1 In the UK, almost all the sulphuric acid, H2SO4, is manufactured by the Contact process.One stage in the Contact process involves the reaction between sulphur dioxide and oxygen.

2SO2(g) + O2(g) 2SO3(g)

Table 1.1 below shows values of the equilibrium constant, Kp, for this equilibrium at different temperatures.

(a) Write an expression for the equilibrium constant, Kp, of this reaction.

(b) In this question, one mark is available for the quality and use of scientific terms.

• The conversion of sulphur dioxide and oxygen into sulphur trioxide is carried out at slightly above atmospheric pressure. Comment on this statement.

• Explain what happens to the equilibrium amounts of SO2, O2 and SO3 as temperature increases at constant pressure.

Deduce the sign of ΔH for the forward reaction in the equilibrium. Explain your reasoning carefully.

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Chemistry H2 9746Tutor TuteeRevision Exercise 9: Miscellaneous

(c) An equilibrium is set up for the SO2, O2, SO3 equilibrium at 400 °C.At this temperature

• the equilibrium partial pressure of SO2 is 10 kPa• the equilibrium partial pressure of O2 is 50 kPa• Kp = 3.0 × 102 kPa–1.

Calculate the equilibrium partial pressure of SO3 at 400 °C. Hence determine the percentage of SO3 in the equilibrium mixture at this temperature.

(d) In the UK, almost all the sulphuric acid manufactured uses sulphur as a starting material for SO2 production. In some countries, metal ores such as zinc sulphide, ZnS, are used instead to form SO2 by heating with air.

(i) Construct a balanced equation to show the reaction that takes place when zinc sulphide is heated in air.

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(ii) Suggest why countries may find it more economic to manufacture sulphuric acid from zinc sulphide.

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2 Natural rain is slightly acidic because of the carbon dioxide dissolved in it. Rain which is more acidic than natural rain is called acid rain and this causes damage to the ecosystem in a number of ways. One effect of acid rain is the leaching of aluminium ions from clay soils into water-courses.

(a) Carbon is in the second period of the Periodic Table. Give the formula of a basic oxide of an element in the same period.

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(b) The main equilibrium that is set up when carbon dioxide dissolves in water is shown below.

H2O(aq) + CO2(aq) H+(aq) + HCO3–(aq)

(i) Write the equation for the acidity constant, Ka, for this reaction and give its units.

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(ii) The numerical value of Ka in the above units is 4.5 × 10–7 at room temperature.Calculate the pH of unpolluted rain where [CO2(aq)] = 1.2 × 10–5mol dm–3.

(c) A clay material, such as kaolinite, reacts with a solution containing hydrogen ions as shown.

Al2Si2O5(OH)4(s) + 6H+(aq) 2Al3+(aq) + 2SiO2(s) + .............

(i) Complete equation 3.2 by writing on the dotted line above. Include the appropriate state symbol.

(ii) An equilibrium constant for this equation is given by the following expression.

Kc =

Show that the value of Kc is approximately 1 × 1016 mol–4 dm12 given that

[Al3+(aq)] = 9 × 10–11 mol dm–3 when the pH = 6.

(iii) This equilibrium is very sensitive to pH.

Calculate the concentration of aluminium ions in solution when[H+] = 1 × 10–4 mol dm–3 (i.e. at pH = 4). Assume Kc = 1 × 1016 mol–4 dm12.

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(d) The ionic radius of an isolated (unhydrated) Al3+ ion is shown below, together withvalues for ions of two other metals in Period 3.

(i) In this question, two marks are available for the quality of the use and organisation of scientific terms.

Explain why the metals form ions with these charges. Say, in terms of their atomic structure, what the ions have in common and explain the trend in ionic radius across the period from Na+ to Al3+.

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(ii) Explain why aluminium ions are the most hydrated of the three ions.

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3 Most of the chemical elements found on Earth were produced in stars. Chemists have arranged the elements into a Periodic Table which allows them to make predictions about the behaviour of the elements and their compounds.

(a)(i) Calcium in Group 2 reacts with water to produce a solution of calcium hydroxide and bubbles of hydrogen gas.

Predict a balanced equation for the reaction of radium, Ra, with water. Include state symbols. Write your equation in the space below.

