Balancing of chemical equation practice: Stoichiometry is a section of chemistry that involves using relationships between reactants and/or products in a chemical reaction to determine desired quantitative data. In Greek, stoikhein means element and metron means measures, so stoichiometry literally translated means the measure of elements.
- balancing equations
- chemical reactions
- worked chemistry problems
A chemical equation describes what happens in a chemical reaction. The equation identifies the reactants (starting materials) and products (resulting substance), the formulas of the participants, the phases of the participants (solid, liquid, gas), and the amount of each substance.
Balancing a chemical equation refers to establishing the mathematical relationship between the quantity of reactants and products. The quantities are expressed as grams or moles. It takes practice to be able to write balanced equations. There are essentially three steps to the process:
- Write the unbalanced equation.
Chemical formulas of reactants are listed on the lefthand side of the equation.
Products are listed on the righthand side of the equation.
Reactants and products are separated by putting an arrow between them to show the direction of the reaction. Reactions at equilibrium will have arrows facing both directions.
Balance the equation.
Apply the Law of Conservation of Mass to get the same number of atoms of every element on each side of the equation. Tip: Start by balancing an element that appears in only one reactant and product.
Once one element is balanced, proceed to balance another, and another until all elements are balanced.
Balance chemical formulas by placing coefficients in front of them. Do not add subscripts, because this will change the formulas.
Indicate the states of matter of the reactants and products.
Use (g) for gaseous substances.
Use (s) for solids.
Use (l) for liquids.
Use (aq) for species in solution in water.
Write the state of the matter immediately following the formula of the substance it describes.
Worked Example Problem
Tin oxide is heated with hydrogen gas to form tin metal and water vapor. Write the balanced equation that describes this reaction.
1. Write the unbalanced equation. SnO2 + H2 → Sn + H2O
Refer to Table of Common Polyatomic Ions and Formulas of Ionic Compounds if you have trouble writing the chemical formulas of the products and reactants.
SnO2 + H2 → Sn + H2O unbalanced equation
If you look very well you will see that all the element is balance apart from oxygen which is not balanced, so multiply H2O by 2
SnO2 + H2 → Sn + 2H2O unbalanced equation
The product is balance but the reactant is not balanced so multiply H2 by 2
SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g) balanced equation
Look at the equation and see which elements are not balanced. In this case, there are two oxygen atoms on the lefthand side of the equation and only one on the righthand side. Correct this by putting a coefficient of 2 in front of water:
SnO2 + H2 → Sn + 2 H2O
This puts the hydrogen atoms out of balance. Now there are two hydrogen atoms on the left and four hydrogen atoms on the right. To get four hydrogen atoms on the right, add a coefficient of 2 for the hydrogen gas. Remember, coefficients are multipliers, so if we write 2 H2O it denotes 2×2=4 hydrogen atoms and 2×1=2 oxygen atoms.
SnO2 + 2 H2 → Sn + 2 H2O
The equation is now balanced. Be sure to double-check your math! Each side of the equation has 1 atom of Sn, 2 atoms of O, and 4 atoms of H.
You have to Indicate the physical states of the reactants and products. To do this, you need to be familiar with the properties of various compounds or you need to be told what the phases are for the chemicals in the reaction. Oxides are solids, hydrogen forms a diatomic gas, tin is solid, and the term ‘water vapor’ indicates that water is in the gas phase:
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