Chemical Equations and Reactions

Chapter Eight

Describing Chemical Reactions

What is a chemical reaction?

-the process by which one or more substances are changed into different substances

There are two parts:

Reactants

-the original substances

Products

-resulting substances

Law of Conservation of Mass

-the total mass of the reactants must equal the total mass of the products

How is a chemical reaction expressed?A chemical equation represents, with symbols

and formulas, the identities and relative amounts of the reactants and products in a chemical reaction

2Ca + O2 2CaO

reactants

yield

products

Indications of a Chemical Reaction

Evolution of heat and light: presence of both is strong evidence; one by itself is not necessarily a sign as many physical changes also do this

Our Reaction:(evolution of heat and light)

Reactants: (known facts)

Mg + O2

(react to yield) Products: MgO

Overall Reaction: Mg + O2 MgO

Checks and “Balances” for Correctly Written

Equationsknown facts are represented: all

reactants and products identified Mg + O2 MgO

correct formulas written; diatomics in molecular form

Law of Conservation of Mass satisfied; add coefficients where necessary

2Mg + O2 2MgO

Indications

Production of a gas.

The gases produced by factories contribute to air pollution and the formation of acid rain.

Our Reaction(production of a gas)

Reactants: Mg + HCl (react to yield) Products: MgCl2 + H2

Overall Reaction:Mg + HCl MgCl2 + H2

Checks and “Balances” for Correctly Written

Equationsknown facts are represented: all

reactants and products identified Mg + HCl MgCl2 + H2

correct formulas written; diatomics in molecular form

Law of Conservation of Mass satisfied; add coefficients where necessary

Mg + 2HCl MgCl2 + H2

Indications

Formation of a precipitate.

A precipitate is considered a solid. It often appears as a cloudy substance in the solution.

Our Reaction(formation of a

precipitate)Reactants:CuSO4 + NaOH

react to yield Products: Cu(OH)2 + Na2SO4

In this reaction the cations (Cu2+ and Na+) simply change places

Overall Reaction:CuSO4 + NaOH Cu(OH)2 + Na2SO4

Checks and “Balances” for Correctly Written

Equationsknown facts are represented: all

reactants and products identified CuSO4 + NaOH Cu(OH)2 + Na2SO4

correct formulas written; diatomics in molecular form

Law of Conservation of Mass satisfied; add coefficients where necessary

CuSO4 + 2NaOH Cu(OH)2 + Na2SO4

Indications

Color change.

This often indicates that a new substance has been formed.

Color ChangeA common example

of a chemical reaction indicated by color change is the bright copper of roofs changing to a blue-green color over time. This is actually Cu+1 being changed to Cu+2.

Review Indications of a Chemical Reaction

• Evolution of heat and light Examples: burning, light sticks, hand

warmers• Production of a gas Examples: food spoiling, decomposition• Formation of a precipitate Examples: rusting, tarnishing, deposits in

tea kettle, chunky milk• Color change Examples: leaves changing color, bleach

on colors

Characteristics of Chemical Equations

1. Must represent known facts. All reactants and products identified.

2. Must contain correct formulas. Diatomics must be in molecular form (H2, Br2…). Sulfur is S8 and phosphorus is P4.

3. Law of conservation of mass must be satisfied. To equalize #s of atoms, coefficients are added where necessary.

Coefficient: small whole # that appears in front of a formula in a chemical equation.

Look Again!Color change and precipitate (rust) formed. (ppt)

When acid rain hits the marble statue it dissolves the marble and gives off carbon dioxide (gas production).

Color change in leaves indicates a chemical change in the production of chlorophyll brought on by a reduction in the amount of daily sunlight.

Burning substances of any kind frequently produces gases as indicated in this factory’s pollution.

Heat and light are two of the products in this chemical reaction known as combustion.

Precipitate formation is one of the most common indications of a reaction that you will observe in the laboratory. Many of our reactions will be carried out in solution where precipitates at the molecular level can be seen.