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(ii) Use your knowledge of atomic structure to explain why the reaction of calcium with water is less vigorous than the reaction of radium with water.

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(b) Many properties such as first ionisation enthalpy vary in a regular way across a period.

(i) Write an equation representing the first ionisation enthalpy for sodium, including state symbols.

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(ii) Explain the general increase in first ionisation enthalpy as the Period 3 (sodium to argon) is crossed from left to right.

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(c) Atoms react together to form molecules in the dense gas clouds in interstellar space.Molecules of H2S, NH3 and OCS (similar in structure to CO2) have been detected.

(i) Complete the dot-and-cross diagram for each molecule in the boxes below.

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(ii) Use the theory of ‘electron pair repulsion’ to decide which of the possible shapes below represents the shape of each molecule.

Write the formula of each of the molecules H2S, NH3 and OCS underneath its shape.

(iii)

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4 The question below relates to chlorides of some of the elements in Period 3 of the PeriodicTable.

(a) Draw ‘dot and cross’ diagrams to show the bonding in magnesium chloride and silicon(IV) chloride. Only draw the outer shell electrons.

(b) Describe the difference in behaviour when magnesium chloride and silicon(IV)chloride are added separately to cold water.

You may include in your answer• the pH of any resulting solution,• relevant chemical equations,• experimental observations,• the name of the process taking place.

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(c) The melting point of magnesium chloride is much higher than that of silicon(IV) chloride.

Explain this difference in terms of structure and bonding.

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(d) Hot aluminium reacts with dry chlorine to give a white compound which has a relative molecular mass of 267.

(i) Deduce the molecular formula for the white compound.

(ii) Write an equation for the reaction between aluminium and dry chlorine.

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(iii) Explain why solid aluminium chloride does not conduct electricity, but when aluminium chloride is added to water, the resulting solution will conduct electricity.

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(e) Phosphorus reacts with excess chlorine to form a compound with an empirical formulaPCl5. The solid compound has positive and negative ions.

The positive ion has the formula PCl4+.

The formula of the negative ion includes one phosphorus atom.

Suggest the formula of the negative ion.

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5 The carbonates and nitrates of Group 2 elements decompose when heated.

(a) Calcium oxide is manufactured by the decomposition of calcium carbonate.

(i) Write the equation for this decomposition.

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(ii) Explain why the decomposition temperature of calcium carbonate is much lower than that of barium carbonate.

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(b) Barium nitrate decomposes when heated to make barium oxide, nitrogen dioxide and oxygen.

2Ba(NO3)2(s) → 2BaO(s) + 4NO2(g) + O2(g)

(i) Use oxidation states to explain why this decomposition reaction involves both oxidation and reduction.

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(ii) Calculate the enthalpy change of reaction, Hr, in kJ mol–1, for the thermal decomposition of barium nitrate using the enthalpy changes of formation, Hf, given in the table.

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(c) A student investigates the volume of gas formed when barium nitrate is heated.The diagram shows the apparatus the student uses.

(i) A 1.31 g sample of barium nitrate is completely decomposed.

Calculate the volume, in cm3, of gas formed at room temperature and pressure.1 mol of gas molecules occupies 24 000 cm3 at room temperature and pressure.

(ii) Suggest one problem that the student may encounter when carrying out the investigation.

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(d) Barium nitrate has a higher decomposition temperature than calcium nitrate. One of the reasons for this is the difference between the lattice enthalpy of barium oxide and that of calcium oxide.

(i) Explain what is meant by the term lattice enthalpy.

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(ii) Explain why the lattice enthalpy of barium oxide is much less exothermic than that of calcium oxide.

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6 The element titanium, Ti, atomic number 22, is a metal that is used in the aerospace industry for both airframes and engines.

A sample of titanium for aircraft construction was analysed using a mass spectrometer and was found to contain three isotopes, 46Ti, 47Ti and 48Ti. The results of the analysis are shown in Table 1.1 below.

(a)(i) Explain the term isotopes.

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(ii) Complete the table below for atoms of two of the titanium isotopes.

(b) Using the information in Table 1.1, calculate the relative atomic mass of this sample oftitanium.Give your answer to three significant figures.

(c) Complete the electronic configuration of a titanium atom.

