1. Chemical Equations and Reactions
Chapter 8

2. Chemical Reactions
Chemical reaction: process by which one or more substances are changed into one or more different substances
Beginning substances: reactants
Ending substances: products
Chemical equation: represents, with symbols and formulas, the identities and relative molecular or molar amounts of the reactants and products in a chemical reaction

3. Indications of a chemical reaction
Energy released as heat and light
Production of a gas
Formation of a precipitate
Precipitate: a solid that is produced as a result of a chemical reaction in solution and that separates from the solution
Color change

5. Characteristics of Chemical Equations
Equation must represent known facts
Whether something is solid, liquid, gas, etc…
Must contain the correct formulas for the reactants and products
Law of conservation of mass must be satisfied
Number of atoms on one side of the equation must equal number of atoms on the other side of the equation

6. Word vs. Formula Equation
Word: reactants and products are represented by a sentence
Example: Methane plus oxygen yields carbon dioxide and water.
Formula: reactants and products are represented by their symbols or formulas
Example:
CH4 (g) + O2 (g)  CO2 (g) + H2O (g)

7. Symbol
Explanation 
Yields or equals
Indicates a reversible reaction
(s)
Solid state
Alternative to (s), but only used to indicate a precipitate
(l)
Liquid state
(aq)
Reactant or product is dissolved in water
(g)
Gaseous state
Alternative to (g), but only used to indicate a gaseous product Δ
Reactants are heated
Atm 
Pressure at which a reaction is carried out
Pressure 
Pressure at which a reaction is carried out exceeds normal atmospheric pressure
0º C 
Temperature at which a reaction is carried out, in this case 0º C
MnO2
Formula of a catalyst, in this case manganese dioxide, used to alter the rate of the reaction

8. Sample Problem
Write the word and formula equations for the chemical reactio nthat occurs when solid sodium oxide is added to water at room temperature and forms sodium hydroxide (dissolved in the water). Include the symbols for physical states in the formula equation. Then balance the formula equation to give a balanced chemical equation.

9. Significance of a chemical equation
Coefficients of a chemical reaction indicate relative, not absolute, amounts of reactants and products.
Relative masses of the reactants and products of a chemical reaction can be determined from the reactions coefficients.
Reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.

10. Significance of a chemical equation
Samples
H2 (g) + Cl2 (g)  2HCl (g)
1 molecule of hydrogen reacts with 1 molecule of chlorine to produce 2 molecules of hydrogen chloride
1 molecule H2: 1 molecule Cl2: 2 molecules HCl

11. Significance of a chemical equation
2. Samples
H2 (g) + Cl2 (g)  2HCl (g)
1 mole H2 x 2.02 g/mol = 2.02 g H2
1 mole Cl2 x g/mol = g Cl2
2 mol HCl x g/mol = g HCl

12. Balancing a Chemical Equation
You cannot change the subscripts to balance an equation
You can balance them by putting coefficients in front of the molecules
CH4 + 2O2  CO2 + 2H2O
Coefficients are the large numbers in front of the molecules.

13. Steps to balancing a chemical equation
Identify the names of the reactants and products. Write a word equation if necessary.
Write a formula equation.
Balance the formula equation according to the law of conservation of mass.
Count the atoms to make sure that the equation is balanced.

14. Helpful Hints Balance the different types of atoms one at a time
First balance the atoms of elements that are combined and that appear only once on each side of the equation
Balance polyatomic ions that appear on both sides of the chemical equation as single units
Balance H and O atoms last

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

16. Sample Problem
Solid aluminum carbide, Al4C3, reacts with water to produce methane gas and solid aluminum hydroxide. Write a balanced chemical equation for this reaction.

17. Sample Problem
Aluminum sulfate and calcium hydroxide are used in a water-purification process. When added to water, they dissolve and react to produce two insoluble products, aluminum hydroxide and calcium sulfate. These products settle out, taking suspended solid impurities with them. Write a balanced chemical equation for the reaction.

18. Synthesis Reaction
Two or more substances combine to form a new compound
A + B  AB
2Na + Cl2  2NaCl
(B) (AB)
Always join compounds
Final product is always more complex than the reactants

19. Decomposition Reaction
A single compound breaks down into two or more simpler substances
AB  A + B
2H2O  2H2 + O2
(AB) (A) (B)
Above reaction is called electrolysis

20. Combustion Reactions Require oxygen to occur “Explosions”
Release heat and often water
Depends on the amount of oxygen present
A + O2  A + H2O + heat
2CH4 + 3O2  2CO + 4H2O

22. Single-displacement Reaction
Elements trade places
A more reactive element will take the place of a less reactive element
Alkali metals react with water to form ions
AX + B  BX + A
3CuCl2 + 2Al  2AlCl3 + 3Cu

23. Double-displacement reactions
Ions are exchanged between compounds
AX + BY  AY + BX
Pb(NO3)2 + K2CrO4  PbCrO4 + 2KNO3

24. Tips for Predicting Products
What does the first part of the equation look like?
Two single elements? Probably a synthesis
Large complex molecule? Probably decomposition
Molecule with carbon & an oxygen? Probably combustion
Ionic compounds? Single or double displacements

25. Activity Series of the Elements
Activity series: a list of elements organized according to the ease with which the elements undergo certain chemical reactions
Most reactive elements are placed at the top
Least reactive at the bottom
Used to help predict whether chemical reactions will occur
The more reactive element will replace the less reactive element
See Table 3 p. 286

26. Sample Problem
Using the activity series shown in Table 3, explain whether each of the possible reactions listed below will occur. For those reactions that will occur, predict what the products will be.