Question

Write a hypothesis that answers the lesson question, “While observing a chemical reaction, how can you tell which reactant is limiting?” Hypothesis: If a substance is the limiting reactant, then . . . because . . .

Answer 1

Answer: If a substance is the limiting reactant, then it will be completely used by the time the reaction completes because it is the reactant that reacts completely and the reaction stops after it is completely used up.

Explanation:

Limiting reagent is a reagent as it limits the formation of product and Excess reagent is the reagent which is preset in excess.

For example: if 0.75 moles of HCl and 1.5 moles of magnesium are reacted together.

$Mg(s)+2HCl(aq)\rightarrow MgCl_2(aq)+H_2(g)$

According to stoichiometry:

As 2 mole of $HCl$ reacts with 1 mole of $Mg$

0.75 moles of $HCl$ will react with =$\frac{1}{2}\times 0.75=0.38moles$ of $Mg$

$HCl$ is a limiting reagent as it limits the formation of product and Mg is an excess reagent as (1.5-0.38)=1.12 moles of magnesium are left unreacted.

Answer 2

Answer:  If a substance is the limiting reactant, then it will be fully consumed by the time the reaction completes because it is the reactant that reacts completely and the reaction cannot proceed further.