This is a big, tough lesson based on the material introduced in the previous lesson, "Calculations 3: Reactions". There are two key concepts. The first is that for any substance, you end up with what you started with plus the change (positive or negative). The second is that the changes in a chemical reaction are related by the coefficients in the balanced equation, and are negative for reactants, positive for products.
The first concept is introduced in terms of cash in a bank account: how deposits and withdrawals affect the balance. A series of questions establishes the principle that if you know any two of "start", "change" and "finish" you can deduce the third. This is then repeated in terms of moles of ammonia.
The other key concept is that the balanced chemical equation relates only the changes. Changes are positive on one side of the equation and negative on the other, and, in moles, they're in proportion to the coefficients in the equation.
The reaction table is used to organize all of this. The student is guided through examples.
Start by entering all you know, and then deduce the rest of the entries step by step. The student is guided through this procedure, one step at a time. At each step it is possible to put a number into any of the remaining entries in the table. As you can see in the figure, suitable feedback is given if the student attempts to enter a number that cannot yet be deduced from the data available.
This is done first in moles, and then with masses added.
Various types of exercises are presented, and guidance and feedback are given. It is made clear that the starting amounts need not be in stoichiometric proportions, and that the reaction need not go to "completion".
This section ends with a series of problems, in which the student must use the reaction table. Comprehensive help is available.
Now the concept of limiting reactant is introduced. Reactions are animated, and shown to stop when one reactant is used up, even if others are still present. So a "quantitative" reaction proceeds until the limiting reactant runs out. The trick is to find the limiting reactant. A systematic procedure to do this is presented. This section ends with a comprehensive quiz.
Finally, the notion of percent yield is introduced, as 100´ (what you got)/(maximum possible), where the maximum was determined by the limiting reactant. There are a couple of practice problems, in which the limiting yield has been worked out for you.
The final review quiz asks for percent yields, since this calculation involves all of the concepts and techniques introduced in this lesson.
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Updated July 24, 2000