8.1.1 Standard enthalpy changes and enthalpy level diagrams Quiz: Pearson Edexcel Chemistry, Unit 8

The 20 questions

1. What are standard conditions for enthalpy change data?

   • 1 atm and 0 K, the conditions at absolute zero where all molecular motion has stopped
   • 101.3 kPa and 273 K, the conditions of one atmosphere at the normal freezing point of water
   • 10 kPa and 500 K, a low pressure and high temperature used in many industrial gas reactions
   • 100 kPa and a specified temperature, usually 298 K

2. What is meant by the enthalpy change of a reaction?

   • The heat energy change at constant volume only
   • The energy needed to start a reaction
   • The total mass of products formed in a reaction
   • The heat energy change at constant pressure

3. Which definition matches the standard enthalpy change of formation?

   • The enthalpy change when one mole of a compound is formed from its elements in their standard states
   • The energy needed to break one mole of bonds in a molecule in the gas phase at standard conditions
   • The enthalpy change when one mole of acid reacts with one mole of alkali to form one mole of water
   • The enthalpy change when one mole of a compound is burned completely in oxygen to form carbon dioxide and water

4. What is the standard enthalpy change of formation of an element in its standard state?

   • Zero
   • Equal to its ionisation energy
   • Equal to its bond enthalpy
   • Always negative

5. Which definition matches the standard enthalpy change of combustion?

   • The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
   • The enthalpy change when one mole of a compound is formed from its elements in their standard states
   • The enthalpy change when one mole of water is formed from an acid and an alkali in a neutralisation
   • The energy change when one mole of gas is ionised by removal of an electron in the gas phase

6. What is the standard enthalpy change of neutralisation when one mole of water is formed from a strong acid and a strong alkali?

   • About -57 kJ mol-1
   • About +57 kJ mol-1
   • About -286 kJ mol-1
   • About -890 kJ mol-1

7. In an enthalpy level diagram for an exothermic reaction, where is the products level drawn relative to the reactants?

   • Higher than the reactants level
   • Lower than the reactants level
   • At the same level
   • Above the activation energy line only

8. A reaction has an enthalpy change of +50 kJ mol-1. Which statement is correct?

   • The reaction is exothermic and the products have lower enthalpy than the reactants, so energy is released
   • The activation energy is 50 kJ mol-1, which is the energy needed to start the reaction in the flask
   • The reaction is catalysed and the products have less energy than the reactants because a catalyst is present
   • The reaction is endothermic and the products have higher enthalpy than the reactants

9. Which statement about the enthalpy level diagram is correct?

   • Activation energy is not shown on an enthalpy level diagram, but is shown on a reaction profile
   • The activation energy is always shown on an enthalpy level diagram as a labelled arrow between the two levels
   • Enthalpy level diagrams show the rate of reaction directly by the spacing of the energy levels on the axis
   • Reaction profile diagrams never show enthalpy change, which appears only on enthalpy level diagrams

10. The enthalpy of combustion of methane is -890 kJ mol-1. How much heat is released when 4.00 g of methane (Mr = 16.0) is burned completely?

    • 3560 kJ
    • 223 kJ
    • 55.6 kJ
    • 890 kJ

11. Which standard enthalpy change is defined for the reaction C(graphite) + O2(g) -> CO2(g)?

    • Enthalpy of atomisation, because carbon is converted into a gas of single atoms in the reaction
    • Enthalpy of neutralisation, because carbon reacts with oxygen to form a salt in the reaction mixture
    • Enthalpy of ionisation, because electrons are removed from the carbon atoms in the reaction
    • Enthalpy of combustion of graphite and of formation of CO2 both apply, because the same equation fits both definitions

12. What is the sign of the enthalpy change for the neutralisation of an acid by an alkali?

    • Positive, because water is formed
    • Negative, because heat is released to the surroundings
    • Zero, because neutralisation is an equilibrium
    • Positive, because heat is absorbed from the surroundings

13. Which standard state is defined for chlorine in its standard enthalpy of formation?

    • Cl-(aq), a chloride ion in solution
    • Cl2(l), liquid chlorine
    • Cl(g), a single chlorine atom
    • Cl2(g), the gas at 100 kPa and 298 K

14. For the enthalpy of formation of water, which equation is correct?

    • H2(g) + 1/2 O2(g) -> H2O(l)
    • 2H2(g) + O2(g) -> 2H2O(l)
    • H2O(l) -> H2(g) + 1/2 O2(g)
    • H(g) + O(g) -> H2O(l)

15. Which equation represents the standard enthalpy of combustion of ethanol?

    • C2H5OH(l) -> C2H4(g) + H2O(l)
    • 2C2H5OH(l) + 6O2(g) -> 4CO2(g) + 6H2O(l)
    • C2H5OH(l) + O2(g) -> CH3CHO(l) + H2O(l)
    • C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)

16. What is the enthalpy change of a reaction that gives 20.0 kJ of heat released per 0.100 mol of reactant?

    • +200 kJ mol-1
    • -200 kJ mol-1
    • -2.00 kJ mol-1
    • -20.0 kJ mol-1

17. Which statement about a standard enthalpy change is correct?

    • It applies only to gases at very high temperature
    • It is measured in joules per gram
    • It is quoted with a sign and units, such as -285.8 kJ mol-1
    • It is always a positive value

18. What information must be shown on an enthalpy level diagram for an exothermic reaction?

    • The activation energy between the reactants and products levels, drawn as a separate arrow above the peak
    • The number of moles of catalyst used in the reaction, which must be recorded on the diagram
    • The reactants above the products, with an arrow showing a negative enthalpy change
    • The volume of gas produced at 298 K, which should be shown on the vertical axis of the diagram

19. The standard enthalpy of formation of carbon dioxide is -394 kJ mol-1. What does this value mean?

    • Burning one mole of carbon absorbs 394 kJ of energy from the surroundings in the reaction vessel
    • The bond enthalpy of C=O is 394 kJ mol-1, which is the energy needed to break one mole of the bond
    • Carbon dioxide is 394 times less stable than oxygen, so it decomposes readily at room temperature
    • Forming one mole of CO2 from carbon and oxygen releases 394 kJ at standard conditions

20. Which standard enthalpy change is defined for a reaction of one mole of acid with one mole of alkali to form water?

    • Standard enthalpy of atomisation
    • Standard enthalpy of combustion
    • Standard enthalpy of formation
    • Standard enthalpy of neutralisation

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