Compounds and Chemical Reactions

Posted on February 25, 2009

0


>
Essentially all elements combine to form compounds
Compounds are of two types:
Molecular, which involve shared electrons and consist of electrically neutral, discrete particles called molecules
Ionic compounds, which involve electron transfer and charged particles called ions
Chemical formulas are collections of chemical symbols that are used to describe elements and compounds
Free elements are not combined with other elements in a compound
Examples: Fe (iron), Na (sodium), and K (potassium)
Many nonmetals occur as diatomic molecules
Chemical formulas specify the composition of a substance
NaCl is composed of the elements sodium and chlorine in a one-to-one (atom) ratio
Fe2O3 is composed of the elements iron and oxygen in a two-to-three ratio
CO(NH2)2 expands to CON2H4, but there are good reasons to write some compounds with parentheses
Hydrates are crystals that contain water molecules, for example plaster: CaSO4 • 2H2O
When all the water is removed (by heating), the solid that remains is said to be anhydrous (without water)
Chemical equations describe what happens in a chemical reactions
Hydrogen and oxygen combine to form water
Hydrogen and oxygen are called reactants
Water is called the product
Reactants are separated from products with “”
2 H2 + O2  2 H2O
Note that the “” is like an equal sign because both sides of the equation have the same number of each type of atom
This can be represented as:
It is sometimes useful to include the physical state of reactants and products
For solids use s, liquids use l, gases use g, and for aqueous solutions use aq.
For example, the reaction between stomach acid (an aqueous solution of HCl) and sodium carbonate (an antacid) can be written
Almost all chemical reactions either absorb or give off energy, often as heat or light
Kinetic and potential energy are both important in chemistry
Kinetic energy is the energy an object has when moving
Potential energy is the energy an object has due to its position
Potential energy is “stored energy” because it can be converted into kinetic energy
Energy must also be conserved
The Law of Conservation of Energy:
Energy cannot be created or destroyed; it can only be converted from one form to another
Heat and temperature are related to kinetic energy
The temperature of an object is proportional to its average kinetic energy (average speed of its atoms)
Heat or thermal energy is transferred between objects with different temperatures
Heat flow spontaneously from hot to cold objects
Chemical energy is a form of potential energy
The analysis of temperature changes in chemical reactions can provide information about the potential energy changes that occur
The kinetic molecular theory of matter provides more details about chemical energy changes and is discussed in Chapter 7
Energy can also be transferred as light, which will be covered later in the book
As a general rule, molecular compounds are formed when nonmetallic elements combine
Many molecular compounds contain hydrogen:
Organic chemistry is a major specialty that deals with compounds containing mostly carbon and hydrogen
Hydrocarbons contain only hydrogen and carbon and are organic compounds
Alkanes are the simplest hydrocarbons
General formula is CnH2n+2
Other classes of hydrocarbons exist
Different classes of organic compounds are derived from hydrocarbons by replacing hydrogen
For example alcohols result when a H is replaced by OH in a hydrocarbon
Inorganic compounds are substances not considered to be derived from hydrocarbons
The rules for naming, or nomenclature, of simple inorganic compound is covered now (organic nomenclature is covered later)
Binary compounds are compounds comprised of two different elements
The goal is to be able to convert between the chemical formula and the name
The first element in the formula is identified by its English name, the second by appending the suffix –ide to its stem
The number of each type of atom is specified with Greek prefixes
The subscripts in the formula of an ionic compound always specifies the smallest whole-number ratio of the ions because molecules don’t exist in ionic compounds
The smallest unit of a compound is called the formula unit
Positively charged ions have more protons than electrons and are called cations
Negatively charged ions have more electrons than protons and are called anions
The formula unit of an ionic compound always contains both cations and anions
Ionic compounds are composed of charged particles (ions)
Ions can be formed from the reaction of metal with a nonmetal
The metals form cations and the nonmetals form anions
The charges on many representative elements can be predicted:
Metals form cations
The positive charge on the cation is the same as the “A” group number of the metal
Nonmetals form anions
The negative charge on the anion is equal to the number of spaces to the right we have to move in the periodic table to get to a noble
Ionic compounds must be electrically neutral
Rules for writing Formulas of Ionic Compounds:
1) The positive ion is given first in the formula.
2) The subscripts in the formula must produce an electrically neutral formula unit.
3) The subscripts should be the set of smallest whole numbers possible.
4) The charges on the ions are not included in the finished formula of the substance.
Ions formed by transition metals (Group IIIB – VIIIB) and post-transition metals:
Some polyatomic ions (ions with two or more atoms):
Naming ionic compounds
The name of the cation is given first followed by the name of the anion
Cations:
If the metal forms only one positive ion, the cation name is the English name for the metal
If the metal forms more than one positive ion, the cation name is the English name followed, without a space, by the numerical value of the charge written as a Roman numeral in parentheses (this is for the Stock system)
Anions:
For monoatomic anions, the name is created by adding the “–ide” suffix to the stem name for the element.
For polyatomic ions, use the names in Table 2.5
To name a compound, you can use this flowchart:
Summary of Properties
Hardness and brittleness
Molecular compounds tend to be soft and easily crushed because the attractions between molecules are weak and molecules can slide past each other
Ionic compounds are hard and brittle because of the strong attractions and repulsions between ions
Melting points
To melt the particles in the solid must have sufficient kinetic energy to overcome the attractions between particles
Molecular compounds tend to have weak attractions between particles and so tend to have low melting points
Many molecular compounds are gases at room temperature
Ionic compound tend to have strong attractions so they have high melting points
Nearly all ionic compounds are solids at room temperature
Electrical conductivity requires the movement of electrical charge
Ionic compounds:
Do not conduct electricity in the solid state
Do conduct electricity in the liquid state
The ions are free to move in the liquid state
Molecular compounds:
Do not conduct electricity in the solid or liquid state
Molecules are comprised of uncharged particles

Posted in: KIMIA