Questions and Answers
What is the actual yield in a chemical reaction?
The amount of product actually obtained from a chemical reaction.
What is an excess reactant in a chemical reaction?
A reactant present in a greater amount than necessary.
What are stoichiometric amounts?
Quantities of reactants in the same relative amounts as those represented in the balanced chemical equation.
What is stoichiometry?
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What is the mole ratio in a balanced equation?
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What is the excess reactant in a chemical reaction?
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What is the limiting reactant in a chemical reaction?
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What is the percent yield in a chemical reaction?
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What is the theoretical yield in a chemical reaction?
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How many mole ratios can be written for a chemical reaction involving a total of n substances?
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How to solve mole-to-mole, mole-to-mass, and mass-to-mass stoichiometric problems?
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What is the conversion factor for mole-to-mole conversions?
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How to determine the limiting reactant?
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How to calculate the product when a reactant is limiting?
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How to calculate the amount of excess reactant left after a reaction?
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Why use an excess of a reactant?
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When would you have to look for limiting reactants?
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Why do reactions not always yield as much product as expected?
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Study Notes
Stoichiometry
- Stoichiometry is the study of quantitative relationships between reactants and products in a chemical reaction, based on the law of conservation of mass.
Reactants and Yield
- Actual yield is the amount of product obtained from a chemical reaction, determined experimentally.
- Excess reactant is a reactant present in a greater amount than necessary.
- Stoichiometric amounts are quantities of reactants in the same relative amounts as those in the balanced chemical equation.
Mole Ratio and Stoichiometric Calculations
- Mole ratio is the ratio between the numbers of moles of any two substances in a balanced equation.
- To solve mole-to-mole, mole-to-mass, and mass-to-mass stoichiometric problems, follow these steps:
- Write a balanced chemical equation.
- Convert from mass to moles, if necessary.
- Use the mole ratio to convert from moles of the given substance to moles of the unknown substance.
- Convert from moles to mass, if necessary.
Limiting Reactant
- A limiting reactant is a reactant that is totally consumed during a chemical reaction, limiting the extent of the reaction and determining the amount of product.
- To determine the limiting reactant:
- Find the number of moles of each reactant.
- Compare the actual mole ratio with the theoretical mole ratio from the balanced equation.
- Determine which reactant will get used up first.
Calculating Product and Excess Reactant
- To calculate the product when a reactant is limiting:
- Determine which reactant is limiting.
- Convert from grams to moles of the limiting reactant, if necessary.
- Convert from moles of the limiting reactant to moles of the product.
- Convert from moles of product to grams, if necessary.
- To calculate the amount of excess reactant left after a reaction:
- Determine which reactant is limiting.
- Convert from grams to moles of the limiting reactant, if necessary.
- Convert from moles of the limiting reactant to moles of the excess reactant.
- Convert from moles to grams, if necessary.
- Subtract the amount of excess reactant that reacted from the total amount to determine how much is left over.
Excess Reactant and Reaction Yield
- Chemists often use an excess of the least expensive reactant to drive reactions to continue until all of the limiting reactant is used up.
- Using an excess of one reactant can also speed up a reaction.
- Reactions do not always yield as much product as expected due to:
- Liquid reactants and products adhering to surfaces or evaporating.
- Formation of products other than the intended one by competing reactions.
- Solid product left behind on filter paper or lost in purification.
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Description
Review chemistry concepts related to stoichiometry, including actual yield and excess reactants, with these Chapter 11 flashcards from Chemistry Matter and Change.