Module 2:Foundations in chemistry OCR A CHEMISTRY ALEVEL

Atomic number

Number of protons in the atom (proton number)

Isotope

Atoms of the same element with different numbers of neutrons and different masses but the same proton number

Mass number

Sum of the number of protons and neutrons in the nucleus (nucleon number)

Ion

A positively or negatively charged atom where the number of electrons is different to the number of protons

Cation

A positively charged ion with fewer electrons than protons

Anion

A negatively charged ion with more electrons than protons

Relative isotopic mass

The mass of an isotope compared with one twelfth of the mass of an atom of carbon-12

Relative atomic mass

The weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12

Mass spectrometer

An instrument used to determine the relative masses of atoms by the deflection of their ions on a magnetic field

Binary compound

A compound containing two elements only

Polyatomic ion

An ion containing more than one atom

Diatomic molecule

A molecule consisting of two atoms

Formula unit

The lowest whole-number ratio of ions in an ionic compound

Amount of substance

The quantity whose unit of the mole. Chemists use amount of substance as a means of counting atoms

Avogadro's constant

6.02 x 10^23 mol -1, the number of atoms per mole of carbon-12

Molar mass

The mass per mole of a substance, in units of g mol-1

Molecular formula

A formula that shows the number and type of atoms of each element present in a molecule

Empirical formula

A formula that shows that the simplest whole-number ratio atoms of each element present in a compound

Relative molecular mass

The weighted mean mass of a molecule of a compound compared with one-twelfth of the mass of an atom of carbon-12

Relative formula mass

The weighted mean mass of the formula unit of a compound compared with one-twelfth of the mass of an atom of carbon-12

Analysis

A detailed examination of the elements or structure of something...................................................

Hydrated

A crystalline compound containing water molecules

Water of crystallisation

Water molecules that are bonded into a crystalline structure of a compound

Anhydrous

Containing no water molecules

Volume

The amount of space that a substance or object occupies, or that is enclosed within a container

Concentration

A measurement of how much solute exists within a certain volume of solvent

Standard solution

A solution of a known concentration

Molar gas volume

The volume per mole of gas molecules at a stated temperature and pressure

Stoichiometry

The ratio of the amount, in moles, of each substance in a chemical reaction

Theoretical yield

The yield resulting from complete conversion of reactants into products

Percentage yield

(actual yield of a product/theoretical yield of product) x100

Actual yield

The amount of product obtained from a reaction

Limiting reagent

The reagent that is not in excess,which will be used up first and stop the reaction

Atom economy

(Sum of molar masses of desired products)/(sum of molar masses of all products) x 100%

Strong acid

An acid that dissociates completely in solution

Dissociation

The separation of a substance into two or more simpler substances, or of a molecule into atoms or ions, by the action of heat or a chemical process

Weak acid

An acid that dissociates only partially in solution

Base

A compound that neutralises an acid to form a salt

Neutralises

The reaction between an acid and a base to produce a salt

Salt

The product of a reaction in which the H+ ions from the acid are replaced by metal or ammonium ions

Alkali

A type of base that dissolves in water to release OH- ions

Titration

A measured amount of a solution of unknown concentration is added to a known volume of a second solution until the reaction between them is just complete

Oxidation number

A measure of the number of electrons that an atom uses to bond with atoms of another element

Redox reaction

A reaction involving reduction and oxidation

Oxidation

Loss of electrons or an increase in oxidation number

Reduction

Gain of electrons or a decrease in oxidation number

Atomic orbitals

A region around the nucleus that can hold up to two electrons with opposite spins

Sub-shells

A group of orbitals of the same type within a shell

Blocks

There a 4; s, p, d and f

Ionic bonding

The electrostatic attraction between positive and negative ions

Giant ionic lattice

a three-dimensional structure of oppositely charged ions, bonded together by strong ionic bonds

Covalent bonding

The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms

Molecule

The smallest part of a covalent compound that can exist while retaining its chemical identity, consisting of two or more atoms covalently bonded together

Displayed formula

A formula that shows the relative positioning of all the atoms in a molecule and the bonds between them

Lone pairs

An outer shell pair of electrons that is not involved in chemical bonding

Coordinate bond

A shared pair of electrons in which the bonded pair has been provided by one of the bonding atoms only (dative bond)

Electronegativity

A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

Pauling electronegativity values

A value assigned as a measure of the relative attraction of a bonded atom for the pair of electrons in a covalent bond

Non-polar bond

With no charge separation across a bond or in a molecule

Pure covalent bond

A covalent bond with equal sharing of electrons between atoms

Polar covalent bond

A bond with a permanent dipole, having positive and negative partial charged=s on the bonded atoms

Polar molecule

A molecule with an overall dipole, having taken into account any dipoles across bonds and the shape of the molecule

Dipole

A separation in electrical charge so that one atom of a polar covalent bond, or one end of a polar molecule, has a small positive charge (delta plus) and the other has a small negative charge (delta negative)

Permanent dipole

A small charge difference that does not change across a bond , with positive and negative partial changes on the bonded atoms : the result of the bonded atoms having different electronegativites

Intermolecular forces

An attractive force between molecules. Intermolecular forces can be London forces, permanent dipole-dipole interactions or hydrogen bonding

London forces

Attractive forces between induced dipoles in different molecules (induced dipole-dipole interactions)

Permanent dipole-dipole interactions

Attractive forces between the permanent dipoles in different molecules

Hydrogen bond

A strong dipole-dipole attraction formed when the slightly positive hydrogen atom of a polar covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent bond in another molecule. (N, O, F)

Instantaneous dipole

An atom or a molecule can have all, most or majority of its electrons shifted to one side by random chance of the electron movements. When this happens a temporary dipole is created causing the side with more concentration of electrons to become more electronegative and the opposite relatively electropositive.

