Chemical Equations and reactions

Chapter 8

8.1 DESCRIBING CHEMICAL REACTIONS

• A chemical reaction is the process by which one or more substances are changed into one or more different substances.

• In any chemical reaction, the original substances are known as the reactants and the resulting substances are known as the products.

• A chemical equation represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction.

• Example: The following chemical equation shows that the reactant ammonium dichromate yields the products nitrogen, chromium(III) oxide, and water.

(NH4)2Cr2O7 N2 + Cr2O3 + 4 H2O

• The arrow () is used to separate the reactants from the products. (Read as “produces” or “yields”)

• What causes a chemical reaction to occur? mixing substances adding energy to a substance (heat/electric

current)

Indications of a Chemical Reaction

• Certain easily observed changes usually indicate that a chemical reaction has occurred.1. Evolution of energy as heat and light2. Production of a gas3. Color change4. Formation of a precipitate.

• A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is known as a precipitate.

Characteristics of Chemical Equations

1. Equation must represent the known facts. (Reactants and products must be identified)

2. Equation must contain correct formulas for the reactants and products.

3. Law of conservation of mass must be satisfied.

Law of Conservation of Mass

• Atoms are neither created nor destroyed in ordinary chemical reactions. (just rearranged)

• Same number of atoms of each element must appear on both sides of the equation.

• In order for an equation to correctly describe a reaction, it must be “balanced.”

• Coefficient – number placed in front of a symbol or formula to balance the equation.

• Do not confuse with subscript.

• Example:

H2 + O2 H2O

2 H2 + O2 2 H2O

• Never alter a subscript when trying to balance a chemical equation. Why?

Word Equations

• Sometimes the reactants and products in a chemical reaction are given in words.

• You must convert into chemical formulas before you can balance the equation.

• There are seven elements that occur in nature as diatomic molecules.

Elements That Normally Exist as Diatomic Molecules

Element Formula

Hydrogen H2

Nitrogen N2

Oxygen O2

Fluorine F2

Chlorine Cl2

Bromine Br2

Iodine I2

Some Example Word Equations1. magnesium + oxygen magnesium oxide

2. potassium iodide + lead (II) nitrate potassium nitrate + lead (II) iodide

Symbols Used in Chemical Equations

Other Symbols Used in Chemical Equations

Some Example Equations

1. Iron metal in chlorine gas reacts to form solid iron (III) chloride.

2. Aqueous solutions of barium chloride and sodium chromate react to produce a precipitate of barium chromate and an aqueous solution of sodium chloride.

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Your turn1. Convert the following word equations into formula

equations and balance. Be sure to include state labels and reactions conditions in the equation.

a) When heated, solid aluminum carbonate decomposes to form solid aluminum oxide and carbon dioxide gas.

b) Hydrogen gas and iron (III) oxide powder react to form liquid water and iron powder.

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Balancing Chemical Equations

Most chemical equations can be balanced by inspection. (Insert coefficients as needed.)

Examples:

Zn + HCl ZnCl2 + H2

N2 + H2 NH3

However, some equations can be more difficult to balance.

Some helpful hints:

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

Atoms listed more than one time on a side should be balanced last.

Examples:

AlBr3 + K2SO4 KBr + Al2(SO4)3

CH4 + O2 H2O + CO2

One Minute Response

• You have one minute to answer these two questions concerning today’s lesson.

• What was the most important thing you learned?

• What is one question you would still like answered?