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Heat Generation in Iron-Based Oxygen Absorbers

This post includes advanced chemistry and physics content.

In a small packaging volume or with large amounts of absorbers, this heat can raise the temperature inside the package, potentially affecting its contents.

The heat produced by an iron-based oxygen absorber is tied to the enthalpy change of the chemical reaction involved. In these absorbers, iron undergoes oxidation to form iron(III) oxide (rust), releasing heat in the process.

The Oxidation Reaction of Iron:

The primary reaction is as follows:4Fe+3O2→2Fe2O34 \text{Fe} + 3 \text{O}_2 \rightarrow 2 \text{Fe}_2\text{O}_34Fe+3O2​→2Fe2​O3​

To determine the heat generated, we examine the standard enthalpy change (ΔH\Delta HΔH) for this reaction. The standard enthalpy of formation (ΔHf∘\Delta H_f^\circΔHf∘​) of Fe2O3\text{Fe}_2\text{O}_3Fe2​O3​ is approximately −824.2 kJ/mol-824.2 \, \text{kJ/mol}−824.2kJ/mol.

Using this value, we calculate the enthalpy change for the full reaction:ΔHreaction=2×ΔHf∘(Fe2O3)\Delta H_{\text{reaction}} = 2 \times \Delta H_f^\circ (\text{Fe}_2\text{O}_3)ΔHreaction​=2×ΔHf∘​(Fe2​O3​)

Substituting the enthalpy of formation:ΔHreaction=2×(−824.2 kJ/mol)=−1648.4 kJ\Delta H_{\text{reaction}} = 2 \times (-824.2 \, \text{kJ/mol}) = -1648.4 \, \text{kJ}ΔHreaction​=2×(−824.2kJ/mol)=−1648.4kJ

This result indicates that 1648.4 kJ of heat is released when 4 moles of iron react with 3 moles of oxygen to form iron(III) oxide.

Calculating Heat Generated by 1 Gram of Iron

To find the heat generated by a specific mass of iron, say 1 gram, we use the following steps:

  1. Determine the Moles of Iron in the Absorber:
    • Assume we have 1 gram of iron.
    • The molar mass of iron (Fe) is approximately 55.85 g/mol.
    • The number of moles of iron, nnn, is:
    n=massmolar mass=1 g55.85 g/mol=0.0179 moln = \frac{\text{mass}}{\text{molar mass}} = \frac{1 \, \text{g}}{55.85 \, \text{g/mol}} = 0.0179 \, \text{mol}n=molar massmass​=55.85g/mol1g​=0.0179mol
  2. Calculate Heat Generated per Mole of Iron:
    • From the reaction, 4 moles of iron release 1648.4 kJ of heat.
    • Therefore, the heat released per mole of iron is:
    Heat per mole of Fe=1648.4 kJ4=412.1 kJ/mol\text{Heat per mole of Fe} = \frac{1648.4 \, \text{kJ}}{4} = 412.1 \, \text{kJ/mol}Heat per mole of Fe=41648.4kJ​=412.1kJ/mol
  3. Calculate Heat Generated for 0.0179 Moles of Iron:
    • For 0.0179 moles of iron, the heat generated is:
    Heat generated=0.0179 mol×412.1 kJ/mol=7.38 kJ\text{Heat generated} = 0.0179 \, \text{mol} \times 412.1 \, \text{kJ/mol} = 7.38 \, \text{kJ}Heat generated=0.0179mol×412.1kJ/mol=7.38kJ

Thus, approximately 7.38 kJ of heat is released by 1 gram of iron reacting fully with oxygen in an iron-based oxygen absorber. In a small packaging volume or with large amounts of absorbers, this heat can raise the temperature inside the package, potentially affecting its contents.