When matter changes it can◦ Release energy in the form of

Heat and/or Light

Example: A burning match, the combination of naturalgas and oxygen in the air to produce a flame on the burner or the combi-nation of sugar and sulfuricacid as in the picture

When gas is produced when two substances are combined is often evidence of a chemical reaction. ◦For example, putting zinc metal in

hydrochloric acid and hydrogen gas is produced

◦Adding vinegar to baking soda, carbon dioxide gas is produced.

When a solid appears when two liquid solutions are mixed, it is called a precipitate. The appearance of a solid is evidence that a new substance that does not dissolve in water has formed. It is a sign that a chemical reaction has occurred. ◦For example, the mixing of water

solutions of ammonium sulfide and cadmium nitrate results in the appearance of a yellow precipitate (cadmium sulfide).

A change in color is often a sign of a chemical reaction as in the example of the yellow precipitate

Chemical equations summarize the changes in a reactions

Unbalanced Balanced

1. Word Equation Characteristics◦ Only descriptive◦ The reactants and products are

represented by words◦ Does not give the whole story◦ Example:◦ Hydrogen + Oxygen ⇨ water

2. Formula Equation Characteristics◦ Use of symbols and/or formulas to

represent reactants and products◦ example:

CH4(g) + O2(g) ⇨ CO2 (g)+ H2O(g)The g in parenthesis tells you that it is a gasHowever the equation is not balanced.

3. Chemical Equation Characteristics: ◦ Like a formula equation but it

is balanced. The number of atoms of each kind on the right is equal to the number of each atom on the left.

CH4(g) + O2(g) ⇨ CO2 (g)+ 2H2O(g)

The equation above follows the law of conservation of mass and is BALANCED

CH4(g) + O2(g) ⇨ CO2 (g)+ H2O(g) This equation above is not balanced. The number of each atom of the left is NOT equal to

the number of the same atom on the right. For example, there are 4 H on the left and 2 H on the right.

To balance the equation we use the trial and error method

By changing the coefficients (the number in front of the formula)

The resulting number of atoms are counted on both sides of the equation. When the numbers are equal then it is a balanced equation. http://funbasedlearning.com/chemistry/chemBalancer/ques5.htm

CH4(g) + O2(g) ⇨ CO2 (g)+ H2O(g)1. Balance the different types of atoms one at a

time.2. First balance the atoms of elements that are

combined and appear only once on each side of the equation

3. Balance polyatomic ions that appear on both sides of the equation as single units.

4. Balance the H and the O atoms after atoms of all other elements have been balanced.

5. Let’s look at examples p. 251, 252-254 in the book (online) my.hrw.com and http://www.nclark.net/ChemicalReactions

1. The coefficients of a balanced chemical reaction indicates relative amounts of reactants and products

For example,

H2 + Cl2 2HClTells you that there are:1 molecule of

H2 :1molecule of Cl2:2 molecules of HCl

2. The relative masses of the reactants and products of a chemical reaction can be calculated from the reaction’s coefficients.

1 mol of H2 x 2.02g H2 = 2.02g H2

mol H2

3. The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.

. The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.

Type of Reaction

Definition Equation

Synthesis

A + B AB

Decomposition

AB A + B

Single Replacement

AB + C AC + B

Double Replacement

AB + CD AC + BD

Two or more elements or compounds combine

to make a more complex substance

Compounds break down into simpler substances

Occurs when one element replaces another one in a

compoundOccurs when different atoms in two different

compounds trade places

A = Red B = Blue C = Green D = Yellow

2. Use colored pencils to circle the common atoms or compounds in each equation to help you determine the type of reaction it illustrates. Use the code below to classify each reaction.

S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement

 ____ P + O2 → P4O10 ____ Mg + O2 → MgO

____ HgO → Hg + O2 ____ Al2O3 → Al + O2

____ Cl2 + NaBr → NaCl + Br2 ____ H2 + N2 → NH3

____ P + O2 → P4O10 ____ Mg + O2 → MgO

____ HgO → Hg + O2 ____ Al2O3 → Al + O2

____ Cl2 + NaBr → NaCl + Br2 ____ H2 + N2 → NH3

2. Use colored pencils to circle the common atoms or compounds in each equation to help you determine the type of reaction it illustrates. Use the code below to classify each reaction.

S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement