18.1 Further Aspects of Acids & Bases

Ethanoic acid, CH₃COOH (aq) , is titrated with 0.16 mol dm^-3 potassium hydroxide and the following graph is obtained.

Explain why the equivalence point is greater than pH 7 for this titration.

Explain what is meant by a buffer solution and describe where the ‘buffer region’ on the graph would occur. 

Explain the shape of the pH curve up to the equivalence point. 

Explain why potassium hydroxide can act as a Brønsted-Lowry base and Lewis base.

This question is about Brønsted-Lowry acids and bases.
 

Give the meaning of the term Brønsted-Lowry acid. 

Explain the term weak acid.

When an acid and a base react they produce a conjugate base and a conjugate acid. 

Write an equation to show how hydrochloric acid behaves as a strong acid when it reacts with water, and state the role of water in this reaction.

Ethanoic acid is a weak acid. Hydrogen carbonate ions can also act as a weak acid if in an aqueous solution.

Write equations for each of these weak acids at equilibrium.

A solution was made up containing sodium hydrogen carbonate and sodium carbonate. Explain how this solution would act as a buffer if a small amount of acid was added to it. 

Explain how a solution containing ethanoic acid and ethanoate ions can act as a buffer. 

A student performed a titration of 25.0 cm³ of 0.100 mol dm^-3 hydrochloric acid, HCl (aq) , with 0.100 mol of sodium hydroxide, NaOH (aq).
 

Draw the expected pH curve on the graph and indicate the equivalence point for this.

  

Explain why the salt produced in this reaction is neutral.

The student repeated the titration using two different chemicals, 25.0 cm³ of 0.100 mol dm^-3 nitric acid, HNO₃ (aq) , and 0.100 mol dm^-3 ammonia, NH₃ (aq).
 

State the equation for this reaction. 

Explain why the salt produced in this reaction is acidic.

State the equation for reaction between nitric acid and water, this reaction and identify the conjugate acid formed in the reaction. 

The image below shows the hexaaquaaluminium ion, [Al(H₂O)₆]³⁺. Explain why this can behave as an acid.

25.0 cm³ of 0.100 mol dm^-3 propanoic acid, CH₃CH₂COOH (aq) , is titrated with of 0.100 mol dm^-3 sodium hydroxide. The pH curve for this titration is shown below.

Label the equivalence point and half equivalence point on the curve
 

Explain what is meant by the half equivalence point.

The student used the pH range of the indicators to determine which was the best one to use for this titration. 

Using Section 22 of the Data Booklet highlight on the graph the pH range of bromocresol green for this titration.
 

Using Section 22 of the Data Booklet suggest a suitable choice of indicator for this titration and state the colour change you would expect to see. 

The end point of an indicator depends on its pK_a 

Explain the connection between the pH range of an indicator that is a weak acid and
the value of pK_a for the indicator. 

Explain how the student can calculate the K_a of propanoic acid by using the pH curve.

A buffer solution contains a mixture of propanoic acid and its salt. A small amount of nitric acid is added to the buffer.

Write an equation, including state symbols, showing how this buffer can resist the change in pH.