Balancing Chemical Equations.pdf

7/21/2019 Balancing Chemical Equations.pdf

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Balancing Chemical Equations

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Chemical Equations

A chemical equation is a method of describing what occurs in a chemical reaction via the use of

symbols for the participating chemicals (atoms, molecules, ions). The reaction identifies both

the reactants (starting material) and products (resulting materials). The reactants are shown on

the left and products are shown on the right, whereby both are separated by an arrow. The

arrow is usually read as “yields” A single arrow () is used for irreversible reactions, whereas a

double arrow (⇌) is used for reversible reactions. The chemical equation also depicts the

formulas and phases of the participant as well as the quantity of each.

w A + x B conditions (e.g. temp.) y C + zD(Reactants) (Products)

The plus sign (+) indicates that substance A and B have reacted with each other in some way. The lower

case letters w, x, y and z show the amount of molecules present and are called stoichiometric

coefficients. The stoichiometric sum is the sum of the coefficients of the reactants minus the sum of thecoefficient of the products [(w + x) – (y + z)]. When the stoichiometric sum is zero, the equation is

balanced.

Note: The quantities expressed by the stoichiometric coefficient can either be expressed in grams or moles

Writing and Balancing Equations

1.

Write the equation in words

e.g.: Carbon + Oxygen Carbon dioxide

Zinc Hydroxide + Hydrochloric acid Zinc chloride + water

Hydrogen + Oxygen water

2.

Write the correct formula for each reactant and product

C + O2 CO2

Zn(OH)2 + HCl ZnCl2 + H2O (not balanced)

H2 + O2 H2O (not balanced)

3.

From looking at the equations, it is noticeable that some of the representations are not consistent

with the law of conservation of mass. The law of conservation of mass states that the mass in a

closed system will remain constant over time, regardless of the processes acting in that system.

Therefore the equation must now be balanced in order to ensure that there is the same number of

atoms or ions of each element on either side of the equation. This is achieved by placing coefficients

(simple whole numbers) in front of the formula so as to alter the proportions of the reactants and

products.




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C + O2 CO2

Reactants Products

C = 1 C=1

O = 2 O=2

The above reaction is therefore balanced since there are equal amounts of atoms on either side ofthe equation.

Zn(OH)2 + HCl ZnCl2 + H2O

Reactants Products

Zn= 1 Zn= 1

O= 2 O=1

H= 2+1=3 H= 4

Cl=1 Cl=2

The (O) oxygen, (H) hydrogen, and (Cl) atoms do not balance since there is an unequal distributionof atoms on either side of the equation.

First to balance the oxygen atoms, a coefficient “2” is placed in front of the formula for water.

Zn(OH)2 + HCl ZnCl2 + 2H2O

In the above equation, the coefficient “2” not only increases the number of oxygen on the right to

two, but it also increases the amount of hydrogen atoms to four, still causing the reaction to be

unbalanced. To balance the amount of hydrogen atoms, the coefficient “2” is placed in front of the

HCl on the left. This will increase the amount of hydrogen atoms on the left, to be four which is

equivalent to the number of hydrogen atoms on the right.

Zn(OH)2 + 2HCl ZnCl2 + 2H2O

Note: Previously the distribution of the Cl atoms on either side of the equation was unequal;

however by balancing off the amount of hydrogen atoms, we indirectly balanced the amount of

chlorine atoms. This is sometimes the case, when dealing with compounds.

H2 + O2 H2O

Reactants Products

H = 2 H = 2O = 2 O = 1

In this equation, the amount of reactants and products, are not balanced. Two oxygen atoms are

shown on the left and only one is present on the right. To balance the equation we start with

balancing the amount of oxygen atoms, therefore the coefficient “2” must be placed in front of the

formula for water.




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H2 + O2 2H2O

The amount of oxygen atoms present is now balanced; however there are now four hydrogen atoms

in the product and only two hydrogen atoms as reactants. Therefore to balance the equation, a

coefficient of “2” must be placed in front of the H2 atom on the left.

2H2 + O2 2H2O

The equation is now balanced.

4.

The physical state of each reactant and product must be shown by placing a state symbol after each

formula:

Solid (s)

Liquid (l)

Gas (g)

Aqueous solution (aq)

C(s) + O2(g) CO2(g)

Zn(OH)2(s) + 2HCl(aq) ZnCl2(aq) + 2H2O(l)

2H2(g) + O2(g) 2H2O(l)

ExampleStep 1: Zinc + Nitric Acid Zinc Nitrate + Hydrogen

Step 2: Zn + HNO3 Zn(NO3)2 + H2

Step 3: Zn + 2HNO3 Zn(NO3)2 + H2

Step 4: Zn(s) + HNO3(aq) Zn(NO3)2(aq) + H2(g)

Hints for Balancing Equations

1.

Start with only the elements that occur in only one formula on each side of the equation:

e.g.: NH4Cl + CuO CuCl2 + H2O + NH3

The chlorine (Cl) atom only occurs in one formula on each side of the equation and does not

balance. Therefore it is balanced by placing the coefficient “2” in front of NH4Cl.




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2NH4Cl + CuO CuCl2 + H2O + NH3

However the nitrogen (N) is not balanced now, but this can easily be balanced by putting a

coefficient of “2” in front of NH3.

2NH4Cl + CuO CuCl2 + H2O + 2NH3

Now the equation is balanced.

2.

Another hint is to treat radicals which remain intact during the reaction as single entities, meaning

that no attempt should be made to try and balance them as individual atoms.

e.g.: Pb(NO3)2 + NaOH Pb(OH)2 + NaNO3

The OH does not balance, however it can be balanced by placing a coefficient of “2” in front of

NaOH.

The NO3 also does not balance, but it can be balance by placing a coefficient of “2” in front of

NaNO3:

Pb(NO3)2 + 2NaOH Pb(OH)2 + 2NaNO3

Revision Problems

Balance the following molecular equations. Make sure to include the state symbols

for each compound.

1.

Calcium + oxygen calcium oxide

2.

Sodium + Oxygen sodium oxide

3.

Hydrogen + lead [II] oxide lead + steam

4.

Calcium carbonate + sulphuric acid calcium sulphate + carbon dioxide + water

5.

Iron + chlorine iron [III] chloride

6.

Carbon + oxygen carbon dioxide

7.

Hydrogen + chlorine hydrogen chloride

8.

Sodium hydroxide + copper [II] sulphate sodium sulphate + copper [II] hydroxide

9.

Sodium hydroxide + phosphoric acid sodium phosphate + water

10.

Copper [II] carbonate + hydrochloric acid copper [II] chloride + carbon dioxide + water

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