ELI5: Why Is the Reaction Written as 2H2 + O2 = 2H2O and Not 2H2O2?
The reaction 2H2 + O2 = 2H2O represents the formation of water, not hydrogen peroxide, because it balances the number of hydrogen and oxygen atoms correctly according to the law of conservation of atoms.
Meaning of the Numbers in the Equation
The numbers before the molecules are called coefficients. They tell how many molecules you have:
- 2H2 means 2 hydrogen molecules, each with 2 hydrogen atoms, so a total of 4 hydrogen atoms.
- O2 is 1 oxygen molecule with 2 oxygen atoms.
- 2H2O means 2 water molecules, each with 2 hydrogen atoms and 1 oxygen atom.
Atom Counting and Balancing
In the reactants:
- Hydrogen atoms: 4 (from 2H2)
- Oxygen atoms: 2 (from O2)
In the product 2H2O:
- Hydrogen atoms: 4 (2 molecules × 2 H atoms)
- Oxygen atoms: 2 (2 molecules × 1 O atom)
The atom count is equal on both sides, satisfying the law of conservation of atoms.
If the product were 2H2O2 (hydrogen peroxide):
- Hydrogen atoms: 4 (same as before)
- Oxygen atoms: 4 (2 molecules × 2 O atoms)
This would require 4 oxygen atoms on the reactant side to balance, but only 2 oxygen atoms are present in O2.
Molecular Structure Representation
The balanced equation shows how atoms rearrange:
H-H + O=O → H-O-H (water)
The formation of hydrogen peroxide would need a different reaction path and more oxygen atoms.
Correct Chemical Notation
It is important to write chemical formulas accurately:
- H2O for water (2 hydrogens, 1 oxygen)
- H2O2 for hydrogen peroxide (2 hydrogens, 2 oxygens)
Using the right formula helps understand the actual chemical process happening in the reaction.
Key Takeaways
- Coefficients show the number of molecules; subscripts show atoms in each molecule.
- The equation 2H2 + O2 = 2H2O balances hydrogen and oxygen atoms correctly.
- Forming 2H2O2 would require twice as many oxygen atoms as reactants provide.
- The law of conservation of atoms requires the same number of each atom before and after the reaction.
- Water is the correct product under these conditions, not hydrogen peroxide.
Why can’t the reaction 2H2 + O2 produce 2H2O2 instead of 2H2O?
Because 2H2O2 contains 4 oxygen atoms while the reactants only have 2 oxygen atoms. This doesn’t balance the atoms on both sides, which is required by the law of conservation of atoms.
What do the numbers in the chemical equation 2H2 + O2 = 2H2O mean?
The numbers in front are coefficients showing how many molecules. “2H2” means two hydrogen molecules (4 hydrogen atoms). “O2” means one oxygen molecule (2 oxygen atoms). The equation balances atoms on both sides.
How does balancing atoms help decide the right product in the reaction?
The atoms must be the same before and after the reaction. Here, 4 hydrogens and 2 oxygens on left must equal the right. 2H2O matches this, but 2H2O2 would have too many oxygen atoms.
Is H2O the only possible product when hydrogen reacts with oxygen?
In simple combustion, yes. Water (H2O) forms because the reaction favors a balance with 2 hydrogens per oxygen atom. Other products like H2O2 form under different conditions, not in this basic reaction.
What does the subscript “2” in H2O signify?
It shows each water molecule has 2 hydrogen atoms. The “O” has no number, meaning just one oxygen atom. This helps keep track of atom counts for balancing equations properly.
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