6.1 Describing Chemical Reactions Chemical reactions are everywhere, resulting in changes that we see all around us: fires burn, grass grows, joggers run, milk sours, autumn leaves change colour, and marshmallows toast to a golden brown (Figure 1). In a chemical reaction, one or more substances change into different substances. One of the most familiar types of chemical reaction is combustion (burning). We use combustion reactions to cook our food, heat our homes, and travel long distances. Combustion reactions can also lead to many health and environmental problems. Chemists have developed a set of rules that enable people around the world to communicate and share information about chemical reactions. In this chapter, you will learn some of these rules of communication. Figure 1 Combustion is a chemical reaction that releases energy. Describing Chemical Reactions with Equations chemical reaction a process in which substances interact, causing the formation of new substances with new properties Chemists use equations to describe chemical reactions. You will learn how to write and interpret two kinds of equations. In word equations, the names of the chemicals are written out in full. In chemical equations, chemical formulas are used to represent the chemicals. word equation a way of describing a chemical reaction using the names of the reactants and products chemical equation a way of describing a chemical reaction using the chemical formulas of the reactants and products Examples of Word and Chemical Equations During a chemical reaction, reactant particles collide, allowing their atoms (or ions) to rearrange and form products. Reactants are the substances that are used up during the reaction. Products are the substances that are produced during the reaction. Figure 2 shows the reaction that occurs when a mixture of powdered iron and sulfur is heated. The reactants in this case are iron and sulfur. The product is iron(II) sulfide. Chemical reactions can either absorb or release energy. In this reaction, more energy is released than is absorbed. The word “energy” is therefore written on the right side of the equation, with the products. If energy is absorbed, it is written with the reactants. We can communicate the reaction between iron and sulfur using either a word equation or a chemical equation. Since energy is released in this reaction, “energy” is written on the right side of the chemical equation. Word equation: Chemical equation: (a) Reactants yields Products iron + sulfur → iron(II) sulfide + energy Fe + S → FeS reactant a chemical, present at the start of a chemical reaction, that is used up during the reaction product a chemical that is produced during a chemical reaction LeaRning Tip Clues of a Chemical Reaction Look back to Table 2 in Section 5.1. These clues indicate that a chemical reaction may be taking place. + energy (b) (c) Figure 2 (a) Powdered iron (black) is mixed with powdered sulfur (yellow). (b) Heating the mixture starts the chemical reaction. (c) The final product is iron(II) sulfide. NEL 6.1 Describing Chemical Reactions 225 Word equations and chemical equations have a lot in common: •Anarrowindicatesthedirectioninwhichthechemicalreaction is going. The arrow is read as “yields,” “forms,” or “produces.” •Substancestotheleftofthearrowarecalledreactants. •Substancestotherightofthearrowarecalledproducts. •Ifseveralreactantsareinvolved,“+”signsareplacedbetweenthereactants. This indicates that the reactants must be in contact with each other. •Ifseveralproductsareformed,“+”signsareplacedbetweentheproducts. state symbol a symbol indicating the physical state of the chemical at room temperature (i.e., solid (s), liquid (l), gas (g), or aqueous (aq)) Table 1 Common State Symbols in Chemical Equations State symbol Meaning (s) solid (l) liquid (g) gaseous (aq) aqueous (dissolved in water) Reading Tip Making Inferences Look for context clues such as significant words, comparisons, or contrasts. For example, in the comparison of word and chemical equations, you notice an arrow pointing right. From the direction of the arrow, you infer that the reactants on the left of the arrow lead to the products on the right of the arrow. You infer that the direction of the arrow is a visual symbol to help you understand these equations. Both equations list the reactants and products of the reaction. The chemical equation, however, provides far more detail: it gives the chemical formulas of the reactants and products as well as their state. State symbols tell us the state, or form, of each substance in a chemical equation. For example, the state symbol (s) means “solid.” All the chemicals involved in this reaction are solids. Table 1 summarizes the most common state symbols. Now look at Figure 3. Pale green