Module 5: Acids, Bases and Buffers V2 Flashcards
When nitric acid is added to methanoic acid, an acid-base equilibrium is set up.
Write an equation for this equilibrium and Use this equilibrium to identify and explain what is meant by the term conjugate acid-base pair.
Define what is meant by a “Bronsted-Lowry Acid”
Proton donor ✓
Define what is meant by a “Strong Bronsted-Lowry acid”
Proton donor that completely dissociates ✓
Define what is meant by a “Weak Bronsted-Lowry acid”
Proton donor that partially dissociates ✓
State the pH of an acid that is “strong” and for an acid that is “weak”
Strong acids have a pH <1 ✓
Weak acids have a pH >1 ✓
The strength of an acid is the proportion of dissociation in water ✓
Write an equation to show the dissociation of a hydrochloric acid
strong acid, so completely dissociates
Write an equation to show the dissociation of ethanoic acid
Weak acid, so partially dissociates (equilibrium arrow)
Define what is meant by the “acid dissociation constant”
Measure of the extent of dissociation. ✓
The larger the Ka (or the smaller the pKa), the stronger the acid ✓
Define what is meant by a “Bronsted-Lowry base”
Proton acceptor ✓
Define what is meant by a “alkali”
Releases OH- ions into solution ✓
Define what is meant by a “salt”
When the H+ in an acid is replaced by a metal ion/ ammonium ion ✓
Define what is meant by a monobasic acid
Each molecule of acid can only release one proton ✓
Define what is meant by a dibasic acid
Each molecule of acid can release two protons ✓
Carbonic acid is a weak acid and the dissociation of this acid occurs in two stages.
Write the equation for the dissociation of carbonix acid.
Write a balanced ionic equation for the following reaction
Write a balanced ionic equation for the following reaction
Define what is meant by a “conjugate acid”
A species formed when a proton is added to a base. ✓
Define what is meant by a “conjugate base”
A species formed when a proton is removed from an acid. ✓
Define what is meant by a conjugate acid-base pair.
Two species that differ by H+ (aq). ✓
Identify the conjugate acid-base pairs in the following
Identify the conjugate acid-base pairs in the following
Explain what is meant by a buffer solution
A buffer solution minimises the change in pH when small amounts of acid or alkali are added. ✓
State three features of a buffer solution
A high concentration of a weak acid, HA. ✓
A high concentration of its conjugate base, A-. ✓
A low concentration of H+ ions. ✓
State when a buffer works best
Describe what would happen with the addition of small amounts of acid to the following buffer solution
Describe what would happen with the addition of small amounts of alkali to the following buffer solution
Describe how you would make a ethanoic acid buffer, using a suitable base.
Give the equation for the partial neutralisation.
Weak acid i.e. CH3COOH is in excess ✓
and partially neutralised to make ethanoate ions (salt) with a metal, metal oxide, metal hydroxide or metal carbonate. ✓
Some acid remains. ✓
Explain using equations how a buffer solution forms when excess butanoic acid is mixed with sodium hydroxide and how the buffer controls pH when acid or alkali are added
Explain using equations how a buffer solution forms when excess propanoic acid is mixed with sodium hydroxide and how the buffer controls pH when acid or alkali are added [9]
State the name of the buffer present in blood
carbonic acid-hydrogen carbonate buffer ✓
Describe what would happen with the addition of small amounts of acid to the following buffer solution
Describe what would happen with the addition of small amounts of alkali to the following buffer solution
How does phenolphthalein act as an indicator in acid and alkali conditions.
Sketch a pH curve for:
1. Strong base added to a strong acid
- Strong acid added to a strong base
Sketch a pH curve for:
1. Weak base added to a weak acid
- Weak acid added to a weak base
Sketch a pH curve for
1. Weak base added to a strong acid
- Strong acid added to a weak base
Sketch a pH curve for
1. Strong base added to a weak acid
- Weak acid added to a strong base
A student is conducting a titration.
He can choose two indicators, phenolphtalein (8.2-10 range) or Methyl orange (3.2-4.4 range).
The vertical section (equivalence point) runs from pH 7-10.5.
State and explain which indicator he should use.
Use phenolphthalein as pH range matches vertical section ✓
Colour change matches vertical section ✓
A student is conducting a titration with a weak acid and weak alkali.
He can choose two indicators, phenolphtalein (8.2-10 range) or Methyl orange (3.2-4.4 range).
The vertical section (equivalence point) runs from pH 6.8-7.2
State and explain which indicator he should use.
