Understanding Limiting Reactants

# The products are increasing so a plus sign is placed

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minus sign is placed in front of them. The products are increasing so a plus sign is placed in front of them. Line 4: This line contains what will be left and what will be formed when the reaction is complete. The values of this line are obtained by adding lines 2 and 3. This line will provide all the answers, in Final moles, to the problems.

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Line 1 2  H 2 + O 2 2  H 2 O Line 2 S tarting moles 2 2 0 Line 3 R eacting moles - 2 x - 1 x + 2 x Line 4 F inal moles 2-2x 2-1x 0+2x Figure 24 Sample SRF Table When the reaction is completed, either the hydrogen will be consumed or the oxygen will be consumed. That means that either 2 – 2x = 0 (hydrogen)    or     2 - 1x = 0 (oxygen) If we solve for x in both equations the smallest x value will be the limiting reactant. The larger value will represent an amount in excess of what is possible with the given ingredients. So for this example, x = 1 (hydrogen) or x = 2 (oxygen). Since x = 1 is smallest, it is the value we choose. This step also identifies the substance that will run out first, the limiting reactant. In this case the hydrogen is the limiting reactant.      Using the value of x = 1, we can determine the final amounts.  The amount of product is 2x =  2(1) = 2 moles of H 2 O.  The reactant in excess is O 2  and it is excess by 2 – 1x = 2 – 1(1) = 1 mole  of O 2 This method of working the problem gives the same result as the visual inspection—hydrogen is the limiting reactant, and 2.00 mol H
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