Chemistry 3.9 - Acid Base Equilibria

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Give the Lowry-Bronsted definition of an acid.
A proton donor.
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Give the Lowry-Bronsted definition of a base.
A proton acceptor.
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What is a conjugate acid?
The species formed after a base accepts a proton.
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What is a conjugate base?
The species formed when an acid donates a proton.
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What is a monoprotic acid?
When one mole of an acid gives up one mole of H+ ions.
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What is a diprotic acid?
When one mole of an acid gives up two moles of H+ ions.
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What is a strong acid?
An acid that totally dissociates in water.
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What is a weak acid?
An acid that partially dissociates in water.
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A strong acid and a weak acid can have the same concentration but differing pH values. How?
A strong acid will have a greater number of dissociated H+ ions and so a lower pH.
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A concentrated acid should not be confused with a strong acid and visa versa. Why?
Dilute/concentrated acids refer to concentration, whilst strong/weak acids are to do with dissociation.
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What does concentrated mean?
That a large amount of acid/base has been dissolved in a set volume of water.
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What does dilute mean?
That a small amount of acid/base has been dissolved in set volume of water.
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Will the Ka value for a strong acid be low or high?
HIGH - greater dissociation, so a greater concentration of products.
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Will the Ka value for a weak acid be low or high?
LOW - less dissociation, and so a lower concentration of products.
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Complete the sentence: since the amount of water that dissociates is minimal, the concentration of water can be described as...
CONSTANT!
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At 298K, Kw is a constant. What is the value of this constant?
1 x 10-14 mol2dm-6.
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So, what is the concentration of hydrogen ions in pure water?
1 x10-7 moldm-3.
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What is a buffer?
A solution that resists changes in pH when a small amount of acid or alkali is added.
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What does an acidic buffer consist of?
A weak acid and one of its salts. The weak acid will partially dissociate, setting up an equilibrium which is important for the buffer to function.
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How does an acidic buffer resist changes when a small amount of acid is added?
The position of equilibrium will shift to the left to reduce the change by using the extra H+ to combine with the negative ions and produce more acid molecules. The pH is restored.
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How does an acidic buffer resist changes when a small amount of alkali is added?
The position of equilibrium will shift to the right to reduce the disturbance. This will replace the H+ ions that reacted with the OH- ions and restore the pH.
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What does a basic buffer consist of?
A weak base and one of its salts. The weak base will partially dissociate to produce a negative ion and OH- ions. This is important for the buffer's function.
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How does a basic buffer resist changes when a small amount of acid is added?
If acid is added the position of equilibrium will shift to the right to reduce the disturbance. The OH- ions reaction with the H+ added and so need to be replaced, restoring the pH.
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How does a basic buffer resist changes when a small amount of base is added?
If base is added then position of equilibrium will shift to the left to reduce the disturbance. The excess OH- is used up to return the pH to normal.
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When working out the pH for weak acid buffers, what is used?
Ka. The concentration of the salt (if provided) is used for the negative ion as it fully dissociates.
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When a strong acid reacts with a strong base...
A neutral salt is produced.
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When a strong acid reacts with a weak base...
A SLIGHTLY ACIDIC SALT is produced.
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Use the salt NH4Cl to explain why.
NH4Cl will dissolve, releasing NH4+ ions and Cl- ions. Ammonia is a weak base and so will partially dissociate, setting up an equilibrium in which H+ ions are released. The release of H+ ions makes the solution slightly acidic.
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When a weak acid is added to a strong base...
A SLIGHTLY BASIC SALT is produced.
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Using the example of CH3COONa, explain why.
With CH3COONa, it dissolves to produce CH3COO- ions. These can combine with H+ ions in solution, increasing the pH.
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The following questions are about acid base titration curves, when a BASE IS ADDED to an acid.
.
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Strong acid-strong base graph: Equivalence point?
E.P = pH 7.
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At what pH does the graph start?
pH 1.
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At what pH does the vertical section begin and end?
Starts at pH 2, ends at pH 12.
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At what pH does the graph end?
pH 13.
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What is an equivalence point?
When the number of moles of acid equal the number of moles of alkali.
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Weak acid-strong base curve: equivalence point?
pH > 7
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At what pH does the graph start?
pH 2.
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At what pH does the vertical section begin and end?
Begins at pH 7, ends at pH 12.
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At what pH does the graph end?
pH 13.
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There is a vertical section that begins at pH 4 due to buffering effect. Explain this.
A acidic buffer is formed, which resists changes in pH. This is because unreacted weak acid is present alongside its salt, formed by neutralisation.
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Strong acid-weak base: equivalence point?
pH
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At what pH value does the curve start?
pH 1.
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At what pH does the vertical section begin and end?
Begins at pH 2, ends at pH 7.
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The graph then levels off before increasing again to pH 12. This is due to the buffering effect. Explain this.
A basic buffer is formed. There is weak base and its salt (formed from neutralisation) left in solution.
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Weak acid-weak base graph.
Lacks a clear vertical region, making it much harder to study using an indicator. Instead a pH probe should be used.
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What is an indicator?
A WEAK ACID that changes colour as the pH changes.
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Card 4

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Card 5

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