1. Chapter 4 Chemical Quantities and Aqueous Reactions

2. Reaction Stoichiometry
the numerical relationships between chemical amounts in a reaction is called stoichiometry
the coefficients in a balanced chemical equation specify the relative amounts in moles of each of the substances involved in the reaction
2 C8H18(l) + 25 O2(g)  16 CO2(g) + 18 H2O(g)
Microscopic: 2 molecules of C8H18 react with 25 molecules of O2 to form 16 molecules of CO2 and 18 molecules of H2O
Macroscopic: 2 moles of C8H18 react with 25 moles of O2 to form 16 moles of CO2 and 18 moles of H2O
Write the molar stoichiometry ratio between C8H18 and O2

3. Stoichiometry: Chemical Arithmetic
Chapter 3: Formulas, Equations, and Moles
Stoichiometry: Chemical Arithmetic
11/14/2018
Stoichiometry: The relative proportions in which elements form compounds or in which substances react.
aA + bB
cC + dD
Moles of A
Moles of B
Grams of B
Grams of A
Definition from the Oxford Dictionary of Chemistry, Oxford University Press, 2000.
Its origin is Greek for “element” “measure.”
“grams-to-moles-to-moles-to-grams”
You need the coefficients from the balanced chemical equation.
You can relate any of the substances in the chemical equation to each other.
Molar Mass of A
Molar Mass of B
Mole Ratio Between A and B (Coefficients)
Copyright © 2008 Pearson Prentice Hall, Inc.

4. Stoichiometry: Chemical Arithmetic
Chapter 3: Formulas, Equations, and Moles
Stoichiometry: Chemical Arithmetic
11/14/2018
Aqueous solutions of sodium hypochlorite (NaOCl), best known as household bleach, are prepared by reaction of sodium hydroxide with chlorine gas:
2NaOH(aq) + Cl2(g)
NaOCl(aq) + NaCl(aq) + H2O(l)
How many grams of NaOH are needed to react with 25.0 g Cl2?
Grams of
Cl2
Moles of
Cl2
Moles of
NaOH
Grams of
NaOH
Mole Ratio
Molar Mass
Molar Mass
Copyright © 2008 Pearson Prentice Hall, Inc.

5. Example
Magnesium hydroxide, the active ingredient in milk of masnesia, neutralizes stomach acid, primarily HCl, according to the following reaction:
Mg(OH)2(aq) HCl(aq)  2 H2O(l) + MgCl2(aq)
What mass of HCl, in grams, can be neutralized by a dose of milk of magnesia containg 3.26g Mg(OH)2?

6. C6H12O6(s) + 6 O2(g) ® 6 CO2(g) + 6 H2O(l)
Example
According to the following equation, how many milliliters of water are made in the combustion of 9.0 g of glucose?
C6H12O6(s) + 6 O2(g) ® 6 CO2(g) + 6 H2O(l)
density of water = 1.00 g/mL

7. Yields of Chemical Reactions
Chapter 3: Formulas, Equations, and Moles
11/14/2018
Yields of Chemical Reactions
Actual Yield:
Theoretical Yield:
The amount actually formed in a reaction.
The amount predicted by calculations from the
limiting reactant.
actual yield
Percent Yield =
x 100%
theoretical yield
Maximum percent yield is 100%.
Copyright © 2008 Pearson Prentice Hall, Inc.

8. Reactions with Limiting Amounts of Reactants
Chapter 3: Formulas, Equations, and Moles
11/14/2018
Reactions with Limiting Amounts of Reactants
Limiting Reactant: The reactant that is present in limiting amount. The extent to which a chemical reaction takes place depends on the limiting reactant.
Excess Reactant: Any of the other reactants still present after determination of the limiting reactant.
Example: How many sandwiches can be made from 10 slices of bread and 8 slices of cheese? Which one is the limiting reactant?
The balanced chemical equation is
2Bd + Ch Bd2Ch
Copyright © 2008 Pearson Prentice Hall, Inc.

9. Reactions with Limiting Amounts of Reactants
Chapter 3: Formulas, Equations, and Moles
11/14/2018
Reactions with Limiting Amounts of Reactants
At a high temperature, ethylene oxide reacts with water to form ethylene glycol which is an automobile antifreeze and a starting material in the preparation of polyester polymers:
C2H4O(aq) H2O(l)
C2H6O2(l)
Copyright © 2008 Pearson Prentice Hall, Inc.

10. Limiting and Excess Reactants in the Combustion of Methane
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
If we have 5 molecules of CH4 and 8 molecules of O2, which is the limiting reactant? H C + O ?

