Matter, Atoms, and Bonds

Chemical Reactions

Chemical reactions are constantly taking place, forming molecules from atoms that chemically bond together.
A chemical reaction occurs when two or more atoms form chemical bonds or when chemical bonds between atoms break. In all chemical reactions, the chemical identity of at least one substance changes. Some chemical reactions include the oxidation of iron (also known as rusting) and the release of chlorine gas and other compounds from the mixing of bleach and ammonia.
In chemical reactions, bonds are broken and formed between atoms. Chemical equations are used to represent chemical reactions. In this reaction, methane and oxygen react to produce carbon dioxide and water. Chemical reactions may or may not be reversible.
Chemical equations are used to represent chemical reactions. A reactant is a substance that reacts in a chemical reaction. Reactants are found on the left side of the equation. A product is a substance formed as the result of a chemical reaction. Products are found on the right side of the equation. For example, the equation CH4 + 2O2 → CO2 + 2H2O represents the combustion (burning) of methane. Methane (CH4) and oxygen gas (O2) are the reactants, so they are listed on the left side of the equation. The arrow represents the fact that the chemical reaction is irreversible; that is, once the products have been formed, they cannot be returned to their original state. The products are carbon dioxide (CO2) and water vapor (H2O), which are listed on the right side of the equation.

Chemical equations can be written unbalanced, that is, with an unequal number of atoms in the reactants versus the products, or balanced, with an equal number of atoms on each side of the equation. A balanced chemical equation adheres to the law of conservation of matter. This law states the number of atoms before and after a chemical reaction are equal to ensure atoms are not created or destroyed. Therefore, a balanced equation better represents the way the reaction takes place in the real world. In the example of the combustion of methane, the unbalanced chemical equation reads CH4 + O2 → CO2 + H2O. In this equation, there are four hydrogen atoms in the reactants, but only two hydrogen atoms in the products. Adding a coefficient of two to H2O corrects the number of hydrogen atoms, but means that the number of oxygen atoms is no longer balanced. Thus, a coefficient of two must be added to oxygen in the reactants as well. The equation is now fully balanced.

Chemical Equilibrium

Chemical equilibrium occurs when chemical reactions occur in the forward and reverse directions at the same rate.
Chemical equilibrium is a state of balance in which forward and reverse chemical reactions are happening at the same rate. Chemical equilibrium plays a role in the reversibility of a chemical reaction. The concentration ratio of reactants and products is also stabilized during equilibrium. The arrow's direction in a chemical equation indicates how a chemical reaction proceeds. Chemical reactions are irreversible when the arrow only points toward the products. This indicates that the reaction will proceed until all reactants are used. For example, when sodium hydroxide (NaOH) is combined with hydrochloric acid (HCl), sodium chloride (NaCl) and water (H2O) are produced. Sodium chloride and water do not react to produce sodium hydroxide and hydrochloric acid.
NaOH+HClNaCl+H2O\mathrm{NaOH}+\mathrm{HCl}\;\rightarrow\mathrm{NaCl}+{\mathrm H}_2\mathrm O
Sometimes a double arrow is present in a chemical equation. This indicates that the reaction is reversible. Thus, the chemical reaction can proceed in the forward and reverse direction. For example, nitrous oxide (NO2) converts to dinitrogen tetroxide (N2O4), which converts back to nitrous oxide. The (g) in the equation shows that the compound is a gas.
2NO2(g)N2O4(g)2\;{\mathrm{NO}}_{2(\mathrm g)}\;\rightleftarrows{\mathrm N}_2{\mathrm O}_{4(\mathrm g)}
During reversible chemical reactions, reactants can convert to products, and products can convert to reactants, after a certain threshold of the reaction is reached. This process will occur until equilibrium is established between the reactants and products. The equilibrium constant is a number that expresses the relationship between the amounts of products and reactants present when a reversible chemical reaction is in balance.