1. Chapter 8 Chemical Equations and Reactions
8.1
Describing Chemical Reactions

2. Chemical Reaction
Chemical Reaction – the process by which one or more substances change to produce one or more different substances
Evidence of chemical change
Formation of a gas, E change, solid formed, color change in system

3. Chemical Reactions
A new substance results from a chem rxn. to prove a rxn has occurred
Reactants  Products
Energy changes
Collisions must occur
Law of Conservation of Mass

4. Chemical Equation
Chemical Equation – A representation of a chemical reaction that uses symbols to show the relationship between the reactants and products
Word equations: Ethanol + oxygen  carbon dioxide + water

5. Chemical Equations Tells you… Number and type of atoms
State of reactants and products (s,l,g)
If it is dissolved in water (aq)
If heat or catalyst are used

6. Equations C2H6 (g) C2H4 (g) + H2 (g) Physical Catalyst State Products
800 K
C2H6 (g) C2H4 (g) + H2 (g)
Physical Catalyst
State Products
Reactants
Cr2O3

7. Assignment
Do the section review on page 266 for homework. Do only numbers 1-5, This will be due on Monday.

8. Balancing Chemical Equations
8.2

9. Equations _CH3CH2OH + _O2  _CO2 + _H2O This eqn needs balanced
Coefficients – integer/numeral used in a chemical eqn to indicate relative amts of reactants or products.
Never change subscripts

10. Balancing an Equation Write the word eqn
Write the unbalanced formula eqn.
Count the # of each element
Insert coefficients for atoms
Check to make sure both sides are the same.

11. Tips for Balancing Equations
Delay balancing elements (like H and O) that occur in several reactants or products.
If the same polyatomic ions appear on both sides of the equation, treat them as single units, like monatomic ions.

12. Tips for Balancing Equations
3. After considering 1&2, balance elements left to right.
4. Remember, balancing 1 element may unbalance others.
5. For ionic equations, be sure charges are balanced.

13. Practice Balancing
Ammonia plus oxygen yields nitrogen monoxide plus water plus heat.
Ionic equations
Need to make sure ionic charges are balanced
HCl + H2O  Cl- + H3O+