Chemical Equations

Student Expectation

The student is expected to use the law of conservation of mass to write and balance chemical equations.

Key Concepts

    • All chemical equations must follow the law of conservation of mass, which states that mass is neither created nor destroyed in a non-nuclear change. The total mass of the reactants must equal the total mass of the products.

    • A chemical equation is written by placing the reactants on the left side, the products on the right side, and a reaction arrow in the middle between the reactant side and the product side. Due to the law of conservation of mass, the same type and number of elements must be present in the reactants and products of a chemical reaction.

    • To balance a chemical equation, count the number of atoms of each element on the reactant side and compare that to the number of atoms of the same elements on the product side of the reaction. Use coefficients in front of the chemical formulas of the reactants or products to make the number of atoms on each side of the arrow equal.

CHEMICAL EQUATIONS

Chemical Equations Represent Chemical Reactions

Chemical equations are written to represent the chemical changes that occur during chemical reactions. A chemical equation consists of the chemical formulas of the reactants (the starting substances) and the chemical formula of the products (substances formed in the chemical reaction). The two are separated by an arrow symbol (→), usually read as “yields,” and each individual chemical formula is separated from others by a plus sign. Below is an example of a chemical reaction that occurs when rocket fuel is burned. This reaction is represented by the chemical equation shown, which also shows the formation of water. All chemical equations must follow the law of conservation of mass, which states that mass is neither created nor destroyed in a non-nuclear reaction. The total mass of the reactants must equal the total mass of the products.

The Law of the Conservation of Matter

In chemistry, a balanced chemical equation is the expression of the law of conservation of mass. In the late 1700s, a scientist named Antoine Lavoisier confirmed the law of conservation of mass. Lavoisier found that during the oxidation of metal, the mass gained by the metal came from the reaction of oxygen with the metal. Using a closed system, Lavoisier was able to demonstrate that the mass of the products equaled the mass of the reactants.

Reactants and Products

A chemical equation is written by placing the reactants on the left side, the products on the right side, and a reaction arrow in the middle between the reactant side and the product side. Reactants are the starting substances: the substances before the reaction happens. Products are the resulting substances: the substances after the reaction happens.

Chemical Equation Example: Burning Rocket Fuel

Houston is the home of the National Aeronautics and Space Administration (NASA). NASA was the former command center for the space shuttle. While the space shuttle program was still in use, NASA used liquid hydrogen as the fuel to launch the giant space shuttle. NASA will continue using liquid hydrogen for the next-generation heavy-lift rocket Space Launch System (SLS), which will have its first test flight at the end of 2017.

Liquid hydrogen is a very dangerous compound to work with. As hydrogen usually exists in a gaseous state, it takes extreme measures to liquefy it. Hydrogen gas must be cooled to, and stored at, temperatures below 250oC. Any exposure to heat will cause the liquid hydrogen to expand very quickly. Also, as liquid hydrogen has such a low molecular mass, it can leak through even the smallest openings, making the containment of liquid hydrogen a tricky issue. The fact that scientists at NASA discovered ways to work with this dangerous chemical points to the amazing scientific achievements and advances in twentieth century American rocketry. Propellants, such as liquid hydrogen, work by having a fuel, such as liquid hydrogen, and an oxidizer, such as liquid oxygen. When the fuel and the oxidizer are combined, a chemical reaction occurs that causes the propulsion of the rocket.

Below is an example of a chemical reaction that occurs when the fuel liquid hydrogen is combined with the oxidizing agent liquid oxygen. This reaction is represented by the chemical equation shown. This chemical equation also represents the formation of water:

H2(l) + O2(l) → H2O(g)

The letters in parentheses next to the chemical formula for each molecule describe the physical state of each substance and are called state symbols. For example, (aq) stands for aqueous. It refers to a substance in solution. In addition, (s) refers to a solid, (l) refers to a liquid, and (g) refers to a gas.

Due to the law of conservation of mass, the same type and number of elements must be present in the reactants and products of a chemical reaction. Therefore, we must balance the equation. To balance a chemical equation, count the number of atoms of each element on the reactant side, then compare that to the number of atoms of the same elements on the product side. Use coefficients in front of the chemical formulas of the reactants and products to make the number of atoms on each side of the arrow equal.

In the example above, we need to balance the chemical equation for the reaction of the rocket propellant liquid hydrogen with the oxidizer liquid oxygen as follows:

2 H2 + O2 → 2 H2O

This gives the quantitative relationship of reactants (liquid hydrogen and liquid oxygen) and the product (water).

Reaction Types

There are several types of chemical reactions that every chemistry student needs to know. They all require balanced equations according to the law of conservation of mass, as the number of reactant atoms must equal the number of product atoms.

    • Combination Reactions, or synthesis reactions, are reactions in which two or more reactants combine during a chemical change to create a single product. Here is the general equation:

    • Decomposition Reactions are chemical reactions in which a single compound on the reactant side breaks down (decomposes) in to two or more products.

    • Combustion reactions, or burning reactions, occur when a compound reacts with oxygen. In other words, oxygen is always a reactant in a combustion reaction. Most often, the other reactant is a hydro carbon (a compound containing carbon, hydrogen, and sometimes oxygen). If the hydrocarbon reacts completely with the oxygen, carbon dioxide gas and water vapor are produced. The combustion of hydrocarbons also releases large amounts of energy in the form of heat and light.

    • Single replacement reactions occur when one element replaces another during a chemical change. In these types of reactions there are two reactants: one single element and one compound. In order for element A to replace element B, element A must be more reactive than element B. The reactivity of metals may be found in a table known as an activity series of metals, or simply an activity series. Any reactive metal will replace any metal listed below it on the activity series. In the example shown below, iron (Fe) is listed higher in the activity series than copper (Cu), making it more reactive than copper. Therefore, Fe will replace Cu in this reaction.

    • Double-Replacement Reactions occur when the reacting compounds exchange ions during the chemical reaction. Solutions of two different reacting compounds are mixed, and the positive ions from one compound combine with the negative ions from the other compound. By convention, the metals are written first and the negative ions switch places.

    • Precipitation reactions, also sometimes known as a metathesis reaction, occurs in aqueous solutions of ionic compounds, where one of the reactants formed is a solid that “precipitates” out of solution. In a precipitation reaction, the rules on solubility are followed to determine which reactants produce a solid when mixed. The formations of solids are caused by a stronger interaction between solutes than between solute and solvent.

    • Acid-base reactions are another type of double-replacement reaction that occur when equal amounts of an acid are added to a base so that the acid and the base neutralize each other. An acid-base reaction is also known as a neutralization reaction in which water and salt are produced, thus neutralizing the pH of the solution.

    • Oxidation-reduction reactions, also known as redox reactions, involve a change in the oxidation states of each species involved in a chemical reaction. There are several different types of reactions that may also be considered redox reactions. These types of reactions include the transfer of oxygen atoms and/or electrons. The most current definition of oxidation and reduction says that the term oxidation refers to the loss of electrons or to the gain of oxygen, where the term reduction refers to the gain of electrons or to the loss of an oxygen. A decrease in the oxidation state means an electron is gained (reduction), and an increase in the oxidation state means an electron is lost (oxidation). Below are several examples of redox reactions.