OCR Chemistry A-Level - Modules 1-4 Questions and Answers

OCR Chemistry A-Level - Modules 1-4 Questions and Answers 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 - low melting and boiling points as intermolecular forces are weak - do not conduct as they have no mobile ions or delocalised electrons - nonpolar molecules will dissolve in nonpolar solvents why is ice less dense than water? - when water freezes a network of hydrogen bonds form which holds the H2O molecules apart in open lattice structure - this spacious lattice gives ice a lower density than water why does water have a RELATIVELY high boiling point? - between water molecules there are London forces as well as hydrogen bonds - a lot of energy is required to overcome these bonds give the general oxidation states group 1 element: +1 group 2 element: +2 group 3 element: +3 fluorine: ALWAYS -1 hydrogen: +1 (except with metals when it's -1) oxygen: -2 (except with F when it's +2) chlorine: -1 (except with O or F when it's positive) roman numerals: used to show the positive oxidation state of an element when it can have more than one oxidation state what is a concentrated solution? a solution which contains a high number of moles of solute dissolved in a given volume of solution give the ideal gas equation and its units PV=nRT P = pressure in Pa V = volume in m3 n = moles R = 8.314 T = temperature in K