LimitingReactants

When this occurs, the calculations relating to this condition are more time-consuming and complex. These types of problems are called Limiting reactant problems.

These calculations follow the Basic Reaction Stoichiometry Rules except for some additional steps. Since these calculations are the most time-consuming and require the most steps, it is important to known when you should do these calculations compared to Basic Reaction Stoichiometry.

• When you need to do the following Limiting Reactant Calculations
  • You only need to do Limiting Reactant calculations when YOU HAVE INFORMATION ON BOTH REACTANT ONLY.
• "When do you NOT need to do Limiting Reactant Calculations.
  • Only give information on one reactant, and assume there is enough of other reactant(s).
  • They give information on one reactant and says the others are "in excess or enough to react,etc"
  • You are given product information (in this case you don't need to know which is limiting)

• Definitions

• Limiting Reactant- The reactant that limits the reaction or runs out first (thereby stopping the making of product).

• Reactant in Excess- The reactant which is not completely consumed during the reaction (one reactant that does not run out when you stop making product)

Special Rules of Reaction Stoichiometry for Limiting Reactant Problems

• Step 1 - Write the balanced equation out unless you are given it. (same as before)

• Step 2 - Need to determine Limiting reactant
  • a. Convert known infor for both reactants to moles (general reaction A + B -> C + D )
  • b.Ask question, one of two reactants is limiting reactant, therefore it will go to completion.(i.e. Is A limiting reactant? (A goes to completion))
  • c.Calculate how much of other reactant (i.e. B) is needed to completely react all "A". Calculate using molar ratio.
  • d. Compare what you need to what you have (from above) to determine limiting reactant
    • i.If you do have enough B (yes to question), A is limiting reactant
    • ii.If you do not have enough B (no to question), B is limiting reactant.
  • e.Need to state which one is limiting reactant and how you know it.
  • In the example above, If there was enough of B to completely react all A then you need to write some thing similar to the following.
    

• Step 3 - Only special rules to finding the information about the product(s) once you find the limiting reactant is:
  • ALWAY USE LIMITING REACTANT MOLES TO DETERMINE MOLES OF PRODUCTS, and then follow Basic Reaction Stoichiometry Rules

• Step 1 - Finding amount of Excess reactant (excess reactant=not limiting reactant)
  • Step 1a - Use the Equation: Initial - reacted = left over
    • You can first calculate moles left over then convert to unit you want or visa versa.
    • You have the initial information from above
    • Depending on which reactant you assumed, you either have already calculated the amount of excess reactant that has reacted or need to do now using the same method as above,

• Plug value(s) into equation (Initial - reacted = left over) to get moles (or actual info) of excess reactant.
• Convert excess reactant moles to actual info.