ASChemistryOCRExam Questions3. Periodic Table & Energy3.4 Enthalpy Changes

Enthalpy Changes (OCR AS Chemistry)

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3.4 Enthalpy Changes

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3.4 Enthalpy Changes

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11 mark

The enthalpy change of a chemical reaction can be found using the following relationship:

q = mcΔT

In this expression, which of the following is true?

  • The unit of q is kJ

  • m represents the amount of substance in moles

  • c is the specific heat capacity of the substance

  • The temperature is measured in Centigrade

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21 mark

Which equation represents both an enthalpy change of formation and combustion?

  • 2CH4 (g) + 3O2 (g) → 2CO (g) + 4H2O (l)

  • 4Na (s) + O2 (g) → 2Na2O (s)

  • HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

  • CO2 (g) + C (s) → 2CO (g)

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31 mark

Which processes have a negative enthalpy change?

 

The combustion of an alcohol

The reaction between hydrochloric acid and sodium hydroxide

Water vapour condensing 

A

✔

✔

⨯

B

✔

⨯

✔

C

⨯

✔

✔

D

✔

✔

✔

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    41 mark

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

    combustion-of-methanol

    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

    412

    360

    463

    496

    743

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

    • 40

    • -40

    • -503

    • 886

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    51 mark

    Which is the correct definition for the standard enthalpy of combustion?

    • The enthalpy change when the reactants in a stoichiometric equation react to give the products, under standard conditions

    • The enthalpy change when one mole of a substance is burnt in excess oxygen, under standard conditions

    • The enthalpy change when one mole of water is formed by reacting an acid and an alkali, under standard conditions

    • The enthalpy change when one mole of a product is formed from its elements, under standard conditions

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    61 mark

    25.0 cm3 of nitric acid is reacted with 25.0 cm3 of potassium hydroxide. The temperature change for the reaction is 7.2 K.

    Assume that the resulting solution has a density of 1.0 g cm3

    The specific heat capacity of water is 4.18 J K-1 g-1. Using the equation q = mcΔT, how much energy is released in this experiment?

    • 1505 J

    • 1505 kJ

    • 752 kJ

    • 752 J

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    71 mark

    1.20 g of ethanol is combusted releasing 35 500 J of energy.

    What is the molar enthalpy change for the combustion of ethanol?

    • -1360 kJ mol-1

    • -1360153.3 kJ mol-1

    • -29.5 kJ mol-1

    • -29583.3 kJ mol-1

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    81 mark

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

    hesss-law-cylcle

    The student used the following enthalpy of combustion data.

     

    C2H4 (g)

    H2 (g)

    C2H6 (g)

    ΔHcϴ / 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

    ΔHrϴ

    - 1411

    - 286

    - 1560

    B

    ΔHcϴ

    - 1411

    + 286

    - 1560

    C

    ΔHcϴ

    + 1411

    - 286

    + 1560

    D

    ΔHrϴ

    - 1411

    - 286

    + 1560

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      91 mark

      What is the definition of average 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 specified type of bond in a gaseous molecule. 

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      101 mark

      A reaction profile is shown below. 

      reaction-profile-unlabelled

      Identify the correct labels and type of reaction shown. 

        X Y Reaction
      A Activation energy Enthalpy change Exothermic
      B Enthalpy change Activation energy Exothermic
      C Activation energy Enthalpy change Endothermic
      D Enthalpy change Activation energy Endothermic

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        11 mark

        Which enthalpy change(s) is / are endothermic?

        1. The bond enthalpy of the C–H bond
        2. The second electron affinity of oxygen
        3. The standard enthalpy change of formation of magnesium

        • 1, 2 and 3

        • Only 1 and 2

        • Only 2 and 3

        • Only 1

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        21 mark

        Enthalpy values are provided below.

        H2 (g) + I2 (g) → 2HI (g)    ΔrH = –9 kJ mol–1 

        Bond Bond enthalpy / kJ mol–1
        H-H +436
        I-I +151


        What is the bond enthalpy, in kJ mol–1, of the H–I bond?

        • -596

        • -298

        • +298

        • +596

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        3
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        The table shows standard enthalpy changes of combustion, ΔcH.

        Substance ΔcH/kJ mol–1
        C (s)  –393.5
        H2 (g) –285.8
        C4H10 (g)  –2876.5

        What is the enthalpy change for the following reaction?

