Chapter 9 Stoichiometry

Stoichiometry

The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It is a VERY mathematical part of chemistry, so be prepared for lots of calculator use.

Meaning of a Balanced Equation

The coefficients in a balanced equation have two meanings. The first is that it represents the ACTUAL number of atoms or

compounds involved in a chemical reaction. The second more common meaning is that it represents the

number of moles of atoms or compounds involved in a chemical reaction.

1N2(g) + 3H2(g) → 2NH3(g) The coefficients above mean either that one molecule of nitrogen

reacts with three molecules of hydrogen to produce two molecules of ammonia or one mole of nitrogen reacts with three moles of hydrogen to produce two moles of ammonia.

Meaning of a Balanced Equation

WARNING WARNING WARNING WARNING

The coefficients in a balanced chemical equation do NOT represent the mass proportion of reactants and products.

1N2(g) + 3H2(g) → 2NH3(g)

In the above equation, since a mole of nitrogen is significantly more massive than even three moles of hydrogen (28.02 amu to 6.06 amu) you can see that more mass of nitrogen is needed which is a different relationship than the mole ratio of 1N2:3H2.

Balanced Equation Ratios

A balanced equation is used to set up mole ratios between products and reactants.

Three mole ratios can be set up with the following

1N2(g) + 3H2(g) → 2NH3(g)

2

2

3

1

H

N

3

2

2

1

NH

N

3

2

2

3

NH

H

Balanced Equation Ratios

What Ratios can be set up with the following balanced equation?

2H2O2 → 2H2O + 1O2

OH

OH

2

22

2

22

22

1

2

O

OH

2

2

1

2

O

OH

Calculating Moles with Mole Ratios

The mole ratios of a balanced equation are used to calculate the number of unknown moles of product or reactant.

1N2(g) + 3H2(g) → 2NH3(g) How many moles of ammonia can be produced from the

reaction of 2.0 moles of nitrogen? Multiply the known by the mole ration between the known

and the unknown just as you did with conversions.

2.0 N2 (2NH3/1N2) = 4.0 NH3

Stoichiometry

Homework Terms Chapter 9 P310 [2, 3, 5, 6, 7]