Proton Transfer Reactions (HL IB Chemistry)

Malonic acid is a naturally occurring acid found in fruits and vegetables and is shown in Figure 1.

Figure 1 

The first dissociation of malonic acid is:

C₃H₄O₄ (aq) + H₂O (l) ⇌ C₃H₃O₄^- (aq) + H₃O⁺ (aq)

Identify one conjugate acid-base pair from the equation.

The equilibrium constant for the first dissociation of malonic acid is 1.48 x 10^-3.

State, with a reason, the strength of malonic acid.

The anion C₃H₃O₄^- may be classified as amphiprotic. Explain the meaning of amphiprotic and write equations, using C₃H₃O₄^-_, to illustrate your answer.

Salicylic acid has the structure shown below in Figure 1.                             

Figure 1

Draw the structure of the conjugate base of salicylic acid, showing all the atoms and all the bonds.

Predict what would be seen if a small amount of copper (II) oxide was added to an aqueous solution of salicylic acid, HOC₆H₄COOH, and warmed.

Write a balanced equation for the reaction.

Suggest, with a reason, whether salicylic acid is likely to be soluble in water.

Determine the relative molecular mass, M_r, of salicylic acid using Table 6 from the Data book.

Write balanced equations to show the separate reactions between ethanoic acid and calcium carbonate, CaCO₃, magnesium oxide, MgO, and aluminium hydroxide, Al(OH)₃.

In Table 1 below, suggest the names and formulae of the acids and bases needed to make the specified salts.

Table 1

The ethanoate ion, CH₃COO^-, carbon dioxide, CO₂, and the ethoxide ion, CH₃CH₂O^-, all contain carbon oxygen bonds. 

Deduce the order in carbon to oxygen bond length from shortest to longest and explain your answer.

Ethanoic acid, CH₃COOH, shows two absorptions in an infrared spectrum that are not present in the spectrum of ethanol.

Using Section 20 of the Data book, state the wavenumber range of these absorptions and the bonds that cause them.

Glycolic acid, C₂H₄O₃, is a colourless, odourless crystalline solid that is highly soluble in water and behaves as a Brønsted–Lowry acid.

The systematic IUPAC name for glycolic acid is 2-hydoxyethanoic acid.

Draw the structural formula for its conjugate base, showing all the atoms and bonds.

Write an equation for the reaction between glycolic acid, C₂H₄O₃, and limescale, CaCO₃. State and explain one observation you would make.

State one reason why you would use glycolic acid to remove the limescale in a kettle at home, but not hydrochloric acid.

An alkaline solution is formed when sodium hydrogencarbonate is dissolved in water.

Write an equation for the reaction and explain why the solution is alkaline.

State whether the HCO₃^- ion is behaving as a Brønsted–Lowry acid or as a base and give a reason for your answer.

Carbon dioxide gas dissolves in rainwater to form carbonic acid. State the formula of the conjugate base of carbonic acid.

Carbonic acid and sulfuric acid can be described as diprotic acids. Explain the meaning of diprotic.

The equilibrium constant for the first dissociation of formic acid is 1.8 x 10^-4 mol dm^-3.

State, with a reason, the strength of formic acid.

Outline one laboratory method used to distinguish between equimolar solutions of formic acid and hydrochloric acid, giving the expected observations.

Formic acid has the chemical formula HCOOH. Identify the conjugate base of formic acid and state whether it is a weak or strong conjugate base.

Draw the structure of formic acid and give its systematic IUPAC name.

The pH of an aqueous solution of salicylic acid at 298 K is 3.85. Determine the concentration of hydroxide ions in the solution, using Section 2 of the Data booklet.

A and B are two solutions of the same concentrations that have pH values of 3 and 6 respectively. 

The variation of conductivity and concentration of a strong and weak monoprotic acid are shown in Figure 1.

Identify the strong and weak acid from the information given and justify your choices.

For acid 1 and acid 2 in part (c) compare the volume of 0.2 mol dm^-3 NaOH required to neutralise 20 cm³ of 0.1 mol dm^-3 solutions of the acids.

The concentrations of solutions of weak acids can be determined by titration against standard solutions of alkalis, such as sodium hydroxide.

A solution of 25.0 cm³ ethanoic acid was titrated against 0.150 mol dm^-3 NaOH (aq) and it was found that 22.35 cm³ of the NaOH was needed for complete neutralisation.

