[PDF] [PDF] 41 Writing and Balancing Chemical Equations

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represented by a chemical equation using formulas (-top).

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πâwγ1 oâr βé γvwβ sΔπoγwéâ °oV ps qéααsqγzV αso2r wâ °oâV -oVβ1 wâqzπrwâuE176Chapter 4 Stoichiometry of Chemical Reactions3DEO?KJPAJPEO=R=EH=>HABKNBNAA=PDPPL/content/col11760/1.9

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A balanced chemical equation often may be derived from a qualitative description of some chemical reaction by a

fairly simple approach known as balancing by inspection. Consider as an example the decomposition of water to yield

molecular hydrogen and oxygen. This process is r-epresented qualitatively by antmaZkZmbdcchemical equation:

=cZdZek KZVXkVekj IifYlXkj :VcVeXZY8

H 1 UAO

. wJK

The numbers of H atoms on the reactant and product sides of the equation are equal, but the numbers of O atoms

are not. To achieve balance, thebndeehbhdmsrof the equation may be changed as needed. Keep in mind, of course, that

theenqltkZ rtarbqhosrdefine, in part, the identity of the substance, and so these cannot be changed without altering

the qualitative meaning of the equation. For example, changing the reactant formula from H

2O to H2O2would yield

balance in the number of atoms, but doing so also changes the reactant's identity (it's now hydrogen peroxide and not

water). The O atom balance may be achieved b-y changing the coefficient for H

2O to 2.

H.2 = 4 1 wJK

. UAO

The H atom balance was upset by this change, but it is easily reestablished by changing the coefficient for the H

2 product to 2.

H 2.2 = 2 4 = 4, yes

O 2 UAO

These coefficients yield equal numbers of both H and O atoms on the reactant and product sides, and the balanced

equation is, therefore:

:VcVeXae] ;_ZdaXVc =hlVkafej178Chapter 4 Stoichiometry of Chemical Reactions3DEO?KJPAJPEO=R=EH=>HABKNBNAA=PDPPL/content/col11760/1.9

Write a balanced equation for the reaction of molecular nitrogen (N

2) and oxygen (O2) to form dinitrogen

pentoxide.

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First, write the unbalanced equation.

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N 1 UAO

. wJK

Though nitrogen is balanced, changes in coefficients are needed to balance the number of oxygen atoms. To

balance the number of oxygen atoms, a reasonable first attempt would be to change the coefficients for the

O

2and N2O5to integers that will yield 10 O atoms (the least common multiple for the O atom subscripts in

these two formulas).

N 132 = 4 2 ≠ 4, no

O62 = 1035 = 10 10 = 10, yes

The N atom balance has been upset by this change; it is restored by changing the coefficient for the reactant

N

2to 2.

N32 = 4 2 UAO

. UAO

The numbers of N and O atoms on either side of the equation are now equal, and so the equation is balanced.

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Write a balanced equation for the decomposition of ammonium nitrate to form molecular nitrogen,

molecular oxygen, and water. (Hint: Balance oxygen last, since it is present in more than one molecule on

the right side of the equation.)

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It is sometimes convenient to use fractions instead of integers as intermediate coefficients in the process of balancing

a chemical equation. When balance is achieved, all the equation's coefficients may then be multiplied by a whole

number to convert the fractional coefficients to integers without upsetting the atom balance. For example, consider

the reaction of ethane (C

2H6) with oxygen to yield H2O and CO2, represented by the unbalanced equation:

TFQE QEB /

2reactant to yield an odd number, so a fractional coefficient,

FP RPBA FKPQB>A QL VFBIA > MOLSFPFLK>I

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bndeehbhdmsr. Although the equation for the reaction between molecular nitrogen and molecular hydrogen to produce

ammonia is, indeed, balanced, additional practice balancing equations.

9YYakafeVc Be[fidVkafe ae ;_ZdaXVc =hlVkafej

The physical states of reactants and products in chemical equations very often are indicated with a parenthetical

abbreviation following the formulas. Common abbreviations includerfor solids,kfor liquids,ffor gases, andZp

for substances dissolved in water ( Zptdntr rnktshnmr, as introduced in the preceding chapter). These notations are illustrated in the example equation here: ڮ

Special conditions necessary for a reaction are sometimes designated by writing a word or symbol above or below the

equation's arrow. For example, a reaction carried out by heating may be indicated by the uppercase Greek letter delta

(Δ) over the arrow.

Huyfxnsrw ksv Lsrnh Ujfhxnsrw

Given the abundance of water on earth, it stands to reason that a great many chemical reactions take place in aqueous

media. When ions are involved in these reactions, the chemical equations may be written with various levels of detail

appropriate to their intended use. To illustrate this, consider a reaction between ionic compounds taking place in an

aqueous solution. When aqueous solutions of CaCl

2and AgNO3are mixed, a reaction takes place producing aqueous

Ca(NO

3)2and solid AgCl:

represent the ionic species that are present in solution. When ionic compounds dissolve in water, they maySXeeaRXPfT

into their constituent ions, which are subsequently dispersed homogenously throughout the resulting solution (a

thorough discussion of this important process is provided in the chapter on solutions). Ionic compounds dissolved in

water are, therefore, more realistically represented -as dissociated ions, in this case:

Explicitly representing all dissolved ions results in aSb_c]UgU YbaYS UdhQgYba. In this particular case, the formulas

for the dissolved ionic compounds are replaced by -formulas for their dissociated ions:

2+(Pc) and

Ч4EBPBfcUSgQgbe Ybaf - ions whose presence is required to maintain charge neutrality - are

neither chemically nor physically changed by the process, and so they may be eliminated from the equation to yield a

more succinct representation called aaUg YbaYS UdhQgYba: $M

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