The Henderson-Hasselbalch Equation and Aromatic Carboxylic Acids

Question 1

What does the pH of a solution relate to?

Answer 1

How many hydronium ions there are in a solution.

Question 2

What will a high level of hydronium ions do to the equilibrium?

Answer 2

By Le Chatelier’s Principle, more H⁺ ions will force the equilibrium back to HA (or HB⁺ depending on system)

Question 3

What will a low level of hydronium ions do to the equilibrium?

Answer 3

By Le Chatelier’s Principle, less H⁺ ions will force the equilibrium towards A^- (or B)

Question 4

What determines how far equilibrium will shift in response to a change in a number of hydronium ions?

Answer 4

The equilibrium constant (pK_a)

Question 5

What does it mean if the pH is less than the pK_a of the compound?

Answer 5

When the pH is less than the pK_a, this means the acid is largely undissociated, and therefore fewer H⁺ ions exist in the solution. According to LeChatelier’s principle, that means there is more HA in solution

Question 6

What does it mean if the pH is greater than the pK_a of the compound?

Answer 6

When the pH is greater than the pK_a, there are more H⁺ ions in the solution. According to LeChatelier’s principle, this means there is more A^- in the solution.

Question 7

How can we figure out which form predominates?

Answer 7

We can figure out which form predominates using the Henderson-Hasselbalch equation: pH=pK_a+log([A^-]/[HA])

Question 8

When is pH=pK_a?

Answer 8

When [A^-} and [HA] are equal (or [B] and [HB⁺])

Question 9

What is it important to remember when considering induction and resonance?

Answer 9

If you’re considering one, you need to consider the other.

Question 10

Alcohols are not normally acidic, but a phenol (alcohol with a benzene ring) is different, why?

Answer 10

The electrons in the lone pair of oxygen can be delocalized into the ring by resonance. This puts a positive charge on the oxygen, which repels the hydrogen and makes it more likely to be donated. Hence phenols are acidic

Question 11

What two things are important to remember about the transfer of electrons?

Answer 11

All substituents either donate electrons or withdraw electrons via induction

All substituents either donate electrons or withdraw electrons via resonance

Question 12

How can lone pairs interact with benzene rings?

Answer 12

If a substituent directly attached to a benzene has a lone pair, the lone pair can be delocalized into the ring. This places a positive charge on the electronegative atom donating lone pair and makes the proton more easily donated. It also puts more electrons into the ring, which will decrease the acidity of a carboxylic acid attached or increase basicity of an amine attached

Question 13

How can double and triple bonds interact with benzene rings?

Answer 13

If the substituent attached to the benzene ring is also double or triple bonded to a more electronegative atom, the p electrons from the ring are delocalised onto the substituent These groups result in fewer electrons in the ring, these will mean that a carboxylic acid attached will be more acidic as charge moves to the basic group. In the case of an amine, it will not want to accept a positively charged proton as there will be repulsion

Question 14

How do the induction and resonance of lone pairs with benzene rings relate?

Answer 14

Groups with lone pairs adjacent to the ring donate electrons into the ring by resonance and withdraw electrons inductively, donation of electrons by resonance is just much greater than the inductive withdrawal of electrons

Question 15

How do alkyl groups donate electrons?

Answer 15

Alkyl groups only donate electrons inductively

Question 16

How do groups with delta positive charges interact with benzene rings?

Answer 16

Groups with a delta positive charge with draw electrons from the ring

Question 17

How do strongly electron-withdrawing group withdraw electrons?

Answer 17

Strongly electron withdrawing groups withdraw electrons from the ring both inductively and by resonance

Question 18

What kind of substituents are halogens?

Answer 18

Weakly electron withdrawing. They donate electrons into a benzene ring by resonance. The withdraw electrons from the ring via induction, the withdrawal of electrons inductively is just much greater than the donation of electrons by resonance.

Question 19

What is aromaticity a measure of?

Answer 19

Aromaticity is a measure of stability and it really relates to the conjugation of a system.

Question 20

What conditions must be satisfied for a system to be fully-conjugated?

Answer 20

A fully-conjugated system has to be cyclic, it has to be planar, and it must have full conjugation, meaning every atom must but conjugated to its neighbour. To do this we need p orbitals

Question 21

What does a small pK_a number suggest?

Answer 21

A strong acid.

Question 22

What does it mean if you call a base a strong acid?

Answer 22

You are talking about the conjugate acid.

Question 23

What occurs when an acid dissociates?

Answer 23

For any acid, it will dissociate into its conjugate base and hydronium ions

Question 24

What is the extent to which an acid dissociates determined by?

Answer 24

The extent to which an acid dissociates is determined by the pK_a. This depends on where the equilibrium is positioned. If it’s very far to the right, you’ll have a very small pK_a

Question 25

At equilibrium, what is the formula for the equilibrium constant?

Answer 25

At equilibrium, we can write the equilibrium constant, which is [product]/[substrate] (If you take the log of that number, it’s called the pK_a)

Question 26

When the pH of the surrounding environment is lower than the pK_a, what does the pH depend on?

Answer 26

When the pH of the surrounding environment is lower than the pK_a then the pH depends on the Hydronium ions in the solution.

Question 27

What does the pK_a of a compound depend on?

Answer 27

The pK_a depends on the properties of the compound: how does it dissociate?

Question 28

What happens if you add an acid to a reaction at equilibrium?

Answer 28

If you add acid, this lowers the pH, forcing equilibrium to the left

Question 29

How can equilibrium be manipulated?

Answer 29

Equilibrium can be manipulated by adding acid or adding base This will change the pH, but not the pK_a. The pK_a is a property, like the boiling point of a compound – constant.

Question 30

What is the relationship between pH and pK_a based on?

Answer 30

The relationship between pH and pK_a is based on the extent of dissociation in the solution. This is known as the Henderson-Hasselbalch equation

Question 31

What does an alkali produce in water?

Answer 31

An alkali produces Hydroxide ions in solution, and will consume the H⁺

Question 32

What happens if you dissociate a pure acid in water?

Answer 32

If you take a pure acid and put it into water, then let it equilibrate, it pH = pK_a

Question 33

What will the log of any number lower than 1 be?

Answer 33

A negative number.

Question 34

If you have more of the conjugate base than the acid in an equilibrium reaction, what will the relationship between pH and pK_a be?

Answer 34

If you have more of the conjugate base than the acid, the pH will be higher than the pK_a