Write an equation for the reaction and determine the concentration of the ethanoic acid.

A solution of 0.1 mol dm^-3 ammonia has a pH of approximately 11. Predict how the pH value of 0.1 mol dm^-3 sodium hydroxide solution would compare and calculate its value.

Write an equation for the reaction between ammonia and water and use it to classify each product as a Brønsted–Lowry acid or base.

Glycolic acid, C₂H₄O₃, is an organic acid sometimes used to remove limescale, CaCO₃, from electric kettles and coffee machines.

Predict, with a reason, a difference in the reaction between the same concentration of sulfuric acid and glycolic acid with samples of calcium carbonate.

Another acid that is sometimes used to descale kettles is sulfamic acid, NH₂SO₃H. Sulfamic is classed as a strong monoprotic acid.

A solution of hydrochloric acid has a pH of 1 and a solution of carbonic acid has a pH of 5. Determine the ratio of hydrogen ion concentrations of hydrochloric acid to carbonic acid.

Outline two ways, apart from using pH, which could allow you to distinguish between two solutions of carbonic acid and hydrochloric acid that have the same concentration.

Four solutions of acids with identical concentrations are prepared. The equilibrium constants of these acids are given in Table 1.

Table 1

Write down the acid dissociation equation for HCN.

Use the information in part (a) to complete this question.

State the name and formula of all the chemical species present in the solution of CH₃COOH.

Write the name and formula of the conjugate base of HF.

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.

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

acid + base  ⇋  conjugate base + 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.

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).

[2]

 

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 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.

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

The end point of an indicator depends on its pK_a 

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.

At 298K, water molecules dissociate into equal quantities of ions, and the pH is 7.

The ionic product of water, K_w, can be used to find the pH of a strong base. Changing the temperature will affect the value for K_w.

Determine the pH of pure water at 40 °.

K_w of pure water at 40 ° is 2.92 x 10^-14 mol² dm^-3

Strong bases fully ionise in water, as shown by the equation of dissociation of sodium hydroxide:

NaOH (aq) →  Na⁺ (aq) + OH^- (aq)

At 298K, K_w is 1 x 10^-14 mol² dm^-6.

Calculate the pH of a 0.05 mol dm^-3  solution of NaOH at 298 K.      

Weak acids do not fully ionise in solution. The acid dissociation constant, K_a is used to determine the hydrogen ion concentration.

Write an expression for the acid dissociation constant, K_a for the acid HA.

The pH of a 0.15 mol dm^-3 solution of HCN is 5.08 at 298 K. Calculate the value of K_a for HCN at 298 K.

Give your answer to two decimal places.

A sample of 0.01 mol dm^-3 butanoic acid has a K_a value of 1.51 x 10^-5 mol dm^-3.

0.50 moles of ammonia was dissolved in water to make a 1.00 dm³ solution. This solution has a hydroxide ion concentration of 6.40 x 10^-3 mol dm^-3. 

The pH curve shown below was obtained when a 0.150 mol dm^–3 solution of sodium hydroxide was added to 25.0 cm³ of an aqueous solution of ethanoic acid. The half equivalence point is where half of the volume of sodium hydroxide required for neutralisation has been added to the ethanoic acid.

A different titration was performed using 0.100 mol dm^-3 ammonia solution, NH₃ (aq) (K_b = 4.75 at 298K) and 25.00 cm³ of 0.100 nitric acid, HNO₃ (aq). 

Calculate the pH of the ammonia solution before it was added to the nitric acid.

The titration is repeated using 0.200 mol dm^-3 sodium hydroxide, NaOH (aq), instead of ammonia.

Determine whether the salt formed in this titration will be acidic, basic or neutral.

Determine the pH of the solution if 150 cm³ of 0.30 mol dm^-3 sodium hydroxide, NaOH (aq), is mixed with 200 cm³ 0.10 mol dm^-3 of nitric acid, HNO₃ (aq).

Monochloroacetic acid, ClH₂COOH, is a skin irritant that is used in “chemical peels” intended to remove the top layer of dead skin from the face and ultimately improve the complexion.

Write an expression for the acid dissociation constant, K_a, of monochloroacetic acid.

