1. Chemical Reactions Chapter 11
General Chemistry

2. Objectives
Balance chemical equations by applying the law of conservation of mass.
Recognize synthesis, decomposition, single displacement, double displacement, and neutralization reactions.
Write chemical equations from words.
Predict the products of chemical reactions.
Write net ionic equations for reactions.

3. Chemical Reactions Basics
Environmental Chemistry unit 2 Sect C

4. Chemical Reactions A chemical reactions are chemical changes.
Recall: chemical changes are when one or more substances are changed into different substances
Examples: wood burning, metal rusting

5. Indications of a Chemical Reaction
Certain easily observed changes indicate that a chemical reaction has occurred:
evolution of energy by heat and/or light
color change
production of gas
production of precipitate
A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is known as a precipitate
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6. Law of Conservation of Matter
Chemical reactions obey the Law of Conservation of Matter
in all chemical and physical changes, matter is neither created or destroyed
The total mass in a chemical reaction remains constant
Environmental Chemistry unit 2 Sect C

7. Chemical Equations
A chemical equation represents, with symbols and formulas, the identities and the amounts of the reactants and products in a chemical reaction.
Has two parts:
Reactants: the substances you start with
Products: the substances you end up with
The reactants turn into the products
4 Fe O2  2 Fe2O3
Reactants  products
Environmental Chemistry unit 2 Sect C

8. Ways to Express a Chemical Reaction
The way atoms are joined is changed in a chemical reaction.
Can be described several ways:
1. In a sentence
Copper reacts with chlorine to form copper (II) chloride.
2. In a word equation
Copper + chlorine ® copper (II) chloride
3. In formula equation:
Cu + Cl2  CuCl2
Environmental Chemistry unit 2 Sect C

9. Symbols Used in Chemical Equations

10. Additional Symbols Used in Equations
shows that heat is supplied to the reaction
is used to indicate a catalyst is supplied, in this case, platinum.
Environmental Chemistry unit 2 Sect C

11. What is a catalyst?
A catalyst is a substance that speeds up a reaction, without being changed or used up by the reaction.
Enzymes are biological or protein catalysts.
Environmental Chemistry unit 2 Sect C

12. Study Buddy Review
What are four things that indicate a chemical reaction has taken place?
What is the Law of Conservation of mass?
What does the symbol (l) mean?
What does (aq) mean?
Environmental Chemistry unit 2 Sect C

13. Balancing Chemical Equations
Environmental Chemistry unit 2 Sect C

14. Skeleton and Balanced Equations
Skeleton equations: do not indicate how many of each element/compound
Balanced equations: the number of atoms of each element is the same on both sides of the reaction.
Environmental Chemistry unit 2 Sect C

15. Numbers in Balanced Equations
Use coefficients to balance equation
Coefficients: large whole numbers placed in front of formula which represents number of units of that compound
Subscript: small whole number placed in chemical formula to represent number of atoms of an element in a compound
Environmental Chemistry unit 2 Sect C

16. 4 Fe + 3 O2  2 Fe2O3 subscript Coefficient
Environmental Chemistry unit 2 Sect C

17. Balanced Equation
A balanced equation has the same number of atoms of each element on both sides of the equation
Atoms can’t be created or destroyed
All the atoms at the beginning must appear in the end
Environmental Chemistry unit 2 Sect C

18. ® O + C C O O C + O2 ® CO We need one more oxygen in the products.
Can’t change the formula, because it describes what it is (carbon monoxide in this example)
Environmental Chemistry unit 2 Sect C

19. ® Must be used to make another CO But where did the other C come from?+ C O C O
Must be used to make another CO
But where did the other C come from?
Environmental Chemistry unit 2 Sect C

20. ® Must have started with two C 2 C + O2 ® 2 CO C C O O + O C C O
Environmental Chemistry unit 2 Sect C

21. Rules for balancing:
Assemble, write the correct formulas for all the reactants and products
Count the number of atoms of each type of element appearing on both sides
Balance the atoms of an element one at a time by adding coefficients (the numbers in front) - save H and O until LAST!
Check to make sure it is balanced.
Environmental Chemistry unit 2 Sect C

22. Never Never change a subscript to balance an equation.
If you change the formula you are describing a different reaction.
H2O is a different compound than H2O2
Never put a coefficient in the middle of a formula
2 NaCl is okay, Na2Cl is not.
Environmental Chemistry unit 2 Sect C

23. Balancing Equations Examples
H2 (g) + O2 (g)  H2O (l)
Zn + HCl  H ZnCl2
Pb (NO3) K2S  PbS + KNO3
Environmental Chemistry unit 2 Sect C

24. Balancing Equations Answers
2 H2 (g) + O2 (g)  2 H2O (l)
Zn HCl  H ZnCl2
Pb (NO3)2 + K2S  2PbS + 2KNO3
Environmental Chemistry unit 2 Sect C

25. Word Equation Examples
Write a balanced chemical equation for the following:
Aqueous solutions of sulfuric acid and sodium hydroxide react to form aqueous sodium sulfate and water.

