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Chemical Reactions
Section 9.1 Reactions and Equations
Section 9.2 Classifying Chemical Reactions
Section 9.3 Reactions in Aqueous Solutions
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2. Section 9.1 Reactions and Equations
Recognize evidence of chemical change.
chemical change: a process involving one or more substances changing into a new substance
Represent chemical reactions with equations.
Balance chemical equations.
chemical reaction
reactant
product
chemical equation
coefficient
Chemical reactions are represented by balanced chemical equations.
Section 9-1

3. Evidence of a chemical reaction Change in temperature (hot or cold)
Chemical Reactions
The process by which one or more substances are rearranged to form different substances is called a chemical reaction.
Evidence of a chemical reaction
Change in temperature (hot or cold)
Change in color (but not from dilution or mixing of colored solutions)
Odor, gas, or bubbles may form.
Formation of a precipitate (solid from two liquids)
Section 9-1

4. Representing Chemical Reactions
Chemists use statements called equations to represent chemical reactions.
Reactants are the starting substances.
Products are the substances formed in the reaction.
Reactants  Products
Section 9-1

5. This table summarizes the symbols used in chemical equations.

6. Representing Chemical Reactions (cont.)
In word equations, aluminum(s) + bromine(l) → aluminum bromide(s) reads as:
“aluminum and bromine react to produce aluminum bromide”.
Skeleton equations use symbols and formulas to represent the reactants and products. Al(s) + Br(l) → AlBr3(s)
Skeleton equations lack information about how many atoms are involved in the reaction.

7. Representing Chemical Reactions (cont.)
A chemical equation is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction.

8. Balancing Chemical Equations (cont.)
The most fundamental law in chemistry is the law of conservation of mass.
Balanced equations show this law.
A coefficient in a chemical equation is the number written in front of a reactant or product, describing the lowest whole-number ratio of the amounts of all the reactants and products.

9. Describing Equations Describing Coefficients: individual atom = “atom”
covalent substance = “molecule”
ionic substance = “unit”
3CO2 
2Mg 
4MgO 
3 molecules of carbon dioxide
2 atoms of magnesium
4 units of magnesium oxide

10. Balancing Chemical Equations
This figure shows the balanced equation for the reaction between aluminum and bromine.

11. Balancing Steps Write the unbalanced equation.
Count atoms on reactant side.
Count atoms on product side.
Add coefficients to make #s equal.
Coefficient  subscript = # of atoms
Reduce coefficients to lowest whole # ratio, if necessary.
Double check your work!!!

12. Helpful Tips Balance one element at a time.
Update ALL atom counts after adding a coefficient.
If an element appears more than once per side, balance it last.
Balance polyatomic ions as single units.
“1 SO4” instead of “1 S” and “4 O”

13. Balancing Example 2 Al + CuCl2  Cu + AlCl3 Al Cu Cl 3 3 2 2  1 1 2 3
Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2
Al CuCl2  Cu AlCl3 Al Cu Cl 3 3 2
2 
 2
 6
3 
6 
 3

14. Balancing Chemical Equations (cont.)
Section 9-1

15. Balance the following:

16. A B C D Section 9.1 Assessment
Which of the following is NOT a chemical reaction?
A. a piece of wood burning
B. a car rusting
C. an ice cube melting into water
D. red litmus paper turning blue A B C D
Section 9-1

17. A B C D Section 9.1 Assessment
What is the coefficient of bromine in the equation 2Al(s) + 3Br2(l) → 2AlBr3(s)?
A. 1
B. 2
C. 3
D. 6 A B C D
Section 9-1

18. End of Section 9-1

19. Section 9.2 Classifying Chemical Reactions
Classify chemical reactions.
metal: an element that is a solid at room temperature, a good conductor of heat and electricity, and is generally shiny
Identify the characteristics of different classes of chemical reactions.
Section 9-2

20. Section 9.2 Classifying Chemical Reactions (cont.)
synthesis reaction
combustion reaction
decomposition reaction
single-replacement reaction
double-replacement reaction
precipitate
There are five types of chemical reactions: synthesis, combustion, decomposition, single and double replacement reactions.
Section 9-2

21. Types of Chemical Reactions
Chemists classify reactions in order to organize the many types.
A synthesis reaction is a reaction in which two or more substances react to produce a single product.
Section 9-2

22. Types of Chemical Reactions (cont.)
In a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light.
Heated hydrogen reacts with oxygen to produce heat and water in a combustion reaction. This is also a synthesis reaction.
Section 9-2

23. A + O2 AO Types of Chemical Reactions (cont.)
The burning of any substance in O2 to produce heat
Reactants  Products
Metal + Oxygen = Oxide of the metal
Nonmetal + Oxygen = Oxide of the nonmetal
Compound + Oxygen = Two or more oxides
A + O2 AO

24. AB  A + B Decomposition Reactions
A decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds.
Decomposition reactions often require an energy source, such as heat, light, or electricity, to occur.
AB  A + B
Section 9-2

25. Replacement Reactions
A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single replacement reaction.
Metals replace metals (and Hydrogen)
Nonmetals replace nonmetals (usually group 17 elements)
A + BX → AX + B
Section 9-2

26. Replacement Reactions (cont.)
A metal will not always replace a metal in a compound dissolved in water because of differing reactivities.
An activity series can be used to predict if reactions will occur.
Section 9-2

27. Replacement Reactions (cont.)
Double replacement reactions occur when ions exchange between two compounds.
Section 9-2

28. Replacement Reactions (cont.)
The solid product produced during a chemical reaction in a solution is called a precipitate.
All double replacement reactions produce either water, a precipitate, or a gas.
Section 9-2

29. Replacement Reactions (cont.)
Section 9-2

30. Decomposition = AB  A + B Single-Replace = A + BX  AX + B
Replacement Reactions (cont.)
Synthesis = A + B  AB
Combustion = A + O2  AO
Decomposition = AB  A + B
Single-Replace = A + BX  AX + B
Double-Replace = AX + BY  AY + BX
Section 9-2

31. A B C D Section 9.2 Assessment
Which of the following is NOT one of the four types of reactions?
A. deconstructive
B. synthesis
C. single replacement
D. double replacement A B C D
Section 9-2

32. A B C D Section 9.2 Assessment
The following equation is what type of reaction?
KCN(aq) + HBr(aq) → KBr(aq) + HCN(g)
A. deconstructive
B. synthesis
C. single replacement
D. double replacement A B C D
Section 9-2