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OCR (A) A-Level
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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

5.2.3 Redox and electrode potentials

Electrode potentialsRedoxRedox titrationsStorage and fuel cells

Storage and Fuel Cells

Specification Reference Topic 5.2.3 (j)–(k)

Quick Notes

  • Storage cells (e.g. lithium-ion) use reversible redox reactions to produce a voltage.
  • In discharging, electrical energy is released.
  • In charging, an external electrical current is used to reform the reactants.
  • Fuel cells use energy from the reaction of the fuel with oxygen to create a voltage and drive an electrical current (e.g. hydrogen–oxygen):
    • Fuel is oxidised
    • Oxygen is reduced
    • They provide continuous voltage as long as reactants are supplied

Full Notes

Storage cells, also known as rechargeable batteries, are electrochemical cells that store energy for use when needed. They involve two half-cells, and their operation is based on redox reactions.

In discharging, the cell acts like a galvanic (voltaic) cell, releasing electrical energy.

In charging, an external current is applied to reverse the redox reaction.

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Matt’s exam tip

The half equations that occur in a storage cell during charging are simply the reverse direction of the half equations that occur when discharging.

Example: Lithium-Ion Battery

Common batteries (such as in mobile phones) contain lithium and are called ‘lithium ion cells’.

The electrode reactions occurring can be simplified to:

Positive electrode (cathode): Li+ + CoO2 + e− → Li[CoO2]−

OCR (A) A-Level Chemistry diagram showing lithium-ion cell setup.

Negative electrode (anode): Li → Li+ + e−

OCR (A) A-Level Chemistry lithium-ion positive electrode reaction diagram.

When a lithium ion battery is recharged, an external input of electricity is used to force the above reactions to happen in reverse – reforming the original reactants.

Photo of Matt
Matt’s exam tip

For OCR (A) you don’t need to remember any specific equations or data for storage (and fuel) cells. However, you do need to make sure you are comfortable with how they work in principle.

Fuel Cells

Fuel cells produce electricity by reacting a fuel with oxygen, without combustion. The most common type is the hydrogen–oxygen fuel cell.

They operate continuously as long as fuel and oxygen are supplied. They are highly efficient and produce only water as a waste product.

The energy released from the reaction of the fuel with oxygen is used to create a voltage and this drives an electrical current.

Example: Hydrogen fuel cell in acidic conditions:

OCR (A) A-Level Chemistry hydrogen fuel cell diagram showing electrodes and reactions.

Reaction summary

Clean energy: only water as by-product

Efficient and environmentally friendly

Summary