When a substance undergoes a chemical change, it takes part in a chemical reaction. Recognizing Chemical Reactions Chemical Reactions and Equations: Basic Concepts After it reacts, it no longer has the same chemical identity. Topic 8 Topic 8

Recognizing Chemical Reactions While it may seem amazing that a substance can undergo a change and become part of a different substance, chemical reactions occur around you all the time. Many important clues indicate when chemical reactions occur. None of them alone proves that such a change occurs because some physical changes involve one or more of these signs. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Writing Chemical Equations In order to completely understand a chemical reaction, you must be able to describe any changes that take place. Part of that description involves recognizing what substances react and what substances form. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Writing Chemical Equations A substance that undergoes a reaction is called a reactant. When reactants undergo a chemical change, each new substance formed is called a product. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Writing Chemical Equations For example, a familiar chemical reaction involves the reaction between iron and oxygen (the reactants) that produces rust, which is iron(III) oxide (the product). The simplest reactions involve a single reactant or a single product, but some reactions involve many reactants and many products. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Word Equations The simplest way to represent a reaction is by using words to describe all the reactants and products, with an arrow placed between them to represent change. Reactants are placed to the left of the arrow, and products are placed to the right. Plus signs are used to separate reactants and also to separate products. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Word Equations Vinegar and baking soda are common names. The compound in vinegar that is involved in the reaction is acetic acid, and baking soda is sodium hydrogen carbonate. These scientific names can also be used in a word equation. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Chemical Equations Word equations describe reactants and products, but they are long and awkward and do not adequately identify the substances involved. Word equations can be converted into chemical equations by substituting chemical formulas for the names of compounds and elements. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Chemical Equations The equation for the reaction of vinegar and baking soda can be written using the chemical formulas of the reactants and products. By examining a chemical equation, you can determine exactly what elements make up the substances that react and form. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Chemical Equations It may also be important to know the physical state of each reactant and product. How can we indicate the bubbles we see during this reaction are CO 2 ? Symbols in the parentheses are put after formulas to indicate the state of the substance. Solids, liquids, gases, and water (aqueous) solutions are indicated by the symbols (s), (l), (g), and (aq). Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Chemical Equations The following equation shows these symbols added to the equation for the reaction of vinegar and baking soda. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Chemical Equations Now the equation tells us that mixing an aqueous solution of acetic acid (vinegar) with solid sodium hydrogen carbonate (baking soda) results in the formation of an aqueous solution of sodium acetate, liquid water, and carbon dioxide gas. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Energy and Chemical Equations Noticeable amounts of energy are often released or absorbed during a chemical reaction. Some reactions absorb energy. If energy is absorbed, the reaction is known as an endothermic reaction. For a reaction that absorbs energy, the word energy is sometimes written along with the reactants in the chemical equation. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Energy and Chemical Equations For example, the equation for the reaction in which water breaks down into hydrogen and oxygen shows that energy must be added to the reaction. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Energy and Chemical Equations Reactions that release heat energy are called exothermic reactions. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8 Click box to view movie clip.

Energy and Chemical Equations When writing a chemical equation for a reaction that produces energy, the word energy is sometimes written along with the products. Some of this energy is in the form of light. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Energy and Chemical Equations Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8 Click box to view movie clip.

Energy and Chemical Equations You may have also noticed that the word energy is not always written in the equation. It is used only if it is important to know whether energy is released or absorbed. Chemical Reactions and Equations: Basic Concepts Topic 8 Topic 8

Basic Assessment Questions Question 1 Write a word equation and a skeleton equation for each of the following descriptions of chemical reactions. Topic 8 Topic 8

Basic Assessment Questions Solid lithium reacts with chlorine gas to produce solid lithium chloride. Answer 1a Question 1a Topic 8 Topic 8