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Limiting Reactants

Limiting Reactants. Stoichiometry. Definitions. Limiting Reactant: The reactant in a chemical reaction that limits the amount of product that can be formed. The reaction will stop when all of the limiting reactant is consumed

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Limiting Reactants

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  1. Limiting Reactants Stoichiometry

  2. Definitions • Limiting Reactant: The reactant in a chemical reaction that limits the amount of product that can be formed. The reaction will stop when all of the limiting reactant is consumed • Excess Reactant: The reactant in a chemical reaction that remains when a reaction stops when the limiting reactant is completely consumed. The excess reactant remains because there is nothing with which it can react.

  3. Which is limiting?Which is excess?Why?

  4. Solve in Chemistry • Treat a LR problem like TWO stoichiometry problems • Pick one of the products to solve for- it doesn’t matter which one, but you have to use the same product for both calculations • Calculate # of gramsof that product for each piece of info you are given • The reactant that gives the smallest # of grams producedis the limiting reactant

  5. example • A 2.00 g sample of ammonia is mixed with 4.00 g of oxygen. Which is the limiting reactant and how much excess reactant remains after the reaction has stopped? 4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g) Known: 2.00g ammonia; 4.00 g oxygen Unknown: grams of product (I’ll pick NO- you could pick water. IT DOESN’T MATTER)

  6. Example continued • First, calculate how many grams of NO are produced from 2.00g of ammonia: 2.00gNH3 x x x = 3.53 g NO • Next, calculate how many grams of NO are produced from 4.00 g of oxygen: 4.00g O2x x x = 3.00 g NO 4.00g of oxygen is the LR because it will produce less NO than 2.00g of ammonia

  7. Now, how much excess is left? • All the oxygen was used up to make the 3.00g NO, that makes it the limiting reactant. That means some ammonia is left over (excess), lets figure out how much!! • We need to figure out how much ammonia is used when 4.00g of oxygen react. That makes: • Known: 4.00g of oxygen • Unknown: g of ammonia

  8. Set up yet another stoich problem… 4.00g O2x x x = 1.70 g We're not finished yet though. 1.70 g is the amount of ammonia that reacted, not what is left over. To find the amount of excess reactant remaining, subtract the amount that reacted from the amount in the original sample: 2.00 g - 1.70 g = 0.30 g remaining

  9. Helpful hints • Do not circle 3.00g of NO in the previous problem and say this is the limiting reactant. The LR has to be a REACTANT, not the product. • Read the whole problem, as often you will be asked to calculate how much of one of the products will be produced. This will save you from having to do extra work if you arbitrarily pick the other product to use in the LR calculation

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