copper(II) carbonate absorbs energy to produce carbon dioxide gas and copper(II) oxide. The equations for this reaction are: Word equation: energy + copper(II) carbonate → carbon dioxide + copper(II) oxide Chemical equation: energy + CuCO3(s) CO2(g) + CuO(s) The state symbols in this case tell you that the reactant is a solid and that the products are a gas and a solid. Since energy must be absorbed for this reactiontooccur,theenergytermiswrittenontheleftsideoftheequation. One final example: if a piece of zinc metal is placed in a solution of copper(II) sulfate, a fuzzy reddish-brown coating forms on the zinc (Figure 4). Afterabout20min,theentirezincstripiscoveredbythisnewsolid. Word equation: zinc + copper sulfate → zinc sulfate + copper + energy Chemical equation: Zn(s) + CuSO4 (aq) → ZnSO4 (aq) + Cu(s) + energy The state symbol (aq) tells us that the chemical is dissolved in water. Both reactants and products can be aqueous. Figure 3 Pale green copper(II) carbonate reacts to become black copper(II) oxide when it is heated. 226 → Chapter 6 • Chemicals and Their Reactions Figure 4 The first test tube contains copper(II) sulfate solution. The second test tube contains the same solution and also a strip of zinc. The blue colour of the solution fades as a reddishbrown solid forms on the zinc. What is this solid? NEL in SUMMARY •Chemicalreactionsalwaysinvolveoneormore reactants changing to give one or more products. •Statesymbolsareoftenwrittenafterachemical formula to indicate the state of the substance. •Wecanusewordequationsorchemicalequations to describe chemical reactions. In both, the reactantsarewrittenontheleftandanarrow points right, toward the products. CHECK YOUR LeaRning 1. What is the purpose of the arrow in a chemical equation? 2. Write word equations for the following reactions: K/U K/U 5. Consider the following chemical equation: Zn(s) + H2SO4(aq) → H2(g) + ZnSO4(aq) + energy K/U (a) Acetic acid (vinegar) and sodium hydrogen carbonate (baking soda) react to form water, carbon dioxide, and sodium acetate. (a) Name the products of this reaction. (b) Aluminum metal reacts with oxygen from the air to form a protective coating called aluminum oxide. (c) What evidence would indicate that this reaction is occurring? (c) Water and carbon dioxide are produced when propane burns in oxygen. 3. Some barbecues cook food by burning charcoal. (Charcoal is mostly carbon.) The chemical equation for this reaction is C(s) + O2(g) → CO2(g) K/U T /I (a) Write the word equation, including an energy term, for this reaction. (b) Write the state of each substance in the reaction. (c) What evidence suggests that a chemical change is taking place? (d) What would you expect to see when this reaction is complete? 4. Consider the reaction in Figure 5: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) K/U (a) Name the reactants and products in this reaction. (b) Name the chemicals that are dissolved in water. (c) Name the white solid. (d) What physical property do both reactants have in common? (b) What liquid is also present in the reaction vessel, along with the reactants and products? (d) Will the test tube in which this reaction is occurring become warmer or cooler during the reaction? Why? (e) What evidence would indicate that the reaction has stopped? (f) How does the quantity of zinc metal change as the reaction proceeds? 6. Take another look at the figures in this section. For each figure, what evidence suggests that a chemical change has taken place? K/U 7. In a burning marshmallow (Figure 1 on page 225), sugar breaks down into carbon (the black residue) and water vapour. K/U T/I (a) Write a word equation for this reaction. (b) What evidence tells you that the reaction is complete? (c) Write a hypothesis predicting how the mass of a marshmallow will change from before to after burning. Include an explanation in your prediction. 8. Bread rises due to the action of a single-celled organism called yeast. Yeast converts some glucose molecules in bread dough into carbon dioxide and ethanol. Carbon dioxide and ethanol then bubble through the dough, making it rise. K/U (a) Write a word equation for this reaction. (b) Is the action of carbon dioxide a physical or chemical change? Explain. 9. Under the right conditions, some chemical reactions can be reversed. For example, K/U A (a) An important step in bottling carbonated soft drinks is bubbling carbon dioxide gas into cold water and then sealing the bottle. Inside, a solution of hydrogen carbonate (also know as carbonic acid), H2CO3(aq), soon forms. Write the chemical equation for this reaction. Figure 5 When two aqueous reactants mix, they sometimes form a solid product. NEL (b) Describe two things you could do to quickly reverse this reaction. 6.1 Describing Chemical Reactions 227