*stoichiometric quantities: molar quantities of reagents which, when reacted together, are completely consumed.
Example
Pb + S PbS 1 mol 1mol 1 mol 207.2 g 32.1 g 239.3g BUT
Pb + S PbS 1 mol 2mol 1 mol 207.2 g 64.2 g 239.3g
limiting excess reagent
*limiting reagent: -all of it is consumes in a reaction.
-determine the maximum amount of product that can be formed.
*reagent in excess: - some is left over at the end of the reaction.
Example
What mass of HCl is produced when 4.50 g of H2(g) and 140.0 g of Cl2(g) are reacted according to the following equation?
H2(g) + Cl2(g) 2HCl(g) 4.50 g 140.0g i) Determine the number of moles of each reactant.
ii) Calculate the number of moles of HCl expected from each reagent. From H2
From Cl2
iii) Which is the limiting reagent?
The one producing the least number of moles.
Therefore, Cl2 is the limiting reagent and H2 is in excess. iv) Answer the question.
Therefore, 144 g of HCl is produced. Example
What mass of S is produced when 15.6 g of KMnO4, 7.95 g of H2S, and 15.3 g of H2SO4 are reacted according to the following equation?
2KMnO4 + 5H2S + 3H2SO4 K2SO4 + 2MnSO4 + 8H2O + 5S Mass (g) 15.6 7.95 15.3 0 0 0 0 Moles (n/M) 0.0987 0.233 0.156 -- -- -- 0.233 Therefore, HOMEWORK: p.330, 1-7 day 1 p.335, 1-11 day 2
Complete Investigation: Limiting and Excess Reactants
Complete: Investigation Which Reagent is Limiting and How Much Precipitate is Formed
Limiting and Excess Reactants
In this investigation, you will predict and observe a limiting reactant. You will use the single displacement reaction of aluminum with aqueous copper(II) chloride.
2Al(s) + 3CuCl2(aq) 3Cu(s) + 2AlCl3(aq)
Note that copper(II) chloride, CuCl2, is light blue in aqueous solution. This is due to the
Cu2+(aq) ion. Aluminum chloride is colourless in aqueous solution.
Question
How can observations tell you which is the limiting reactant in the reaction of aluminum with aqueous copper(II) chloride?
Materials 250 mL beaker stirring rod 0.51 g CuCl2 0.25 g Al foil Safety precautions
The reaction mixture may get hot. Do not hold the beaker as the reaction proceed. Procedure
1. To begin the reaction, add about 150 mL of water to the beaker that contains the aluminum foil and copper(II) chloride.
2. Record the colour of solution and any metal that is present at the beginning of the reaction.
3. Record any colour changes as the reaction proceeds. Stir occasionally with the stirring rod.
4. When the reaction is complete, return the beaker, with its contents, to your teacher for proper disposal. Do not pour anything down the drain.
Observations
Beginning of the reaction: During the reaction:
Analysis
1. According to your observations, which reactant was present in excess? Which reactant was the limiting reactant?
2. Do stoichiometric calculations to support your observations of the limiting reactant.
3. Magnesium and hydrogen chloride react according to the following skeleton equation:
Mg(s) + HCl(aq) MgCl2(aq) + H2(g)
a) Balance the skeleton equation.
b) Examine the equation carefully. What evidence would you have that a reaction was taking place between the hydrochloric acid and the magnesium?
c) You have a piece of magnesium of unknown mass, and a beaker of water in which is dissolved an unknown amount of hydrogen chloride. Design an experiment to determine which reactant is the limiting reactant.
Which Reagent is Limiting and How Much Precipitate is Formed?
The purpose of this Investigation is to test gravimetric stoichiometry by predicting and determining the mass of precipitate produced by the reaction of aqueous strontium nitrate and aqueous copper(II) sulfate pentahydrate.
Question
What is the mass of precipitate produced by the reaction of 2.00 g of strontium chloride with 2.00 g of copper(II) sulfate pentahydrate?
Materials 3 beakers
2.00 g strontium chloride
2.00 g copper(II) sulfate pentahydrate eyedropper hot plate funnel filter paper Erlenmeyer flask Safety Precautions
Careful with hot liquids. Never leave hot plate unattended. Procedure
1. Measure 100 mL of water in a beaker. Heat the water using a hot plate. 2. In a beaker, measure 2.00 g of strontium chloride using an electronic balance. 3. In a second beaker, measure 2.00 g of copper(II) sulfate hydrate.
4. Once the water is warm, transfer water into the beaker containing strontium chloride using an eyedropper. Transfer enough water to totally dissolve the solid. Do not add more water than necessary.
5. Repeat step 4 for the beaker containing copper(II) sulfate hydrate. 6. Turn off the hot plate.
7. Mix the two solutions together by transferring the copper(II) sulfate hydrate solution to the strontium chloride solution.
8. Wash the walls of the beaker that contained the copper(II) sulfate hydrate solution using warm water and the eyedropper. Transfer to the beaker containing the two solutions.
9. Put the reaction beaker in an ice bath for 5 minutes to maximize the recovery of crystals.
10. Measure out the mass of your filter paper. Record the mass.
11. Filter your solution, washing the beaker with water to recover most of the crystals. 12. Let the filter paper dry overnight.
Observation
Mass of Strontium chloride (g) Mass of copper(II) sulfate hydrate (g) Mass of filter paper (g)
Mass of filter paper + precipitate (g) Analysis
1. Write the chemical equation for the reaction studied in this experiment.
2. Do stoichiometric calculations to determine which reactant is limiting. What mass of the other reactant will remain after the reaction is complete?
3. Based on the limiting reactant, predict the mass of precipitate expected.
4. Compare the actual amount produced (actual yield) to the expected mass (theoretical yield). Calculate the percentage yield (actual yield/theoretical yield x 100%).
7.5 The yield of a chemical reaction
*Most reactions do not produce the exact amount of product that is predicted by the balanced chemical equation
1) Theoretical Yield :
2) Actual Yield :
3) Percentage Yield :
Example
Bromine was made according to the following reaction: HBrO3 + 5HBr 3Br2 + 3H2O
If 10.0 g of HBrO3 was reacted with an excess of HBr and 26.3 g of Br2 was produced, what was the percentage yield of the reaction?
i) Determine number of moles of reactant used.
ii) Use the molar ratio to compare the limiting reagent to the product.
mass of Br2 expected iv) Determine % Yield