1. Chemical Reactions: An Introduction
Chapter 7
Chemical Reactions: An Introduction

2. Objective
To learn the signals that show a chemical reaction has occurred

4. Objectives
To learn to identify the characteristics of a chemical reaction
To learn the information given by a chemical equation

5. Chemical reactions involve a rearrangement of the ways atoms are grouped together.
A chemical equation represents a chemical reaction.
Reactants are shown to the left of an arrow.
Products are shown to the right of the arrow.

6. In a chemical reaction, atoms are not created or destroyed.
All atoms present in the reactants must be accounted for in the products.
Same number of each type of atom on both sides of arrow

7. Balancing a Chemical Equation
Unbalanced equation

8. Balancing a Chemical Equation
Balancing the equation
The balanced equation CH O2  CO H2O

9. Physical states of compounds are often given in a chemical equation.

10. Objective
To learn to write a balanced equation for a chemical reaction

11. A chemical reaction is balanced by using a systematic approach:
Write the formulas of the reactants and products to give the unbalanced chemical equation.
Balance by trial and error starting with the most complicated molecule(s).
At the end check to be sure the equation is balanced (same numbers of all types of atoms on the reactant and product sides).

12. Example 
Hydrogen gas and oxygen gas combine to form liquid water.
Write the formulas of the reactants and products to give the unbalanced chemical equation.
H2(g) + O2(g)  H2O(l)

13. Example 
Hydrogen gas and oxygen gas combine to form liquid water.
Balance by trial and error starting with the most complicated molecule(s).

14. Example 
Hydrogen gas and oxygen gas combine to form liquid water.
At the end check to be sure the equation is balanced (same numbers of all types of atoms on the reactant and product sides).
2H2(g) + O2(g)  2H2O(l)

15. Exercise
Which of the following correctly balances the chemical equation given below? There may be more than one correct balanced equation. If a balanced equation is incorrect, explain what is incorrect about it.
CaO + C  CaC2 + CO2
I. CaO2 + 3C  CaC2 + CO2
II. 2CaO + 5C  2CaC2 + CO2
III. CaO + (2.5)C  CaC2 + (0.5)CO2
IV. 4CaO + 10C  4CaC2 + 2CO2
II, III, and IV are correct. I is not correct because you cannot use subscripts to balance a chemical equation.

16. I. The number of molecules is conserved.
Concept Check
Which of the following are true concerning balanced chemical equations? There may be more than one true statement.
I. The number of molecules is conserved.
II. The coefficients tell you how much of each substance you have.
III. Atoms are neither created nor destroyed.
IV. The coefficients indicate the mass ratios of the substances used.
V. The sum of the coefficients on the reactant side equals the sum of the coefficients on the product side.
Only III is correct.