IBChemistryDPSLExam QuestionsWhat Drives Chemical Reactions?Energy Cycles in Reactions

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

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Energy Cycles in Reactions

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11 mark

Enthalpy changes that are difficult to measure directly can often be determined using Hess’ Law to construct an enthalpy cycle. 

Which enthalpy change is indicated by X in the enthalpy cycle shown?

q1_5-2_ib_sl_easy_mcq

  • + 1 x Enthalpy of formation of water

  • - 1 x Enthalpy of formation of water

  • + 3 x Enthalpy of formation of water

  • - 3 x Enthalpy of formation of water

Choose your answer

21 mark

A student drew a Hess cycle to calculate the enthalpy of reaction to produce ethane from ethene and hydrogen.

q2_5-2_ib_sl_easy_mcq

 The student used the following enthalpy of combustion data 

 

C2H4 (g)

H2 (g)

C2H6 (g)

ΔHϴc / kJ mol-1

- 1411

- 286

- 1560

 What are the correct labels for the arrows for the student’s Hess cycle?

 

Arrow 1

Arrow 2

Arrow 3

Arrow 4

A

ΔHϴr

- 1411

- 286

- 1560

B

ΔHϴc

- 1411

+ 286

- 1560

C

ΔHϴc

+ 1411

- 286

+ 1560

D

ΔHϴr

- 1411

- 286

+ 1560

    Choose your answer

    31 mark

    The diagram shows two possible reaction pathways for the reaction of A → D:

    q7_5-2_ib_sl_easy_mcq

     Which of the following statements are correct? 

    I. A → D            ΔH = +45 kJ

    II. C → D            ΔH = -25 kJ

    III. D → C            ΔH = -65 kJ

    • I and II only

    • I and III only

    • II and III only 

    • I, II and III

    Choose your answer

    41 mark

    The thermal decomposition of calcium carbonate is very slow and requires a high temperature to go to completion. This makes it impractical to measure the enthalpy change for the direct reaction. 

    The enthalpy change for the thermal decomposition of calcium carbonate can be determined by two chemical reactions with dilute hydrochloric acid.          

    q9_5-2_ib_sl_easy_mcq

     Which set of chemicals correctly completes the Hess cycle diagram?

    • CaCO3 + CaO + HCl

    • CaCl + H2O + CO2 

    • CaCl2 + H2O

    • CaCl2 + H2O + CO2

    Choose your answer

    51 mark

    Hess’s Law can be used to calculate the enthalpy change for reactions that are difficult to measure experimentally, such as the conversion of graphite to diamond.

    q10_5-2_ib_sl_easy_mcq

     Which equation shows the correct application of Hess’s law to calculate the enthalpy change for the conversion of graphite to diamond?

    • ΔHr = ΔH1 + ΔH2

    • ΔHr = ΔH1 - ΔH2

    • ΔHr = ΔH2 - ΔH1

    • ΔHr = ΔH1 x ΔH2

    Choose your answer

    61 mark

    Hydrogen atoms bond covalently to iodine atoms to form hydrogen iodide as shown in the equation below:

    H2(g) + I2(g)  →  2HI(g) 

    Which statement best describes what is meant by the average HI bond enthalpy?

    • The energy stored in a covalent bond.

    • The energy required to break one covalent bond in the gas phase. 

    • The energy required to break one mole of the HI bonds in the gas phase. 

    • The energy released when two atoms form a covalent bond.

    Choose your answer

    71 mark

    Which is the correct definition of mean bond enthalpy?

    • The amount of energy required to break a specific covalent bond in the gas phase

    • The energy required to break one mole of a specific covalent bond with all chemicals in their standard states

    • The amount of energy required to break a specific covalent bond with all chemicals in their standard states

    • The energy required to break one mole of a specific covalent bond in the gas phase, with all chemicals in their standard states

    Choose your answer

    11 mark

    Combustion of ethene proceeds via the reaction shown in the Hess cycle diagram.

    Diagram showing the combustion of ethene with oxygen forming carbon dioxide and water, annotated with arrows labelled P, Q, R, and S pointing to Elements.

     What are the correct elements missing from the Hess cycle?

    • 2C (g) + 2H2 (g) + 3O2 (g)

    • C (s) + H2 (g) + O2 (g)

    • 2C (s) + 2H2 (g) + 3O2 (g)

    • C2 (s) + 2H2 (g) + 3O2 (g)

    Choose your answer

    21 mark

    A basic definition of Hess’s Law states that the overall enthalpy change for a reaction is the same independent of the route taken. 

