Compounds and Chemical Reactions

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

Atoms and Elements

Chemistry is a science that studies the composition and properties of matter
Matter is anything that takes up space and has mass
Mass is a measure of the amount matter in a sample
Chemistry holds a unique place among the sciences because all things are composed of chemicals
A knowledge of chemistry will be valuable whatever branch of science you study
Chemistry is constantly changing as new discoveries are made by researchers
Researchers use a commonsense approach to the study of natural phenomena called the scientific method
A scientific study normally:
Begins with a question about nature
Involves a search of the work of others
Requires observing the results of experiments
Often results in a conclusion, or a statement based on what is thought about a series of observations
Experiments provide empirical facts
Facts are called data
A broad generalization based on the results of many experiments is called a (scientific) law
Laws are often expressed as mathematical equations
Laws summarize the results of experiments
Theoretical models attempt to explain why substances behave as they do
A hypothesis is a tentative explanation
A theory is an experimentally tested explanation of the behavior of nature
Chemical substances are comprised of atoms
Atoms combine to form molecules which can be represented in a number of ways, including:
Characteristics or properties of materials distinguish one type of substance from another
Properties can be classified as physical or chemical
Physical properties can be observed without changing the chemical makeup of the substance
Chemical properties involve a chemical change and result in different substances
Chemical changes are described by chemical reactions
Properties can also be described as intensive or extensive
Intensive properties are independent of sample size
Examples: sample color and melting point
Extensive properties depend on sample size
Examples: sample volume and mass
In general, intensive properties are more useful in identifying a substance
Matter is often classified by properties
The three common physical states of matter have different properties:
Solids have a fixed shape and volume
Particles are close together and have restricted motion
Liquids have indefinite shape but fixed volume
Particles are close together but are able to flow
Gases have indefinite shape and volume
Particles are separated by lots of empty space
Elements are substances that cannot be decomposed by chemical means into simpler substances
Each element is assigned a unique chemical symbol
Most are one or two letters
First letter is always capitalized
All remaining letters are lowercase
Names and chemical symbols of the elements are listed on the inside front cover of the book
Compounds are substances formed from two or more different elements combined in a fixed proportion by mass
The physical and chemical properties of a compound are, in general, different than the physical and chemical properties of the elements of which it is comprised
Elements and compounds are examples of pure substances whose composition is the same, regardless of source
A mixture consists of varying amounts of two or more elements or compounds
Homogeneous mixtures or solutions have the same properties throughout the sample
Heterogeneous mixtures consist of two or more phases
Matter can be classified:
We take for granted the existence of atoms and molecules
The concept of the atom had limited scientific usefulness until the discovery of two important laws: the Law of conservation of mass and the Law of Definite Proportions
These laws summarized the results of the experimental observations of many scientists
Law of Conservation of Mass:
No detectable gain or loss of mass occurs in chemical reactions. Mass is conserved.
Law of Definite Proportions:
In a given chemical compound, the elements are always combined in the same proportions by mass.
In the sciences mass is measured in units of grams (symbol, g)
One pound equals 453.6 g
The laws of conservation of mass and definite proportions provided the experimental foundation for the atomic theory
Dalton’s Atomic Theory:
Matter consists of tiny particles called atoms.
Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not themselves break apart.
In any sample of a pure element, all the atoms are identical in mass and other properties.
The atoms of different elements differ in mass and other properties.
In a given compound the constituent atoms are always present in the same fixed numerical ratio.
It follows from Dalton’s Atomic Theory that atoms of an element have a constant, characteristic atomic mass or atomic weight
For example, for any sample of hydrogen fluoride:
F-to-H atom ratio: 1 to 1
F-to-H mass ratio: 19.0 to 1.00
This is only possible if each fluorine atom is 19.0 times heavier than each hydrogen atom
It turns out that most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses
Atoms of the same element with different masses are called isotopes
For example: there are 3 isotopes of hydrogen and 4 isotopes of iron
Chemically, isotopes have virtually identical chemical properties
The relative proportions of the different isotopes are essentially constant
A uniform mass scale for atoms requires a standard
For atomic mass units (amu, given the symbol u) the standard is based on carbon:
1 atom of carbon-12 = 12 u (exactly)
1 u = 1/12 mass 1 atom of carbon-12 (exactly)
This definition results in the assignment of approximately 1 u for the mass of hydrogen (the lightest atom)
Example: Naturally occurring chlorine is a mixture of two isotopes. In every sample of this element, 75.77% of the atoms are chlorine-35 and 24.23% are chlorine-37. The measured mass of chlorine-35 is 34.9689 u and that of chlorine-37 is 36.9659 u. Calculate the average atomic mass of chlorine.
Experiments have been performed that show atoms are comprised of subatomic particles
There are three principal kinds of subatomic particles:
Proton – carries a positive charge, found in the nucleus
Electron – carries a negative charge, found outside the nucleus, about 1/1800 the mass of a proton
Neutron – carries no charge, found in the nucleus, a bit heavier than a proton, about 1800 times heavier than an electron
An element can be defined as a substance whose atoms all contain the identical number of protons, called the atomic number (Z)
Isotopes are distinguished by mass number (A):
Atomic number, Z = number of protons
Mass number, A = (number of protons) + (number of neutrons)
For charge neutrality, the number of electrons and protons must be equal
This information can be summarized
Example: For uranium-235
Number of protons = 92 ( = number of electrons)
Number of neutrons = 143
Atomic number (Z) = 92
Mass number (A) = 92 + 143 = 235
Chemical symbol = U
Summary for uranium-235:
The Periodic Table summarizes chemical and physical properties of the elements
The first Periodic Tables were arrange by increasing atomic mass
The Modern Periodic table is arranged by increasing atomic number:
Elements are arranged in numbered rows called periods
The vertical columns are called groups or families (group labels vary)
Modern Periodic Table with group labels and chemical families identified
Some important classifications:
A groups = representative elements or main group elements
I A = alkali metals
II A = alkaline earth metals
VII A = halogens
VIII = noble gases
B groups = transition elements
Inner transition elements = elements 58 – 71 and 90 – 103
58 – 71 = lanthanide elements
90 – 103 = actinide elements
Classification as metals, nonmetals, and metalloids
Metals
Tend to shine (have metallic luster)
Can be hammered or rolled into thin sheets (malleable) and can be drawn into wire (ductile)
Are solids at room temperature and conduct electricity
Nonmetals
Lack the properties of metals
React with metals to form (ionic) compounds
Metalloids
Have properties between metals and nonmetals