Lesson 8.3.1

8.3.1 Mean bond enthalpies and enthalpy change of reaction Quiz: Pearson Edexcel Chemistry, Unit 8

20 questions

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Lesson 8.3.1, Mean bond enthalpies and enthalpy change of reaction: 20 multiple choice questions for the Pearson Edexcel Chemistry (9CH0), Unit 8: Energetics I, written with Revision Ninja.

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The 20 questions

  1. What is a mean bond enthalpy?

    • The energy released when one mole of a bond forms in a liquid at standard temperature and pressure
    • The energy needed to break a bond in the solid state only, without any reference to the gas phase
    • The enthalpy change for the complete combustion of one mole of the bond in a flame at high temperature
    • The average energy needed to break one mole of a given bond in gaseous molecules

  2. Which equation gives the enthalpy change of reaction from mean bond enthalpies?

    • delta H = sum of bond enthalpies broken - sum of bond enthalpies formed
    • delta H = sum of bond enthalpies formed - sum of bond enthalpies broken, with the same sign
    • delta H = sum of bond enthalpies of reactants only
    • delta H = sum of bond enthalpies broken + sum of bond enthalpies formed

  3. Using mean bond enthalpies H-H = 436, Cl-Cl = 242, H-Cl = 431 kJ mol-1, what is the enthalpy change of H2(g) + Cl2(g) -> 2HCl(g)?

    • -184 kJ mol-1
    • -862 kJ mol-1
    • -678 kJ mol-1
    • +184 kJ mol-1

  4. Using C=C = 612, H-H = 436, C-C = 346 and C-H = 413 kJ mol-1, what is the enthalpy change for C2H4(g) + H2(g) -> C2H6(g)?

    • +124 kJ mol-1
    • -124 kJ mol-1
    • -172 kJ mol-1
    • -1048 kJ mol-1

  5. Methane atomisation requires 1660 kJ mol-1 to break all four C-H bonds in CH4(g). What is the mean C-H bond enthalpy?

    • 332 kJ mol-1
    • 1660 kJ mol-1
    • 415 kJ mol-1
    • 830 kJ mol-1

  6. What is a main limitation of calculating enthalpy changes with mean bond enthalpies?

    • The method gives the rate of the reaction, so it cannot be used to calculate the enthalpy change directly
    • The values are averages, so the bond strength in a particular molecule may differ
    • The method cannot be used for any molecule with a double bond, because the values are not defined for C=C
    • The method ignores all reactants in the gas state, so it can only be applied to solid and liquid reactants

  7. Using mean bond enthalpies H-H = 436, Br-Br = 193 and H-Br = 366 kJ mol-1, what is the enthalpy change for H2(g) + Br2(g) -> 2HBr(g)?

    • -629 kJ mol-1
    • -732 kJ mol-1
    • -103 kJ mol-1
    • +103 kJ mol-1

  8. A reaction breaks bonds with total enthalpy 1500 kJ mol-1 and forms bonds with total enthalpy 1620 kJ mol-1. What is the sign of the enthalpy change, and why?

    • Positive, because bond formation absorbs energy from the surroundings and so the overall change is endothermic
    • Negative, because more energy is released in forming bonds than is needed to break them
    • Zero, because the bonds broken and the bonds formed have exactly the same total energy in this case
    • Positive, because bond breaking releases energy into the surroundings and so the total is raised

  9. Which process is always endothermic when considered with mean bond enthalpies?

    • Forming a C-H bond in a gas-phase reaction
    • Forming a C=O bond in CO2
    • Forming a covalent bond
    • Breaking a covalent bond

  10. Why are mean bond enthalpies of C-H measured in gaseous molecules?

    • Gaseous species avoid intermolecular forces, so the bond energies are compared on the same basis
    • Gaseous molecules have no bonds to break, so the bond energies are measured in an empty container instead
    • The bond energies are always higher in gases because the molecules are further apart and less bonded
    • Liquids and solids do not contain covalent bonds, so their bond energies cannot be compared with gases

  11. Using mean bond enthalpies C-H = 413, Cl-Cl = 242, C-Cl = 339 and H-Cl = 431 kJ mol-1, what is the enthalpy change for CH4 + Cl2 -> CH3Cl + HCl, where the reaction breaks one C-H bond and one Cl-Cl bond?

    • -115 kJ mol-1
    • -770 kJ mol-1
    • -655 kJ mol-1
    • +115 kJ mol-1

  12. Which statement about the enthalpy change calculated from mean bond enthalpies is correct?

    • It is independent of the bonds present in the reactants, since only the products determine the result
    • It is exact, because bond enthalpies do not vary between molecules and so the result is always precise
    • It always equals the standard enthalpy of formation, because the two methods are defined in the same way
    • It is an approximation, because mean values are averaged over many compounds

  13. Calculate the mean O-H bond enthalpy in water if breaking both O-H bonds of H2O(g) needs 926 kJ mol-1.

    • 463 kJ mol-1
    • 926 kJ mol-1
    • 1852 kJ mol-1
    • 232 kJ mol-1

  14. Given a mean C-H bond enthalpy of 413 kJ mol-1, how much energy is needed to break all the C-H bonds in one mole of ethane, C2H6?

    • 826 kJ
    • 1239 kJ
    • 413 kJ
    • 2478 kJ

  15. What does a positive value of the enthalpy change calculated from bond enthalpies show?

    • The reaction is catalysed, because the bond energies in the products are lower than those in the reactants
    • The reaction is exothermic, because more bonds are formed in the products than were broken in the reactants
    • The reaction is endothermic, because more energy is needed to break bonds than is released in forming them
    • The reaction has no activation energy, because the energy needed to break bonds is provided by the bond formation

  16. Which pair of bond enthalpies is used to calculate the enthalpy change for the combustion of H2 with O2 to form H2O(g)?

    • Cl-Cl and H-Cl formed only
    • C-H and C-C broken only
    • C=C and C-C broken, and C-H formed
    • H-H and O=O broken, and O-H formed

  17. Using mean bond enthalpies C=O = 743, O=O = 498 and C-H = 413, why is it difficult to calculate the precise enthalpy change of combustion of methane?

    • O=O bonds are not present in oxygen, so the bond enthalpy for oxygen cannot be used in the sum
    • Methane contains no bonds to break, so the enthalpy change depends only on the products formed
    • C=O in CO2 differs from the mean value, so the answer is only approximate
    • C-H bonds cannot be broken in methane, so the calculation of the enthalpy change is impossible to carry out

  18. Which statement about breaking and forming bonds is correct?

    • Energy is absorbed in both bond breaking and bond forming
    • Energy is released when bonds break and absorbed when bonds form
    • Energy is released in both bond breaking and bond forming
    • Energy is absorbed when bonds break and released when bonds form

  19. Using mean bond enthalpies, what is the enthalpy change for 2H2(g) + O2(g) -> 2H2O(g) if H-H = 436, O=O = 498 and O-H = 463 kJ mol-1?

    • -482 kJ mol-1
    • +482 kJ mol-1
    • -1852 kJ mol-1
    • -934 kJ mol-1

  20. What is the advantage of using bond enthalpies rather than standard enthalpy data to estimate an enthalpy change?

    • It shows the rate of reaction, because the bond energies determine how fast the reactants combine
    • It removes the need to know the reactants, since only the bonds in the products affect the result
    • It can be used when standard enthalpy data are not available for a compound
    • It gives exact values for all compounds, because bond enthalpies are measured for each molecule individually

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