Embed Size (px)
DESCRIPTION
Limiting Reagents. Because life rarely gives you exactly the right amount of stuff. Limiting Reagents. At this point, you will have done the S'mores activity in class, hopefully. - PowerPoint PPT Presentation
Citation preview
Limiting ReagentsBecause life rarely gives you exactly the right amount of stuff
Limiting ReagentsAt this point, you will have done the S'mores activity in class, hopefully.
Given that, you should have some idea that because things can be mixed in whatever proportion you want, but only react in a certain proportion, you will often end up running out of one reactant before the other.
Limiting Reagents
Therefore, this presentation is just about some techniques you can use.
Identifying the Limiting ReagentLet's start with a sample reaction:
2Na3PO
4 + 3CaO → Ca
3(PO
4)
2 + 3Na
2O
10 moles 12 moles
Identifying the Limiting ReagentLet's start with a sample reaction:
2Na3PO
4 + 3CaO → Ca
3(PO
4)
2 + 3Na
2O
10 moles 12 moles
Method 1: Just calculate the yield using each compound normally:
10 moles Na3PO
4 yields 5 moles Ca
3(PO
4)
2 and 15 moles Na
2O
12 moles CaO yields 4 moles Ca3(PO
4)
2 and 12 moles Na
2O
Identifying the Limiting ReagentLet's start with a sample reaction:
2Na3PO
4 + 3CaO → Ca
3(PO
4)
2 + 3Na
2O
10 moles 12 moles
Method 1: Just calculate the yield using each compound normally:
10 moles Na3PO
4 yields 5 moles Ca
3(PO
4)
2 and 15 moles Na
2O
12 moles CaO yields 4 moles Ca3(PO
4)
2 and 12 moles Na
2O
Whichever one gives you less is the one that will run out first (limiting)
Identifying the Limiting ReagentLet's start with a sample reaction:
2Na3PO
4 + 3CaO → Ca
3(PO
4)
2 + 3Na
2O
10 moles 12 moles
Method 1: How much of the Na
3PO
4 will be used up? Use the ratio
8 moles of Na3PO
4 will react with the 12 moles of CaO
Identifying the Limiting ReagentLet's start with a sample reaction:
2Na3PO
4 + 3CaO → Ca
3(PO
4)
2 + 3Na
2O
10 moles 12 moles
Method 2: Figure out how many times you can do the reaction, based on the amount
you have, and how many it takes each time.
10 moles Na3PO
4 / 2 = enough to do it 5 (moles of) times
12 moles CaO / 3 = enough to do it 4 (moles of) times
Identifying the Limiting ReagentLet's start with a sample reaction:
2Na3PO
4 + 3CaO → Ca
3(PO
4)
2 + 3Na
2O
10 moles 12 moles
Method 2: Take this number of times, and multiply it by the coefficient of anything you're interested in:
4 (moles of) times * 2 = 8 moles Na3PO
4 used
4 (moles of) times * 1 = 4 moles of Ca3(PO
4)
2
4 (moles of) times * 3 = 12 moles of Na2O
Either Way We Get ThisLet's start with a sample reaction:
2Na3PO
4 + 3CaO → Ca
3(PO
4)
2 + 3Na
2O
Starting amount: 10 moles 12 moles 0 0Amount Used: 8 moles 12 molesFinal Amount: 2 moles 0 4 moles 12 moles
Or, In GramsLet's start with a sample reaction:
2Na3PO
4 + 3CaO → Ca
3(PO
4)
2 + 3Na
2O
Starting amount: 1640g 673g 0 0 Total: 2313g
Final Amount: 328g 0 1241g 744g Total: 2313g
Conservation of mass still applies, but you get less products out than the mass you put in, because some of it never reacted.
Summary• Two methods. Both give the same results.
• Everything still reacts in the reaction equation proportions.
• You will have less products than what you put in, but conservation of mass still applies—some just isn't used.
• S'mores are delicious.
