chemistry definitions

Question 1

Activation Energy

Answer 1

Minimum energy (1)
required before a reaction can occur or go or start (1)

Question 2

Atomic number

Answer 2

Number of protons in one atom or nucleus (1)
Allow protons & electrons
do not allow protons + electrons or electrons 1

Question 3

Catalyst

Answer 3

A substance that speeds up the reaction / alters the rate but is chemically unchanged at the end / not used up Both ideas needed

Question 4

Dynamic Equilibrium

Answer 4

Rate of forward reaction = rate backward reaction (1)
concentration remains constant (1)
NOT ‘Equal’, Allow ‘The same’ if clear that means constant 2

Question 5

Electronegativity

Answer 5

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

Question 6

Empirical Formula

Answer 6

(simplest) ratio of atoms of each element in compound (1)

Question 7

Enthalpy Change

Answer 7

Heat energy change (1)
Not energy on its own
measured at constant pressure (1)
Mark separately, ignore constant temperature statements 2

Question 8

First Ionisation Energy

Answer 8

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^–

Question 9

Hess’s Law.

Answer 9

(The enthalpy change for a reaction is) independent of the route (1)

Question 10

Isotope

Answer 10

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

Question 11

Le Chatelier’s Principle

Answer 11

Equilibrium opposes a change; (Q of L mark) 1

Question 12

Mass Number

Answer 12

p + n / number of nucleons
(accept protons and neutrons)
(Incorrect reference to electrons = contradiction)

Question 13

Mean Bond Enthalpy

Answer 13

(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

Question 14

Oxidation

Answer 14

Loss (of electrons) (1)

Question 15

Oxidising Agent

Answer 15

Species that Gains electrons (or removes electrons) 1

Question 16

Periodicity

Answer 16

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

Question 17

Rate of Reaction

Answer 17

(Measured) change in concentration (of a substance) in unit time
/ given time
May be written mathematically
OR the gradient of the concentration (against) time1

Question 18

Reducing Agent

Answer 18

A reducing agent loses (donates) electrons (1)

Question 19

Reduction

Answer 19

Reduction involves gain of electrons (1)

Question 20

Relative Atomic Mass

Answer 20

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

Question 21

Relative Molecular Mass

Answer 21

(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)

Question 22

Standard Enthalpy of Combustion

Answer 22

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

Question 23

Standard Molar Enthalpy of Formation

Answer 23

(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)

Question 24

Electron orbital

Answer 24

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.

Question 25

Electron shell

Answer 25

An energy level representing the distance of a group of electrons from the nucleus of an atom.

Question 26

Ionic bond

Answer 26

A bond formed by the electrostatic forces of attraction between a positively charged ion and a negatively charged ion

Question 27

covalent bond

Answer 27

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.

Question 28

Dative covalent bond

Answer 28

A bond formed when one atom contributes both electrons in a covalent bond ( forms a shared pair of electrons)