In Which Reaction Does The Oxidation Number Of Hydrogen Change

The oxidation number of hydrogen changes in reactions where hydrogen either gains or loses electrons, altering its charge state within a compound. Understanding when and how these changes occur provides key insights into redox (reduction-oxidation) chemistry and the behavior of hydrogen in various chemical environments.

Oxidation Number Basics

Before diving into specific reactions, let’s establish some foundational principles about oxidation numbers. The oxidation number, also known as the oxidation state, represents the hypothetical charge an atom would have if all bonds were completely ionic. Here are some rules to remember:

• The oxidation number of an atom in its elemental form is always 0. For example, H₂ has an oxidation number of 0.
• The oxidation number of a monatomic ion is equal to its charge. For example, Na⁺ has an oxidation number of +1, and Cl⁻ has an oxidation number of -1.
• Oxygen usually has an oxidation number of -2 in compounds, except in peroxides (like H₂O₂) where it is -1, or when combined with fluorine, where it is positive.
• Hydrogen usually has an oxidation number of +1 in compounds, except when combined with metals, where it is -1.
• The sum of the oxidation numbers in a neutral compound is 0, and in a polyatomic ion, it equals the charge of the ion.

Reactions Where the Oxidation Number of Hydrogen Changes

Hydrogen’s oxidation number changes when it is either oxidized (loses electrons, oxidation number increases) or reduced (gains electrons, oxidation number decreases). Here are several types of reactions where these changes occur:

1. Formation of Hydrides

What is a Hydride?

A hydride is a compound in which hydrogen is bonded to a more electropositive element. In these compounds, hydrogen has an oxidation number of -1.

Reaction with Active Metals

When hydrogen reacts with active metals (such as alkali metals and alkaline earth metals), it forms ionic hydrides. In this process, hydrogen gains electrons and is reduced.

• Example: Formation of Sodium Hydride (NaH)

  2Na(s) + H₂(g) → 2NaH(s)

  In this reaction:

  • Na starts with an oxidation number of 0 and ends with +1.
  • H₂ starts with an oxidation number of 0 and ends with -1.

  Here, hydrogen is reduced from 0 to -1, while sodium is oxidized from 0 to +1. This is a clear example of a redox reaction where hydrogen’s oxidation number changes.

Reaction with Transition Metals

Hydrogen can also form hydrides with transition metals, although these are often non-stoichiometric and have more complex structures.

• Example: Formation of Titanium Hydride (TiH₂)

  Ti(s) + H₂(g) → TiH₂(s)

  In this reaction:

  • Ti starts with an oxidation number of 0 and ends with +2.
  • H₂ starts with an oxidation number of 0 and ends with -1.

  Again, hydrogen is reduced, and its oxidation number changes from 0 to -1.

2. Reactions with Oxygen

Formation of Water

One of the most common reactions involving hydrogen is its reaction with oxygen to form water.

• Example:

  2H₂(g) + O₂(g) → 2H₂O(l)

  In this reaction:

  • H₂ starts with an oxidation number of 0 and ends with +1.
  • O₂ starts with an oxidation number of 0 and ends with -2.

  Here, hydrogen is oxidized from 0 to +1, while oxygen is reduced from 0 to -2. This is an exothermic reaction, releasing a significant amount of energy, often observed as an explosion when uncontrolled.

Formation of Hydrogen Peroxide

Hydrogen can also react with oxygen to form hydrogen peroxide (H₂O₂).

• Example:

  H₂(g) + O₂(g) → H₂O₂(l)

  In this reaction:

  • H₂ starts with an oxidation number of 0 and ends with +1.
  • O₂ starts with an oxidation number of 0 and ends with -1.

  Again, hydrogen is oxidized from 0 to +1, but in this case, oxygen is reduced to -1.

3. Reactions with Halogens

Formation of Hydrogen Halides

Hydrogen reacts with halogens (fluorine, chlorine, bromine, iodine) to form hydrogen halides.

