1. Comprehensive Science Mr. Knighton
Chemical Reactions
Comprehensive Science
Mr. Knighton

2. Chemical Reactions are all around us!
Examples:
Growth, cooking, decaying, burning,……
Food + O energy + CO2 + H2O
This is a Chemical Reaction!! It is what takes place in your body when you eat!

3. Another Example Cooking: Yeast + Sugar + Flour Bread (CO2 + acid)
Chemical Reaction: Substances undergo a chemical change to form new substances.

4. Signs of a Chemical Reaction
Production of gas
Ex) CO2 bubbles are produced to make the bread rise.
Color Change
Ex) The White dough changes color to a golden brown after cooking.

5. Chemical Reactions Rearrange Atoms
Example) Photosynthesis!
The Chemical Equation:
CO2 + H2O C6H12O6 + O2

6. Parts of a Chemical Equation
CO2 + H2O C6H12O6 + O2
To the Left of the Arrow are the REACTANTS (what goes in)
The Arrow is called a YIELD SIGN
To the Right of the yield sign are the PRODUCTS (what comes out)

7. Rearranging the Atoms CO2 + H2O C6H12O6 + O2
Notice that no new atoms appear when the products are made. The atoms just rearrange themselves to form new compounds!

8. How does this happen?
Energy must be added to break the bonds of the reactants.
When new bonds form the products, energy is then released.

9. 2 Major Types of Reactions
Exothermic: more energy is released then was absorbed. This produces HEAT!
Endothermic: More energy is abosrbed the released. Results in COLD

10. Classifying Chemical Reactions
There are 5 general types of chemical reactions.
Synthesis
Decomposition
Combustion
Single Displacement
Double Displacement

11. Synthesis Reaction
A reaction of at least 2 substances that forms a new, more complex compound.
(Synthesis means to make)
Example: Hydrogen + oxygen water
General Form: A+ B AB
2Na + Cl NaCl

12. 2. Decomposition Reaction
A reaction in which one compound breaks into at least 2 products.
Example: digestion (breaking down your food molecules)
General Form: AB A + B
Ex.) 2H2O H2 + O2
The Decomposition of Water

13. 3. Combustion A reaction in which a compound and oxygen burn.
Oxygen is a reactant in every combustion reaction.
Example) the burning of methane
2CH4 + 4O CO2 + 4H2O

14. 4. Single-Displacement Reaction
A reaction in which atoms of one element take the place of atoms of another element in a compound.
General Form: XA + B AB + X
Example: CuCl2 + Al AlCl3 + Cu

15. 5. Double-displacement reaction
A reaction where a compound is formed from the exchange of ions between 2 compounds.
General form: AX + BY AY + BX
Example:
Pb(NO3)2 + K2CrO PbCrO4 + 2KNO3

16. Balancing Equations

17. Review…. What is a chemical Equation?
The use of chemical formulas and symbols to show the reactants and products in a chemical reaction.
Write an equation and label the reactants, products, and yield sign.

18. Why do we need to balance equations?
Equations must be balanced to account for the Conservation of Mass!

19. For Example….. CH4 + O2 CO2 + H2O This equation is not balanced.
It appears that we have lost Hydrogens and gained Oxygens…..does this agree with the Law of Conservation of Energy?

20. How to Balance Equations
CH4 + O CO2 + H2O
Count the # of atoms on each side of the yield sign.
C = 1 C = 1
H = 4 H = 2
O = 2 O = 3

21. You can NOT change the subscripts!
Now what?
You can NOT change the subscripts!
2. Place Coefficients to balance the equation.
Coefficient = a # in front of atoms or compounds.

22. Using Coefficients CH4 + O2 CO2 + H2O
There are 4 Hydrogens on the left side, how many more do you need to get 4 on the right side?
Answer = 2

23. Using Coefficients Place a 2 in front of H2O. CH4 + O2 CO2 + 2H2O
3. Now Re-count atoms for both sides!
C = 1 C = 1
H = 4 H = 4
O = 2 O = 4

24. How to Balance the Oxygen
We need to add another Coefficient.
CH4 + O CO2 + 2H2O
Oxygen needs 2 more on the left side.
Place a 2 in front of the O2.

25. 4. Re-count atoms for both sides.
CH4 + 2O CO2 + 2H2O
C = C = 1
H = H = 4
O = O = 4
All is balanced!

26. Try this! Mg + O2 MgO KI + Br2 KBr + I2
3. What types of reactions are these?
4. P.168 # 1, 2a. , 3