9.1.2 Catalysts and reaction profiles Quiz: Pearson Edexcel Chemistry, Unit 9

The 20 questions

1. What is the role of a catalyst in a chemical reaction?

   • It increases the yield of the product at equilibrium, by shifting the position of the reaction
   • It increases the enthalpy change of the reaction, so the products are more stable than before
   • It is used up completely in the reaction, so a fresh supply must be added at each step
   • It provides an alternative reaction route with lower activation energy

2. On a reaction profile diagram, what does a catalysed reaction show compared with an uncatalysed one?

   • A product level that is higher than the reactants, showing that the catalysed route is endothermic
   • The same peak but with a different enthalpy change, showing that the catalyst alters the energy of the products
   • A lower peak representing a lower activation energy, with the same overall enthalpy change
   • A higher peak with a more negative enthalpy change, showing that the catalyst makes the products more stable

3. Why does a catalyst not change the enthalpy change of a reaction?

   • It increases the number of molecules involved in the reaction, which changes the overall energy balance
   • It removes the need for activation energy, so the reaction can take place without any energy input
   • It does not change the energy of the reactants or products
   • It makes the reaction reversible, so the forward and backward reactions both proceed at equal rates

4. A heterogeneous catalyst is used in a gas-phase industrial reaction. What is the main role of its solid surface?

   • It dissolves the gases to form a liquid solution in which the reaction can take place more easily
   • It provides a surface where reactant molecules are adsorbed and brought together to react
   • It raises the temperature of the gas so that the molecules have enough energy to react on contact
   • It removes the products from the reaction vessel so that the equilibrium shifts forward continuously

5. What is a homogeneous catalyst?

   • A catalyst in a different phase from the reactants
   • A catalyst in the same phase as the reactants
   • A catalyst that is used only in biological systems
   • A catalyst that is always a solid

6. Which statement about the Maxwell-Boltzmann distribution and catalysts is correct?

   • A catalyst changes the total number of molecules in the sample, which increases the number of collisions
   • A catalyst increases the proportion of molecules with energy above the lower activation energy
   • A catalyst moves the Maxwell-Boltzmann curve to higher energies so that more molecules have enough energy
   • A catalyst has no effect on the distribution of molecular energies, and it works only by changing the enthalpy

7. Which catalyst is used in the hydrogenation of alkenes, and why is it described as heterogeneous?

   • Potassium manganate(VII), because it is a liquid
   • Hydrogen gas, because it is a catalyst for alkenes
   • Concentrated sulfuric acid, because it dissolves in the reactants
   • Nickel, because it is a solid while the reactants are gases or liquids

8. A reaction is catalysed by a substance that lowers the activation energy from 80 kJ mol-1 to 50 kJ mol-1. What is the change in activation energy?

   • An increase of 30 kJ mol-1
   • No change, because the enthalpy change is the same
   • A decrease of 130 kJ mol-1
   • A decrease of 30 kJ mol-1

9. What does the activation energy represent on a reaction profile diagram?

   • The difference between the products and reactants levels, which is the enthalpy change of the reaction
   • The energy released at the end of the reaction, which is shown as the drop to the product level
   • The height of the energy barrier between the reactants and the highest point of the reaction path
   • The energy of the catalyst alone, measured from the bottom of the energy axis of the diagram

10. Which statement about catalysts in the context of collisions is correct?

    • A catalyst raises the average kinetic energy of molecules
    • A catalyst lowers the number of collisions but makes them more energetic
    • A catalyst increases the fraction of successful collisions, which increases the rate
    • A catalyst increases the number of collisions at any temperature by a factor of 10

11. In heterogeneous catalysis, why does a finely divided solid catalyst work better than a single large lump?

    • It has a larger surface area, so more reactant can be adsorbed at once
    • It has a larger mass, so more energy is released when the catalyst surface reacts with the reactants
    • It has a lower enthalpy of formation, so it reacts more readily with the reactant molecules present
    • It removes more heat from the reaction, so the temperature stays lower and the catalyst works better

12. Which description best matches a catalyst in a homogeneous system?

    • It is a solid surface on which reactants adsorb, so the reaction takes place at the surface of the solid
    • It is always removed by filtration after the reaction, which is a standard step in homogeneous catalysis
    • It is dissolved in the same solution as the reactants and forms intermediate species
    • It has no effect on the reaction pathway, but it does change the rate by altering the temperature

13. What is the economic benefit of a catalyst in an industrial reaction?

    • It allows lower temperature and pressure to be used, saving energy and equipment costs
    • It eliminates all waste from the process, so there is no by-product or unused reactant at the end
    • It increases the mass of product without any energy input, by forming extra product from the reactants
    • It removes the need for any raw materials, because the catalyst supplies the atoms for the product

14. Which feature is shown on a reaction profile for a reaction that releases heat overall?

    • The activation energy is negative
    • Products lie lower than reactants on the energy axis
    • The energy axis is not needed
    • Products lie higher than reactants on the energy axis

15. Which phrase describes a catalyst that remains unchanged at the end of the reaction?

    • It is consumed to form the product, which is why the catalyst must be replaced after each run
    • It is converted into the product, and so the amount of catalyst falls steadily during the reaction
    • It is regenerated, so it is not used up in the overall reaction
    • It reacts only once and then decomposes, which makes it a poor choice for repeated industrial use

16. Why is the catalyst surface in a heterogeneous reaction important for the reactant molecules?

    • The surface converts gases into solids, which then react more easily with the remaining reactants
    • The surface increases the enthalpy of the reaction, which makes the products more stable than before
    • Adsorption onto the surface weakens bonds in the reactants, so they react more easily
    • The surface removes all the energy from the reactants, so the reaction proceeds without any activation energy

17. Why is a larger activation energy generally associated with a slower reaction?

    • The catalyst is inactive at high activation energy, so no reaction takes place in the flask
    • A smaller fraction of collisions has enough energy to react
    • All collisions become exothermic, so the reaction releases heat more quickly and the rate falls
    • The reactants have higher enthalpy, so the collisions are less effective at forming products

18. Which statement describes a catalyst in a reaction profile compared with the uncatalysed route?

    • The catalysed route has a higher highest energy point, which is why the reaction becomes slower overall
    • The catalysed route has different products, so the catalyst changes what is formed in the reaction
    • The catalysed route has intermediate steps with a lower highest energy point
    • The catalysed route has no intermediate steps, and it simply lowers the energy of the reactants

19. Why do many enzymes act as catalysts in living systems?

    • They are consumed in every reaction in the cell, so their concentration falls quickly over time
    • They lower activation energies of biochemical reactions so they occur at body temperature
    • They change the enthalpy of the cell's reactions, making each reaction release more energy overall
    • They raise the temperature of cells to speed reactions, so that the enzyme molecules move faster

20. A reaction profile for an uncatalysed reaction starts with reactants at 0 kJ mol-1, has products at -40 kJ mol-1 and a peak at +60 kJ mol-1. What is the activation energy?

    • 60 kJ mol-1
    • 20 kJ mol-1
    • 40 kJ mol-1
    • 100 kJ mol-1

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