        4C (s) + 5H2 (g) → C4H10 (g)

        • –2197.2 kJ mol–1

        • –126.5 kJ mol–1

        • +126.5 kJ mol–1

        • +2197.2 kJ mol–1

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        4
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        The table below shows standard enthalpy changes of formation, ΔfH.

        Compound NH4NO3 (s) H2O (g) CO2 (g)
        ΔfH / kJ mol-1  -366 -242 -394

        What is the enthalpy change for the following reaction?

        2NH4NO3 (s) + C (s) → 2N2 (g) + 4H2O (g) + CO2 (g)

        • –630 kJ mol–1

        • –96 kJ mol–1

        • +96 kJ mol–1

        • +630 kJ mol–1

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        5
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        The table below shows enthalpy changes of formation, ΔfH.

        Compound  TiCl4 (l) H2O (l)  TiO2 (s) HCl (g)
        ΔfH / kJ mol-1  -804 -286 -945 -92

        What is the value of the enthalpy change of reaction, ∆rH, for the reaction in the following equation?

        TiCl4 (l) + 2H2O (l) → TiO2 (s) + 4HCl (g)

        • –63 kJ mol–1

        • –53 kJ mol–1

        • +53 kJ mol–1

        • +63 kJ mol–1

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        61 mark

        Four possible reactions (A, B, C and D) of the following equation are measured at the same temperature.

                              X (g) + Y (g) → Z (g)

        Which reaction pathway diagram shows the reaction occurring rapidly with an overall negative enthalpy value?

        A 4zU~gKEJ_enthalpy-profile-option-a
        B Bw3EKQVy_enthalpy-profile-option-b
        C BTtKUeVT_enthalpy-profile-option-c
        D screenshot-2022-06-06-104039

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          71 mark

          The table below discusses three types of enthalpy change:

          ‘+’ means that this type of standard enthalpy change can only have positive values,

          ‘–’ means that this type of standard enthalpy change can only have negative values,

          ‘+ / –’ means that either positive or negative values are possible.

          Which row is correct?

            Formation Combustion Neutralisation
          A + + + / - 
          B + / - + + / -
          C + / -  - -
          D - - +

           

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            8
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            A student carried out an experiment to determine the enthalpy change for the combustion of ethanol. In the experiment, ethanol in a spirit burner was ignited and used to heat water in a copper calorimeter.

            Using the following results obtained by the student, how much of the heat energy produced, in kilojoules, by the burning of ethanol went into the water?

            The specific heat capacity of water is 4.18 J g−1 K−1.

             Initial temperature of the water

            21.0 °C

             Final temperature of the water

            54.0 °C

             Mass of water in the calorimeter

            100 g

             Mass of spirit burner before burning

            259.75 g

             Mass of spirit burner after burning

            259.18 g

            • 8.78

            • 13.8

            • 35.75

            • 13800

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            9
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            The combustion of ethanol (C2H5OH) is increasingly being used to fuel cars. 

            • The enthalpy change of combustion of hydrogen is –286 kJ mol-1
            • The enthalpy change of combustion of carbon is –394 kJ mol-1
            • The enthalpy change of combustion of ethanol is – 1367 kJ mol-1

            Using the enthalpy of combustion data, what is the enthalpy change for the formation of ethanol?

            • 687 kJ mol-1

            • - 279 kJ mol-1

            • - 2047 kJ mol-1

            • - 3013 kJ mol-1

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            10
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            Nitrogen reacts with fluorine according to the following equation.

            N2 (g) + 3F2 (g) → 2NF3 (g)   ΔrH = -246 kJ

            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? 

            • 225 kJ mol-1

            • 278 kJ mol-1

            • 1350 kJ mol-1

            • 1668 kJ mol-1

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            1
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            The equation for the reaction of aluminium sulfide, Al2S3, with oxygen is shown below.


            2Al2S3(s) + 9O2(g) → 2Al2O3(s) + 6SO2(g)


            The table shows standard enthalpy changes of formation, ΔfHstraight theta

             Substance   Al2S3 (s) O2 (g) Al2O3 (s) SO2 (g)
            ΔfH / kJ mol-1  -723.8 0 -1675.7 -296.8


            What is the standard enthalpy change of combustion of Al2S3(s), in kJmol−1?

