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Chemical Bonding, I: Ionic Bonding and
Covalent Bond
LEARNING OBJECTIVES:
- Discuss the formation of ionic bond and the relation of lattice energy towards its stability.
- Discuss the formation of covalent bond.
- Draw Lewis structures of common molecules.
- Draw Lewis structures of molecules not adhering to the octet rule
Lewis dot symbol consists of the symbol of an element and one dot for each valence electron in an atom of the element.
THE IONIC BOND
An ionic bond is the electrostatic force that holds ions together in an ionic compound.
THE COVALENT BOND
A covalent bond is a bond in which two electrons are shared by two atoms.
Types of Covalent bond:
**1. Polar Covalent Bond
- Nonpolar Covalent Bond**
Lone pairs are pairs of valence electrons that are not involved in covalent bond formation.
OCTET RULE
An atom other than hydrogen tends to form bonds until it is surrounded by eight valence electrons.
ELECTRONEGATIVITY
Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond.
ELECTRONEGATIVITY DIFFERENCE AND
BONDING
- An ionic bond forms when the electronegativity difference between the two bonding atoms is 2.0 or more.
- A polar covalent bond forms when the electronegativity difference between the atoms is in the range of 0.5–1.6.
- If the electronegativity difference is below 0.3, the bond is normally classified as a nonpolar covalent bond, with little or no polarity.
WRITING LEWIS STRUCTURE
Rule 1
Write the skeletal structure of the compound, using chemical symbols and placing bonded atoms next to one another. In general, the least electronegative atom occupies the central position. Hydrogen and fluorine usually occupy the terminal (end) positions in the Lewis structure.
Rule 2
Count the total number of valence electrons present. For polyatomic anions, add the number of negative charges to that total.
(For example, for the CO 32 -ion we add two electrons because the 2-charge indicates that there are two more electrons than are provided by the atoms.)
For polyatomic cations, we subtract the number of positive charges from this total. (Thus, for NH 4 +^ we subtract one electron because the 1+charge indicates a lossof one electron from the group of atoms.) Rule 3
Draw a single covalent bond between the central atom and each of the surrounding atoms. Complete the octets of the atoms bonded to the central atom. Electrons belonging to the central or surrounding atoms must be shown as lone pairs if they are not involved in bonding. Rule 4
After completing steps 1–3, if the central atom has fewer than eight electrons, try adding double or triple bonds between the surrounding atoms and the central atom, using lone pairs from the surrounding atoms to complete the octet of the central atom.
FORMAL CHARGES AND LEWIS STRUCTURE An atom’s formal charge is the electrical charge difference between the valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure_._ Assigning the number of electrons on an atom in a Lewis structure:
- All the atom’s nonbonding electrons are assigned to the atom.
- We break the bond(s) between the atom and other atom(s) and assign half of the bonding electrons to the atom. EXAMPLES:
Assign formal charge to O atom in H 3 O+
Intermolecular Forces and Properties of
Liquids
LEARNING OBJECTIVES:
- compare the properties of liquids and solids with those of gases;
- apply the kinetic molecular theory to describe liquids and solids;
- describe the various intermolecular forces and factors that affect their strengths;
- identify the types of intermolecular forces that may operate in a given molecular substance;
- rank substances according to strength of intermolecular forces; and
- illustrate the intermolecular forces between molecules of a compound.
KINETIC MOLECULAR THEORY
- All matter is made of tiny particles.
- These particles are in constant motion.
- The speed of particles is proportional to temperature. Increased temperature means greater speed.
- Solids, liquids, and gases differ in distances between particles, in the freedom motion of particles, and in the extent to which the particles interact.
INTERMOLECULAR FORCES
These are attractive forces between molecules.
INTRAMOLECULAR FORCES These are attractive forces that hold atoms together in a molecule. DIPOLE-DIPOLE FORCES
Dipole-dipole forces are attractive forces between polar molecules, that is, between molecules that possess dipole moments.
Why is acetonitrile has a higher bp compared to propane though both have almost the same MW?
Acetonitrile (polar) has a higher boiling point than propane (nonpolar) because there’s a dipole-dipole present in acetonitrile.
ION-DIPOLE FORCES
Forces that attract an ion (whether cation or anion) and a polar molecule to each other.
DISPERSION FORCES (also referred as LONDON DISPERSION FORCES )
Dispersion forces are very weak interactions due to momentary changes in electron density in a molecule.
POLARIZABILITY
Polarizability refers to the ease with which the electron distribution int the atom/molecule can be distorted.
HYDROGEN BONDING
Hydrogen bonding occurs when a hydrogen atom is electrostatically attracted to an O, N, or F atom in another molecule.
Organic Chemistry: An Introduction
LEARNING OBJECTIVES:
- Differentiate organic compounds from inorganic compounds;
- Identify several sources of organic compounds;
- Write structural formulas of organic compounds;
- Name organic compounds given their structural formula; and
- Compare and contrast the different functional groups.
ORGANIC CHEMISTRY
Organic chemistry is the scientific study of the structure, properties, composition, reactions, and synthesis of organic compounds that by definition contain carbon.
SOURCES OF ORGANIC COMPOUNDS
Plants and animals
Many organic compounds are obtained directly from plant and animals’ sources via suitable methods of isolation.
Natural gas and Petroleum
Through synthetic organic reaction, natural gas and petroleum are used for the production of hundreds of useful organic substances such as solvents, synthetic rubber, explosives, and plastics.
Coal
Coal-tar products form the starting materials for the manufacture of thousands of useful aromatic compounds including perfumes, drugs, and dyes among others.
Synthesis
Simple organic compounds derive from petroleum or coal that have been converted into thousands of useful materials by synthetic methods.
FUNCTIONAL GROUPS
In organic chemistry, a functional group is a set of atoms within molecules that function together to react in predictable ways.
Functional groups undergo the same chemical reactions no matter how large or small the molecule is.
Covalent bons link the atoms within functional groups and connect them to the rest of the molecule.
HYDROXYL GROUP (also known as ALCOHOL GROUP)
Oxygen atom bonded to a hydrogen atom.
Hydroxyls are often written as -OH structures and chemical formulas.
Examples of common compounds containing hydroxyl groups are alcohols and carboxylic acids.
ALDEHYDE GROUP
Aldehydes are made up of carbon and oxygen double-bonded together and hydrogen bonded to the carbon.
KETONE GROUP (CARBONYL FUNCTIONAL GROUP)
A ketone is a carbon atom double bonded to an oxygen atom that appears as a bridge between two other parts of a molecule.
AMINE GROUP
Amine functional groups are derivatives of ammonia (NH3) where one or more of the hydrogen atoms are replaced by an alkyl or aryl functional group.
AMIDE GROUP
Amides are a combination of a carbonyl group and an amine functional group.
ETHER GROUP
An ether group consists of an oxygen atom forming a bridge between two different parts of a molecule. Ethers have formula ROR.
ESTER GROUP
An ester is derived from substitution reaction of a carboxylic acid and an alcohol. Esters have formula RCO 2 R.
