ORGANIC ACIDS

Exam Objectives• describe the useful chemical and physical properties of

organic acids, such as pH, acid behaviour and miscibility

Organic Acids• You should be familiar with Ethanoic Acid – the main

component of vinegar• This is the most common of the ORGANIC ACIDS which

can also be called CARBOXYLIC acids• These are acids made from hydrocarbons – the mainly

contain Carbon and Hydrogen• Other examples include:

• Methanoic acid – aka formic acid, found in bee stings • Lactic acid – aka 2-hydroxypropanoic acid, found in milk and made

during anaerobic respiration• Citric acid – aka 3-carboxy-3-hydroxypentanedioic acid, found in

citrus fruits and sherbet

Properties of Organic Acids• They are weak acids (NB: this is different from

concentration, you can be a weak acid but very concentrated)

• Like alcohols, the smaller organic acids (like the examples given earlier) are miscible in water.

• Larger organic acids are only soluble in organic solvents like alcohols or oily compounds

• The smaller acids are liquids at room temperature, while the larger ones are solid – the BP increases as the number of C atoms increases

What is a weak acid?• pH measures how strong an acid is by how many H+ ions

are dissolved in water.• A strong acid has a low pH and lots of H+ ions, while a

weak acid has a higher pH (but still less than 7) and fewer H+ ions.

• So, hydrochloric acid is a strong acid, while organic acids are weak acids

In the picture, a strong acid has lots of free H+ ions, while in the weak acid more of the H+ ions are still attached to the negative ions. Unless they are free, the H+ ions won’t react, and so the acid is said to be weaker.

Hydrogen Bonding• This is a brief explanation, but helps you understand the

difference in properties of the alkanes compared to the carboxylic acids

• Alcohols, acids and Esters all form hydrogen bonds.• These are stronger than the forces that alkanes have

between the molecules.• This means they can be soluble in water, and have higher

boiling points than similar alkanes.

The picture shows hydrogen bonds between water molecules because it contains O and H. Acids, alcohols and esters also contain O and H atoms and this means they also have hydrogen bonding.

Hydrogen Bonding and Miscibility• When an acid is very long (more

than 4 carbons), it cannot form enough hydrogen bonds to be stable – it becomes hydrophobic

• This means its solubility in water decreases

• However, with longer carbon chains it can form bonds with organic solvents which are also made up of lots of carbons.

• It now becomes more soluble in organic solvents than water.

Reactions of Organic Acids• Are the same as for the inorganic acids we already know• Acid + Alkali Salt + water• Acid + Metal Salt + hydrogen• Acid + Metal Carbonate Salt + Carbon Dioxide + water• Acid + Metal Hydrogencarbonate Salt + CO2 + water

• The only thing that changes is the name:• Ethanoic acid + sodium hydroxide sodium ethanoate +

water• Methanoic acid + potassium carbonate potassium

methanoate + carbon dioxide

+ water

Questions to practice• Imagine you are carrying out the following tests. Write

down what your observations would be and explain them. It might be easier to put this in a table:

• A Test a little ethanoic acid with universal indicator. What is the pH?

• B Fill a tube about one-third full with dilute ethanoic acid. Add a piece of magnesium. Note your

• observations and test the gas that is given off.• C Fill a tube about one-third full with dilute ethanoic acid.

Add a spatula of sodium hydrogen• carbonate. Note your observations and test the gas that is

given off.• D Repeat step C using sodium carbonate.

RESULTS TABLEReaction with ethanoic

acidObservations

(including any tests for gases made)

Explanation

A: test with UI

B: add magnesium

C: add sodium hydrogen carbonate

D: add sodium carbonate

Finish off by writing a word and balanced chemical equation for the reactions in B, C and D