chemistry definitions Flashcards
Activation Energy
Minimum energy (1)
required before a reaction can occur or go or start (1)
Atomic number
Number of protons in one atom or nucleus (1)
Allow protons & electrons
do not allow protons + electrons or electrons 1
Catalyst
A substance that speeds up the reaction / alters the rate but is chemically unchanged at the end / not used up Both ideas needed
Dynamic Equilibrium
Rate of forward reaction = rate backward reaction (1)
concentration remains constant (1)
NOT ‘Equal’, Allow ‘The same’ if clear that means constant 2
Electronegativity
Tendency or strength or ability or power of an atom/element/nucleus to
attract/withdraw electrons / e– density / bonding pair / shared pair 1
In a covalent bond (tied to M1 – unless silly slip in M1)
(If molecule/ion then = CE = 0) (NOT electron (singular) for M1)
Mark as 2 + 2 1
Empirical Formula
(simplest) ratio of atoms of each element in compound (1)
Enthalpy Change
Heat energy change (1)
Not energy on its own
measured at constant pressure (1)
Mark separately, ignore constant temperature statements 2
First Ionisation Energy
The energy required to remove 1 electron from every atom in 1 mole of gaseous atoms to form a mole of gaseous 1+ ions
X –> X^+ + e^–
Hess’s Law.
(The enthalpy change for a reaction is) independent of the route (1)
Isotope
Atoms with the same number of protons / proton number (1)
NOT same atomic number
with different numbers of neutrons (1)
NOT different mass number / fewer neutrons
Le Chatelier’s Principle
Equilibrium opposes a change; (Q of L mark) 1
Mass Number
p + n / number of nucleons
(accept protons and neutrons)
(Incorrect reference to electrons = contradiction)
Mean Bond Enthalpy
(Energy required) to break a given covalent bond into gaseous atoms (1)
averaged over a range of compounds (1)
Penalise first mark if ‘energy’ / ‘enthalpy’ evolved 2
Oxidation
Loss (of electrons) (1)
Oxidising Agent
Species that Gains electrons (or removes electrons) 1
Periodicity
Pattern in the change in the properties of a row of elements (1)
OR Trend in the properties of elements across a period
Repeated in the next row (1)
OR element underneath (or in same group) has similar properties
Rate of Reaction
(Measured) change in concentration (of a substance) in unit time
/ given time
May be written mathematically
OR the gradient of the concentration (against) time1
Reducing Agent
A reducing agent loses (donates) electrons (1)
Reduction
Reduction involves gain of electrons (1)
Relative Atomic Mass
Average/mean mass of (1) atom(s) (of an element) 1
1/12 mass of one atom of 12C 1
OR
(Average) mass of one mole of atoms
1/12 mass of one mole of 12C
OR
(Weighted) average mass of all the isotopes
1/12 mass of one atom of 12C
OR
Average mass of an atom/isotope compared to C-12 on a
scale in which an atom of C-12 has a mass of 12
Not average mass of 1 molecule
Allow the wording Average mass of 1 atom of an element compared to 1/12 mass atom of 12C (or mass 1/12 atom of 12C)
Allow if moles of atoms on both lines
Accept answer in words
Can have top line × 12 instead of bottom line ÷12
If atoms/moles mixed, max = 1
Relative Molecular Mass
(ii) Mean /average mass of a molecule/entity/formula 1
1/12th mass of atom of 12C
[Not 1/12th mass of molecule of 12C]
(mark independently) 1
OR Mass of 1 mole of molecules/entities (1)
1/12thmass of 1 mole of 12C (1)
OR Average mass of a molecule/entity (1)
Relative to the mass of a 12C atom taken as 12 / 12.000 (1)
(Mean/average = stated or explained)
(mass = stated or explained)
(Penalise ‘weight’ once only)
(Ignore ‘average ‘ mass of 12C)
(Do not allow ‘mass of average molecule)
Standard Enthalpy of Combustion
Enthalpy change when 1 mol of a substance
(or compound) (QL mark) 1
is (completely) burned in oxygen (or reacted in excess oxygen) 1
at 298 K and 100 kPa (or under standard conditions) 1
Standard Molar Enthalpy of Formation
(Enthalpy change) when 1 mol (1) of a compound is formed
from its constituent elements (1) in their standard states (1)
Allow energy or heat, Ignore evolved or absorbed
Mark each point independently (b)
Electron orbital
a subdivision of a sub shell and shell. Each orbital is defined by its energy, shape and direction in space. Each orbital contains up to two electrons.
Electron shell
An energy level representing the distance of a group of electrons from the nucleus of an atom.
Ionic bond
A bond formed by the electrostatic forces of attraction between a positively charged ion and a negatively charged ion
covalent bond
Formed when atoms share a pair of electrons. The atoms are held together by the attraction between positive charges in the nuclei and the negative charge on the shared electron pair.
Dative covalent bond
A bond formed when one atom contributes both electrons in a covalent bond ( forms a shared pair of electrons)