Calculate the pH of a 0.05 M solution of monochloric acid. 

The value of K_a for monochloroacetic acid is 1.35 x 10^-3 mol dm^-3

Using Section 2 of the Data Booklet, calculate the value of [OH^-] for the solution of monochloric acid.

Calculate the percentage dissociation for the solution of monochloric acid.

State the relationship between the following expressions for conjugate acid-base pair

Use Table 1 to calculate the following for the conjugate bases at 298 K.

Table 1

A student performs a titration using a 0.10 mol dm^-3 ammonia, NH₃ (aq), and a hydrochloric acid and 0.10 hydrochloric acid, HCl (aq).

Volume of HCl (cm³)

The student repeats the titration with 0.10 mol dm^-3 ethanoic acid, CH₃COOH (aq) which has a pK_a value of 4.76. A sketch of the pH curve obtained is shown below.

Explain why it is difficult to determine the equivalence point for this reaction accurately.

Explain why an ammonium ion can not behave as a Brønsted-Lowry base.

State and explain the acid-base character of aqueous ammonia at 298 K.

Acids can be classed as monoprotic, diprotic and triprotic. Sulfuric acid is a diprotic acid. 

The second ionisation step is for the ionisation of sulfuric acid is as follows.

HSO₄^- (aq) + H₂O (aq) ⇌ SO₄^2- (aq) + H₃O⁺ (aq)

Suggest why the second ionisation step reaches equilibrium.

Sodium hydrogen carbonate solution, NaHCO₃ (aq) , can act as an amphiprotic species. State the equation for the reaction fo NaHCO₃ (aq) with the following compounds:

Using your answer to part a) i) and ii), explain why NaHCO₃ is amphiprotic. 

Phosphine is usually prepared by heating white phosphorus, one of the allotropes of phosphorus, with concentrated aqueous sodium hydroxide.

The equation for the reaction is.

P₄ (s) + 3OH⁻ (aq) + 3H₂O (l) → PH₃ (g) + 3H₂PO₂⁻ (aq)

Identify the amphiprotic species in this reaction giving the formulas of both species it is converted to when it behaves in this manner.

1.68 g of white phosphorus was used to make phosphine

Oxalic acid, H₂C₂O₄ , is a weak diprotic acid and can be used in titrations. State the equation for the reaction of oxalic acid with sodium hydroxide. 

The ionisation of oxalic acid occurs in two steps. State equations for both of these steps. 

Tartaric acid shown below behaves as a Brønsted-Lowry acid when it reacts with calcium hydroxide, Ca(OH)₂. Sketch the structure of the salt formed from this reaction. 

Using ionic equations state how HPO₄^2- can behave as an amphiprotic and amphoteric species.

Gallium oxide behaves as an amphoteric oxide. State two equations to show how gallium oxide reacts with a strong monoprotic acid and strong base.

Reaction with strong monoprotic acid .........................................................................................................................................

Reaction with strong base .........................................................................................................................................

Identify the Br∅nsted-Lowry acids in the following reaction.

CH₃CH₂O^- (aq) + H₂O (l) CH₃CH₂OH (aq) + OH^- (aq)

A solution of hydrochloric acid of concentration 0.001 mol dm^-3 has a pH value of 3. Suggest, giving a reason, the pH of the following solutions of acids:

A solution of 0.01 mol dm^-3 ethanoic acid has a concentration of hydrogen ion of 1 x 10^-4 mol dm^-3.  Determine the percentage of ethanoic acid molecules that have dissociated.

Two separate titrations are carried out using 25.00 cm³ of 0.01 mol dm^-3 solutions of hydrochloric acid followed by ethanoic acid, against 0.01 mol dm^-3 sodium hydroxide. 

State what difference(s) would be observed in the two titrations.  

Suggest a suitable indicator for the titration of hydrochloric acid and sodium hydroxide in part c), and state the colour changes observed.

Show how the ionic product for water is derived from the dissociation of water and give it units.

Determine the pH of 0.001 mol dm^-3 sodium hydroxide.

Suggest, with a reason, how the magnitude of K_w changes with increasing temperature.

Malonic acid is a weak dibasic carboxylic acid with the formula C₃H₄O₄. Draw the displayed structure of malonic acid.

Suggest, with a reason, which of the two acids, ethanoic or malonic, has a higher pH?