Chapter 12 Flashcards
What does an acid release?
Protons
What does a base accept?
Protons
What are Bronsted-Lowry acids?
Proton donors
What do Bronsted-Lowry acids release when they are mixed with water?
They release hydrogen ions
What are Bronsted-Lowry bases?
Proton acceptors
When Bronsted-Lowry bases are in solution, what do they grab?
Hydrogen ions from water molecules
What do strong acids do?
They dissociate almost completely in water-nearly all the H+ ions will be released
Give an example of a strong acid:
Hydrochloric acid
What is the equation for the dissociation of hydrochloric acid?
HCl –> H+ + Cl-
Give an example of a strong base:
Sodium hydroxide
What do strong bases do?
They dissociate almost completely in water
What is the equation for the dissociation of sodium hydroxide?
NaOH –> Na+ + OH-
What do weak acid do?
They only partially dissociate -dissociate very slightly in water, so only small numbers of H+ ions form
What is the main difference for the dissociations of strong acids and weak acids equations?
For weak acids an equilibrium forms which lies well over to the left, representing the partial dissociation
What do weak bases do in water?
They only slightly protonate in water, and the equilibrium lies well over to the left
How do acids get rid of a proton?
They can only get rid of a proton if there’s a base to accept them
What does HA + B react to form?
BH+ +A-
What are conjugate pairs?
They are species that are linked to the transfer of a proton
What is the species that has lost a proton?
The conjugate base
What is the species that has gained a proton?
The conjugate acid
In the reaction HCl+H2O –> H30+ + Cl-, what is the conjugate base of HCl?
Cl-
When acids and bases react, what is formed?
A salt and water
If the concentration of H+ ions produced is equal to the concentration of OH- ions produced, what type of solution has been produced?
A neutral solution
What is a neutral solution?
One where (H+) = (OH-)
What is the standard enthalpy change of neutralisation?
The enthalpy change when standard solutions of an acid and a base react together, under standard conditions, to produce 1 mole of water
What is a salt?
A compound consisting of an ionic assembly of anions and cations
What is the standard enthalpy change of neutralisation for weak acids and weak bases to do with?
The reaction between H+ and OH- ions, and the enthalpy to do with dissociation
What happens when strong acids and strong bases react togehter in neutralisation reaction?
As both the strong acids and bases fully dissociate in solution, there is no dissociation enthalpy for the acid or base, just enthalpy for the reaction of the H+ and OH- ions
Why is the standard enthalpy of neutralisation of neutralisation very similar for all the reactions of strong acids and strong bases?
When they react together in neutralisation reactions, it is just the enthalpy for the reaction of H+ and OH- ions, which is very similar for all the reactions of strong acids and strong bases H+ + OH- –> H20
Why does the standard enthalpy change of neutralisation for reactions involving weak acids or bases vary?
The enthalpy of dissociation varies, depending on the acid or base used, as the standard enthalpy change of neutralisation includes enthalpy to do with the reaction between H+ and OH- ions, and the enthalpy change to do with dissocotiation
What is the pH a measure of?
The hydrogen ion concentration of the solution
What is the equation to work out the value of pH?
pH = -log(H+)
What is a monoprotic acid?
Where each mole of acid produces one mole of hydrogen ions
For strong, monoprotic acids, what does the concentration of H+ ions equal?
The acid concentration, as they dissociate fully
What cab polyprotic acids do?
They can lose more than one proton
What does each molecule of a strong diprotic acid release?
Two protons when it dissociates
What is Ka?
The dissocotiation constant
For a weak aqueous acid, what equilibrium do you get?
HA with H+ and A-
For weak acids, is the concentration of H+ ions equal to the acid concentration?
No
For a weak acid, as only a tiny amount of HA disocotiates, what can we assume?
(HA)»_space; (H+) so (HA) at start = (HA) at end
For the dissocotiation of the weak acid HA, what is the equation of Ka?
Ka = (H+)^2 / (HA)
For weak acids, why do we assume (H+) = (A-)?
We assume that the dissocotiation of the acid is much greater than the dissocotiation of water, so we can assume all the H+ ions came from the acid, so (H+) = (A-)
What does the ionic product of water depend on?
The concentration of H+ and OH- ions
What is Kw?
The ionic product of water
What is the equation for Kw?
Kw = (H+)(OH-)
For pure water, what is the ratio of H+ ions to OH- ions?
1:1 ratio
At 298K, what is the value of Kw always?
10^-14
What is pKw?
-logKw
What is pKa?
-logKa
How do you get the value of Ka from pKa?
Ka = 10^-pKa
What is the value of pKw at 298K?
14
What is a pH meter?
A device which tells you the pH of a solution
How do you make sure a pH meter is calibrated correctly?
You place it in deionised water, and adjust the reading so that it is 7.00
Do the same with a standard solution of pH 4 and pH10
What is the pH of 1 moldm^-3 HCl?
0.00
What is the pH of 1 moldm^-3 C2H5COOH?
2.44
What is the pH of 1 moldm-3 NaCl?
7.00
What is the pH of 1 moldm-3 NaOH?
14
What does 10^-pH equal?
(H+)
When a strong acid is diluted by a factor of 10, what happens to the pH?
