ic3.31.3 weak or strong? © oxford university press 2011 weak or strong?
TRANSCRIPT
IC3.31.3 Weak or strong?
© Oxford University Press 2011
Weak or strong?
IC3.31.3 Weak or strong?
© Oxford University Press 2011
The functional group gives carboxylic acids their acid properties.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
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.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
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)
IC3.31.3 Weak or strong?
© Oxford University Press 2011
Acids that are fully ionised in solution are strong acids.
Nitric acid, sulfuric acid, and hydrochloric acid are all strong acids.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
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)
IC3.31.3 Weak or strong?
© Oxford University Press 2011
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.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
Name and concentration of acid pH
hydrochloric acid, 0.1 mol/dm3 1.0
ethanoic acid, 0.1 mol/dm3 2.9
Two acids with the same concentration are measured for pH.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
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.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
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.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
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.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
1. 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.
IC3.31.3 Weak or strong?
© Oxford University Press 2011
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.