UALR 1402: General Chemistry I
- Understanding Stoichiometric Proportions
- Understanding Limiting and Excess Reagents
- Predict quantities of products produced or reactants consumed based on complete consumption of limiting reagent (on both mole and mass basis)
- Predict quantities of excess reagents left over after complete consumption of limiting reagents.
Stoichiometric Proportions, Limiting and Excess Reagents
A balanced chemical equation describe the ratios at which products and reactants are respectively produced and consumed. That said, the coefficients of the balanced equation have nothing to do with the actually quantity of reactants you start with, as you can mix any amount you choose, but clearly the maximum yield must be limited by the reactant that gets consumed up first, the limiting reagent. This can be easily understood by the analogy of making bicycles, where each bike requires 2 tires and one frame. Lets look at a bike shop that has 10 frames. The "equation" becomes:
1 frames + 2 tires --> 1 bike
- 10 frames and 16 tires makes 8 bikes, with 2 frames left over. So tires are the limiting reagent and there are excess frames.
- 10 frames and 20 tires makes 10 bikes with nothing left over (they are in stoichiometric proportions)
- 10 frames and 30 tires makes 10 bikes with 10 tires left over. So frames are the limiting reagent and tires are excess.
Stoichiometric Proportions: Reactants are mixed in the ratios defined by their stoichiometric coefficients. If a reaction proceeds to completion, everything is consumed.
NonStoichiometric Proportions: Reactants are mixed in ratios that are different than the stoichiometric coefficients. One species runs out first (Limiting Reagent), while another is not completely consumed (Excess Reagent).
Theoretical Yield: the maximum possible yield based on the complete consumption of the limiting reagent
Are the limiting reagents always completely consumed? No, only if the reaction goes to completion. There can be many different reasons why the limiting reagent is not completely consumed, and these will be covered in detail in later chapters of this text. The Theoretical Yield is the maximum amount of product that would be produced through the complete consumption of the limiting reagent.
Excess Reagent: The quantity (mole or mass) left over after the complete consumption of the limiting reagent.
Quantity Excess = Initial Quantity - Consumed Quantity.
Where quantity can be moles or mass.
Limiting Reagent Problem Strategies:
- Identify moles of all reactants present.
If given mass, divide by formula weight to convert moles (this is the mass to mole step from the section 4.1.
- Divide moles of each reactant by it's stoichiometric coefficient.
This is the denominator of the mole-to-mole step in section 4.1.
- Smallest number indicates limiting reagent.
- Multiply by stoichiometric coefficient of species you are solving for, and answer the question .
This is the numerator of the mole-to-mole step in section 4.1. If you are after moles, you are finished, if you are after mass, you need to use the molar mass of product to convert moles product to grams mass product, which is the mass-to-mole step in section 4.1.
Limiting Reagent Problem
Silver tarnishes in the presence of hydrogen sulfide and oxygen due to the following reaction
4Ag + 2H2S + O2 ----> 2Ag2S + 2H2O
What is the Limiting Reagent and Theoretical Yield of Ag2S if 2.4 g Ag, 0.48 g H2S and 0.16g O2 react?
6:07 minute YouTube video showing shortcut method for determining limiting reagent and theoretical yield. [corrigenda: after dividing moles Ag by stoichiometric coef. the value is 0.00556, not 0.0556, but this this did not effect the solution as both are less than the value for oxygen of 0.00500]
What quantities of excess reagents are left over after the complete consumption of the limiting reagent if 2.4 g Ag, 0.48 g H2S and 0.16g O2 react?
4:36 minute YouTube determining the excess reagents after the complete consumption of the limiting reagent.
To Calculate moles of Excess reagent you subtract the amount consumed by the complete consumption of the limiting reagent from the initial quantity of the excess reagent.
Do the Following Worksheet