 moldm^-3 of hydrochloric acid is diluted to 100cm^3 with water. What is the change in pH?
100/50 = 2
- 1/50 = 0.05
- log (0.1) = 1.00
- log (0.05) = 1.30 - 30 - 1.00 = 0.30 which is the change in pH
What does the strength of an acid describe?
The proportion of dissociation of the acid.
What’s the units of Ka?
moldm^-3
What does the [H^+] depend upon for weak acids?
- The concentration of the acid, [HA]
- The acid dissociation constant, Ka
How can Ka be deduced using equilibrium concentrations?
Ka = [H^+]eqm [A^-]eqm / [HA]eqm Ka = [H^+]eqm [A^-]eqm / [HA]start - [H^+]eqm
What are the approximations made so that the following calculation can be used for some weak acids…
Ka = [H^+]^2/ [HA]
Approximation 1:
HA dissociates to produce equal equilibrium concentrations of H^+ and A^-. There’s a small [H^+] from the dissociation of water, but this is so small that it is neglected compared to the [H^+] from the acid.
[H^+]eqm = [A^-]
Approximation 2:
[HA]eqm = [HA]start - [H^+]eqm
As the dissociation of weak acids is small you can assume that [HA]start»_space; [H^+] and you can neglect any decrease in the concentration of HA from dissociation.[HA]eqm = [HA]start
When do the approximations break down so you can’t use the following calculation…
Ka = [H^+]^2/ [HA]
- Approximation 1 is that [H^+]eqm = [A^-] . It breaks down for very weak acids or very dilute solutions because the dissociation of [H^+] from water will be significant compared to that of the acid.
- Approximation 2 is that [HA]eqm = [HA]start. It breaks down for stronger weak acids or very dilute solutions because the [H^+] becomes significant so there is a real difference between [HA]eqm and [HA]start.
What factor impacts Ka?
Temperature.
What is special about pure water at 25 degrees C that can be used to deduce the concentration of [H^+] or [OH^-]?
Kw = 1x10^-14
[H^+] = [OH^-]
so
[H^+] or [OH^-] = (1x10^-14)^1/2
When is a buffer solution most effective?
When [HA] = [A^-] so pH = pKa
The operating pH of the buffer is typically over about 2 pH units, centred at the pH of the pKa value.
What pH does blood plasma need to be maintained at?
pH 7.35-7.45
What is the typical pH of healthy blood.
7.40
What happens if blood pH falls below 7.35?
A condition called acidosis develops which can cause fatigue, shortness of breath and in extreme cases, shock or death.
What happens if blood pH rises above 7.45?
A condition called alkalosis develops which can cause muscle spasms, light headaches and nausea.
What buffer system is in place in the blood?
carbonic acid- hydrogen carbonate buffer system
Describe a pH titration curve when a base is added to an acid.
- When the base is first added, the acid is in great excess and the pH increases very slightly.
- As the vertical section is approached the pH starts to increase more quickly as the acid is used up more quickly.
- Eventually the pH increases rapidly during the addition of a very small volume of base (a few drops), producing the vertical section.
- after the vertical section the pH will rise slightly as the base is now in excess.
Where is the equivalence point on a pH titration curve?
It is the centre point of the vertical section of the pH curve.
Describe an equation that represents the indicator methyl orange and label the colours associated with each part.
HA → H^+ + A^-
←
red orange yellow
How do you choose the correct indicator for a neutralisation reaction?
The end point of the indicator must occur within the same pH range of the vertical section of the graph. Ideally the end point and equivalence point would be the same, but this is not always possible.
why is no indicator suitable for weak acid-weak base titrations?
There is no vertical section and even at its steepest the pH requires several cm^3 to pass through a typical pH indicator range of 2pH units.