1s22s22p6 ...............................................................................................................................

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(d) Titanium has metallic bonding.

(i) Explain what is meant by metallic bonding. Use a diagram in your answer.

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(ii) How does metallic bonding allow titanium to conduct electricity?

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(e) A student reacted 1.44 g of titanium with chlorine to form 5.70 g of a chloride X.

(i) Determine the empirical formula of X.

(ii) Construct a balanced equation for the reaction between titanium and chlorine.

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(iii) At room temperature, X is a liquid which does not conduct electricity. What does this information suggest about the bonding and structure in X?

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7 The Group 2 element radium, Ra, is used in medicine for the treatment of cancer. Radium was discovered in 1898 by Pierre and Marie Curie by extracting radium chloride from its main ore pitchblende.

(a) Predict the formula of radium chloride.

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(b) Pierre and Marie Curie extracted radium from radium chloride by reduction.

Explain what is meant by reduction, using this reaction as an example.

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(c) Radium reacts vigorously when added to water.

Ra(s) + 2H2O(l) Ra(OH)2(aq) + H2(g)

(i) Use the equation to predict two observations that you would see during this reaction.

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(ii) Predict a pH value for this solution.

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(d) Reactions of the Group 2 metals involve removal of electrons. The electrons are removed more easily as the group is descended and this helps to explain the increasing trend in reactivity.

(i) The removal of one electron from each atom in 1 mole of gaseous radium atoms is

called the ...............................................................................................................

The equation for this process in radium is:

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(ii) Atoms of radium have a greater nuclear charge than atoms of calcium.

Explain why, despite this, less energy is needed to remove an electron from a radium atom than from a calcium atom.

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8 A student had a stomach-ache and needed to take something to neutralise excess stomach acid. He decided to take some Milk of Magnesia, which is an aqueous suspension of magnesium hydroxide, Mg(OH)2.

(a) The main acid in the stomach is hydrochloric acid, HCl(aq), and the unbalanced equation for the reaction that takes place with Milk of Magnesia is shown below.

.....Mg(OH)2(s) + .....HCl(aq) → .....MgCl2(aq) + .....H2O(l)

Balance the equation by adding numbers where necessary in the unbalanced equation above.

(b) The student’s stomach contained 500 cm3 of stomach fluid with an acid concentration of 0.108moldm–3. The student swallowed some Milk of Magnesia containing 2.42 g Mg(OH)2. He wondered whether this dose was sufficient to neutralise the stomach acid. Assume that all the acid in the stomach fluid was 0.108moldm–3 hydrochloric acid.

(i) Calculate the mass of Mg(OH)2 necessary to neutralise this stomach fluid.

(ii) Determine whether the student swallowed too much, too little, or just the right amount of Milk of Magnesia to neutralise the stomach acid.

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(c) Chewing chalk has been used for many years to combat excess stomach acid and indigestion tablets often contain calcium carbonate, CaCO3. Suggest, with the aid of an equation, how these tablets work.

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9 All the carbonates of the elements in Group 2 thermally decompose to form oxides.

(a) Write the equation, including state symbols, for the thermal decomposition of magnesium carbonate.

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(b) What is the trend in the ease of thermal decomposition of Group 2 carbonates?

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(c) Arrange the following compounds in order of their lattice enthalpy. Put the most exothermic first.

barium carbonatebarium oxide

magnesium oxide

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Explain your answer.

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(d) Aluminium carbonate does not exist at room temperature.Suggest why in terms of polarisation.

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10 One of the largest uses of phosphorus is in boxes of safety matches. A safety match ignites when it is rubbed against the striking surface of the match box.The friction between the match head and the striking surface generates enough heat for the phosphorus to burn. This in turn provides enough energy for the decomposition of potassium chlorate(V), KClO3, on the match head.

2KClO3(s) 2KCl(s) + 3O2(g)

Sulphur on the match stick ignites and sufficient heat is generated to ignite paraffin wax and then the wood in the match.

(a) When the phosphorus burns, phosphorus(V) oxide forms. What is the formula of phosphorus(V) oxide?

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(b) Calculate the volume of oxygen, measured at room temperature and pressure, that forms when 0.368 g of potassium chlorate(V) is decomposed.One mole of any gas occupies 24 000 cm3 at room temperature and pressure.

