1. Chapter 3 Mass Relationships in Chemical Reactions
Chemistry: McMurry and Fay, 6th Edition
Chapter 3: Mass Relationships in Chemical Reactions
2/21/2019 7:15:56 PM
Chapter 3 Mass Relationships in Chemical Reactions
Copyright © 2011 Pearson Prentice Hall, Inc.

2. Chemical Reaction In a chemical reaction
a chemical change produces one or more new substances.
there is a change in the composition of one or more substances.
old bonds are broken and new bonds are formed.
atoms in the reactants are rearranged to form one or more different substances.

3. Chemical Equations A chemical equation gives
the formulas of the reactants on the left of the arrow.
the formulas of the products on the right of the arrow.
Reactants Product
O2 (g)
CO2 (g)
C(s)

4. Symbols Used in Equations
Symbols in chemical
equations show
the states of the reactants.
the states of the products.
the reaction conditions.
TABLE

5. Representing Chemistry on Different Levels
2H2O(l)
2H2(g) + O2(g)
2 molecules of hydrogen gas react with 1 molecule of oxygen gas to yield 2 molecules of liquid water.
Microscopic:
Macroscopic:
2 moles of hydrogen gas react with 1 mole of oxygen gas to yield 2 moles of liquid water.

6. Chemical Equations are Balanced
In a balanced
chemical reaction
no atoms are lost or gained.
the number of reacting atoms is equal to the number of product atoms.

7. Balancing Chemical Equations
Chapter 3: Formulas, Equations, and Moles
2/21/2019
Balancing Chemical Equations
A balanced chemical equation shows that the law of conservation of mass is adhered to.
In a balanced chemical equation, the numbers and kinds of atoms on both sides of the reaction arrow are identical.
Remember, a chemical reaction can be thought of as a rearrangement of the atoms to form new compounds.
Copyright © 2008 Pearson Prentice Hall, Inc.

8. Balancing Chemical Equations
Chapter 3: Formulas, Equations, and Moles
2/21/2019
Balancing Chemical Equations
Write the unbalanced equation using the correct chemical formula for each reactant and product.
H2O(l)
H2(g) + O2(g)
Find suitable coefficients—the numbers placed before formulas to indicate how many formula units of each substance are required to balance the equation.
2H2O(l)
2H2(g) + O2(g)
Copyright © 2008 Pearson Prentice Hall, Inc.

9. Balancing Chemical Equations
Chapter 3: Formulas, Equations, and Moles
Balancing Chemical Equations
2/21/2019
Reduce the coefficients to their smallest whole-number values, if necessary, by dividing them all by a common denominator.
4H2O(l)
4H2(g) + 2O2(g)
divide all by 2
2H2O(l)
2H2(g) + O2(g)
Check your answer by making sure that the numbers and kinds of atoms are the same on both sides of the equation.
Since balancing at this level is often trial-and-error, the coefficients need to be checked to make sure they are the smallest whole-number ones. An alternative is to use fractions but some students do not do well with fractional coefficients at this stage.
2H2O(l)
2H2(g) + O2(g)
left side:
4 H
2 O
right side:
4 H
2 O
Copyright © 2008 Pearson Prentice Hall, Inc.

10. Balancing Chemical Equations
Chapter 3: Formulas, Equations, and Moles
2/21/2019
Balancing Chemical Equations
Do not change subscripts when you balance a chemical equation. You are only allowed to change the coefficients.
H2O(l)
H2(g) + O2(g)
unbalanced
2H2O(l)
2H2(g) + O2(g)
H2O2(l)
H2(g) + O2(g)
Balancing chemical equations only determines the proper numerical ratios between all of the substances (both reactants and products). It does not change any of the substances.
Balanced properly
Chemical equation changed!
Copyright © 2008 Pearson Prentice Hall, Inc.

11. Examples Balance the coefficients from reactants to products.
__N2(g) __H2(g) __ NH3(g)
B. __Co2O3(s) + __ C(s) __Co(s) + __CO2(g)
Write a balanced equation for the reaction between carbon dioxide and potassium hydroxide to form potassium carbonate and water.

12. Chemical Symbols on Different Levels
Chapter 3: Formulas, Equations, and Moles
2/21/2019
Chemical Symbols on Different Levels
2H2O(l)
2H2(g) + O2(g)
2 molecules of hydrogen gas react with 1 molecule of oxygen gas to yield 2 molecules of liquid water.
microscopic:
0.56 kg of hydrogen gas react with 4.44 kg of oxygen gas to yield 5.00 kg of liquid water.
macroscopic:
How can we relate the two to each other?
The mole relates microscopic to macroscopic.
Copyright © 2008 Pearson Prentice Hall, Inc.

13. Formula Mass versus Molar mass
The average mass of a molecule or formula unit in amu
also known as molecular mass or molecular weight (MW)
whole = sum of the parts!
Molar mass
Total mass of a compound in gram per 1 mol of its molecules or formula unit

14. Molar Mass of Compounds
the relative masses of molecules can be calculated from atomic masses
Formula Mass = 1 molecule of H2O
= 2(1.01 amu H) amu O = amu
since 1 mole of H2O contains 2 moles of H and 1 mole of O
Molar Mass = 1 mole H2O
= 2(1.01 g H) g O = g
so the Molar Mass of H2O is g/mole
Why multiplying 2?

15. Examples Calculate the molar mass of HCl C2H4 Element # moles
Atomic mass
Total mass
Element
# moles
Atomic mass
Total mass

16. Molar Mass of Na2SO4 Calculate the molar mass of Na2SO4. Element
Number of Moles
Atomic Mass
Total Mass in each element Na S O
Total mass in 1 mol Na2SO4

17. Stoichiometry: Chemical Arithmetic
Chapter 3: Formulas, Equations, and Moles
2/21/2019
Stoichiometry: Chemical Arithmetic
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
Molar Mass of A
Molar Mass of B
Mole Ratio Between A and B (Coefficients)
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.
Copyright © 2008 Pearson Prentice Hall, Inc.

18. Stoichiometry: Chemical Arithmetic
Chapter 3: Formulas, Equations, and Moles
2/21/2019
Stoichiometry: Chemical Arithmetic
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
Molar Mass
Mole Ratio
Molar Mass
Copyright © 2008 Pearson Prentice Hall, Inc.

19. Stoichiometry: Chemical Arithmetic
Chapter 3: Formulas, Equations, and Moles
Stoichiometry: Chemical Arithmetic
2/21/2019
The commercial production of iron from iron ore involves the reaction of Fe2O3 with CO to yield iron metal plus carbon dioxide:
Fe2O3(s) + 3CO(g) Fe(s) + 3CO2(g)
Predict how many grams of CO will react with moles Fe2O3?
Remind students about significant figures.
Copyright © 2008 Pearson Prentice Hall, Inc.

20. Yields of Chemical Reactions
Chapter 3: Formulas, Equations, and Moles
2/21/2019
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.

21. Reactions with Limiting Amounts of Reactants
Chapter 3: Formulas, Equations, and Moles
2/21/2019
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.

22. Reactions with Limiting Amounts of Reactants
Chapter 3: Formulas, Equations, and Moles
2/21/2019
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.

23. 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

24. Reactions with Limiting Amounts of Reactants
Chapter 3: Formulas, Equations, and Moles
2/21/2019
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.

25. 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)