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Limiting reactant

Posted: Thu Oct 04, 2018 6:18 pm
by Marsenne Cabral 1A
I understand that even when gram amounts for different reactants differ, it is not automatically the smallest one that becomes the limiting reactant, but what is the reason that that is the case?

Re: Limiting reactant

Posted: Thu Oct 04, 2018 6:42 pm
by 005199302
We are looking to compare the amount/number of atoms in each of the reactants, not how much all of the atoms weigh together. This is why we need to convert to moles. Looking at just grams would be inaccurate because atoms of different elements differ in their atomic mass. If the grams of reactant 1 exceeds the grams of reactant 2, but each atom in reactant 2 weighs more, it is possible the total amount of the reactant 2 exceeds the reactant 1.

Re: Limiting reactant  [ENDORSED]

Posted: Thu Oct 04, 2018 7:06 pm
by timschaeffer Dis 1J
What matters most for these problems is that you know:

1) how many atoms/molecules/moles of each reactant you have at the beginning of the process, and

2) how many atoms/molecules/moles of each reactant are consumed for each cycle of the chemical reaction

Some reactants might be naturally heavier than others, but they might also be consumed multiple times faster than other reactants

For example: Lets say a made-up reaction consumes 2 moles of lead and 1 mole of hydrogen each cycle, and you start out with two moles of each reactant. All of the lead will be consumed in only 1 cycle while it would take 2 cycles to consume all of the hydrogen. This means that the sample of lead would run out first, even though it weighs much more than the sample of hydrogen.