1. Chemical Equations & Reactions
Chapter 8

2. Chemical Reactions
A chemical reaction is a process by which one or more substances is changed into one more different substances
Reactants: original substances
Products: resulting substances

3. Indications of a chemical reaction
Evolution of heat & light
Production of a gas
Formation of a precipitate
Precipitate: a solid that is produced as a result of a chemical reaction

4. Chemical equation Describes a chemical reaction
Represents with symbols & formulas the identities & relative amounts of the reactants & products

5. Characteristics of a chemical equation
Must represent known facts
Must contain the correct formulas
Law of conservation of mass must be satisfied
The same number of atoms of each element must appear on each side
To equalize number of atoms, coefficients are added
Placing a coefficient in front of a formula specifies the number of moles of a substance

6. Equations
Word equation: an equation in which the reactants & products in a chemical reaction are represented by words
Only qualitative meaning
Formula equation: represents the reactants & products by their symbols or formulas
Must be balanced!

7. Problem Write word & formula equations for the chemical reaction that occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide.

8. Significance of a chemical equation
Coefficients indicate relative, not absolute, amounts of reactants & products as moles
The relative masses of the reactants & products of a chemical reaction can be determined from the reactions coefficients.
The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.

9. Cont’d…
An equation gives no indication of whether a reaction will actually occur.
An equation can be written for a reaction that may not even occur.
An equation gives no info about the speed of the reaction.

10. Balancing equations Write a word equation. Write a formula equation.
Balance the formula equation
Balance atoms that appear only once on each side.
Balance polyatomic ions that appear on both sides as single units.
Balance the different types of atoms one at a time.
Balance H & O atoms after all other atoms are balanced.
Count atoms to make sure the equation is balanced.

11. Common mistakes Writing incorrect formulas
Trying to balance an equation by changing the subscripts

12. Problem The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride & hydrogen gas. Write a balanced chemical equation for the reaction.

13. Balance the equations:
_Al + _Fe2O3  _Al2O3 + _Fe
_Zn(NO3)2  _ZnO + _NO2 + _O2
_KClO3  _KCl + _O2
_H3PO4  _H2O + _P4O10
_C4H10 + _O2  _CO2 + _H2O
_Cu + _AgNO3  _Cu(NO3)2 + _Ag
_N2 + _H2  _NH3

14. Types of chemical reactions

15. (1) Synthesis Two or more substances combine to form a new compound
A + X  AX
Na + Cl NaCl

16. (2) Decomposition
A single compound breaks down into two or more simpler substances
AX  A + X
NaCl Na + Cl

17. (3) Single-replacement
One element replaces a similar charged element in a compound
A + BX  AX + B
Na + KCl  K + NaCl

18. (4) Double-replacement
The ions of two compounds exchange places in an aqueous solution to form two new compounds
AX + BY  AY + BX
NaCl + KBr  NaBr + KCl

19. (5) Combustion
A substance combines with oxygen, releasing a large amount of energy in the form of light & heat.
A compound reacts with oxygen to form carbon dioxide & water
C2H8 + O2  CO2 + H2O

20. Identify the type of reaction
C3H6O + 4O2  3CO2 + 3H2O
2KClO3  2KCl + 3O2
2CaO + MnI4  MnO2 + 2CaI2
O2 + 2H2  2H2O
5. LiNO3 + Ag  AgNO3 + Li