• Example: Formation of Hydrogen Chloride (HCl)

  H₂(g) + Cl₂(g) → 2HCl(g)

  In this reaction:

  • H₂ starts with an oxidation number of 0 and ends with +1.
  • Cl₂ starts with an oxidation number of 0 and ends with -1.

  Hydrogen is oxidized from 0 to +1, while chlorine is reduced from 0 to -1. The reactivity of halogens with hydrogen decreases down the group (F > Cl > Br > I), with fluorine reacting explosively and iodine reacting more slowly.

4. Reactions with Nitrogen

Haber-Bosch Process

The Haber-Bosch process is an industrial process for producing ammonia (NH₃) from nitrogen and hydrogen.

• Example:

  N₂(g) + 3H₂(g) → 2NH₃(g)

  In this reaction:

  • H₂ starts with an oxidation number of 0 and ends with +1.
  • N₂ starts with an oxidation number of 0 and ends with -3.

  Hydrogen is oxidized from 0 to +1, while nitrogen is reduced from 0 to -3. This reaction is crucial for the production of fertilizers and has had a significant impact on agriculture.

5. Displacement Reactions

Displacement of Hydrogen from Acids

Metals that are more reactive than hydrogen can displace hydrogen from acids.

• Example: Reaction of Zinc with Hydrochloric Acid

  Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

  In this reaction:

  • Zn starts with an oxidation number of 0 and ends with +2.
  • H⁺ in HCl starts with an oxidation number of +1 and ends as H₂ with 0.

  Here, zinc is oxidized from 0 to +2, while hydrogen is reduced from +1 to 0. The hydrogen gas is displaced from the acid by the more reactive zinc.

Displacement of Hydrogen from Water

Very active metals can also displace hydrogen from water.

• Example: Reaction of Sodium with Water

  2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

  In this reaction:

  • Na starts with an oxidation number of 0 and ends with +1.
  • H in H₂O starts with an oxidation number of +1 and ends as H₂ with 0.

  Sodium is oxidized from 0 to +1, while hydrogen is reduced from +1 to 0. This reaction is highly exothermic and can be dangerous due to the rapid release of heat and the formation of flammable hydrogen gas.

6. Organic Reactions

Hydrogenation Reactions

Hydrogenation is a chemical reaction where hydrogen is added to a molecule, typically an unsaturated organic compound, in the presence of a catalyst.

• Example: Hydrogenation of Ethene to Ethane

  C₂H₄(g) + H₂(g) → C₂H₆(g)

  In this reaction:

  • The oxidation number of hydrogen in H₂ starts at 0 and changes to +1 in ethane.
  • Carbon’s oxidation number changes from -2 in ethene to -3 in ethane.

  Here, hydrogen is added across the double bond of ethene, converting it to ethane. The oxidation number of hydrogen changes as it forms new bonds with carbon.

Reduction Reactions in Organic Chemistry

Hydrogen is often involved in reduction reactions of organic compounds, such as the reduction of aldehydes and ketones to alcohols.

• Example: Reduction of Acetaldehyde to Ethanol

  CH₃CHO(aq) + H₂(g) → CH₃CH₂OH(aq)

  In this reaction:

  • Hydrogen’s oxidation number changes from 0 in H₂ to +1 in ethanol.
  • Carbon’s oxidation number in the carbonyl group is reduced as it gains bonds to hydrogen.

  Hydrogen is used to reduce the carbonyl group (C=O) to an alcohol (C-OH).

7. Complex Redox Reactions

Reactions Involving Complex Ions

In complex redox reactions, hydrogen can be part of complex ions or molecules that undergo oxidation or reduction.

• Example: Redox Reaction with Permanganate in Acidic Solution

  5H₂C₂O₄(aq) + 2KMnO₄(aq) + 6HCl(aq) → 2MnCl₂(aq) + 10CO₂(g) + 8H₂O(l) + 2KCl(aq)

  In this reaction:

  • Manganese (Mn) is reduced from +7 in KMnO₄ to +2 in MnCl₂.
  • Carbon (C) is oxidized from +3 in H₂C₂O₄ to +4 in CO₂.
  • Hydrogen’s oxidation number remains +1, but it is a crucial component of the reactants and products.

