Evidence for a Chemical Reaction Section 7.1 Steven S. Zumdahl Susan A. Zumdahl Donald J. DeCoste Gretchen M. Adams University of Illinois at Urbana-Champaign.

Evidence for a Chemical Reaction Section 7.1

Steven S. Zumdahl

Susan A. Zumdahl

Donald J. DeCoste

Gretchen M. Adams • University of Illinois at Urbana-Champaign

Chapter 7

Chemical Reactions: An Introduction

Evidence for a Chemical Reaction Section 7.1

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

Objective

Evidence for a Chemical Reaction Section 7.1

Section 7.2Chemical Equations

1. To learn to identify the characteristics of a chemical reaction

2. To learn the information given by a chemical equation

Objectives

Section 7.2Chemical Equations

• 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.

Section 7.2Chemical Equations

• 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

Section 7.2Chemical Equations

Balancing a Chemical Equation

• Unbalanced equation

Section 7.2Chemical Equations

Balancing a Chemical Equation

• Balancing the equation

• The balanced equation CH4 + 2O2 CO2 + 2H2O

Section 7.2Chemical Equations

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

Section 7.3Balancing Chemical Equations

1. To learn to write a balanced equation for a chemical reaction

Objective

Section 7.3Balancing Chemical Equations

• 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).

Section 7.3Balancing Chemical Equations

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)

Section 7.3Balancing Chemical Equations

Example 

Hydrogen gas and oxygen gas combine to form liquid water. Balance by trial and error starting with the most

complicated molecule(s).

Section 7.3Balancing Chemical Equations

Example 

Hydrogen gas and oxygen gas combine to form liquid water.

2H2(g) + O2(g) 2H2O(l)

At the end check to be sure the equation is balanced (same numbers of all types of atoms on the reactant and product sides).

Section 7.3Balancing Chemical Equations

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

Section 7.3Balancing Chemical Equations

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.