26. Five General Types of Chemical Reactions

27. Five General Types of Chemical Reactions
Synthesis (Composition)
Decomposition
Single-Replacement
Double-Replacement
Combustion
By knowing the type of reaction that is occurring, you can predict the products that will be formed.

28. General Types of Chemical Reactions
Combination: A + B  AB
Decomposition: AB  A + B
Single Replacement :
AX +B  A + BX
Double Replacement:
AX + BY  AY + BX
Combustion:
CxHy + O2  CO2 + H2O

29. Synthesis (Combination)
General form: A + B  AB
(two reactants make a single product)
A, B = elements or compounds
AB = compound consisting of A and B
This is the only type of chemical reaction in which there is a single product formed. This single product is always more complex than the reactants.

30. Examples of Synthesis Reactions
a. calcium + oxygen yields calcium oxide
2Ca + O2  2CaO
b. carbon dioxide + water yields carbonic acid
CO H2O  H2CO3
Notice: All equations show two (or more) reactants, but only one product.

31. Decomposition Reactions
(Decomposing = breaking down into smaller parts; microorganisms)
A reaction in which a single compound is broken into 2 or more smaller compounds or elements. The opposite of direct combination/synthesis.
You can identify this reaction because there is only one reactant.
General form: AB  A + B
AB = compound
A, B = elements or simpler compounds

32. Examples of Decomposition Reactions
Water yields hydrogen and oxygen
2H2O  H O2
marble (rock) yields (with heating) calcium oxide and carbon dioxide
CaCO  CaO CO2
Notice: single compound decomposes into two (or more) products.

33. Single-Replacement Reactions
One element and one compound recombine (switch partners)
General Form: A + BX  AX + B
AX, BX = ionic compounds
A, B = elements
X = ion that switches partners

34. Examples of Single-Replacement Reactions
Magnesium metal and copper (II) sulfate
Mg + CuSO4  MgSO4 + Cu
Iron metal and copper (II) sulfate)
Fe + CuSO4  FeSO4 + Cu

35. Double-Replacement Reactions
Positive ions in two compounds are exchanged.
Look for: 2 compounds as reactants and 2 compounds as products.
Reactants are usually ionic compounds.
Specific example: neutralization reactions between acid and base
General form: AX + BY  AY + BX
(Notice: X and Y “change partners”)

36. Examples of Double Replacement Reactions
Calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid.
CaCO3 + 2HCl  CaCl2 + H2CO3

37. Rules of Double-Displacement Reactions
Reactants must be dissolved in water (releasing the ions).
Will occur if:
One of the products is a molecule (covalent),
A precipitate (solid comes out of solution), or
An insoluble gas.

38. C2H5OH (gasohol) + 3O2  3H2O + 2CO2
Combustion Reactions
Reactants: a fuel (usually a hydrocarbon) and oxygen.
Products: water, carbon dioxide, and lots of energy! (heat, light)
Examples:
CH4 (methane) + 2O2  2H2O + CO2
C2H5OH (gasohol) + 3O2  3H2O + 2CO2

39. Study Buddy Review Describe each of the following to your study buddy:
Synthesis (combination)
Decomposition
Single-Replacement
Double-Replacement
Combustion

40. Activity Series

41. Activity Series and Single Replacement Reactions
Al + CuCl2  ???
Cu + AlCl3  ???
Only one of the following reactions will occur
How do you know which one?

42. Activity Series of Metals

44. Activity Series Rules Elements at top of series are most active
Elements at bottom of series are least active (coinage metals)
Elements at the top of the series will replace elements below it in a chemical reaction.
Example: Al + CuCl2  will occur
Cu + AlCl3  will NOT occur

45. Activity Series Examples
Using the activity series, predict whether each of the possible reactions will occur:
Cr + H2O 
Pt + O2 
Cd + 2HBr 

46. Activity Series Examples
Using the activity series, predict whether each of the possible reactions will occur:
Cr + H2O  NR
Pt + O2  NR
Cd + 2HBr  CdBr2 + H2

47. References
Dr. Stephen L. Cotton Charles Page High School
Mrs. Lijek