Ib Chemistry Definitions (Sl)

Front 1

Relative atomic mass (Ar) /
Atomic weight /
Average atomic mass

Back 1

average mass of an atom, taking into account relative abundances of all naturally occurring isotopes of the element, relative to one atom of C-12

Front 2

Relative molecular mass (Mr) /
Molecular weight /
Average molecular mass

Back 2

equivalent to molar mass of a substance in grams

the average mass of a molecule calculated by adding the Ar of its constituent atoms

Front 3

Mass number (A)

Back 3

sum of an atom's protons & neutrons

Front 4

Atomic number (Z)

Back 4

number of protons in atom's nucleus (determines identity of atom)
equal to the number of electrons it contains

Front 5

Isotopes of an element

Back 5

atoms of the same element (same atomic number /Z) with a different number of neutrons (different mass number /A)

Front 6

First ionisation energy

Back 6

the minimum energy required to remove a mole of electrons from a mole of gaseous atoms to form a mole of univalent cations in the gaseous state

it is the enthalpy change for the reaction : X(g) → X^+ + e^-

Front 7

Electronegativity

Back 7

measure of the tendancy of an atom in a moloecule to attract a shared pair of electrons towards itself

Front 8

Exothermic reaction

Back 8

Reaction that releases heat to surroundings
Enthalpy change = -ve
Forms products with stronger bonds than the reactants

Front 9

Endothermic reaction

Back 9

Reaction in which energy is absorbed from surroundings
Enthalpy change = +ve
Forms products with weaker bonds than the reactants

Front 10

Standard enthalpy change of reaction (delta H)

Back 10

the heat transferred during a reaction carried out under standard conditions:

temperature (298K)
pressure (100 kPa or 1atm)
all substances pure and in their standard state

Front 11

Average bond enthalpy

Back 11

the energy required to break 1 mole of covalent bonds in the reactant, all reactants and products being in the gaseous state.

It is an average value because it takes into account the different energies in a bond between the same atoms in different molecules

Front 12

Standard state

Back 12

standard state of an element is its most stable state under the specified conditions

A precisely defined reference state as:
- Temperature of a substance at exactly 298K
- Pressure of gaseous substance are exactly 1 atm
- liquids & solids are pure
- aqueous solutions are exactly 1 mol per dm cubed

Front 13

Rate of reaction

Back 13

the increase in concentration of a reactant or product per unit time

Front 14

Activation energy (Ea)

Back 14

the minimum energy required for a chemical reaction to take place

Front 15

Bronsted-Lowry acid

Back 15

a proton donor

Front 16

Bronsted-Lowry base

Back 16

a proton acceptor

Front 17

Lewis acid

Back 17

electron-pair acceptor

Front 18

Lewis base

Back 18

electron-pair donor

Front 19

Oxidation

Back 19

loss of electrons

Front 20

Reduction

Back 20

gain of electrons

Front 21

Oxidising agent

Back 21

accepts electrons and becomes reduced

Front 22

Reducing agent

Back 22

donates electrons and becomes oxidised

Front 23

Solute

Back 23

the substance dissolved in a solvent forming a solution

Front 24

Solvent

Back 24

the liquid that dissolves another substance or substances to form a solution

Front 25

Solution

Back 25

a homogeneous mixture of a liquid (solvent) with another substance (solute)

there is usually some interaction between the solvent and solute molecules

Front 26

Concentration

Back 26

the amount of solute in a known volume of solution

expressed as dm^-3

Front 27

Continuous Spectrum

Back 27

shows an unbroken sequence of frequencies

(eg. the spectrum of visible light)

Front 28

Line Spectrum

Back 28

an emission spectrum that has only certain frequencies of light.

produced by excited atoms and ions as they fall back to a lower energy level

Front 29

Group

Back 29

vertical column of elements in the Periodic table

the atoms of the elements in the same group all have the same outer shell structure but an increasing number of inner shells.

Front 30

Period

Back 30

horizontal row of elements in the Periodic table

within a period the atoms of the elements have the same number of shells but with an increasing number of electrons in the outer shell

Front 31

Strong acids and bases

Back 31

almost completely dissociated

Front 32

Weak acids and bases

Back 32

only partially dissociated

Front 33

Buffer

Back 33

A solution that maintains a relatively constant pH when small amounts of strong acid or strong base are added to it

Front 34

unsaturated

Back 34

containing at least one C=C double bond

Front 35

saturated

Back 35

containing no C=C double bonds

Front 36

nucleophile

Back 36

Reagents that have a non-bonding pair of electrons; attracted to the C atom in the halogenoalkanes during substitution reactions