Reactants

General reaction equation:

Products

Bricks + lumber + cement Building

Law of conservation of mass: matter is neither created nor destroyed during a physical or chemical change or….What goes in must come out.

Ex: 23 g of sodium metal is reacted with 35 g of chlorine gas to form sodium chloride. How much sodium chloride was formed?

23 g Na (s) + 35 g chlorine (g) 58 g of NaCl

reactants products

Remember Chlorine gas is actually diatomic. Its formula is Cl2.

Can you remember the other diatomic elements?

H2, N2, O2, F2, Br2, I2

Let’s look at that reaction again…

Na + Cl2 NaCl

Imagine that we’re reacting atoms

Na Cl Cl+ ClNa

Na ClClNa

2 Na + Cl2 2 NaCl

Now the reaction is balanced.

____ H2 + ____ O2 ____ H2O

2 H

2 O

2 H

R P

1 O2 x

2

2 x

= 2 O

= 4 H2 x = 4 H

2

• The large numbers in front of the chemical formulas are called coefficients.

• They act like multipliers when balancing equations.

1) Zinc metal and an aqueous solution of lead(II) nitrate react to form an aqueous solution of zinc(II) nitrate and lead metal.

__ Zn (s) + __ Pb(NO3)2 (aq) __ Zn(NO3)2 (aq) + __ Pb (s)

1 Zn

1 Pb

1 Zn

R P

1 Pb

1 NO3 1 NO3

This reaction equation is already balanced!

STOP

Zn (s) + Pb2+ NO31-(aq)

(aq) Zn2+ NO31- Pb (s)+

2) Solid aluminum bromide and chlorine gas react to form solid aluminum chloride and bromine liquid.

___ AlBr3 (s) + ___ Cl2 (g) ___ AlCl3 (s) + ___ Br2 (l)

1 Al

3 Br

1 Al

R P

2 Br

2 Cl 3 Cl

2 x = 6 Br 3 x = 6 Br

2 3

2 x = 2 Al 2 x = 2 Al

2

2 x = 6 Cl3 x = 6 Cl

3

This reaction equation is balanced!

STOP

(s) Al3+ Br1- + Cl2 (g)

(s) Al3+ Cl1- + Br2 (l)

An alternative balancing technique: Balancing by inspection

Phosphoric acid and sodium hydroxide react to form water and sodium phosphate.

____H3PO4 + ____ NaOH

____H2O + ____Na3PO4

2nd pass: Balance non-phosphate oxygens (phosphates are already balanced)

3rd pass: Check hydrogens

1st pass: Balance sodiums

PO43- H+ Na1+ OH1- H2O

Na1+ PO43-

3 3

(3 on both sides)

(6 H on both sides)

(3 on both sides)

+ +

Balance the following by inspection:

1. ___C2H4 + ___O2 ___CO2 + ___H2O

• Balance carbons

2

• Balance hydrogens

2

• Balance oxygens

3

2. ___P + ___O2 ___P2O5

• Balance oxygens

• Balance phosphorus

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