(c) Suggest why the match head contains potassium chlorate(V).

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(d) Write the equation for the reaction between sulphur and oxygen.

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(e) Phosphorus(V) oxide is a simple molecular oxide.

(i) Suggest a physical property of phosphorus(V) oxide.

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(ii) Suggest a chemical property of phosphorus(V) oxide.

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11 The compound FeSO4.7H2O can be used to kill moss in grass. Iron(II) ions in a solution ofFeSO4.7H2O are slowly oxidised to form iron(III) ions.

(a) Describe a test to show the presence of iron(III) ions in a solution of FeSO4.7H2O.

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(b) The percentage purity of an impure sample of FeSO4.7H2O can be determined by titration against potassium dichromate(VI), K2Cr2O7, under acid conditions, using a suitable indicator.

During the titration, Fe2+(aq) ions are oxidised to Fe3+(aq) ions.

• Stage 1 – A sample of known mass of the impure FeSO4.7H2O is added to a conical flask.

• Stage 2 – The sample is dissolved in an excess of dilute sulphuric acid.• Stage 3 - The contents of the flask are titrated against K2Cr2O7(aq).

(i) The reduction half equation for acidified dichromate(VI) ions, Cr2O72–, is as follows.

Cr2O72–(aq) + 14H+(aq) + 6e– 2Cr3+(aq) + 7H2O(l)

Construct the balanced equation for the redox reaction between Fe2+(aq), Cr2O72–(aq) and

H+(aq).

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(ii) In Stage 1, a student uses a 0.655 g sample of impure FeSO4.7H2O.

In Stage 3, the student uses 19.6 cm3 of 0.0180moldm–3 Cr2O72– to reach the end-point.

One mole of Cr2O72– reacts with 6 moles of Fe2+.

Calculate the percentage purity of the impure sample of FeSO4.7H2O.

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12 Methanoic acid, HCOOH, is a weak organic acid which occurs naturally in ants and stinging nettles.

(a) Use an equation for the dissociation of methanoic acid to show what is meant by a weak acid.

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(b) A 1.50 × 10–2 mol dm–3 solution of HCOOH has [H+] = 1.55 × 10–3 mol dm–3.

(i) Calculate the pH of this solution and give one reason why the pH scale is a more convenient measurement for measuring acid concentrations than [H+].

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(ii) Write the expression for Ka for methanoic acid.

(iii) Calculate the values of Ka and pKa for methanoic acid.

(iv) Estimate the percentage of HCOOH molecules that have dissociated in thisaqueous solution of methanoic acid.

(c) A student titrated the 1.50 × 10–2 mol dm–3 methanoic acid with aqueous sodium hydroxide.

A 25.00 cm3 sample of the HCOOH(aq) was placed in a conical flask and the NaOH(aq)was added from a burette until the pH no longer changed

(i) Write a balanced equation for the reaction between HCOOH(aq) and NaOH(aq).

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(ii) Part of the pH curve for this titration is shown below.

Calculate the concentration, in mol dm–3, of the aqueous sodium hydroxide.

(iii) Calculate the pH of the aqueous sodium hydroxide.Kw = 1.00 × 10–14 mol dm–3

(iv) The pH ranges in which colour changes for three acid-base indicators are shown below.

Explain which of the three indicators is suitable for this titration.

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13 The preparation of hydrogen iodide, HI(g), from hydrogen and iodine gases is a reversible reaction which reaches equilibrium at constant temperature.

H2(g) + I2(g) 2HI(g)

(a) Write the expression for Kc for this equilibrium.

(b) A student mixed together 0.30 mol H2(g) with 0.20 mol I2(g) and the mixture was allowed to reach equilibrium. At equilibrium, 0.14 mol H2(g) was present.

(i) Complete the table below to show the amount of each component in the equilibriummixture.

(ii) Calculate Kc to an appropriate number of significant figures. State the units, if any.

(c) The student compressed the equilibrium mixture so that its volume was reduced. The temperature was kept constant.

Comment on the value of Kc and the composition of the equilibrium mixture under these new conditions.

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(d) The student repeated the experiment at a higher temperature and found that less HI was present at equilibrium.

