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Chemical Formula and Chemical Equations

Chemical Formula
  • The chemical formula of a substance is a representation of the substance using symbols.
  • Two atoms of hydrogen combine together to form a molecule of hydrogen. A molecule of hydrogen is represented by H2.
  • One atom of oxygen combines with two atoms of hydrogen to form one molecule of water. A molecule of water is represented by the chemical formula H2O.
Combining Power (Valency)
  • The ability of atoms of elements to combine with hydrogen is referred to as their combining power or valency.
  • One atom of oxygen can combine with two atoms of hydrogen, H2O. Oxygen here is said to have the combining power of two.
  • One atom of nitrogen can combine with three atoms of hydrogen, NH3. Nitrogen here is said to have the combining power of three.
  • Chemical formula, like chemical symbols, are written in a particular way to obey certain rules.
  • In a compound, where we have more than one type of atom chemically combined, the formula of such a substance is again written to express the type and number of atoms present.
  • Chemical formulas always have both letters and numbers; a combination of letters and numbers representing symbols and the number of atoms in the molecule that are used. For example:
    • CH4 (Methane) – This contains one carbon atom (C) combined with four atoms of hydrogen (H).
    • C4H10 (Butane) – This contains four atoms of carbon combined with ten atoms of hydrogen.
Chemical Equations
  • A chemical equation is used to describe the initial substance and end products in a chemical reaction.
Rules to be Followed in Writing and Balancing a Chemical Equation
  • The reactants and products must be known.
  • Write a word equation.
  • Write the formula of all the reactants and all the products.
  • Check the number of each type of atom of the reactant(s) and the product(s).
  • Balance the equation by supplying coefficients that will make the number of atoms of each element, in the reactants and products, the same. For example:
    • H2 + O2 → H2O
      2H2 + O2 → 2H2O
    • N2 + H2 → NH3
      N2 + 3H2 → 2NH3
    • KClO3 → KCl + O2
      2KClO3 → 2KCl + 3O2
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