    The addition of which of the following statements makes the definition of Hess’s Law more complete? 

    I. Providing that the reactants are the same

    II. Providing that the products are the same

    III. Providing that the conditions at the start and the end of the reaction are the same

    • I and II only 

    • I and III only 

    • II and III only 

    • I, II and III

    Choose your answer

    31 mark

    The hydration enthalpy of anhydrous copper(II) sulfate cannot be measured directly. It can be found indirectly by determining the solution enthalpies of anhydrous and hydrated copper(II) sulfate.

    q9_5-2_mcq_medium_ib-chemistry-sl

    Which are the correct labels for the enthalpies shown in the Hess cycle? 

     

    1

    2

    3

    A

    ΔHӨsol

    ΔHӨhyd

    ΔHӨhyd

    B

    ΔHӨr

    ΔHӨhyd

    ΔHӨhyd

    C

    ΔHӨhyd

    ΔHӨsol

    ΔHӨhyd

    D

    ΔHӨr

    ΔHӨsol

    ΔHӨsol

      Choose your answer

      41 mark

      The hydration enthalpy of anhydrous copper(II) sulfate, labelled as ΔHexp, cannot be measured directly. It can be found indirectly by determining the solution enthalpies of anhydrous and hydrated copper(II) sulfate.

      q10_5-2_mcq_medium_ib-chemistry-sl

      Which of the following statements correctly explains why the value for ΔHexp for this reaction cannot be measured directly?

      • Measuring the temperature change in a solid is difficult

      • The reaction is very slow

      • The reaction has high energy requirements

      • The reaction is endothermic

      Choose your answer

      51 mark

      The standard enthalpy change, ΔHӨ, for the following reaction is -246 kJ. 

      N2(g)  +  3F2(g)  →  2NF3(g)     

      The bond energy of  N≡N is 945 kJ mol-1 and F–F is 159 kJ mol-1 

      What is the bond energy of the N–F bond?

      • 196

      • 588

      • 278

      • 1668

      Choose your answer

      61 mark

      The complete combustion of ethyne, C2H2, is shown in the equation below. 

      H−C≡C−H + 2½ O═O  →  H−O−H + 2O═C═O 

      Using the average bond enthalpies given in the table, what is the enthalpy change of combustion of ethyne?

      bond

      average bond enthalpy / kJ mol-1

      C−H

      a

      C≡C

      b

      O=O

      c

      C=O

      d

      O−H

      e

      • (2e + 4d) - (2a + b + 2½c)

      • (2a + b + 2½c) - (-2e - 4d)

      • (2a + b + 2½c) - (2e + 4d)

      • - e - d + (2a + b + 2½c)

      Choose your answer

      71 mark

      The incomplete combustion of methanol can produce carbon monoxide and water: 

      Chemical reaction of methanol and oxygen forming carbon monoxide and hydrogen molecules, depicted with molecular structures and an arrow indicating reaction.

      Some enthalpy data is given in the table.

       

      C-H

      C-O

      O-H

      O=O

      Cidentical toO

      Mean bond dissociation enthalpy / kJ mol−1

      414

      358

      463

      498

      1072

      What is the enthalpy change for this incomplete combustion of methanol?

      • -466 kJ mol-1

      • -288 kJ mol-1

      • -144 kJ mol-1

      • -368 kJ mol-1

      Choose your answer

      11 mark

      Some bond energy values are listed below.

      bond

      bond energy / kJ mol-1

      Br–Br 

      Cl–Cl

      C–H

      C–Cl

      193

      242

      414

      324

      These bond energy values relate to the following four reactions. 

      W

      Br2 → 2Br

      X

      2Cl → Cl 2

      Y

      CH3 + Cl → CH3Cl

      Z

      CH4 → CH3 + H 

      What is the correct order of enthalpy changes of the above reactions from most negative to most positive? 

      • Y → Z → W → X

      • Z → W → X → Y

      • Y → X → W → Z

      • X → Y → Z → W

      Choose your answer

      21 mark

      Given the following enthalpy changes:

      I2 (s) → I2 (g)   ∆HƟ = +38 kJ mol–1 

      I2 (g) + 3Cl2 (g) → 2ICl3 (s)   ∆HƟ = –214 kJ mol–1                  

      What is the correct value for ∆HfƟ of iodine trichloride, ICl3 (s)? 