            • -3684.6

            • -1842.3

            • +1842.3

            • +3684.6

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            21 mark

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

            S (s) + O2 (g) → SO2 (g) ∆Hf⦵ = –297 kJ mol–1

            S (s) + 1½O2 (g) → SO3 (g) ∆Hf⦵ = –395 kJ mol–1

            What is the enthalpy change of reaction, ∆Hf⦵, of 2SO2 (g) + O2 (g) → 2SO3 (g)?

            • +98 kJ mol–1

            • -98 kJ mol–1

            • +196 kJ mol–1

            • -196 kJ mol–1

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            31 mark

            The reaction of hydrochloric acid with sodium hydroxide reached a maximum temperature of 297.4 K.

            Given the following data, how much thermal energy was evolved during this reaction?

            Initial temperature of 25.0 cm3 hydrochloric acid = 17.6 oC

            Initial temperature of 25.0 cm3 sodium hydroxide = 18.5 oC 

            The specific heat capacity of water is 4.18 J K-1 g-1.

            • 1327.2 J

            • 1421.2 J

            • 1233.1 J

            • 663.6 J

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            41 mark

            The temperature rise when 0.1 MJ of heat energy is absorbed by 2.5 kg of solid aluminium is 44.4 oC.

            What is the specific heat capacity of aluminium?

            • 9 x 102 J K−1 g−1

            • 9 x 10-4  J K−1 g−1

            • 9 x 10-1 J K−1 g−1

            • 9 x 10-7 J K−1 g−1

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            51 mark

            The table shows the standard enthalpy of formation, ΔHfθ, for some of the substances in the reaction.


            4FeS2 (s) + 11O2 (g) → 2Fe2O3 (s) + 8SO2 (g) ΔHrθ = -3312 kJ mol-1

             

            FeS2 (s)

            Fe2O3 (s)

            ΔHfθ / kJ mol-1 

            -178

            -824

            What is the standard enthalpy of formation, in kJ mol-1, for SO2 (g)?

            • -2376 kJ mol-1

            • -952 kJ mol-1

            • -297 kJ mol-1

            • -119 kJ mol-1

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            61 mark

            When completing experiments to measure enthalpy change for exothermic processes, which of the following is not a reason for the experimental data to be lower than the expected value?

            • Heat transfer to the system

            • Heat loss through convection

            • Water vapour released as a product

            • An inadequate supply of oxygen during combustion

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            71 mark

            Heptathiepane, S7, can undergo complete combustion to form sulfur dioxide.

            S7 (s) + 7O2 (g) → 7SO2 (g)

            Which enthalpy changes can be used to describe this reaction?

             

            ΔHfθ

            ΔHcθ

            ΔHr

            A

            x

            x

            ✔

            B

            x

            ✔

            x

            C

            x

            ✔

            ✔

            D

            ✔

            x

            ✔

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              81 mark

              Consider the reactions

              C2H4 (g) + 2O2 (g) → 2CO (g) + 2H2O (g) ∆H = −758 kJ mol−1

              2C (s) + 2H2 (g) → C2H4 (g) ∆H = +52 kJ mol−1

              2C (s) + O2 (g) → 2CO (g) ∆H = −222 kJ mol−1

              What is the enthalpy of formation of water?

              • -524 kJ mol-1

              • -484 kJ mol-1

              • -262 kJ mol−1

              • -242 kJ mol−1

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              91 mark

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

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

              Some mean bond enthalpy data is given in the table.

               Bond

              C-C

              C-H

              H-H

               Mean bond enthalpy / kJ mol-1

              348

              412

              436

              Use the information in the table and the stated enthalpy change to calculate the bond enthalpy for the C=C bond in buta-1,3-diene.

              • 786 kJ mol-1

              • 612 kJ mol-1

              • 1224 kJ mol-1

              • 830 kJ mol-1

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              10
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              In the gas phase, phosphorus pentachloride can be thermally decomposed into gaseous phosphorus trichloride and chlorine. 

              PCl5 → PCl3 + Cl2

              Given the following data, what is the enthalpy change of the reaction?

              Bond 

              P–Cl (in both chlorides)

              Cl – Cl 

              Bond energy / kJ mol–1

              328

              241

              • –415 kJ mol–1

              • +415 kJ mol–1

              • 95 kJ mol–1

              • –95 kJ mol–1

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