Explain what additional information this tells you about the reaction.

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(e) Hydroiodic acid, HI(aq), is a strong acid that is an aqueous solution of hydrogen iodide.In the laboratory, hydroiodic acid can be prepared by the method below.

(i) Construct a balanced equation, with state symbols, for the preparation of hydroiodic acid.

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(ii) Determine the percentage yield of hydroiodic acid.

(iii) Calculate the pH of the hydroiodic acid fraction.

14 This question looks at some chemicals found in food.1 mole of gas molecules occupies 24.0 dm3 at room temperature and pressure.The Avogadro constant = 6.02 × 1023 mol–1.

(a) A chemistry student bought a bar of chocolate. The student looked on the label and found that the main ingredient listed was ‘sugars’, making up 47.0% by mass of the 43.0 g chocolate bar. Throughout this question you can assume that all the sugars are present as sucrose, C12H22O11.

(i) How many sucrose molecules were in the bar of chocolate?

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A mixture of 480 g of iodine and 600 cm3 of water was put into a flask. The mixture was stirred and hydrogen sulphide gas, H2S(g), was bubbled through for several hours.

The mixture became yellow as sulphur separated out. The sulphur was filtered off and the solution was purified by fractional distillation. A fraction of HI(aq) was collected containing 440 g of HI in a total volume of 750 cm3.

(ii) The student ate the bar of chocolate. The standard enthalpy change of combustion of sucrose is –5640 kJ mol–1. On food labels, the energy content is measured in Calories.

1 Calorie = 4.18 kJ.

• Write an equation for the chemical change involved in the standard enthalpy change of combustion of sucrose.

• How much energy, in Calories, is available to the student from the sugars in the chocolate bar?

(b) An oxide of nitrogen is used as the propellant in whipped cream. This oxide contains 63.64% N by mass, and has a density of 1.833 g dm–3 at room temperature and pressure.

What is the molecular formula of this gas? Show your working.

(c) Chocolate mousse contains gelatine and a compound to promote fast setting of the mousse.

Compound A is such a setting agent. It has two acidic hydrogen atoms per molecule and is one of the six acids listed below.

oxalic acid HOOCCOOHmalonic acid HOOCCH2COOHsuccinic acid HOOC(CH2)2COOHglutaric acid HOOC(CH2)3COOHadipic acid HOOC(CH2)4COOHpimelic acid HOOC(CH2)5COOH

The student analysed a sample of compound A by titration.

The student dissolved 2.82 g of compound A in water and made the solution up to 250 cm3

in a volumetric flask. He titrated 25.0 cm3 of this solution with 0.175 mol dm–3 NaOH. 22.05 cm3 of NaOH were required for complete neutralisation.

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Use the results of the student’s analysis to identify compound A from the list above.Show all of your working.

15 Barium, Ba, was discovered by Davy in 1808. The element gets its name from the Greek ‘barys’ meaning ‘heavy’.

The table below compares some properties of barium with caesium.

(a)(i) Why do caesium and barium have different atomic numbers?

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(ii) State the block in the Periodic Table in which caesium and barium are found.

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(iii) Explain why the atomic radius of barium is less than the atomic radius of caesium.

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(iv) Predict and explain whether a barium ion is larger, smaller or the same size as a barium atom.

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(b) Barium reacts with water in a redox reaction.

Ba(s) + 2H2O(l) Ba(OH)2(aq) + H2(g)

(i) Explain, in terms of electrons, what is meant by oxidation.

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(ii) Which element has been oxidised in this reaction? Deduce the change in its oxidation number.

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(iii) Sulphuric acid was added to aqueous barium hydroxide until the solution was just neutralised, forming the insoluble salt, BaSO4, and water.

Ba(OH)2(aq) + H2SO4(aq) BaSO4(s) + 2H2O(l)

The electrical conductivity of the solution steadily decreased as the sulphuric acid was added.

Explain why the electrical conductivity decreased.

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(c) Barium metal can be extracted from barium oxide, BaO, by reduction with aluminium.

6BaO + 2Al 3Ba + Ba3Al2O6

Calculate the mass of barium metal that could be produced from reduction of 500 g of barium oxide using this method.

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