Reactions of acids

This question is about redox reactions and electrode potentials.

The flowchart shows some reactions of barium and its compounds.

• Write balanced equations for Reaction 1 and Reaction 2.

• Identify the type of reaction in Reaction 3.

Reaction 1: equation........

Reaction 2: equation........

Reaction 3: type of reaction........

Potassium iodate tablets prevent the uptake of radioactive iodine in the human body following a nuclear accident.

The mass of potassium iodate(V), KIO₃, in a tablet can be determined by reaction with an aqueous solution of potassium iodide, K I, in the presence of acid.

A chemist finds that two KIO₃ tablets react with exactly 26.2 cm³ 0.150 mol dm^–3 KI.

Calculate the mass, in mg, of KIO₃ in one tablet.

Give your answer to the nearest whole number.

Standard electrode potentials are measured by comparison with a reference half-cell.

Draw a labelled diagram to show how the standard electrode potential could be measured for the redox system below.

Include details of the apparatus, solutions and the standard conditions needed when measuring this standard electrode potential.

Standard conditions........

Many electric vehicles are powered by lithium-ion cells.

Hydrogen-oxygen fuel cells can also be used to power vehicles.

Six redox systems are shown in the table. State symbols have been omitted.

Redox system	Half-equation	E / V
1	Li+ + e– Li	–3.04
2	2H2O + 2e– H2 + 2OH–	–0.83
3	2H+ + 2e– H2	0.00
4	O2 + 2H2O + 4e– 4OH–	+0.40
5	Li+ + CoO2 + e– LiCoO2	+1.16
6	O2 + 4H+ + 4e– 2H2O	+1.23

A lithium-ion cell involves redox systems 1 and 5.

Construct the overall cell equation for a lithium-ion cell.

Hydrogen-oxygen fuel cells can operate in acidic or in alkaline conditions.

Show that for acidic and alkaline hydrogen–oxygen fuel cells, the standard cell potentials, and the overall cell equations, are the same.

Acidic........

Alkaline........

Oil of wintergreen is a liquid used in medicine to relieve muscle pain.

Compound H is a component in oil of wintergreen and can be synthesised from compound G, as shown below. The boiling point and density of compound H are stated.

Compound G
Compound H boiling point = 222 °C density = 1.174 g cm⁻³

A student prepares a sample of compound H by the method below.

Step 1 Reflux 8.97 g of compound G for 30 minutes with an excess of methanol in the presence of a small amount of sulfuric acid as a catalyst.

Step 2 Add an excess of aqueous sodium carbonate, Na₂CO₃(aq). Two layers are obtained.

Step 3 Purify the impure compound H that forms from the resulting mixture.

The student follows this method and obtains 5.32 g of pure compound H.

Why does the student use reflux in Step 1 ?

In Step 2, Na₂CO₃(aq) removes the sulfuric acid catalyst and any unreacted compound G from the mixture.

Write equations for this removal.

Removal of sulfuric acid

Removal of unreacted compound G

Compound G

Another student suggests that adding aqueous sodium hydroxide would be more effective in removing the sulfuric acid catalyst than Na₂CO₃(aq).

Comment on whether the student’s suggestion is an improvement for the preparation of compound H.

Calculate the percentage yield of compound H.

Give your answer to three significant figures.

Describe how to purify the impure compound H from the two layers in Step 2.

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.

This question is about nitric acid, hydrochloric acid and sulfuric acid.

Dilute nitric acid reacts with aluminium oxide to form a solution of aluminium nitrate.

Write an equation for this reaction.

The solution contains nitrate ions, NO₃^–.

Draw a ‘dot-and-cross’ diagram for the NO₃^– ion.

Use a different symbol for the extra electron.

Balance the half-equation for the oxidation process in this reaction.

In the reduction process in this reaction, HNO₃ and H⁺ react together to form 2 oxides: X (M_r = 30) and Z (M_r = 18).

Determine the formulae of X and Z and write the half-equation for this reduction.

* Three reactions involving sulfuric acid are shown below.

Reaction 1 Dilute sulfuric acid is reacted with nickel(II) hydroxide to form a green solution.

The solvent is allowed to evaporate leaving hydrated crystals of compound D, with the percentage composition by mass: Ni, 22.33%; S, 12.20%; O, 60.87%; H, 4.60%.

Reaction 2 Concentrated sulfuric acid is reacted with hydrogen bromide, HBr, to form three products:

• an element which exists as diatomic molecules

• a gaseous compound E

• a liquid.

At RTP, 1.00 dm³ of compound E has a mass of 2.67 g.

Reaction 3 Concentrated sulfuric acid acts as a catalyst when 2-hydroxypropanoic acid reacts to form compound F (M_r = 144).

