Transcript Unit 6: Stoichiometry

UNIT 7: STOICHIOMETRY
KEY TERMS
• Actual yield - Amount of product was actually made in a reaction
• Dimensional analysis - The practice of checking relations among physical quantities by identifying
their dimensions
• Excess reagent - Sometimes when you do a chemical reaction, there's some of one reagent left over
• Limiting reagent - the chemical that determines how far the reaction will go before the chemical in
question gets "used up", causing the reaction to stop
• Molar mass - The number of grams of a substance in 1 mole
• Mole Ratio - The ratio of moles of any two substances in a chemical reaction
• Percent Yield - Percentage of product produced in the lab with relationship to theoretical yield
• Stoichiometric calculations - Calculation of the quantities of reactants and products in a chemical
reaction
• Stoichiometry - The relationship between the relative quantities of substances taking part in a
reaction or forming a compound, typically a ratio of whole integers
• Theoretical Yield - The amount of product that can theoretically be produced in a chemical reaction
found by stoichiometric calculation
MOLE TO MASS CONVERSION
• Dimensional analysis process based on Molar Mass
• Since Molar Mass is the mass of one mole of a substance, mass can
be converted to moles through dividing by Molar Mass
Mass
X
1 mole
Molar Mass
= Moles
• Since Molar Mass is the mass of one mole of a substance, the number
of moles can be converted to mass through multiplying by Molar
Mass
Moles
X
Molar Mass
1 mole
= Mass
STOICHIOMETRY
• Stoichiometry is the process of using the principles of molar quantities to predict the theoretical
amounts of reactants and products in a chemical reaction.
• Why? – All chemical reactions involve real amounts of reactants and products. Based on knowing
only one amount (one reactant or one product), the theoretical amounts of all other reactants and
products can be determined.
• In many instances, it is important to know more than just what products are formed.
• It can be used to determine the amount of pollutants produced.
• It can be used to determine the amount of a second substance that is needed to produce a valuable product.
• Regardless of the purpose, the concept of molar conversions and mole ratios are the keys.
STOICHIOMETRY
• All stoichiometry is based on the mole ratios of the substances in a chemical reaction.
• A balanced chemical equation identifies the ratio of moles of each reactant and product in
that chemical reaction.
• By comparing the number of moles of any two substances, a known amount of one
substance will identify how many moles of a second substance are involved in that
chemical reaction.
EXAMPLE
• If the mole ratio of two substances is 3:1 in a balanced chemical equation, then the 3:1 ratio
cannot change regardless of how many moles of any one substance is involved in that reaction.
• If 6 moles of the first substance is present, then two moles of the second substance must be present:
3 : 1 ratio =
6 : 2 ratio
• If 0.5 moles of the second substance is present, then 1.5 moles of the first must be present:
3 : 1 ratio =
1.5 : 0.5 ratio
STOICHIOMETRY
• What if you are not given moles of the known element, but instead you are given grams?
• Since all reactions involve moles of substances and not grams, the grams must first be converted to
moles. Once that is accomplished, the same process is used to determine the moles of the second
substance.
• What if you are asked for the mass of the unknown substance instead of moles?
• Since all reactions involve moles of substances and not grams, once the number of moles of the
unknown substance has been determined using the mole ratio, the number of moles can be converted
into grams as requested.
STOICHIOMETRY
LIMITING AND EXCESS REAGENTS
• Reactions will only occur as long as there are sufficient reactants capable of reacting.
• Typically, one reactant will run out before running out of the others.
• The one that runs out first is the limiting reagent (or limiting reactant).
• All of the others are excess reactants.
• Once the limiting reactant is completely used, the reaction will stop.
• To determine the limiting reactant, mole to mole calculations are performed based on the amounts of reactants
present.
• The reactant that would produce the least product is the limiting reactant.
ACTUAL, THEORETICAL, AND PERCENT
YIELD
• Theoretical yield – a calculated maximum amount of product possible from a certain amount of
limiting reactant.
• The information needed is obtained from the chemical equation and a given amount of one of the
reactants or products
• Actual Yield – the measured amount of product produced from a certain amount of reactant
• This value is obtained from experimental data. This is only done by actually performing the experiment.
Actual Yield
Percent Yield = ---------------------- x 100%
Theoretical Yield