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.
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6.1 Describing Chemical Reactions
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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.
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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
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