Syllabus Edition

First teaching 2023

First exams 2025

|

Energy Cycles in Reactions (HL IB Chemistry)

Exam Questions

5 hours79 questions
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

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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

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    31 mark

    Which is the correct equation to calculate the enthalpy of reaction using enthalpy of formation data? 

    • ΔHr = ΣΔHf products + ΣΔHf reactants

    • ΔHr = ΣΔHf products - ΣΔHf reactants

    • ΔHr = ΣΔHf reactants - ΣΔHf products 

    • ΔHr = ΣΔHf reactants + ΣΔHf products 

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    41 mark

    Iron(II,III) oxide, Fe3O4, is reduced by graphite according to the following equation: 

    Fe3O4 (s) + 2C (s) → 3Fe (s) + 2CO2 (g) 

    The standard enthalpy of formation values for iron(II,III) oxide and carbon dioxide are -1118 kJ mol-1 and -394 kJ mol-1 respectively. 

    The standard enthalpy of reaction can be determined using which of the following calculations?

    • -330

    • 1512

    • -1906

    • 330

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    51 mark

    Hydrogen peroxide slowly decomposes to form water and oxygen: 

    2H2O2 (l) → 2H2O (l)  + O2 (g)

     

    ΔHӨf, kJ mol-1

    H2O2 (l)

    -188

    H2O (l)

    -286

     

    Which calculation gives the correct enthalpy of reaction for the decomposition of hydrogen peroxide?

    • -98

    • -196

    • -662

    • 196

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    61 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? 

    1. A → D            ΔH = +45 kJ
    2. C → D            ΔH = -25 kJ
    3. D → C            ΔH = -65 kJ
    • I and II only

    • I and III only

    • II and III only 

    • I, II and III

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    71 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

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    81 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

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    9
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    1 mark

    What is the correct definition of lattice enthalpy? 

    • Enthalpy change when one mole of solid ionic compound is separated into its ions in their standard state 

    • Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms

    • Enthalpy change when one mole of solid ionic compound is formed from its gaseous ions under standard conditions

    • Enthalpy change when one mole of a compound is formed from its elements

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    10
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    1 mark

    Which steps are endothermic in the Born-Haber cycle for the formation of LiCl?

    1. ½Cl2 (g)  → Cl (g)  
    2. Cl (g) + e- → Cl- (g)
    3. Li (s)  → Li (g)
    • I and II only 

    • I and III only

    • II and III only 

    • I, II and III 

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    111 mark

    Which equation represents the second electron affinity of nitrogen? 

    • ½N2 (g) + 2e → N2- (g) 

    • N (g) + 2e → N2- (g) 

    • N2 (g) + 4e → 2N2- (g) 

    • N- (g) + e → N2- (g) 

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    121 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.

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    131 mark

    The enthalpy of atomisation of a compound can be calculated using a range of different enthalpy changes. 

    Which statement below correctly describes the enthalpy change of atomisation?

    • The energy stored in a covalent bond

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

    • The energy required to break all bonds in an element  

    • The energy released when two atoms form a covalent bond

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    141 mark

    Which quantity gives the best indication of the relative strength of the hydrogen bonds between water molecules in the liquid state?

    • Enthalpy changes of vaporisation 

    • Bond dissociation energies 

    • Enthalpy of formation  

    • Activation energy

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    151 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

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    11 mark

    The first stage in the industrial production of nitric acid from ammonia can be represented by the following equation. 

    4NH3 (g) + 5O2 (g) ⇌ 4NO (g) + 6H2O (g) 

    Using the following standard enthalpy change of formation data, what is the value of the standard enthalpy change, ΔHθ, for this reaction?

    Compound

     increment H subscript f superscript ⦵/kJ mol– 1

    NH3 (g)

    -46.1

    NO (g)

    +90.3

    H2O (g)

    -241.8

    • -1274

    • -905.2

    • 905.2

    • -378.2

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    21 mark

    Titanium occurs naturally as the mineral rutile, TiO2. One possible method of extraction of titanium is to reduce the rutile by heating with carbon. 

    TiO2 (s) + 2C (s) → Ti (s) + 2CO (g) 

    The standard enthalpy changes of formation of TiO2(s) and CO(g) are –890 kJ mol-1 and –110.5 kJ mol-1 respectively. 

