The statement "one plus one equals two" can be represented by ilie mailiematicalequation 1 + 1 = 2. Mathematicians use symbols for numbers and operations

as a kind of shoriliand iliat conveniently and clearly expresses mailiematicalconcepts. Scientists use a different type of shoriliand to represent chemicalconcepts. As you have seen in this unit, scientists use chemical symbols, suchas Hand 0, to represent elements. They use chemical formulas, such as H2O,to represent compounds. They represent chemical reactions using a combinationof chemical formulas and symbols. For example, ilie reaction of hydrogen willioxygen to form water is represented as follows:

2H2(g) + 02(g) ~ 2H20(f)

To represent chemical reactions accurately, chemical equations should bewritten so iliat iliey reflect ilieories and laws iliat relate to chemical reactions.One such law is ilie law of conservation of mass. The law was first developedby French chemist Antoine Lavoisier (1743-1794).

A painting depicting Lavoisier at work is shown in Figure 3.9. One ofLavoisier's most successful and influential techniques as a scientist was his carefulmeasurement of mass. He stressed the importance of measuring ilie mass ofall ilie substances involved in a chemical change. These measurements werecrucial for making accurate inferences about what happened to ilie substances.By generalizing many observations of the same results, Lavoisier wrote hisversion of ilie law of conservation of mass.

When Lavoisiercarried out reactions in closedsystems, he observed that thesystem did not gain or lose mass.

One of Lavoisier's innovations was carrying out his reactions in closedsystems. A closed system does not allow any exchange of matter betweenthe system and its surroundings. A chemical system includes all the substancesinvolved in a reaction and the container that holds them. The surroundingsinclude everything that is not part of the system. A tightly capped test tubecontaining a solution is a closed system. Gases, liquids, and solids cannotenter or exit the system. An open system allows the exchange of substancesbetween the system and its surroundings. An open test tube containing asolution is an example of an open system.

The law of conservation of mass is a vital part of modern chemical theory.In Inquiry Investigation 3-B on the following pages, you can carry out aninvestigation to illustrate the law of conservation of mass.

94 MHR . Unit 1 Energy and Matter in Chemical Change

To review procedures for reading,recording, and interpreting experi-mental measurements, turn to SkillFocus 4.

Laws such as the law of conservation of mass are generalizations basedon numerous experiments that have given the same results. In thisinvestigation, you will consider why it is important for scientists toanalyze the appropriateness of their procedure and the precision oftheir instruments when interpreting results.

Skill

If you need help with rules forworking with significant digits,turn to Skill Focus 2.

QuestionHow do you decide whether observations of a specific chemicalreaction demonstrate the law of conservation of mass?

Hypothesis. State the law of conservation of mass and describe and

explain the conditions necessary to demonstrate the law.. Explain hqw you expect the law of conservation of mass

will apply to this investigation.

Prediction. Predict how d1e mass of your system will compare before

and after d1e reaction. Remember, you should be able to

explain your prediction using your hypod1esis.. Once you have measured d1e initial mass of your system

(procedure step 4), record your prediction as a specific mass.

Apparatuselectronic balanceErlenmeyer flask

stoppersmall test tube

tongs

Materialscopper(ll) sulfate solutionsodium hydroxide solution

Safety Precautions [s;I L - -. Sodium hydroxide solution is caustic. Avoid

skin contact.. Copper(ll) sulfate is toxic.

. Always report spills of any chemicals to yourteacher immediately.

. If any solutions touch your skin, rinse immediatelywith plenty of cold water. Report the accident toyour teacher.

. Wash your hands thoroughly at the end ofthis investigation.

. Dispose of materials as your teacher directs.

CONTINUED ...

Chanter 3 Chemical Reactions. MHR 95

To finally balance this chemical equation, you need to add one more coefficient:

2Hz + Oz ~ 2HzO

Figure 3.12 shows why this equation is balanced. Often it is useful to knowthe state of the chemicals that are involved in a chemical reaction. Add thisinformation to a chemical equation by inserting the appropriate abbreviationin parentheses after each chemical formula. Table 3.3 provides the abbreviations.

The final balanced equation for the formation of water, if the reactants andproduct are at room temperature, is:

2H20 2Hz(g) + Oz(g) ~ 2HzO(t'}022H2

A

Why is the scalefinally balanced?

Table 3.3 State Abbreviations

Chemical equations sometimes also show whether a reaction is exothermicor endothermic. If a reaction is endothermic, energy is written as a reactant.If a reaction is exothermic, energy is written as a product. For example, thedecomposition of copper(II) oxide into copper and oxygen is endothermic.The reaction can be written like this:

CuO(s) + thermal energy ~ CU(s) + Oz(g)

The exothermic reaction of sulfuric acid and sodium hydroxide to formwater and sodium sulfate can be written as follows:

HZSO4(aq) + 2NaOH(aq) ~ 2HzO(t') + NaZSO4(aq) + thermal energy

Writing Balanced Chemical EquationsBalancing a chemical equation requires patience, practice, and perseverance.Here are some guidelines and examples to help.