There is no vertical section and no suitable indicator for titration of a weak acid/weak base. ✓
Use of a pH meter is preferred as the pH changes slowly over the equivalence point. ✓
Explain when a buffer works best
pH = pKa. ✓
This happens when [HA] = [A-] in the buffer ✓
Explain what is meant by an ‘acid-base indicator’
weak acid ✓
The weak acid and its conjugate base, have different colours ✓
The pH of pure water at 60 degrees celcius is 6.52 and at 10 degrees celcius is 7.27
Determine whether the ionisation of water is an endothermic or exothermic process. Is water neutral at these two temperatures?
Endothermic because Kw increases with temperature.
Water is neutral as [H+] = [OH-]
Give the assumption relating the concentrations of OH- and strong bases [XOH]
[XOH] = [OH-]
Note: when dealing with group 2 hydroxides, you have x2[OH-]
For the dibasic acid H2SO4, the first dissociation is complete. The second dissociation has Ka = 1.20 x 10-2 mol dm-3. The pH of 0.01 moldm-3 H2SO4 is 1.84.
i) Suggest why the pH of H2SO4 is not simply –log (2 x 0.01) = 1.70. What approximation has been made for this strong dibasic acid?
ii) Describe the steps that would be needed to calculate the pH of H2SO4 and how this could also be measured experimentally.
The actual pH of 1.84 is a weaker acid than the calculated pH of 1.70 ✓
The second dissociation must not be complete ✓
The approximation for a strong dibasic acid was complete dissociation of both protons ✓
ii) calculate [H+] from both first and second dissociations ✓
Add the [H+] together and calculate the actual pH using pH = -log [H+] ✓
Experimentally this could be done using a pH meter ✓
The pH of 0.1moldm-3 solution of chlorous acid HClO2 is measured as being 1.53 from experiment. Ka = 1.2 x 10-2 moldm-3.
(i) Calculate the pH of chlorous acid using the approximations for weak acids.
(ii) Comment on whether the approximations are justified by finding the percentage difference in [H+] values.
(iii) Suggest when the approximations for a weak acid can be justified
i) pH = 1.46 ✓
ii) Approximation not justified ✓
There is a significant difference (18%) between the [H+] values ✓
iii) Approximation only justified when for weak acids, with a lower Ka value and not for stronger acids with higher Ka values where [H+] is significant ✓
Also only justified when there is no significant dissociation of water ✓
Explain the significance of a large Ka value and a small Ka value
large: Extent of dissociation is large, so the acid is strong ✓
small: Extent of dissociation is small, so the acid is weak ✓
A student decided to carry out a titration using an acid-base indicator.
What important factor does the student need to consider when deciding the most suitable indicator to use for this titration?
pH range of indicator matches vertical section. ✓
What is meant by the strength of an acid? Use an equation for the dissociation of benzoic acid to support your answer.
The strength of an acid is the proportion of dissociation in water ✓
Write a balanced equation and an ionic equation for the reaction between hydrochloric acid and sodium carbonate
Write a balanced equation and an ionic equation for the reaction between nitric acid and potassium hydroxide
Write a balanced equation and an ionic equation for the reaction between ethanoic acid and lithium carbonate
Write a balanced equation and an ionic equation for the reaction between hydrobromic acid and calcium
Write a balanced equation and an ionic equation for the reaction between sulfuric acid and sodium oxide.
Write a balanced equation and an ionic equation for the reaction between sulfuric acid and lithium
Using the data provided, explain whether the dissociation of water is endothermic or exothermic.
Endothermic ✓
Kw increases with temeperature ✓
Many experimental measurements use published data, such as Kw, measured at 25 degrees celcius.
Often these measurements have been taken at different temperatures, especially in experimental work carried out at body temperature.
What is the consequence of this for published scientific work?
Work is invalid. ✓
because Kw varies with temperature. ✓
Write an equation for the dissociation of water.
What is the difference between strong acids and weak acids
Strong acids are proton donors that completely dissociate ✓
Weak acids are proton donors that partially dissociate ✓
Write a balanced equation and an ionic equation for the reaction between nitrous acid and calcium hydroxide.
Nitrous acid, has a smaller acid dissociation constnat, Ka, compared to that of nitric acid.
Complete the equation for the equilibrium that would be set up and label the conjugate acid-base pairs
The following can react as a bronsted-lowry acid in addition to a bronsted-lowry brase. Explain this statement. Use equations for the reaction between this and H+ and with OH- to show how.