Induced dipole

A dipole temporarily created in an otherwise non polar molecule induced by a neighboring charge.

Simple molecular lattice

A three-dimensional structure of molecules, bonded together by weak intermolecular forces

What structure has covalent bonding?

Simple molecules
Giant covalent structure

What is the principle of electron pair repulsion

-Electron pairs repel each other and will settle in a position to minimise repulsion, i.e. maximise the bond angle
-Two lone pairs will repel the most, followed by a bonding pair and a lone pair and two bonding pairs repel the least

Linear bond

-180° bond angle
-2 bonding pair and no lone pair

Trigonal planar

-120° bond angle

-3 bonding pairs and no lone pairs

Tetrahedral

-109.5° bond angle

-4 bonding pairs and no lone pairs

Octrahedral

-90° bond angle

-6 bonding pairs and no lone pairs

Pyramidal

-107° bond angle

-3 bonding pairs and 1 lone pair

non linear

-104.5° bond angle

-2 bonding pairs and 2 lone pair

Trigonal bipyramidal

-120° and -90° bond angles

-5 bonding pairs and no lone pairs

What is VSEPR used to determine?

The shape of covalent molecules with three or more atoms based on electron pair repulsion.

Rules of VSEPR

• Valence shell electrons are those electrons that are found in the outer shell

• Electron pairs repel each other as they have the same charge

• Lone pair electrons repel each other more than bonded pairs

• Repulsion between multiple and single bonds is treated the same as for repulsion between single bonds

• Repulsion between pairs of double bonds are greater

• The most stable shape is adopted to minimize the repulsion forces

Order of repulsion

one pair – lone pair > lone pair – bond pair > bond pair – bond pair

How does the electronegativity change across the periodic table?

-It is lowest in the bottom left (metals) of the periodic table because there is the lowest effective nuclear charge (smallest Mr) and high shielding
-It is highest in the top right (non-metals in particular Nitrogen, Oxygen, and Fluorine) because there is the highest effective nuclear charge (highest Mr) as well as low shielding

How does electronegativity affect bonding?

-In metals and non-metals, their difference in electronegativity is so large that it causes electrons to be transferred from the non metal to the metal
-In only non metals, if the two atoms are of the same element, they have the same electronegativity. This means they are non-polar, have no dipole and the electrons are shared equally within the covalent bond
-However, if the two elements in the covalent bond are different, they will differ in electronegativity. The more electronegative element will attract the bonding pair of electrons more and so the pair will be shared unequally within the covalent bond. This reults in the formation of a dipole across the molecule (more electronegative is delta negative and less electronegative is delta positive) and so the molecule is polar

What molecules have london dispersion forces?

-All molecules as the electron pair is constantly moving around so at any given moment in time, they can have an instantaneous dipole. This repels the electron pair away from the other atom or molecule inducing a dipole. The attraction between the two atoms/molecules forms the london dispersion force
-This is the weakest type of force (hydrogen bonds then permanent dipole forces then london dispersion forces)
-Its strength varies with size
-Monotomic elements have them as they cannot be polar
-Molecules with two of the same element have them as there is no difference in the electronegativity between the two elements so no permanent dipole as they electron pair is shared equally
-Larger non-polar molecules have them as they are symmetrical, so despite polar bonds they are overall non-polar and have no permanent dipole
-The strength of the london dispersion forces increases with the size of the molecule as its Mr increases. This means that more electrons are involved in the formation of london dispersion so the london dispersion forces are stronger and require more energy to overcome

What type of molecules have permanent dipole forces?

-Polar molecules
-These have a permanent dipole and include two atom molecules of different elements or larger, non-symmetrical molecules (with polar bonds)
-This intermolecular force is stronger than london dispersion forces (which will also be present in these molecules) but weaker than hydrogen bonds
-Because they take more energy to overcome than london dispersion forces, it can cause these molecules to have a higher Tm/Tb than expected

What type of molecules show hydrogen bonding?

-Molecules where a hydrogen atom is bonded directly to a nitrogen, oxygen or fluorine atom
-Because there is such a large difference in the electronegativity of the atoms, it is the strongest intermolecular force
-Essentially it is just a very strong dipole as there is such great attraction between the very delta positive hydrogen and the very delta negative nitrogen/oxygen/fluorine
-It causes molecules to have a much greater Tm/Tb than expected
-The main examples include Hydrogen fluoride, water and ammonia (hydrogen bond forms from the lone pair of the atom if it has one e.g. in water and ammonia)

Which organic molecules are hydrogen bonded?

-Alcohols (OH functional group)
-Carboxcylic acids (COOH functional group)
-Amines (NH2 functional group)