Electrochemical Cells

The electricity is generated through a step-by-step mechanism:

• First, the two metals have different tendencies to lose electrons based on their position in the reactivity series.
• Then, the more reactive metal loses electrons more easily, becoming oxidised and acting as the negative electrode.
• Next, the electrons flow through the external wire (the circuit) from the more reactive metal to the less reactive metal (the positive electrode).
• Finally, ions move through the electrolyte to complete the circuit.

Factors Affecting Cell Voltage

Why do some combinations of metals produce a tiny spark, while others can power a motor? The voltage produced by a cell depends heavily on the difference in reactivity between the two metal electrodes.

If you look at the reactivity series, the greater the difference in reactivity between the two metals, the higher the voltage produced. For example, a cell using magnesium and copper will produce a higher voltage than a cell using zinc and copper, because magnesium is much more reactive than zinc.

Other factors also affect the output, such as the type of electrolyte and the concentration of ions within it. However, if both electrodes are made of the exact same metal, the potential difference is exactly $0\text{ V}$ because there is no difference in their tendency to lose electrons.

Calculating Battery Voltage

It is common in everyday life to call a single AA cylinder a "battery", but in chemistry, this is technically incorrect. A battery is strictly defined as two or more cells connected together in series.

Cells are connected in series to provide a greater overall voltage than a single cell could provide alone. The total voltage of a battery is simply the sum of the voltages of its individual cells.

Worked Example:

Calculate the total voltage of a battery made of three identical cells, each producing $1.5\text{ V}$.

Step 1: Write down the formula for series voltage.

• $\text{Total Battery Voltage} = V_1 + V_2 + \dots + V_n$

Step 2: Substitute the known values.

• $\text{Total Battery Voltage} = 1.5\text{ V} + 1.5\text{ V} + 1.5\text{ V}$

Step 3: Calculate the final answer with units.

• $\text{Total Battery Voltage} = 4.5\text{ V}$

Non-Rechargeable vs Rechargeable Batteries

You throw away standard TV remote batteries when they die, but you plug your phone into the wall every night to revive it. This difference comes down to whether the chemical reactions inside the cells are reversible.

Feature	Non-rechargeable Cells	Rechargeable Cells
Reaction Type	Feature an irreversible reaction (it only goes one way).	Feature a reversible reaction.
Why they stop	The chemical reactions stop when one of the reactants is used up.	They stop producing electricity when reactants are depleted, but can be restored.
How they restart	Cannot be restarted; they must be disposed of.	Can be recharged by applying an external electrical current to reverse the reactions.
Common Examples	Alkaline batteries.	Lithium-ion batteries, lead-acid batteries.

In non-rechargeable cells, once a chemical reactant is depleted, the cell can no longer produce electricity. In rechargeable cells, the external electrical current forces the chemical reactions to run backwards, regenerating the original reactants so the cell can produce a potential difference again.