Acid-base Equilibria

Question 1

What is a Brønsted-Lowry acid?

Answer 1

Proton donor

Question 2

What is a Brønsted-Lowry base?

Answer 2

Proton acceptor

Question 3

What is a conjugate acid?

Answer 3

A species which has gained a proton in an acid-base reaction is the conjugate acid, as it can lose the proton again if the reaction runs backwards.

Question 4

What is a conjugate base?

Answer 4

A conjugate base is a species which has lost a proton in an acid-base reaction, so can gain the proton back if the reaction runs backwards.

Question 5

What are conjugate pairs?

Answer 5

A base and its conjugate acid or an acid and its conjugate base

Question 6

Why is the enthalpy of neutralisation similar for all strong acid/strong base reactions?

Answer 6

When strong acids and bases are in solution, they are fully dissociated, so when they react, the enthalpy change comes from the formation of one mole of water, which has the same value, regardless of which acids or bases you use.

Question 7

Why is the enthalpy of neutralisation different for reactions involving weak acids/weak bases rather than strong acids/bases?

Answer 7

For weak acids and weak bases, their enthalpies of neutralisation are less exothermic as in order to neutralise them, the acid needs to dissociate, the ions must be hydrated and then the water can be formed. The dissociation is endothermic, which makes the overall process less exothermic. As different weak acids have different dissociation enthalpies, the enthalpy of reaction varies for different weak acid/base reactions.

Question 8

What is pH?

Answer 8

pH=-log[H+]

Question 9

How would you calculate the pH of a strong acid?

Answer 9

Work out the concentration of the acid, then see if this is the same as the [H+] (for monoprotic acids, it is the same, for diprotic (e.g. H2SO4) multiply by 2, for triprotic (e.g. H3PO4) multiply by 3 etc.). Use the formula pH=-log[H+] to calculate pH

Question 10

How do you find the pH of a weak acid?

Answer 10

For a weak acid, the dissociation equilibrium is HA —> H+ + A-
The acid dissociation constant is given as Ka = [H+][A-]/[HA]
Work out the [HA] and multiply by Ka to find [H+]^2 (assuming [H+] and [A-] are equal and [HA]initial = [HA]eqm). Square root this value and plug into pH formula to find the pH.

Question 11

What is the ionic product of water?

Answer 11

Kw=[H+][OH-]
This comes from the equation for the dissociation of water H2O —> H+ + OH- (since H2O is a pure liquid it is not included in the equation). At 298K (standard temperature), Kw=1x10^-14 mol^2 dm^-6

Question 12

How do you find the pH of a strong base?

Answer 12

Calculate the concentration of the base and multiply if necessary to get the [OH-]. Divide Kw by [OH-] to get [H+]. Sub into pH equation to find the pH. If the temperature is 298K, you can also find [OH-] then negative log to find the pOH. Subtract the pOH from 14 to find the pH.

Question 13

How do you find the pH of a weak base?

Answer 13

Use Kb in a similar way to Ka to find the [OH-] then use Kw to find the [H+]. Sub this into the equation for pH to find the pH.

Question 14

What is pKa?

Answer 14

pKa=-logKa

Question 15

How does the pKa value relate to how strong/weak an acid is?

Answer 15

High pKa = weaker
Low pKa = stronger

Question 16

How do you work out the pH of diluted acids/bases?

Answer 16

Use v1 x c1 = v2 x c2 (the number of moles of acid/base is unchanged when you dilute). Once you have the new concentration, work out the pH as normal (method depends on if you have a strong or weak acid or base).

Question 17

How does the pH change if you dilute a strong acid by a factor of 10?

Answer 17

Increases by 1

Question 18

How does the pH of a weak acid change if you dilute by a factor of 100?

Answer 18

Increase by 1

Question 19

What are titration curves?

Answer 19

Titration curves plot pH against volume of acid/base added in a titration. The straight portion of the graph occurs when the same number of moles of acid and alkali have been added (the equivalence point). This section should pass through pH 7, but this may not occur at the centre of the straight portion depending on how weak/strong the acids and bases are relative to each other.

Question 20

What is the half-equivalence point in an acid-base reaction?

