Energy Cycles in Reactions (DP IB Chemistry: SL): Exam Questions

Nitrogen oxides produced by combustion are largely nitrogen monoxide or nitrogen dioxide.

i) Draw Lewis diagrams for nitrogen monoxide and nitrogen dioxide.

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ii) Using the diagrams, explain the meaning of the term free radical.

Platinum and rhodium are found in catalytic converters and facilitate the conversion of Carbon monoxide and nitrogen monoxides to nitrogen and carbon dioxide.

i) Write an equation for this reaction.

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ii) State the changes in oxidation state for each carbon and nitrogen.

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Using your answer to part (bi) and the average bond enthalpies from section 12 of the data booklet,determine the enthalpy change for the reaction between carbon monoxide and nitrogen monoxide.

Hess's Law provides a method for calculating enthalpy changes for reactions that cannot be measured directly by constructing an alternative reaction pathway. This often involves the use of standard enthalpy of formation (ΔH_f^ө) data.

State Hess’s Law.

Define the term standard enthalpy of formation, ΔH^ϴ_f.

Construct a Hess’s Law cycle for the reaction of calcium fluoride, CaF₂ (s) , and sulfuric acid, H₂SO₄ (aq).

CaF₂ (s) + H₂SO₄ (aq) → 2HF (g) + CaSO₄ (s)

Define the term standard enthalpy of combustion, ΔH^ϴ_c.

Write an equation for the complete combustion of propan-1-ol, CH₃CH₂CH₂OH (l).

Construct a Hess’s Law cycle for the complete combustion of propan-1-ol.

The industrial production of hydrogen cyanide can be achieved by the reaction of methane and ammonia.

i) Write the balanced chemical equation for the conversion of one mole of methane and one mole of ammonia to form hydrogen cyanide and hydrogen.

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ii) Draw the Lewis structure for the hydrogen cyanide molecule, HCN.

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Use Hess’s Law and the information below to calculate the enthalpy change for this reaction.

Using your answer to part (b), draw a labelled reaction profile diagram for this reaction.

Define the term standard enthalpy of reaction, ΔH^ϴ_r.

Butane, C₄H₁₀,  is typically used as fuel for cigarette lighters and portable stoves, a propellant in aerosols, a heating fuel, a refrigerant, and in the manufacture of a wide range of products.

Write a balanced chemical equation for the complete combustion of butane.

The enthalpy change for the hydrogenation of butene to form butane is shown below.

C₄H₈ + H₂  → C₄H₁₀

Using section 12 of the data booklet, calculate the enthalpy of hydrogenation of butene.

The accepted value for the enthalpy of hydrogenation of butene is -30.3 kJ mol^-1.

Suggest one reason why your value calculated using average bond enthalpies in part (b) is different from this accepted value.

The enthalpy of combustion for ethene gas can be determined theoretically using average bond enthalpies.

Write the balanced chemical equation for the complete combustion of ethene, C₂H₄, assuming liquid water is formed.

Define the term average bond enthalpy.

Using your answer to (a) and section 12 of the data booklet, calculate the enthalpy of combustion of ethene.

Bond enthalpies can be found using Hess’s Law or from experimental data.

Outline the difference between the two ways of finding bond enthalpy.

Describe the structure and bonding in calcium chloride.

Vanadium is commonly found in different ores such as magnetite, vanadinite and patronite. The vanadium is commonly extracted from these ores by reduction and displacement.

Vanadium can be extracted by the reduction of vanadium pentoxide, V₂O₅, with calcium at high temperatures, according to the following equation.

V₂O₅ (s) + 5Ca (s) → 2V (s) + 5CaO (s)

The enthalpy of formation of vanadium pentoxide is -1560 kJ mol^-1 and the standard enthalpy change for the reaction is -1615 kJ mol^-1. 

Construct a Hess’s Law cycle for this reaction.

The compound diborane, B₂H₆, is used as a rocket fuel. The equation for the combustion of diborane is shown below.

B₂H₆ (g) + 3O₂ (g) → B₂O₃ (s) + 3H₂O (l)

Calculate the standard enthalpy change of this reaction using the following data

I. 2B (s) + 3H₂ (g) → B₂H₆ (g)          ΔH = 36 kJ mol^-1

II. H₂ (g) + ½O₂ (g) → H₂O (l)          ΔH = -286 kJ mol^-1

III. 2B (s) + 1½O₂ (g) → B₂O₃ (s)       ΔH = -1274 kJ mol^-1

Coal can be converted into "coal gas", a fuel containing carbon monoxide and hydrogen.

