3.4 Limiting reactant calculations

Question 1

N2 (g) + 3H2 (g) –> 2NH3 (g)

This is saying 1 mole of nitrogen gas reacts w/ 3 moles of hydrogen gas to produce 2 moles of ammonia gas

The reaction never tells you how much you have, it just tells you the mole to mole ratio in which things react
* We don’t know how many reactants or products we have, just the ratios 1:3:2
* the coefficents in these problems only give us a new way of finding mole to mole ratios
* You can compare any to specifics in the equation and get a mole to mole ratio out of that. Comparing hydrogen gas to ammonia we have a 2:3 ratio, compairing nitrogen to ammonia we get a 1:3 ratio

EX: You have 6 moles of N2 gas. How many moles of hydrogen gas would I need for that 6 moles to react completely (using the equation above). The key is based on that 1:3 ratio. For every 1 mole of N2 gas, we need 3 moles of hydrogen gas to react fully to form ammonia. 6 moles of N2 gas needs 18 moles of hydrogen gas to react fully
* must create a mol to mol ratio - shown below

This is how you would get that 18 # w/o doing it in your head.

Question 2

N2 (g) + 3H2 (g) –> 2NH3 (g)

Using the equation above, you have 6mols N2 gas. What is the maximum # of moles of 2NH3 that can be produced (ammonia gas)

Answer 2

Key is knowing how to set up the mol to mol ratio

Question 3

The prior 2 problems have been in moles. However, what happens if were in grams

N2 (g) + 3H2 (g) –> 2NH3 (g)

You start w/ 56gN2. How many moles of H2 do I need for it to react completely

so the first thing we need to do is get that 56g to moles to we can take advantage of those mol:mol ratios

Question 4

The reason were converting to moles instead of leaving it in grams is because the chemical reactions happen between particles, not individual masses.

EX:

N2 + 3H2 –> 2NH3

This means:
* 1 mole of N2 reactions w/ 3 moles of hydrogen to produce 2 moles of NH3
* it doesnt mean that 1 gram of nitrogen reacts with 3 grams of hydrogen to produce 2 grams of NH3. This wouldnt be correct because N2 and 3H have different molar masses

When a problem gives you grams of two reactant, you:
1) convert them both to moles so you can:
2) Compare how amny moles you actually have vs how many moles you need, based on the reaction

This is how you figure out which reactant runs out first - aka the limiting reactant

You convert to moles in limiting reactant problems because
1) Reactions occur based on number of moelcules, not their mass
2) The banclced equation gives you mole rations, not grams
3) Comparing grams would be like comparing apples and organges - only moles tell you how many atoms/molecules you’re really dealing with

essentailly 1 gram of carbon would have a different # of molecules than 1 mole of hydrogen, however, 1 mole of hydrogen would have the exact same number of atoms in it as 1 mole of carbon - letting us compare even amounts of molecules
* in limiting reactant problems you’re asking “ how many atoms or molecules do I actually have of each substance? - so you would need to be in moles to compare because thats the only thing that actually lets you see the individual molecules

Question 5

1 Frame + 2 tires –> 1 Bike

You have 10 frames, how many tires do you need to use up all your frames?

Question 6

1 Frame + 2 tires –> 1 bike

You have 10 frams and 22 tires. How many bikes can you make? You need to find the limiting reactant or the thing that limits the # of bikes that can be made (whichever one is going to run out first).

So hes going to take the approach of seeing how many bikes he can make by using up all the frames, then using up all the tires to see which one creates less bikes. So start by setting your ratio for frames to bikes then tires to bikes

Question 7

N2 (g) + 3H2 (g) —> 2NH3 (g)
* in this formula these are moles (this is assumed)

112g N2
30g H2

Question 1: If 112 grams of N2 and 30g of H2 react completely, which is the limiting reagent (meaning which one stops more of the product from being produced)? Limiting reactant = limiting reagent

Question 2: How product is formed if these do react completely

Question 3: How much of the reagent of excess (other reactant) is left over (meaning how much of that unused reactant do we have)

its called the theoretical yield because its the most possible amount of product we can make - however - there are often side reactions that occur that keep this maximum from occuring - also lots of reactions dont go to completetion

One thing we do after the calculation is the % yield calculation

% yield = Actual yield
—————– x 100
Theoretical yield

So actual yield should be less than theoretical yield because there are going to be some of those side rxns happening and other things that keep it from being the max possible yield (which is what theoretical yield is)
* we got a theoretical yield of 8mols NH3 = 136g NH3
* Say we only got 102 grams of NH3 what is our yield %?

yield % = (102g/136g) x 100 = 75%
* this may done in moles as well

Question 8

Answer 8

Question 9

Answer 9

Question 10

missed

Answer 10

so in this reaction you need 2 mols of carbon for every 3 mols of hydrogen (shown in the equation)

So if you want your 10 moles of H2 to react completely you need that much carbon (makes since using that 2:3 ratio)

Question 11

missed

Answer 11

Question 13

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Question 14

Answer 14

Question 15

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Question 16

Answer 16

Question 17

Answer 17

first step is determining the limiting reagent, because even when they say 1 reacts completely i guess that doesnt automatically mean its the limiting reagent

Question 18

Answer 18

Question 19

Answer 19