11. Things Don’t Always Go as Planned!
many things can happen during the course of an experiment that cause the loss of product
the amount of product that is made in a reaction is called the actual yield
generally less than the theoretical yield, never more!
the efficiency of product recovery is generally given as the percent yield

12. Example Ammonia, NH3, can be synthesized by the following reaction:
2 NO(g) H2(g)  2 NH3(g) H2O(g)
Starting with 86.3 g NO and 25.6 g H2
Determine the limiting reactant
Find the theoretical yield of NH3 in grams

13. Example
What is the percent yield if you experimentally produce grams of copper when 1.87 grams of Aluminum reacts with 9.65 grams of copper (II) sulfate
Al(s) + Cu(SO4)2(aq)  Cu(s) + Al2(SO4)3(aq)

14. Reactions with Limiting Amounts of Reactants
Chapter 3: Formulas, Equations, and Moles
11/14/2018
Reactions with Limiting Amounts of Reactants
Lithium oxide is used aboard the space shuttle to remove water from the air supply according to the equation:
Li2O(s) + H2O(g)
2LiOH(s)
If 80.0 g of water are to be removed and 65.0 g of Li2O are available, which reactant is limiting? How many grams of excess reactant remain? How many grams of LiOH are produced?
Copyright © 2008 Pearson Prentice Hall, Inc.

15. Example
The reaction between aluminum and iron (III) oxide can generate temperatures approaching 3000oC and is used in welding metals:
2 Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(s)
In one process, 124 g Al are reacted with 601 g of Fe2O3.
a. Which reactant is limited?
b. What is the theoretical yield of Al2O3(s)?

16. Concentrations of Reactants in Solution: Molarity
Chapter 3: Formulas, Equations, and Moles
Concentrations of Reactants in Solution: Molarity
11/14/2018
Molarity: The number of moles of a substance dissolved in each liter of solution. In practice, a solution of known molarity is prepared by weighing an appropriate amount of solute, placing it in a container called a volumetric flask, and adding enough solvent until an accurately calibrated final volume is reached.
Solution: A homogeneous mixture.
Solute: The dissolved substance in a solution.
Solvent: The major component in a solution.
It’s the volume of solution that’s needed and not simply the volume of water (assuming water is the solvent). Either pictures or showing a volumetric flask might be helpful.
Copyright © 2008 Pearson Prentice Hall, Inc.

17. Concentrations of Reactants in Solution: Molarity
Chapter 3: Formulas, Equations, and Moles
Concentrations of Reactants in Solution: Molarity
11/14/2018
Molarity converts between mole of solute and liters of solution:
molarity =
moles of solute
liters of solution
1.00 mol of sodium chloride placed in enough water to make 1.00 L of solution would have a concentration equal to:
1.00 mol
mol
= 1.00
or 1.00 M
1.00 L L
Copyright © 2008 Pearson Prentice Hall, Inc.

18. Example
Find the molarity of a solution that has 25.5 g KBr dissolved in 1.75 L of solution
Calculate the concentration of a solution made by adding 45.4g of NaNO3 to a flask and dissolving with water to a total volume 250.0mL

19. Using molarity in solution
molarity shows the relationship between the moles of solute and liters of solution
If a sugar solution concentration is 2.0 M, then 1 liter of solution contains 2.0 moles of sugar
Express 2.0 M as a conversion factor
Examples:
How many liters of M NaOH contains mol NaOH?
How many grams of solute would you use to prepare 1.50 L of M glucose, C6H12O6?

20. moles solute in solution 1 = moles solute in solution 2
Dilution
often, solutions are stored as concentrated stock solutions
to make solutions of lower concentrations from these stock solutions, more solvent is added
the amount of solute doesn’t change, just the volume of solution
moles solute in solution 1 = moles solute in solution 2
the concentrations and volumes of the stock and new solutions are inversely proportional
M1∙V1 = M2∙V2

21. Example
To what volume should you dilute L of 15.0 M NaOH to make 3.00 M NaOH?
Sulfuric acid is normally purchased at a concentration of 18.0 M. How would you prepare mL of M aqueous H2SO4?

22. Solution Stoichiometry
Chapter 3: Formulas, Equations, and Moles
Solution Stoichiometry
11/14/2018
In aqueous reactions, quantities of reactants and products are often specified in terms of volume and concentrations.
We can use the stoichiometric coefficients in the chemical equation to convert to the amount of another reactants of products in moles.
aA + bB
cC + dD
Volume of
Solution of A
Moles of A
Moles of B
Volume of
Solution of B
These problems still come back to moles.
Molarity of A
Mole Ratio Between A and B (Coefficients)
Molarity
of B
Dang
Copyright © 2008 Pearson Prentice Hall, Inc.

23. Example
What volume of M KCl is required to completely react with L of M Pb(NO3)2 in the reaction
2 KCl(aq) + Pb(NO3)2(aq)  PbCl2(s) + 2 KNO3(aq)

24. Examples
What is the minimum amount (L) of 6.0 M H2SO4 necessary to produce 25.0 g of H2 . Assumed H2SO4 is limited
2 Al(s) H2SO4(aq)  Al2(SO4)3(aq) H2(g)

25. Titration The base solution is the titrant in the burette.
As the base is added to
the acid, the H+ reacts with
the OH– to form water.
But there is still excess
acid present so the color
does not change.
At the titration’s endpoint,
just enough base has been
added to neutralize all the
acid. At this point the
indicator changes color.

26. Example
The titration of mL of HCl solution of unknown concentration requires mL of M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?

27. Example
The titration of a 20.0 mL sample of an H2SO4 solution of an unknown concentration requires mL of a 0.158M KOH solution to reach the equivalent point. What is the concentration of the unknown H2SO4 solution?