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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.