Tuesday, February 12th
Bellwork: Complete the Limiting Reactants Transparency worksheet (pg 64/64)
RARELY does anything work out 100% fair in life. Someone is always a little ahead of someone else.
In nature reactants are very rarely present in the same ratios that are specified by the balanced equation.
­One reactant will always be in excess (over abundant) compared to the other.
Limiting reactant­ limits the extent of the reaction and in turn determines the amount of product formed.
The reactants that are leftover when a reaction stops are called excess reactant(s) .
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Limiting Reactions Calculations
Example: S8(l) + 4Cl 2(g) 4S2Cl2(l)
If 200.0g of sulfur reacts with 100.0g of chlorine, what mass of disulfur dichloride is produced?
1st: Determine which reactant is the limiting factor. Why?­Because this will determine the amount of product that you can have.
**REMEMBER the only way to compare the masses of the reactants is to turn them into moles!!**
200.0g S 8 1 mol S 8
256.5g S 8
100.0g Cl 2 1 mol Cl 2
70.91g Cl 2
= 0.780 mol S 8
=1.410 mol Cl 2
2
= 1.410 mol Cl 2
= 0.780 mol S 8
2nd: We must find out if the reactants are in the same mole ratio as given in the balanced chemical equation. (According to the chemical equation the ratio should be 4 mol chlorine:1 mol sulfur)
1.410 mol Cl 2
0.780 mol S 8
=
1.808 mol Cl 2 available
1 mol S 8 available
Since the answer should be 4:1. Chlorine is the limiting factor.
Note: How do you know which one should go on top?...doesn't really matter...but make it easy on yourself. I want a 4 to 1 ratio. So If I divide Chlorine by Sulfur I should get a whole number of 4. If I do it the other way, you are looking for .25 as an answer.
3rd: Now that you know the limiting factor, calculate the amount of product that can be formed.
­Use the number of moles the limiting factor and the mole ratio of the limiting factor and the product being formed.
1.410 mol Cl 2 4 mol S 2Cl2 135.0g S 2Cl2
4 mol Cl 2 1 mol S 2Cl2
= 190.4g S 2Cl2
Statement: 190.4g S2Cl2 forms when 1.410 mol Cl 2 reacts with excess S8.
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4th: How much of the excess reactant, reacted?
1.410 mol Cl 2 1 mol S 8 265.5g S 8
4 mol Cl 2 1 mol S 8
= 90.42g S 8
How much was excess? 200.0g ­90.42g = 109.6g S 8
Why use an excess amount of the reactant?
­If chemists used a cheaper reactant they can ensure that the reaction continues until a limiting reaction stops it.
­Speeds up the reaction
Homework: Practice Problems 23 & 24 on page 383
12 & 13 on page 983
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11.4 Percent Yield
Example: When your playing basketball, ideally you want to make every single free­throw that you have the opportunity to make­­the reality is this will not happen, most likely. Let's say you have the opportunity to go to the free­throw line 5 times in one game so theoretically you should be able to gain 10pts for your team. But you actually only make 8pts. Your accuracy would be 80%. 8/10 x100 = 80%
IN CHEMISTRY: you are also very unlikely to actually produce all of the product that you should be able to get. We can calculate how much you actually got in class verses what you should have gotten.
Actual Yield­ the amount of product produced when the chemical reaction is carried out in an experiment (reality)
Theoretical Yield­ the amount of product you should have been able to produce.
Theoretically Percent Yield­ gives us the efficiency of the experiment, it is the ratio of actual vs. theoretical that is expresed as a %.
% Yield = actual theoretical
x 100
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Example: Silver Chromate (Ag 2CrO4) forms when potassium chromate (K 2CrO4) is added to a solution containing 0.500g of Sliver nitrate (AgNO 3). Determine the theoretical yield of Ag 2CrO4. Then calculate the % yield of the reaction if 0.455g of Ag 2CrO4 is actually produced.
2AgNO 3 + K2CrO4 Ag 2CrO4 + 2KNO 3
0.500gAgNO 3 1 mol AgNO 3 1 mol Ag 2CrO4 331.7g Ag 2CrO4
169.9g AgNO 3 2 mol AgNO 3 1 mol Ag 2CrO4.
= .488g Ag 2CrO4 .455g (actual)
.488g (theoretical)
x 100
= 93.2% Ag 2CrO4
Why is it important to be able to do % yield?
­Cost effectiveness fo many industrial manufacturing processes. (The cost to make certain chemicals affects the cost to the consumer.)
CLASSWORK & HOMEWORK
Practice Problems #28­30 on page 387 Also complete pre­lab for Limiting Reactants Lab
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