Download CHAPTER 2: Chemical Equations and Reaction Yields

CHAPTER 2:
Chemical Equations and
Reaction Yields
•Chemical Formulas
•Percent Composition and Elemental Analysis
•Balanced Equations and Stoichiometry
•Combustion Reactions
•Volume Relationships of Gases
•Limiting Reagents
•Percent Yield
CHEM 1310 A/B Fall 2006
Chemical Formulas
• Molecular formula: Gives exact # of atoms
for each element in one molecule of the
substance. Assumes molecule made of
well-defined molecules.
• Empirical formula: Gives smallest whole
numbers consistent with the ratios of the
#’s of each type of element. Used when
substance is not made of distinct
molecules.
CHEM 1310 A/B Fall 2006
Percent Composition and
Elemental Analysis
• Compute the fraction of the total mass due
to silver atoms in AgNO3
• A 100g sample of a compound is found to
contain 85.63g of C and 14.37g of H
atoms. What’s the empirical formula?
CHEM 1310 A/B Fall 2006
Balanced Chemical Equations
• “Sodium drop”
__ Na + __ H2O → __ NaOH + __ H2
• Balanced chemical equation needs to reflect:
(a) atoms not created/destroyed, (b) mass is
conserved
• Algebraic approach to balancing equations
CHEM 1310 A/B Fall 2006
Stoichiometry
• The balancing of chemical equations or the
study of the relationships between chemical
species in a balanced chemical equation
• Lots of possible stoichiometry problems;
frequently convert to moles, use relationships
between moles of different species, then convert
back to grams
• E.g., How many g of H2 are produced if 10g of
Na is dropped into a large beaker of H2O?
• E.g., How many g of products are produced from
the complete combustion of 10g of acetylene?
CHEM 1310 A/B Fall 2006
Volume relationships of gases
• Recall the law of combining volumes says
the volumes of gaseous reactants and
products stand in ratios of simple integers
(assuming those volumes measured at
same temperature & pressure)
• 2 C2H2 + 5 O2 → 4 CO2 + 2H2O,
for every 2L of C2H2, 4L of CO2 result, etc.
• Why do volumes behave just like moles?
CHEM 1310 A/B Fall 2006
Limiting Reagents
Reaction cannot go to completion because
one reagent is limiting…
CHEM 1310 A/B Fall 2006
Identifying the Limiting Reactant
•
•
Procedure:
1. Compute # of moles of each reactant
2. Divide each by the coefficient in the
balanced chemical equation
3. The smallest value corresponds to the
limiting reagent
Example: 10g of C2H2 reacts with 20g of O2 to
undergo complete combustion. Which reactant
is limiting, and how many g of CO2 are
produced?
CHEM 1310 A/B Fall 2006
Percent Yield
• Amount of product may be less than computed
by stoichiometry:
1. reaction not run to completion
2. competing side reactions
3. product lost in separation/purification
• If we expected 22g of CO2 in the combustion
reaction and only got 20g, what’s the % yield?
• Cumulative effect of low yields in multi-step
reactions
CHEM 1310 A/B Fall 2006