Now that you understand the basic structural differences between different kinds of substances, you are ready to begin learning about the chemical changes that take place as one substance is converted into another. Chemical changes are chemists’ primary concern. They want to know what, if anything, happens when one substance encounters another. Do the substances change? How and why? Can the conditions be altered to speed the changes up, slow them down, or perhaps reverse them? Once chemists understand the nature of one chemical change, they begin to explore the possibilities that arise from causing other similar changes. For example, let’s pretend that you just bought an old house as is, with the water turned off. On moving day, you twist the hot water tap as far as it will go, and all you get is a slow drip, drip, drip. As if the lack of hot water weren’t enough to ruin your day, you also have a toothache because of a cavity that you haven’t had time to get filled. As a chemist in training, you want to know what chemical changes have caused your troubles. In this chapter, you will read about the chemical change that causes a solid to form in your hot water pipes, eventually blocking the flow of water through them. In Chapter 5, you will find out about a chemical change that will dissolve that solid, and a similar change that dissolves the enamel on your teeth
Chapter 5 will also show you how fluoride in your toothpaste makes a minor chemical change in your mouth that can help fight cavities. Chemical changes, like the ones mentioned above, are described with chemical equations. This chapter begins with a discussion of how to interpret and write chemical equations.
The presentation of information in this chapter assumes that you can already perform the tasks listed below. You can test your readiness to proceed by answering the Review Questions at the end of the chapter. This might also be a good time to read the Chapter Objectives, which precede the Review Questions. Write the formulas for the diatomic elements. (Section 2.5) Predict whether a bond between two atoms of different elements would be a covalent bond or an ionic bond. (Section 3.2) Describe attractions between H2O molecules. (Section 3.3) Describe the structure of liquid water. (Section 3.3) Convert between the names and formulas for alcohols, binary covalent compounds, and ionic compounds. (
A chemical change or chemical reaction is a process in which one or more pure substances are converted into one or more different pure substances. Chemical changes lead to the formation of substances that help grow our food, make our lives more productive, cure our heartburn, and much, much more. For example, nitric acid, HNO3, which is used to make fertilizers and explosives, is formed in the chemical reaction of the gases ammonia, NH3, and oxygen, O2. Silicon dioxide, SiO2, reacts with carbon, C, at high temperature to yield silicon, Si—which can be used to make computers—and carbon monoxide, CO. An antacid tablet might contain calcium carbonate, CaCO3, which combines with the hydrochloric acid in your stomach to yield calcium chloride, CaCl2, water, and carbon dioxide. The chemical equations for these three chemical reactions are below.
In chemical reactions, atoms are rearranged and regrouped through the breaking and making of chemical bonds. For example, when hydrogen gas, H2(g), is burned in the presence of gaseous oxygen, O2(g), a new substance, liquid water, H2O(l ), forms. The covalent bonds within the H2 molecules and O2 molecules break, and new covalent bonds form between oxygen atoms and hydrogen atoms (Figure 4.1).
Chemical equations show the formulas for the substances that take part in the reaction. The formulas on the left side of the arrow represent the reactants, the substances that change in the reaction. The formulas on the right side of the arrow represent the products, the substances that are formed in the reaction. If there are more than one reactant or more than one product, they are separated by plus signs. The arrow separating the reactants from the products can be read as “goes to” or “yields” or “produces.” The physical states of the reactants and products are provided in the equation. A (g) following a formula tells us the substance is a gas. Solids are described with (s). Liquids are described with (l ). When a substance is dissolved in water, it is described with (aq) for aqueous, which means “mixed with water.”
In chemical reactions, atoms are neither created nor destroyed; they merely change partners. Thus the number of atoms of an element in the reaction’s products is equal to the number of atoms of that element in the original reactants. The coefficients we often place in front of one or more of the formulas in a chemical equation reflect this fact. They are used whenever necessary to balance the number of atoms of a particular element on either side of the arrow. For an example, let’s return to the reaction of hydrogen gas and oxygen gas to form liquid water. The equation for the reaction between H2(g) and O2(g) to form H2O(l ) shows there are two atoms of oxygen in the diatomic O2 molecule to the left of the arrow, so there should also be two atoms of oxygen in the product to the right of the arrow. Because each water molecule, H2O, contains only one oxygen atom, two water molecules must form for each oxygen molecule that reacts. The coefficient 2 in front of the H2O(l ) makes this clear. But two water molecules contain four hydrogen atoms, which means that two hydrogen molecules must be present on the reactant side of the equation for the numbers of H atoms to balance (Figure 4.2 on the previous page).
The substances that mix to make a solution, such as our solution of sodium chloride and water, are called the solute and the solvent. In solutions of solids dissolved in liquids, we call the solid the solute and the liquid the solvent. Therefore, the NaCl in an aqueous sodium chloride solution is the solute, and the water is the solvent. In solutions of gases in liquids, we call the gas the solute and the liquid the solvent. Therefore, when gaseous hydrogen chloride, HCl(g), is dissolved in liquid water to form a mixture known as hydrochloric acid, HCl(aq), the hydrogen chloride is the solute, and water is the solvent. In other solutions, we call the minor component the solute and the major component the solvent. For example, in a mixture that is 5% liquid pentane, C5H12, and 95% liquid hexane, C6H14, the pentane is the solute, and the hexane is the solvent