Answer 20

The half-equivalence point is when the acid/base has been half-neutralised (to work this out you can use a titration curve (half-equivalence point will occur at half the volume of the equivalence point) or using a pH meter (keep adding acid/base until pH 7, then add the same moles of acid/base that you started with to get to half-neutralisation). At this point, the pH=pKa, which can be used to work out the pH of the solution.

Question 21

What is the buffer region of a titration curve?

Answer 21

The buffer region appears if weak acids or bases are used. This is because, if you add base to a weak acid, the acid will dissociate more (equilibrium shifts right) to increase the [H+] back to its starting value to maintain the Ka value. This means that the pH does not change very much until the buffer is broken (i.e. the acid becomes fully dissociated). A similar logic can be applied to adding acids to weak bases.

Question 22

How does the shape of a titration curve change with the strength of the acids and bases used?

Answer 22

The stronger the acid, the lower down the curve starts (if you are adding base to acid) or the lower down it will finish (if you are adding acid to base). The stronger the base, the higher the curve ends (for base to acid) or starts (for acid to base). A stronger acid/base will have a flatter line until the equivalence point, while a weaker acid or base will have more of a curve.

Question 23

How do you determine which indicator to use for a titration?

Answer 23

Do the titration and plot a titration/pH curve. The pH values on the straight portion of the graph should be within the range of the colour change of your chosen indicator. For example, if you use a strong acid, methyl orange will work, if you use strong base, phenolphthalein will work. For weak acid/weak base titrations, there are no indicators which work, so a pH meter is required. The values for the colour changes of each indicator is given in the data booklet.

Question 24

What is a buffer solution?

Answer 24

A buffer solution is a solution which minimises changes in pH when small amounts of acid or alkali are added.

Question 25

What are acidic and alkaline buffers made of?

Answer 25

Acidic buffers consist of an acid and its conjugate base. Alkaline buffers consist of a base and its conjugate acid.

Question 26

How do buffers work?

Answer 26

For an acidic buffer, adding alkali shifts the dissociation equilibrium to the right to produce more H+, meaning the pH remains constant. Adding acid means the equilibrium shifts left to produce more HA, so the pH remains constant. Similar logic can be applied to alkaline buffers.

Question 27

What is an example of a buffer system in the body?

Answer 27

The blood is pH buffered at around 7.4. It is very important that this pH is maintained or your enzyme-controlled reactions (which is most metabolic reactions) will not occur and you would die. HCO3- + H+ <—> CO2 + H2O. If the blood becomes too acidic, the equilibrium shifts right and the pH would decrease back to a normal level. If the blood becomes too alkaline, the equilibrium shifts to the left to produce more H+.

Question 28

How do you calculate the pH of a buffer formed from an acid and its conjugate base?

Answer 28

Work out [HA] and [A-] from given information and sub into Ka to find [H+] then find pH. [H+] and [A-] are not equal in a buffer system as the H+ only comes from the acid dissociation, the A- comes from the salt and the acid dissociation.

Question 29

How do you calculate the pH of a buffer formed by adding an alkali to a weak acid?

Answer 29

Work out initial moles of the acid and alkali. The acid dissociation equilibrium will shift to the right when the alkali is added, so the mol HA eqm=mol HA initial - mol alkali. Use molar quantities in the Ka expression to get the [H+] (you don’t need to calculate concentrations as the volumes cancel out). Use pH equation to find the pH.

Question 30

How do you calculate the pH when you add a small amount of acid/alkali to a buffer?

Answer 30

Work out the moles of the buffer and the acid/alkali. Work out which way the buffer equilibrium will shift and use this to work out equilibrium moles of each species. Sub this into the Ka equation and use the [H+] to work out pH.

Question 31

What does it mean to break a buffer?

Answer 31

If you add more acid/alkali to a buffer than it can neutralise, the buffer will be broken (pH changes will no longer be resisted). E.g. if you had a buffer with 0.1 moles of a weak acid and 0.15 moles of its salt, you could add 0.1 moles of OH- or 0.15 moles of H+ before it breaks.

Question 32

How do you work out the volumes of each solution required to make a buffer of a certain pH?

Answer 32

Work out H+ from the desired pH and do Ka/[H+] = [A-]/[HA]. This gives you the ratio of A- to HA. Do the total volume divided by the sum of both parts of the ratio and multiply by each part to get the volume of each solution required (these volumes should sum to the total volume).