# 4.1: Chemical Reactions and Chemical Equations


$2H_2 + O_2 \rightarrow 2H_2O$

Chemical formulas and other symbols are used to indicate the starting materials, or reactants, which by convention are written on the left side of the equation, and the final compounds, or products, which are written on the right. An arrow points from the reactant to the products. The chemical reaction for the ammonium dichromate volcano in Figure $$\PageIndex{1}$$ is

$(NH_4)_2Cr_2O_7 \rightarrow Cr_2O_3 + N_2 + 4H_2O \label{4.1.1}$

$reactant \, \, \, \, \,\, \, \, \, \, \, \,\,\, products$

The arrow is read as “yields” or “reacts to form.” Equation 4.1.1 indicates that ammonium dichromate (the reactant) yields chromium(III) oxide, nitrogen, and water (the products). The equation for this reaction is even more informative when written as follows:

$(NH_4)_2Cr_2O_7(s) \rightarrow Cr_2O_{3\;(s)} + N_{2\;(g)} + 4H_2O_{(g)} \label{4.1.2}$

Equation 4.1.2 is identical to Equation 4.1.1 except for the addition of abbreviations in parentheses to indicate the physical state of each species. The abbreviations are (s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution, a solution of the substance in water.

Consistent with the law of conservation of mass, the numbers of each type of atom are the same on both sides of Equations 4.1.1 and 4.1.2. Each side of the reaction has two chromium atoms, seven oxygen atoms, two nitrogen atoms, and eight hydrogen atoms.

In a balanced chemical equation, both the numbers of each type of atom and the total charge are the same on both sides. Equations 4.1.1 and 4.1.2 are balanced chemical equations. What is different on each side of the equation is how the atoms are arranged to make molecules or ions. A chemical reaction represents a change in the distribution of atoms, but not in the number of atoms. In this reaction, and in most chemical reactions, bonds are broken in the reactants (here, Cr–O and N–H bonds), and new bonds are formed to create the products (here, O–H and N≡N bonds). If the numbers of each type of atom are different on the two sides of a chemical equation, then the equation is unbalanced, and it cannot correctly describe what happens during the reaction. To proceed, the equation must first be balanced.

of two substances in a chemical reaction is the ratio of their coefficients in the balanced chemical equation.

Introduction to Chemical Reaction Equations: https://youtu.be/5mjawuf7K2Q

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