Ch. 10.1 – Chemical Reactions

II. Balancing Equationsread:

(p. 278 – 283)

A. Words to Skeletons First

Read the statement and underline all reactants and products first.

“Iron and chlorine react to produce iron(III) chloride.”

Write a word equation, then a skeleton:

iron(s) + chlorine(g) → iron(III) chloride(s)

Fe(s) + Cl2(g) → FeCl3(s)

B. Balancing Steps

1. Write the unbalanced skeleton equation. Check formulas; look for diatomic elements.

2. Count atoms on each side.

3. Never change a formula (don’t change subscripts); add coefficients to make

#s equal.

Coefficient subscript = # of atoms

4. Reduce coefficients to lowest possible ratio, if necessary.

5. Double check atom balance!!! YOU MUST OBEY THE LAW.

C. Helpful Tips

Balance one element at a time, working left to right except for H and O. Save H for next to last, and O until last.

Update ALL atom counts after adding a coefficient. NEVER change a formula.

Balance polyatomic ions as single units if they appear on both sides “1 SO4” instead of “1 S” and “4 O”

D. Law of Conservation of Mass

Fe(s) + Cl2(g) → FeCl3(s)2 23

D. Law of Conservation of Mass

Fe(s) + Cl2(g) → FeCl3(s)2 23

Fe = 1

Cl = 2

Fe = 1Cl = 3

X 2 = 2

= 6

X 2 = 2

X 3 = 6

Atoms are placed in “boxes” to ensure the formula is not changed; the number of “boxes” can change, but not the insides of the boxes.

Potassium chromate reacts with lead(II) nitrate to produce potassium nitrate and solid lead(II) chromate. The reaction occurs in water.

Write a skeleton equation with correct formulas.

D. Law of Conservation of Mass

K1+(CrO4)2- + Pb2+(NO3)1- →K1+(NO3)1- + Pb2+(CrO4)2-

K2CrO4(aq) + Pb(NO3)2(aq) → KNO3(aq) + PbCrO4(s)

K2CrO4(aq) + Pb(NO3)2(aq) → KNO3(aq) + PbCrO4(s)

K = 2

CrO4= 1

Pb = 1

NO3 = 2

K = 1

NO3 = 1

Pb = 1

CrO4 = 1

x 2 = 2

= 2

2

2 K 1

1 CrO4 1

1 Pb 1

2 NO3 1

/ 2

2/

Al + CuCl2 Cu + AlCl3

Al

Cu

Cl

1 1

1 1

2 3

2

3

6

3

33 2

E. Balancing Example

Aluminum and copper(II) chloride react to form copper and aluminum chloride.

2

2

6

Zn

Cl

H

1 1

1 2

1 2

2

121

E. Balancing Example

Solid zinc reacts with hydrogen chloride to produce zinc chloride and hydrogen gas

Zn + HCl → ZnCl2 + H2

2

1

Your turn…..

1. Sodium phosphate reacts with potassium hydroxide to produce sodium hydroxide and potassium phosphate

2. Magnesium fluoride reacts with lithium carbonate to produce magnesium carbonate and lithium fluoride

3. Lead (II) hydroxide reacts with hydrogen chloride to produce dihydrogen monoxide and lead (II) chloride

The answers…..

1. 1 Na3PO4 + 3 KOH 3 NaOH + 1 K3PO4

2. 1 MgF2 + 1 Li2CO3 1 MgCO3 + 2 LiF

3. 1 Pb(OH)2 + 2 HCl 2 H2O + 1 PbCl2