3.1.11a Electrode potentials and electrochemical cells Quiz: AQA Chemistry, Unit 1

The 20 questions

1. In an electrochemical cell, how do electrons pass from the reducing agent to the oxidising agent?

   • Directly, by collision between the two species in the same solution
   • Through the salt bridge only, as ions carry electrons across it
   • By evaporation of the reducing agent into the air above the cell
   • Indirectly, through an external circuit

2. Why does a potential difference develop in an electrochemical cell?

   • Electrons are driven through the external circuit because the two half-reactions are separated
   • The cell contains a catalyst that raises the energy of the electrons in both half-cells
   • Gases are produced at both electrodes, which creates a pressure difference
   • The temperature of the two half-cells differs, so the electrons move from hot to cold

3. What is the main purpose of an electrochemical cell as described in this topic?

   • To separate gases by diffusion through a membrane
   • To store heat energy from an exothermic reaction for later use
   • To produce a catalyst for use in industrial reactions
   • To provide a portable supply of electrical energy from a redox reaction

4. Which everyday device is commonly powered by an electrochemical cell?

   • A stone wall that stores heat energy
   • A wooden table lamp that uses only mains gas
   • A mobile phone or laptop
   • A glass beaker used for heating water

5. In a voltaic cell, in which direction do electrons flow in the external circuit?

   • From the negative electrode, where oxidation occurs, to the positive electrode, where reduction occurs
   • From the salt bridge into the external wire at both electrodes
   • Around the cell in a closed loop with no net direction
   • From the positive electrode, where oxidation occurs, to the negative electrode, where reduction occurs

6. In an electrochemical cell, which electrode is the site of oxidation?

   • The electrode where the most positive half-equation is written
   • The positive electrode, which releases electrons into the external circuit
   • Whichever electrode is made of the less reactive metal
   • The negative electrode, which releases electrons into the external circuit

7. What is the role of the salt bridge in a simple electrochemical cell?

   • To supply the reactants that are consumed at both electrodes during the reaction
   • To stop the cell reaction by keeping the two half-cells completely separate
   • To complete the electrical circuit by allowing ions to move between the two half-cells
   • To carry electrons directly from one electrode to the other through the solution

8. In the cell Zn|Zn2+||Cu2+|Cu, which species is oxidised?

   • Copper(II) ions, which change from +2 to 0 as they gain electrons
   • Zinc(II) ions, which change from +2 to 0 as they gain electrons
   • Copper metal, which changes from 0 to +2 as it loses electrons
   • Zinc, which changes from 0 to +2 as it loses electrons

9. Why is a cell described as a source of electricity from a redox reaction, rather than from a simple mixing of reactants?

   • Electrons are transferred through the external circuit rather than directly, so the energy is available as electrical work
   • The reactants must be heated to very high temperatures before any electrical energy is released into the circuit
   • Mixing releases no energy, so a cell must use a separate heat source to function and produce any electrical output
   • A cell reaction produces only gases, which are collected in a tank and then used as the electricity source directly

10. Which statement about a cell's electrodes and their chemistry is correct?

    • Both electrodes carry out oxidation, so electrons are generated at both of them and flow into the salt bridge
    • Both electrodes carry out reduction, so no electrons are needed in the external circuit and the cell stops working
    • Each electrode carries out one half of the overall redox reaction, with electrons passing between them through the circuit
    • The electrodes are inert, so the redox reaction takes place only in the salt bridge and not in either half-cell

11. A cell produces a current because a redox reaction takes place. What drives the electrons from one electrode to the other?

    • The temperature of the salt bridge, which pushes the electrons along
    • The potential difference between the electrodes, created by the redox reaction
    • The mass of the electrodes, which attracts the electrons
    • The pressure of the gas above the electrolyte

12. Which statement about a cell is correct when it is connected to a load and producing a current?

    • The current stops at once, because the potential difference becomes zero as soon as any load is connected to it
    • Electrons flow only in the salt bridge, and the external wire carries no current, so the load receives no energy
    • Electrons flow through the load, and the redox reaction continues until the reactants are used or the cell is recharged
    • No reaction occurs, because a cell can only store electricity and never produce it, so the load receives none

13. What is the effect of connecting a cell to a voltmeter of high resistance?

    • Very little current is drawn, so the reading is close to the cell's electromotive force
    • The cell is destroyed, because high resistance blocks all of the electrons
    • The reading is always zero, because a voltmeter cannot measure any potential
    • The cell reaction speeds up, because the voltmeter supplies extra reactants

14. Which pair describes how a redox reaction is split in an electrochemical cell?

    • The oxidation half-reaction takes place at one electrode and the reduction half-reaction takes place at the other
    • The oxidation and reduction half-reactions take place at the same point in the solution
    • The half-reactions take place in the gas phase above the cell only
    • Both half-reactions take place at one electrode, and the second electrode is used only as a wire

15. What does the overall equation for an electrochemical cell consist of?

    • The oxidation half-equation and the reduction half-equation added together, with electrons cancelled
    • The salt bridge reaction combined with the electrolyte reaction
    • The reduction half-equation only, because oxidation happens without electrons
    • The two electrode materials written side by side, with no change in oxidation state

16. Which is a commercial application of electrochemical cells?

    • Heating a building with a gas-fired boiler
    • Powering a vehicle with a large battery or fuel cell
    • Making steel from iron ore in a blast furnace
    • Producing ammonia from nitrogen and hydrogen in a reactor

17. Why are electrochemical cells so useful for portable electronic devices?

    • They provide electricity from a compact chemical source without needing a power station connection
    • They provide heat from a continuous flame, which keeps the device warm and lets it run without any electricity
    • They provide a constant supply of water for cooling the device, which keeps the internal parts from overheating
    • They provide light from a photochemical reaction in the device casing, which powers the screen without any wires

18. In the cell Zn|Zn2+||Cu2+|Cu, which electrode is the positive electrode?

    • The salt bridge, which carries the positive charge
    • Neither electrode, because both electrodes are negative
    • Zinc, where Zn is oxidised and loses electrons
    • Copper, where Cu2+ is reduced and gains electrons

19. What does the EMF of a cell measure?

    • The rate of the redox reaction at the electrodes, measured in mol per second
    • The total mass of the reactants that are consumed in one second
    • The temperature rise of the electrolyte during the reaction
    • The potential difference between the electrodes when no current is flowing

20. Why does the copper ion gain electrons in the cell Zn|Zn2+||Cu2+|Cu?

    • Cu2+ is the species oxidised at the negative electrode, so it releases electrons into the circuit
    • Cu2+ is the species reduced at the positive electrode, so it accepts electrons from the external circuit
    • Cu2+ is the solvent, which takes electrons from water at the electrode
    • Cu2+ is the salt bridge component, which passes electrons to zinc without reaction

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