STEST 404CHAPTER 4 NOTES

DETERMINING THE LIMITING AND EXCESS REAGENTS

When two or more subtsances react, the quantity of ______is determined by the ______.

______: The reactant in a chemical reaction that reacts completely.

The number of ______of LIMITING REAGENT is used to determine the quantity of product that will form.

______: The reactant that is present in more than the required amount.

Sample Problem:

Cadmium sulfide, which is usedinlight meters, solar cells, and other light-sensitive devices, can be made by the direct combination of the two elements:

Cd(s)+ S(s)  CdS(s)

If we start out with 2.00 g of cadmium and 2.00 g of sulfur,

a)which reactant is in excess and by how much?

b)what massof CdS will be produced?

Solution to a)Determining which reactant is in excess and by how much.

STEP 1: Determine the number of moles of each reactant. (Make sure the equation is balanced first!!!)

STEP 2: Use the moles of either reactant in a mole to mole ratio to determine how many

mole of the other reactant is needed.

How many moles of _____do we actually HAVE? ______

How many moles of _____ do we really NEED? ______

Do we have too much or too little _____? ______

Final answer:

______is the LIMITING REAGENT!

______is IN EXCESS (INXS)!

STEP 3: Determine the mass of sulfur, S, actually needed.

STEP 4: Subtract the actual amount from the amount needed.

Final Answer: ______is in excess by ______g.

Solution to b) Determine the mass of cadmium sulfide produced using the moles of the

limiting reagent.

STEP 1: Use the moles of limiting reagent in a mole to mole ratio to determine the moles

of cadmium sulfide (CdS) produced.

STEP 2: Convert the moles to grams.

APPLICATIONS

  1. A mixture is prepared from 25.0 g of aluminum and 85.0 g of Fe2O3. The reaction that occurs is described by the equation:

Fe2O3(s) + Al(s)  Al2O3(s) + Fe(s)

How much iron, Fe, is produced in the reaction?

  1. How many grams of silver chromate,Ag2CrO4, will precipitate when 150 mL of 0.500 M silver nitrate, AgNO3, are added to 100 mL of 0.400 M potassium chromate,K2CrO4?

2 AgNO3(aq) + K2CrO4(aq)  Ag2CrO4(s) + 2 KNO3(aq)

Limiting Reagent Worksheet

For the following reactions, find the following:

a)Which of the reagents is the limiting reagent?

b)What is the maximum amount of each product that can be formed?

c)How much of the other reagent is left over after the reaction is complete?

1)Consider the following reaction:

3 NH4NO3 + Na3PO4 (NH4)3PO4 + 3 NaNO3

Answer the questions above, assuming we started with 30 grams of ammonium nitrate and 50 grams of sodium phosphate.

2)Consider the following reaction:

3 CaCO3 + 2 FePO4 Ca3(PO4)2 + Fe2(CO3)3

Answer the questions at the top of this sheet, assuming we start with 100 grams of calcium carbonate and 45 grams of iron (III) phosphate.

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