Balancing Chemical Balancing Chemical EquationsEquations

Balancing Chemical Balancing Chemical EquationsEquations

Part IIPart II

Balancing Chemical Reactions

Follow these steps 1. Check for diatomic gases 2. Balance formulas of compounds 3. Conservation of atoms 4. Add coefficients if needed 5. Conservation of Mass

A. Check For Diatomic Gases

• Diatomic Gases – molecules containing only two atoms.• You must include the subscript “2” when the symbol is

written by itself.

• Seven Diatomic Gasesnitrogen, N2 bromine, Br2

oxygen, O2 iodine, I2 fluorine, F2 hydrogen, H2

chlorine, Cl2

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Diatomic Gases

Example H2(g) + O2(g) → H2O(g)

Hydrogen and oxygen are diatomic gases when written ALONE.

You must include the subscript “2”.Note: The equation isn’t balanced.

Balancing A Decomposition Equation

A. Check for diatomic gases

H2O(ℓ) → H + O

Diatomic gases: hydrogen & oxygen. Add a subscript “2” to each

H2O(ℓ) → H2(g) + O2(g)

B. Check The Formulas of Compounds

• The compound in the equation is water.• The formula is balanced.

H2O(ℓ) → 2H2(g) + O2(g)

C. Same Elements on Both Sides

H2O(ℓ) → H2(g) + O2(g)

Reactants Products

hydrogen hydrogen

oxygen oxygen

D. Conservation of Atoms

H2O(ℓ) → 2H2(g) + O2(g)

2 – H 2 - H

1 – O 2 – O

Atoms aren’t conserved. Equation isn’t balanced.

E. Balanced Equation 2H2O(ℓ) → 2H2(g) + O2(g)

The coefficients “2” are placed in front of water,

and hydrogen.

4 – H 4 – H

2 – O 2 – O

The equation is balanced.

F. Law of Conservation of Mass

Mass of Reactants = Mass of Products

2H2O(ℓ) → 2H2(g) + O2(g)

2(18 g) = 4 g + 32 g

36 g = 36 g

Equation is balanced.

Balancing A Synthesis Equation

• Equation: Na + Cl → NaCl

A. Diatomic Gas: Cl is a diatomic gas. Na isn’t a diatomic gas. Na is a metal.

Na + Cl2 → NaCl Formula of compound balanced? Yes. Na+Cl-

Balancing A Synthesis Equation

B. Put a “2” in front of the Na and a “2” in front of NaCl. Check number of each element again.

2Na + Cl2 → 2NaCl Na = 2 Na = 2 Cl = 2 Cl = 2

The equation is balanced.

Balancing A Synthesis Equation

C. Check number of each element on both sides. Na + Cl2 → NaCl Na = 1 Na = 1 Cl = 2 Cl = 1Not Balanced.

Balancing A Synthesis Equation

D. Conservation of Mass

2Na + Cl2 → 2NaCl

(2 x 23)g + 71g = 2(23 + 35.5)g 117 g = 117 g

Balancing A Single Replacement Equation

C. Same elements on both sides

NaI + F2(g) → NaF + I2(g)

sodium sodium

iodine iodine fluorine fluorine

Balancing A Single Replacement Equation

A. Check for diatomic gases

NaI + F → NaF + I

F and I are diatomic gases. Add “2” as a subscript on both.

NaI + F2(g) → NaF + I2(g)

Balancing A Single Replacement Equation

B. Balance the formulas of compounds + - + -

Na I + F2(g) → Na F + I2(g)

The compounds, NaI and NaF are balanced.

Balancing A Single Replacement Equation

D. Same number of atoms or ions on both sides

NaI + F2(g) → NaF + I2(g)

1 – Na 1 – Na 1 – I 2 – I 2 – F 1 – F

Equation is incorrect.

Balancing A Single Replacement Equation

“2” is needed in front of NaI and NaF.

2NaI + F2(g) → 2NaF + I2(g)

2 – Na 2 – Na 2 – I 2 – I 2 – F 2 – F

Equation is correct.

Balancing A Single Replacement Equation

E. Same mass on both sides

2NaI + F2(g) → 2NaF + I2(g)

2(23 + 127)+ 2(19) = 2(23 + 19) + 2(127)

338 g = 338 g

Balancing A Double Replacement Equation

A. Check for diatomic gases.

Fe S(s) + H Cl(aq) → H S(g) + Fe Cl(aq)

No diatomic gases. Chlorine isn’t ALONE.

B. Check The Formulas of Compounds

All formulas must be balanced.Incorrect: 2+ 2- + - + 2- 2+ -

Fe S(s) + H Cl(aq) → H S(g) + Fe Cl(aq)

Correct: 2+ 2- + - 2+ 2- 2+ 2-

Fe S(s) + H Cl(aq) → H2 S(g) + Fe Cl2(aq)

C. Same Elements on Both Sides

FeS(s) + HCl(aq) → H2S(g) FeCl2(aq)

iron iron sulfur sulfur hydrogen hydrogen chlorine chlorine

D. Conservation of Atoms

The number of ions or atoms each element must be the same on both sides

Fe S(s) + H Cl(aq) → H2 S(g) + Fe Cl2(aq)

Reactants Products Fe =1 Fe = 1 S = 1 S = 1 H = 1 H = 2 Cl = 1 Cl = 2

This equation is incorrect.

D. What’s Needed?

• We need 2 hydrogen and 2 chlorine.

• You can’t write in subscripts. This is incorrect. H2Cl2.

This formula isn’t balanced. Hydrogen and chlorine are written with the subscript “2” when they are alone in an equation.

• You must write the 2 in front of HCl as a coefficient.

• You have 2-H and 2-Cl.

E. Balanced Equation

Balanced Formula

Fe S(s) + 2HCl(aq) → H2S(g) + FeCl2(aq)

Reactants Products Fe =1 Fe = 1 S = 1 S = 1 H = 2 H = 2 Cl = 2 Cl = 2

This equation is correct.

F. Law of Conservation of Mass

Calculate the total molar mass on each side.They must be equal. Fe S(s) + 2HCl(aq) → H2S(g) + FeCl2(aq)

(56+32) + (2 + 71) = (2 +32) + (56 + 71)

88g + 73g = 34g + 127g 161g = 161g