1. Chapter 6
Chemical Reactions

2. Section 1: The Nature of Chemical Reactions
Chemical reactions occur when substances undergo chemical changes to form NEW substances.
Examples: Grow, ripen, decay , burn, rust, change color, produce a gas, digest
Atoms are rearranged as bonds are broken and new ones are formed.

3. Reaction Format:
H2O > H2 and O2
Reactants Products

4. Energy and Reactions Endothermic reactions:
When more energy is required to break bonds in the reactants than is given off by forming bonds in the products
Example: photosynthesis
Products might feel colder

5. Exothermic reactions:
More energy is released as the products form than is absorbed to break bonds in the reactants
Examples: zinc reacting with HCl, reaction of isooctane and oxygen
Temperature in the surroundings rises
Quiz

6. Section 2: Reaction Types
Synthesis reaction: substances join to form a more complex compound than the reactants
A + B > AB
2Na + Cl2 ---> 2NaCl
2H2 + O2 ---> 2H2O

7. Decomposition Reaction
More complex substances are broken apart to form simpler products
AB ---> A + B
H2O ---> H2 + O2

8. Combustion Oxygen is always a reactant, water is also a common product
The oxidation reaction of an organic compound releasing heat
CH4 + O2 ---> CO2 + H2O

9. Single-Replacement Reactions
One element takes the place of another element in a compound
AX + B ---> A + BX
3CuCl2 + 2Al ---> 2AlCl3 + 3Cu

10. Double-Replacement Reaction
A compound forms from the apparent exchange of atoms between 2 compounds
AX + BY  BX + AY
Pb(NO3)2 + K2CrO4  PbCrO4 + KNO3

11. REDOX reaction
Oxidation-reduction reaction: a chemical change in which a substance is oxidized (loses electrons) and another is reduced (gains electrons):
Fe + O2  Fe2O3
Other examples: respiration and combustion
Review
Drawings

12. Diatomic Elements
These elements, when alone, exist diatomically (2 atoms – share electrons):
H2, N2, O2, Cl2, Br2, I2, F2

13. Section 3: Balancing Chemical Equations
Law of Conservation of Mass: the number of atoms on the left side must equal the number of atoms on the right.
Rules for balancing:
ONLY coefficients (in front of the formula) can be added to either or both sides of the equation.
NO chemical formulas can be changed (subscripts cannot be changed)
Hints:
Save solo elements for last.
If there is a certain polyatomic ion on both sides, don’t count the atoms within the ion separately.
Know diatomic elements: H2, N2, O2, F2, Cl2, Br2, I2 .

14. Try some balancing:
Quiz
More

15. Section 4: Rate of reactions
Chemical reactions can be increased by:
Increasing temperature
Increasing the concentration of one substance in an equilibrium reaction favors the reaction that produces less of that substance: Le Chatelier’s principle
Adding energy
Increasing pressure
Catalysts
Increasing surface area

16. Equilibrium
Chemical equilibrium is a state of balance in which the rate of reaction is equal in both directions and the concentration of products and reactants remains the same
Different factors can affect equilibrium:
Increasing temperature favors a reaction that absorbs energy
Increasing pressure favors a reaction that reduces the production of gas
Increasing concentration favors a reaction that produces less of that substance