1. Skeleton Equation: Write the skeleton equation. Be sure you have thecorrect formulas for all the compounds and elements. Each reactant andproduct make up a "term" in the equation.

2. Balancing: Identify unbalanced atoms and polyatomic ions. Writecoefficients to balance them. The steps below will help for more

complex equations.. Look for an unbalanced atom or a polyatomic ion that appears just once

on each side of the equation. Add balancing coefficients to the two termsthat contain the element or polyatomic ion. Leave atoms that appear aselements in the equation until later (e.g., Oz, Hz, or Na).

. Pick another unbalanced element that appears once on each side. Choosecoefficients to balance the atoms of this element as well. Adjust coefficientsso that the terms you balanced in. the previous step remain balanced.Repeat for any other elements that appear once on each side.

. Balance any remaining unbalanced elements or ions. Use fractionalcoefficients if necessary.

. Clear any fractional coefficients by multiplying the whole equation by thesame factor. The resulting coefficients should be the smallest possiblewhole numbers.

3. Check It: Check that the equation is balanced. Make a table of the atomsof each element on each side of the equation.

98 MHR . Unit 1 Energy and Matter in Chemical Change

Examples of Balancing Chemical Equations

Write a balanced chemical equation to represent the following word equation.

sodium + water -7 sodium hydroxide + hydrogen gas

1. Skeleton Equation

First write a skeleton equation. Be sure you have used the correct fonnulas.

Na(s) + HzO(t') ~ NaOH(aq) + Hz(g)

Remember: Hydrogen gas is diatomic.

2. Balancing. Identify unbalanced atoms and polyatomic ions.

Na(s) + HzO(t') ~ NaOH(aq) + Hz(g)

The only atoms that do not balance on each side of the skeleton equationare hydrogen atoms.

. There are two hydrogen atoms on the reactant side, and three hydrogen

atoms on the product side. On the product side, hydrogen appears as anelement, Hz, and also as part of sodium hydroxide, NaOH.

. Since the atoms of all other elements are balanced, begin to balance the

equation by changing the coefficient in front of HzO(t') to 2. There arenow four hydrogen atoms on the reactant side. Balance the hydrogenatoms by changing the coefficient in front ofNaOH(aq) to 2. The hydrogenatoms are balanced.

Na(s) + 2HzO(t') ~ 2NaOH(aq) + Hz(g)

. The sodium atoms are now unbalanced. There are two sodium atoms on

the product side, but only one on the reactant side. Balance the sodiumatoms by placing a coefficient of 2 in front of N a(s) on the reactant side.

2Na(s) + 2HzO(t') ~ 2NaOH(aq) + Hz(g)

The equation now appears to be balanced.

Pause&3. Check It

Your final step should always be to check the total number of atoms oneach side of the equation. Make a chart to list the reactants and products,and record the number of atoms and ions to see if they match.

2 Na+ ions

20 atoms

4 H atoms

2 Na atoms

20 atoms

4 H atoms The equation is balanced.

. MHR QQ,,'..r ~ rh..mi" Rp

Sometimes students try to balancea chemical equation by changingthe formula of a reactant orproduct. For example, you mightbe tempted to balance the skeletonequation for water by changingH2O to H202. This approachseems simple, and the numbersbalance. So why is it incorrect?

1. Skeleton EquationCU(s) + AgNQ3(aq) ~ Cu(NQ3)2(aq) + Ag(s)

2. Balancing

On the left and right sides, there are compounds that contain nitrate ions,NO3-. Balance these ions as a unit first.

CU(s) + 2AgNO3(aq) ~ Cu(NO3)2(aq) + Ag(s)

Then balance the silver atoms.

CU(s) + 2AgNO3(aq) ~ Cu(NO3)2(aq) + 2Ag(s)

The copper atoms are balanced without any further changes.

3. Check It

- : ~ fm1 ~- :my; ITTij ~1 Cu atom 1 Cu+ ion

~ This photo showsthe reaction in Model Problem 2.Which ions are in the solution?What is the precipitate that isforming on the wire?

The equation is balanced.

Write a chemical equation for the following word equation.

calcium nitrate + sodium hydroxide ~ calcium hydroxide + sodium nitrate

1. Skeleton EquationCa(NQ3)2(aq) + NaQH(aq) ~ Ca(QH)2(s) + NaNQ3(aq)

2. Balancing

First balance the nitrate ions and the hydroxide ions.

Ca(NO3)2(aq) + 2NaOH(aq) ~ Ca(OH)2(s) + 2NaNO3(aq)

The equation appears to be balanced without further changes.

3. Check It

- : {;:fiTii f;m ~- :l7iTi 11TH ~

1 Ca+ ion 1 Ca+ ion

The equation is balanced.

100 MHR . Unit Energy and Matter in Chemical Change

Write a chemical equation for the following word equation.

copper + silver nitrate ~ copper(I1) nitrate + silver

In 1995 a scientist spent 15days in a sealed chamber. Hisoxygen was supplied entirelyby photosynthesis, courtesy of30 000 wheat plants. In turn,the carbon dioxide needed for thephotosynthesis came from thescientist's breath, as a productof his cellular respiration.

