Weak or strong?

1. Weak or strong?

2. The functional group gives carboxylic acids their acid properties.

3. When an ethanoic acid molecule dissolves in water, a hydrogen ion may leave its functional group. • The acid molecule has split up to form ions. It has ionised.

4. Hydrochloric acid is completely ionised in solution. • Every hydrogen chloride molecule has split up to make hydrogen ions and chloride ions. • The acid is fully ionised. • HCl(g) + (aq)  H+(aq) + Cl–(aq)

5. Acids that are fully ionised in solution are strong acids. • Nitric acid, sulfuric acid, and hydrochloric acid are all strong acids.

6. Carboxylic acids are different. • Fewer than 1% of their molecules ionise when they dissolve in water. • This means they are weak acids. • The equilibrium for the solution of ethanoic acid lies towards the left:  CH3COOH(l) + (aq) CH3COO–(aq) + H+(aq)

7. A solution is acidic if its pH is less than 7. The lower the pH, the more acidic the solution. • pH is a measure of the concentration of hydrogen ions, H+, in solution. • The greater the concentration of hydrogen ions, the lower the pH.

8. Two acids with the same concentration are measured for pH.

9. The hydrochloric acid has a lower pH. • It is more acidic. • This is because all the hydrogen chloride molecules are ionised. • The concentration of hydrogen ions is relatively high. • Universal indicator turns red.

10. The ethanoic acid has a higher pH. • It is less acidic. • This is because fewer than 1% of the ethanoic acid molecules are ionised. • The concentration of hydrogen ions is relatively low. • Universal indicator turns orange.

11. Remember: • There is a difference between a strong acid and a concentrated acid. • A strong acid, such as hydrochloric acid, ionises completely in solution. • A concentrated acid has a greater amount of acid dissolved in a certain volume of solution than a more dilute solution of the same acid.

12. Predict which will have the lower pH: (a) a 1 mol/dm3 solution of ethanoic acid or (b) a 2 mol/dm3 solution of ethanoic acid. Solution (b) because it has a greater amount of acid dissolved in it, and so it has a higher concentration of H+ ions and is more acidic.

13. 2. Predict which will have the lower pH: • (a) a 1 mol/dm3 solution of propanoic acid or • (b) a 1 mol/dm3 solution of sulfuric acid. • Solution (b) because sulfuric acid molecules ionise much more than propanoic acid molecules, and soit has a higher concentration of H+ ions and is more acidic.