balancing chemical equations. balancing by inspection most chemical equations we will encounter can...

Balancing Chemical Equations

Balancing by Inspection

Most chemical equations we will encounter can be “balanced by inspection.”

This involves a step-by-step approach.

We will use the decomposition of water as our example.

water hydrogen + oxygen


1. Identify the names of the reactants and the products, and write a word equation.

In our example reaction, water is being decomposed into hydrogen and oxygen.

water hydrogen +

Write a skeleton equation by substituting correct formulas for the names of the reactants and the products.

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

water hydrogen + oxygen



2.

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



2. Write a skeleton equation by substituting correct formulas for the names of the reactants and the products.

3. Balance the skeleton equation according to the law of conservation of mass.a. Balance the different types of atoms one at a time.

b. First balance the atoms of elements that are combined and that appear only once on each side of the equation.

c. Balance polyatomic ions that appear on both sides of the equation as single units.

d. Balance H atoms and O atoms after atoms of all other elements have been balanced.

Let’s look at the H’s first.

There are two H’s on the left and two H’s on the right - balanced.

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








There is 1 O on the left and 2 O’s on the right - unbalanced

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








We put a “2” in front of the H2O to balance the O’s.

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








2

We now need to rebalance the H’s.

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








2

We place a “2” in front of the H2 to balance the H’s.

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








2 2

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




3. Balance the skeleton equation according to the law of conservation of mass.

2 2

4. Count atoms to be sure that the equation is balanced.

On the left -4 H’s

2 O’s

On the right -4 H’s

2 O’s

It’s balanced!

Sample Problem 1The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas. Write a balanced chemical equation for the reaction.

First, we write the word equation:

Next, we write the skeleton equation:

zinc + hydrochloric acid ➔ zinc chloride + hydrogen

Zn(s) + HCl(aq) ➔ ZnCl2(aq) + H2(g)

Next, we balance the skeleton equation.Zinc is balanced.

We have 2 Cl’s on the right and only 1 Cl on the left.






Next, we balance the skeleton equation.

We balance the Cl’s by putting a “2” in front of the HCl.







2

This also balances the H’s.







2

Finally, we count atoms.







2

on the left -1 Zn2 H’s2 Cl’s

on the right -

1 Zn2 H’s2 Cl’s

It’s balanced

Sample Problem 2Solid aluminum carbide, Al4C3, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.



aluminum carbide+ water ➔ methane + aluminum hydroxide

Al4C3(s) + H2O(l) ➔ CH4(g) + Al(OH)3(s)


We have 4 Al’s on the left and 1 Al on the right.

We put a “4” in front of the Al(OH)3 to balance the Al’s.







4

We have 3 C’s on the left and only 1 C on the right.







4

We put a “3” in front of the CH4 to balance the C’s.







43

We have 2 H’s on the left and (12 + 12 = 24) H’s on the right.







43

We put a “12” in front of the H2O to balance the H’s.







4312

We have 12 O’s on the left and (4x3 = 12) O’s on the right.







4312

The O’s are balanced.







4312

Finally, we count atoms to check the balance.







on the left -4 Al’s3 C’s

24 H’son the right -

4 Al’s3 C’s

24 H’sIt’s balanced

4312

12 O’s 12 O’s

Sample Problem 3Aluminum sulfate and calcium hydroxide are used in a water-purification process. When added to water, they dissolve and react to produce two insoluble products, aluminum hydroxide and calcium sulfate. These products settle out, taking suspended solid impurities with them. Write a balanced chemical equation for the reaction.First, we write the word equation:


aluminum sulfate+ calcium hydroxide ➔ calcium sulfate+aluminum hydroxide

Al2(SO4)3(aq) + Ca(OH)2(aq) ➔ CaSO4(s)+Al(OH)3(s)


We have 2 Al’s on the left and 1 Al on the right.

We put a “2” in front of the Al(OH)3 to balance the Al’s.

Aluminum sulfate and calcium hydroxide are used in a water-purification process. When added to water, they dissolve and react to produce two insoluble products, aluminum hydroxide and calcium sulfate. These products settle out, taking suspended solid impurities with them. Write a balanced chemical equation for the reaction.First, we write the word equation:





2

Sample Problem 3

We have 3 SO4’s on the left and only 1 SO4 on the right.






2

Sample Problem 3

We put a “3” in front of the CaSO4 to balance the SO4’s.






2 3

Sample Problem 3

We have 3 Ca’s on the right and 1 Ca on the left.






2 3

Sample Problem 3

We put a “3” in front of the Ca(OH)2 to balance the Ca’s.






2 33

Sample Problem 3

We have 6 OH’s on the left and 6 OH’s on the right.






2 33

Sample Problem 3

The OH’s are balanced.






2 33

Sample Problem 3

Finally, we count atoms to check the balance.






on the left -2 Al’s3 Ca’s

3 SO4’son the right -

2 Al’s3 Ca’s

3 SO4’sIt’s balanced6 OH’s 6 OH’s

2 33

Sample Problem 3

Practice Problems

Balance each of the following chemical equations.

1.Pb(s) + AgNO3(aq) ➜ Ag(s) + Pb(NO3)2(aq)

2.Pb(NO3)2(aq) + KI(aq) ➜ PbI2(s) + KNO3(aq)

3.C3H8(g) + O2(g) ➜ CO2(g) + H2O(l)

4.H2SO4(aq) + NaOH(aq) ➜ Na2SO4(aq) + H2O(l)

5.HgO(s) ➜ Hg(l) + O2(g)

Pb(s) + 2 AgNO3(aq) ➜ 2 Ag(s) + Pb(NO3)2(aq)

Pb(NO3)2(aq) + 2 KI(aq) ➜ PbI2(s) + 2 KNO3(aq)

C3H8(g) + 5 O2(g) ➜ 3 CO2(g) + 4 H2O(l)

H2SO4(aq) + 2 NaOH(aq) ➜ Na2SO4(aq) + 2 H2O(aq)

2 HgO(s) ➜ 2 Hg(s) + O2(g)

balancing chemical equations. balancing by inspection most chemical equations we will encounter can...

Documents

skeleton equation

word equation

balance h atoms

atoms of elements

o atoms

different types of atoms

balance polyatomic ions

correct formulas