It increases by 1
When a weak acid is diluted by a factor of 10, what happens to its pH?
The pH increases by 0.5
What do titrations of acids and bases allow you to find?
The exact amount of base required to neutralise and acid
For a titration curve which plots pH against volume of base/acid added, what does the initial pH depend on?
The strength of the acid. A strong acid will have a lower starting pH
What does the titration curve of a strong acid against a strong base look like?
Starts at a low pH, ends at a high pH. Large change in pH at equivalence
What does the titration curve of a strong acid against a weak base look like?
Starts at a low pH, ends at a fairly neutral pH (7-9). Large change in pH at equivalence point, not as large as strong acid against strong base curve.
What does the titration curve of a weak acid against a strong base look like?
Starts at a fairly neutral pH (5-7) ends at a high pH. Large change in pH but not as large as strong acid against strong base curve
What does the titration curve of a weak acid against a weak base look like?
Starts at fairly neutral pH (5-7) and ends at fairly neutral pH (7-9). Change in pH at equivalence point is not very large
What is the point at the centre of the equivalence line for a titration of acid aganist base line?
The equivalence point
What is the equivalence point?
Where the concentration of H+ ions = the concentration of OH- ions. The point where the acid has been neutralised
What does the final pH of a titration curve of acid against base depend on?
The strength of the base
When carrying out a titration, how do you know when your sample has been neutralised?
You use an indicator which changes colour to show when your sample has been neutralised
At which point in your titration do you need the indicator to change colour?
At the end point of your titration, therefore you need to pick an indicator that changes colour over a narrow pH range
What are the two main indicators that are used for acid-base titrations?
Methyl orange and phenophalein
What colour is methyl orange at a low pH?
Red
What colour is phenolphthalein at a low pH?
Colourless
What colour is methyl orange at a high pH?
Yellow
What colour is phenolphthalein at a high pH?
Pink
What is the approximate pH colour change for methyl orange?
3.1 to 4.4
What is the approximate pH colour change for phenolphthalein?
8.3 to 10
For a strong acid-strong base titration, what indicator would you use?
Either methyl orange or phenolphthalein, as there’s a rapid pH change for both indicators
For a strong acid-weak base titration, what indicator would you use?
Only methyl orange as as the pH changes rapidly for the methyl orange pH range
For a weak acid-strong base titration, what indicator would you use?
Phenolphthalein
For a weak acid-weak base titration, what indicator would you use?
Neither, so a pH meter should be used
How can you work out the pKa of a weak acid?
You can use the titration curve for a weak acid/strong base titration. At the half-equivalnce point the pH = pKa
What is the half-equivalence point?
The stage of the titration where half of the acid has been neutralised
For a weak acid that dissociates HA to A- and H+, what does the concentration of HA equal at the half-equivalence point?
Concentration of HA= concentration of A-
At the half equivaence point, what does Ka equal?
The concentration of hydrogen ions, so pKa = pH
What does a pH chart allow you to do in a titration?
It allows you to follow pH changes
What is a buffer?
A solution that minimises changes in pH when a small amount of an acid or base are added
Do buffer solutions only work for small amounts or large amounts of acid and base solutions?
They only work for slight changes
What do acidic buffers contain?
A weak acid and its conjugate base
When creating a buffer solution, what can you mix an excess of a weak acid with?
A strong base
Give an example of a weak acid and strong base you can mix together to form a buffer solution?
Ethanoic acid and sodium hydroxide
Give an example of an equilibrium formed in a buffer solution?
CH3COOH in equilibium with CH3COO- + H+
In a buffer solution, what happens when a small amount of an acid is added to the equilibrium?
The concentration of H+ ions increases. Most of the extra H+ ions combine with the CH3COO- ions to form CH3COOH. This shifts the equilibrium to the left, reducing the H+ ions concentration, lowering it close to its original value so the pH doesn’t change by much
In a buffer solution, what happens when you add a small amount of a base to the equilibrium?
The OH- ion concentration increases. Most of the extra OH- ions react with H+ ions to form water, removing H+ ions from the solution. This causes more CH3COOH to dissociate to form H+ ions, shifting the equilibrium to the right, causing the H+ ion concentration to increase to its original value, so the pH doesn’t change by much
How are alkaline buffers made?
From a weak base and one of its salts
Give an example of 2 compounds that can be used to form an alkaline buffer:
Ammonia solution (base) and ammonium chloride (a salt of ammonia)
When reacting ethanoic acid(weak acid) with sodium hydroxide(strong base) why does the pH change quickly to start?
The base is strong and contains with a lot of H+ ions
When reacting ethanoic acid(weak acid) with sodium hydroxide(strong base) why does the pH change slow down after the start?
A buffer solution of sodium ethanoate in ethanoic acid which resists further dramatic pH change
Why does the pH change drastically at the equivalence point?
All the ethanoic acid is used up and the equivalence point is reached
How is the pH in blood controlled?
Using a carbonic acid-hydrogencarbonate buffer system
How are the levels of H2CO3 controlled?
By respiration
How are the levels of HCO3- controlled?
By the kidneys
What is the equation required to calculate the pH of a buffer solution?
pH = pKa + log (A-/HA)
