1. Unit 6: Chemical Reactions
Chapter 9

2. Unit 6: Chemical Reactions
The Nature of Chemical Reactions (9-1)
A. Definition: process in which 1 or more substances are converted to new substances with different chemical and physical properties
B. Common Examples:
C. Components
1. Reactant: substance that enters a chemical reaction
2. Product: substance that is produced from a chemical reaction

3. D. Reason for Reactions
* Depends on: valence electrons & energy
II. Chemical Equations (9-2)
A. Definition: sentences that represent reactions
B. Indicates: what substances are involved in the reaction & how many

4. C. Types of Equations
1. Word Equations
a. Name chemicals involved in the reaction
b. Only a qualitative description
Ex: hydrogen + oxygen  water
(reactants) (yields) (product)
2. Formula Equations
a. Gives the formulas of the chemicals involved and the ratios

5. b. Gives a quantitative description
Ex: H2 + O2  H2O
(reactants) (yields) (product)
** This is not balanced, it must be! Why?
** Law of Conservation of Mass/Atoms: in ordinary chemical reactions:
1.) Total mass of reactants is equal to total mass of products

6. 2.) Number of elements for reactants is equal to the number of elements for products
** Atoms are not created nor destroyed, but they simply rearrange.
** Coefficients: numbers in front of the formulas in chemical equations that give the ratios of the substances involved in the reaction

7. 2 molecules 1 molecule 2 molecules
Ex: 2H O > H2O
2 molecules molecule 2 molecules or
2 moles mole 2 moles
Ex: Interpret the following chemical reaction
2 NaI + Cl2 ---> 2NaCl + I2
** Note: What is the difference between 2CO and CO2

8. D. Writing and Balancing Chemical Equations
1. Review: Indicate how many atoms of each element are present in the following:
a. 5NH3 --->
b. 4Ca(OH)2 --->
c. 3BaSO >

9. 2. Review: Diatomics (H2,O2,N2,Cl2,F2,Br2,I2)
*Exist as a molecule of 2 atoms when they are by themselves (natural/elemental form)
3. Steps to writing chemical equations
a. Write the correct chemical formulas for compounds involved in the reaction
*All compounds should have a neutral charge- subscripts are used to balance the charge on ionic compounds

10. b. Balance equation according to the law of
b. Balance equation according to the law of conservation of atoms – coefficients are used
c. Indicate the phase of each compound
(s) ---> solid
(l) ---> liquid
(g) ---> gas
(aq) ---> solid salt dissolved in water
d. Show the energy change in the equation
Endothermic ---> Energy written on left side
Exothermic ---> Energy written on right side

11. III. Classifying Chemical Reactions (9-3)
1. Synthesis (S), Composition, or Direct Combination – two or more substances combine to form a more complex substance
General Form: A + B ---> AB
Examples: 2H2 + O2 ---> 2H2O
4Fe + 3O2 ---> 2Fe2O3 (rusting)
2CO + O2 ---> 2CO2

12. 2. Decomposition (D) – a complex substance breaks down to form two or more simple substances
General Form: AB ---> A + B
Examples: 2H2O ---> 2H O2
2KClO3 ---> 2KCl + 3O2
2HgO ---> 2Hg + O2

13. 3. Single Replacement (SR) – one element is replaced in a compound by another element
General Form: A + BX ---> AX + B or
Y + BX ---> BY + X
Examples: Fe + CuSO4 ---> FeSO4 + Cu
Cl KI ---> 2KCl + I2
Cu + AgNO3 ---> CuNO3 + Ag

14. 4. Double Replacement (DR) or Ionic – ionic compounds swap partners/pairings
General Form: AB + XY ---> AY + XB
Examples: NaCl + AgNO3 ---> NaNO3 + AgCl
Ba(OH)2 + NaBr --> BaBr2 + 2NaOH
(NH4)2CO3 + CaCl2 --> 2NH4Cl + CaCO3