Representing Chemical Reactions

Chemical Reactions

The process by which the atoms of one or more substances are rearranged to form different substances is called a chemical reaction. A chemical reaction is another name for a chemical change. Chemical reactions affect every part of your life. They break down your food, producing the energy you need to live. They produce natural fibers such as cotton and wool in the bodies of plants and animals. In factories, they produce synthetic fibers such as nylon and polyesters. Chemical reactions in the engines of cars and buses provide the energy to power the vehicles.

How can you tell when a chemical reaction has taken place? Although some chemical reactions are hard to detect, many reactions provide evidence that they have occurred. A temperature change can indicate a chemical reaction. Many reactions, such as those that occur during a forest fire, release energy in the form of heat and light. Other reactions absorb heat.

In addition to a temperature change, other types of evidence may indicate that a chemical reaction has occurred. One indication of a chemical reaction is a color change. For example, you may have noticed that the color of some nails that are left outside changes from silver to orange-brown in a short time. The color change is evidence that a chemical reaction occurred between the iron in the nail and the oxygen in air. Odor, gas bubbles, and/or the appearance of a solid are other indications of chemical change.

Representing Chemical Reactions

Chemists use statements called equations to represent chemical reactions. Their equations show a reaction’s reactants, which are the starting substances, and products, which are the substances formed during the reaction. Chemical equations do not express numerical equalities as do mathematical equations because during chemical reactions the reactants are used up as the products form. Instead, the equations used by chemists show the direction in which the reaction progresses. Therefore, an arrow rather than an equal sign is used to separate the reactants from the products. You read the arrow as “react to produce” or “yield”. The reactants are written to the arrow’s left, and the products are written to its right. When there are two or more reactants, or two or more products, a plus sign separates each reactant or each product. These elements of equation notation are shown below.

reactant 1 + reactant 2 ® product 1 + product 2

In equations, symbols are used to show the physical states of the reactants and products. Reactants and products can exist as solids, liquids, and gases. When they are dissolved in water, they are said to be aqueous. It is important to show the physical states of a reaction’s reactants and products in an equation because the physical states provide clues about how the reaction occurs. Some basic symbols used in equations are shown in Table 10-1.

Word equations You can use statements called word equations to indicate the reactants and products of chemical reactions. The word equation below describes the reaction between iron and chlorine, which is shown in Figure 10-2. Iron is a solid and chlorine is a gas. The brown cloud in the photograph is composed of the reaction’s product, which is solid particles of iron(III) chloride.

reactant 1 + reactant 2 ®product 1

iron(s) + chlorine(g) ® iron(III) chloride(s)

This word equation is read, “Iron and chlorine react to produce iron(III) chloride.”

Skeleton equations Although word equations help to describe chemical reactions, they are cumbersome and lack important information. A skeleton equation uses chemical formulas rather than words to identify the reactants and the products. For example, the skeleton equation for the reaction between iron and chlorine uses the formulas for iron, chlorine, and iron(III) chloride in place of the words.

iron(s) + chlorine(g) ® iron(III) chloride(s)

Fe(s) + Cl₂(g) ® FeCl₃(s)

How would you write the skeleton equation that describes the reaction between carbon and sulfur to form carbon disulfide? Carbon and sulfur are solids. First, write the chemical formulas for the reactants to the left of an arrow. Then, separate the reactants with a plus sign and indicate their physical states.

C(s) + S(s) ®

Finally, write the chemical formula for the product, liquid carbon disulfide, to the right of the arrow and indicate its physical state. The result is the skeleton equation for the reaction.

C(s) + S(s) ® CS₂(l)

This skeleton equation tells us that carbon in the solid state reacts with sulfur in the solid state to produce carbon disulfide, which is in the liquid state.

Chemical equations Writing a skeleton equation is an important step toward using an equation to completely describe a chemical reaction. But, like word equations, skeleton equations also lack important information about reactions. Recall from Chapter 3 that the law of conservation of mass states that in a chemical change, matter is neither created nor destroyed. Chemical equations must show that matter is conserved during a reaction, and skeleton equations lack that information.

Look at Figure 10-3. The skeleton equation for the reaction between iron and chlorine shows that one iron atom and two chlorine atoms react to produce a substance containing one iron atom and three chlorine atoms. Was a chlorine atom created in the reaction? Atoms are not created in chemical reactions, and to accurately show what happened, more information is needed.

To accurately represent a chemical reaction by an equation, the equation must show how the law of conservation of mass is obeyed. In other words, the equation must show that the number of atoms of each reactant and each product is equal on both sides of the arrow. Such an equation is called a balanced chemical equation. A chemical equation is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction. It is chemical equations that chemists use most often to represent chemical reactions.

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