  While hydrogen’s oxidation number does not change directly, its presence in water and oxalic acid is essential for the reaction to proceed.

Factors Affecting the Oxidation Number of Hydrogen

Several factors influence whether hydrogen’s oxidation number will change in a reaction:

1. Electronegativity: Hydrogen’s oxidation number depends on the electronegativity of the element it is bonded to. When bonded to more electronegative elements (like oxygen, chlorine), it has an oxidation number of +1. When bonded to more electropositive elements (like sodium, calcium), it has an oxidation number of -1.
2. Reaction Conditions: The conditions under which the reaction occurs (e.g., temperature, pressure, presence of catalysts) can affect the outcome. For example, high temperatures and pressures are required for the Haber-Bosch process to produce ammonia.
3. Availability of Electrons: The presence of strong oxidizing or reducing agents can influence the transfer of electrons to or from hydrogen. Strong oxidizing agents (like fluorine) will readily oxidize hydrogen, while strong reducing agents (like alkali metals) will reduce hydrogen.

Importance of Understanding Changes in Oxidation Number

Understanding when and how the oxidation number of hydrogen changes is crucial for several reasons:

• Predicting Reaction Outcomes: By knowing the oxidation states of reactants, you can predict the products of a reaction and whether it will proceed spontaneously.
• Balancing Redox Equations: Assigning oxidation numbers is essential for balancing complex redox equations, ensuring that the number of electrons lost equals the number of electrons gained.
• Understanding Reaction Mechanisms: Changes in oxidation numbers provide insights into the mechanisms of chemical reactions, showing how electrons are transferred between atoms.
• Industrial Applications: Many industrial processes, such as the production of ammonia, the refining of metals, and the synthesis of organic compounds, rely on redox reactions involving hydrogen.

Conclusion

The oxidation number of hydrogen changes in a variety of chemical reactions, including the formation of hydrides, reactions with oxygen and halogens, and displacement reactions. In these processes, hydrogen can be either oxidized (loses electrons, oxidation number increases) or reduced (gains electrons, oxidation number decreases), depending on the electronegativity of the other reactants and the reaction conditions. Understanding these changes is fundamental to comprehending redox chemistry and its applications in various scientific and industrial fields.

FAQ

1. In what compounds does hydrogen have an oxidation number of -1?

   • Hydrogen has an oxidation number of -1 in hydrides, which are compounds formed with more electropositive metals such as alkali and alkaline earth metals (e.g., NaH, CaH₂).

2. Why does hydrogen usually have an oxidation number of +1?

   • Hydrogen usually has an oxidation number of +1 because it is generally bonded to more electronegative elements like oxygen and halogens, causing it to lose an electron and attain a positive charge state.

3. Can hydrogen have an oxidation number other than +1 or -1?

   • Yes, hydrogen can have an oxidation number of 0 in its elemental form (H₂). Additionally, in some exotic compounds, it can exhibit fractional oxidation states, though these are rare.

4. How does the oxidation number of hydrogen change in the formation of water?

   • In the formation of water (2H₂ + O₂ → 2H₂O), hydrogen is oxidized from 0 in H₂ to +1 in H₂O.

5. What is the significance of understanding the oxidation number changes in redox reactions?

   • Understanding oxidation number changes is crucial for predicting reaction outcomes, balancing redox equations, understanding reaction mechanisms, and optimizing industrial applications that rely on redox reactions.

6. What role does electronegativity play in determining the oxidation number of hydrogen?

   • Electronegativity is a key factor. When hydrogen is bonded to a more electronegative element, it has a +1 oxidation number because it loses an electron. Conversely, when bonded to a more electropositive element, it has a -1 oxidation number because it gains an electron.

7. In organic chemistry, how does hydrogenation affect the oxidation number of hydrogen?

   • In hydrogenation reactions, hydrogen is added to unsaturated organic compounds, and its oxidation number changes from 0 in H₂ to +1 as it forms new bonds with carbon, reducing the carbon atoms in the process.