Limiting Reactantpreap

What is a limiting reactant? – Limiting Reactant – limits or determines the amount of product that can be formed in a reaction. – Excess Reactant – the reactant that is not completely used up in a reaction. – Limiting reactant determines the theoretical yield – how much should be formed.

• The reaction will stop when the reactants are used up. • If one reactant is used up before the other, the reaction then stops. • The first reactant used up is the limiting reactant, use it for the calculation. • The other reactant is the excess reactant, it is unimportant in the calculation.

Sample Problem: 2 Na (s)

100. g Na

+

Cl2 (g)  2 NaCl (s)

100. g CI2

Which is the limiting reactant? Plan: grams-grams calculation from Na to NaCl, then Cl2 to NaCl; see which results in the smaller amount of product

100. g Na 1 mol Na 22.99 g Na

2 mol NaCl 58.44 g = 2 mol Na 1 mol NaCl 254 g NaCl

100. g Cl2 1 mol CI2 70.90 g Cl2

2 mol NaCl 58.44 g = 1 mol Cl2 1 mol NaCl

Limiting Reactant is Cl2 Excess Reactant is Na Theoretical yield is 164 g NaCl

164 g NaCl

Theoretical vs. Actual Theoretical Yield – The maxiumum possible that could be formed Actual Yield = amount that is actually produced Percent yield = actual x 100 theoretical Reasons: Reaction is reversible Conditions are not ideal Side reactions occur Human error

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