OCR Chemistry A-Level - Modules 1-4 with complete
solution
what is water of crystallisation?
water molecules that make up part of the crystal structure of a solid
define relative isotopic mass
the ass of an atom of an isotope of an element relative to one twelfth the mass of an
atom of C-12
why is relative atomic mass written as a decimal?
it is a weighted average mass
define molecular formula
the actual number of atoms of each element in a compound
define what an orbital is
a region within an atom that can hold up to two electrons of opposite spins
what shape are S orbitals?
spherical
what shape are orbitals?
figure of 8
how can you make out the electron structure of an element using the periodic
table?
period = energy level
columns = electrons
group = subshell
show the oxidation of sodium and the reduction of chlorine
Na -> Na+ + e-
Cl + e- -> Cl-
ions always go on the RHS whether oxidation or reduction
define ionic bond
the electrostatic force of attraction between positive and negative ions
ionic bonding occurs between metals and non-metals but which ion is the
exception?
NH4+
define covalent bond
the strong electrostatic force of attraction between the shared pair of electrons and the
nuclei of the bonded atoms
when non-metals covalently bond, what is their aim?
the octet rule:
- to reach a noble gas configuration (8 electrons in the outer shell)
what are there exceptions to the octet rule?
- NO (odd number of electrons)
- BF3 (boron has less than 8 outershell electrons)
- SF6 (sulfur has more than 8 outershell electrons)
why can sulfur have more than 8 electrons in its outershell?
it has an empty 3d subshell
, define a dative covalent bond
a shared pair of electrons which has been provided by only one of the bonding atoms
give some common examples of ions with a dative bond
NH4+
H3O+
what is a base?
proton (H+) acceptors
what is the shape of an atom determined by?
- the number of electron pairs
- electron pairs repel to become as far apart as possible
- lone pairs of electrons repel more strongly than bonding pairs
define electronegativity
the ability of an atom to attract the bonding electrons in a covalent bond
what creates a polar bond?
- lone pairs offset symmetry by 102.5 degrees each
- asymmetrical molecule
- dipoles do not cancel out
- bonded atoms with different electronegativities
what are London forces?
- uneven distribution of electrons causes a temporary dipole on one molecule
- this induces an opposite dipole on the next molecule
which type of molecules are London forces common to?
- all molecules
- however London forces are most obvious in nonpolar molecules
what are permanent dipole-dipole forces?
- the permanent dipole on one molecule will attract the opposite permanent dipole in a
neighbouring molecule
- this includes polar molecules
what is hydrogen bonding?
the attraction between the positive dipole on an H atom and the lone pair of electrons on
an O, F or N on a different molecule
between which type of molecules do intermolecular forces occur?
simple covalent molecules
give the properties of giant ionic compounds
- high melting and boiling points as the lattice of strong ionic bonds requires a lot of
energy to overcome
- soluble in polar solvents as the ions interact with the dipole on water molecules
- do not conduct when solid as the ions are fixed in place in the lattice
- conduct when molten/aqueous solution as the ions are mobile
why does boiling point increase down group 17?
- the number of electrons in the molecules increase
- larger dipoles are induced which means stronger London forces
- more energy is required to overcome the intermolecular forces
give the properties of simple covalent molecules
solution
what is water of crystallisation?
water molecules that make up part of the crystal structure of a solid
define relative isotopic mass
the ass of an atom of an isotope of an element relative to one twelfth the mass of an
atom of C-12
why is relative atomic mass written as a decimal?
it is a weighted average mass
define molecular formula
the actual number of atoms of each element in a compound
define what an orbital is
a region within an atom that can hold up to two electrons of opposite spins
what shape are S orbitals?
spherical
what shape are orbitals?
figure of 8
how can you make out the electron structure of an element using the periodic
table?
period = energy level
columns = electrons
group = subshell
show the oxidation of sodium and the reduction of chlorine
Na -> Na+ + e-
Cl + e- -> Cl-
ions always go on the RHS whether oxidation or reduction
define ionic bond
the electrostatic force of attraction between positive and negative ions
ionic bonding occurs between metals and non-metals but which ion is the
exception?
NH4+
define covalent bond
the strong electrostatic force of attraction between the shared pair of electrons and the
nuclei of the bonded atoms
when non-metals covalently bond, what is their aim?
the octet rule:
- to reach a noble gas configuration (8 electrons in the outer shell)
what are there exceptions to the octet rule?
- NO (odd number of electrons)
- BF3 (boron has less than 8 outershell electrons)
- SF6 (sulfur has more than 8 outershell electrons)
why can sulfur have more than 8 electrons in its outershell?
it has an empty 3d subshell
, define a dative covalent bond
a shared pair of electrons which has been provided by only one of the bonding atoms
give some common examples of ions with a dative bond
NH4+
H3O+
what is a base?
proton (H+) acceptors
what is the shape of an atom determined by?
- the number of electron pairs
- electron pairs repel to become as far apart as possible
- lone pairs of electrons repel more strongly than bonding pairs
define electronegativity
the ability of an atom to attract the bonding electrons in a covalent bond
what creates a polar bond?
- lone pairs offset symmetry by 102.5 degrees each
- asymmetrical molecule
- dipoles do not cancel out
- bonded atoms with different electronegativities
what are London forces?
- uneven distribution of electrons causes a temporary dipole on one molecule
- this induces an opposite dipole on the next molecule
which type of molecules are London forces common to?
- all molecules
- however London forces are most obvious in nonpolar molecules
what are permanent dipole-dipole forces?
- the permanent dipole on one molecule will attract the opposite permanent dipole in a
neighbouring molecule
- this includes polar molecules
what is hydrogen bonding?
the attraction between the positive dipole on an H atom and the lone pair of electrons on
an O, F or N on a different molecule
between which type of molecules do intermolecular forces occur?
simple covalent molecules
give the properties of giant ionic compounds
- high melting and boiling points as the lattice of strong ionic bonds requires a lot of
energy to overcome
- soluble in polar solvents as the ions interact with the dipole on water molecules
- do not conduct when solid as the ions are fixed in place in the lattice
- conduct when molten/aqueous solution as the ions are mobile
why does boiling point increase down group 17?
- the number of electrons in the molecules increase
- larger dipoles are induced which means stronger London forces
- more energy is required to overcome the intermolecular forces
give the properties of simple covalent molecules
