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General Chemistry

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Chemical Reactions

11 out of 15

Passage 1

Information regarding the amounts of substances that actually react to form the products are of extreme help. Analyzing reactions and having the balanced equations help chemists to determine the correct and optimum proportions of reactants to be used. There are various other factors besides the amount of reactants, which determine the efficiency of reactions.

Experiment 1

Student 1 used 40.5 g of aluminum and 80 g of Fe2O3 for Reaction I. A second student conducted the same reaction with a different amount for one of the reactants. The second student used 40.5 g of aluminum, and 90 grams of Fe2O3. A third student also conducted the same reaction with 54 g of aluminum and the same amount of Fe2O3 used by the first student.

Experiment 2

Reaction II involved the production of Al2O3 from aluminum hydroxide This was done by heating aluminum hydroxide.

Experiment 3

Based on the given information, which of the following are true regarding Experiment 1?

I. The second student had the highest yield for aluminum oxide.

II. The third student had a higher yield for aluminum oxide than the first student.

III. The first and the third students had the same yield for aluminum oxide.

A I only
B I and II only
C I and III only
D II and III only

Ans. C Look at the items one by one. Item I says that the second student had the highest yield of products. In Experiment 1, the first student used 40.5 g of Al and 80 g of Fe2O3. This translates to 1.5 moles of Al and roughly 0.5 moles of Fe2O3. Here Fe2O3 is the limiting reagent. What was the second student doing? Well, the second student increased the amount of Fe2O3 used, keeping the same amount of Al. Since Fe2O3 is the limiting reagent, an increase of Fe2O3 will increase the product yield. So the second student must have had more yield than the first student. The first and the third students have the same yield for aluminum oxide, because the amount of limiting reagent present is the same in both cases. So items I and II are true.

Chemical Reactions Flashcard List

15 flashcards