Balance Chemical Equations

A chemical equation is a written symbolic representation of a chemical reaction. The reactant chemical(s) are given on the left-hand side and the product chemical(s) on the right-hand side. The law of conservation of mass states that no atoms can be created or destroyed in a chemical reaction, so the number of atoms that are present in the reactants has to balance the number of atoms that are present in the products. Follow this guide to learn how to balance chemical equations differently.

Steps

Traditional balance

  1. Write down your given equation. For this example, you will use:
    • C3H8 + O2 --> H2O + CO2
    • This reaction occurs when propane (C3H8) is burned in the presence of oxygen to produce water and carbon dioxide.
  2. Write down the number of atoms per each element that you have on each side of the equation. Look at the subscripts next to each atom to find the number of atoms in the equation.
    • Left side: 3 carbon, 8 hydrogen and 2 oxygen.
    • Right side: 1 carbon, 2 hydrogen and 3 oxygen.
  3. Always leave hydrogen and oxygen for last.
  4. If you have more than one element left to balance: select the element that appears in only a single molecule of reactants and in only a single molecule of products. This means that you will need to balance the carbon atoms first.
  5. Add a coefficient to the single carbon atom on the right of the equation to balance it with the 3 carbon atoms on the left of the equation.
    • C3H8 + O2 --> H2O + 3CO2
    • The coefficient 3 in front of carbon on the right side indicates 3 carbon atoms just as the subscript 3 on the left side indicates 3 carbon atoms.
    • In a chemical equation, you can change coefficients, but you must never alter the subscripts.
  6. Balance the hydrogen atoms next. You have 8 on the left side. So you'll need 8 on the right side.
    • C3H8 + O2 --> 4H2O + 3CO2
    • On the right side, you now added a 4 as the coefficient because the subscript showed that you already had 2 hydrogen atoms.
    • When you multiply the coefficient 4 times by the subscript 2, you end up with 8.
    • The other 6 atoms of Oxygen come from 3CO2.(3x2=6 atoms of oxygen+ the other 4=10)
  7. Balance the oxygen atoms.
    • Because you've added coefficients to the molecules on the right side of the equation, the number of oxygen atoms has changed. You now have 4 oxygen atoms in the water molecules and 6 oxygen atoms in the carbon dioxide molecules. That makes a total of 10 oxygen atoms.
    • Add a coefficient of 5 to the oxygen molecule on the left side of the equation. You now have 10 oxygen atoms on each side.
    • C3H8 + 5O2 --> 4H2O + 3CO2.
    • The carbon, hydrogen, and oxygen atoms are balanced. Your equation is complete.

Algebraic balance

  1. Write the equation by symbol and formula. Say a=1 and write the equation base on the formula.
  2. Substitute the numbers by their variable.
  3. Check the number of elements found in the reactant side and also in the product side.
    • Example: aPCl5 + bH2O = cH3PO4 + dHCl let a=1 b= c= d= and separate the elements they are P, Cl, H, O, so you get a=1 b=4 c=1 d=5.



Tips

  • If you're stuck, you can type the equation into the online balancer to balance it. Just remember that you won't have access to an online balancer when you're taking an exam, so don't become dependent on it.
  • Remember to simplify!
  • Better to balance with algebraic balance.
  • Always write down the method it will help you!!

Warnings

  • Never use fractions as coefficients in a chemical equation--you never make half of a molecule or half of an atom in a chemical reaction.
  • During the balancing process, you may use fractions to assist you, but the equation is not balanced as long as there are still coefficients using fractions.
  • To get rid of fractions, multiply the entire equation (both the left and right sides) by the number in the denominator of your fraction.

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Sources and Citations

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