Transcript Chapter 9 Stoichiometry - Las Lomas Science Home Page

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)
1N 2
3H 2
1N 2
2 NH 3
3H 2
2 NH 3
Balanced Equation Ratios
 What Ratios can be set up with the following balanced
equation?
2H2O2 → 2H2O + 1O2
2 H 2O 2
2 H 2O
2 H 2O 2
1O 2
2 H 2O
1O 2
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 [4, 6, 7, 8, 9]