Chemical cells for AQA A-level Chemistry

Chemical cells

This page covers the following topics:

1. Fuel cells
2. Electrochemical series
3. Electrochemical cells

A fuel cell is a system that uses energy released during a combustion reaction to create a potential difference. During the combustion of hydrogen water steam is produced that causes little to no harm to the environment. During the combustion of fossil fuels within fuel cells, not only water but also greenhouse gas CO₂ is produced, which is not environmentally friendly. Since the process of producing fuel from the products of combustion is more complex that just recharging a battery using electricity, fuel cells are non-rechargeable. Fuel cells have an increased safety risk as the fuel may react with oxygen at any point causing an explosion.

Fuel cells

Electrochemical series is a list of half equations and their corresponding electrode potentials. Standard electrode potential is the voltage that is created when a specific reduction or oxidation is occurring in comparison to hydrogen oxidation. The aforementioned potentials are usually measured for 1.00 mol/dm³ ion solutions at standard room temperature and pressure. The greater the electrode potential, the more favourable a particular redox process is and the stronger oxidising agent the initial reactant is.

Electrochemical series

An electrochemical cell is a system that uses chemical reactions to create a potential difference across its electrodes. A cell diagram shows both reduced and oxidised species, for example, Mg(s) | Mg²⁺(aq) || Ag⁺(aq) | Ag(s).

The combined potential difference of an electrochemical cell can be measured using a voltmeter or calculated by combining relevant electrode potentials. If a half-equation provided with a potential is the opposite to the one needed, an opposite electrode potential value is taken. If the voltage of a cell is positive, the reaction within the cell is feasible. The calculated value is often different from the measured one due to deviations from the standard conditions at which electrode potentials are measured.

Electrochemical cells

1

Determine the potential difference of Mg(s) | Mg²⁺(aq) || Ag⁺(aq) | Ag(s). A fragment of electrochemical series is provided below.

Ag⁺ + e⁻ → Ag 0.80 V
Cu²⁺ + 2e⁻ → Cu 0.34 V
Ni²⁺ + 2e⁻ → Ni −0.23 V
Al³⁺ + 3e⁻ → Al −1.68 V
Mg²⁺ + 2e⁻ → Mg −2.37 V
Li⁺ + e⁻ → Li −3.05 V

In this cell magnesium is oxidised and silver is reduced.

Ag⁺ + e⁻ → Ag 0.80 V
Mg²⁺ + 2e⁻ → Mg −2.37 V

If a half-equation provided with a potential is the opposite to the one needed, an opposite electrode potential value is taken.

Ag⁺ + e⁻ → Ag 0.80 V
Mg → Mg²⁺ + 2e⁻ +2.37 V

The combined potential difference of an electrochemical cell can be calculated by combining relevant electrode potentials.

V = 0.80 + 2.37 = 3.17 V

3.17 V

Determine the potential difference of Mg(s) | Mg²⁺(aq) || Ag⁺(aq) | Ag(s). A fragment of electrochemical series is provided below.

Ag⁺ + e⁻ → Ag 0.80 V
Cu²⁺ + 2e⁻ → Cu 0.34 V
Ni²⁺ + 2e⁻ → Ni −0.23 V
Al³⁺ + 3e⁻ → Al −1.68 V
Mg²⁺ + 2e⁻ → Mg −2.37 V
Li⁺ + e⁻ → Li −3.05 V

2

Provide the conditions at which standard electrode values are given at.

Standard electrode potentials are usually measured for 1.00 mol/dm³ ion solutions at standard room temperature and pressure.

1.00 mol/dm³ concentration, room temperature and pressure

Provide the conditions at which standard electrode values are given at.

3

Jordan has calculated an electrochemical cell to have a potential difference of 1.21 V. When he has measured the voltage of the same cell in an experiment by using a voltmeter, he has obtained 1.19 V. Suggest a reason for the difference between the theoretical and experimental values.

The calculated value is often different from the measured one due to deviations from the standard conditions at which electrode potentials are measured. For example, the temperature or pressure during the experiment might have been different from standard conditions.

There might have been not a standard temperature using during the experiment.

Jordan has calculated an electrochemical cell to have a potential difference of 1.21 V. When he has measured the voltage of the same cell in an experiment by using a voltmeter, he has obtained 1.19 V. Suggest a reason for the difference between the theoretical and experimental values.

4

What is the relationship between the standard electrode potential and the reducing ability of a reactant involved in a corresponding equation?

The greater the electrode potential, the more favourable a particular redox process is and the stronger oxidising agent the initial reactant is. Thus, the greater the potential, the weaker the reducing ability of the reactant.

The greater the potential, the weaker the reducing ability.

What is the relationship between the standard electrode potential and the reducing ability of a reactant involved in a corresponding equation?

5

Alex has connected two electrodes into their corresponding nitrate solutions to make an electrical cell as displayed in the image. They did remember that one of the metals is copper but couldn't determine the other one. Determine the name of the other metal if the total voltage of the cell is 2.02 V and copper ions are reduced in the process. A fragment of electrochemical series is provided below.

Ag⁺ + e⁻ → Ag 0.80 V
Cu²⁺ + 2e⁻ → Cu 0.34 V
Ni²⁺ + 2e⁻ → Ni −0.23 V
Al³⁺ + 3e⁻ → Al −1.68 V
Mg²⁺ + 2e⁻ → Mg −2.37 V
Li⁺ + e⁻ → Li −3.05 V

The combined potential difference of an electrochemical cell can be calculated by combining relevant electrode potentials. If a half-equation provided with a potential is the opposite to the one needed, an opposite electrode potential value is taken.

Reduction of copper ions provides 0.34 V, which leaves 2.02 − 0.34 = 1.68 V for the oxidation reaction. A matching metal is aluminium with an opposite voltage value for its reduction reaction.

aluminium

Alex has connected two electrodes into their corresponding nitrate solutions to make an electrical cell as displayed in the image. They did remember that one of the metals is copper but couldn't determine the other one. Determine the name of the other metal if the total voltage of the cell is 2.02 V and copper ions are reduced in the process. A fragment of electrochemical series is provided below.

Ag⁺ + e⁻ → Ag 0.80 V
Cu²⁺ + 2e⁻ → Cu 0.34 V
Ni²⁺ + 2e⁻ → Ni −0.23 V
Al³⁺ + 3e⁻ → Al −1.68 V
Mg²⁺ + 2e⁻ → Mg −2.37 V
Li⁺ + e⁻ → Li −3.05 V

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