Balancing Chemical Equations

Chemical Equations Quick Check

A chemical equation shows the starting materials called reactants and the products made by the reaction. The arrow on the left side of the equal sign indicates that the reaction proceeds in one direction.

A balanced chemical equation obeys the law of conservation of mass and is written so that the numbers of atoms of each element are the same on both sides of the equation. This is achieved by adding stoichiometric coefficients to the formulas.

Subscripts

In chemical equations, the little number that follows an element symbol is called a subscript. It indicates how many atoms of that element are in a molecule. For example, the 2 in H2O shows that there are two oxygen atoms per water molecule.

Students should always know that the big number in front of a chemical formula (coefficient) is multiplied by the little number that follows the symbol for the element to find the total number of atoms in the molecule of the compound or substance. The result should match the numbers in the reactants and products sides of the chemical equation.

The term stoichiometric coefficients refers to the number of atoms or molecules of each element that is needed in a reaction to balance it. When balancing chemical equations, students should focus on balancing the stoichiometric coefficients first. This strategy is sometimes referred to as “balancing by inspection”. It involves looking at the numbers of atoms of each type of molecule in the reactants and products, then adding the stoichiometric coefficients to make sure that the total number of atoms is the same on both sides of the chemical equation.

Coefficients

A chemical equation is a written description of what happens during a reaction. It lists the starting materials, called reactants, on one side and the products made by the reaction on the other. A plus sign separates the two sides and an arrow indicates the direction of the reaction. Chemical reactions often break bonds and produce new molecules. The numbers of each type of atom on each side must match to comply with the Law of Conservation of Mass, which states that matter is never created or destroyed but always rearranged.

The small whole numbers that appear in front of the formulas are known as coefficients. They help you balance the chemical equation by matching the number of atoms of each type on both sides. For example, if there are 3 carbon atoms on the left side of the equation and 2 on the right, you can balance it by adding a coefficient of 3. This increases the number of carbon atoms on the right side of the equation to match the number of oxygen atoms in the products.

Symbols

Chemical equations show the relationship between the initial substances (reactants) and the new substances that form when a reaction takes place. The reactant substances are written on the left side of the equation and the product substances are on the right. A line (called an arrow) separates the two sides of the equation. An arrow pointing to the right means that the reaction is proceeding forward, while an arrow pointing down means that the reaction is occurring backward.

The arrow is balanced by changing the coefficients on either side of the arrow. A coefficient is a number that appears in front of a chemical formula and provides the ratio between the chemical species on each side of the arrow.

In addition to the arrow, chemical equations also contain symbols that indicate the state of matter for each substance. If a chemical is a solid, the symbol is (s). If it is liquid, the symbol is (l). If it is gaseous, the symbol is (g). The shorthand notation for an aqueous solution is (aq). In some chemical reactions, a symbol called delta can be used above the arrow to indicate that the reaction requires energy in the form of heat.

Directions

A chemical equation is a written representation of a chemical reaction. It shows the reactants on the left and the products on the right. A plus sign separates them and an arrow indicates the direction of the reaction (reactants form products). The law of conservation of mass states that the number of atoms in the reactants must be equal to the number of atoms in the products.

To balance an unbalanced chemical equation, start with one of the least complex substances and work your way up to the most complex. Then, use a trial and error method of adjusting the coefficients until the number of atoms is the same on both sides of the equation.

The process of balancing an equation sounds simple enough, but it’s actually quite difficult to do. It requires a lot of practice and common sense. It’s important to check your work, especially if you’re using fractional values for the stoichiometric coefficients.

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