The overall equation for coal gasification is

H₂O (l) + C (s) → CO (g) + H₂ (g) 

Using the equations below, calculate the standard enthalpy change for this reaction. 

I. 2C (s) + O₂ (g) → 2CO (g)            ΔH = -222 kJ

II. 2H₂ (g) + O₂ (g) → 2H₂O (g)        ΔH = -484 kJ

III. H₂O (l) → H₂O (g)                       ΔH = +44 kJ

The coal gas produced is then combusted:

CO (g) + H₂ (g) + O₂ (g) → CO₂ (g) + H₂O (g) 

Calculate the standard enthalpy change for this combustion, using the following data.

I. 2C (s) + O₂ (g) → 2CO (g)       ΔH = -222 kJ

II. C (s) + O₂ (g) → CO₂ (g)         ΔH = -394 kJ

III. 2H₂ (g) + O₂ (g) → 2H₂O (g) ΔH = -484 kJ

E85 is a biofuel blend containing a high percentage of ethanol in gasoline.

If E85 is vaporised before combustion, predict if the energy released would be greater than, less than, or the same as combusting liquid E85. Explain your reasoning.

A 1.00 kg sample of E85 is found to contain 60.0% ethanol by mass. Using sections 7 and 14 of the data booklet:

i) Calculate the amount, in moles, of ethanol in the sample.

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ii) Calculate the energy released, in kJ, from the complete combustion of the ethanol in the sample.

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Strontium salts have a number of applications such as fireworks, flares, glow in the dark paint and toothpaste for sensitive teeth. The strontium required for these salts can be extracted from the ore strontia, SrO, by displacement with powdered aluminium in a vacuum.

i) Write a balanced symbol equation, including state symbols, for the reaction of strontia with aluminium.

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ii) State the role of the aluminium in this reaction.

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The standard enthalpy change for this extraction of strontium is 99.3 kJ mol^-1 and the standard enthalpy of formation of aluminium oxide is -1676.7 kJ mol^-1

Use this information to calculate the standard enthalpy of formation, ΔH_f, in kJ mol^-1 of strontia.

Manganese is too brittle for use as a pure metal, so it is often alloyed with other metals. Manganese is used in steel to increase the strength and resistance to wear. Manganese steel (13% Mn) is extremely strong and used for railway tracks, safes and prison bars. Alloys of 1.5% manganese with aluminium are used to make drinks cans due to the improved corrosion resistance of the alloy. Manganese is extracted from different ores by reduction with carbon monoxide.

Mn₂O₃ (s) + 3CO (g) → 2Mn (s) + 3CO₂ (g)

The enthalpy of formation, ΔH_f, of Mn₂O₃ (s) is −971 kJ mol^-1. Use this information and section 13 of the data booklet to calculate the enthalpy change of reaction, ΔH_r, in kJ mol^-1. 

The reaction in part c) reaches equilibrium at high temperatures.

Using your answer to part c, explain how temperature can be altered to increase the yield of the reaction and explain the effect that this would have on the rate of reaction.

Fluorine, the most electronegative element, forms a compound with oxygen called oxygen difluoride, OF₂.

Define the term average bond enthalpy.

The standard enthalpy change for the formation of oxygen difluoride is +28 kJ mol^-1.

F₂ (g) + ½O₂ (g) → OF₂ (g)

Using section 12 of the data booklet, determine the bond enthalpy of the O–F bond.

Ethanol reacts with ethanoyl chloride, CH₃COCl, to produce the ester ethyl ethanoate and hydrogen chloride gas.

Write the balanced chemical equation for this reaction.

Using section 12 of the data booklet, calculate the overall enthalpy change for this reaction.

Methane reacts violently with fluorine to form carbon tetrafluoride and hydrogen fluoride

Write the equation for this reaction. 

Using your answer to part a) and section 12 of the data booklet, calculate the standard enthalpy change for this reaction

Sketch a labelled energy diagram for the reaction of methane and fluorine.

Explain why strontium chloride, SrCl₂ , has a much greater lattice enthalpy than rubidium chloride, RbCl.