      • -352

      • 352

      • -88

      • 88

      Choose your answer

      31 mark

       Using the following information: 

      CO (g) + ½O2 (g) → CO2 (g)           ∆HƟ  = –283 kJ mol–1 

      H2 (g) + ½O2 (g) → H2O (I)            ∆HƟ  = –286 kJ mol–1 

      H2O (g) → H2O (I)                        ∆HƟ = –44 kJ mol–1 

      What is the enthalpy change, ∆HƟ, for the following reaction? 

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

      • -613

      • 41

      • -47

      • -525

      Choose your answer

      41 mark

      Iodine trichloride, ICl3, is made by reacting iodine with chlorine. 

                                          I2(s) + Cl2(g) → 2ICl(s)        ∆Ho = +14 kJ mol–1

                                                      ICl(s) + Cl2(g) → ICl3(s)      ∆Ho= –88 kJ mol–1

      By using the data above, what is the enthalpy change of the formation for solid iodine trichloride?

      • –162 kJ mol–1

      • –81 kJ mol–1

      • –74 kJ mol–1

      • –60 kJ mol–1

      Choose your answer

      51 mark

      Shown below are three enthalpy changes: 

      CH4 (g)     +    O2 (g)   →  HCHO (l)   +   H2O (l)                 ΔH = x 

      HCHO (l)   +  ½O2 (g)  → HCOOH (l)                                 ΔH = y 

      2HCOOH (l)  +  ½O2 (g)  → (COOH)2 (l) + H2O (l)             ΔH = z 

      Use the information given to deduce the correct expression for the enthalpy change of the following reaction: 

      2CH4 (g)   +   3½ O2 (g)    →   (COOH)2 (l)  + 3H2O (l)

      • x + y + z

      • 2x + y + z

      • 2x + 2y + z

      • 2x + 2y + 2z

      Choose your answer

      61 mark

      The hydration enthalpy of anhydrous copper(II) sulfate, labelled as ΔHexp, cannot be measured directly. It can be found indirectly by determining the solution enthalpies of anhydrous and hydrated copper(II) sulfate.

      q8_5-2_ib_sl_hard_mcq

      Which of the following statements correctly explains why the value for ΔHexp for this reaction cannot be measured directly? 

      I. Hydrated copper(II) sulfate is not produced in a controlled manner

      II. Dissolving of the solid is difficult to avoid

      III. Heat energy is trapped inside the solid copper(II) sulfate

      • I and II only  

      • I and III only  

      • II and III only 

      • I, II and III

      Choose your answer

      71 mark

      Bond energy calculations show the enthalpy of combustion for propene to be -1572.0 kJ mol-1.

      Compound

      C3H6 (g)

      CO2 (g)

      H2O (l)

      H2O (g)

      ΔHӨf / kJ mol-1

      +20

      -394

      -286

      -242

      Using the enthalpy of formation data, what is the percentage error between propene’s enthalpy of combustion values obtained from bond energy calculations and Hess’s Law calculations?

      Assuming the bond energy calculation value is correct.

      • 45%

      • 123%

      •  120%

      • 23%

      Choose your answer

      81 mark

      Butane can be produced by the hydrogenation of buta-1,3-diene.

      C4H6 (g) + 2H2 (g) → C4H10 (g)                 ΔH = -248 kJ mol-1

      Bond

      C-C

      C-H

      H-H

      Mean bond enthalpy / kJ mol-1

      346

      414

      436

      Using the information, which calculation shows the bond enthalpy for the C=C bond in buta-1,3-diene?

      • 1228

      • 614

      • 1724

      • 862

      Choose your answer

      91 mark

      The enthalpy changes for the formation of methane and propane from their constituent gaseous atoms are shown.

      C (g) + 4H (g) → CH4 (g)              ΔHr = -1656 kJ mol-1

      3C (g) + 8H (g) → C3H8 (g)           ΔHr = -4004 kJ mol-1

      Using this data, what is the average bond enthalpy of a C–C bond in kJ mol-1?

      • 173

      • 346

      • 414

      • 692

      Choose your answer

      101 mark

      Water has two different bond enthalpies for the two O-H bonds that it contains:

      H2O (g) → H (g) + OH (g)             ΔH = +502 kJ mol-1

      OH (g) → H (g) + O (g)                 ΔH = +427 kJ mol-1

      The average O-H bond enthalpy from an accepted data table is 463 kJ mol-1.

      Which answer correctly shows the percentage difference between the average O-H bond enthalpy of water and the data table average bond enthalpy value?

      • 0.0032%

      • 0.32%

      • 0.99%

      • 32%

      Choose your answer