1 Aluminium hydride was at one time investigated by the US Air Force in the 1960s as a potential rocket fuel. With air, aluminium hydride forms aluminium oxide and water; with water it forms aluminium hydroxide and hydrogen.

(a)(i) Write down the equation to show the reaction of aluminum trihydride with water.

.................................................................................................................................................

(ii) Write down the equation to show the reaction of aluminium trihydride with oxygen.

.................................................................................................................................................

(b)(i) Use the standard molar enthalpies of formation shown below to calculate the enthalpy change for the reaction between aluminium trihydride and an excess of oxygen under standard conditions.

Species Hf/ kJ mol-1

AlH3 (g) +123

Al2O3 (s) -1677

H2O (l) -286

(ii) Comment on the potential use of aluminium trihydride as a rocket fuel and the likely disadvantage of having aluminium oxide as a combustion product.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

1

Class Reg Number

Candidate Name .......................................................................

Chemistry H2 9746Tutor TuteeRevision Exercise 10: Miscellaneous

2 The explosive RDX (C3H6N6O6) is a white crystalline solid first prepared in 1899, but was not used as an explosive until 1920 by a German chemist called Herz. RDX, like most other chemical explosives, contains oxygen and nitrogen, together with the oxidisable elements (fuels) carbon and hydrogen.

(a)(i) During the explosion, the nitrogen and oxygen atoms combine with the carbon and hydrogen atoms to form carbon monoxide and nitrogen.

Write an equation showing the explosive decomposition of RDX into carbon monoxide, steam and nitrogen.

.................................................................................................................................................

(ii) Use the enthalpies of formation shown below to calculate the standard molar enthalpy change for the explosive decomposition of RDX.

Compound Hf/ kJ mol-1

C3H6N6O6 (s) +62

CO (g) -111

H2O (g) -242

(iii) Data tables usually quote the enthalpy of formation for chemical explosives in kJ kg-1. Convert the enthalpy of formation of RDX from kJ mol-1 to kJ kg-1.

[The relative molecular mass of RDX is 222 u]

(b) A mixture is made of RDX and ammonium nitrate, such that the oxygen balance is zero, that is, the only products from its explosion are carbon dioxide, water and nitrogen.

(i) By considering how many oxygen atoms each RDX molecule needs, and how many oxygen atoms each formula unit of ammonium nitrate can give when it forms nitrogen and water, write a balanced equation for the reaction.

.................................................................................................................................................

2

(ii) Hence, calculate the percentage by mass of ammonium nitrate in the mixture.

(iii) Why do you think designers of underground explosive mixtures are particularly concerned with calculating oxygen balances?

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

(c)(i) When heated gently by itself, ammonium nitrate decomposes into ‘laughing gas’ and steam. Write down an equation showing the decomposition.

.................................................................................................................................................

(ii) Name another important commercial use for ammonium nitrate, other than as an explosive additive.

.................................................................................................................................................

3 Fe3O4 is a dark blue solid. An intense colour like this is typical of compounds that have the same metal in two different oxidation states. Fe3O4 occurs naturally as the minerals magnetite and lodestone. The former name indicates its unusual property of being ferromagnetic. Its tendency to align in a particular direction was known, in about the first century BC to the Chinese who used it as a simple compass on ships.

(a) Fe3O4 can be formed by passing hydrogen gas over heated iron(III) oxide. Write down the equation for this reaction.

.................................................................................................................................................

(b) Calculate the H for the reaction using the enthalpies of formation Hf, shown below.

Compound Hf/ kJ mol-1

H2O (l) -286

Fe3O4 (s) -1117

Fe2O3 (s) -822

3

(c) What type of redox reagent is hydrogen gas behaving as? Explain your answer.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

(d) Suggest a simple physical test and its results that could be performed to distinguish the reactant from the product.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

4 Borane, BH3, might be expected to be the simplest hydride, since boron has three valence electrons to share with three hydrogen atoms. However, this compound cannot be isolated and the simplest boron hydride is B2H6.