    What is the standard enthalpy change of the extraction of titanium?

    • + 669 kJ mol–1

    • + 779.5 kJ mol–1

    • – 779.5 kJ mol–1

    • – 669 kJ mol–1

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    31 mark

    The combustion of ethanol (C2H5OH) is increasingly being used to fuel cars. The combustion reaction is shown below.

    C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l) 

    • The standard enthalpy change of formation of carbon dioxide is a kJ mol–1.
    • The standard enthalpy change of formation of water is b kJ mol–1.
    • The standard enthalpy change of formation of ethanol is c kJ mol–1

    What is the standard enthalpy change of combustion for ethanol?

    • 2a + 3b + c

    • 2a + 3b − c

    • −2a − 3b + c

    • −2a − 3b −c

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    41 mark

    Propanone has the molecular formula C3H6O. Use the following information to calculate the enthalpy change for the formation of propanone? 

    The enthalpy change of combustion of carbon is x kJ mol -1

    The enthalpy change of combustion of hydrogen is y kJ mol -1

    The enthalpy change of combustion of propanone is z kJ mol -1

    • x + y - z

    • 3x + 3y - z

    • z - 3x + 3y 

    • 3x + 3y + z

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    51 mark

    Combustion of ethene proceeds via the reaction shown in the Hess cycle diagram.q5_5-2_ib_sl_medium_mcq

     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)

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    61 mark

    Combustion of propene proceeds via the reaction shown in the Hess cycle diagram.q6_5-2_ib_sl_medium_mcq

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

     

    P

    Q

    R

    S

    A

    ΔHӨc

    ΔHӨr

    ΔHӨc

    ΔHӨc

    B

    ΔHӨc

    ΔHӨf

    ΔHӨr

    ΔHӨr

    C

    ΔHӨc

    ΔHӨf

    ΔHӨc

    ΔHӨf

    D

    ΔHӨr

    ΔHӨr

    ΔHӨf

    ΔHӨf

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      71 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? 

      1. Providing that the reactants are the same
      2. Providing that the products are the same
      3. 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

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      81 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

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        91 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

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        101 mark

        Thermodynamic data for the components for magnesium oxide are

        Name of enthalpy change

        Energy change (kJ mol-1)

        Enthalpy of formation of magnesium oxide

        -602

        Enthalpy of atomisation of magnesium

        150

        First and second ionisation energy of magnesium 

        2188

        Enthalpy of atomisation of oxygen

        248

        First and second electron affinity of oxygen

        702

        Which is the value of the lattice enthalpy of magnesium oxide?

        • -2486

        • -3890

        • -3766

        • -4618

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        111 mark

        Using the information in the table to answer the question 

        Enthalpy Change

        Value (kJ mol-1)

        ΔHӨlatt CaF2

        2651

        ΔHӨhyd Ca2+

        -1616

        ΔHӨhyd F-

        -504

         

        Which of the following is the correct value for the enthalpy change of solution, ΔHӨsol, of calcium fluoride, CaF2?

        • 531

        • 4771

        • 27

        • 5275

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        121 mark

        Which of the following changes is not endothermic?

        • K (g) → K+ (g) + e-

        • Cl (g) + e- → Cl - (g)

        • Ca (s) → Ca (g)

        • O- (g) + e- → O2- (g)

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        131 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

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        141 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)

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        151 mark

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

         q6_5-3_ib-sl_medium_mcq

        Some enthalpy data is given in the table.

         

        C-H

        C-O

        O-H

        O=O

        C=O

        Mean bond dissociation enthalpy / kJ mol−1

        414

        358

        463

        498

        804

         

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

        • -466

        • -288

        • -144

        • -95

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        161 mark

        Which of the following statements about bond enthalpy are true? 

        1. A bond enthalpy may be inaccurate when compared to data tables because it is not an average value
        2. Average bond enthalpies are valid for all states of matter
        3. Bond enthalpies may be inaccurate as they do not account for intermolecular forces
        • I and II only  

        • I and III only  

        • II and III only 

        • I, II and III

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        11 mark

        The equations below show the formation of sulfur oxides from sulfur and oxygen. 

        S (s) + O2 (g) → SO 2(g)                   increment H subscript f superscript ⦵ = –297 kJ mol–1 

        S (s) + 1½O2 (g) →SO3 (g)               increment H subscript f superscript ⦵ = –395 kJ mol–1 

        What is the enthalpy change of reaction, ΔHΘ, of 2SO2 (g) + O2 (g) → 2SO3 (g) in kJ mol–1?

