Bronsted-Lowry acids and bases

This question is about the chemistry of compounds containing phosphorus.

Phosphorus forms several acids including H₃PO₄ and H₃PO₃.

H₃PO₄ is a tribasic acid. The equilibria for the dissociations are shown below.

During the equilibria, H₂PO₄^– behaves both as an acid and as a base.

Explain this statement, using the equilibria 1, 2 and 3, as required.

In a H₃PO₃ molecule, the O atoms are covalently bonded to the P atom. The H atoms are bonded to the O atoms.

Draw the structure of a H₃PO₃ molecule, showing all the bonds.

On your diagram, add the values for the O–P–O and P–O–H bond angles.

The systematic name of H₃PO₄ is phosphoric(V) acid.

What is the systematic name of H₃PO₃?

Phosphine, PH₃, is a poisonous gas.

Phosphine reacts with oxygen gas to form phosphorus(V) oxide and water.

Write the equation for this reaction.

Aqueous silver nitrate, AgNO₃, is reduced by PH₃. The unbalanced equation is shown below.

Balance the equation and use oxidation numbers to explain why this is a redox reaction.

Explanation........

When phosphorus(V) chloride, PCl₅, and ammonium chloride are heated together, the compound P₃N₃Cl₆ is formed, together with HCl gas.

P₃N₃Cl₆ has a cyclic structure, like the Kekulé structure of benzene.

Write an equation for the reaction of PCl₅ and ammonium chloride to form P₃N₃Cl₆.

Calculate the percentage by mass of P in P₃N₃Cl₆.

Give your answer to 2 decimal places.

Suggest one example of evidence that could show that P₃N₃Cl₆ has a Kekulé structure rather than a delocalised structure.

In a molecule of P₃N₃Cl₆ all the N and Cl atoms are bonded to P atoms.

Suggest a possible structure for a molecule of P₃N₃Cl₆.

This question is about acids and bases.

Table 20.1 shows the ionic product, K_w, of water at 25 °C and 40 °C.

Table 20.1

Temperature /°C	Kw / mol2 dm–6
25	1.00 10−14
40	2.92 10−14

Calculate the pH of water at 40 °C.

Give your answer to 2 decimal places.

Table 20.1 shows different K_w values at 25 °C and at 40 °C. A student suggests that water is neutral at these temperatures.

Explain why this student is correct.

A student reacts strontium metal with water to make a 250.0 cm³ solution of aqueous strontium hydroxide, Sr(OH)₂. The solution contains 0.145 g of strontium hydroxide.

• Write an equation for the reaction of strontium with water.

• Calculate the pH of this 250.0 cm³ solution of strontium hydroxide at 40 °C. You should refer back to Table 20.1 at the start of (a).

Give your answer to 2 decimal places.

Equation........

Calculation

A student reacts 1.00 g of strontium carbonate, SrCO₃, with an excess of dilute nitric acid, HNO₃. A gas is produced.

Construct the equation for this reaction.

The student then reacts 1.00 g of calcium carbonate, CaCO₃, with an excess of dilute nitric acid, HNO₃.

Explain why the student’s two reactions produce different volumes of gas.

A student reacts an excess of magnesium with 25.0 cm³ of 0.500 mol dm^–3 hydrochloric acid, HC l.

The student also reacts an excess of magnesium with 25.0 cm³ of 0.500 mol dm^–3 ethanoic acid, CH₃COOH.

Construct an ionic equation for the reaction of magnesium with an acid.

Explain why these two reactions of magnesium produce the same volume of gas but at different rates.

Butanoic acid, CH₃CH₂CH₂COOH, is a weak monobasic acid.

Explain what is meant by the term monobasic acid.

A buffer solution is prepared by dissolving 3.39 g of potassium hydroxide in 250 cm³ of 0.376 mol dm^–3 butanoic acid.

This buffer solution has a pH of 5.07 at 25 °C.

Calculate the acid dissociation constant, K_a, of butanoic acid at 25 °C.

Assume that the volume of the solution remains constant at 250 cm³ when the potassium hydroxide is dissolved.

K_a =........ mol dm^–3

A buffer solution has a pH of 4.50.

When a small volume of water is added to this buffer solution, the pH does not change.

Explain why the pH does not change.

Ammonia and water react to set up an acid–base equilibrium.

What are the Brønsted–Lowry acids in the equilibrium mixture?

A H₂O and OH^–

B OH^– and NH₃

C NH₄⁺ and H₂O

D NH₄⁺ and NH₃

Your answer

This question is about two different types of acid found in organic compounds, carboxylic acids and sulfonic acids, as shown in Fig. 6.1.

Complete Table 6.1 to predict bond angles a and b and name the shapes which makes these bond angles in the functional groups of carboxylic acids and sulfonic acids.

Ethanoic acid, CH₃COOH, and methanesulfonic acid, CH₃SO₂OH, are both monobasic acids. The pK_a values are shown in the table.

A student suggests that 1.0 mol dm⁻³ CH₃SO₂OH should have a lower pH value than 1.0 mol dm⁻³ CH₃COOH.

Write an equation, showing conjugate acid–base pairs, for the equilibrium of CH₃SO₂OH with water and explain, with reasons, whether the student is correct.

Label the conjugate acid–base pairs: A1, B1 and A2, B2.

Carboxylic acids and sulfonic acids both form esters.

Sulfonic acid esters can be hydrolysed by aqueous alkali. The equation shows the alkaline hydrolysis of a sulfonic acid ester.

CH₃SO₂OCH₃ + OH⁻ → CH₃SO₂O⁻ + CH₃OH

In the 3 boxes below, add curly arrows to show the mechanism for this reaction.

In the first box, the hydroxide ion acts as a nucleophile.

The equation shows the dissociation of the acid H₃AsO₄ in water.

Which pair is a conjugate acid–base pair?

A H₃AsO₄ and H₂O

B H₂AsO₄⁻ and H₃O⁺

C H₃AsO₄ and H₃O⁺

D H₃O⁺ and H₂O

Your answer

In Step 1, sodium carbonate, Na₂CO₃, makes the reaction mixture alkaline.

Write an ionic equation to show how carbonate ions form an alkaline solution in water.

In Step 2, explain why the mixture must be acidified so that crystals of benzoic acid appear.

Write the overall equation for the preparation of benzoic acid from phenylmethanol.

Use [O] for the oxidising agent.

Calculate the percentage yield of benzoic acid.

Give your answer to 3 significant figures.

In Step 3, describe how the student can recrystallise the impure crystals to obtain pure benzoic acid.

This question is about organic reactions.

Give the systematic name for compound A.

What type of reaction has taken place?

A similar reaction takes place when propanone, (CH₃)₂CO, is mixed with OH^–(aq) ions.

Draw the structure of the organic product of this reaction.

* Many organic reactions use electrophiles as reagents.

Explain the role of electrophiles in organic chemistry.

Your answer should include one reaction of an aliphatic compound and one reaction of an

aromatic compound, including relevant mechanisms.