Hydrogen • Topic 3 of 3

Hydrogen Peroxide & Hydrogen as Fuel

Hydrogen peroxide (H2O2) is one of the most useful oxygen compounds of hydrogen, and dihydrogen itself is increasingly viewed as the clean fuel of the future.

Preparation

In the laboratory, dilute sulphuric acid is added to barium peroxide: BaO2·8H2O + H2SO4 → BaSO4 + H2O2 + 8H2O; the insoluble BaSO4 is filtered off. Industrially it is made by the auto-oxidation of 2-ethylanthraquinol (the anthraquinone process).

Structure (open-book)

The H2O2 molecule is non-planar with an open-book structure: the two O–H bonds lie in two different planes that meet along the O–O bond, like a half-open book. In the gas phase the dihedral angle is about 111° (about 90° in the solid), the O–O bond about 148 pm and each O–H bond about 95 pm. Pure H2O2 is a colourless, syrupy liquid, denser than water and miscible with it; it is sold as a 30% solution or a dilute 3% antiseptic.

Chemical properties — both oxidising and reducing

The oxygen in H2O2 is in the −1 state, midway between 0 and −2, so it can either gain or lose electrons, acting as both oxidising and reducing agent.

  • Oxidising (acidic): 2Fe2+ + H2O2 + 2H+ → 2Fe3+ + 2H2O; it also turns black PbS to white PbSO4, restoring old paintings.
  • Reducing (acidic): it decolourises acidified KMnO4: 2MnO4− + 5H2O2 + 6H+ → 2Mn2+ + 5O2 + 8H2O.
  • Reducing (basic): 2MnO4− + 3H2O2 → 2MnO2 + 3O2 + 2OH− + 2H2O.

Storage and uses

H2O2 decomposes (2H2O2 → 2H2O + O2) in light, dust, alkali or metal ions, so it is stored in dark, wax-lined bottles, kept cool, with a trace of stabiliser (urea or phosphoric acid). It is used as an antiseptic, a mild bleach, in restoring paintings, rocket propellant and pollution control.

Dihydrogen as a fuel

Hydrogen is an excellent fuel: its combustion (2H2 + O2 → 2H2O) releases a very high energy per gram (about 143 kJ g−1, far more than petrol) and the only product is water, so it is non-polluting. The vision of a hydrogen economy is to transport and store energy as hydrogen rather than as electricity or fossil fuels. In a fuel cell hydrogen and oxygen react electrochemically to generate electricity directly and efficiently, with water as the only by-product — the technology that powered the Apollo spacecraft. The main challenges are the safe, compact storage of a light, flammable gas and producing hydrogen cheaply without releasing CO2.

Open-book (non-planar) structure of hydrogen peroxide showing the dihedral angleOOHHdihedral ≈ 111°O–O 148 pmfront planeback plane
Solved Examples
1
Worked Example
Describe the open-book structure of hydrogen peroxide and state the gas-phase dihedral angle.
Solution
  1. H2O2 is non-planar: the two O–H bonds lie in two different planes that meet along the central O–O bond.
  2. The shape resembles a half-open book, hence the name open-book structure.
  3. In the gas phase the angle between the two planes (the dihedral angle) is about 111°.

Answer: A non-planar open-book structure with a gas-phase dihedral angle of about 111°.

2
Worked Example
Write the laboratory preparation of hydrogen peroxide from barium peroxide.
Solution
  1. Add cold dilute sulphuric acid to hydrated barium peroxide.
  2. The reaction is BaO2·8H2O + H2SO4 → BaSO4 + H2O2 + 8H2O.
  3. The insoluble BaSO4 is filtered off, leaving a solution of H2O2.

Answer: BaO2·8H2O + H2SO4 → BaSO4↓ + H2O2 + 8H2O.

3
Worked Example
Why can hydrogen peroxide act both as an oxidising agent and as a reducing agent?
Solution
  1. In H2O2 the oxidation state of oxygen is −1.
  2. This is intermediate between 0 (in O2) and −2 (in H2O).
  3. So oxygen can either be reduced to −2 (acting as oxidiser) or oxidised to 0 (acting as reducer), depending on the other reactant.

Answer: Because oxygen is in the intermediate −1 state, H2O2 can both gain and lose electrons.

4
Worked Example
Write the equation showing hydrogen peroxide acting as a reducing agent towards acidified potassium permanganate.
Solution
  1. Acidified KMnO4 (Mn in +7) is reduced to Mn2+.
  2. H2O2 is oxidised, releasing O2.
  3. Balanced: 2MnO4− + 5H2O2 + 6H+ → 2Mn2+ + 5O2 + 8H2O.

Answer: 2MnO4− + 5H2O2 + 6H+ → 2Mn2+ + 5O2 + 8H2O; the pink colour is discharged.

5
Worked Example
Why is hydrogen peroxide stored in dark, wax-lined bottles, and what stabiliser is added?
Solution
  1. H2O2 decomposes to water and oxygen: 2H2O2 → 2H2O + O2.
  2. Decomposition is speeded up by light, heat, dust, alkali and metal ions, so it is kept cool in dark plastic or wax-lined bottles away from rough glass surfaces.
  3. A trace of stabiliser such as urea or phosphoric acid is added to slow the decomposition.

Answer: To prevent light/surface-catalysed decomposition; a trace of urea or phosphoric acid is added as stabiliser.

6
Worked Example
State two advantages and one challenge of using dihydrogen as a fuel.
Solution
  1. Advantage 1: it has a very high energy output per gram (about 143 kJ g−1), much higher than petrol.
  2. Advantage 2: its combustion (2H2 + O2 → 2H2O) gives only water, so it is non-polluting.
  3. Challenge: it is a light, highly flammable gas that is difficult and expensive to store and transport safely, and producing it cleanly is costly.

Answer: Advantages: very high energy per gram and clean (only water); challenge: safe, compact storage and cheap clean production.

Key Points

  • H2O2 is prepared in the lab from BaO2·8H2O + H2SO4 and industrially by the anthraquinone (2-ethylanthraquinol) process.
  • It has a non-planar open-book structure with a gas-phase dihedral angle of about 111° and an O–O bond of about 148 pm.
  • Because oxygen is in the −1 state, H2O2 acts both as an oxidising agent (e.g. PbS → PbSO4) and as a reducing agent (e.g. decolourising acidified KMnO4).
  • It is stored in dark, wax-lined bottles, kept cool, with urea or phosphoric acid as stabiliser, because it decomposes to water and oxygen.
  • Dihydrogen is a clean, high-energy fuel (only water on combustion) central to the hydrogen economy and fuel cells; safe storage and cheap clean production are the main hurdles.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.The structure of hydrogen peroxide is best described as:
Explanation: H2O2 is non-planar, with the two O–H bonds in different planes meeting along the O–O bond, like a half-open book.
Q2.The oxidation state of oxygen in hydrogen peroxide is:
Explanation: Oxygen is in the −1 state, intermediate between 0 and −2, which lets H2O2 act as both oxidiser and reducer.
Q3.When H2O2 decolourises acidified KMnO4, it behaves as:
Explanation: It reduces Mn in +7 to Mn2+ while being oxidised to O2, so here H2O2 is the reducing agent.
Q4.A major advantage of dihydrogen as a fuel is that its combustion produces:
Explanation: 2H2 + O2 → 2H2O gives only water, making hydrogen a clean, non-polluting fuel.
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