By now you are all wizards at balancing equations. You know the number of atoms of each element must be the same on the reactant (left) side of the arrow as on the product (right) side of the arrow. Let’s use a reaction you should be familiar with from the conservation of mass lab (Lesson 71): aqueous calcium chloride reacts with aqueous sodium carbonate to produce solid calcium carbonate and aqueous sodium chloride:
Here’s the balanced equation:
CaCl2(aq) + Na2CO3(aq) → CaCO3(s) + 2NaCl(aq)
Quick vocabulary review and a piece of important new learning:
- (aq) means aqueous (in water)
- (s) means solid
- In a chemical equation, the arrow points from the reactants toward the products. As written above, the reactants are on the left and the products are on the right.
- The letters represent element symbols from the periodic table
- The numbers represent subscripts and coefficients.
- Subscripts are the small integers (whole numbers) to the lower right of an element symbol and they cannot be changed. Subscripts tell how many atoms of each element are present in a given molecule. In the equation above, the small 2 after Cl tells us there are two chlorine atoms bonded with 1 calcium atom in one molecule of CaCl2.
- Coefficients are the regular-sized integers to the left of a molecule. This is new and important: Coefficients tell us the correct mole ratio in which the reactants combine to form the products. In the equation above, we now know that one mole of aqueous calcium chloride reacts with one mole of with aqueous sodium carbonate to produce one mole of solid calcium carbonate and two moles of aqueous sodium chloride.
Another example: N2(g) + 3H2(g) → 2NH3(s) This equation tells us that one mole of nitrogen gas reacts with 3 moles of hydrogen gas to produce one mole of solid ammonia.
Even more examples: