Chemistry Notes: Chemical Reactions

Chemistry 2014-2015

Balancing EquationsW 12/10

In a chemical reaction equation… Reactants are written on the left and

products are written on the right Ex. NaOH + HCl H2O + NaCl

Sodium hydroxide and hydrochloric acid are the reactants, water and sodium chloride are the products. An arrow acts as the “equals sign” of the

reaction equation, pointing from the reactants to the products. This arrow means “yields” or “produces”.

The numbers written in front of each reactant or product are called coefficients. A coefficient indicates how many atoms, molecules, or formula units of each substance are present in the reaction. If no coefficient is written, the coefficient is implied to be one.

Ex. Al2(SO4)3 + 3 CaCl2 2 AlCl3 + 3 CaSO4

Review

We can use the symbols (s), (l), (g), and (aq) to show that an element or compound is a solid, liquid, gas, or aqueous solution, respectively. For example, liquid water is written H2O(l) while ice is written H2O(s).

The Law of Conservation of Mass states that matter cannot be created or destroyed, but it can be changed from one form to another or moved around. In a chemical reaction equation, this means that the number of atoms of each element have to be the same on the products side and the reactants side.

Reactants: ____ atom(s) of carbon, ____ atom(s) of hydrogen, ____ atom(s) of oxygen

Products: ____ atom(s) of carbon, ____ atom(s) of hydrogen, ____ atom(s) of oxygen

Ex. CH4 + 2 O2 CO2 + 2 H2O

Reactants: ____ atom(s) of aluminum, ____ sulfate ion(s), ____ atom(s) of calcium, ____ atom(s) of chlorine

Products: ____ atom(s) of aluminum, ____ sulfate ion(s), ____ atom(s) of calcium, ____ atom(s) of chlorine

You can keep polyatomic ions grouped together—

Ex Al2(SO4)3 + 3 CaCl2 2 AlCl3 + 3 CaSO4

In order to balance a chemical reaction equation, you write coefficients in front of each reactant/product until you have an equal number of each element or polyatomic ion on both sides of the equation. Begin balancing with the most complicated-looking

group. Save the elemental (single elements) reactant and

products for last, especially if it is hydrogen or oxygen. If you get stuck, double the most complicated-looking

group and try again. Finally, make sure that all coefficients are in the

lowest-possible ratio. You cannot change subscripts in a chemical

reaction equation.

____ H2 (g) + ____ O2 (g) ____ H2O (l)

____ AgNO3 (aq) + _____Cu (s) _____ Cu(NO3)2 (aq) + _____ Ag (s)

____ Al + _____ O2 _____ Al2O3

Practice Problems: Balance the following chemical reaction equations.