In this reaction, 2 mol of 2-hydroxypropanoic acid forms 1 mol of compound F and 2 mol of water.

Identify compounds D, E and F and construct equations for the reactions.

Show structures for any organic compounds.

This question is about the reactions of Group 2 metals and their compounds.

A student adds magnesium to dilute hydrochloric acid in one test tube. The student adds calcium to dilute hydrochloric acid in a second test tube.

A redox reaction takes place in each test tube.

Suggest two observations from the student’s experiment that would show that calcium is more reactive than magnesium.

Write half-equations for the reaction of magnesium with hydrochloric acid.

Oxidation half-equation:........

Reduction half-equation:........

A sample of barium oxide is added to distilled water at 25 °C. A colourless solution forms containing barium hydroxide, Ba(OH)₂.

The solution is made up to 250.0 cm³ with distilled water. The pH of this solution is 13.12.

Determine the mass of barium oxide that was used.

Give your answer to 3 significant figures.

10 cm³ of dilute sulfuric acid is added to 10 cm³ of the colourless solution of Ba(OH)₂

Write an ionic equation, including state symbols, for the reaction.

Limestone and huntite are two calcium minerals.

A typical sample of limestone contains 95.0% by mass of calcium carbonate, CaCO₃

Fertiliser Z, Ca₅NH₄(NO₃)₁₁•10H₂O (M_r = 1080.5 g mol^–1) can be made from limestone.

Calculate the mass, in g, of limestone needed to make 1.50 kg of fertiliser Z.

Give your answer to 3 significant figures.

Huntite is a carbonate mineral with the chemical formula Mg₃Ca(CO₃)₄.

Huntite reacts with dilute hydrochloric acid to produce bubbles of a gas and a colourless solution.

Construct the equation for the reaction. Include state symbols.

In Step 1, the student observed that bubbles of gas were produced.

Suggest the formula of the copper(II) compound which reacted with HNO₃ to form the gas, and write a full equation for the reaction.

Formula:........

Equation:........

Write an ionic equation, including state symbols, for the reaction in Step 3.

Suggest a suitable indicator for this titration and state the colour change at the end point in Step 4.

Indicator:........

Colour from........ to........

Determine the percentage, by mass, of copper in the copper ore. Give your answer to an appropriate number of significant figures.

Explain whether the calculated percentage by mass of copper would be higher, lower or the same if the following changes were made to the method.

The potassium iodide was not in excess, in Step 3.

The burette readings were read from the top of the meniscus, in Step 4.

The student then modifies the method in order to obtain a more accurate value for the percentage by mass of copper in the ore. The student decides to use 25.00 g of the copper ore in Step 1.

What further modifications should the student make to produce a more accurate value for the percentage by mass of copper in the ore?

A student carries out an investigation to identify two metals, M and X, by two different methods.

In Stage 1, a redox reaction takes place between M and HC l (aq), forming hydrogen and a solution containing M in the +2 oxidation state.

Write an overall equation, with state symbols, for this reaction. Write half-equations for the oxidation and reduction processes.

Overall equation........

Oxidation half-equation........

Reduction half-equation........

In Stage 1, suggest two observations that would confirm that all of metal M has reacted.

In Stage 3, write the ionic equation for the reaction taking place in the titration.

Metal M can be identified following the steps below.

1. The amount, in mol, of excess HC l (aq) that remains after the reaction of M with HC l (aq).

2. The amount, in mol, of HC l (aq) that reacted with M.

3. The identity of metal M.

Analyse the results to identify metal M.

The student is provided with the carbonate of an unknown metal, X₂CO₃.

The student measures the mass loss when the X₂CO₃ is reacted with an excess hydrochloric acid. The equation is shown below.

X₂CO₃(s) + 2HCl (aq) → 2XCl (aq) + CO₂(g) + H₂O(l)

The reaction is carried out using this method:

Step 1 Add 100 cm³ HCl (aq) to a conical flask and weigh.

Step 2 Add X₂CO₃ to the conical flask and immediately reweigh.

Step 3 After 5 minutes, reweigh the conical flask and contents.

Calculate the amount, in mol, of CO₂ released in the reaction.

After analysing the results, the student was told that their molar mass of X₂CO₃ was incorrect.

The student evaluated the experiment for possible reasons for the incorrect result.

The student wondered whether the reaction was complete when the mass was recorded after 5 minutes (Step 3).

How could the student modify the experimental procedure to be confident that the reaction was complete?

The student finds out that carbon dioxide is slightly soluble in water.

State and explain how the solubility of CO₂ would affect the calculated molar mass of X₂CO₃.