Human-carrying interplanetaryexpeditions will likely dependon similar closed systems tosupply and recycle water,oxygen, and many nutrients.

5. Balance each of the following equations.(a) Li(s) + H20(t') ~ LiOH(aq) + H2(g)

(b) Fe(s) + 02(g) ~ Fe203(s)

(c) Zn(s) + HC!(aq) ~ ZnCI2(aq) + H2(g)

(d) (NH4)3PO4(aq) + Ba(OH)2(aq) ~ Ba3(PO4)2(s) + ~OH(aq)

(e) Pb(NO3)2(aq) + KI(aq) ~ PbI2(s) + KNO3(aq)

(f) Cu(NO3)2(s) ~ CuO(s) + NO2(g) + 02(g)

6. Aqueous copper(I1) nitrate reacts with aqueous potassium hydroxide toform aqueous potassium nitrate and solid copper(I1) hydroxide. Writea balanced chemical equation to represent this reaction.

7. In section 3.1, you learned that plants use the Sun's energy to make theirown food through the process of photosynthesis. Both plants and animalsuse food to make energy through the process of cellular respiration.

(a) The skeleton equation for photosynthesis is below. Balance theequation. Hint: Leave oxygen for last.

COz(g) + HzO(t') ~ C6HIZO6(aq) + OZ(g)

(b) The skeleton equation for cellular respiration is below. Balancethe equation.

C6HIZO6(aq) + Oz(g) ~ COz(g) + HzO(t')

(c) Classify each reaction as exothermic or endothermic. For eachequation, add" + energy" on the product side or reactant side.

8. Figure 3.14 below shows the contact process for producing sulfuricacid, an important and versatile industrial chemical.

(a) Read about the reactions that take place in each step. For eachof the four steps, write a chemical equation to represent the reaction.

(b) Identify three dangerous substances that are used or producedin the contact process. Explain why they are dangerous.

(c) The contact process produces no harmful waste substances thatmust be disposed of. Explain what happens to the dangeroussubstances you identified in question (b).

~

:hemical Reactions. MHR 101:hapter

Step 1 Liquid sulfur, S(1:), isburned in air. The sulfur reactswith oxygen to produce sulfurdioxide, SO2(g), a stable, poisonousgas. The sulfur can be obtainedfrom underground deposits oras a byproduct of natural

gas production.

Step 2 After impurities areremoved, the sulfur dioxideis reacted with more oxygento produce toxic sulfur trioxidegas, SO3(g). To speed up thereaction, the gases are heatedto a temperature of 400°C.

Step 3 After cooling, the sulfurtrioxide is bubbled throughhighly concentrated sulfuric acidH2S04(aq). The reaction producespyrosulfuric acid, H2S207(aq).

Step 4 Water is added to thepyrosulfuric acid, producinghighly concentrated sulfuricacid, H2S04(aq).

Section 3.2 SummaryIn this section, you learned that the law of conservation of mass states thatthe mass of a system remains constant during a chemical reaction. You learnedhow to write balanced chemical equations to reflect this law. In Section 3.3,you will continue to work on your balancing skills as you learn to classifychemical reactions.

Check Your Understanding1. Create a chart that summarizes the key features of word equations,

skeleton equations, and balanced equations, showing the advantagesand disadvantages of each type of equation.

2. Explain why a balanced chemical equation is consistent with the law ofconservation of mass. Use an example of a balanced chemical equationill your answer.

3. Each of the following chemical equations is balanced, but is incorrect insome other way. State what is wrong and then write the equation correctly.

1(a) H20(f,) ~ H2(g) + 202(g)

(b) NH3(g) ~ N(g) + 3H(g)

(c) 2C(s) + 202(g) ~ 2CO2(g)

4. When you light a campfire, you are burning carbon compounds in thewood. The main compound in the wood is cellulose, a compound containingcarbon, hydrogen, and oxygen. The products are carbon dioxide and watervapour. There is one other reactant besides cellulose. What is it? Write aword equation to describe the burning of cellulose.

5. Apply Given the following word and skeleton equations,write balanced chemical equations for each reaction.

(a) copper(ll) oxide ~ copper + oxygen gasCuO(s) ~ CU(s) + 02 (g)

(b) methane + oxygen ~ carbon dioxide + waterC~(g) + 02 (g) ~ CO2 (g) + H20(g)

(c) ammonia gas + oxygen ~ nitrogen + waterNH3(g) + 02 (g) ~ N2(g) + H20(f)

(d) sulfur dioxide + oxygen ~ sulfur trioxideS02(g) + 02 (g) ~ S03(g)

6. Thinking Critically Suppose that you measure the mass of a chemical inan open container, and then heat it for a few minutes over a Bunsen burnerflame. After the container and contents have cooled, you find that the massis larger than before.

(a) If you accept the law of conservation of mass, how can you explainyour observation?

(b) Is there a way to carry out the same reaction so that your results illus-trate the law of conservation of mass? Explain your answer.

1 02 MHR . Unit 1 Energy and Matter in Chemical Change