        • 200

        • 691

        • -98

        • -196

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        21 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

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        31 mark

        A student calculated the standard enthalpy change of formation of propane, C3H8, using a method based on standard enthalpy changes of combustion. 

        He used correct values for the standard enthalpy change of combustion of propane (–2219 kJ mol–1) and hydrogen (–286 kJ mol–1) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was –158 kJ mol–1

        What did he use for the standard enthalpy change of combustion of carbon?

        • -917

        • -306

        • 306

        • 917

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        41 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

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        51 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

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        61 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

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        71 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

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        81 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? 

        1. Hydrated copper(II) sulfate is not produced in a controlled manner
        2. Dissolving of the solid is difficult to avoid
        3. 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

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        91 mark

        Lithium iodide solution can be produced by two different reaction paths, according to the following diagram:q9_5-2_ib_sl_hard_mcq

        Which row shows the correct labels for the diagram?

         

        ΔH1

        ΔH2

        ΔH3

        A

        +364 kJ mol-1

        ΔHhyd

        +82 kJ mol-1

        B

        ΔHhyd

        ΔHsol

        +82 kJ mol-1

        C

        ΔHhyd

        -307 kJ mol-1

        ΔHsol

        D

        -364 kJ mol-1

        ΔHsol

        ΔHhyd

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          101 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%

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          111 mark

          In the gas phase, phosphorus pentachloride can be thermally decomposed into gaseous phosphorus trichloride and chlorine.

          PCl→ PCl3 + Cl2

          The table below gives the relevant bond energies found in these compounds.

          bond

          bond energy / kJ mol–1

          P–Cl (in both chlorides)

          x

          Cl–Cl

          y

          What is the enthalpy change in the decomposition of the reaction?

          • y minus 2 x

          • 2 x minus y

          • 8 x plus y

          • x plus y

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          121 mark

          The diagram shows the skeletal formula of cyclobutane.
           q3_5-3_mcq_hard_ib-chemistry-sl

          The enthalpy change of formation of cyclobutane is +75.1 kJ mol–1, and the enthalpy change of atomisation of graphite is +712 kJ mol–1.

          The bond enthalpy of C–H is 414 kJ mol–1 and of H–H is 436 kJ mol–1.

          What is the average bond enthalpy of the C–C bond in cyclobutane?

          • 1123

          • 715

          • 301

          • 245

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          131 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

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          141 mark

          The equations to form methane and propane from their gaseous atoms are:

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

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

          What is the bond enthalpy of a C-C bond?

          • -2900

          • 346

          • -346

          • -2438

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          151 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%

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          16
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          1 mark

          Which value represents the first electron affinity of iodine? 

          Name of enthalpy change Enthalpy change / kJ mol-1
          Enthalpy of atomisation of calcium  +178
          1st ionisation energy of calcium  +590
          2nd ionisation energy of calcium +1145
          Enthalpy of atomisation of iodine +107
          Lattice energy of calcium iodide -2074
          Enthalpy of formation of calcium iodide  -534
          • -587

          • -480

          • -293.5

          • 240.5

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          17
          Sme Calculator
          1 mark

          Which equation represents the correct working to determine the lattice enthalpy of magnesium chloride, ΔHθlatt(MgCl2)? 

          Enthalpy change  Representation 
          ΔHθsol(MgCl2) x
          ΔHθhyd(Mg2+) y
          ΔHθhyd(Cl-) z
          • x - (y + z)

          • x + (y + z)

          • x + (y + 2z)

          • x - (y + 2z)

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          18
          Sme Calculator
          1 mark

          Which row of the table correctly represents the equations for the lattice enthalpy of substance W2X and the ionisation energy of atom W?

            Lattice enthalpy Ionisation energy 
          A. 2W (s) + X(g) → W2X W (g) → W+ (g) + e-
          B. W2X (s) → 2W+ (g) + X2- (g)  W (g) → W2+ (g) + 2e-
          C. W2X (s) → 2W+ (g) + X2- (g)  W (g) → W+ (g) + e-
          D. W2X (s) → 2W (g) + X (g)  W (g) → W2+ (g) + 2e-

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