(a) B4H10 is best prepared by storing B2H6 under pressure at 25oC for ten days.

(i) Write a balanced equation for this process assuming the other product of the reaction is hydrogen.

.................................................................................................................................................

(ii) In an experiment, it was found that 1.12 g of B4H10 produced 0.162 g of B5H11. Calculate the percentage yield of B5H11. What volume of hydrogen, in cm3, would be produced for this yield?

(b) Another compound, pentaborane(5), B5H9, was also considered as possible rocket fuel because of its high standard molar enthalpy of combustion. Pentaborane(5) can be made form diborane according to the equation below.

5B2H6 (g) 2B5H9 (g) + 6H2 (g)

(i) State Hess’s Law.

.................................................................................................................................................

.................................................................................................................................................

4

(ii) Given the following standard molar enthalpies of combustion, use Hess’s Law to calculate the enthalpy change for the synthesis of pentaborane(5) according to the above equation.

Equation Hc/ kJ mol-1

2B5H9 (g) + 12O2 (g) 5B2O3 (s) + 9H2O (l) -9015

B2H6 (g) + 3O2 (g) B2O3 (s) + 3H2O (l) -2152

H2 (g) + ½O2 (g) H2O (l) -286

5 Coffee contains chlorogenic acid, C16H18O9, which behaves effectively as a weak monoprotic (monobasic) acid according to the equation shown below.

C16H18O9 (aq) H+ (aq) + C16H17O9- (aq)

[Ka = 3.00 x 10-4 mol dm-3]

(a) Explain what is meant by the term monoprotic (monobasic).

.................................................................................................................................................

.................................................................................................................................................

(b) For 250 cm3 of solution containing 3.54 g of chlorogenic acid,

(i) Calculate the concentration of chlorogenic acid in mol dm-3.

(ii) Calculate the concentration of hydrogen ions in mol dm-3.

5

(iii) Calculate the pH of the chlorogenic acid solution.

(d) A 15 cm3 sample of the chlorogenic acid solution is then titrated with 0.02 mol dm-3

aqueous sodium hydroxide.

(i) Calculate the volume of sodium hydroxide solution needed to just react with the chlorogenic acid present.

(ii) Predict whether at the equivalence point the solution is acidic, basic or neutral. Explain your answer.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

(e) Sketch a graph showing the change in pH when 0.1 mol dm-3 aqueous chlorogenic acid solution is gradually added to 10 cm3 of 0.1 mol dm-3 aqueous barium hydroxide solution.

6 Alkanes are important hydrocarbons since they are used as fuels in homes and in industry. It is important that the enthalpy changes involved in alkane reactions are known.

(a)(i) Define the term enthalpy change of formation of a compound.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

6

(ii) Write the equation, including state symbols, that accompanies the enthalpy change of formation of hexane, C6H14(l).

.................................................................................................................................................

(b) The standard enthalpy change of formation of hexane is –199 kJ mol–1.

Using the axes below, show the enthalpy profile diagram for the formation of hexane.On your diagram label the enthalpy change of reaction, H, and the activation energy, Ea.

(c) Enthalpy changes can be calculated using enthalpy changes of combustion. The table below shows some values for standard enthalpy changes of combustion.

Use these values to calculate the standard enthalpy change of the reaction below.

C(s) + 2H2(g) CH4(g)

7

(d) The equations for the combination of gaseous atoms of carbon and hydrogen to form methane, CH4, and ethane, C2H6, are shown below.

C(g) + 4H(g) CH4(g) H = −1652 kJ mol−1

2C(g) + 6H(g) C2H6(g) H = −2825 kJ mol−1

Use these data to calculate:

(i) the bond enthalpy of a C−H bond

(ii) the bond enthalpy of a C−C bond.

7 Many industrial processes, used to manufacture important chemicals, involve equilibrium reactions. Chemists use their understanding of rates of reaction and of yields at equilibrium to find the most economic conditions for the reactions.

Chemists were investigating the production of a chemical, X2Y, that could be formed from X2 and Y2 as shown in equilibrium 7.1 below.

