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CIE iGCSE Co-ordinated Sciences-C4.2 Hydrogen–oxygen fuel cells- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-C4.2 Hydrogen–oxygen fuel cells – Study Notes

CIE iGCSE Co-ordinated Sciences-C4.2 Hydrogen–oxygen fuel cells – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

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CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Hydrogen–Oxygen Fuel Cell

A hydrogen–oxygen fuel cell is a device that converts the chemical energy of hydrogen and oxygen directly into electrical energy. The only chemical product of the reaction is water, making it a clean energy source.

How it works

  • Hydrogen gas (\( \text{H}_2 \)) is supplied to the anode (negative electrode).
  • Oxygen gas (\( \text{O}_2 \)) is supplied to the cathode (positive electrode).
  • At the anode, hydrogen molecules are oxidized, releasing electrons and forming hydrogen ions (protons):
    \( \text{H}_2 \rightarrow 2\text{H}^+ + 2e^- \)
  • The electrons flow through an external circuit, producing electric current.
  • At the cathode, oxygen molecules react with the electrons and hydrogen ions to form water:
    \( \dfrac{1}{2}\text{O}_2 + 2\text{H}^+ + 2e^- \rightarrow \text{H}_2\text{O} \)
  • Overall reaction in the fuel cell:
    \( 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} \)

Advantages

  • Produces electricity efficiently with minimal energy loss.
  • The only chemical product is water, so it does not produce harmful emissions.
  • Can operate continuously as long as hydrogen and oxygen are supplied.

Applications

  • Fuel cells are used in hydrogen-powered vehicles, such as cars and buses.
  • They provide backup power for buildings and remote installations.
  • They are used in spacecraft for both electricity and water generation.

Example

In a hydrogen–oxygen fuel cell, 4 moles of hydrogen react with oxygen. Calculate:

  • The moles of water produced.
  • The volume of water vapor produced at r.t.p. (1 mole of gas = 24 \( \text{dm}^3 \)).
▶️ Answer/Explanation

Balanced equation: \( 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} \)

  • Mole ratio H₂ : H₂O = 2 : 2 = 1 : 1
  • Moles of H₂ given = 4 → moles of H₂O produced = 4
  • Volume of water vapor at r.t.p.: \( V = n \times 24 = 4 \times 24 = 96\,\text{dm}^3 \)

Example

A hydrogen–oxygen fuel cell is used to power a vehicle. 10 moles of hydrogen react completely with oxygen. Calculate:

  • The moles of water produced.
  • The volume of water vapor produced at r.t.p. (1 mole of gas = 24 \( \text{dm3} \)).
  • The total number of water molecules produced.
▶️ Answer/Explanation

Step 1: Write the balanced chemical equation

\( 2\text{H2} + \text{O2} \rightarrow 2\text{H2O} \)

Step 2: Determine the mole ratio

  • Mole ratio H2 : H2O = 2 : 2 = 1 : 1
  • Moles of H2 given = 10 → moles of H2O produced = 10

Step 3: Calculate the volume of water vapor at r.t.p.

  • \( V = n \times 24 = 10 \times 24 = 240\,\text{dm3} \)

Step 4: Calculate the total number of water molecules

  • Number of molecules = moles × Avogadro constant
  • \( N = 10 \times 6.02 \times 10^{23} \approx 6.02 \times 10^{24} \) molecules

Advantages and Disadvantages of Hydrogen–Oxygen Fuel Cells Compared to Gasoline Engines

Advantages

  • Produces electricity efficiently with minimal energy loss because chemical energy is directly converted into electrical energy without intermediate combustion.
  • The only chemical product is water (\( \text{H2O} \)), so it does not emit carbon dioxide (\( \text{CO2} \)) or other greenhouse gases, reducing global warming potential.
  • Reduces air pollution in urban areas compared to petrol engines that produce carbon monoxide (\( \text{CO} \)), nitrogen oxides (\( \text{NOx} \)), and particulate matter.
  • Can operate continuously as long as hydrogen (\( \text{H2} \)) and oxygen (\( \text{O2} \)) are supplied, making it suitable for long-duration use.
  • Quiet operation due to no combustion, leading to lower noise pollution.
  • Refueling with hydrogen can be faster than charging electric batteries, making it convenient for certain applications like buses and trucks.

Disadvantages

  • Hydrogen storage and transport are challenging because hydrogen (\( \text{H2} \)) is highly flammable and requires high-pressure tanks or cryogenic storage.
  • Fuel cell vehicles currently have higher initial costs compared to conventional petrol vehicles due to expensive catalysts like platinum.
  • Hydrogen production is often energy-intensive and may rely on fossil fuels, which can reduce the overall environmental benefit if not produced sustainably.
  • Limited refueling infrastructure for hydrogen (\( \text{H2} \)) compared to widespread petrol stations, restricting travel range and convenience.
  • Fuel cells can degrade over time, and replacement costs can be high.

Example

A hydrogen–oxygen fuel cell car uses 4 moles of hydrogen. Compare the energy output and environmental impact with a petrol car that burns an equivalent amount of fuel. Calculate the mass of water produced and the total number of water molecules.

▶️ Answer/Explanation

Step 1: Balanced chemical equation

\( 2\text{H2} + \text{O2} \rightarrow 2\text{H2O} \)

Step 2: Mole ratio and water production

  • Moles of H2 = 4 → moles of H2O produced = 4 (1:1 ratio)
  • Molar mass of H2O = 18 g/mol → mass of water = 4 × 18 = 72 g

Step 3: Number of water molecules

  • Number of molecules = moles × Avogadro constant
  • \( N = 4 \times 6.02 \times 10^{23} \approx 2.41 \times 10^{24} \) molecules

Step 4: Environmental comparison

  • Fuel cell produces only water, no CO2 or NOx emissions.
  • Petrol combustion releases CO2, CO, and NOx, contributing to air pollution and global warming.
  • Hydrogen fuel cell car is more environmentally friendly and efficient.
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