2X2(g) + Y2(g) 2X2Y(g) equilibrium 7.1

(a) State Le Chatelier’s principle.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

(b) State and explain the effect on equilibrium 7.1 of a decrease in pressure on:

(i) the equilibrium position of the reaction,

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

8

(ii) the rate of the reaction.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

(c) The chemists measured the percentage conversion of X2 at various temperatures. The results are shown in the graph below.

(i) Use the graph to predict the percentage conversion at 350°C.

.................................................................................................................................................

(ii) The forward reaction in equilibrium 7.1 is exothermic. Explain how the graph supports this statement.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

(d) The chemists decided to use a catalyst in the process. State, and explain, the effect of using a catalyst on:

(i) the rate of conversion of X2 and Y2 into X2Y,

.................................................................................................................................................

.................................................................................................................................................

(ii) the percentage conversion at equilibrium of X2 and Y2 into X2Y.

.................................................................................................................................................

.................................................................................................................................................

9

8 Until recently, large amounts of bromomethane, CH3Br, were being sprayed onto agricultural land to kill pests, weeds and micro-organisms that affected crop growth. An international agreement has now been reached to stop production of bromomethane, because it was realised that its contribution to ozone depletion is more significant than previously thought.

(a) Name the homologous series that bromomethane belongs to.

.................................................................................................................................................

Bromomethane is produced by reacting hydrogen bromide with methanol. The hydrogen bromide for this process is made industrially by reacting hydrogen and bromine.

(b) Suggest a source for the hydrogen used in this process.

.................................................................................................................................................

(c) The bromine can be obtained from sea water by the steps outlined in the table below.

Use information from the table above to answer the following questions.

(i) In step 1, chlorine, Cl2, is added to convert the bromide ions to bromine molecules.

Write the ionic equation for this reaction.

.................................................................................................................................................

(ii) In step 3, the reaction below takes place.

Br2 (g) + SO2 (g) + 2H2O (l) H2SO4 (aq) + 2HBr (aq)

Give the oxidation state of sulphur before and after the reaction. Explain how you can tell that the sulphur has been oxidised.

Before ....................................

After ....................................

.................................................................................................................................................

10

(iii) Calculate the concentration of bromide ions in sea water at the start of step 1, giving your answer in mol dm-3.

(iv) From your answer to (c)(iii), work out how many times more concentrated the Br- ion is in the solution at step 3 than in the original sea water sample.

(v) Give the colour of the bromine after step 4 of the process.

.................................................................................................................................................

(d) The hydrogen bromide for this process is made industrially by passing a mixture of hydrogen and bromine vapour over a heated catalyst.

Using ideas of collision theory, explain how the use of a catalyst makes the reaction occur more quickly.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

(e) International agreements, to reduce the amount of halogenated compounds being prodiced, concentrated initially on CFCs. The release of bromomethane at the Earth’s surface was not thought to be as serious as the release of CFCs.

Explain how CFCs cause the breakdown of the ozone layer and why bromomethane has a smaller effect.

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

11

9 Many copper minerals are found in hydrothermal deposits where they were formed by crystallisation from very hot solutions which were trapped underground at high pressures.

One such copper mineral is chalcopyrite, CuFeS2, which contains both copper and iron in the +2 oxidation state. The mineral is smelted in modern works by heating with air:

CuFeS2 + 10.5O2 4Cu + 2FeO + Fe2O3 + 8SO2

(a) Write the electron configurations, in terms of s, p and d electrons for

(i) a copper atom

.................................................................................................................................................

(ii) a Cu2+ ion

.................................................................................................................................................

(b) Copper is reduced in this reaction. Complete the chart of oxidation states below to show two elements that are oxidised in the reaction.

(c) Calculate the percentage of copper by mass in a sample of rock containing 0.50% by mass of chalcopyrite, assuming this is the only source of copper in the rock.

(d) Suggest an environmental problem which arises as a result of your answer to (c).

.................................................................................................................................................

.................................................................................................................................................

.................................................................................................................................................

End of Paper

12